The Clinical Utility of Partial Pressure of End-Tidal Carbon Dioxide as a Substitute for Partial Pressure of Arterial Carbon Dioxide in Anesthetized Birds of Prey
Service de médecine zoologique, Faculté de médecine vétérinaire, Université de Montréal, Saint-Hyacinthe, QC, Canada
We evaluated the relationship between partial pressure of end-tidal CO2 (ETCO2) and partial pressure of arterial CO2 (PaCO2) in isoflurane-anesthetized raptors. The PaCO2 were determined in serial arterial samples from isoflurane-anesthetized birds and compared to concomitant ETCO2 measured with a handheld microstream sidestream capnograph (NPB-75®, Nellcor Puritan Bennett, Pleasanton, CA, USA). Ventilation rates (VR) were modified to reach different levels of ETCO2. Forty-eight paired samples, taken from 11 birds of prey, weighing 416–2062 g, were subjected to linear regression analysis and the Bland-Altman method for assessment of clinical suitability of the two methods (i.e., PaCO2 and ETCO2 determinations). Limits of agreement between ETCO2 and PaCO2 were also calculated. A strong correlation was observed between the two measurements (r=0.94; p<0.0001). The level of agreement was more variable when the ventilatory rate was lower than the third of the bird’s theoretical respiratory rate (TRR):a1: mean difference between ETCO2 and PaCO2 was -6.43±18.64 mm Hg (mean±2SD; n=28). When birds were ventilated with a rate above 0.3 TRR, the capnograph accuracy increased and values of ETCO2 obtained overestimated the concomitantly measured values of PaCO2 by 5.54±6.10 mm Hg (mean±2SD; n=20), which is in agreement with the normal avian respiratory physiology. These results indicate that the ETCO2 measured by this microstream sidestream capnograph provides a noninvasive, sufficiently accurate estimation of PaCO2, at least in birds over 400 g receiving manual positive ventilation above the third of the TRR with a Bain system.
a1TRR = BW-0.31 x 17.2 (BW = body weight in kg)