Contents Classification History Introduction Indications Key terms- compliance , ventilatory work Components Control mechanism Variables Triggering Factors to consider in mechanical ventilation Wave-forms
Classification According to Robert chatburn Broadly classified into Negative pressure ventilators And according to the manner in which Positive pressure ventilators they support ventilation
Negative pressure ventilators Exert a negative pressure on the external chest Decreasing the intrathoracic pressure during inspiration allows air to flow into the lung, filling its volume Physiologically, this type of assissted ventilation is similar to spontaneous ventilation It is used mainly in chronic respiratory failure associated with neuromascular conditions such as poliomyleitis, muscular dystrophy, a myotrophic lateral sclerosis, and mysthenia gravis.
The iron lung, often referred to in the early days as the "Drinker respirator", was invented by Phillip Drinker(1894 – 1972) and Louis Agassiz Shaw Junior, professors of industrial hygiene at the Harvard School of Public Health . The machine was powered by an electric motor with air pumps from two vacuum cleaners. The air pumps changed the pressure inside a rectangular, airtight metal box, pulling air in and out of the lungs
Biphasic cuirass ventilation Biphasic cuirass ventilation (BCV) is a method of ventilation which requires the patient to wear an upper body shell or cuirass, so named after the body armour worn by medieval soldiers. The ventilation is biphasic because the cuirass is attached to a pump which actively controls both the inspiratory and expiratory phases of the respiratory cycle .
Disadvantages Complex and Cumbersome Difficult for transporting Difficult to access the patient in emergency claustrophobic
Positive pressure ventilators Inflate the lungs by exerting positive pressure on the airway, similar to a bellows mechanism, forcing the alveoli to expand during inspiration Expiration occurs passively. modern ventilators are mainly PPV s and are classified based on related features, principles and engineering.
History Andreas Vesalius (1555) Vesalius is credited with the first description of positive- pressure ventilation, but it took 400 years to apply his concept to patient care. The occasion was the polio epidemic of 1955, when the demand for assisted ventilation outgrew the supply of negative-pressure tank ventilators (known as iron lungs). In Sweden, all medical schools shut down and medical students worked in 8-hour shifts as human ventilators, manually inflating the lungs of afflicted patients. Invasive ventilation first used at Massachusetts General Hospital in 1955. Thus began the era of positive-pressure mechanical ventilation (and the era of intensive care medicine).
INTRODUCTION TO MECHANICALVENTILATION: CONVENTIONAL MECHANICAL VENTILATION Mechanical ventilation is a useful modality for patients who are unable to sustain the level of ventilation necessary to maintain the gas exchange functions- oxygenation and carbon dioxide elimination The first positive-pressure ventilators were designed to inflate the lungs until a preset pressure was reached. In contrast, volume-cycled ventilation, which inflates the lungs to a predetermined volume, delivers a constant alveolar volume despite changes in the mechanical properties of the lungs.
INDICATIONS FOR MECHANICALVENTILATION Respiratory Failure Cardiac Insufficiency Neurologic dysfunction Rule 1. The indication for intubation and mechanical ventilation is thinking of it. Rule 2. Endotracheal tubes are not a disease, and ventilators are not an addiction
Key terms Ventilatory work- During inspiration , the size of the thoracic cage increases overcoming the elastic forces of the lungs and the thorax and resistance of the airways. As the volume of the thoracic cage increases, intrapleural pressure becomes more negative, resulting in lung expansion. Gas flows from the atmosphere into the lungs as a result of transairway pressure gradient. During expiration, the elastic forces of the lung and thorax cause the chest to decrease in volume and exhalation occurs as a result of greater pressure at the alveolus compared to atm. Press.
This ventilatory work is proportional to the pressure required for inspiration times the tidal volume. LOAD- The pressure required to deliver the tidal volume is referred to as the load that the muscles or ventilator must work against. load elastic ( α volume & inv. Prop t0 compliance) resistance (α Raw & inspiratory flow)
Equation of motion for respiratorysystem Muscle pressure + ventilator pressure = (volume / compliance)+ (resistance x flow) Flow- it’s the unit of volume by unit of time. Resistance- it is the force that must be overcome to move the gas through the conducting airways. It is described by the poiseulle’s law.
