The effect of progressive exercise on the equilibrium rebreathing cardiac output method

The equilibrium CO2 rebreathing method has been used in the study of exercise cardiac output (Q) in health and disease, but the requirement for a steady state has usually limited its application to step function exercise. This limits testing to only a few levels of exertion in a laboratory session. We have devised a treadmill test where the grade is flat at the beginning and rises continuously at 1%.min-1 after stable measurements are obtained during level walking. The walking speed is determined by the subject's comfort and tolerance for fast walking. In series I seven normal subjects were studied with this protocol, and an automated system was employed for estimation of Q and oxygen uptake (VO2). For comparison of steady state and progressive tests, an interpolation method was devised which furnished a value of Q (progressive) that could exist at the value of VO2 obtained during the steady state. The average difference in Q so estimated between the steady state and the unsteady state was 0.3 l.min-1, SD = 1.7. This difference was less than the difference between averages of duplicate values of Q obtained during the steady state (1.11.min-1, SD = 16), and the difference between the two differences was not significant. Based on 51 determinations of Q and VO2 in eight normal subjects the unsteady state procedure furnished an average value of the relation slope of delta Q/delta VO2(-1) of 4.8 l.l-1. We conclude that the equilibrium CO2 method can be used to depict the Q, VO2 relationship over the range of VO2 at which rebreathing is tolerable.