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“…an opening must be attempted in the trunkof the trachea, into which a tube of reed or caneshould be put; you will blow into this, so that the lung may rise again… and the heart becomes strong…” Andreas Vesalius (1555)MODES OF MECHANICAL VENTILATION Dr. Nikhil Yadav
IntroductionMechanical ventilation forms a mainstay of critical care in patientswith respiratory insufficiency.Ventilator must generate inspiratory flow to deliver tidal volume.Transairway pressure (PTA) = PAO – PALVPTA = 0, at the end of expiration and beginning of inspiration.Mechanical ventilator produce either negative or positive pressuregradient.
Negative pressure ventilation PTA gradient is created by decreasing PALV to below PAOe.g.- Iron lungsDisadvantages- poor patient access,bulky size, cost, dec. cardiac output(Tank shock). Chest cuirass or chest shell
Positive pressure ventilationAchieved by applying positive pressure at airwayopening which produces PTA gradient that generatesinspiratory flow.Inspiratory flow results in the delivery of tidal volume.
Modes of ventilation Ventilator mode is a set of operating characteristics that controls how the ventilator functions. An operating mode describes the way a ventilator is-• triggered into inspiration• cycled into exhalation• what variables are limited during inspiration• allowing mandatory or spontaneous breaths or both
Controlled Mode Ventilation Volume controlThe ventilator delivers a preset TV at a specific R/R andinspiratory flow rate.It is irrespective of patients’ respiratory efforts.In between the ventilator delivered breaths the inspiratory valveis closed so patient doesn’t take additional breaths.PIP developed depends on lung compliance and respiratorypassage resistance.
Volume controlled CMV Indications-• In initial stage when patients “fighting” or “bucking” with the ventilator• Tetanus or other seizure activity• Crushed chest injury Disadvantages-• Asynchrony• Barotrauma d/t high PAW & dec. lung compliance• Haemodynamic disturbances• V/Q mismatch• Total dependence on ventilator
Pressure Controlled CMVVentilator gives pressure limited, time cycled breaths thuspreset inspiratory pressure is maintained.Decelerating flow pattern.Peak airway/alveolar pressure is controlled but TV, minutevolume & alveolar volume depends on lung compliance, airwayresistance, R/R & I:E ratio.
PC-CMVAdvantages- Less PAW, thus chances of barotrauma and hemodynamic disturbances are less. Even distribution of gases in alveoli In case of leakage, compensation for loss of ventilation is better as gaseous flow increases to maintain preset pressure.Disadvantages- Asynchrony TV dec. if there is dec. lung compliance or inc. airway resistance, thus causes hypoventilation and alveolar collapse. V/Q mismatch.
ASSIST-CONTROL MODE Ventilation (A-C Mode) Ventilator assists patient’s initiated breath, but if not triggered, it will deliver preset TV at a preset respiratory rate (control). Mandatory mechanical breaths may be either patient triggered (assist) or time triggered (control) If R/R > preset rate, ventilator will assist, otherwise it will control the ventilation.
A-C Mode VentilationAdvantages- Dec. patients work of breathing. Better patient ventilator synchrony. Less V/Q mismatch. Prevents disuse atrophy of diaphragmatic muscle.Disadvantages- Alveolar hyperventilation Development of high intrinsic PEEP in obstructed pts. Inc. mean airway pressure causes hemodynamic disturbances.
Intermittent Mandatory Ventilation (IMV) Ventilator delivers preset number of time cycled mandatory breaths & allows patient to breath spontaneously at any tidal volume in between.Advantages- Lesser sedation Lesser V/Q mismatch Lesser hemodynamic disturbancesDisadvantage- Breath stacking- lung volume and pressure could increase significantly, causing barotrauma.
Synchronized Intermittent Mandatory Ventilation (SIMV)Ventilator delivers either assisted breaths to the patient at thebeginning of a spontaneous breath or time triggered mandatorybreaths.Synchronization window- time interval just prior to timetriggering.Breath stacking is avoided as mandatory breaths aresynchronized with spontaneous breaths.In between mandatory breaths patient is allowed to takespontaneous breath at any TV.
SIMV It provides partial ventilatory supportAdvantages- Maintain respiratory muscle strength and avoid atrophy. Reduce V/Q mismatch d/t spontaneous ventilation. Decreases mean airway pressure d/t lower PIP & inspiratory time Facilitates weaning.
SIMVDisadvantages- Desire to wean too rapidly results in high work of spontaneous breathing & muscle fatigue & thus weaning failure.
Positive End Expiratory Pressure (PEEP) An airway pressure strategy in ventilation that increases the end expiratory or baseline airway pressure greater than atmospheric pressure. Used to treat refractory hypoxemia caused by intrapulmonary shunting. Not a stand-alone mode, used in conjugation with other modes.Indications- Refractory hypoxemia d/t intrapulmonary shunting. Decreased FRC and lung compliance
Physiology of PEEPPEEP reinflates collapsed alveoli & maintain alveolar inflation during exhalation. PEEP Increases alveolar distending pressure Increases FRC by alveolar recruitment Improves ventilation Increases V/Q Improves oxygenation Decreases work of breathing
PEEPComplications Dec. venous return and cardiac output. Barotrauma Inc. ICP d/t impedance of venous return from head. Alteration of renal function & water imbalance.
Continuous Positive Airway Pressure (CPAP)PEEP applied to airway of patient breathing spontaneouslyIndications are similar to PEEP, to ensure patient must haveadequate lung functions that can sustain eucapnic ventilation.
