Dreaming like Sleeping Beauty What’s the safest equine anesthetic available to veterinarians? BRIAN FRAY by Clare Illingworth H orses, like humans, are at risk from anesthetic-related complications during surgery.
Historically, about one per cent of equine surgeries around the world end in mortality.
A University of Guelph researcher is making progress in trying to further reduce that number, by determining the safest anesthetic available to veterinarians.
Wayne McDonell, Clinical Studies, looked at recovery rates in horses undergoing arthroscopy procedures (a technique that uses a video camera and long instruments to visualize joint spaces with minimal invasiveness) and found a certain anesthetic that appears to work best in elective surgeries. “The type of anesthetic used can have great impact on the success of a surgery,” says McDonell.“There have been few studies that objectively compare inhaled anesthesia drugs and regimens during and after actual surgical procedures.” Two inhalant anesthetics are popular with equine veterinarians.
Halothane, the most common, is preferred because it’s effective and relatively inexpensive.
Isoflurane is more frequently used in small animal surgeries and with critically ill horses, but it’s 14 RESEARCH Spring 2003 becoming a more popular choice for elective surgeries because its cost is declining and many veterinarians say they’ve noticed fewer side effects in certain situations.
The problem, says McDonell, is that the benefits of both drugs are poorly researched in horses.
Both anesthetic types are inhaled gases that lower nerve impulses (the way the brain communicates with the body), rendering the horse unconscious and relaxed.
The exact molecular process isn’t fully understood, but it is known that although both drugs require recovery time, horses receiving isoflurane tend to waken too soon, before the drug has been fully expelled. “The sedative prevents the drowsy horse from standing prematurely and injuring itself.” – Wayne McDonell “For this reason, veterinarians commonly use a sedative to allow the drug extra time to be expelled,” says McDonell.“The sedative prevents the drowsy horse from standing prematurely and injuring itself.” This seems to work.
Researchers studied eight horses undergoing two similar arthroscopic surgeries three months apart.
The horses received halothane for one of the surgeries and isoflurane for the other, then were evaluated on reactions such as cardiovascular function, recovery characteristics, 48-hour post-recovery differences, gut mobility and overall well-being.
Preliminary results show isoflurane involves fewer complications in recovery when coupled with a sedative. “In the past, isoflurane was thought to be the poorer anesthetic choice because the recovery wasn’t handled properly,” says McDonell.“Now we know how to handle the post-operative care, and we’re having great success with recovery after post-arthroscopic surgeries.” Others involved in this research include Profs.
Carolyn Kerr and Melissa Sinclair and graduate student Sumit Durongphongtorn of the Department of Clinical Studies.
This research is sponsored by the Ontario Ministry of Agriculture and Food and the Ontario Horse Racing Industry Association.
R Equine Guelph THE OPERATING THEATRE Healthy hearts beat the odds Heart rate range is critical under anesthetic Putting the heart before the horse AN INNOVATIVE HEART-MONITORING TECH-nique — 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 RESEARCH Spring 2003 15 OLIVIA BROWN by Clare Illingworth A n equine athlete’s heart is so efficient at pumping blood through the body that it requires fewer beats.
In fact, the 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 cardiovascular 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 beating the odds when it comes to a slowly beating heart, by combining studies of cardiac function and the effectiveness of new drugs.
Wayne McDonell and graduate student Francisco Teixeira-Neto of the departments of Clinical Studies, are determining the critical heart rate range in horses under anesthetic, information to be used by veterinarians 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.” Graduate student Francisco Teixeira-Neto, Department of Biomedical Sciences, is at the University of Guelph on a scholarship from CAPES, the Ministry of Education of Brazil, and a national scholarship from his home university there.
Inhaled anesthesia used during equine surgeries may bring the heart rate below its optimum performance level, which TeixeiraNeto says ranges between 30 and 45 beats per minute under anesthetic.
The researchers 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 postsurgical 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.
Bill Black, Biomedical Sciences, is also involved in the study.
R 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. Equine Guelph THE OPERATING THEATRE When nature’s calling at the wrong time New minimally invasive surgery will repair ruptured bladders in foals Stories by Clare Illingworth L ike all babies, foals are often born with a full bladder.
During delivery, a strong contraction by the mare can burst the baby’s bladder, putting newborns in grave danger and requiring immediate surgery.
University of Guelph surgeons are exploring the use of a new technique to repair the problem while boosting a foal’s chance of survival.
Ludovic Bouré of the Department of Clinical Studies is determining if laparoscopy, a minimally invasive technique, will improve recovery success compared with the currently used laparotomy method (see sidebar on page 18). “Current surgical treatment may be considered invasive and can cause complications,” says Bouré. “We would like to determine if the less invasive laparoscopy avoids many of the complications now experienced.” Symptoms of a ruptured bladder can go unnoticed for several days.
The split most often occurs on the top and down the middle, making it difficult to access with con-
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