ASK THE VET

Anesthesia and the Horse:
A Unique Problem

by DR. GREG FERRARO

Most horsemen, when faced with decisions regarding surgery for their horse, focus all their attention on the surgeon and the injury to be repaired. They immediately inquire as to what surgeon is available, the reputation of that individual and how much experience the person has had with this particular surgical exercise. They often ask about the relative merits of the proposed method of repair, the success rate of that procedure and the time required for recuperation and rehabilitation.

Horse owners rarely ask the equally important questions relating to the identity of the anesthesiologist, the relative merits of the agents to be employed and the possibility for anesthetic complications. Surgeons may be able to mechanically correct specific injuries, but the life of the patient and oftentimes, their well-being following surgical intervention is in the hands of another individual. In today’s horse world, where surgery is commonplace and elective procedures are carried out with high rates of success; we tend to focus on the local injury and its prospects for a functional repair. All of us have developed a "take it for granted" attitude toward the well being of the patient in total. We forget to address those issues which are paramount to the successful outcome of any surgery, particularly in surgeries involving the severely ill patient.

The problem for the average horse owner, is that they rarely have any idea of what is involved regarding anesthesia in the horse, let alone enough knowledge to ask the proper questions of an anesthesiologist that they have probably just met. Additionally, anesthesiologists who deal with issues of physiological dynamics are being asked to make explanations of medical abstractions that frequently require the use of unfamiliar scientific terms. It is no wonder then, that the poor horsemen becomes frustrated and confused. Most often, people feel that their best choice is to trust the surgeon’s judgement in the selection of an anesthesiologist and hope that good luck and fate will take care of the rest. While this usually results in satisfactory results, it does nothing towards educating the horseman in this important area of equine medicine.

It is in the context of this background that some of the basic anesthetic issues of concern to any horse owner when considering surgery for his animal will be discussed. The reason why the horse poses such a difficult anesthetic problem for the veterinarian will be explained as well as some of the physiological characteristics which make equine anesthesia unique. While no attempt will be made to provide a broad range of knowledge regarding anesthesia in the horse, the more important factors involved in a successful outcome will be highlighted.

There are two major factors which delineate the difference between anesthesia in the horse and that in most other mammals. One is the considerable size and weight of the animal and the other is the horse’s unique response to the opiate family of drugs. Size and weight dictates the basis for all fundamental anesthetic methodology and the horse’s aversion to narcotic type drugs eliminates those techniques most commonly used for sedation and pain management in most other animals, including man.

The issue of size and weight in the management of equine anesthesia is profoundly important because of its effect upon the balance of air intake and blood flow within the lung and because of its restrictive effects upon circulation of blood to other organs. When a horse or any other animal breathes, they draw air into the lung where it is exchanged with CO2 in the blood stream. The flow of gas and blood is unrestricted and evenly matched throughout the lung because there is no impedance upon the movement of the chest wall and diaphragm, or to the flow of blood through the lung tissue.

When you change the horse from a standing to a recumbent position for the purposes of surgery, profound changes in the dynamics of blood and oxygen flow occur. If a horse is placed on its side during surgery, the weight of the horse compresses the downside lung which severely restricts the amount of oxygen that can enter, while at the same time gravity is exerting its effects on the circulatory system such that most of the blood flow will go to that dependent lung. On the other hand, the reverse is true for the topside lung; it receives plenty of air during respiration, but the circulation is insufficient to adequately oxygenate the blood.

Things are not much different when the horse is placed on its back. The dependent portion of the lung still receives all the blood flow, while the top half of the lung fields get the air. Additionally, the entire weight of the intestinal tract (which is considerable) is now pressing against the horses diaphragm, restricting its movement. Combine this with the fact that cardiac function and circulation is also compromised in this upside down position, and you can readily see why the maintenance of a stable anesthetic state is difficult at best.

In addition to worrying about proper oxygen exchange within the lung, one also has to be cognizant of the fact that all the musculature, nerves and other soft tissue structures that are on the downside of the horse and in contact with the table surface are going to have compromised circulation and compression induced damage due to the weight of the animal upon them. These effects progress in severity as the length of the surgery increases, such that serious muscle damage can occur after exceptionally long surgical procedures.

