Ventilatory Failure & Hypoxia

1. Applied Sciences Lecture Course Ventilatory Failure & Hypoxia Mahesh Nirmalan MD, FRCA, PhD Consultant, Critical Care Medicine Manchester Royal Infirmary

2. Objectives • Respiratory failure is one of the commonest manifestations of acute illness • Hypoxia and CO2 retention • Failure of oxygen transfer • Failure of effective alveolar ventilation • Pathophysiology • Differences in management approach

3. Ventilation Moving an adequate volume of air Minute ventilation Alveolar ventilation Oxygenation Transfer of O2 across the alveoli Dusky colour Cyanosis Low SpO2 Low arterial PaO2 Respiratory rate Tidal volume or chest expansion Arterial PaCO2 Respiration or Breathing

4. Type 2 Type 1 Mixed Hypoxia & Hypercarbia Hypercarbia PaCO2>7kPa Hypoxia PaO2<8kPa Respiratory Failure

5. Treatment of Respiratory failure • Type 1 • Cause • O2 supplementation • PEEP • Type 2 • Cause • Ventilatory assistance • Pharmacological • Mechanical: IPPV • Mixed

6. FRC

7. Lung volumes FRC is a balance between two forces Reduced compliance Reduced FRC Increased compliance Increase in FRC

8. Inflammation and oedema within the lung parenchyma Low compliance and low FRC

9. Hepatisation Fibrinous exudate H’ge

10. Hyaline.membrane Normal lung Interstitial oedema Organising oedema Alveolar oedema Haemorrhage Neutrophil infiltartion Histological changes: reduced lung compliance

11. Reduced compliance • Pulmonary oedema • Pneumonia • ARDS and ALI • Fibrosis Tachypnoea Increased work of breathing Hypoxia

12. Extensive pneumonia: Reduced lung compliance→↓FRC

13. Pneumonia: ↓lung compliance

14. Decreased lung compliance • Tendency for the alveoli to collapse • May involve large parts of the lung • Reduction in FRC is an important factor • Increased work of breathing • Common cause for failure in oxygenation • Type 1 respiratory failure

15. Loss of elastic tissue within the lung parenchyma Increased lung compliance

16. COPD: Increase in compliance→↑FRC

17. Increased lung compliance: Increased FRC Hyper-inflation Low set diaphram Reduced lung markings

18. Lung volumes

19. Hypoxia: Failure in oxygenation

20. Hypoxia: failure of tissue oxygenation • Hypoxic hypoxia: Pulmonary oxygen transfer • Stagnant hypoxia: Poor blood flow • Anaemic hypoxia: poor oxygen carriage • Histotoxic hypoxia: Sepsis, Cyanide

21. Oxygen cascade in an ideal lung Diffusion, shunt, ventilation perfusion mismatch High Altitude Hypoventilation

22. CO2 retention: Ventilatory failure Treat the cause: Opiates, pain, airway obstruction Ventilatory support: Non-Invasive: BiPAP Invasive: Mechanical ventilation OXYGENATION: HYPOXIA Treat the cause: Infection, oedema ↑FiO2 PEEP Treatment of respiratory failure

23. Summary Failure of oxygenation • Hypoventilation • Diffusion • Shunt and V/Q mismatch • Treat the cause • Supplemental oxygenation & PEEP Failure of ventilation • Respiratory depression • Increase in physiological dead space • Treat the cause • Ventilatory assistance

24. Pathophysiology of hypoxia Venous blood Oxygenated blood

25. Pathophysiology of hypoxia Venous blood Venous blood

26. Ventilation/perfusion or V/Q mismatch Partially oxygenated blood Venous blood

27. Shunt and V/Q mismatch Alveolar oedema Shunt: blood that goes through unventilated lung units V/Q mismatch: Blood going through poorly ventilated units

28. Causes of Hypoxia Clinically how does one distinguish between shunt and V/Q mismatch? Effect of increasing FiO2 Hypoventilation Diffusion defects Ventilation-perfusion mismatch Shunts

29. 45 years old male: Breathless, pyrexial, unwell, (breathing 50% O2)Pulse oximetry: 90% saturation • pH=:7.15 • PCO2: 3.3 kPa • PO2: 13.47kPa • HCO3-: 17 mmol.l-1 • Hb: 10.8 g.dl-1 • Glucose: 12.8mmol.l-1 • Lactate: 0.9mmol.l-1 Shunt and V/Q mismatch

30. Ventilatory failure: CO2 retention

31. Physiological dead space Wasted ventilation Extension of dead space Ventilated but not perfused alveolar units Physiological dead space Dead space ventilation does not clear CO2 Extension of dead space will lead to CO2 retention

32. COPD: Increase in compliance→↑FRC

33. Pulmonary embolism: Typically increase in Physiological dead space When large also causes significant V/Q mismatch Hypoxia and CO2 retention

34. Most organic parenchymal diseases:Increase in V/QSome shuntingIncrease in physiological dead space

35. Ventilatory Failure • Hypoventilation • Depression of respiratory centre: opiates • Pain: upper abdominal surgery, Rib fractures • Prolonged increase in work of breathing • Tachypnoea • Reduced lung compliance • Severe asthma • Extension of physiological dead space • COPD

36. COPD: 25% O2 pH=:7.15 PCO2: 12.3 kPa PO2: 13.47 kPa HCO3-: 32mmol.l-1 Hb: 18.8 g.dl-1 Glucose: 9.8mmol.l-1 Lactate: 0.9mmol.l-1

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