CHANGES IN PCO2 WITH ACUTE CHANGES IN CARDIAC INDEX

Purpose: A direct relationship between cardiac index (CI) and end-tida l PCO2 (PETCO(2)) shortly after decreased CI was reported, but arteria l PCO2 was not measured. Our purpose was to supply the missing informa tion on the immediate effects of alterations in CI on PaCO2, PETCO(2) and thus on Pa-PETCO(2). Methods: We measured CI, Pa and PETCO(2) and calculated the difference in 20 patients scheduled for elective heart surgery just before and immediately after the sternotomy. The measurem ents were made using standard methods: thermodilution for CI, infra-re d and blood gas analysis for PET and PaCO2 respectively. The results w ere analyzed by linear regression. Results: Very significant, direct a nd immediate changes in PET and PaCO2 with changes in CI were noted. T he ratios were 3.8 and 4.2 mmHg L(-1) respectively. The calculated val ues of r were 0.75 (P < 0.001) for PETCO(2) and 0.64 (P < 0.005) for P aCO2. The magnitude of individual change in PCO2 varied considerably s uch that the alterations in Pa-PETCO(2) were also variable, without an y correlation with the direction or magnitude of change in CI. Conclus ion: Our results explain the reported wide variations in Pa-PETCO(2) t hat accompany perturbations of cardiac output. Our observations pertai n to the unsteady state only. The results suggest that PETCO(2) cart b e used to estimate changes in CI with a reasonable degree of confidenc e.