Ak. Khanna et al., CARDIOPULMONARY EFFECTS OF HYPERCAPNIA DURING CONTROLLED INTERMITTENTPOSITIVE PRESSURE VENTILATION IN THE HORSE, Canadian journal of veterinary research, 59(3), 1995, pp. 213-221
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.