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### ventilator

3. 3. CLASSIFICATION OF VENTILATOR MODES: UPDATE AND PROPOSAL FOR IMPLEMENTATIONmake the purchasing decisions. The result is that the term system. Indeed, the equation shows that for any mode,“assist/control” continues to be associated with mode se- only one variable (ie, pressure, volume, or flow) can belection on new ventilators, even though the meaning of the controlled at a time, which greatly simplifies our under-term has changed from its historical roots to the point of standing of ventilator operation. We can simplify mattersvirtual uselessness. Originally, “assist/control” meant vol- even more by recognizing that volume and flow are in-ume-controlled continuous mandatory ventilation (CMV). verse functions (ie, flow is the derivative of volume as aNow it can also refer to pressure control as well. In fact, function of time, and volume is the integral of flow), suchthe term “assist/control” only means that a breath may be that we only need to speak about pressure control versuseither machine-triggered or patient-triggered and thus tech- volume control. It is quite possible to have a very goodnically does not distinguish continuous mandatory venti- clinical understanding of patient-ventilator interaction withlation from intermittent mandatory ventilation (IMV). The nothing more than a conceptual (ie, nonmathematical) ap-term could apply to any of numerous new modes, and thus preciation of this model. Of course, it would be ideal tooffers little of its former descriptive utility. The most prac- understand that the model is a linear differential equationtical uses for the word “control” are to describe how the and all that this implies.ventilator manages pressure, volume, and flow deliverywithin a breath or to describe how the ventilator manages Modethe sequence of mandatory and spontaneous breaths tocreate specific breathing patterns. Perhaps no other word in the mechanical ventilation lexicon is more used and less understood than “mode.”Equation of Motion Intuitively, a ventilation mode must refer to a predefined pattern of interaction between the patient and the ventila- The interaction between the patient and ventilator dur- tor. To be specific, the pattern of interaction is the breath-ing inspiration (and expiration) in terms of pressure, vol- ing pattern. Even more specifically, the breathing patternume, flow, and the time course of these variables is com- refers to the sequence of mandatory and spontaneousplex. Yet these variables can be adequately represented by breaths. Thus, a mode description reduces to a specifica-a mathematical model called the equation of motion for tion of how the ventilator controls pressure, volume, andthe respiratory system.22–25 The simplest version of this flow within a breath, along with a description of how themodel assumes that the complicated respiratory system breaths are sequenced. Indeed, as Beier et al have suggest-can be modeled as a single resistance (R, representing the ed,2 a complete mode description should have 3 compo-artificial airway and natural airways) connected in series nents: (1) a description of the breathing sequence and con-with a single elastance (E, representing lung and chest wall trol variables within breaths, (2) a description of controlelastance). A force balance equation for this model relat- type used within and between breaths, (3) a detailed de-ing the pressure generated by the ventilator at the airway scription of adjunctive control algorithms. This 3-levelopening (Pvent), the pressure generated by the ventilatory mode specification provides the scalability mentionedmuscles (Pmus), the elastic load (PE) and the resistive load above. At the bedside we need only refer to a mode briefly,(PR) can be written as: using the first component. To distinguish among similar modes and brand names we would need to use at least the first and second components. For a complete and unique P vent P mus PE PR mode specification we would use all 3 components.(see the Glossary entry for Equation of motion for a more The Proposalprecise version of this equation relating the variables pres-sure, volume, and flow, along with the parameters elas- This proposal describes a system for specifying venti-tance and resistance). lation modes primarily for educational purposes. Ventila- This model has 2 main functions in mechanical venti- tor manufacturers will probably not adopt this system forlation: (1) to calculate the lung mechanics parameters of creating names for new or existing modes, nor would it beresistance and compliance given information about pres- very practical to do so. But manufacturers would find itsure, volume, and flow, and (2) to predict pressure, vol- helpful to use this system (ie, achieve consensus) to ex-ume, and flow given values for resistance and compliance. plain their products’ capabilities in a way that is consistentThe first application is widely implemented on newer ven- across the field, thereby improving understanding not onlytilators to monitor the patient’s course during changing on the part of customers but also among their own staffs.pathology or in response to treatment. The second appli- The outline in Table 1 defines the proposed classifica-cation is the very basis of ventilator-control theory and tion scheme for ventilation modes. The terms used in thethus a key component of the proposed mode-classification outline are defined in the glossary, which appears after theRESPIRATORY CARE • MARCH 2007 VOL 52 NO 3 303
4. 4. CLASSIFICATION OF VENTILATOR MODES: UPDATE AND PROPOSAL FOR IMPLEMENTATIONTable 1. A 3-Level Ventilator-Mode Classification Scheme* Table 2. All Ventilation Modes Can Be Identified By One of These 8 Breathing Patterns1. Breathing pattern a. Primary breath-control variable Breath-Control Breath Sequence Acronym i. Volume Variable ii. Pressure Volume Continuous mandatory ventilation VC-CMV iii. Dual Intermittent mandatory ventilation VC-IMV b. Breath sequence Pressure Continuous mandatory ventilation PC-CMV i. Continuous mandatory ventilation (CMV) Intermittent mandatory ventilation PC-IMV ii. Intermittent mandatory ventilation (IMV) Continuous spontaneous ventilation PC-CSV iii. Continuous spontaneous ventilation (CSV) Dual Continuous mandatory ventilation DC-CMV2. Control type Intermittent mandatory ventilation DC-IMV a. Tactical control (within breaths) Continuous spontaneous ventilation DC-CSV i. Set point ii. Auto-set-point iii. Servo turer should specify the control variable for each mode. b. Strategic control (between breaths) i. Adaptive For simple set-point control (see the section