ISOCAPNIC AND SMALL HYPERCAPNIC SINGLE-BREATH STIMULI - EVIDENCE FAR AN INHIBITORY INFLATION REFLEX IN CONSCIOUS MAN

We wished to find out if a deep inspiration had any influence on subse quent breathing which was mediated by neural rather than chemical stim uli. We therefore compared the effect on ventilation of a deep isocapn ic breath with that of a similar breath containing 6% CO2, and with th e effect of two successive tidal volume breaths of 6% CO2. We studied five normal subjects, each of whom repeated the three manoeuvres 20 ti mes, and we used ensemble averaging to increase the signal-to-noise ra tio. The isocapnic deep inspiration was followed by a significant inhi bition of ventilation in the group in the second post-stimulus breath, and in 4 of the 5 subjects in first and second post-stimulus breaths. This was due to an increase in both inspiratory and expiratory time, with a variable effect on tidal volume. A similar initial ventilatory inhibition was seen in the response to a deep breath of 6% CO2. When t he isocapnic response was subtracted from the hypercapnic response, th e result was similar to that observed from two tidal volume breaths of 6% CO2. We conclude that a single deep inflation of the lungs in awak e man inhibits subsequent ventilation by a neural mechanism, and that this may affect the CO2 response measured by single-breath techniques using such manoeuvres.