Mechanical Ventilation

1. Mechanical Ventilation

2. Overview • Intro • NIV • Basic Modes • Settings • Specific Conditions • Ventilators • Other modes

3. Acute respiratory failure • Hypoxia (PO2 < 60mmHg) • Low inspired O2 • Hypoventilation – CNS, peripheral neuro, muscles, chest wall • V/Q mismatch • Shunt – pneumonia, APO, collapse, contusions • Alveoli perfused but not ventilated • Venous admixture • Anatomical shunt – cardiac anomaly • Increased dead space (hypercapnia) – hypovolaemia, PE, poor cardiac function • Diffusion abnormality – severe destructive disease of the lung – fibrosis, severe APO, ARDS • Hypercapnia (PCO2 >50mmHg) • Hypoventilation • Dead space ventilation • Increased CO2 production

4. 0 mmHg 450 mmHg 100% 70% 70% 70% 85% Shunt

5. Mechanical Ventilation • Pump gas in and letting it flow out • Function • Gas exchange • Manage work of breathing • Avoid lung injury • Physics • Flow needs a pressure gradient • Pressure to overcome airway resistance and inflate lung • Pressure (to overcome resistance) = Flow x Resistance • Alveolar pressure = (Volume/Compliance) + PEEP • Airway pressure = (Flow x Resistance) + (V/C) + PEEP

9. Gas Exchange • Oxygenation – get O2 in • FiO2 • Ventilation (minor effect) – alveolar gas equation, CO2 effect • Mean alveolar pressure • Mean airway pressure – surrogate marker, affected by airway resistance • Pressure over inspiration + expiration • Set Vt or inspiratory pressure • Inspiratory time • PEEP • Reduce shunt • Re-open alveoli – PEEP • Prolonging inspiration – improve ventilation of less compliant alveoli • Ventilation – get CO2 out • Alveolar ventilation = RR x (Tidal volume – Dead space)

11. Adverse Effects • Barotrauma • High alveolar pressure • High tidal volume • Shear injury – • Repetitive collapse + re-expansion of alveoli • Tension at interface between open + collapsed alveoli • Pneumothorax, pneumomediastinum, surgical emphysema, acute lung injury • Gas trapping • Insufficient time for alveoli to empty • Increase risk • Airflow obstruction – asthma, COPD • Long inspiratory time • High respiratory rate • Progressive • Hyperinflation • Rise in end-expiratory pressure – intrinsic-PEEP, auto-PEEP • Result – Barotrauma, Cardiovascular compromise (high intrathoracic pressure) • Oxygen toxicity • Acute lung injury due to high O2 concentrations • Cardiovascular effects • Preload – positive intrathoracic pressure reduces venous return • Afterload - positive intrathoracic pressure reduces afterload • Cardiac Output – depends on LV contractility • Normal – IPPV decreases CO • Reduced – IPPV increases CO • Myocardial O2 consumption - reduced

12. Gas Trapping

13. NIV • CPAP • Similar to PEEP • Splint alveoli open – reduce shunt • Spontaneous breathing at elevated baseline pressure • BiPAP • Ventilatory assistance without invasive artificial airway • Fitted face/nasal mask • Initial settings 10/5

14. NIV

15. Indicator of success Known benefits Younger age Lower APACHE score Cooperative Intact dentition Moderate hypercarbia (pH<7.35, >7.10) Improvement within first 2 hrs Contraindications Cardiac/Resp arrest Non-respiratory organ failure Encephalopathy GCS <10 GIH Haemodynamically unstable Facial or neurological surgery, trauma or deformity High aspiration risk Prolonged ventilation anticipated Recent oesophageal anastamosis NIV

16. NIV Benefits • General • COPD • Cardiogenic pulmonary oedema • Hypoxaemic respiratory failure • Asthma • Post-extubation • Immunocompromised • Other diseases

17. What is a Mode? • 3 components • Control variable • Pressure or volume • Breath sequence • Continuous mandatory • Intermittent mandatory • Continuous spontaneous • Targeting scheme (settings) • Vt, inspiratory time, frequency, FiO2, PEEP, flow trigger

18. Volume Control Ventilation • Set tidal volume • Minimum respiratory rate • Assist mode – both ventilator and patient can initiate breaths • Advantage • Simple, guaranteed ventilation, rests respiratory muscle • Disadvantages • Not synchronised – ventilator breath on top of patient breath • Inadequate flow – patient sucks gas out of ventilator • Inappropriate triggering • Decreased compliance – high airway pressure • Requires sedation for synchrony

