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Patho-Physiology of Respiratory Failure.

Patho-Physiology of Respiratory Failure. John KOLBE Respiratory Services Green Lane Hospital, & Faculty of Health Sciences University of Auckland.

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Patho-Physiology of Respiratory Failure.

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  1. Patho-Physiology of Respiratory Failure. John KOLBE Respiratory Services Green Lane Hospital, & Faculty of Health Sciences University of Auckland.

  2. Someone told me that each equation I included in the book would halve the sales. Stephen Hawking “A Brief History of Time”: 1988.

  3. RESPIRATORY FAILURE Def: When the lungs fail to adequately oxygenate the arterial blood and/or fail to prevent undue CO2 retention. In practical Terms; PaO2 < 8kPa (60 mmHg) (Hypoxic,TypeI) PaCo2 > 6.6 kPa (50mmHg) (Hypercapnic, Type II)

  4. Relationship of PaCO2 and (Alveolar) Ventilation PaCO2 = Vco2k VA i.e. PaCO2 1 VA

  5. Relationship of PaCO2 and (Alveolar) Ventilation VT = VA + VD

  6. I could never make out what those damned dots meant. Lord Randolph Churchill (referring to decimal points).

  7. Respiratory Drive Neuromuscular Transmission Load

  8. HYPOVENTILATION  Drive  Neuromuscular Transmission  Muscle weakness/ (fatigue = reversible) + LOAD.

  9. HYPERCAPNIA  Hypoventilation - Respiratory Drive - Neuro-muscular (in)competence (- Drive) - Neuromuscular Transmission - Muscle weakness/fatigue - Abnormal load  V/Q mismatch (Multi-factorial)

  10. HYPOXIA • Reduced F1o2 • Hypoventilation • Diffusion • V/Q mismatch • R-L Shunt

  11. Alveolar-Arterial Oxygen Gradient. • Adequacy of Gas Exchange!

  12. Alveolar-Arterial Oxygen Gradient. ie, at sea level, breathing air; PAO2 = 20 - PaCO2/0.8 A-a Gradient = 20 - PaCO2/0.8 -PaO2 (Normal A-a gradient = 1-2 kPa)

  13. ALVEOLAR GAS EQUATION PaO2 = (Pb - PH2O)  FIO2 - PaCo2 + k R  20 kPa -PaCo2 0.8 (Normal A-a gradient = 1-2 kPa)

  14. DIFFUSION Depends on - gas - diffusion distance/thickness - surface area - (Hb) - capillary volume Use of CO - diffusion (and not perfusion) limited - soluble - avidly binds to Hb  zero back pressure

  15. V/Q mismatch - most important cause - 3 compartment model of Lung V/Q = 00 V/Q = 1 V/Q = 0 Dead Space “Ideal” Shunt

  16. V/Q =  V/Q = 1 V/Q = 0 Co2 - N      Cco2 - N   N  Compensatory Hyperventilation (of V/Q = 1)

  17. Worsening Disease  Unable to Undertake compensatory Hyperventilation  Pa CO2 (pH is normalised by rental (and other) mechanisms) Dangers: - High inspired O2 - Monitoring O2 saturations only

  18. Relationship of PaCO2 and (Alveolar) Ventilation Ve = Va + Vd Ve = k  Vco2 PaCo2 (1 - Vd/Vt)

  19. RIGHT TO LEFT SHUNTS - distinguish from V/Q mismatch by administration of 100% O2 - normally Pao2 rises to >600 mmHg - role of - absorption atelectasis - relaxation of hypoxic vasoconstriction

  20. Science is built up of facts, as a house is built of stones; but an accumulation of facts is no more a science than a heap of stones is a house. Henri Poincare “Science and Hypothesis”: 1905.

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