Objective To determine, in mildly hypercapnic
horses under isoflurane–medetomidine balanced
anaesthesia, whether there is a difference in cardiovascular
function between spontaneous ventilation
(SV) and intermittent positive pressure ventilation
Study design Prospective randomized clinical study.
Animals Sixty horses, undergoing elective surgical
procedures under general anaesthesia: ASA classification
I or II.
Methods Horses were sedated with medetomidine
and anaesthesia was induced with ketamine and
diazepam. Anaesthesia was maintained with isoflurane
and a constant rate infusion of medetomidine.
Horses were assigned to either SV or IPPV
for the duration of anaesthesia. Horses in group
IPPV were maintained mildly hypercapnic (arterial
partial pressure of carbon dioxide (PaCO2) 50–
60 mmHg, 6.7–8 kPa). Mean arterial blood pressure
(MAP) was maintained above 70 mmHg by
an infusion of dobutamine administered to effect.
Heart rate (HR), respiratory rate (fR), arterial blood
pressure and inspiratory and expiratory gases were
monitored continuously. A bolus of ketamine was
administered when horses showed nystagmus.
Cardiac output was measured using lithium
dilution. Arterial blood-gas analysis was performed
regularly. Recovery time was noted and recovery
Results There were no differences between groups
concerning age, weight, body position during
anaesthesia and anaesthetic duration. Respiratory
rate was significantly higher in group IPPV. Significantly
more horses in group IPPV received supplemental
ketamine. There were no other significant
differences between groups. All horses recovered
from anaesthesia without complications.
Conclusions There was no difference in cardiovascular
function in horses undergoing elective surgery
during isoflurane–medetomidine anaesthesia with
SV in comparison with IPPV, provided the horses
are maintained slightly hypercapnic.
Clinical relevance In horses with health status ASA
I and II, cardiovascular function under general
anaesthesia is equal with or without IPPV if the
PaCO2 is maintained at 50–60 mmHg.