Clinical Evaluation of Adequate Value of Arterial Partial Pressure of Carbon Dioxide during Inhalation Anesthesia in Horses

Abstract

An appropriate level of arterial partial pressure of carbon dioxide (PaCO₂) under isoflurane anesthesia was assessed during clinical case operation of 31 thoroughbred racing horses. Horses were assigned to either one of three PaCO₂ conditions, 40, 50 and 60 mmHg during intermittent positive pressure ventilation (IPPV) . There was no difference in arterial to end-tidal CO₂ tension (P [a-et] CO₂) between horses maintained at 40 mmHg and other PaCO₂ values. Compared with other PaCO₂ values, the ratio of physiologic dead space ventilation to minute ventilation (V_D/V_T) and the dosage of dobutamine for hypotension were higher in the 40 mmHg group. One horse at a PaCO₂ of 40 mmHg showed in junctional premature contraction. Though the minute ventilation volume of horses in the 60 mmHg group was significantly lower than other PaCO₂ values, the arterial partial pressure of oxygen (PaO₂) was maintained at more than 200mmHg. Neither cardiac arrythmias nor postanesthetic complications developed, but spontaneus breathing was observed in horses maintained at PaCO₂ of 60mmHg. Based on these findings, a clinically appropriate level of PaCO₂ may be estimated to be approximately 50 mmHg during inhalation anesthesia in horses.