Maximal ventilation assessment in healthy calves

In order to define a reliable method for estimating maximal ventilation in cattle, 12 healthy calves underwent a rebreathing trial and injections of i ncreasing doses of lobeline, a respiratory analeptic. The effects of these tests on the main ventilatory parameters (tidal volume, V-T; respiratory fr equency, f(R) and minute volume, (V)over dot(E)) recorded during the 15 s o f maximal response were studied and compared. The sharp rise in (V)over dot (E) (4.8 times higher than the resting value) observed during the rebreathi ng trial was mainly due to an increase in V-T. This rise in ventilation was the highest ever reported in calves. Lobeline dose-dependently enhanced ve ntilation up to a threshold dose of 0.25 mg/kg, which always produced a max imal response. This maximal response (3.7 times higher than the resting val ue), reflecting both an increase in f(R) and V-T, was reproducible at an in terval of 12 h and was highly correlated with that observed during the rebr eathing trial (R = 0.98, P < 0.001). These results suggest that: (1) rebrea thing trial is a reliable method to induce and measure maximal ventilation in calves; and (2) lobeline administration (0.25 mg/kg) is a reliable means of accurately estimating this variable. Lobeline administration, unlike th e rebreathing trial, is safe and easy to standardize, and the test therefor e seems to be the preferred way of studying maximal ventilation in calves.