on control ii. Optimal type, below), the control variable can be identified as fol- c. Intelligent control (between patients) lows: If the peak inspiratory pressure remains constant as i. Knowledge-based the load experienced by the ventilator changes, then the ii. Artificial neural network control variable is pressure. If the peak pressure changes3. Operational algorithms as the load changes but VT remains constant, then the a. Phase variables control variable is volume. Volume control implies flow i. Trigger control and vice versa, but it is possible to distinguish the ii. Limit two on the basis of which signal is used for feedback iii. Cycle control. Some primitive ventilators cannot maintain either iv. Baseline constant peak pressure or VT and thus control only inspira- b. Conditional variables c. Computational logic tory and expiratory times (ie, they may be called time controllers). The control variable should not be confused*These elements can be used to characterize modes of ventilator operation. The specification with the manipulated variable.7 For example, a ventilatorof a mode should begin with, and may just be limited to, a description of the mandatorybreaths. However, a complete specification includes descriptions of both mandatory and manipulates flow to control pressure based on a pressurespontaneous breaths. feedback signal. As mentioned above, the ventilator may control pres- sure or volume during inspiration, but not both. However, it may switch from one control variable to the other duringSummary section below. As mentioned above, the scheme a single inspiration, which leads to the designation of dualis scalable, in that a mode can be describe in increasing control. I first coined the term “dual control” while writingdetail using 1, 2, or all 3 levels, as appropriate for the a chapter in the second edition of Respiratory Care Equip-situation. The following are some specific guidelines for ment.8 At the time it seemed appropriate to consider con-implementation: trol types that automatically adjust the pressure limit to meet a target tidal volume as a form of dual control. How-1. Breathing Pattern ever, in practice this may be confusing. The control vari- able designation is based on the equation of motion, which Given 3 possible control variables (volume, pressure, describes the events within a breath. Adjusting the pres-dual control) and 3 breath sequences (CMV, IMV, and sure limit to meet a target tidal volume is something (atcontinuous spontaneous ventilation [CSV]), there are 8 least at present) that occurs between breaths and is a func-possible breathing patterns (Table 2). (Note that VC-CSV tion of the control type. Thus, the term dual control, as partis not possible, because the definition of volume control of a level 1 description, should be restricted to situations inwould conflict with the definition of a spontaneous breath. which inspiration starts out as volume control and thenVolume control implies that the ventilator determines the switches to pressure control before the end of the breathtidal volume [VT], whereas in a spontaneous breath the (or vice versa). However, it would still be convenient topatient determines the VT.) have a general term that describes automatic adjustment of 1a. Control variable. The control variable is the variable the pressure limit over several breaths to meet a target tidalthat the ventilator uses as a feedback signal to control volume as implemented with adaptive, optimal and knowl-inspiration (ie, pressure, volume, or flow). The manufac- edge based control types. The term “volume targeted pres-304 RESPIRATORY CARE • MARCH 2007 VOL 52 NO 3
5. 5. CLASSIFICATION OF VENTILATOR MODES: UPDATE AND PROPOSAL FOR IMPLEMENTATIONsure control” has been used to describe these schemes. Inother words, pressure is the control variable during inspi-ration. The term “target” is appropriate because the ven-tilator may miss the volume goal for several reasons,whereas the pressure is always controlled to the specificsetpoint during normal operating conditions. A simplerand more general term for any control scheme that allowsautomatic adjustment of setpoints would be “self-adjustingmode.” 1b. Breath sequence. The acronym “CMV” has beenused to mean a variety of things by ventilator manufac-turers. The most logical usage in this classification systemis “continuous mandatory ventilation,” as part of a contin-uum from full ventilatory support to unassisted breathing.The acronym “IMV” has a long history of consistent use tomean intermittent mandatory ventilation (ie, a combina-tion of mandatory and spontaneous breaths). However, thedevelopment of the “active exhalation valve” and otherinnovations has made it possible for the patient to breathespontaneously during a mandatory breath. This is primar-ily a feature to help ensure synchrony between the venti-lator and patient in the event that the mandatory breathparameters (eg, preset inspiratory time, pressure, volume,or flow) do not match the patient’s inspiratory demands.This blurs the historical distinction between CMV andIMV. The key difference now between CMV and IMV isthat with CMV the clinical intent is to make every inspi-ration a mandatory breath, whereas with IMV the clinicalintent is to partition ventilatory support between manda-tory and spontaneous breaths. This means that during CMV,if the patient makes an inspiratory effort after a mandatorybreath cycles off, another mandatory breath is triggered.Thus, if the operator decreases the ventilatory rate (oftenconsidered to be a safety “backup” rate in the event ofapnea), the level of ventilatory support is unaffected solong as the patient continues triggering mandatory breathsat the same rate (ie, each breath is assisted to the samedegree). With IMV, the rate setting directly affects thenumber of mandatory breaths and hence the level of ven-tilatory support, assuming that spontaneous breaths are not Fig. 1. Algorithm to distinguish among the 3 types of breath se- quence: continuous mandatory ventilation (CMV), intermittent man-assisted to the same degree as mandatory breaths (origi- datory ventilation (IMV), and continuous spontaneous ventilationnally spontaneous breaths could not be assisted during (CSV).IMV). CMV is normally considered a method of full ven-tilatory support, whereas IMV is usually viewed as a methodof partial ventilatory support (eg, for weaning). Thus, for Patient triggering can be specified in the level-3 descrip-classification purposes, if spontaneous breaths are not al- tion, under phase variables.lowed between mandatory breaths, the breath sequence is There has been no consistent acronym to signify a breath-CMV; otherwise the sequence is IMV (Fig. 1). Given that ing pattern composed of all spontaneous breaths. The log-almost every ventilator may be patient-triggered, it is no ical progression would be from CMV to IMV to CSVlonger necessary to add the letter S (as in SIMV) to des- (continuous spontaneous ventilation).ignate “synchronized” IMV (ie, the patient may trigger Note that the definitions for assisted breath and spon-mandatory breaths). Such usage was important in the early taneous breath are independent. That is, an assisted breathdays of mechanical ventilation but is an anachronism now. may be spontaneous or mandatory. A spontaneous breathRESPIRATORY CARE • MARCH 2007 VOL 52 NO 3 305
