CARDIOPULMONARY EFFECTS OF HYPERCAPNIA DURING CONTROLLED INTERMITTENTPOSITIVE PRESSURE VENTILATION IN THE HORSE

The cardiopulmonary effects of eucapnia (arterial CO2 tension [PaCO2] 40.4 +/- 2.9 mm Hg, mean +/- SD), mild hypercapnia (PaCO2, 59.1 +/- 3. 5 mm Hg), moderate hypercapnia (PaCO2, 82.6 +/- 4.9 mm Hg), and severe hypercapnia (PaCO2, 110.3 +/- 12.2 mm Hg) were studied in 8 horses du ring isoflurane anesthesia with volume controlled intermittent positiv e pressure ventilation (IPPV) and neuromuscular blockade. The sequence of changes in PaCO2 was randomized. Mild hypercapnia produced bradyca rdia resulting in a significant (P <0.05) decrease in cardiac index (C I) and oxygen delivery (DO2), while hemoglobin concentration (Hb), the hematocrit (Hct), systolic blood pressure (SBP), mean blood pressure (MBP), systemic vascular resistance (SVR), and venous admixture (Q(S)/ Q(T)) increased significantly. Moderate hypercapnia resulted in a sign ificant rise in CI, stroke index (SI), SEP, MBP, mean pulmonary artery pressure (PAP), Hct, Hb, arterial oxygen content (CaO2), mixed yenous oxygen content (C (v) over bar O-2), and DO2, with heart rate (HR) st aying below eucapnic levels. Severe hypercapnia resulted in a marked r ise in HR, CI, SI, SEP, PAR, Hct, Hb, CaO2, C (v) over bar O-2, and DO 2. Systemic vascular resistance was significantly decreased, while MBP levels were not different from those during moderate hypercapnia. No cardiac arrhythmias were recorded with any of the ranges of PaCO2. Nor epinephrine levels increased progressively with each increase in PaCO2 , whereas plasma cortisol levels remained unchanged. It was concluded that hypercapnia in isoflurane-anesthetized horses elicits a biphasic cardiopulmonary response, with mild hypercapnia producing a fall in CI and DO2 despite an increase in MBP, while moderate and severe hyperca pnia produce an augmentation of the cardiopulmonary performance and DO 2.