19. VCV

20. Pressure Control Ventilation • Set inspiratory pressure • Constant pressure during inspiration • High initial flow • Inspiratory pause – built in • Advantages • Simple, avoids high inspiratory pressures, improved oxygenation • Disadvantages • Not synchronised • Inappropriate triggers • Decreased compliance – reduced tidal volume

21. PCV

22. Pressure Support • Set inspiratory pressure • Patient initiates breath • Back-up mode – apnoea • Cycle from inspiration to expiration • Inspiratory flow falls below set proportion of peak inspiratory flow • Advantages • Simple, avoids high inspiratory pressure, synchrony, less sedation, better haemodynamics • Disadvantages • Dependent on patient breaths • Affected by changes in lung compliance

23. PS

24. Synchronised Intermittent Mandatory Ventilation • Mandatory breaths – VCV, PCV • Patient breaths – depends on SIMV cycle • Synchronised mandatory breath • Pressure support breath • Advantages • Synchrony, guaranteed minute ventilation • Disadvantages • Sometimes complicated to set

25. SIMV

26. VCV vs PCV

27. VCV vs PCV

28. PCV + PS Variable flow Reduced WOB Max Palveolar = Max Pairway (or less) Palveolar controlled Variable I-time & pattern (PS) Better with leaks VCV vs PCV - Advantages • VCV • Consistent TV • changing impedance • Auto-PEEP • Minimum min. vent. (f x TV) set • Variety of flow waves

29. PCV + PS Variable tidal volume Too large ortoo small No alarm/limit for excessive TV (except some new gen. vents) Some variablity in max pressures (PC, expir. effort) VCV vs PCV - Disadvantages • VCV • Variable pressures • airway • alveolar • Fixed flow pattern • Variable effort = variable work/breath • Compressible vol. • Leaks = vol. loss

30. Settings • FiO2 – start at 1.0 • RR – average 12, higher for those with sepsis/acidosis • Tidal volume – 500ml, 8ml/kg, smaller volumes in ARDS • Inspiratory pressure - <30cmH2O, sum of PEEP + Pinsp • Inspiratory time • I:E – normally 1:2, simulates normal breathing – synchrony • PCV – easy to set • VCV – complicated, Time = Volume/Flow • PEEP • Start at 5cmH2O • Higher – APO, ARDS • Lower – asthma, COPD • Triggering • Flow triggering – more sensitive, synchrony, -2cmH2O • Pressure triggering • Inappropriate triggering – triggering when no patient effort • Oxygenation • FiO2, PEEP, Insp Time, InspP, Insp pause • Problems – CVS effects, gas trapping, barotrauma • Ventilation • Tidal volume, RR, eliminate dead space • Problems – barotrauma, gas trapping (reduced minute ventilation)

31. Total PEEP Pressure PEEPi PEEPe Time Troubleshooting • Airway pressure • Ventilator – settings, malfunction • Circuit – kinking, water pooling, wet filter • ETT – kinked, obstructed, endobronchial intubation • Patient – bronchospasm, compliance (lungm, pleura, chest wall), dysynchrony, coughing • Inspiratory pause pressure - Estimate of alveolar pressure • Tidal volume • Reduced – respiratory acidosis • Monitor in PCV/PS • Changes in compliance – anywhere in system • Expired Vt – more accurate • Minute ventilation – determined by RR + Vt • Apnoea – important in PS • Intrinsic PEEP (gas trapping) • Expiratory pause hold • Hypotension – after initiating IPPV • Hypovolaemia/Reduced VR • Drugs • Gas trapping – disconnect • Tension pneumothorax • Dysynchrony • Patient factors • Ventilator – settings, eg I:E • PS > SIMV > PCV/VCV

32. Troubleshooting • Desaturation • Patient causes • All causes of hypoxic respiratory failure • Endobronchial intubation, PTx, collapse, APO, bronchospasm, PE • Equipment causes • FIO2 1.0 • Sat O2 waveform • Chest moving? • Yes – Examine patient, treat cause • No – Manually ventilate • No – ETT/Patient problem • Yes – Ventilator problem – setting, failure, O2 failure

33. Maquet VCV PCV PRVC PS/CPAP SIMV (VC) + PS SIMV (PC) + PS SIMV (PRVC) + PS MMV NAVA Evita PS PCV+ SIMV PCV+A Autoflow Ventilators

34. Adaptive Modes - PRVC • PCV unable to deliver guaranteed minimum minute ventilation • Changing lung mechanics + patient effort • Pressure controlled breaths with target tidal volume • Inspiratory pressure adjusted to deliver minimum target volume • Not VCV - average minimum tidal volume guaranteed • Like PCV – constant airway pressure, variable flow (flow as demanded by patient)

