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Monitoring the Anesthetized Animal
Veterinary & Aquatic Services Department, Drs. Foster & Smith
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Various techniques are used to monitor the animal's vital signs during anesthesia and through recovery. A skilled technician is able to use his or her senses of touch, hearing, and sight to monitor an animal. Electronic devices are also used.

Heart rate and rhythm

The heart rate and rhythm may be monitored by directly feeling the chest wall, listening with a regular stethoscope or an esophageal stethoscope, or by using an electrocardiogram (EKG, ECG). An esophageal stethoscope is an inexpensive tool that consists of a tube placed through the mouth and down the esophagus to the level of the heart. This special tube can be attached to a device which amplifies the sound of the heart, which is then transmitted through the speaker on the device. An EKG measures the electric currents generated by the heart. It is used to monitor the heart rate, rhythm, and changes in the nerve impulses in the heart. Continuous monitoring with an EKG allows early recognition of electrical changes associated with disorders of conduction in the heart and arrhythmias that may need to be treated.

Blood flow

The passage of blood through the vessels of an organ is called tissue perfusion. Several methods may be used to monitor tissue perfusion. The pulse may be palpated in the animal (just like you take your own pulse at your wrist or neck), and with practice, be characterized as strong or weak. Using the strength of the pulse along with monitoring the color of the gums and internal organs during surgery will help tell the anesthetist how well the blood is getting to the tissues.

The capillary refill time (CRT) measures the rate of return of color to a mucous membrane such as the gums after the application of gentle pressure. It is the same as if you take a finger and press it against another and release: the skin goes pale for a second and then the normal color returns as the blood returns. With good tissue perfusion, the CRT is typically about 1 second.

Taking the animal's blood pressure during surgery will also help determine tissue perfusion as well as depth of anesthesia. If the pet is in a light surgical plane (under light anesthesia), he may have an increase in blood pressure when the surgeon starts the surgery or manipulates the tissues. The surgeon also monitors the blood loss that is occurring during the procedure, since that also impacts tissue perfusion.


normal pink color of a dog's gumsPulse oximetry is a way to estimate how much oxygen is being carried by the blood. In humans, a sensory clip is placed on a person's finger and by photoelectronic means, the oxygenation of the blood can be determined. In animals, the sensory probe needs to be placed on the pet's pink, nonpigmented tongue. Examining the color of the mucus membranes also helps assess oxygenation. The membranes should be pink.


The respiratory system can be monitored by watching the chest wall and counting the number of breaths taken per minute and by watching the movement of the reservoir bag on the gas anesthetic machine. The breathing should be smooth and regular.


taking the temperature of a dogThe body temperature falls during anesthesia due to the effect of the anesthesia and procedures associated with the surgery. Prior to surgery, the hair is shaved and the skin washed with antiseptic and alcohol solutions; this results in heat loss from the animal. Internal organs may be exposed to the room air during surgery further decreasing the animal's temperature. The animal cannot replace the lost heat through shivering or muscle movement due to the anesthesia. The temperature should be monitored throughout surgery and continued until it is back to normal after surgery.

Several methods are available to help decrease heat loss. They include using warm IV (intravenous) fluids, placing the animal on pads containing circulating warm water, using hot water bottles, and placing padding between the animal and the metal surgery table. As the anesthesia wears off, the animal can start to shiver. Shivering during recovery is normal as the body regulates its temperature.

Depth of anesthesia

The vital signs can be a clue as to how deep the animal is anesthetized. The anesthetist will also monitor other parts of the animal's body during the surgery. The amount of muscle relaxation, different reflexes, and pupil position will all help determine the depth of anesthesia.


As you can see, anesthesia monitoring is not a simple process. It requires training and experience. Correct monitoring will help make sure that any complications that occur are detected and treated early so the outcome is a healthy pet and happy owner.

References and Further Reading

McKelvey, D; Hollingshead, W.K. Small Animal Anesthesia Canine and Feline Practice. Mosby. St. Louis, MO; 1994.

Noninvasive Monitoring Techniques in Anesthetized Animals. Veterinary Medicine. April 1996. 

Stages of Anesthesia in Animals 
Types of Anesthesia, Sedatives & Tranquilizers in Animals 
How Anesthetic Gases Work in Animals 
Preanesthetic Planning & Screening Tests in Animals 
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