Patho-Physiology of Respiratory Failure. John KOLBE Respiratory Services Green Lane Hospital, & Faculty of Health Sciences University of Auckland.
<ul><li>Someone told me that each equation I included in the book would halve the sales. </li></ul>Stephen Hawking “ A Brief History of Time”: 1988.
RESPIRATORY FAILURE Def: When the lungs fail to adequately oxygenate the arterial blood and/or fail to prevent undue CO 2 retention. In practical Terms; PaO 2 < 8kPa (60 mmHg) (Hypoxic,TypeI) PaCo 2 > 6.6 kPa (50mmHg) (Hypercapnic, Type II)
Relationship of PaCO 2 and (Alveolar) Ventilation PaCO 2 = Vco 2 k VA i.e. PaCO 2 1 VA
Relationship of PaCO 2 and (Alveolar) Ventilation VT = VA + VD
<ul><li>I could never make out what those damned dots meant. </li></ul>Lord Randolph Churchill (referring to decimal points).
ALVEOLAR GAS EQUATION PaO 2 = (P b - PH 2 O) F I O 2 - PaCo 2 + k R 20 kPa - PaCo 2 0.8 (Normal A-a gradient = 1-2 kPa)
DIFFUSION <ul><li>Depends on </li></ul><ul><ul><li>- gas </li></ul></ul><ul><ul><li>- diffusion distance/thickness </li></ul></ul><ul><ul><li>- surface area </li></ul></ul><ul><ul><li>- (Hb) </li></ul></ul><ul><ul><li>- capillary volume </li></ul></ul><ul><li>Use of CO </li></ul><ul><li>- diffusion (and not perfusion) limited </li></ul><ul><li>- soluble </li></ul><ul><li>- avidly binds to Hb zero back pressure </li></ul>
V/Q = 00 V/Q = 1 V/Q = 0 V/Q mismatch - most important cause - 3 compartment model of Lung Dead Space “Ideal” Shunt
V/Q = V/Q = 1 V/Q = 0 Co2 - N Cco2 - N N Compensatory Hyperventilation (of V/Q = 1)
Worsening Disease Unable to Undertake compensatory Hyperventilation Pa CO 2 (pH is normalised by rental (and other) mechanisms) Dangers: - High inspired O 2 - Monitoring O 2 saturations only
Relationship of PaCO 2 and (Alveolar) Ventilation Ve = Va + Vd Ve = k Vco 2 PaCo 2 (1 - Vd/Vt)
RIGHT TO LEFT SHUNTS - distinguish from V/Q mismatch by administration of 100% O 2 - normally Pao 2 rises to >600 mmHg - role of - absorption atelectasis - relaxation of hypoxic vasoconstriction
<ul><li>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. </li></ul>Henri Poincare “ Science and Hypothesis”: 1905 .
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