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When a gas mixture containing CO2 is breathed by a healthy subject, alveolar ventilation is increased. The ratio delta ventilation/delta PCO2 has been termed the ventilatory response to CO2 (S). Since the development of a rebreathing method in 1967, it has proved easy to measure this index of the regulation of ventilation. This index, although showing considerable interindividual variation (range, 0.5-9 liters/min/mm PCO2), tends to be consistent over time for any given individual. Positive correlations between S and personality traits such as anxiety, extroversion, and aggression and negative correlations between S and certain scales of the Minnesota Multiphasic Personality Inventory (MMPI: depression, psychopathology, psychasthenia, and social introversion) have been found in previous studies. It was hypothesized that S may be a psychobiologic marker of the respiratory system. A project in which 82 normal subjects (50 male, 32 female) were studied with both psychologic (Eysenck Personality Inventory, Catell 16PF, and 168-item MMPI) and rebreathing tests was undertaken to investigate this possibility further. A strong correlation between S and neurotic personality traits was found, but this correlation appeared valid only in female subjects.

The prominence of respiratory symptoms in patients with neurotic psychiatric disorders is noted and the literature on the control of respiration is reviewed to attempt to explain this finding. A previous study demonstrated a positive correlation between the ventilatory response to CO2 (S) and neurotic personality traits in a group of normal subjects. This study attempted to follow up this finding and hypothesized that a group of neurotically disturbed patients would have a higher S value and a group of individuals who practiced a calming technique such as transcendental meditation (TM) would have a lower S value than normal subjects. The second hypothesis was confirmed, but not the first, in that the neurotically disturbed patients had the lowest mean values for S of the three groups, rather than the highest. Particular characteristics of the sample of psychiatric patients cast doubt, however, on the validity of this finding. Three additional findings of this study were that anxious, depressive, and hyperventilating subject groups were no different from one another in terms of S values; that very experienced TM practitioners (sidhas) could significantly lower their ventilatory response to CO2 in the meditating state as compared to the nonmeditating alert state; and that the S value did not increase in two male subjects with endogenous depression after successful treatment with electroconvulsive therapy.