Increases in cardiac output and pulmonary blood flow result in better perfusion of the alveoli and a rise in PETCO2.
Under conditions of constant lung ventilation, PETCO2 monitoring can be used as a monitor of pulmonary blood flow
Increases in cardiac output and pulmonary blood flow result in better perfusion of the alveoli and a rise in PETCO2.1,2 Consequently alveolar dead space is reduced as is (a-ET)C02 The decrease in (a-ET)PC02 is due to an increase in the alveolar C02 with a relatively unchanged arterial C02 concentration, suggesting better excretion of C02 into the lungs. The improved C02 excretion is due to better perfusion of upper parts of the lung.2 Relationship between PETCO2 and pulmonary artery blood flow was studied during separation from cardiopulmonary bypass.4 This showed that PETCO2 is a useful index of pulmonary blood flow. A PETCO2 greater than 30 mm Hg was invariably associated with a cardiac output more than 4 L/min or a cardiac index > 2 L/min.4 Furthermore, when PETCO2 exceeded 34 mm Hg, pulmonary blood flow was more than 5 L/min (CI > 2.5 L).4
Thus, under conditions of constant lung ventilation, PETCO2 monitoring can be used as a monitor of pulmonary blood flow.4-8
Recently, using Fick's Principle, attempts were made to determine cardiac output non-invasively implementing periods of CO2 rebreathing during which CO2 partial pressure of oxygenated mixed venous blood was obtained from the measured exponential rise of the PET value. In addition, oxygen uptake, carbon dioxide elimination, end-tidal PCO2, oxygen saturation, and tidal volume were determined. The results are encouraging in patients with healthy lungs.9 Whereas the results are controversial when the lungs are diseased.10
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