Factors affecting the relationship between arterial and end-tidal carbon dioxide pressures in the anaesthetised horse

To assess the effects of the duration of anaesthesia, position of recumbency, mode of ventilation, anaesthetic drug protocol, patient age and type of surgical procedure on the usefulness of capnometry as a measure of the partial pressure of arterial carbon dioxide (P(a)co(2)) during general anaesthesia in horses.

A prospective study compared the P(a)co(2) values with those of partial pressure of end-tidal carbon dioxide (ETco(2)) in horses anaesthetised for elective or emergency surgical procedures. The difference between P(a)co(2) and ETco(2) (P(a)co(2)- ETco(2)) and the physiological dead space to tidal volume ratio (V(D)/V(T)) were calculated. The effects of the study parameters on these variables was determined.

The agreement between P(a)co(2) and ETco(2) was poor. P(a)co(2)- ETco(2) and V(D)/V(T) during the first 60 min of anaesthesia was significantly less than after 60 min of anaesthesia. Mode of ventilation, position of recumbency, anaesthetic drug protocol, patient age and type of procedure did not have a significant affect on either value.

P(a)co(2)- ETco(2) in anaesthetised horses can be large, making ETco(2) unreliable as a predictor of P(a)co(2) and for assessment of pulmonary ventilation. For anaesthesia lasting less than 60 min at least one blood gas analysis of an arterial blood sample is required to assess P(a)co(2)- ETco(2). Arterial blood gas analysis should be repeated after 60 min of general anaesthesia.