Transient ventilatory and heart rate responses to moderate nonabrupt pseudorandom exercise

Abstract

Dynamic responses of inspired minute ventilation, CO2 and O2 end-tidal gas fractions, and heart rate were obtained from six normal human volunteers in response to a complex dynamic exercise challenge. Subjects pedalled a chair ergometer at constant frequency. The retarding torque applied to the ergometer pedals was controlled by a low-pass-filtered pseudorandom binary sequence (fPRBS), which provided a complex, nonanticipatory exercise stimulus containing sufficient high- and low-frequency energy to excite the small signal, broadband ventilatory response. The exercise range was chosen to produce a mean level of O2 consumption at or below 50% maximum O2 consumption. Cross-covariant analysis of the fPRBS exercise with breath-by-breath ventilation provided an estimate of the dynamic (impulse) response to exercise, which contained both fast phase 1 and slow phase 2 components. The initial, phase one, hyperpnea occurred within the same breath as the exercise transition and preceded a hypocapnic response. The phase one hyperpnea represented 26% of the total ventilatory response. The secondary, phase 2, hyperpnea was delayed several breaths from the onset of phase 1. It contained slower dynamics and followed a hypercapnic response. Heart rate increased abruptly during phase 1, peaked near the phase 1-to-2 boundary, and then decreased rapidly. The experimental protocol was designed to minimize the subjective response and provide an adequate stimulus for the faster time constants. Results obtained from these experiments were consistent with a nonhumoral induced phase 1 exercise hyperpnea.