VENTILATION FOR THE SURGICAL RESIDENT

1. VENTILATION FOR THE SURGICAL RESIDENT POS review lecture 2008-2009 Heather Whittingham

2. OBJECTIVES • Respiratory physiology • oxygen delivery • abnormalities of gas exchange • review of lung volumes • chest wall and respiratory mechanics • Mechanical Ventilation • indications • nomenclature • ventilation modes: invasive and non-invasive • special circumstances: ARDS, refractory hypoxemia and BPF • complications: High pressures, VILI, Auto-PEEP, VAP • weaning

3. RESPIRATORY PHYSIOLOGY REVIEW

4. OXYGEN DELIVERY • oxygen is carried in the blood in two forms: • bound to Hb (SpO2) * • dissolved in plasma (PaO2) • oxygen content (CaO2) is the sum of both: • oxygen delivery is a product of both the arterial O2 content and cardiac output harder to unload O2 [Hb] x SpO2 x (1.36) + easier to unload O2 (PaO2) x (0.003)

5. OXYGENATION • Hypoxia is a state of tissue oxygen deprivation • anaerobic metabolism  lactic acidosis • can lead to cellular, tissue and organ death • Hypoxia can result from: • low PaO2 • anemia or abnormal Hb • low cardiac output states/ impaired perfusion • inability to utilize O2 (eg. cyanide) • Hypoxemia refers to low PaO2 in the blood

6. ABNORMAL GAS EXCHANGE • Efficiency of gas exchange: the a-a gradient • P(A-a)O2 = PAO2 – PaO2 • PAO2 = [713 x FiO2] – [1.25 x PaCO2] • cumbersome, normal values not known for supplemental O2 • Often use P/F ratio instead: PaO2/FiO2 • normal on FiO2 0.21 is 450-500 range • tells us nothing about alveolar ventilation (PCO2) • will be dependent on level of PEEP/ CPAP

7. ABNORMAL GAS EXCHANGE

8. INTRAPULMONARY SHUNT

9. VENTILATION • Ventilation refers to CO2 clearance • Alveolar ventilation • air that meets perfused alveoli and participates in gas exchange • Dead space ventilation • air doesn’t contact perfused alveoli to participate in gas exchange • anatomic+ alveolar + equipment • “wasted” ventilation • Minute Ventilation (MV) • RR x VT • total gas (L/min) of ventilation • normal 6-8 L/min

10. ABNORMAL GAS EXCHANGE • rarely causes hypercapnia in absence of other ventilatory defect HYPERCAPNIA • Mechanisms: • Increased CO2 production • malignant hyperthermia • thyroid storm • Decreased CO2 clearance • low minute ventilation (RR x VT) • high dead space ventilation • low respiratory drive • CNS depression • drugs • OHS/ CSA • respiratory mechanical failure • fatigue • neuromuscular disease • chest wall abnormality • underlying lung pathology • COPD • ILD • pulmonary embolism • pulmonary vascular disease

11. LUNG VOLUMES • TLC: amount of gas in lungs after maximal inspiration • RV: amount of gas in lungs after maximal expiration • VC: volume of gas expired going from TLC to RV • FRC: volume of gas in lungs at the resting state (end-expiration) • TV: amount of gas inhaled in a normal inspiration

12. PULMONARY COMPLIANCE • Defined as the ability of the lung to stretch (change in volume) relative to an applied pressure • Factors affecting compliance: • lung volume (overdistention vs. atelectasis) • interstitial pathology (CHF, ILD) • alveolar pathology (pneumonia, CHF, blood) • pleural pathology (pleural effusion, fibrosis) • chest wall mechanics • diaphragm mobility • chest wall deformities • abdominal pressures

13. RESPIRATORY FAILURE

14. RESPIRATORY FAILURE • Acute respiratory failure: “any impairment of O2 uptake or CO2 elimination or both that is severe enough to be a threat to life” • The signs and symptoms of respiratory failure are non-specific and often non-respiratory • reflect end-organ dysfunction of neurologic and cardiovascular systems RESPIRATORY FAILURE HYPOXEMIC HYPERCAPNIC Won’t breathe Can’t breathe

