Healthy Hearts Beat the Odds.

Heart rate range is critical under anesthetic.

An article by Clare Illingworth, SPARK writer, University of Guelph.


An equine athlete's heart is so efficient at pumping blood through the body that it requires fewer beats. In fact, fittest athletes have heart rates as low as 22 to 24 beats per minute at rest (compared with human athletes, who average 50 to 60 beats per minute). But when this slow heart rate is coupled with inhaled anesthesia for surgery — which depresses the cardiovascu-lar system and tends to further reduce heart rate — the result can be fatal.

Now, two lines of research at the Ontario Veterinary College could lead to horses beat-ing the odds when it comes to a slowly beat-ing heart, by combining studies of cardiac function and the effectiveness of new drugs.

Prof. Wayne McDonell and graduate stu-dent Francisco Teixeira-Neto of the depart-ments of Clinical Studies, are determining the critical heart rate range in horses under anesthetic, information to be used by veteri-narians deciding whether treatment should be given to raise rates.

“All equine surgeries are associated with a greater risk while the animal is under inhalant anesthetic,” says Teixeira-Neto, “but now we are testing new treatments that may lower this risk and reduce mortality rates.”

Inhaled anesthesia used during equine surgeries may bring the heart rate below its optimum performance level, which Teixeira-Neto says ranges between 30 and 45 beats per minute under anesthetic. The research-ers have also determined — in studies using a cardiac pacemaker — that heart rates above 60 beats per minute are likely to harm cardiac performance during anesthesia.

When the heart rate falls below 30, it’s up to the anesthetist to decide whether or not to proceed with drug treatment during surgery, says Teixeira-Neto. Conventional treatments do raise heart rates, but can cause digestive complications after surgery, making gut flow stop and posing a serious threat to the animal’s health.

He is testing a new drug, methoctramine, which he says seems to be effective at improving cardiac function without post-surgical digestive complications.
“The safer alternative may improve care of the horse under general anesthesia,” he says. “This is just one more step towards reducing the risk associated with equine surgery.”

This research was sponsored by the Ontario Ministry of Agriculture and Food and the Brazilian Ministry of Education. Prof. Bill Black, Biomedical Sciences, is also involved in the study.

Putting the heart before the horse.

An innovative heart technique - one that could significantly improve the outcome of general anesthesia in the horse - has been validated at the University of Guelph. Clinical studies professors Wayne McDonell and Carolyn Kerr and graduate students Sumit Durongphongtorn and Francisco Teixiera-Neto have found that a new technology for monitoring heart function —called lithium dilution cardiac output (LiDCO)—measures the amount of blood the heart circulates per minute with repeatable and consistent results.The major advantage of this technique over others is its non-invasiveness.

“This procedure allows us to more easily and effectively assess changes in horses with severe disease before,during and after surgery,” says McDonell. The LiDCO technique involves injecting a lithium-containing compound into a horse ’s bloodstream just before the blood enters the heart.The solution mixes with blood in the heart chamber,and veterinarians can tell how much blood the heart is pumping by measuring the lithium concentration downstream from the heart.

McDonell says the LiDCO technique is an excellent way of measuring cardiac output because it uses a compound that isn ’t naturally found in the bloodstreams of most mammals.That means it ’s easy to detect and veterinarians need to inject only small amounts. Past methods of measuring cardiac output have been costly,time-consuming and riskier to animal health.But McDonell says the new technique minimizes the risks.His research team has found it ’s effective in a variety of cases with underlying medical disease.So far,they ’ve used it to compare the physiological effects of different anesthetic drugs and techniques on horses,but the method has potential applications for many aspects of equine research,including exercise studies and assessment of new surgical techniques.

“This technique is now being used in humans to monitor heart performance in intensive-care patients and people recovering from surgery,” he adds.“It ’s more practical and allows us to monitor a patient ’s health and follow the heart function beat by beat.”

This research is supported by LiDCO Group PLC (England)and the Ontario Ministry of Agriculture and Food.
– Murray Tong.