6. 6. CLASSIFICATION OF VENTILATOR MODES: UPDATE AND PROPOSAL FOR IMPLEMENTATIONmay be assisted or unassisted (a mandatory breath is as- on a Maquet Servo-i ventilator. Ask any knowledgeablesisted by definition). Understanding the difference between person you know to describe the differences and see if youassisted and unassisted breaths is important clinically. For can get an accurate, coherent explanation. Then considerexample, when making measurements for the calculation these simple descriptions: pressure support is PC-CSV withof the rapid-shallow breathing index, the breaths must be set-point control of inspiratory pressure; volume support isboth spontaneous and unassisted. There is a common mis- PC-CSV with adaptive control of inspiratory pressure. Ifconception, evident in the way some people talk about you know the definitions of those terms (and they aremodes, that when the patient triggers an inspiration, he explicitly defined in the glossary below), you can imme-somehow “assists” the ventilator. On the contrary, it is diately understand how different the modes are. Your at-always the ventilator that assists the patient. tention would also be directed to the clinical implications for the patient (eg, what settings are required). A level-22. Control Type description also allows the clinician to see that a ventilator function such as Drager’s AutoFlow feature is not just a ¨ The control type is a categorization of the feedback “supplement” or “extra setting,” as the operator’s manualcontrol function of the ventilator. As shown in Table 1, at would have you believe, but indeed creates a whole dif-least 7 different control types have evolved to date. All but ferent mode. For example, operating the Drager Evita 4 in ¨one (artificial neural network control) are commercially “CMV” yields VC-CMV with set-point control of inspira-available at this time. These control types have been de- tory volume and flow. However, activating AutoFlow whenscribed in detail elsewhere7 and are defined again in the CMV is set (ie, CMV AutoFlow) yields PC-CMV withglossary below, and in Table 3. Ventilator control types adaptive control of inspiratory pressure—and vastly dif-display a definite hierarchy of evolutionary complexity. At ferent clinical ramifications for the patient! Indeed, these 2the most basic level, control is focused on what happens modes are about as different as any 2 modes can be. I havewithin a breath. We can call this tactical control, and there many times seen clinicians befuddled simply because theis a very direct need for operator input of static set points nomenclature and description of AutoFlow in the opera-(eg, pressure and flow limits, VT, timing). The next level tor’s manual and sales literature is so misleading.up is what may be called strategic control. With strategic It is important to note that if the breath sequence iscontrol, the ventilator takes over some of the tactical con- IMV, then a complete level-2 description of the mode willtrol normally managed by the human operator. In strategic include both mandatory and spontaneous breaths. For ex-control, the set points are dynamic in that they may be ample, on the Puritan Bennett 840 ventilator, the modeautomatically adjusted by the ventilator over the course of called Synchronized Intermittent Mandatory Ventilationmany breaths, according to some model of desired perfor- would be described as VC-IMV with set-point control ofmance. The operator is somewhat removed in that inputs volume for mandatory breaths and set-point control ofare entered at the level of the model, and they take effect pressure for spontaneous breaths.over several breaths, instead of at the level of individualbreath control. 3. Operational Algorithms Finally, the highest level so far is what might be con-sidered intelligent control, in which the operator can (in At the highest level of detail, the mode description musttheory) be eliminated altogether by artificial intelligence describe the explicit instructions used by the ventilator’sprograms that take over strategic and/or tactical control. control circuit to generate the breathing pattern. Such aNot only dynamic set points but dynamic models of de- description should include a listing of phase variables,sired performance are permitted (eg, one model for pa- conditional variables, and any special artificial intelligencetients with neurological disorders and another for patients programs used.with chronic obstructive pulmonary disease). The artificial 3a. Phase variables. There are some modes that are sointelligence programmed into the computer condenses the similar that a level-2 description will not suffice to distin-experience of experts who have dealt with many patients, guish them. The most common example might be discern-and there is the possibility of the model learning from its ing VC-IMV with and without pressure support. In eitherown experience so that the control actually spans between case, a level-2 description would be the same: VC-IMVpatients. These ideas are summarized in Figure 2. with set-point control of volume for mandatory breaths Specifying the control type in a level-2 description al- and set-point control of pressure for spontaneous breaths.lows us to easily distinguish between modes that look Even using a level-3 description, both modes may have thenearly identical on a graphics monitor but that present same trigger, limit, and cycle variables for mandatory andconceptual/verbal problems when trying to differentiate spontaneous breaths. The difference is that with VC-IMVthem. For example, it might be difficult to appreciate the plus pressure support, the limit variable for spontaneousdifference between pressure support and volume support breaths is pressure with a setting above baseline pressure.306 RESPIRATORY CARE • MARCH 2007 VOL 52 NO 3
