BREATH-HOLDING TIME AND RESPIRATORY SENSATION DURING EXERCISE IN HUMANS1633

Abstract

We have previously demonstrated that activation of the peripheral chemoreceptors (PCs) by isocapnic hypoxia increases the perception of breathing during moderate exercise to a far greater extent than expected simply from the ventilatory (˙Ve) effects (Ward & Whipp.J. Physiol (Lond) 411:27-43, 1989). The PCs are even more active during heavy exercise (ie. above the lactate threshold,θL), consequent to the metabolic (lactic) acidemia. The purpose of the present investigation was to examine whether: (a) acute changes in PC ventilatory drive, induced by controlled alterations in arterial oxygenation status, influence respiratory sensation during exercise both below and aboveθL; and (b) the duration of a volitional breath-hold (Tbh) provides a less subjective index of respiratory sensation during exercise. Utilizing inspired O2 fractions (FiO2) of 0.21 (normoxia), 0.12 (hypoxia) and 1.00 (hyperoxia), five healthy subjects exercised on a cycle ergometer, completing (a) ramp tests (15 W/min) for the determination of VO2max and work intensity, and (b) constant-load tests at 6-8 work rates (WR) spanning the range between “0” W to maximum. Tests were conducted on different days and in a randomized sequence. Respired volume was measured with a turbine transducer and gas concentrations by mass spectrometry, for on-line determination of ˙Ve and pulmonary gas exchange variables breath-by-breath. PC ventilatory drive was quantified by the transient“Dejours” O2-inhalation technique. At each work rate, subjects were asked (a) to rate the difficulty of their breathing (∂), using a standard visual analog scale; and (b) to perform a volitional breath-hold (BH) maneuver to the limit of tolerance at total lung capacity - the effects of lung shrinkage during the BH were estimated from the difference in vital capacity immediately prior to and immediately following the BH. At each level of oxygenation, ∂ increased in a curvilinear fashion with respect to WR such that the increment in ∂ associated with a given increase in WR became progressively larger the higher the WR; the ∂-˙Ve relationship was similarly curvilinear, but to a lesser degree. In contrast the curvilinear decrease of Tbh with WR was such that the decrement in Tbh became progressively smaller for a given change in WR as WR increased, despite the proportionally greater operating levels of ˙Ve. The hypoxic condition was associated with an increased PC ventilatory drive, with ∂ being higher and Tbh being shorter at any given WR; the converse was seen with hyperoxia. Oxygenation status also altered the ∂-˙Ve relationship; this was shifted upwards by hypoxia and downwards by hyperoxia. We conclude that (a) breath-hold duration is not a functional, and less subjective, correlate of respiratory sensation during exercise; and (b) the sensation of“breathlessness” during exercise is not simply a function of˙Ve, but is influenced by the prevailing level of ventilatory drive from the peripheral chemoreceptors.