RESPIRATORY SHORT-TERM POTENTIATION (AFTER-DISCHARGE) IN ELDERLY HUMANS

During a ventilatory stimulus, respiratory short-term potentiation (STP, after-discharge) develops, so that ventilation after the stimulus is greater than that before the stimulus. When the stimulus is withdrawn STP gradually decays, tending to prevent hypoventilation and therefore stabilizing breathing pattern. STP has been demonstrated in young humans after brief hypoxic stimuli. Since respiratory arrhythmias increase with age, we examined the decay of STP in normal elderly humans (mean age 62), comparing them with young normals (mean age 27). Resting subjects were exposed to 35-50 sec hypoxia (end-tidal P(O2) = 55 Torr) followed by hyperoxia and breathing analyzed during the hyperoxic period, when the subjects were also hypocapnic. With hypoxia, ventilation increased to 152% of control in both the older and younger subjects while end-tidal CO2 fell to 92.0% of control in the older subjects and 94.7% of control in the younger. In both groups the hypoxic increase in ventilation was almost entirely due to an increase of tidal volume. During hyperoxia, ventilation and tidal volume declined over 20-25 sec to control, pre-hypoxic levels, without an apparent undershoot, and there were no consistent differences between the older and younger subjects. Prolonging the hypoxic exposure to 90 sec had no influence on STP in the older subjects. We conclude that neither age nor prolonging the hypoxic stimulus from 50 to 90 sec influenced STP.