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

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Presentation on theme: "Dr. Megha Aggarwal Physiology of positive pressure ventilation & newer modes of ventilation University College of Medical Sciences & GTB Hospital, Delhi."— Presentation transcript:

1 Dr. Megha Aggarwal Physiology of positive pressure ventilation & newer modes of ventilation 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 – a. apnea b. Ac. ventilation failure c. Impending ventilation failure d. 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 ARTIFICIAL 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 Negative pressure ventilation Positive pressure ventilation

5 ventilation without artificial airway -Nasal, face mask adv. 1.Avoid intubation / c/c 2.Preserve natural airway defences 3.Comfort 4.Speech/ swallowing + 5.Less sedation needed 6.Intermittent use Disadv 1.Cooperation 2.Mask discomfort 3.Air leaks 4.Facial ulcers, eye irritation, dry nose 5.Aerophagia 6.Limited P support e.g. BiPAP, CPAP Noninvasive

6 Ventilatory support FULLPARTIAL 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 1) Different P gradients 2) Time constant 3) Airway P ( peak, plateau, mean ) 4) PEEP and Auto PEEP 5) Types of waveforms

8 Pressure gradients

9

10 Distending pressure of lungs

11 Airway pressures Peak insp P (PIP) Highest P produced during insp. P RESISTANCE + 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 H 2 O R = 1cm H 2 O/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 Pendelluft phenomenon Visco-elastic properties of lung End-inspiratory pause Ppeak < 50 cm H2O Pplat < 30 cm H2O Ppeak < 50 cm H2O Pplat < 30 cm H2O

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 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 T i /TCT + PEEP Decreases as spontaneous breaths increase MAP SIMV < MAP ACV Hemodynamic consequences Factors 1. Mandatory breath modes 2. ↑insp time, ↓ exp time 3. ↑ PEEP 4. ↑ Resistance, ↓compliance 5. Insp flow pattern

18 PEEP BENEFITS 1. Restore FRC/ Alveolar recruitment 2. ↓ shunt fraction 3. ↑Lung compliance 4. ↓WOB 5. ↑PaO 2 for given FiO 2 DETRIMENTAL EFFECTS 1.Barotrauma 2.↓ VR/ CO 3.↑ WOB (if overdistention) 4.↑ PVR 5.↑ MAP 6.↓ Renal / portal bld flow 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

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 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 Function of- Ventilator settings – TV, Exp time Lung func – resistance, compliance

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 Flow curve FV loop 1.Allow more time for expiration 2.Increase inspiratory flow rate 3.Provide ePEEP 1.Allow more time for expiration 2.Increase inspiratory flow rate 3.Provide ePEEP

25 Disadv 1. Barotrauma / volutrauma 2. ↑WOB a) lung overstretching ↓contractility of diaphragm b) alters effective trigger sensitivity as autoPEEP must be overcome before P falls enough to trigger breath 3. ↑ MAP – CVS side effects 4. May ↑ PVR Minimising Auto PEEP 1. ↓airflow res – secretion management, bronchodilation, large ETT 2. ↓Insp time ( ↑insp flow, sq flow waveform, low TV) 3. ↑ exp time (low resp rate ) 4. Apply PEEP to balance AutoPEEP

26 Cardiovascular effects of PPV Spontaneous ventilationPPV

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 1. compression of intrathoracic veins (↓ CVP, RA filling P) 2. Increased PVR due to compression by alveolar vol (decreased RV preload) 3. 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 – 1.INCREASE 2.DECREASE 1.Intravascular fluid status 2.Compensation – HR, vasoconstriction 3.Sepsis, 4.PEEP, MAP 5.LV function

31 Effect on other body systems

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

33 Overview 1. Mode of ventilation – definition 2. Breath – characteristics 3. Breath types 4. Waveforms – pressure- time, volume –time, flow-time 5. Modes - Volume & pressure limited 6. Conventional modes of ventilation 7. Newer modes of ventilation

34 What is a ‘ mode of ventilation’ ? A ventilator mode is delivery a sequence of 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 What the ventilator senses to initiate a breath Patient Pressure Flow Machine Time based Recently – EMG monitoring of phrenic Nerve via esophageal transducer -1 to -3 cm H 2 O -1 to -3 L/min

37 CONTROL/ LIMIT Variable not allowed to rise above a preset value Does not terminate a breath  Pressure  Volume 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 - V T set Pressure Variable  Dual Controlled volume targeted (guaranteed) and pressure limited  Pressure Controlled Pressure targeted, pressure limited - Ppeak set Volume Variable  Volume Controlled Volume targeted, volume limited - V T 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 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 Spontaneous Both triggered and cycled by the patient Spontaneous Both triggered and cycled by the patient Control/Mandatory Machine triggered and machine cycled Control/Mandatory Machine triggered and machine cycled Assisted Patient triggered but machine cycled Assisted Patient triggered but machine cycled