8. 8. CLASSIFICATION OF VENTILATOR MODES: UPDATE AND PROPOSAL FOR IMPLEMENTATIONFig. 2. Summary of ventilator control types. For tactical control types, the operator is needed to adjust specific, static set points such asinspiratory pressure limit, tidal volume and inspiratory flow. With strategic control types, set points such as inspiratory pressure limit, andventilatory frequency may change to accommodate changes in the patient’s condition (eg, respiratory-system resistance and compliance).With strategic control, the operator adjusts the parameters of a static mathematical model. The model then adjusts the set points to shapethe breath. With intelligent control, not only are the set points dynamic (ie, machine adjusted) according to a model, but the model itself maychange from one form to another from a list of different models (eg, rule-based expert systems) or it may change by learning (eg, with anartificial neural network).• SmartCare: spontaneous breaths assisted with knowl- able to change the inspiratory time by making either in- edge-based control spiratory or expiratory efforts. If this is not possible, then the breath is, by definition, machine-cycled. For example, One subject of confusion caused by manufacturers has with pressure cycling, the patient can make the inspiratoryto do with pressure limits. For mandatory breaths, the time longer by making an inspiratory effort. Because thepressure limit on some ventilators is set relative to atmo-spheric pressure. But for spontaneous breaths (eg, pressure patient is breathing in, it takes longer for the ventilator tosupport mode), the pressure limit is set relative to the generate the set pressure. (From the ventilator’s point ofpositive end-expiratory pressure (PEEP). Setting the pres- view, it looks like the patient’s compliance has increased.)sure limit relative to PEEP is more useful, because it is the The patient can shorten the inspiratory time by making anchange in pressure relative to baseline (ie, PEEP) during expiratory effort, forcing the pressure to rise more rapidly.inspiration that determines the VT. Thus, a pressure setting Another example of patient cycling is the pressure supportrelative to PEEP carries more information than a pressure mode, in which inspiration ends when flow decays to somesetting relative to atmospheric pressure, because the clini- preset value (ie, flow cycling). Just as with pressure cy-cian must know PEEP to be confident of the implications cling, the patient can either prolong or shorten the timeof the level of ventilation. For example, if PEEP is in- required to reach the threshold flow. If the ventilator iscreased, the VT will decrease for the same peak inspiratory time-cycled, it is by definition machine-cycled, as the pa-pressure setting during PC-IMV on many ventilators. On tient cannot do anything to change the inspiratory timethe other hand, PEEP changes do not affect VT with PC- aside from getting out of bed and turning a knob. VolumeCSV on those same ventilators (Fig. 3). cycling is usually a form of machine cycling, because most When talking about modes, it is sometimes more con- ventilators today deliver the preset volume at a preset flow,venient to say that a breath is either machine-triggered or and this determines the inspiratory time (inspiratorypatient-triggered rather than describe the exact trigger vari- time volume/flow). If a ventilator were designed toable. Similarly, we can use the terms machine-cycled or allow the patient to draw as much flow as needed but stillpatient-cycled. Distinguishing between machine and pa- cycle when the preset volume was delivered, then this typetient triggering is fairly easy, but cycling can be confusing. of volume cycling would be patient cycling, because theFor the breath to be patient-cycled, the patient must be patient could shorten inspiratory time by making an in-308 RESPIRATORY CARE • MARCH 2007 VOL 52 NO 3
9. 9. CLASSIFICATION OF VENTILATOR MODES: UPDATE AND PROPOSAL FOR IMPLEMENTATIONFig. 3. The difference between setting the pressure limit relative to the positive end-expiratory pressure (PEEP) versus relative to atmo-spheric pressure. A: Initial pressure limit and PEEP. B: Pressure limit set relative to PEEP with an increased PEEP. C: Pressure limit setrelative to atmospheric pressure with an increased PEEP. Note that the tidal volume is unchanged in B but decreased in C.spiratory effort. This, however, would not make much cycling and volume cycling are examples of “machinesense from a patient-ventilator-synchrony point of view. cycling.” Pressure and flow cycling are types of “patientThe engineering design response to this situation may be cycling.”to start the breath in volume control with a preset inspira- 3b. Conditional variables. The more complex the mode,tory flow and VT-cycling threshold, then, if the patient the more necessary it is to distinguish it on the basis of thedemands more flow, switch inspiration to pressure control computer logic that manages the events during the differ-with a flow-cycling threshold (eg, as with the Respironics ent phases of the breathing pattern. One way to do this isFlow-Trak feature, a type of dual-control IMV). by specifying conditional variables that are used in pro- As mentioned above, a pressure support breath is cycled grams that determine, for example, if spontaneous minuteoff when the flow decays to a preset value. This is a form ventilation falls below a preset threshold, then deliverof patient cycling, even if the patient is paralyzed (eg, enough mandatory breaths to raise minute ventilation aboveinspiratory time shortens if compliance and/or resistance the threshold. It takes this level of detail to explain, fordecreases). It is also possible that the patient may trigger example, the differences between one form of pressurethe ventilator without using muscle pressure. For example, support breath-ending criteria (eg, with a Newport venti-the Vortran Automatic Resuscitator terminates expiration lator) from corresponding criteria on another brand (eg,(ie, inspiration is triggered on) when the pressure due to with a Siemens ventilator). This level of detail also allowsexpiratory flow against the valve falls below the force the operator to distinguish a feature such as FlowBy on thefrom the spring acting on the other side of the valve. It Puritan Bennett ventilator as a setting for phase variableswould also be possible to build a device that measures (ie, the trigger variable and threshold) rather than being aexpiratory flow and triggers inspiration when a preset mode in and of itself. Of course, any unique combinationthreshold was met (ie, the reverse of flow cycling on pres- of breathing pattern, control type, and operational algo-sure support). The