Anaesthesia in Sighthounds
by Mitchell Kornet, D.V.M.
The prospect of anesthesia can be very frightening for the owner of
a
Sighthound. Veterinarians and owners alike have been warned about the
dangers of any kind of anesthesia for Sighthounds and reports of anesthetic
deaths are common in the Sighthound community. Unfortunately some conditions
such as Cesarean sections, fractured bones, dental disease, and other
surgical conditions require anesthesia for proper treatment. Therefore
veterinarians and pet owners are frequently faced with the dilemma of
whether to treat Sighthounds surgically or to ignore certain health
problems.
Fortunately, modern anesthetic agents have given veterinarians the ability
to perform anesthesia for a variety of surgical problems with relative
safety on Sighthounds. The following discussion will address basic
anesthesiology and some of the anesthetic regimens that are suitable
for
Sighthounds.
Anesthesia is defined as "without pain." In order to perform surgical
procedures on animals safely and humanely, an anesthetic agent must
be
administered. The classes of anesthesia which are available are local,
regional, and general anesthetics.
Local anesthesia is provided by injecting lidocaine ("Novocaine") directly
into the area which has to be anesthetized. Regional anesthesia injects
the
anesthetic into a nerve or area around the spinal cord which supplies
sensation to the surgical site. Only the pain impulses from the region
of
the body being anesthetized are blocked. These forms of anesthesia leave
the
patient conscious and are very safe. Vital signs are not affected.
I recently performed a skin biopsy on a Saluki by using local anesthesia.
The area of surgery was injected with the local anesthetic, a small
incision
was made, and the wound was sutured with little trauma to the patient.
Within 30 minutes of entering my hospital, the Saluki and owner went
home
safe and sound.
The disadvantage of this type of anesthesia is that the doctor is limited
to treating minor problems such as suturing lacerations, or removing
small
skin tumors. The patient is awake and can struggle during the procedure.
This can cause the patient to become injured while being restrained.
In
addition, complications such as excessive hemorrhage can be very difficult
to control.
General anesthetics produces a loss of feeling in the entire body and
produces a state of unconsciousness. They permit veterinarians to perform
complex surgical procedures and have revolutionized the quality of care
offered to pets. However, one of the drawbacks of general anesthesia
is that
there is a risk of a serious adverse reaction to the anesthetic. These
include heart, liver, kidney and neurological problems that could ultimately
lead to death. Because of two unique anatomic features, Sighthounds
can be
more sensitive to certain anesthetics than other dogs. Some general
anesthetics are unpredictable in Sighthounds and have a prolonged duration
of effect. This increases the risk of a complicated recovery. The reason
for
increased anesthetic sensitivity is the low amount of body fat that
Sighthounds possess, and decreased liver metabolism of some anesthetics.
Sighthounds have a higher ratio of lean muscle mass to total body fat.
Body
fat is important in that some anesthetics, such as barbiturates, require
redistribution from the blood to the body fat in order for the patient
to
recover from the anesthetic. Years ago barbiturate anesthetics were
the only
"safe" anesthetics available for veterinary use. With their use, some
Sighthounds were unable to tolerate anesthetic procedures, and had serious
complications. This helped spread the myth that Sighthounds could not
tolerate any kind of general anesthetic.
Luckily, today there are many anesthetics that are safe for Sighthounds.
Veterinarians are able to perform surgery and other medical procedures
with
safety and confidence. The following discussion will focus on some of
the
anesthetics which I have used safely in my practice.
In order to anesthetize a dog, a veterinarian must either inject a liquid
anesthetic agent into a vein or muscle, or administer an anesthetic
gas. In
almost all cases, I will initiate anesthesia with a short acting injectable
agent and then place an endotracheal tube in the trachea (windpipe)
of the
dog in order to administer a gas anesthetic and oxygen. This is the
safest
way to maintain and control the depth of anesthesia. However, using
an
injectable anesthetic as the sole anesthetic agent is appropriate only
for
short procedures.
