Arterial blood PO2 and PCO2 in horses during early halothane-oxygen anaesthesia

Arterial blood was collected from 25 clinically normal horses immediately before and serially throughout the first hour of halothane oxygen anaesthesia. Blood was analysed for oxygen and carbon dioxide partial pressure (PaO2, PaCO2). Measurements of inspired oxygen concentration during anaesthesia permitted direct correlation with blood gases. Horses were divided arbitrarily into two groups based on their age: two to seven years, n = 15; over seven years, n = 10. Average (+/- sd) PaO2 and PaCO2 was 14.1 +/- 1.5 kPa (106 +/- 11 mmHg) and 5.9 +/- 0.6 kPa (44.4 +/- 4.4 mmHg) respectively in conscious, young horses and 14.0 +/- 0.7 and 5.8 +/- 0.5 kPa (105 +/- 5 and 43.3 +/- 3.8 mmHg) respectively in conscious older horses. Arterial oxygen tension decreased to 9.3 +/- 1.0 and 8.5 +/- 1.4 kPa (69.6 +/- 7.8 and 63.7 +/- 10.4 mmHg) in young and older air breathing horses respectively immediately following intravenous anaesthetic induction, recumbency and orotracheal intubation. At this time, PaCO2 was 6.5 +/- 0.5 and 6.0 +/- 0.7 kPa (48.7 +/- 3.5 and 45.1 +/- 4.9 mmHg) respectively. By 30 mins after the start of halothane in oxygen (6 litres/min) anaesthesia PaO2 increased to a maximum in both study groups. Arterial PCO2 increased steadily during anaesthesia and 60 mins after induction PaCO2 was 10.5 +/- 2.4 kPa (78.5 +/- 17.8 mmHg) in the younger horses and 9.2 +/- 1.6 kPa (68.8 +/- 11.8 mmHg) in the older horses. During inhalation anaesthesia PaO2 tended to be greater at comparable time periods in the younger horses despite a slightly greater degree of hypoventilation.