Monitoring the Anesthetized Patient
World Small Animal Veterinary Association World Congress Proceedings, 2003
Sandra Forsyth, BVSc, DACVA
Institute of Veterinary, Animal and Biomedical Sciences, Massey University
New Zealand

Patients undergoing even the most minor procedure require monitoring during the anesthetic period. This doesn't have to be a big deal and most of us do this without even thinking about it i.e., we check respiratory rate and depth, heart rate, pulse strength, mucous membrane color, CRT and jaw tone as second nature. However, when we have a compromised patient or a long procedure this monitoring is insufficient to tell us how well our patient is doing under anesthesia. We need some tools to help.

Frequency of Monitoring

Ideally the animal should be assessed at 5 minute intervals. This is because irreversible cardiovascular changes can occur within 10 minutes in the anesthetized patient and it seems sensible to try to avoid these. In young, healthy animals such as those undergoing desexing surgery the cardiovascular and pulmonary reserves are large and they show few detrimental changes in response to the effects of depressant anesthetic agents and we can be more relaxed about monitoring. The reserves in ill animals are low because of disease and so complications are common and close monitoring is essential to avoid problems. In practice you will probably find that you don't often get asked to monitor healthy animals but only the sick ones so it may be easy to forget the normal responses under anesthesia.

Monitoring to ensure Freedom from Pain

Generally, you can assume that if an animal is not moving then it not experiencing pain and is unlikely to be aware during surgery. This is the case during inhalational anaesthesia, because muscle tone becomes too great to allow surgery at a plane where analgesia is lost. Inhalational agents do not prevent spinal changes that occur in response to painful stimuli. In order to prevent these from occurring it is necessary to provide opioids, local anesthesia or N2O as part of the anesthetic protocol. With injectable agents analgesia is lost at about the same plane as loss of muscle tone so there is a greater chance that analgesia will be inadequate when using injectable agents alone for surgery.

Appropriate analgesia is important because painful procedures that don't include analgesia can result in profound long term pain post operatively.

Monitoring to Maintain Organ Function

Heart rate Normal heart rate: 70-140 (dog), 110-140 (cat). Maintaining a normal heart rate is important because of its effect on cardiac output and arterial blood pressure


Bradycardia may be seen in several circumstances during anesthesia such as excessive depth, intubation, drugs (opioids, α2-agonists), ocular, facial, abdominal surgery, urinary bladder distension, hypothermia, hypoxia and after IV atropine administration. Bradycardia doesn't always require treatment but if blood pressure falls, mucous membranes are pale, grey or cyanotic or if CRT is slow anticholinergic such as atropine are needed. Hypothermia consistently prevents a rise in heart rate with atropine administration.


Tachycardia during anesthesia may be due to a light plane of anesthesia, hemorrhage, hypotension, hypoxia, hyperthyroidism, hyperthermia or drugs (e.g., ketamine, anticholinergics). There is no generic therapy for correcting tachycardia. The underlying cause must be discovered and corrected.


An ECG represents electrical activity of the heart but gives no indication of mechanical activity and ECG activity can remain normal long after contractile ability of the heart is severely compromised i.e., don't rely on the ECG alone to tell you how well the patient is doing under anesthesia. For ECG monitoring during anesthesia the leads do not need to be accurately placed provided that the trace is easy to read. It is important to know what the normal pattern for a particular animal looks like so that any change occurring throughout the anesthetic period is noticed. Good contact is necessary to get a tracing that is devoid of artifacts. Hair clipping is not necessary but ultrasound jelly, alcohol or methylated spirits are needed between animal and leads. Arrhythmias are fairly common during anesthesia. Those most frequently seen aside from bradycardia and tachycardia are premature atrial contractions (PACs) and premature ventricular contractions (PVCs).

PACs and PVCs

These arrhythmias are occasionally seen when halothane anesthesia is light. This is especially so in cats and is due to endogenous catecholamine release. Hypoxia, hypercapnia, exogenous catecholamine administration (e.g., epinephrine/adrenaline), hyperkalaemia (ruptured bladder), drugs (α2 agonists, thiopental), cardiac disease, non cardiac disease (GDV, haemangiosarcoma), endocardial or epicardial stimulation (catheters, pacemaker wires), severe hypothermia (<30oC) and intracranial disorders can cause PACs and/or PVCs to develop. PACs are common and they can generally be ignored unless they affect tissue perfusion (mucous membrane color, CRT) or blood pressure. It is uncommon to have to treat them. PVCs are uncommon unless underlying disease is present, the patient is a cat under light halothane anaesthesia or an ECG is read immediately after thiopental administration. If you see them look for a reason why they might be occurring. Check the plane of anesthesia to ensure that it not too light or too deep, make sure that the animal is not hypoxic or hypercapnic. Medication (lidocaine) may be required to manage them.