40 Waveforms 1. Volume -time 2. Flow - time 3. Pressure - time

41 a) Volume – time graphs 1. Air leaks 2. Calibrate flow transducers

42 b) Flow waveforms 1. Inspiratory flow waveforms

43 Sine Square Decelerating Resembles normal inspiration More physiological Resembles normal inspiration More physiological Maintains constant flow high flow with ↓ Ti & improved I:E Maintains constant flow high flow with ↓ Ti & improved I:E Flow slows down as alveolar pressure increases meets high initial flow demand in spont breathing patient - ↓WOB Flow slows down as alveolar pressure increases meets high initial flow demand in spont breathing patient - ↓WOB Accelerating Produces highest PIP as airflow is highest towards end of inflation when alveoli are less compliant Square - volume limited modes Decelerating – pressure limited modes 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 1. Spontaneous/ mandatory breaths 2. Patient ventilator synchrony 3. Calculation of compliance & resistance 4. Work done against elastic and resistive forces 5. AutoPEEP ( by adding end exp pause)

47 Classification of modes of ventilation Volume controlledPressure controlled TV & inspiratory flow are preset Airway P is preset Airway P depends on above & lung elastance & compliance TV & insp flow depend on above & lung elastance & compliance

48

49 Volume controlledPressure controlled Trigger - patient / machinePatient / machine Limit FlowPressure Cycle Volume / timetime / flow TV Constantvariable Peak P Variableconstant Modes ACV, SIMVPCV, PSV

50 Volume controlledPressure controlled Advantages 1.Guaranteed TV 2.Less atelectasis 3.TV increases linearly with MV Advantages 1.Limits excessive airway P 2.↑ MAP by constant insp P – better oxygenation 3.Better gas distribution – high insp flow ↓Ti & ↑Te,thereby, preventing airtrapping 4.Lower WOB – high initial flow rates meet high initial flow demands 5.Lower PIP – as flow rates higher when lung compliance high i.e early insp. phase Disadvantages 1.Limited flow may not meet patients desired insp flow rate- flow hunger 2.May cause high Paw ( barotrauma ) Disadvantages 1.Variable TV ↑TV as compliance ↑ ↓TV as resistance ↑

51 Conventional modes of ventilation 1. Control mandatory ventilation (CMV / VCV) 2. Assist Control Mandatory Ventilation (ACMV) 3. Intermittent mandatory ventilation (IMV) 4. Synchronized Intermittent Mandatory Ventilation (SIMV) 5. Pressure controlled ventilation (PCV) 6. Pressure support ventilation (PSV) 7. Continuous positive airway pressure (CPAP)

52 1. Control mandatory ventilation (CMV / VCV) Breath - MANDATORY Trigger – TIME Limit - VOLUME Cycle – VOL / TIME Breath - MANDATORY Trigger – TIME Limit - VOLUME Cycle – VOL / TIME Patient has no control over respiration Requires sedation and paralysis of patient Patient has no control over respiration Requires sedation and paralysis of patient

53 2. Assist Control Mandatory Ventilation (ACMV) Patient has partial control over his respiration – Better Pt ventilator synchrony Ventilator rate determined by patient or backup rate (whichever is higher) – risk of respiratory alkalosis if tachypnoea PASSIVE Pt – acts like CMV ACTIVE pt – ALL spontaneous breaths assisted to preset volume Patient has partial control over his respiration – Better Pt ventilator synchrony Ventilator rate determined by patient or backup rate (whichever is higher) – risk of respiratory alkalosis if tachypnoea PASSIVE Pt – acts like CMV ACTIVE pt – ALL spontaneous breaths assisted to preset volume Breath – MANDATORY ASSISTED Trigger – PATIENT TIME Limit - VOLUME Cycle – VOLUME / TIME Breath – MANDATORY ASSISTED Trigger – PATIENT TIME Limit - VOLUME Cycle – VOLUME / TIME Once patient initiates the breath the ventilator takes over the WOB If he fails to initiate, then the ventilator does the entire WOB

54 3. Intermittent mandatory ventilation (IMV) Breath stacking Spontaneous breath immediately after a controlled breath without allowing time for expiration ( SUPERIMPOSED BREATHS) Breath stacking Spontaneous breath immediately after a controlled breath without allowing time for expiration ( SUPERIMPOSED BREATHS) Basically CMV which allows spontaneous breaths in between Disadvantage In tachypnea can lead to breath stacking - leading to dynamic hyperinflation Not used now – has been replaced by SIMV Breath – MANDATORY SPONTANEOUS Trigger – PATIENT VENTILATOR Limit - VOLUME Cycle - VOLUME Breath – MANDATORY SPONTANEOUS Trigger – PATIENT VENTILATOR Limit - VOLUME Cycle - VOLUME