15. RESPIRATORY FAILURE • Clinical signs and Symptoms • hypoxia is relatively easily identified on clinical examination • hypercapnia can be more subtle in its presentation • may not be in respiratory distress (central failure) Respiratory General Cardiovascular Neurologic • tachypnea • dyspnea • diaphoresis • central cyanosis (late) • restlessness • headache • confusion • delirium • tremor • asterixis • seizures • coma • tachycardia • dysrhythmias • hypertension • hypotension • wheeze • dyspnea • cough • accessory muscle use • abdominal paradox

16. MECHANICAL VENTILATION

17. MV: INDICATIONS • Hypoventilation • arterial pH more important than absolute pCO2 • can result from central or mechanical failure • respiratory acidosis with pH <7.25 and pCO2 >50 • Hypoxemia • hypoxemia refractory to conservative measures • pO2 < 60 with FiO2 >60% • Respiratory Fatigue • excessive work of breathing suggestive of impending respiratory failure • Airway Protection

18. MV: INDICATIONS • most absolute criteria for initiation of mechanical ventilation are arbitrary and reflect a line drawn in the sand • fail to account for a spectrum of disease • a PaO2 of 61 is acceptable and 59 is not? • chronic vs acute derangements • fail to account for co-morbid disease management • precise control of PaCO2 in a patient with a head injury • assisted hyperventilation to compensate for a metabolic acidosis • airway maintenance with nasal airway or surgical airway “the patient looked like they need to be placed on a ventilator”

19. NOMENCLATURE • A “mode” is a pattern of breaths delivered by the ventilator • pressure support • pressure control • volume control • To understand the differences, must understand the “phases” of ventilation • expiratory: passive phase, PEEP applied • triggering: change from expiration to inspiration • inspiratory: assisted inspiratory flow • cycling: end of inspiration and change to expiration

20. PHASES OF VENTILATION • Triggering: • patient triggered (flow, pressure) • machine triggered (time) • Inspiration-assisted • Cycling • time (PCV) • volume (VCV) • flow (PSV) • Expiration- passive C INSP B A EXP D

21. VOLUME CONTROL (VCV) • Set tidal volume, cycles into exhalation when target volume has been reached; airway pressure dependent on lung compliance • guarantees a minimum minute ventilation (MV= RR x Vt) • useful for patients with a decreased respiratory drive • post-operative, head-injured, narcotic overdose • Variables: • Trigger: patient or machine controlled • Inspiratory phase: set inspiratory flow rate • Cycling: SET • Expiratory phase: set amount of PEEP • Alarms: high pressure (default into PCV and cycle), high RR

22. PRESSURE CONTROL (PCV) • Inspiratory pressure and inspiratory time are set; tidal volume is dependent on lung compliance • allows for control of peak airway pressures (ARDS) • a longer inspiratory time can allow for better recruitment and oxygenation • Variables: • Trigger: patient or machine controlled • Inspiratory phase: SET- target pressure, generated quickly and maintained throughout; high initial flow rate • Cycling: time • Expiratory phase: set amount of PEEP • Alarms: high and low tidal volumes, high RR

23. PRESSURE SUPPORT (PSV) • Spontaneous mode of ventilation; patient generates each breath and a set amount of pressure is delivered with each breath to ‘support’ the breath • comfortable: determine own RR, inspiratory flow and time • Vt depends on level of pressure support set, lung compliance and patient effort • Variables: • Trigger: patient controlled; must initiate breath • Inspiratory phase: SET support pressure • Cycling: flow cycled (when falls to ~25% of peak) • Expiratory phase: set amount of PEEP • Alarms: apnea and high RR

24. NOMENCLATURE • CMV (Controlled Mechanical Ventilation) • minute ventilation entirely determined by set RR and Vt • patient efforts do not contribute to minute ventilation • AC (Assist/Control) • combination of mandatory (set rate) and patient triggered breaths • patient triggered breaths deliver same Vt or pressure as mandatory breaths • SIMV (Synchronized Intermittent Mandatory Ventilation) • combination of mandatory and patient-triggered breaths • pure SIMV, patient not assisted on additional breaths • can combine SIMV with PSV, so additional breaths are supported

