Diminished hypercapnic drive in endogenous or severe depression.

Resting breathing rate, resting end-tidal PCO2 and ventilatory response to CO2 were studied in patients with primary depression. Patients tended to breathe faster and showed decreased PECO2 levels when compared with control subjects. Carbon dioxide response was determined using a modification of the rebreathing technique of Read. Endogenous depressives, unlike reactive depressives, tended to give low CO2 responses and low CO2 thresholds. Carbon dioxide responses of patients on admission correlated highly with severity of depression. The modification of the standard rebreathing technique, and the relationship between CO2 response and physical and psychological variables are discussed.