key to understanding these examples of rithms is technically a mode, yet it may not be very prac-passive cycling and triggering is to realize that it is the tical to give it a unique name.patient’s respiratory-system time constant that is respon- 3c. Computational logic. As shown in Figure 2, ad-sible for the action. The patient (rather than the machine) vanced control types employ models that specify fairlytriggers and cycles in these examples, because he can change complex interactions between ventilator and patient. Thehis time constant (actively by invoking muscles) or pas- computational logic is a description of the relationshipsively (by disease). between the inputs (eg, settings), feedback signals, and In summary, time triggering is referred to as “machine outputs (eg, breathing pattern), adding detail about howtriggering.” Pressure-triggering, volume-triggering, and the mode operates that is not given in the other compo-flow-triggering (along with rare mechanisms such as chest- nents of the mode specification. For example, the adaptivewall motion, transthoracic impedance, and diaphragm elec- support ventilation mode on the Hamilton Galileo usestrical activity) may be called “patient triggering.” Time work of breathing as the performance function, and it isRESPIRATORY CARE • MARCH 2007 VOL 52 NO 3 309
10. 10. CLASSIFICATION OF VENTILATOR MODES: UPDATE AND PROPOSAL FOR IMPLEMENTATIONTable 4. Specifications for Some of the Modes Found on the Drager Evita 4 Ventilator* ¨ Breathing Mandatory Breaths† Spontaneous BreathsDrager Mode Name ¨ Pattern Control Type Trigger‡ Limit§ Cycle Control Type Trigger Limit CycleCMV VC-CMV Set point Time Flow, Volume Time NA NA NA NA Computational logic: Every breath is volume-controlled and mandatory. Every breath is machine-triggered and cycled.CMV AutoFlow PC-CMV Adaptive Time, Flow Pressure Time NA NA NA NA Computational logic: Mandatory breaths are pressure-controlled, but the patient may trigger a breath. If the target tidal volume is not met, the pressure limit is automatically adjusted.CMV pressure- DC-CMV Auto-set-point Time, Flow Flow, Volume, Pressure Time NA NA NA NA limited ventilation Computational logic: Mandatory breath starts out in volume-control but switches to pressure-control if the airway pressure reaches the set maximum pressure (Pmax).SIMV VC-IMV Set point Time, Flow Flow, Volume Time Set point Pressure Pressure Pressure Computational logic: Mandatory breaths are volume-controlled. Spontaneous breaths may occur within the window determined by the set rate and are not assisted (ie, inspiratory pressure stays at baseline).PC PC-IMV Set point Time, Flow Pressure Time Set point Pressure Pressure Pressure Computational logic: Mandatory breaths are pressure-controlled. Spontaneous breaths may occur within the window determined by the set rate and are not assisted (ie, inspiratory pressure stays at baseline).SIMV AutoFlow PC-IMV Adaptive Time, Flow Pressure Time Set point Pressure Pressure Pressure Computational logic: Mandatory breaths are pressure-controlled and the pressure limit is automatically adjusted if the target tidal volume is not met. Spontaneous breaths may occur, within the window determined by the set rate, and are not assisted (ie, inspiratory pressure stays at baseline).CPAP PC-CSV NA NA NA NA Set point Pressure Pressure Pressure Computational logic: Spontaneous breaths are unassisted.Pressure support PC-CSV NA NA NA NA Set point Flow Pressure Flow Computational logic: Spontaneous breaths are assisted (ie, inspiratory pressure rises above baseline).*Figures 4 through 11 illustrate the corresponding pressure, volume, and flow waveforms.†The patient can take spontaneous breaths during mandatory breaths with PC and AutoFlow, but not with pressure-limited ventilation.‡Flow-triggering may be turned off. When off, mandatory breaths cannot be triggered, but spontaneous breaths are automatically pressure-triggered with factory-set sensitivity.§Volume limit occurs if inspiratory time is set longer than (tidal volume/flow). Volume limit may occur for any volume-controlled breath.NA not available. CMV continuous mandatory ventilation (all breaths are mandatory). VC volume-controlled. PC pressure-controlled. DC dual-controlled. SIMV spontaneousintermittent mandatory ventilation (spontaneous breaths between mandatory breaths). CPAP continuous positive airway pressure. CSV continuous spontaneous ventilation (all breaths arespontaneous).related to lung mechanics, alveolar ventilation, dead-space SmartCare mode on the Drager Evita XL uses a rule-based ¨volume, and breathing frequency.26 As lung mechanics expert system to keep the patient in a “comfort zone,”change, the ventilator finds the optimum frequency (to based on ventilatory rate, VT, and end-tidal carbon dioxideminimize work) and then sets the VT to meet the minute level.27 However, the use of “fuzzy logic” and artificial neu-ventilation requirement. This mode also employs a number ral networks in ventilator control systems may eliminate theof rules that ensure a lung-protective strategy. These rules possibility of generating explicit decision rules and may thuswould be part of the computational logic description. The improve care while making it less understandable.28310 RESPIRATORY CARE • MARCH 2007 VOL 52 NO 3
11. 11. CLASSIFICATION OF VENTILATOR MODES: UPDATE AND PROPOSAL FOR IMPLEMENTATIONFig. 4. Drager Evita 4 mode called CMV, which is a volume-controlled continuous mandatory ventilation mode (VC-CMV) with set-point ¨control (see Table 4). Breaths during VC-CMV can be machine triggered (A) or patient-triggered (B). Although VC-CMV is often chosen torest the patient, patient effort does not necessarily stop. When airway pressure drops below baseline, the patient is doing work on theventilator. When the airway pressure is above baseline, the ventilator is doing work on the patient. The difference in the areas betweenpassive inspiration (A) and active inspiration (C) indicates that the patient does work during inspiration. (From Reference 9, with permission.) Summary modes in operator’s manuals and educational materials. An example of the utility of this scheme is illustrated in Table 4, Ventilator manufacturers and the respiratory care aca- which gives detailed specifications for a sample of modesdemic community have not yet adopted a standardized available on the Drager Evita 4 ventilator. Graphic represen- ¨system for classifying and describing ventilation modes. tations of these modes are given in Figures 4 –11. A goodAs a result, there is much confusion among their custom- example of how the classification scheme can also be applieders, as well as their own staff, with the result that potential to design efficient user interfaces is shown in Figure 12.sales, education, and patient care are all put at risk. Thisproposal summarizes a ventilator-mode classificationscheme and complete lexicon that has been extensively Glossarypublished over the last 15 years. In addition, I have pre-sented practical considerations for implementing the Adaptive control. One set point (eg, the pressure limit)scheme as the primary means of identifying ventilation of the ventilator is automatically adjusted over severalRESPIRATORY CARE • MARCH 2007 VOL 52 NO 3 311