Various injectable anesthetics are available. As discussed, barbiturates,
which are injectable anesthetics are more risky in Sighthounds because
of
their body fat content and liver metabolism. In my practice, barbiturates
are never used. Instead, I have successfully used three separate injectable
anesthetic regimens and a gas anesthetic.
In order to produce a smooth transition from the awake to the anesthetic
state, it is best to give a preanesthetic medication. These drugs reduce
patient fear and stress, and allow the anesthetic drugs to take affect
in a
gradual and predictable manner. Additional benefits of some preanesthetic
drugs is the reduction of postoperative pain.
Butorphanol, an opioid analgesic, is an excellent preanesthetic for
Sighthounds. Given intravenously or intramuscularly before anesthesia,
it
will help ensure that the patient becomes anesthetized smoothly. Butorphanol
is a medication that will reduce post operative pain.
Once a patient receives its preanesthetic, the injectable anesthetic
agent
can be administered. For over ten years, I have used a combination of
ketamine and diazepam (Valium) as an injectable anesthetic. Ketamine
is a
dissociative anesthetic and valium is a tranquilizer. Used together,
they
provide rapid, smooth anesthesia, with minimum depression of cardiac
function. It will induce fewer irregular heartbeats than barbiturates,
and
can be used in patients with cardiovascular disease.[2] Because of its
cardiovascular sparing affects, this anesthetic combination can be used
in
critically ill pets.[3] It is relatively easy to place an endotracheal
tube
in the dog's trachea once they are anesthetized.
Recovery from anesthesia is dependent on liver conversion to inactive
substances and direct excretion by the kidneys. Sighthounds and other
breeds
appear to react alike to this anesthetic combination.
A disadvantage of this drug combination is that young dogs tend to be
anesthetized less deeply than older dogs when the recommended dosage
is
given. Once gas anesthesia is administered, it takes longer for the
dogs to
achieve the proper surgical anesthetic plane.
Ketamine and diazepam can induce seizures in dogs and should not be
used
in patients with epilepsy. It should not be used in patients with liver
or
kidney disease.
Another anesthetic combination used in Sighthounds is oxymorphone and
diazepam. Oxymorphone is a narcotic analgesic, and when used with diazepam,
produces a state of sedation and analgesia that permits the veterinarian
to
perform minor procedures, or place an endotracheal tube for the
administration of gas anesthetics. It should be noted that if this drug
combination is used, the preanesthetic butorphanol should not be used,
because it is in the same drug class as oxymorphone.
One advantage of using oxymorphone is the ability to reverse its affects
by administering naloxone, a narcotic reversal agent. Dogs will wake
up
within minutes of receiving the proper intravenous dosage of naloxone.
This
gives the veterinarian the ability to halt anesthesia if a dog is having
an
adverse reaction.
Although cardiovascular changes are minimal during anesthesia, respiratory
depression may occur. Dogs can stop breathing for a period of time,
and the
veterinarian must be prepared to provide respiratory support if this
happens. When using oxymorphone and diazepam, some dogs will react to
loud
noises in the operating room. It is helpful to administer this anesthetic
combination in a quiet atmosphere.
Oxymorphone and diazepam is suitable for most patients except those
with
severely compromised liver function. These patients tend recover slowly.[7]
The most recent injectable anesthetic used successfully in my practice
is
propofol. It is considered a safe anesthetic in Sighthounds.[6] Propofol
has
many characteristics that make it ideal for use in veterinary practice.
Its
use creates smooth, excitement free anesthesia, and its effects are
very
predictable.[6]
Recovery is rapid and smooth. Termination of propofol's effect has been
attributed to the redistribution of the drug to muscles and fat, and
to
metabolism in the liver. The primary difference between propofol and
barbiturates is due to propofol's rapid clearance in the liver.[1]
Sighthounds will wake up more slowly than other breeds, but propofol
is
still considered a good choice.
When using propofol, the most remarkable aspect is how quickly the patient
becomes anesthetized and how quickly the patient wakes up after the
procedure is finished. It is very easy to place an endotracheal tube,
and
propofol works well with gas anesthetics.
