Is breath-hold time an objective index of exertional dyspnoea in humans?

Abstract.

Since dyspnoeic sensation (δ) increases progressively with work rate (WR) and the duration of a volitional breath-hold (t _BH) shortens, we wished to explore whether t _BH might correlate sufficiently closely with δ to provide a quantitative and descriptor-free index of respiratory sensation during dynamic exercise. Nine healthy males exercised on a cycle ergometer at a series of constant WRs, above and below the lactate threshold. Ventilatory and gas exchange variables were measured breath-by-breath. At each WR, breath-holds to the limit of tolerance were taken; δ was recorded (visual-analog scale) immediately prior to and throughout each breath-hold. During breath-holds, δ increased with time as a "break-away" monoexponential characteristic, reaching the maximum (100%) at the break-point. Despite end-tidal partial pressure of carbon dioxide at the break-point being higher and end-tidal partial pressure of oxygen being lower with increasing WR, the relationship between WR and t _BH declined curvilinearly (i.e. with large falls in t _BH occurring in the low WR range, but far smaller reductions at higher WRs). The t _BH/minute ventilation relationship had a similar form. The relationship between pre-breath-hold δ and t _BH was also complex: the large reductions in t _BH in the low WR range were associated with only modest increases in pre-BH δ while, at higher WRs, the progressively smaller decrements in t _BH were associated with progressively larger increases in δ. We therefore conclude that breath-hold duration is unlikely to provide a useful correlate of exertional dyspnoea during dynamic exercise. Furthermore, the relative prolongation of t _BH at high WRs (accounting for the more-extreme levels of end-tidal gas tensions) may reflect the attention-diverting influence of the exercise per se.