12. 12. CLASSIFICATION OF VENTILATOR MODES: UPDATE AND PROPOSAL FOR IMPLEMENTATIONFig. 5. Drager Evita 4 mode called CMV AutoFlow, which is a pressure-controlled continuous mandatory ventilation mode (PC-CMV) with ¨adaptive control (see Table 4). During steady state, the pressure limit is just high enough to achieve the set tidal volume (A). If the patientsuddenly makes a large inspiratory effort, muscle pressure adds to ventilator pressure and the resultant tidal volume is larger than the settidal volume (B). The ventilator compensates by lowering the pressure limit (C). (From Reference 9, with permission.)breaths to maintain another set point (eg, the target VT) as the exhalation assist feature on the Venturi ventilator oras the patient’s condition changes (eg, pressure-regu- automatic tube compensation on the Drager Evita 4 ven- ¨lated volume control mode on the Maquet Servo-i ven- tilator). In contrast, spontaneous breaths during continuoustilator). Thus, the ventilator adapts to the need for a positive airway pressure (CPAP) are unassisted, becausechanging set point. the ventilator attempts to maintain a constant airway pres- sure during inspiration.Assisted breath. A breath during which all or part ofinspiratory (or expiratory) flow is generated by the venti- Automatic set-point control. The ventilator automati-lator doing work on the patient. In simple terms, if the cally selects the set point enforced at the moment. Forairway pressure rises above end-expiratory pressure dur- example, the ventilator’s output is automatically adjusteding inspiration, the breath is assisted (as in pressure sup- during the breath to maintain the set VT, using either theport mode). It is also possible to assist expiration by drop- set pressure limit or the set inspiratory flow. The breathping airway pressure below end-expiratory pressure (such can start out as pressure-controlled and automatically switch312 RESPIRATORY CARE • MARCH 2007 VOL 52 NO 3
13. 13. CLASSIFICATION OF VENTILATOR MODES: UPDATE AND PROPOSAL FOR IMPLEMENTATIONFig. 6. A: Volume-controlled continuous mandatory ventilation (VC-CMV) versus (B) the Drager Evita 4 mode called CMV pressure limited ¨ventilation, which is a dual-control CMV. Using automatic set-point control, the breath in B starts out in volume control but switches topressure control when airway pressure reaches the set maximum (Pmax). Flow stops when the set tidal volume is met. An inspiratory holdensues until the set inspiratory time is met and the breath is cycled off. Notice that the inspiratory flow time is increased while the inspiratorypause time is decreased, compared to breath in A, to assure tidal volume delivery within the set inspiratory time. Pmax results in a lower peakpressure at the airway opening but the same pressure in the lungs, because of the same tidal volume as the breath in A. (From Reference 9,with permission.)to volume-controlled (eg, the Bird VAPS [volume-assured (PEEP) set by the clinician. AutoPEEP is the pressurepressure support] mode) or vice versa (eg, the Drager pres- ¨ associated with the trapped gas when dynamic hyperinfla-sure-limited ventilation mode). tion occurs. See dynamic hyperinflation.Automatic tube compensation. A feature that allowsthe operator to enter the size of the patient’s endotracheal Auto-trigger. (Sometimes mistakenly called “auto-cy-tube and have the ventilator calculate the tube’s resistance cling.”) A condition in which the ventilator repeatedlyand then generate just enough pressure (in proportion to triggers itself because the sensitivity is set too high. Forinspiratory or expiratory flow) to compensate for the added pressure triggering, the ventilator may auto-trigger dueresistive load. See servo control. to a leak in the system that drops airway pressure below the pressure-trigger threshold. When sensitivity is setAutoPEEP. The positive difference between end-expira- too high, even the heartbeat can cause inadvertent trig-tory alveolar pressure and the end-expiratory pressure gering.RESPIRATORY CARE • MARCH 2007 VOL 52 NO 3 313
14. 14. CLASSIFICATION OF VENTILATOR MODES: UPDATE AND PROPOSAL FOR IMPLEMENTATIONFig. 7. Drager Evita 4 mode called SIMV, which is a volume-controlled intermittent mandatory ventilation (VC-IMV) mode with set-point ¨control. Mandatory breaths may be machine-triggered (A) or patient-triggered (C). When patient triggering is allowed, this mode is oftenreferred to as synchronized IMV (SIMV). Unassisted spontaneous breaths may occur between mandatory breaths (B). The small fluctuationin airway pressure during spontaneous breaths is due to the resistance of the expiratory limb of the patient circuit and/or the opening delayin the demand flow valve. (From Reference 9, with permission.)Breath. A positive change in airway flow (inspiration) CMV. Continuous mandatory ventilation, in which allpaired with a negative change in airway flow (expiration), breaths are mandatory, unless there is a provision forassociated with ventilation of the lungs. This definition spontaneous breaths during mandatory breaths (ie, us-excludes flow changes caused by hiccups or cardiogenic ing a so-called active exhalation valve). The definingoscillations. However, it allows the superimposition of, for characteristic is that spontaneous breaths are not per-example, a spontaneous breath on a mandatory breath or mitted between mandatory breaths, because inspiratoryvice versa. efforts after a mandatory breath always trigger another mandatory breath.Breathing pattern. A sequence of breaths (CMV, IMV, Conditional variable. A variable used by a ventilator’sor CSV) with a designated control variable (volume, pres- operational logic system to make decisions on how tosure, or dual control) for the mandatory breaths (or the manage control and phase variables. Conditional variablesspontaneous breaths in CSV). can be described in terms of “if-then” statements. For314 RESPIRATORY CARE • MARCH 2007 VOL 52 NO 3