Lung compliance Lung compliance: Is the change in volume per unit change in pressure COMPLIANCE = Volume / Pressure
Types Static compliance- is measured when there is no air flow. Reflects the elastic properties of the lung and the chest wall Dynamic compliance is measured when air flow is present Reflects the airway resistance (non elastic resistance) and elastic properties of lung and chest wall Static compliance=Corrected tidal volume Plateau pressure-PEEP Dynamic compliance corrected tidal volume Peak inspiratory pressure-PEEP
What is a mechanical ventilator? A machine or a device that fully or partially substitute for the ventilatory work accomplished by the patients muscles. Components – INPUT POWER DRIVE MECHANISM CONTROL CIRCUIT OUTPUT WAVEFORMS ALARMS
INPUT POWER It can be Pneumatically powered(uses compressed gases) Electrically powered(uses 120 Volts AC/12Volts DC)Here the electric motor drives pistons and compressors to generate gas flows . Microprocessor controlled- combined.Also called as 3rd generation ventilators.
Source of Gas Supply Air - Central compressed air, compressor, turbine flow generator, etc Oxygen – Central oxygen source, O2 concentrator, O2 cylinder Gas mixing unit – O2 blender
DRIVE MECHANISM It’s the system used by the ventilator to transmit or convert the input power to useful ventilatory work. This determines the characteristic flow and pressure patterns produced by the ventilator. It includes pistons bellows reducing valves pneumatic circuits
Pneumatic circuits- uses pressurized gas as power source. these are microprocessor controlled with solenoid valves. use programmed algorithms in microprocessor to open and close solenoid valves to mimic any flow or pressure wave pattern.
Control circuit Its the system that governs the ventilator drive mechanism or output control valve. Classified as- Open circuits- desired output is selected and venti. achieves it without any further input from clinician. Closed circuits- desired output is selected and venti. Measures a specific parameter (flow/vol/press) continuously and input is constantly adjusted to match desired output.a.k.a SERVO controlled.
Control parameters Pressure Volume Flow Time
Ventilators deliver gas to the lungs using positive pressure at a certain rate. The amount of gas delivered can be limited by time, pressure or volume. The duration can be cycled by time, pressure or flow.If volume is set, pressure varies…..if pressure is set, volume varies….. ….according to the compliance…...
Mechanical- employs levers or pulleys to control drive mechanism. Pneumatic Fluidic- applies gas flows and pressure to control direction of other gas flows and to perform logic functions based on the COANDA effect. Electronic- uses resistors and diodes and integrated circuits to provide control over the drive mechanism.
Pressure controller Ventilator controls the trans-respiratory system pressure . This trans-respiratory system gradient determines the depth or volume of respiration. Based on this a ventilator can be positive or negative pressure ventilator.
Volume controller Volume cycled ventilation delivers a: set volume; with a variable Pressure - determined by resistance, compliance and inspiratory effort
Flow controller Allows pressure to vary with changes in patient s compliance and resistance while controlling flow. This flow is measured by vortex sensors or venturi pnemotachometers.Time controllermeasures and controls inspiratory and expiratory time.These ventilators are used in newborns and infants Inspiratory time is a combination of the inspiratory flow period and time taken for inspiratory pause. The following diagram depicts how the addition of an inspiratory pause extends total inspiratory time.
Normal inspiratory time of a spontaneously breathing healthy adult is approximately 0.8- 1.2 seconds, with an inspiratory expiratory (I: E) ratio of 1:1.5 to 1:2 2.Its advantageous to extend the inspiratory time in order to:• improve oxygenation - through the addition of an inspiratory pause; or to•increase tidal volume - in pressure controlled ventilationAdverse effects of excessively long inspiratory times are haemodynamic compromise,patient ventilator dysynchrony, and the development of autoPEEP.
Phase variablesA. Trigger ……. What causes the breath to begin? B CB. Limit …… What regulates gas flow during the breath? AC. Cycle ……. What causes the breath to end?
Phases of ventilator supported breath inspiration change from inspiration to expiration expiration change from expiration to inspirationTypes of ventilator breaths- Mandatory breath Assisted breath Spontaneous breath
Trigger variable It’s the variable that determines start of inspiration Triggering refers to the mechanism through which the ventilator senses inspiratory effort and delivers gas flow or a machine breath in concert with the patient’s inspiratory effort. Can use pressure or volume or time or flow as a trigger. In modern ventilators the demand valve is triggered by either a fall in pressure (pressure triggered) or a change in flow (flow triggered). With pressure triggered a preset pressure sensitivity has to be achieved before the ventilator delivers fresh gas into the inspiratory circuit. With flow triggered a preset flow sensitivity is employed as the trigger mechanism.
