Hemodynamic Effects of Oxygen Therapy in Patients With Acute Exacerbations of Chronic Obstructive Pulmonary Disease: Conclusion
An abnormal relationship between Do2 and Vo2 has been reported in states of tissue hypoxia. When normally functioning mechanisms, such as an increase in the oxygen extraction ratio and blood flow, compensate for changes in Do2, Vo2 is independent of supply. Only when Do2 decreases below the so-called critical value do those mechanisms become exhausted, and Vo2 becomes dependent on Do2. Although criticized by some authors who did not find this abnormal relationship when Vo2 is measured by expired gas analysis, rather than calculated by the modified Fick equation, others accept that in certain clinical conditions (such as sepsis, the adult respiratory distress syndrome, pulmonary hypertension, and chronic congestive heart failure), there is an abnormal dependency of Vo2 on Do2 for a wide range of supranormal values of Do2. Since those patients have normal Do2 values, it has been proposed that this dependency phenomenon reveals a covert tissue oxygen debt. It can be postulated that patients with decompensated COPD suffer such an occult oxygen debt that could be demonstrated by increases in Vo2 when Do2 increases.
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