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Physiology of positive pressure ventilation & newer modes of ventilation. Dr. Megha Aggarwal. University College of Medical Sciences & GTB Hospital, Delhi. Mechanical ventilation – supports / replaces the normal ventilatory pump moving air in & out of the lungs. Primary indications – apnea
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Physiology of positive pressure ventilation & newer modes of ventilation Dr. Megha Aggarwal University College of Medical Sciences & GTB Hospital, Delhi
Mechanical ventilation – supports / replaces the normal ventilatory pump moving air in & out of the lungs. Primary indications – • apnea • Ac. ventilation failure • Impending ventilation failure • Severe oxygenation failure
Goals • Manipulate gas exchange • ↑ lung vol – FRC, end insp / exp lung inflation • Manipulate work of breathing (WOB) • Minimize CVS effects
Negative pressure ventilation • - Creates a transairway P gradient by ↓ alveolar P to a level below airway opening P • - Creates – P around thorax • e.g. iron lung • chest cuirass / shell - Achieved by applying + P at airway opening producing a transairway P gradient Positive pressure ventilation ARTIFICIAL VENTILATION
ventilation without artificial airway • Nasal , face mask • adv. • Avoid intubation / c/c • Preserve natural airway defences • Comfort • Speech/ swallowing + • Less sedation needed • Intermittent use • Disadv • Cooperation • Mask discomfort • Air leaks • Facial ulcers, eye irritation, dry nose • Aerophagia • Limited P support • e.g. BiPAP, CPAP Noninvasive
FULL PARTIAL Ventilatory support All energy provided by ventilator e.g. ACV / full support SIMV ( RR = 12-26 & TV = 8-10 ml/kg) Pt provides a portion of energy needed for effective ventilation e.g. SIMV (RR < 10) Used for weaning WOB total = WOB ventilator (forces gas into lungs)+ WOB patient (msls draw gas into lungs)
Understanding physiology of PPV • Different P gradients • Time constant • Airway P ( peak, plateau, mean ) • PEEP and Auto PEEP • Types of waveforms
Distending pressure of lungs Resistance load Distending pressure Elastance load
Airway pressures Peak insp P (PIP) • Highest P produced during insp. • PRESISTANCE + P INFLATE ALVEOLI • Dynamic compliance • Barotrauma • Plateau P • Observed during end insp pause • P INFLATE ALVEOLI • Static compliance • Effect of flow resistance negated
Time constant • Defined for variables that undergo exponential decay • Time for passive inflation / deflation of lung / unit t = compliance X resistance = VT . peak exp flow Normal lung C = 0.1 L/cm H2O R = 1cm H2O/L/s COAD – resistance to exp increases → time constant increases → exp time to be increased lest incomplete exp ( auto PEEP generates). ARDS - inhomogenous time constants
Why and how to separate dynamic & static components ? • Why – to find cause for altered airway pressures • How – adding end insp pause - no airflow, lung expanded, no expiration
How -End inspiratory hold End-inspiratory pause • Pendelluft phenomenon • Visco-elastic properties of lung Ppeak < 50 cm H2O Pplat < 30 cm H2O Ppeak = Pplat + Paw
Pendulum like movement of air between lung units • Reflects inhomogeneity of lung units • More in ARDS and COPD • Can lead to falsely measured high Pplat if the end-inspiratory occlusion duration is not long enough
Mean airway P (MAP) • average P across total cycle time (TCT) • MAP = 0.5(PIP-PEEP)X Ti/TCT + PEEP • Decreases as spontaneous breaths increase • MAPSIMV < MAP ACV • Hemodynamic consequences Factors • Mandatory breath modes • ↑insp time , ↓ exp time • ↑ PEEP • ↑ Resistance, ↓compliance • Insp flow pattern
PEEP PEEP prevents complete collapse of the alveoli and keep them partially inflated and thus provide protection against the development of shear forces during mechanical inflation BENEFITS • Restore FRC/ Alveolar recruitment • ↓ shunt fraction • ↑Lung compliance • ↓WOB • ↑PaO2 for given FiO2 DETRIMENTAL EFFECTS Barotrauma ↓ VR/ CO ↑ WOB (if overdistention) ↑ PVR ↑ MAP ↓ Renal / portal bld flow
How much PEEP to apply? Lower inflection point – transition from flat to steep part - ↑compliance - recruitment begins (pt. above closing vol) Upper inflection point – transition from steep to flat part - ↓compliance - over distension
Set PEEP above LIP – Prevent end expiratory airway collapse Set TV so that total P < UIP – prevent overdistention Limitation – lung is inhomogenous - LIP / UIP differ for different lung units
Auto-PEEP or Intrinsic PEEP • What is Auto-PEEP? • Normally, at end expiration, the lung volume is equal to the FRC • When PEEPi occurs, the lung volume at end expiration is greater then the FRC