Mandatory Minute Ventilation (MMV) Similar to IMV mode except that minimum minute volume is set rather than R/R. Ventilator measures spontaneous minute volume, if found less than preset mandatory minute volume, the difference b/w two is delivered as mandatory breaths by ventilator at preset flow & TV. Suited for patients with variable respiratory driveDisadvantage- Hypoventilation as either minute volume recorded is not necessary alveolar ventilation.
Pressure Support Ventilation (PSV)Supports spontaneous breathing of the patients.Each inspiratory effort is augmented by ventilator at a presetlevel of inspiratory pressure.Patient triggered, flow cycled and pressure controlled mode.Decelerating flow pattern.Applies pressure plateau to patient airway during spontaneus br.Can be used in conjugation with spontaneous breathing in anyventilator mode.
PSV Commonly applied to SIMV mode during spontaneous ventilation to facilitate weaningWith SIMV, PS- Inc. patient’s spontaneous tidal volume. Dec. spontaneous respiratory rate. Decreases work of breathing. Addition of extrinsic PEEP to PS increases its efficacy.
PSVDisadvantages- Not suitable for patients with central apnea. (hypoventilation) Development of high airway pressure. (hemodynamic distubances) Hypoventilation, if inspiratory time is short.
Adaptive Support Ventilation (ASV)Available on Galileo ventilator.Patient body weight (deadspace) & percent minute volume arefeed in ventilator.Ventilator has pre determined setting of 100ml/kg/min.Test breath measures compliance, airway resistance & i. PEEP.Ventilator selects and provide the frequency, inspiratory time, I:E& sets high pressure limit for mandatory and spontaneousbreaths.May be either time triggered or patient triggered.
Proportional Assist Ventilation (PAV)PAV is a spontaneous breathing mode that offers assistance tothe patient in proportion to the patient’s effort.Inspiratory flow, volume & pressure are variable & pressuresupport changes according to elastance & airflow resistance &patients demand (volume or flow).PAV is set to overcome 80% of elastance & airflow resistance.PAV instantaneously measures the flow and volume being pulledin by the patient, and automatically calculates the complianceand resistance of the respiratory system to determine how muchpressure to provide for each breath.
PAVFlow Assist (FA)- Pressure is provided to meet patient’s inspiratory flow demand. Dec. inspiratory effort to overcome airflow resistance.Volume Assist (VA)- Pressure is provided meet patient’s volume requirement. Dec. inspiratory efforts to overcome systemic elastance.Indications- Spontaneously breathing patient Intact respiratory drive Intact neuromuscular function Generally, a patient considered suitable for pressure support ventilation could be considered for PAV.
PAVAdvantage- The patient ‘drives’ the ventilator Better patient ventilator synchrony as pressure vary to augment flow & demand.Disadvantage- Barotrauma- if elastance & resistance show sudden improvement.
Volume Assured Pressure Support (VAPS)Incorporates inspiratory pressure support ventilation &conventional volume assisted cycles to provide optimalinspiratory flow during assisted/controlled ventilation.Desired TV & pressure support level are preset.Once triggered desired PS level reaches asap & delivered volumeis compared with preset TV.If volume delivered = 0r > preset volume, it is PS breath.If volume < preset volume, ventilator switches to volumelimited, resulting in longer inspiratory time until preset TV isdelivered
Pressure Regulated Volume Control (PRVC)Used to achieve volume support while keeping PIP to lowestlevel.Achieved by altering the peak flow & inspiratory time inresponse to changing airway or compliance characteristics.At constant flow PIP increases d/t inc. airflow resistance, sodecreasing flow reduces the airflow resistance.To compensate for lower flow, inspiratory time is prolonged.
Airway Pressure Release Ventilation (APRV)Similar to CPAP as patient breathes spontaneously.Airway pressure is maintained at moderately high level (15-20 cmH2O)throughout most of respiratory cycle with brief periods of lower pressure toallow deflation of lungs.Inc. pressure ensures alveolar recruitment & oxygenation & brief deflationallows CO2 elimination without alveolar collapse.Indicated as an alternative to conventional volume cycled ventilation forpatients with decreased lung compliance (ARDS), as chances of barotrauma isless d/t less PAW.
Inverse Ratio Ventilation (IRV) Used to promote oxygenation esp. in ARDS. Normal I:E ratio is 1:1.5 – 1:3, in IRV I:E is 2:1 – 4:1 Improves oxygenation by• Reducing intrapulmonary shunting.• Improving V/Q mismatch.• Decreasing deadspace ventilation.• Increasing mean airway pressure.• Presence of auto PEEP.Disadvantages- Barotrauma d/t inc. mPaw & auto PEEP. High rate of transvascular fluid flow. May worsen pulm. oedema
Tracheal Gas Insufflation (TGI)Adjuvant to mechanical ventilation in which O2 enriched gas isinsufflated into trachea to ventilate anatomical dead spaceduring expiration.Decreases PaCO2 at any level of inspiratory minute ventilation.Extra flow may cause inc. airway pressure and hyperinflation.
Independent Lung Ventilation (ILV) It is simultaneous separate ventilation of individual lung. Separation achieved by double lumen tube and two ventilators- synchronized or asynchronized.Indication- Severely diseased one lung which can not be treated with conventional ventilation.e.g.- unilateral pulmonary contusion, aspiration pneumonia,bronchopleural fistula, massive unilateral pulmonary embolism etc.
High frequency ventilation (HFV) For all high frequency techniques during which tidal volume equals or less than anatomical dead space volume and respiratory frequency between 60 to 3000 breaths / minutes.They are 3 types: HFPPV …..60 to 110 breaths/min HFJV…….110 to 600 breaths/min HFO……..600 to 3000 breaths/minAdvantages-Low PAW, less V/Q mismatch, less barotrauma