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N. S. (1975). Thorax, 30,[337][338][339][340][341][342][343] Evaluation of breath holding in hypercapnia as a simple clinical test of respiratory chemosensitivity. Breath holding was used as the basis of a simple test of respiratory chemosensitivity. Breath holding was begun at selected degrees of hypercapnia produced by CO2 rebreathing. In 16 healthy control subjects there was a linear regression of the log of breath-holding time on the Pco2 at the start of breath holding. Breath-holding time (BHT) and the slope of a log BHT/Pco2 plot were closely correlated with the ventilatory response to CO2. In five cases of the idiopathic hypoventilation syndrome, CO2 retention and reduced ventilatory response to CO2 were accompanied by prolonged breath-holding time and the regression of log BHT on Pco0 was abnormally flat. However, in 17 patients with chronic airways obstruction, breath-holding time was never prolonged and the log BHT/Pco2 relationship was normal, even though 13 had a diminished ventilatory response to CO2 and four had chronic CO2 retention. It is concluded that the BHT/Pco2 relationship provides a useful index of respiratory chemosensitivity which is not influenced by airways obstruction. This may be helpful in the detection of impaired chemosensitivity as a cause of CO2 retention even when the ventilation CO2 response is reduced non-specifically by coexisting airways obstruction.Chronic CO2 retention is most frequently due to chronic airways obstruction which hinders the translation of respiratory motor activity into ventilation. Another important cause of chronic hypercapnia is impairment of respiratory chemosensitivity as in the idiopathic hypoventilation syndrome. The recognition of impaired chemosensitivity in cases of this syndrome is sometimes made difficult by the coexistence of chronic airways obstruction (McNicol and Pride, 1965;Rhoads and Brody, 1969). Respiratory chemosensitivity is usually measured by the increment in ventilation elicited by a change in the alveolar or arterial CO2 tension produced by breathing a C02-enriched gas mixture. However, the ventilatory response to CO2 is altered if ventilation is obstructed even when respiratory chemosensitivity is normal (Cherniack and Snidal, 1956). This has stimulated a search for other tests of respiratory 'Present address:

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