One disadvantage of propofol is the development of respiratory depression
when administered; dogs could stop breathing for several minutes. This
may
be avoided by injecting propofol slowly until the desired effect is
obtained. Equipment for assisted ventilation should be available when
propofol is used. Also, propofol should be used with caution in dogs
that
have cardiovascular disease.[5]
Inhalant anesthesia, also known as "gas," offers many advantages over
injectable anesthesia. These include the ability to control the depth
of
anesthesia, the ability to provide a high concentration of oxygen during
the
anesthetic period, and minimal cardiovascular and respiratory depression.[9]
Most of the inhalant anesthetic is removed from the body by exhalation
and
Sighthounds appear to utilize and remove inhalant anesthetics similar
to
other dogs.
Inhalant anesthetics can be used to induce anesthesia by placing a
facemask over the patient's nose and mouth. After anesthesia is achieved,
an
endotracheal tube is placed. In my practice, I never use an inhalant
anesthetic to induce anesthesia because of the risks of chronic exposure
to
humans. Instead, I use the injectable anesthetics described above to
induce
theanesthesia and the inhalant anesthetic delivered with an endotracheal
tube to maintain anesthesia. This method allows me to safely anesthetize
Sighthounds with minimal risk to my staff.
There are several types of inhalant anesthetics available. The inhalant
anesthetic used exclusively in my practice is isoflorane. This is considered
the safest of all of the veterinary inhalant anesthetics because the
patients reach a surgical anesthetic level faster, and tend to exhale
it out
faster, than other inhalant anesthetics. This means better anesthetic
control for the veterinarian and safer anesthesia for the patient.
Isoflorane is eliminated from the body mostly by respiration. Unlike
other
inhalant anesthetics, which can damage the liver or kidney when eliminated,
isoflorane is particularly safe for the patient. Isoflorane can reduce
cardiac function if too much is given, but it does not sensitize the
heart
to irregular rhythms.[8]
I have discussed some of the anesthetic regimens used successfully in
my
practice. Used judiciously, with careful preoperative testing and monitoring
in the operating room, anesthesia can be as safe in sighthounds as other
breeds. Anesthesia is stressful for the patient, owner, and veterinarian.
However, the anesthetics available today allow Sighthounds to be
anesthetized safely, and permit veterinarians to perform complex surgical
procedures. Hopefully this report will help make Sighthound owners more
confident and at ease when anesthesia is required. In the future, research
will develop even safer and more sophisticated anesthetic agents. Ultimately
Sighthounds will live longer and healthier lives because of the advances
in
veterinary anesthesia.
Mid Island Animal Hospital
264 Old Country Road
Hicksville, N.Y. 11801
References:
1. Zoran, D.L., et al: Pharmacokinetics of propofol in mixed-breed dogs
and Greyhounds. Am J Vet Res 54[5]:755-760; 1993
2. Hartsfield, S.M., Advantages and Guidelines for Using Ketamine for
Induction of Anesthesia. Vet. Clin. North Am. 22[2]:266-267
3. Bednarski, R.M., Anesthesia and Pain Control. Vet. Clin. North Am.
19[6]:1223-1237
4. Grandy, J.L., and Dunlop, C.I., Anesthesia of pups and kittens. JAVMA
198[7]:1244-1249
5. Smith, J.A. et al,Adverse effects of administration of propofol with
various preanesthetics regimens in dogs. JAVMA 202[7]:1111-1115
6. Ilkiw, J.E., Other Potentially Useful New Injectable Anesthetic Agents.
Vet Clin. North Am. 22[2]:281-289
7. Wilson, D.V., Advantages and Guidelines for Using Opioid Agonists
for
Induction of Anesthesia. Vet. Clin. North Am.22[2]:269-273
8. Werner, R.E., Isoflurane Anesthesia: A Guide for
Practitioners.Compendium of Continuing Education 9[6]:603-606
9. Evans, A.T., The Case for Maintenance of General Anesthesia With
an
Inhalant Agent. Vet. Clin. North Am. 22[2]:312-313
Source: http://www.salukiclub.org:80/breedinfo/healthan.htm