Pressure Triggering Breath is delivered when ventilator senses patients spontaneous inspiratory effort. sensitivity refers to the amount of negative pressure the patient must generate to receive a breath/gas flow. If the sensitivity is set at 1 cm then the patient must generate 1 cm H2O of negative pressure for the machine to sense the patients effort and deliver a breath. Acceptable range - -1 to -5 cm H2O below patient s baseline pressure If the sensitivity is too high the patients work of breathing will be unnecessarily increased. It is not a reasonable course of action to increase the sensitivity to reduce the patients respiratory rate as it only increases their work of breathing.
Flow Triggering The flow triggered system has two preset variables for triggering, the base flow and flow sensitivity. The base flow consists of fresh gas that flows continuously through the circuit. The patient’s earliest demand for flow is satisfied by the base flow. The flow sensitivity is computed as the difference between the base flow and the exhaled flow Here delivered flow= base flow- returned flow Hence the flow sensitivity is the magnitude of the flow diverted from the exhalation circuit into the patient’s lungs. As the subject inhales and the set flow sensitivity is reached the flow pressure control algorithm is activated, the proportional valve opens, and fresh gas is delivered.
•Flow triggerAdvantages --The time taken for the onset of inspiratory effort to the onset ofinspiratory flow is considerably less. -decreases the work involved in initiating a breath.
Cycle variable Defined as the length of one complete breathing cycle. Inspiration ends when a specific cycle variable is reached. This variable is used as a feedback signal to end inspiratory flow delivery which then allows exhalation to start. Most new ventilators measure flow and use it as a feedback signal. So volume becomes a function of flow and time Volume= flow x inspiratory time
Baseline variable The variable controlled during expiration phase. Mostly its pressure
Basic definitions Airway Pressures Peak Inspiratory Pressure (PIP) Plateau pressures Positive End Expiratory Pressure (PEEP) Continuous Positive Airway Pressure (CPAP) Inspiratory Time or I:E ratio Tidal Volume: amount of gas delivered with each breath
Pressures Mechanical ventilation delivers flow and volume to the patient’s as a result of the development of a positive pressure gradient between the ventilator circuit and the patient’s gas exchange units as illustrated in the diagram above. There are four pressures to be aware of in regards to mechanical ventilation. These are the: Peak Plateau Mean; and End expiratory pressures.
Peak Inspiratory Pressure (PIP)- The peak pressure is the maximum pressure obtainable during active gas delivery. This pressure a function of the compliance of the lung and thorax and the airway resistance including the contribution made by the tracheal tube and the ventilator circuit. Maintained at <45cm H2O to minimize barotrauma Plateau Pressure-The plateau pressure is defined as the end inspiratory pressure during a period of no gas flow. The plateau pressure reflects lung and chest wall compliance.
As the plateau pressure is the pressure when there is no flow within the circuit and patient airways it most closely represents the alveolar pressure and thus is of considerable significance as it desirable to limit the pressure that the alveoli are subjected to. Excessive pressure may result in extrapulmonary air (eg pneumothorax) and acute lung injury. An increase in airways resistance (including ETT resistance) will result in an increase in PIP. An increase in resistance will result in a widening of the difference between PIP and plateau pressure. A fall in compliance will elevate both PIP and plateau pressure.
It is generally believed that end inspiratory occlusion pressure (ie plateau pressure) is the best clinically applicable estimate of average peak alveolar pressure. Although controversial it has been generally recommended that the plateau pressure should be limited to 35 cms H2O.
Mean Airway Pressure- The mean airway pressure is an average of the system pressure over the entire ventilatory period. End Expiratory Pressure- End expiratory pressure is the airway pressure at the termination of the expiratory phase and is normally equal to atmospheric or the applied PEEP level.
PEEP Positive end expiratory pressure (PEEP) refers to the application of a fixed amount of positive pressure applied during mechanical ventilation cycle Continuous positive airway pressure (CPAP) refers to the addition of a fixed amount of positive airway pressure to spontaneous respirations, in the presence or absence of an endotracheal tube. PEEP and CPAP are not separate modes of ventilation as they do not provide ventilation. Rather they are used together with other modes of ventilation or during spontaneous breathing to improve oxygenation, recruit alveoli, and / or decrease the work of breathing
Advantages ability to increase functional residual capacity (FRC) and keep FRC above Closing Capacity. The increase in FRC is accomplished by increasing alveolar volume and through the recruitment of alveoli that would not otherwise contribute to gas exchange. Thus increasing oxygenation and lung compliance The potential ability of PEEP and CPAP to open closed lung units increases lung compliance and tends to make regional impedances to ventilation more homogenous.
