midazolam (0.09 mg/kg bwt/h), ketamine (3.3 mg/kg bwt/h)
and xylazine (Xylapan) 7 at 0.3 mg/kg bwt/h. During
anaesthesia, i.v. lactated Ringer’s solution 8was administered
at 10 ml/kg bwt/h. Horses were breathing ambient air
supplemented with oxygen from a cylinder via insufflation
into the endotracheal tube (spontaneous breathing) or into
the inlet port on the foot pump (IPPV). For economic reasons
a flow of 6 l/min was chosen. Anaesthesia was monitored
clinically. Pulse rate assessed by palpation of the facial artery,
respiratory rate and capillary refill time of the oral mucous
membranes were recorded immediately after induction of
anaesthesia and every 5 min throughout the anaesthesia
period. Oxygen saturation was continuously measured using
a hand-held pulse oximeter with the sensor clip on the
tongue (Masimo Rad 5) 9. Respiratory flow and volumes were
measured using a pitot tube-based sensor for large animals
(Horselite) 10connected to the endotracheal tube. This sensor
was connected to a dedicated monitor that performed
respiratory gas analysis (Datex-Ohmeda Capnomac Ultima) 11.
The pressure-volume (‘compliance’) loops during IPPV shown
on the screen display were recorded with a video printer
(Mitsubishi Model P6 IE, Video Copy Processor) 12. A catheter 13
was placed in a facial artery following induction. Arterial blood
samples were drawn directly after catheter placement, 15
and 30 min after induction and at the end of anaesthesia.
They were analysed immediately with a portable blood
gas analyser (iSTAT Handheld Blood Analyzer) 14. Horses
were breathing spontaneously until blood gas analysis
demonstrated hypercapnia (arterial partial pressure of carbon
dioxide [PaCO 2] >6.65 kPa). Subsequently IPPV was provided
by rapid consecutive compressions of the foot pump aiming to
deliver a tidal volume of about 10 ml/kg bwt. The respiratory
frequency was adjusted to maintain PaCO 2below 6.65 kPa.
Descriptive statistical data are given as mean s.d.
Results
Induction of anaesthesia was smooth and uneventful in all
horses. Duration of anaesthesia was similar in all horses (44
11 min). Four horses out of 5 became hypercapnic (PaCO 2 > 6.65 kPa; 6.7 0.41 kPa) within 15 min after induction of
anaesthesia. With the foot pump, the lungs of these horses
were inflated by 2–4 quick consecutive compressions of
Expiratory valve, open
Expiratory valve, closed
Oxygen supply to foot pump
2.5 l foot pump
Fig 1: Setup for providing ventilatory support and oxygen with a modified foot pump during field anaesthesia.
© 2013 EVJ Ltd
582 EQUINE VETERINARY EDUCATION / AE / NOVEMBER 2013