15. 15. CLASSIFICATION OF VENTILATOR MODES: UPDATE AND PROPOSAL FOR IMPLEMENTATIONFig. 8. Drager Evita 4 mode called PC , which is a pressure-controlled intermittent mandatory ventilation (PC-IMV) mode with set-point ¨control. Mandatory breaths may be machine-triggered (A) or patient-triggered (C). When patient triggering is allowed, this mode is oftenreferred to as synchronized IMV (SIMV). Unassisted spontaneous breaths may occur between mandatory breaths (B). The small fluctuationin airway pressure during spontaneous breaths is due to the resistance of the expiratory limb of the patient circuit and/or the opening delayin the demand flow valve. (From Reference 9, with permission.)example, if minute ventilation is below the set threshold, remains constant, then the control variable is volume. Vol-then deliver a mandatory breath. ume control implies flow control and vice versa, but it is possible to distinguish the 2 on the basis of which signal isControl type. The control type is a categorization of the used for feedback control. Some primitive ventilators can-ventilator’s feedback control function. not maintain either constant peak pressure or tidal volume, and thus control only inspiratory and expiratory times (ie,Control variable. The control variable is the variable they may be called time controllers).that the ventilator uses as a feedback signal to controlinspiration (ie, pressure, volume, or flow). For simple set-point control (see control type), the control variable can be Conventional ventilator. A ventilator that producesidentified as follows: If the peak inspiratory pressure re- breathing patterns that mimic the way humans normallymains constant as the load experienced by the ventilator breathe, at rates and tidal volumes our bodies producechanges, then the control variable is pressure. If the peak during our usual living activities: 12–25 breaths/min forpressure changes as the load changes but tidal volume children and adults, 30 – 40 breaths/min for infants.RESPIRATORY CARE • MARCH 2007 VOL 52 NO 3 315
16. 16. CLASSIFICATION OF VENTILATOR MODES: UPDATE AND PROPOSAL FOR IMPLEMENTATIONFig. 9. Drager Evita 4 mode called SIMV AutoFlow, which is a pressure-controlled intermittent mandatory ventilation (PC-IMV) mode with ¨adaptive control. Spontaneous breaths are allowed between mandatory breaths (A). The small fluctuation in airway pressure duringspontaneous breaths is due to the resistance of the expiratory limb of the patient circuit and/or the opening delay in the demand flow valve.If inspiratory effort decreases, the next mandatory breath (B) will result in a reduced tidal volume. Using adaptive control, the ventilatorautomatically increases the pressure limit to achieve the set tidal volume (C). (From Reference 9, with permission.)CPAP. Continuous positive airway pressure, which is a DC-CMV. Dual-controlled continuous mandatory venti-constant pressure maintained at the airway opening through- lation.out the breathing cycle. CPAP is usually associated withunassisted breathing. DC-CSV. Dual-controlled continuous spontaneous ven- tilation.CSV. Continuous spontaneous ventilation, in which allbreaths are spontaneous. DC-IMV. Dual-controlled intermittent mandatory venti- lation.Cycle. Verb: To end the inspiratory time (and begin ex-piratory flow). Noun: A breath (inspiration and expira- Dual control. The control variable switches betweention). pressure and volume within a breath. Control can switch from volume to pressure (eg, pressure-limited ventila-Cycle variable. The variable (usually pressure, volume, tion mode on the Drager Evita 4) or from pressure to ¨flow, or time) that is measured and used to end inspiration volume (eg, volume-assured pressure support mode on(and begin expiratory flow). the Bird 8400).316 RESPIRATORY CARE • MARCH 2007 VOL 52 NO 3
17. 17. CLASSIFICATION OF VENTILATOR MODES: UPDATE AND PROPOSAL FOR IMPLEMENTATIONFig. 10. Drager Evita 4 mode called CPAP, which is a pressure-controlled continuous spontaneous ventilation (PC-CSV) mode (unassisted). ¨Continuous positive airway pressure (CPAP) allows the patient to breathe spontaneously at a near constant pressure above atmosphericpressure. Because this is a CSV mode, if there is an apnea the patient will not be ventilated (A). Any changes in airway pressure are dueto the fact that ventilators are not perfect pressure controllers. A drop in pressure during inspiration and a rise in pressure during expirationindicate that the patient is doing work on the ventilator, due to resistance of the exhalation valve and/or the opening delay of the inspiratoryflow demand valve. A good pressure controller will keep the airway pressure fluctuations at a minimum regardless of the patient effort. (FromReference 9, with permission.)Dynamic compliance. The slope of the pressure-volume compressible contents relative to some reference amount.curve drawn between 2 points of zero flow (eg, at the start For a linear system it is elastance volume, or, volume/and end of inspiration). compliance. For a container, the overall effective elastance (compliance) includes the elastances (compliances) of itsDynamic hyperinflation. The increase in lung volume structural components and the compressibility of the fluidthat occurs whenever insufficient exhalation time prevents (gas or liquid) within it.the respiratory system from returning to its normal restingend-expiratory equilibrium volume between breath cycles. Equation of motion for the respiratory system. A re- lationship among pressure difference, volume, and flow,Elastic load. The pressure difference applied across a that describes the mechanics of the respiratory system. Thesystem (eg, a container) that sustains the system’s volume simplest and most useful form is a differential equationrelative to some reference volume, and/or the amount of its with constant coefficients that describes the respiratoryRESPIRATORY CARE • MARCH 2007 VOL 52 NO 3 317