55 4.Synchronized Intermittent Mandatory Ventilation (SIMV) Breath – SPONTANEOUS ASSISTED MANDATORY Trigger – PATIENT TIME Limit - VOLUME Cycle – VOLUME/ TIME Breath – SPONTANEOUS ASSISTED MANDATORY Trigger – PATIENT TIME Limit - VOLUME Cycle – VOLUME/ TIME

56 Basically, ACMV with spontaneous breaths (which may be pressure supported) allowed in between Synchronisation window – Time interval from the previous mandatory breath to just prior to the next time triggering, during which ventilator is responsive to patients spontaneous inspiratory effort Weaning Adv  Allows patients to exercise their respiratory muscles in between – avoids atrophy  Avoids breath stacking – ‘Synchronisation window’

57 5.Pressure controlled ventilation (PCV) Breath – MANDATORY Trigger – TIME Limit - PRESSURE Cycle – TIME/ FLOW Breath – MANDATORY Trigger – TIME Limit - PRESSURE Cycle – TIME/ FLOW Rise time Time taken for airway pressure to rise from baseline to maximum Rise time Time taken for airway pressure to rise from baseline to maximum

58 6.Pressure support ventilation (PSV) Breath – SPONTANEOUS Trigger – PATIENT Limit - PRESSURE Cycle – FLOW ( 5-25% OF PIFR) Breath – SPONTANEOUS Trigger – PATIENT Limit - PRESSURE Cycle – FLOW ( 5-25% OF PIFR) After the trigger, ventilator generates a flow sufficient to raise and then maintain airway pressure at a preset level for the duration of the patient’s spontaneous respiratory effort

59 7.Continuous positive airway pressure (CPAP) Breath – SPONTANEOUS CPAP is actually PEEP applied to spontaneously breathing patients. But CPAP is described a mode of ventilation without additional inspiratory support while PEEP is not regarded as a stand-alone mode CPAP is actually PEEP applied to spontaneously breathing patients. But CPAP is described a mode of ventilation without additional inspiratory support while PEEP is not regarded as a stand-alone mode

60 Newer modes of ventilation 1. Volume assured pressure support (VAPS) 2. Volume support (VS) 3. Pressure regulated volume controlled (PRVC) 4. Automode 5. Automatic Tube Compensation (ATC) 6. Airway pressure release ventilation (APRV) 7. Proportional Assist Ventilation (PAV) 8. Biphasic positive airway pressure (BiPAP) 9. Neurally Adjusted Ventilatory Assist (NAVA)

61 Newer modes of ventilation Recent modes allow ventilators to control one variable or the other based on a feedback loop Volume controlled Pressure controlled Feedback loop Is the Airway P exceeding set P limit ? Has the desired/ set TV been delivered ?

62 Dual modes of ventilation Devised to overcome the limitations of both V & P controlled modes Dual control within a breath Switches from P to V control during the same breath e.g. VAPS PA Dual control from breath to breath P limit ↑ or ↓ to maintain a clinician set TV ANALOGOUS to a resp therapist who ↑ or ↓ P limit of each breath based on TV delivered in last breath

63 Dual control within a breath Combined adv – 1. High & variable initial flow rate of P controlled breath ( thereby - ↑ pt – vent synchrony, ↓WOB, ↓sense of breathlessness) 2. Assured TV & MV as in V controlled breaths Starts as P limited breaths but change over to V limited breath by converting decelerating flow to constant flow if minimum preset TV not delivered

64 1.Breath triggered (pt/ time) – 2. P support level reached quickly – 3.ventilator compares delivered and desired/ set TV 4.Delivered = set TV -------- Breath is FLOW cycled as in P controlled modes 5.Delivered < set TV -------- Changeover from P to V limited ( flow kept constant + Ti ↑) P rises above set P support level till set TV delivered

65 Dual control – breath to breath P limited + FLOW cycled Vol support / variable P support P limited + TIME cycled PRVC

66 Volume support Allows automatic weaning of P support as compliance alters. OPERATION – C = V P changes during weaning & guides P support level Preset & constant P support dependent on C compliance ↑ - P support ↓ ↓ - P support ↑ By 3 cm H 2 O / breath Deliver desired TV

67 Limitations – a) MV is fixed, pt may be stuck at that level of support even if pt demand exceeds MV chosen by clinician b) If tachypnoea occurs – ventilator senses it as ↑ MV and ↓ses P support which is exactly OPPOSITE of what is required

68 Pressure regulated volume controlled (PRVC) Autoflow / variable P control Similar to VS except that it is a modification of PCV rather than PSV

69 Had it been 1.Conventional V controlled mode – very high P would have resulted in an attempt to deliver set TV -------- BAROTRAUMA 2.Conventional P controlled mode – inadequate TV would have been delivered

