Measuring central-chemoreflex sensitivity in man: rebreathing and steady-state methods compared

We compared the central-chemoreflex sensitivities estimated from steady-sta te tests with those estimated from rebreathing tests in five subjects, In o ne laboratory, each subject underwent nine dynamic end-tidal forcing experi ments, Three repetitions of 3, 6 and 9 mmHg step changes in the end-tidal p artial pressure of carbon dioxide, from a pre-step partial pressure 1.5 mmH g above resting, were used to establish four points of the steady-state ven tilatory response to carbon dioxide. In another laboratory, each subject un derwent two rebreathing experiments, one using Read's rebreathing technique and the other a modified rebreathing method which included a prior hyperve ntilation. The central-chemoreflex sensitivities, estimated from the slopes of the ventilatory responses to carbon dioxide using different combination s of the four steady-state points, were compared to those estimated from th e slopes of the rebreathing responses, The steady-state sensitivities were significantly lower than the Read rebreathing sensitivities, The ratio of m odified rebreathing sensitivities to steady-state sensitivities was closest to one when steady-state sensitivities were estimated from the two middle points of the ventilatory responses, The mean (SE) ratio of the sensitiviti es was 1.22 (0.21) in this case, We identify a number of factors that may a ffect the estimation of central-chemoreflex sensitivity using each techniqu e, These include a maximum limit of the ventilation response at high partia l pressures of carbon dioxide, an inability to sustain high ventilation for the duration of the steady-state tests and the inclusion of parts of the v entilatory response whose carbon dioxide partial pressures lie below the ce ntral-chemoreflex threshold. We conclude that the modified rebreathing meth od provides the best estimate of central-chemoreflex sensitivity of the thr ee methods. (C) 1999 Elsevier Science B.V. All rights reserved.