18. 18. CLASSIFICATION OF VENTILATOR MODES: UPDATE AND PROPOSAL FOR IMPLEMENTATIONFig. 11. Drager Evita 4 mode called Pressure Support, which is a pressure-controlled continuous spontaneous ventilation (PC-CSV) mode ¨(assisted). Because this is a CSV mode, if there is an apnea the patient will not be ventilated (A). Pressure support breaths are typicallypressure-triggered or flow-triggered, pressure-limited (above the positive end-expiratory pressure) and flow-cycled. A relatively smallinspiratory effort results in a relatively short inspiratory time (B vs C). Usually the flow cycle threshold is preset by the manufacturer as apercentage of peak flow or as an absolute flow setting. Some ventilators allow the operator to set the flow cycle threshold and the pressurerise time (the time required for airway pressure to reach the set pressure limit), to improve ventilator-patient synchrony. Rise time affectsthe shape of the pressure waveform and hence the volume and flow waveforms. A short rise time gives a more rectangular shape, whereasa long rise time produces a more triangular shape (C) (From Reference 9, with permission.).system as a single deformable compartment that includes piratory pressure. This is the pressure gener-the lungs and chest wall. The version shown below, is ated by a ventilator ( pvent) during an assistedoften simplified by ignoring the inertia term (Iv): ¨ breath. pmus ventilatory muscle pressure difference; the the- p TR p mus Ev Rv ˙ Iv ¨ oretical chestwall transmural pressure differ- ence that would produce movements identicalwhere to those produced by the ventilatory muscles pTR the change in transrespiratory pressure differ- during breathing maneuvers (positive during ence (ie, airway opening pressure minus body inspiratory effort, negative during expiratory surface pressure), measured relative to end ex- effort).318 RESPIRATORY CARE • MARCH 2007 VOL 52 NO 3
19. 19. CLASSIFICATION OF VENTILATOR MODES: UPDATE AND PROPOSAL FOR IMPLEMENTATIONFig. 12. Front panel of the Newport e360 ventilator. Note the convenient placement of controls to select the breathing pattern (ie, controlvariable and breath sequence) in the MODE area. The operator may adjust details of the operational algorithms (eg, phase variables andthresholds) to the right of the MODE area. v volume change relative to functional residual where capacity. pvent the transrespiratory pressure generated by the ventilator during an assisted breath. v ˙ flow, which is the first derivative of volume with respect to time, measured relative to the end-expiratory flow (usually zero). Expiratory flow time. The period from the start of ex- v the second derivative of volume with respect ¨ piratory flow to the instant when expiratory flow stops. to time. E elastance (inverse of compliance E 1/C). Expiratory hold. Occlusion of the airway at the moment R resistance. when the next inspiration would start; usually implemented I inertance. to measure intrinsic positive end-expiratory pressure For mechanical ventilation the equation is often ex- (autoPEEP).pressed as: Expiratory pause time. The period from cessation of p vent p mus Ev Rv ˙ expiratory flow to the start of inspiratory flow.RESPIRATORY CARE • MARCH 2007 VOL 52 NO 3 319
20. 20. CLASSIFICATION OF VENTILATOR MODES: UPDATE AND PROPOSAL FOR IMPLEMENTATIONExpiratory time. The period from the start of expiratory Inspiratory time. The period from the start of inspira-flow to the start of inspiratory flow. Expiratory time equals tory flow to the start of expiratory flow. Inspiratoryexpiratory flow time plus expiratory pause time. time equals inspiratory flow time plus inspiratory pause time.Feedback control. Closed-loop control accomplished byusing the output as a signal that is fed back (compared) to Intelligent control. A class of ventilator control typesthe operator-set input. The difference between the 2 is that implement strategic control and/or tactical control,used to drive the system toward the desired output (ie, using artificial intelligence programs.negative feedback control). For example, pressure- Knowledge-based control. A type of ventilator controlcontrolled modes use airway pressure as the feedback sig- that attempts to capture the experience of human ex-nal to manipulate gas flow from the ventilator to maintain perts. It may use various artificial intelligence systems,an inspiratory pressure set point. such as branching logic algorithms, lookup tables, or fuzzy logic.Flow control. Maintenance of an invariant inspiratoryflow waveform despite changing respiratory-system me- Leak. The steady-state difference between the inspiredchanics. VT produced by the ventilator and the expired volume produced by the patient.Flow cycle. Inspiration ends and expiratory flow startswhen inspiratory flow reaches a preset threshold. Limit. To restrict the magnitude of a variable (pressure, volume, or flow) to some preset value.Flow limit. Inspiratory flow reaches a preset value thatmay be maintained before inspiration cycles off. Limit variable. A variable that can reach and be main- tained at a preset level before inspiration ends but does notFlow trigger. Assisted inspiration starts when inspira- end inspiration. Pressure, flow, or volume can be the limittory flow due to patient inspiratory effort reaches a preset variable.threshold. Load. The pressure required to generate inspiration. SeeHigh-frequency jet ventilation. Ventilation by means elastic load and resistive load.of a high-frequency low-volume pulsed jet of gas into thetrachea. Loop display. A graphic display of one variable against another, such as flow on the vertical axis and volume onHigh-frequency oscillatory ventilation. Ventilation by the horizontal axis.means of a piston arrangement (or other mechanism) that Mandatory breath. A breath in which the timing and/ormoves gas back and forth rapidly in the patient’s breathing size of the breath is controlled by the ventilator. That is,circuit and airways, causing pressure to oscillate above the machine triggers and/or cycles the breath.and below baseline pressure. Mandatory minute ventilation. A mode in which theHigh-frequency ventilator. Ventilator that produces ventilator monitors the exhaled minute ventilation as abreathing patterns at frequencies much higher than can be conditional variable. As long as the patient either triggersvoluntarily produced (150 –900 cycles per minute). mandatory breaths or generates his own spontaneous breath often enough to maintain a preset minute ventilation, theIMV. Intermittent mandatory ventilation, in which spon- ventilator does not interfere. However, if the exhaled minutetaneous breaths are permitted between mandatory breaths. ventilation falls below the operator-set value, the ventila-When the mandatory breath is patient-triggered, it is com- tor will trigger mandatory breaths or increase the pressuremonly referred to as synchronized IMV (or SIMV). limit until the target is reached.Inspiratory flow time. The period from the start of in- Mean airway pressure. The average pressure at the air-spiratory flow to the cessation of inspiratory flow. way opening; the mean airway pressure is the area under the pressure-time curve for one breathing cycle divided byInspiratory pause time. The period from when inspira- the total breathing-cycle time (ie, inspiratory time plustory flow stops to the start of expiratory flow. expiratory time).320 RESPIRATORY CARE • MARCH 2007 VOL 52 NO 3