35. Adaptive Modes - PRVC • Consistent tidal volumes • Promotes inspiratory flow synchrony • Automatic weaning • Inappropriate – increased respiratory drive, eg severe metabolic acidosis • Evidence – lower peak inspiratory pressures

36. VCV vs PRVC

37. Adaptive Modes - Autoflow • First breath uses set TV & I-time • Pplateau measured • Pplateau then used • V/P measured each breath • Press. changed if needed (+/- 3) • Dual mode similar to PRVC • Targets vol., applies variable press. based on mechanics measurements • Allows highly variable inspiratory flows • Time ends mandatory breaths • Adds ability to freely exhale during mandatory inspiration (maintains pressure)

38. PCV + Assist • Like PCV, flow varies automatically to varying patient demands • Constant press. during each breath - variable press. from breath to breath • Mandatory + patient breaths the same

39. Inverse Ratio Ventilation • Increased mean airway pressure • Prolonged I:E ratio • Improved oxygenation • Reduced shunting • Improved V/Q matching • Decreased dead space • Heavy sedation, paralysis • Preferred PCV • Benefit – no effect in mortality in ARDS

40. Other Modes • Adaptive support ventilation • Mandatory minute ventilation • Adaptive pressure control • Proportional assist ventilation • Pressure support (spontaneous breaths) • Pressure applied function of patient effort • Automatic tube compensation • adjusts its pressure output in accordance with flow, theoretically giving an appropriate amount of pressure support

41. Airway Pressure-Release Ventilation • High constant PEEP + intermittent releases • Unrestricted spontaneous breaths – reduced sedation • Extreme form of inverse ratio ventilation • E:I – 1:4 • Spontaneous breaths – 10-40% total minute ventilation

42. APRV • Settings – 2 pressure levels, 2 time durations • Uses – ALI, ARDS • Caution – COPD, increased respiratory drive

43. APRV • Increase mean airway pressure • Alveolar recruitment, improve oxygenation • Promote spontaneous breathing • Improved V/Q match, haemodynamics • Improved synchrony • Evidence – no difference in mortality, decreased duration of ventilation

44. High-Frequency Ventilation • 4 types • High frequency jet ventilation • Ventilation by jet of gas • 14-16G cannula, specialised ventilator • 35 psi, RR100-150, Insp 40% • High frequency oscillatory ventilation • High frequency percussive ventilation • HFV + PCV • HFOV – oscillating around 2 pressure levels • Less sedation, better clearance of secretions • High frequency positive pressure ventilation • Conventional ventilation at setting limits

45. High Frequency Oscillatory Ventilation • Ventilator delivers a constant flow (bias flow) • Valve creates resistance – maintain airway pressure • Piston pump oscillates 3-15Hz (RR160-900) • “Chest wiggle” – assess amplitude • Tidal volumes – less than dead space • Ventilation – achieved by laminar flow • Deep sedation, paralysis

46. HFOV • CO2 clearance • Decrease oscillation frequency, increase amplitude, increase inspiratory time, increase bias flow (with ETT cuff leak) • Oxygenation • Mean airway pressure, FiO2 • Settings • Airway pressure amplitude • Mean airway pressure • % inspiration • Inspiratory bias flow • FiO2

47. HFOV • Applications • ARDS • Lung protection – highest mean airway pressure + lowest tidal volumes • Ventilatory failure – FiO2>0.7, PEEP>14, pH <7.25, Vt >6ml/kg, plateau pressure >30) • Contraindicated • Severe airflow obstruction • Intracranial hypertension • Evidence • Animal models – less histologic damage + lung inflammation • Better oxygenation as rescure therapy in ARDS • No difference in mortality

48. Mean Airway Pressure • Main factor in recruitment and oxygenation • Increased surface area for O2 diffusion • Problems • Barotrauma • Haemodynamic instability • Contraindicated patients • Deep sedation, paralysis

49. Specific Conditions • ARDS • Definition • Diffuse bilateral pulmonary infiltrates • No clinical evidence of Left Atrial Hypertension (CWP<18mmHg) • PaO2/FiO2 of 300 or less • Exclusions • Unilateral lung disease • Children (wt less than 25kg) • Severe obstructive lung disease (asthma, COPD) • Raised intracranial pressure • High PEEP, low volumes + pressure • SIMV(PRVC) + PS • Vt 6ml/gk – check plateau pressure • Pins >30cmH2O – reduce Vt • Lowest plateau pressure possible • RR 6-35, aim pH 7.3-7.45 • Evidence – improved mortality

50. Ventilator Induced Lung Injury • Excessive inflation pressure • Mechanical tissue damage • Inflammation – mechano-signaling due to tensile forces • Overstretching of lung units • Shear force at junction of open and collapsed tissue • Repeated opening and closing of small airways under high pressure