A QUASI STEADY-STATE RAMP METHOD FOR THE ESTIMATION OF THE VENTILATORY RESPONSE TO CO2

There are two suitable methods for estimating the ventilatory response to CO2: the steady state or the Read method. The latter is usually applied because of its shorter duration and its stimulus, near to tissue PCO2. From recent studies on a physiological model but also on the dynamics of the ventilatory CO2 response (G liters min-1 kPa-1), it can be shown, both theoretically and experimentally, that the Read method markedly overestimates the steady-state estimate of the ventilatory CO2 response (Gss). We have, therefore, applied two ramp approaches: one with an initial end-tidal PCO2 (PetCO2) step of 0.5 kPa and a mean PetCO2 ramp slope of 0.41 kPa/min (step-ramp method, SR) and one without an initial step and a mean ramp slope of 0.21 kPa/min (ramp method; R). As predicted by theory, the ventilatory CO2 response from the R method (GR) during the second 4-min interval of the 8-min procedure and the ventilatory CO2 response from the SR method (GSR) from the whole 6-min procedure, except for the first 30 s, should yield a good approximation of Gss. In a group of 12 normal volunteers we indeed found no differences between these estimates, and, as predicted by theory, we found both estimates to be larger than GR from the first 4-min interval. From the interindividual spread, only the difference with respect to GSR was significant (p < 0.02). The volunteers preferred the R approach because of the lower PetCO2 attained and the smaller ventilation increase. Our results are, from the theoretical point of view, in accordance with recent direct comparisons of the steady state and rebreathing approaches, using different ramp slopes and initial PetCO2 steps. The rebreathing approaches, however, lasted only 3-4 min with a markedly steeper PetCO2 rise. We conclude that the R approach we used is a possible substitute for the more cumbersome steady state approaches applied until now.