The challenges of general anaesthesia in horses are described in a just-published review.
Horses are considered to be one of the most challenging domestic species to anaesthetize, Elżbieta Stefanik and her colleagues wrote in the journal Animals.
General anaesthesia can cause significant changes to lung function, blood circulation and gas exchange because of the compression of the abdominal organs on the lungs when horses are positioned on their back or side.
Chest wall movement is restricted and it leaves horses particularly prone to cardiopulmonary disorders while under general anaesthesia.
Consequently, post-operative complications and anaesthetic mortality rates are higher for horses than for other commonly anaesthetized companion animals.
Anaesthesia-related complications are responsible for a horse mortality rate of almost 1% – nearly 10 times higher than for dogs and cats.
A crucial contributing factor is a long duration of surgery, which increases the risk of low blood pressure, low oxygen levels in the blood, and acid-base disturbances such as respiratory acidosis.
“There is no ideal method for monitoring respiratory gas concentrations during general anaesthesia,” they said, “so it is important to know the advantages and limitations of individual methods and the factors that affect monitoring devices.”
The authors set out in their review to summarize previously published studies regarding the causes and effects of gas exchange disturbances during general anaesthesia, as well as monitoring methods.
The authors said gas exchange disturbances are considered an important factor contributing to the high anaesthetic mortality rate and many post-anaesthetic side effects.
“Current monitoring methods, such as pulse oximetry, capnography, arterial blood gas measurements and spirometry, may not be sufficient by themselves,” they said, “and only in combination with each other can they provide extensive information about the condition of the patient.”
The review team described near-infrared spectroscopy as a new, promising, complementary method for monitoring horses, being able to measure oxygenation of the crucial blood supply to the brain.
The authors traversed available evidence on the causes and effects of intraoperative gas exchange disturbances, as well as monitoring methods.
“Understanding the changes that occur during general anaesthesia and the factors that affect them, as well as improving gas monitoring techniques, can improve the post-aesthetic survival rate and minimize post-operative complications.”
Significant changes in pulmonary function, haemodynamics and gas exchange seem to be the cause of many post-operative complications and presumably contribute to high post-operative mortality in horses, they said.
The use of specialized monitoring equipment provides valuable information about the condition of the anaesthetized horse.
“Improving the methods of anaesthesiologic supervision, proper interpretation of the results, understanding the limitations of the equipment and understanding the factors that may affect the reading of monitoring devices will contribute to the safety of anaesthetized horses,” they said.
The review team comprised Stefanik, Olga Drewnowska, and Bernard Turek, all with the Warsaw University of Life Sciences in Poland; and Barbara Lisowska, with the National Geriatrics, Rheumatology and Rehabilitation Institute, also in Warsaw.
Stefanik, E.; Drewnowska, O.; Lisowska, B.; Turek, B. Causes, Effects and Methods of Monitoring Gas Exchange Disturbances during Equine General Anaesthesia. Animals 2021, 11, 2049. https://doi.org/10.3390/ani11072049