The effects of halothane and isoflurane on cardiovascular function in laterally recumbent horses

Experimental studies in adult horses have shown that general anaesthesia maintained with isoflurane is associated with less depression of cardiovascular function compared with halothane anaesthesia. Adverse effects of intermittent positive-pressure ventilation (IPPV) have also been demonstrated. Nevertheless, the haemodynamic effects of these agents and the effects of differing modes of ventilation have not been assessed during clinical anaesthesia in horses undergoing surgery.

The haemodynamic effects of isoflurane or halothane anaesthesia during spontaneous or IPPV were studied non-invasively in 32 laterally recumbent horses undergoing elective surgery. Indices of cardiac function and measurements of femoral arterial blood flow and resistance were recorded using transoesophageal and transcutaneous Doppler echocardiography, respectively. Arterial pressure was measured directly using a facial artery catheter.

Cardiac index (CI) was significantly higher during isoflurane anaesthesia than during halothane anaesthesia and was also higher during spontaneous ventilation with isoflurane. CI decreased significantly over time and an inverse relationship was observed between CI and mean arterial pressure (MAP). Horses with higher MAP had a significantly lower CI. During isoflurane anaesthesia, femoral arterial blood flow was significantly higher in both pelvic limbs compared with halothane anaesthesia, and flow in the lower limb was significantly higher during spontaneous ventilation than during IPPV. No significant change in femoral blood flow was observed over time.

The effects of anaesthetics and mode of ventilation on cardiovascular function recorded under surgical conditions in horses are similar to those reported under experimental conditions. However, in contrast with previous experimental studies, CI progressively decreased over time regardless of agent used or mode of ventilation employed.