CARDIAC-OUTPUT DURING EXERCISE MEASURED BY ACETYLENE REBREATHING, THERMODILUTION, AND FICK TECHNIQUES

In dogs during exercise, respiratory rate can reach 200 breaths/min, b lood temperature can exceed 42 degrees C, and hematocrit can approach 60%. To determine whether these changes significantly affect the measu rement of cardiac output by the acetylene rebreathing method (Q(CRB)), we compared estimates of Q(CRB) With those measured by thermodilution and Fick (Q(CFI)) techniques in nine dogs at rest and during steady-s tate exercise on a treadmill up to near-maximal workloads. Solubility of acetylene in blood was corrected to the simultaneously measured blo od temperature and hematocrit. Results were also adjusted for mixing e fficiency. Up to a Q(CFI) Of 20 l/min, Q(CRB) was not significantly di fferent from Q(CFI) (P > 0.05) However, cardiac output measured by the rmodilution was consistently higher than those measured by the other t echniques (P < 0.0001). We conclude that the overall agreement between Q(CRB) and Q(CFI) estimates supports the validity of the rebreathing technique under exercise conditions where body temperature and hematoc rit are changing rapidly and the breathing pattern is unrestrained. Sy stematic error by the thermodilution technique may be related to a var iety of methodological issues as well as possible dissipation of cooli ng into the myocardial tissue and subsequent incomplete washout.