The expired carbon dioxide as a continuous guide of the pulmonary and circulatory systems during anesthesia and surgery. The results are encouraging in patients with healthy lungs.9 Whereas the results are controversial when the lungs are diseased.10ฤก. In addition, oxygen uptake, carbon dioxide elimination, end-tidal PCO 2, oxygen saturation, and tidal volume were determined. Recently, using Fick's Principle, attempts were made to determine cardiac output non-invasively implementing periods of CO 2 rebreathing during which CO 2 partial pressure of oxygenated mixed venous blood was obtained from the measured exponential rise of the PET value. Thus, under conditions of constant lung ventilation, PETCO 2 monitoring can be used as a monitor of pulmonary blood flow.4-8 A PETCO 2 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 PETCO 2 exceeded 34 mm Hg, pulmonary blood flow was more than 5 L/min (CI > 2.5 L).4 The improved C02 excretion is due to better perfusion of upper parts of the lung.2 Relationship between PETCO 2 and pulmonary artery blood flow was studied during separation from cardiopulmonary bypass.4 This showed that PETCO 2 is a useful index of pulmonary blood flow. Increases in cardiac output and pulmonary blood flow result in better perfusion of the alveoli and a rise in PETCO 2.1,2 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. These findings suggest that decrease in PETCO 2 quantitatively reflect the decreases in CO 2 elimination.3 Reduction in cardiac output and pulmonary blood flow result in a decrease in PETCO 2 and an increase in (a-ET)PC02.1,2 The percent decrease in PETCO 2 directly correlated with the percent decrease in cardiac output (slope= 0.33, r2=0.82 in 24 patients undergoing aortic aneurysm surgery with constant ventilation).3 Also, the percent decrease in CO 2 elimination correlated with the percent decrease in cardiac output similarly (slope=0.33, r2=0.84).3 The changes in PETCTO2 and CO 2 elimination following hemodynamic perturbation were parallel.
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