WARREN ISAKOW, MD
Respiratory failure is a common reason for intensive care unit admisson and is the final pathway for a number of disease of differing pathophysiology. an mechanism-based approach enables the clinician to identify the most likely cause for the respiratory failure and to treat appropriately. In general, patients with respiratory failure may be classified into groups, depending on the component of the respiratory system that is involved. Hypercapnic respiratory failure is a consequence of ventilatory failure and is recognized by an elevated PaCO2 above normal (>45mmHg at sea level). This denotes failure of the respiratory pump an can occur with normal lungs. Hypoxemic respiratory failure is a consequence of gas exchange failure and is recognized by hypoxemia ( PaO2 < 60mmHg ) with or without widening of the alveolararterial O2 gradient.
HYPERCAPNIC RESPIRATORY FAILURE
The hallmark of hypercapnic respiratory failure is an elevated PaCO2 above 45mmHg.
PaCO2 = K x [VcO2/(1-Vd/Vt)x VA] where PaCO2 = the partial pressure of carbon dioxide in the blood, K = constant, VcO2 = carbon dioxide production, Vd/Vt = dead-space ratio of each tidal volume breath, VA = minute ventilation. Analysis of the previous equation shows that hypercapnia can occur from three processes.: (a) an increase in CO2 production, (b) a decrease in minute ventilation, and (c) an increase in dead-space ventilation. Understanding of the "respiratory pump" enables the clinician to systematically consider the cause of hypercapnic respiratory failure in different patients, as depicted in Algorithm 7.1.
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