RELATIONSHIP BETWEEN INSPIRATORY PRESSURE AND TIDAL VOLUME IN THE ANESTHETIZED CANINE

Hounds undergoing prolonged or complicated surgical procedures are oft en underventilated, as indicated by blood gas and end-tidal CO2 (CO2) values when using published ventilatory guidelines, We investigated th e relationship between body weight, tidal volume, and inspiratory pres sure delivered by the ventilator (lung inflation pressure) in 59 anest hetized hounds (19 to 33 kg), Animals were ventilated under positive p ressure control and noninvasively instrumented to monitor blood pressu re, EGG, oxygen saturation, CO2, and tidal volume, Weight, sex, and th orax measurements were recorded, All dogs were monitored at lung infla tion pressures of 10, 14, and 18 cm H2O, with measurements recorded on ce CO2 stabilized, Veterinary guidelines recommend tidal volumes of 10 to 15 ml/kg of body weight and lung inflation pressures of 15 to 25 c m H2O, When inflation pressure was below guidelines (10), tidal volume was ''normal'' (10 to 15 ml/kg), but the animals were underventilated , When inflation pressure was ''normal'' (14 or 18 cm H2O), tidal volu me was above guidelines, Physiologic variables were normal only when i nflation pressure was 14 cm H2O. Weight and thorax depth accounted for 32 and 6%, respectively, of tidal volume variability, and tidal volum e varied by +/- 250 ml at any given body weight and inflation pressure , None of the measured physical variables accurately predicted tidal v olume, These data suggest that the inconsistency in tidal volume is du e to a previously undescribed variability in respiratory compliance in the anesthetized hound and that the guidelines for ventilation during surgery need further investigation.