The relation between tidal volume and inspiratory and expiratory times during steady-state carbon dioxide inhalation in man.

1. Ambiguities and discrepancies in the published descriptions of the patterns of breathing in man have been re-investigated.Steady-state hyperpnoea during rest was induced in normal subjects of both sexes by means of CO(2) inhalation, usually in high O(2), but sometimes in low (P(A, O2) approximately 200 and 55 torr respectively).2. The relations between mean tidal volume V(T) and mean times for inspiration T(I) and expiration T(E) were satisfactorily divided into lower and upper parts (ranges 1 and 2) in nineteen out of thirty-three experiments using an objective least-squares method, and polynomials were fitted separately to each of the following pairs of variables V(T,lower), T(I); V(T,lower), T(E); V(T,upper), T(I); V(T,upper), T(E).3. The breakpoints occurred when V(T) was about one third of vital capacity, but there was much variation between subjects.4. In range 2, V(T) was inversely related to both T(I) and T(E) and there appeared to be linkage between T(I) and T(E), all as described by Clark & Euler (1972). The relation suggested by Hey, Lloyd, Cunningham, Jukes & Bolton (1966) over range 2 was not confirmed.5. The main part of the relation described by Hey et al. (1966) is concerned with range 1. Here the relation between V(T) and T(I) was variable; slight, but significant negative correlation predominated. V(T) was, on the other hand, strongly and inversely related to T(E), and plots of (T(E) - T(I)) against V(T) over both ranges showed even less scatter. Hence (a) T(E) is probably related to V(T) through two mechanisms, one involving a link with T(I) and another prominent one being quite independent of T(I), (b) most changes of respiratory frequency in normal man are brought about by change of T(E) rather than of T(I), and (c) there is no region of constant frequency as implied by Clark & Euler (1972).