Peripheral Perfusion

Blood flow to tissue can be assessed by checking mucous membrane color and CRT. Bright pink-red mucous membranes indicate vasodilation (may be innocuous but can result in hypotension). Pale pink to white mucous membranes may indicate anemia (check PCV before surgery), vasoconstriction due to increased sympathetic tone, traction on abdominal viscera, hypovolemia, excitement and pain.

Arterial Blood Pressure

Pulse pressure (systolic-diastolic pressure) can be estimated by palpating the pulse in the femoral, dorsal pedal, ulnar brachial or lingual artery. Pulse pressure gives a less accurate view of tissue perfusion compared to blood pressure but requires no special tools. Blood pressure is the "best" tool for monitoring cardiovascular status of the patient during anesthesia. A fall in blood pressure gives the earliest indication that there is a problem with cardiovascular status, early enough that the situation can be corrected. Relying on changes in heart rate, heart rhythm or respiratory parameters are usually too late to allow remedy of the underlying problem. Blood pressure can be measured by indirect methods such as Doppler or by direct means such as arterial catheter.

Indirect methods for detecting blood pressure

There are minimal complications associated with Doppler BP monitoring. It is easy to carry out, doesn't cause the patient any trauma and it provides an estimate of HR also. However it is less accurate if BP is low (shock or hypothermia) and if the patient is small. Interference occurs at times such as during use of a cautery unit.

Normal blood pressure is about 100-160 mmHg systolic. A systolic blood pressure less than about 90 mmHg is "hypotensive" and vital organ perfusion may be compromised.

Pulse Oximeters

Pulse oximeters provide a continuous assessment of haemoglobin saturation (SpO2) and of pulse rate. They can be attached to the tongue, lip, toe web, toe and tail (depends on model) provided that the site is clipped. Reduced or absent pulsatile flow may result from poor cardiovascular function, poor peripheral perfusion, hypothermia and site compression. (probe with a tight grip). There needs to be a good vascular bed between the probe. Tissue that is too thick will not transmit light and tissue that is too thin doesn't allow a probe to calibrate. There is a high incidence of signal detection failure in pigmented skin. Ambient light and motion can cause false readings.

End tidal CO2

End tidal CO2 (EtCO2) is a non-invasive assessment of ventilation that measures CO2 concentration in end tidal gas. End tidal gas is the last part of the breath on exhalation it is closely reflects alveolar gas and capillary blood. Capnography is the graphic representation of the data and forms a flat top sine wave. The wave form changes depending upon ventilatory pattern, fresh gas flow rate etc.

With all the toys that we can place on patients to aid in monitoring it is easy to forget that we still need to put our hands on the patient to fully assess how they are doing under anesthesia.

Depth of Anesthesia

Assessed by checking muscle tone, generally using jaw tone, heart rate and respiratory response to response to surgical stimulation, eye position and responses. The depth of anesthesia depends on the procedure being undertaken and which anesthetic agents have been given.

Muscle tone decreases with increasing depth of anaesthesia. Inhalational agents generally produce good muscle relaxation whereas the opioids and ketamine do not. Surgical stimulation should cause an increase in sympathetic tone (increased HR, RR, BP) and an increase in parasympathetic tone (decreased HR and BP) is an indication of excessive anesthetic depth.

At light anesthesia the eye is central moving to a ventromedial position as anaesthesia deepens and then back to a central position with deep anesthesia. Light and deep anaesthesia are differentiated by palpebral response and pupillary size.

Minor procedures require light plane of anaesthesia: tight jaw tone, central eye with palpebral, low end-tidal halothane/isoflurane concentration whereas major procedures require deeper plane of anesthesia, moderate jaw tone, ventromedial eye and higher end-tidal gas concentration.


Hypothermia is a common consequence of anesthesia because metabolic activity is decreased, drugs induce vasodilation, the patient is clipped and scrubbed with water based solutions on cold tables in cool rooms.

Hypothermia lowers anesthetic requirements, produces bradycardia and if severe induces cardiac arrhythmias. In recovery shivering may result in hypoxia. The most accurate place for measuring core body temperature in the dog and cat is the esophagus. Care must be taken that over zealous warming of the patient doesn't result in burns.


Don't assume that all will be well the moment that the animal is disconnected from the anesthetic machine. Patients should be monitored continuously until they have fully recovered from anesthesia and are ambulatory. Removal of oxygen may result in hypoxemia, ventilation is no longer support so hypercapnia can occur. Hypothermia readily results if an animal is not actively warmed with heating pads. Vomiting and aspiration can occur.


To ensure that a patient recovers from anesthesia as close to "normal" as possible many parameters ranging from heart rate to temperature should be monitored throughout the entire anesthetic and into the recovery period.

Speaker Information
(click the speaker's name to view other papers and abstracts submitted by this speaker)

Sandra Forsyth, BVSc, DACVA
Institute of Veterinary, Animal and Biomedical Sciences
Massey University
New Zealand

MAIN : Technician Program : Monitoring Anesthesia
Powered By VIN