Auto-PEEP or Intrinsic PEEP Function of- Ventilator settings – TV, Exp time Lung func – resistance, compliance • Why does hyperinflation occur? • Airflow limitation because of dynamic collapse • No time to expire all the lung volume (high RR or Vt) • Lesions that increase expiratory resistance
Auto-PEEP or Intrinsic PEEP • Auto-PEEP is measured in a relaxed pt with an end-expiratory hold maneuver on a mechanical ventilator immediately before the onset of the next breath
Inadequate expiratory time - Air trapping FV loop Flow curve iPEEP Allow more time for expiration Increase inspiratory flow rate Provide ePEEP
Disadv • Barotrauma / volutrauma • ↑WOB a) lung overstretching ↓contractility of diaphragm b) alters effective trigger sensitivity as autoPEEP must be overcome before P falls enough to trigger breath • ↑ MAP – CVS side effects • May ↑ PVR Minimising Auto PEEP • ↓airflow res – secretion management, bronchodilation, large ETT • ↓Insp time ( ↑insp flow, sq flow waveform, low TV) • ↑ exp time (low resp rate ) • Apply PEEP to balance AutoPEEP
Cardiovascular effects of PPV Spontaneous ventilation PPV
Determinants of hemodynamic effects due to – change in ITP, lung volumes, pericardial P severity – lung compliance, chest wall compliance, rate & type of ventilation, airway resistance
Low lung compliance – more P spent in lung expansion & less change in ITP less hemodynamic effects (DAMPNING EFFECT OF LUNG) Low chest wall compliance – higher change in ITP needed for effective ventilation more hemodynamic effects
Effect on CO ( preload , afterload ) Decreased PRELOAD • compression of intrathoracic veins (↓ CVP, RA filling P) • Increased PVR due to compression by alveolar vol (decreased RV preload) • Interventricular dependence - ↑ RV vol pushes septum to left & ↓ LV vol & LV output Decreased afterload 1. emptying of thoracic aorta during insp 2. Compression of heart by + P during systole 3. ↓ transmural P across LV during systole
PPV ↓ preload, ventricular filling ↓ afterload , ↑ventricular emptying • CO – • INCREASE • DECREASE • Intravascular fluid status • Compensation – HR, vasoconstriction • Sepsis, • PEEP, MAP • LV function
Physiology of positive pressure ventilation & newer modes of ventilation Presenter – Megha Aggarwal Moderator – Dr Sujata Chaudhary Dr Asha Tyagi
Overview • Mode of ventilation – definition • Breath – characteristics • Breath types • Waveforms – pressure- time, volume –time, flow-time • Modes - Volume & pressure limited • Conventional modes of ventilation • Newer modes of ventilation
What is a ‘ mode of ventilation’ ? A ventilator mode is delivery a sequenceof breath types & timing of breath
Breath characteristics A= what initiates a breath - TRIGGER B = what controls / limits it – LIMIT C= What ends a breath - CYCLING
TRIGGER • What the ventilator senses to initiate a breath • Patient • Pressure • Flow • Machine • Time based Recently – EMG monitoring of phrenic Nerve via esophageal transducer Pressure triggering -1 to -3 cm H2O Flow triggering -1 to -3 L/min
CONTROL/ LIMIT • Variable not allowed to rise above a preset value • Does not terminate a breath • Pressure • Volume • Pressure Controlled • Pressure targeted, pressure limited - Ppeak set • Volume Variable • Volume Controlled • Volume targeted, volume limited - VT set • Pressure Variable • Dual Controlled • volume targeted (guaranteed) and pressure limited
CYCLING VARIABLE • Determines the end of inspiration and the switch to expiration • Machine cycling • Time • Pressure • Volume • Patient cycling • Flow May be multiple but activated in hierarchy as per preset algorithm
Breath types Control/Mandatory Machine triggered and machine cycled Assisted Patient triggered but machine cycled Spontaneous Both triggered and cycled by the patient
Waveforms • Volume -time • Flow - time • Pressure - time
a) Volume – time graphs • Air leaks • Calibrate flow transducers
b) Flow waveforms 1. Inspiratory flow waveforms
Sine • Resembles normal inspiration • More physiological Square • Maintains constant flow • high flow with ↓ Ti & improved I:E Square- volume limited modes Decelerating – pressure limited modes • Flow slows down as alveolar pressure increases • meets high initial flow demand in spont breathing patient - ↓WOB Decelerating • Produces highest PIP as airflow is highest towards end of inflation when alveoli are less compliant Accelerating Not used
2. Expiratory flow waveform • Expiratory flow is not driven by ventilator and is passive • Is negative by convention • Similar in all modes • Determined by Airway resistance & exp time (Te) Use 1.Airtrapping & generation of AutoPEEP 2.Exp flow resistance (↓PEFR + short Te) & response bronchodilators (↑PEFR)
c) Pressure waveform • Spontaneous/ mandatory breaths • Patient ventilator synchrony • Calculation of compliance & resistance • Work done against elastic and resistive forces • AutoPEEP ( by adding end exp pause)