70 Automode Shifts between P support (flow cycled)& P control (time cycled) mode with pt efforts Combines VS & PRVC If no efforts : PRVC (time cycled) As spontaneous breathing begins : VS (flow cycled) Pitfalls : During the switch from time-cycled to flow cycled ventilation ↓ Mean airway pressure ↓ ↓ hypoxemia may occur

71 Automatic Tube Compensation Compensates for the resistance of ETT Facilitates “ electronic weaning “ i.e pt during ATC mimic their breathing pattern as if extubated ( provided upper airway contorl provided) Operation As the flow ↑ / ETT dia ↓, the P support needs to be ↑to ↓WOB ∆P (P support) α (L / r 4 ) α flow α WOB

72 Static condition – single P support level can eliminate ETT resistance Dynamic condition – variable flow e.g. tachypnoea & in different phases of resp. - P support needs to be continously altered to eliminate dynamically changing WOB d/t ETT 1.Feed resistive coef of ETT 2.Feed % compensation desired 3.Measures instantaneous flow Calculates P support proportional to resistance throughout respiratory cycle Limitation – resistive coef changes in vivo ( kinks, temp molding, secretions) Under/ overcompensation may result.

73 Airway pressure release ventilation (APRV) High level of CPAP with brief intermittent releases to a lower level Conventional modes – begin at low P & elevate P to accomplish TV APRV – commences at elevated P & releases P to accomplish TV

74 Higher plateau P – improves oxygenation Release phase – alveolar ventilation & removal of CO 2 Active patient – spontaneous breathing at both P levels Passive patient – complete ventilation by P release

75 Settings 1.P high (15 – 30 cmH 2 O ) 2.P low (3-10 cmH 2 O ) == PEEP 3. F = 8-15 / min 4. T high /T low = 8:1 to 10:1 If ↑ PaCO 2 -↑ P high or ↓ P low - ↑ f If ↓ PaO 2 - ↑ P low or Fi O 2

76 Advantages 1. Preservation of spontaneous breathing and comfort with most spontaneous breathing occurring at high CPAP 2. breathing occurring at high CPAP 3. ↓WOB 4. ↓Barotrauma 5. ↓Circulatory compromise 6. Better V/Q matching

77 Proportional Assist Ventilation Targets fixed portion of patient’s work during “spontaneous” breaths Automatically adjusts flow, volume and pressure needed each breath

78 WOB Ventilator measures – elastance & resistance Clinician sets -“Vol. assist %” reduces work of elastance “Flow assist%” reduces work of resistance's Increased patient effort (WOB) causes increased applied pressure (and flow & volume) ELASTANCE (TV) RESISTANCE (Flow)

79 Limitations 1. Elastance (E) & resistance (R) cannot be measured accurately. 2. E & R vary frequently esp in ICU patients. 3. Curves to measure E ( P-V curve) & R (P-F curve ) are not linear as assumed by ventilator.

80 Biphasic positive airway pressure (BiPAP)  PCV & a variant of APRV  Time cycled alteration between 2 levels of CPAP BiPAP – P support for spontaneous level only at low CPAP level Bi-vent - P support for spontaneous level at both low & high CPAP Spontaneous breathing at both levels Changeover between 2 levels of CPAP synchronized with exp & insp

81 . Can provide total / partial ventilatory support 1.BiPAP – PCV – if pt not breathing 2.BiPAP – SIMV- spontaneous breathing at lower CPAP + mandatory breaths by switching between 2 CPAP levels 3. CPAP – both CPAP levels are identical in spontaneously breathing patient 4. BiPAP – P support – additional P support at lower CPAP 5. Bi- vent – additional P support at both levels of CPAP

82 BiPAP Bi- vent

83 Advantages 1. Allows unrestricted spontaneous breathing 2. Continuous weaning without need to change ventilatory mode – universal ventilatory mode 3. Synchronization with pt’s breathing from exp. to insp. P level & vice versa 4. Less sedation needed

84 Neurally Adjusted Ventilatory Assist (NAVA) Electrical activity of respiratory muscles used as input Eadi (electrical activity of diaphragm) Cycling on, cycling off: determined by Eadi Synchrony between neural & mechanical inspiratory time is guaranteed Patient comfort

85 References 1. Egan’s – fundamentals of respiratory care 9 th ed. 2. International Anaesthesiology Clinics – Update on respiratory critical care, vol 37, no 3, 1999. 3. Anaesthesia newsletter,Indore city,June 2009, vol 10, no 2 4. David W Chang, Clinical application of mechanical ventilation 2 nd ed 5. Wylie and Churchill Davidson – A Practice of Anesthesia, 5 th ed. 6. Paul L Marino, The ICU Book, 3 rd ed.

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