Physiology of positive pressure ventilation & newer modes of ventilation

1. Physiology of positive pressure ventilation & newer modes of ventilation Dr. Megha Aggarwal University College of Medical Sciences & GTB Hospital, Delhi

2. 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

3. Goals • Manipulate gas exchange • ↑ lung vol – FRC, end insp / exp lung inflation • Manipulate work of breathing (WOB) • Minimize CVS effects

4. 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

5. 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

6. 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)

7. Understanding physiology of PPV • Different P gradients • Time constant • Airway P ( peak, plateau, mean ) • PEEP and Auto PEEP • Types of waveforms

8. Pressure gradients

10. Distending pressure of lungs Resistance load Distending pressure Elastance load

11. 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

12. 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

13. 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

14. 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

15. 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

16. Why

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. Inadequate expiratory time - Air trapping FV loop Flow curve iPEEP Allow more time for expiration Increase inspiratory flow rate Provide ePEEP

25. 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

26. Cardiovascular effects of PPV Spontaneous ventilation PPV

27. 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

28. 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

29. 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

30. PPV ↓ preload, ventricular filling ↓ afterload , ↑ventricular emptying • CO – • INCREASE • DECREASE • Intravascular fluid status • Compensation – HR, vasoconstriction • Sepsis, • PEEP, MAP • LV function

31. Effect on other body systems

32. Physiology of positive pressure ventilation & newer modes of ventilation Presenter – Megha Aggarwal Moderator – Dr Sujata Chaudhary Dr Asha Tyagi

33. 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

34. What is a ‘ mode of ventilation’ ? A ventilator mode is delivery a sequenceof breath types & timing of breath

35. Breath characteristics A= what initiates a breath - TRIGGER B = what controls / limits it – LIMIT C= What ends a breath - CYCLING

36. 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

37. 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

38. 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

39. Breath types Control/Mandatory Machine triggered and machine cycled Assisted Patient triggered but machine cycled Spontaneous Both triggered and cycled by the patient

40. Waveforms • Volume -time • Flow - time • Pressure - time

41. a) Volume – time graphs • Air leaks • Calibrate flow transducers

42. b) Flow waveforms 1. Inspiratory flow waveforms

43. 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

44. Inspiratory and expiratory flow waveforms

45. 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)

46. 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)

47. Classification of modes of ventilation