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

Since dyspnoeic sensation (delta) increases progressively with work rate (W R) and the duration of a volitional breath-hold (t(BH)) shortens, we wished to explore whether t(BH) might correlate sufficiently closely with delta t o 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. Ventil atory and gas exchange variables were measured breath-by-breath. At each WR , breath-holds to the limit of tolerance were taken; delta was recorded (vi sual-analog scale) immediately prior to and throughout each breath-hold. Du ring breath-holds, delta increased with time as a "break-away" monoexponent ial characteristic, reaching the maximum (100%) at the breakpoint. Despite end-tidal partial pressure of carbon dioxide at the break-point being highe r and end-tidal partial pressure of oxygen being lower with increasing WR, the relationship between WR and t(BH) declined curvilinearly (i.e. with lar ge falls in tBH occurring in the low WR range, but far smaller reductions a t higher WRs). The t(BH)/minute ventilation relationship had a similar form . The relationship between pre-breath-hold delta and t(BH) was also complex : the large reductions in t(BH) in the low WR range were associated with on ly modest increases in pre-BH delta while, at higher WRs, the progressively smaller decrements in t(BH) were associated with progressively larger incr eases in delta. We therefore conclude that breath-hold duration is unlikely to provide a useful correlate of exertional dyspnoea during dynamic exerci se. Furthermore, the relative prolongation Of t(BH) at high WRs (accounting for the more-extreme levels of end-tidal gas tensions) may reflect the att ention-diverting influence of the exercise per se.