Airway Pressures (Paw) For gas to flow to occur there must be a positive pressure gradient. In spontaneous respiration gas flow occurs due to the generation of a negative pressure in the alveoli relative to atmospheric or circuit pressure (as in CPAP) (refer to following diagram).
Physiology of PEEP Reinflates collapsed alveoli and maintains alveolar inflation during exhalation PEEP Decreases alveolar distending pressure Increases FRC by alveolar recruitment Improves ventilation Increases V/Q, improves oxygenation, decreases work of breathing
Dangers of PEEP High intrathoracic pressures can cause decreased venous return and decreased cardiac output May produce pulmonary barotrauma May worsen air-trapping in obstructive pulmonary disease Increases intracranial pressure Alterations of renal functions and water metabolism
AutoPEEP During expiration alveolar pressure is greater than circuit pressure until expiratory flow ceases. If expiratory flow does not cease prior to the initiation of the next breath gas trapping may occur. Gas trapping increases the pressure in the alveoli at the end of expiration and has been termed: dynamic hyperinflation; autoPEEP; inadvertent PEEP; intrinsic PEEP; and occult PEEP
effects of autoPEEP can predispose the patient to: an increased risk of barotrauma; fall in cardiac output; hypotension; fluid retention; and an increased work of breathing
I:E ratioThis defines the inspiration to expiration ratio.I:E ratios are normally set as 1:2 as expiration requires a longer time .In severe obstructive disease such as status asthamaticus it can be set as 1:4Factors affecting I:E Ratio-1. Tidal volume2. Respiratory rate3. Flow rate • Increasing inspiration time will increase TV, but may lead to auto-PEEP
Tidal Volume Tidal volume refers to the size of the breath that is delivered to the patient. Normal physiologic tidal volumes are approximately 5-7 ml / kg whereas the traditional aim for tidal volumes has been approximately 10 - 15 ml / kg. The rationale for increasing the size of the tidal volume in ventilated patients has been to prevent atelectasis and overcome the deadspace of the ventilator circuitry and endotracheal tube. Inspired and expired tidal volumes are plotted on the y axis against time as depicted in the following diagram.
The inspired and expired tidal volumes should generally correlate. Expired tidal volumes may be less than inspired tidal volumes if: there is a leak in the ventilator circuit - causing some of the gas delivered to the patient to leak into the atmosphere there is a leak around the endotracheal / tracheostomy tube - due to tube position, inadequate seal or cuff leak there is a leak from the patient, such as a bronchopleural fistula Expired tidal volumes may be larger than inspired tidal volumes due to: the addition of water vapour in the ventilator circuitry from a hot water bath humidifier.
Flow (V) Flow rate refers to the speed at which a volume of gas is delivered, or exhaled, per unit of time. Flow is described in litres per minute . The peak (inspiratory) flow rate is therefore the maximum flow delivered to a patient per ventilator breath. Flow is plotted on the y axis of the ventilator graphics against time on the x axis . In the following diagram that inspiratory flow is plotted above the zero flow line, whereas expiratory flow is plotted as a negative deflection. When the graph depicting flow is at zero there is no gas flow going into or out of the patient.
primary factors to consider whenapplying mechanical ventilation the components of each individual breath, specifically whether pressure, flow, volume and time are set by the operator, variable or dependent on other parameters the method of triggering the mechanical ventilator breath/gas flow, how the ventilator breath is terminated: potential complications of mechanical ventilation. methods to improve patient ventilator synchrony; and the nursing observations required to provide a safe and effective level of care for the patient receiving mechanical ventilation
Time (Ti) Time in mechanical ventilation is divided between inspiratory and expiratory time. Inspiratory Time In most volume cycled ventilators used in the intensive care environment it is not possible to set the inspiratory time. The inspiratory time is determined by the peak inspiratory flow rate, flow waveform and inspiratory pause. Where inspiratory time is able to be set, flow becomes dependent on inspiratory time and tidal volume.
