Putative cerebral cortical involvement in the ventilatory response to inhaled CO2 in conscious man

1. The response of the diaphragm to both transcranial magnetic stimulation and electrical phrenic nerve stimulation was studied in thirteen normal subjects under conditions of either a 'reflex' drive to ventilation with inhaled CO2 or during volitional ventilation of similar magnitude. 2. The induced compound action potential in the diaphragm was recorded using an oesophageal electrode, and in some cases transdiaphragmatic pressure was recorded using oesophageal and gastric balloon catheters. 3. The response of the diaphragm to transcranial magnetic stimulation was invariably facilitated with volitional inspiration; there was either minimal or no response at functional residual capacity. 4. Facilitation with inspiration was also seen during a 'reflex' drive to ventilation with inhaled CO2 in the presumed absence of any volitional contribution to ventilation. A similar degree of facilitation was seen with voluntary ventilation of similar magnitude and pattern. 5. If the facilitation is predominantly a cortical phenomenon, then these results imply that there is a behavioural component in the previously supposed purely 'reflex' drive to ventilation with inhaled CO2. We also discuss the interpretation of these results if some of the facilitation occurs at the phrenic motoneurone.