How then, does the equine veterinarian overcome these obstacles to perform successful anesthesia? Why are so many surgeries completed in such a seemingly routine manner if anesthesia in the horse is so problematical? The answer can be traced back to a pair of discoveries made in the 1960’s. The first was the development of gas anesthetic agents that were suitable for use in the horse. These agents made it possible to anesthetize a horse for an extended period of time with minimal depressant effects to the cardiopulmonary system. The second and more significant discovery was made at the U. C. Davis School of Veterinary Medicine by Dr. Murray Fowler. He and his colleagues developed a closed gas anesthetic delivery system whereby oxygen could be delivered to the horse’s lung under positive pressure. This system actually forces air into the lung as the horse inhales and expands the lung literally from the inside out. The ventilation system does the work of breathing for the horse and thereby insures that sufficient oxygen is delivered to the animal’s blood stream.

This breakthrough was made possible by the steady advancement of equine surgical techniques and procedures that have occurred over the last 30 years. Without this basic system of ventilation, routine surgery of the horse would not be as easily completed and surgical procedures involving critically ill patients (as in colic) would be undertaken at much greater risk. This anesthesia/ventilation system is not perfect, however. While it allows the veterinarian to overcome a major obstacle in anesthetic management, it does not mimic the natural physiological function of the horse’s heart and lungs and it does very little to alleviate the problem of compromised circulation to the downside muscle mass. Consequently, surgeries that require extended periods of anesthesia, those involving severely injured and painful animals compromised by a state of shock, or those dealing with horses that are critically ill, are still a major problem for the equine anesthesiologist.

It is within this group of patients, that the horse’s second unique anesthetic problem appears. In man and most other animals, the use of opiate type drugs (narcotics) is a major tool in the anesthetic management of the severely injured or critically ill patient. These drugs have the ability to alleviate severe pain and create a state of sedated euphoria within the patient with minimal interference to the normal function of the body’s physiological systems. Their use in surgery allows the anesthesiologist to use much lower doses of the anesthetic agent which will more severely compromise these essential body functions. Therefore, in the critically ill patient, the use of opiates are often a key factor for their survival. The ability of these drugs to provide pain relief is also a significant factor in the patient’s ability to move ahead smoothly in the post surgical recovery phase.

In the horse these opiate type drugs are very unreliable. They do not consistently produce euphoric sedation and therefore, cannot be used with confidence in the equine patient. The veterinarian, dealing with the critically ill horse is deprived of a major tool considered essential by anesthesiologist of man and other animals. The equine medical field has yet to discover a drug that can replace the opiates in this functional role. Consequently, anesthetic management in the severely physiologically compromised horse is dangerous and difficult. As a result of this deficiency, horses are suffering more than they should and many are being lost that might otherwise survive.

Scientists at U.C. Davis have been working diligently for several years to solve these problems. The guru for equine anesthesia here at our veterinary school and to many practicing veterinarians throughout the country is Dr. Gene Steffey. He and his team of scientists are recognized worldwide for leadership in this field. Much of the foundation for anesthetic technique and knowledge in the use of anesthetic related drugs was developed by this group.

Over the last 20 years, Dr. Steffey’s efforts, have resulted in the expansion of the group of drugs available for equine anesthesia, from a handful to a broad array of agents. He has provided the veterinary profession with information on the use of drugs for pre-anesthetic sedation, those useful for both intravenous and gaseous anesthesia and others which assist in providing a smoother and safer post-anesthetic recovery period. Dr. Steffey has taken us from the age of "one anesthetic agent does all" to the age of specialization, where the drugs used are tailored to the medical needs of each individual patient. Consequently, the more seriously ill or injured horse has been experiencing an ever increasing chance for survival.

Currently, our equine anesthetic research program is focused on funding a substitute for the opiate class of drugs. This is of paramount importance because anesthetic management of the critically ill and severely compromised equine patient cannot move forward until this problem is solved. An easy answer will not be forthcoming. Only a sustained and concentrated effort will result in the necessary solution. While research in the area of equine anesthesia is not particularly well known or talked about amongst the general population of horsemen, it is vital to their best interests. Further advancements in equine surgery will be delayed considerably until the necessary improvements in anesthetic management are made. Horsemen must realize that the quiet person at the head of the surgery table truly holds the key to their horse’s future.