25. NOMENCLATURE • Comparison of respiratory pattern using different modes:

26. PEEP • Positive End-Expiratory Pressure (PEEP) • constant baseline pressure delivered throughout cycle • by convention: called CPAP if breathing spontaneously and PEEP if receiving positive pressure ventilation • 3-5cm H20 PEEP provided to all intubated patients to overcome the decrease in FRC caused by bypass of glottis • Advantages: • Improve oxygenation by preventing end-expiratory collapse of alveoli and help recruit new alveoli • may prevent barotrauma caused by repetitive opening and closing of alveoli • creates hydrostatic forces to fluid from alveoli into interstitium

27. PEEP- COMPLICATIONS • Potential complications: • may overdistend alveoli: • causing barotrauma • can worsen oxygenation by increasing dead space • decreases venous return (high intrathoracic pressures) • decreasing cardiac output • increases RV afterload • can contribute to RV strain and/or failure associated with severe respiratory failure • lung heterogeneous • some areas may be getting too much, while others not enough

28. PEEP- CONTRAINDICATIONS • Relative contraindications to high PEEP • circumstances where risk may outweigh benefit:

29. NON-INVASIVE VENTILATION • The delivery of PPV without an ETT • avoids complications of intubation, including VAP • Two fundamental types: CPAP and bi-level or BiPAP • CPAP delivers continuous positive pressure throughout respiratory cycle • useful for hypoxemic respiratory failure • BiPAP delivers ‘pressure support’ during inspiration (IPAP), coupled with PEEP during expiration (EPAP) • useful for hypercapneic or combined respiratory failure

30. NIV: INDICATIONS • Has been shown to decrease need for intubation and decrease morbidity & mortality in certain patients: • Acute cardiogenic pulmonary edema (ACPE) • COPD exacerbation • May decrease re-intubation rate after extubation in COPD • Fundamental requirements: • spontaneously breathing patient who can protect airway • potentially reversible condition • ability to improve within a few hours • cooperative patient • no hemodynamic instability, no cardiac ischemia

31. NIV: CONTRAINDICATIONS • Hemodynamic instability or shock • Decreased LOC and inability to protect airway • Inadequate respiratory drive • High risk of aspiration (SBO, UGI bleed) • Facial trauma or craniofacial abnormality • Upper airway obstruction • Uncooperative patient • Inability to clear secretions or excessive secretions

32. NIV: MONITORING • NIV has been successful if the patient’s work of breathing has decreased and blood gas abnormalities are starting to resolve • Clinical improvement is usually evident within the 1st hour • Biochemical improvement usually evident within 2-4 hours of initiation If ongoing evidence of respiratory failure despite NIV within a few hours of initiation… CONSIDER INTUBATION

33. SPECIAL CIRCUMSTANCES

34. ARDS • Definition: • bilateral pulmonary infiltrates • absence of LA hypertension • severe hypoxemia (PaO2/FiO2 ratio <200) • Heterogeneous lung involvement • dependent: atelectatic, consolidated • non-dependent: relatively preserved • Concept of the “baby lung” • high inflation pressures/ volumes used for hypoxemia can damage normal lung (volutrauma, barotrauma) • repetitive opening/closing of marginal areas causes additional trauma (atelectrauma)

35. ARDS: VENTILATION • Important to understand principles of ARDS to minimize ventilator-induced lung injury • Lung protective ventilation (ARDSnet) • compared tidal volume of 12ml/kg (840) and plateau <50 cm H2O vs 6ml/kg (420) and plateau <30 cm H2O • stopped early for benefit • mortality 31 vs 39% (p=0.007) • more vent free days • Mild permissive hypercapneia ok • May require sedation to maintain

36. REFRACTORY HYPOXIA • Some additional modes of ventilation can be tried for hypoxia refractory to conventional ventilation: • recruitment maneuvers • inverse ratio ventilation (I>E) • prone ventilation • airway pressure release ventilation (APRV) • high frequency oscillation ventilation (HFOV) • None to date have shown an increased mortality, but can improve oxygenation

37. APR VENTILATION • APRV ventilates by time-cycled switching between two pressure levels (Phigh and Plow) • degree of ventilator support is determined by the duration of the two pressure levels and the tidal volume delivered • tidal volume determined by Δ P and respiratory compliance • permits spontaneous breathing in any phase • better ventilation of posterior, dependent lung regions after 24h • improves recruitment • lower sedation required • C/I if deep sedation needed, COPD?