The following example illustrate how these parameters effect inspiratory time. Ventilator settings · Tidal volume 1000mls · Peak Flow 60 lpm · Flow Waveform square / constant · Insp. Pause 0 secs The inspiratory time for this patient would be 1 second because gas is constantly being delivered at a flow rate of 60 lpm, which equals 1 litre per second. If an inspiratory pause of 0.5 seconds were applied then the inspiratory time would be increased to 1.5 seconds. Changing the patients flow waveform from a square to a decelerating flow waveform, without changing the flow rate, will result in an increase in inspiratory time, because the flow of gas is only initially set at 60 lpm and decreases throughout inspiration
Output waveforms Graphical representation of the control or phase variables in relation to time. presented as pressure flow waveforms volume The ventilator determines the shape of control variable whereas the other two depend on the patient compliance and resistance. Conventionally flow above X-axis is inspiration.
Advantages • Allows user to interpret, evaluate, and troubleshoot the ventilator and the patient’s response to ventilator. • Monitors the patient’s disease status (C and Raw). • Assesses patient’s response to therapy. • Monitors ventilator function • Allows fine tuning of ventilator to decrease WOB, optimize ventilation, and maximize patient comfort.
Flow Waveforms inspiratory flow is controlled by setting the peak flow and flow waveform. The peak flow rate is the maximum amount of flow delivered to the patient during inspiration, whereas the flow waveform determines the how quickly gas will be delivered to the patient throughout various stages of the inspiratory cycle. There are four different types of flow waveforms available. These include the square, decelerating (ramp), accelerating sine/sinusoidal waveform
Square waveform- The square flow waveform delivers a set flow rate throughout ventilator inspiration. If for example the peak flow rate is set at 60 lpm then the patient will receive 60 lpm throughout ventilator inspiration. Decelerating waveform The decelerating flow waveform delivers the peak flow at the start of ventilator inspiration and slowly decreases until a percentage of the peak inspiratory flow rate is attained.
Accelerating waveform- The accelerating flow waveform initially delivers a fraction of the peak inspiratory flow and steadily increasing the rate of flow until the peak flow has been reached. Sine / sinusoidal waveform- The sine waveform was designed to match the normal flow waveform of a spontaneously breathing patient.
Setting the Peak Flow and FlowWaveform The flow rate should be set to match the patient’s inspiratory demand. Where the patient’s inspiratory flow requirements exceed the preset flow rate there will be an imposed work of breathing which may cause the patient to fight the ventilator and become fatigued. Where flow rate is unable to match the patient’s inspiratory flow requirements the pressure waveform on the ventilator graphics screen may show a depressed or “scooped out” pressure waveform. This is often referred to as flow starvation.
The decelerating flow waveform is the most frequently selected flow waveform as it produces the lowest peak inspiratory pressures of all the flow waveforms. This is because of the characteristics of alveolar expansion. Initially a high flow rate is required to open the alveoli. Once alveolar opening has occurred a lower flow rate is sufficient to procure alveolar expansion. Flow waveforms which produce a high flow rate at the end of inspiration (ie. square and accelerating flow waveforms) exceed the flow requirements for alveolar expansion, resulting in elevated peak inspiratory pressures
Pressure waveforms Rectangular Exponential rise Sine• Can be used to monitor-• Air trapping (auto-PEEP)• Airway Obstruction• Bronchodilator Response• Respiratory Mechanics (C/Raw)• Active Exhalation• Breath Type (Pressure vs. Volume)• PIP, Pplat• CPAP, PEEP• Asynchrony• Triggering Effort
References Guide to mechanical ventilation- chang s Breathing and mechanical support- wolfgang oczenski Internet references
Advantages of Volume Cycled Ventilation Ease of Use Set Volumes: One of the major advantages of volume cycled ventilation is the ability to set a tidal volume. This is of critical importance to patient’s requiring tight regulation of carbon dioxide elimination. Neurosurgical patients - post surgery / head injury and patients suffering a neurological insult (eg post cardiac arrest) often require CO2 regulation. This is because carbon dioxide is a potent vasodilator. Increased levels of carbon dioxide, in these groups of patients, may therefore increase cerebral blood volume with a concomitant elevation of intracranial pressure. A raised intracranial pressure may decrease the delivery of oxygenated blood to the brain - resulting in cerebral ischaemia. Conversely a low CO2 may cause constriction of the cerebral vasculature also resulting in decreased oxygen delivery and cerebral ischaemia. For these reasons volume cycled ventilation is often the mode of choice for patients requiring CO2 regulation.
Disadvantages The major disadvantages of volume cycled ventilation are the variable pressure and set flow rate. It is therefore a necessary part of nursing practice to closely monitor the patients inspiratory pressure and observe the patient for signs of “flow starvation”.
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