38. HFO VENTILATION • HFOV achieves gas transport by rapidly oscillating a small Vt (~anatomic dead space) achieving rapid gas mixing in the lung • gas transport occurs along partial-pressure gradients • oscillates around a constant high mean airway pressure (mPaw) to maintain alveolar recruitment, avoiding big Δ P • risk of barotrauma and hemodynamic compromise limilar to conventional ventilation • O2: mPaw and FiO2 • CO2: frequency and ΔP

39. BRONCHOPLEURAL FISTULA • Presence of a persistent air-leak >24h after insertion of a CT is highly suggestive of a bronchopleural fistula • after exclusion of an external leak • Weaning from PPV entirely is optimal • When not possible, select strategy to minimize minute ventilation and intrathoracic pressure

40. BPF- MANAGEMENT • Wean ventilatory support as much as tolerates • PSV may be preferable to full ventilation • limit mean airway pressure and number of high pressure breaths • avoid alkalosis; consider permissive hypercapnia • minimize PEEP (intrinsic and extrinsic); treat bronchospasm • Limit VT to 6-8 ml/kg • Minimize inspiratory time (keep I:E ratio low, use high flows) • Use lowest CT suction that maintains lung inflation • Explore positional differences that minimize leak

41. BPF- MANAGEMENT • Consider specific or unconventional measures for physiologically significant leaks: • independent lung ventilation • endobronchial approach to sealing leak • surgical closure • Treat underlying cause of respiratory failure

42. BPF in ARDS • Usually a measure of severity of underlying disease will -- -often doesn’t improve until ARDS improves • BPF nearly always improves without specific therapy • BPF usually not physiologically significant (<10%), even in presence of hypercapnia • Reducing the size of the leak has minimal effect on gas exchange • No specific measures have been shown to affect outcome • Patients almost never die of BPF… they die with BPF

43. COMPLICATIONS OF VENTILATION

44. HIGH AIRWAY PRESSURES • Decreased Compliance • pneumothorax • mainstem intubation • dynamic hyperinflation • CHF • ARDS • consolidation • pneumonectomy • pleural effusion • abdominal distention • chest wall deformity • Increased Resistance • bronchospasm • secretions • small ETT • mucosal edema • biting ETT

45. VILI VENTILATOR-INDUCED LUNG INJURY • multiple recognized forms: • barotrauma: • high ventilation pressures result in global or regional overdistention  can result in alveolar rupture • may be gross (PTX, BPF, subcut emphysema) or microscopic • volutrauma/atelectrauma: • ventilation at low lung volumes causes repetitive opening and closing of alveoli • may lead to shear stress, disruption of surfactant and epithelium • biotrauma: • mechanical stretch or shear injury lead to inflammatory mediator release and cellular activation

46. VILI • Prevention: • low VT ventilatory strategies • minimize peak and plateau pressures • PEEP for recruitment and minimize end-expiratory collapse • tolerate mild to moderate permissive hypercapnia to achieve above goals: • allowing PCO2 to rise into high 40’s to 50’s to reduce driving and plateau pressures • generally considered safe at low levels • contraindications: increased ICP, acute or chronic cardiac ischemia, severe PH, RV failure, uncorrected severe metabolic acidosis, TCA overdose, pregnancy

47. AUTO-PEEP • aka: intrinsic PEEP or dynamic hyperinflation • Seen when a patient has failed to expire full VT and subsequent breaths delivered result in increasing hyperinflation

48. AUTO-PEEP • Making the diagnosis: • inspection: continuous inward movement of chest until start of next breath • auscultation: persistence of breath sounds until start of next ventilator breath • failure to return to baseline on waveform before delivery of next breath “Auto-PEEP” “normal”

49. AUTO-PEEP

50. AUTO-PEEP: MANAGEMENT • Lengthen time for exhalation • slow controlled rate on ventilator • lengthen I:E ratio (shorten I time) • may require patient sedation if patient-driven • Treat bronchospasm • bronchodilators • corticosteroids if asthma or AECOPD • Match intrinsic PEEP to minimize gas trapping by dynamic collapse