Assessment and monitoring during anaesthesia
THE VITAL SIGNS OF LIFE: To assess an animal’s health under anaesthesia it is necessary to have a working knowledge of its “vital signs” i.e. the signs of life. A practical knowledge of an animal’s vital signs and the physiological processes giving rise to them is essential. In simple terms, vital signs are the clinical indicators of the existence and stability of life.
Life in all cells of the body depends on a good supply of oxygen and nutrients that fuel cellular metabolism. The waste products of this metabolism must be removed from the cells’ immediate environment to avoid them being poisoned. Cellular metabolism, which is basically a series of complex chemical reactions, can only occur within a limited temperature range.
The circulatory or cardiovascular system (the heart, blood vessels and the blood flowing through them) provides the means by which oxygen and nutrients can be distributed to every cell in the body and the waste products, such as carbon dioxide (CO2) and lactic acid, removed. It is only in the extensive network of tiny blood vessels (capillaries) between the small arteries and small veins that this delivery and removal service occurs; therefore, normal function in the capillary network is vital for the life of any tissue.
Breathing (respiration in its narrowest sense) is the mechanism by which oxygen is taken into the lung and delivered to the red blood cells in the lungs’ capillaries for distribution throughout the body. Simultaneously, waste CO2 is released from the blood and voided as the animal breathes out.
An animal’s vital signs provide information about the functional state of the cardiovascular and respiratory systems, thereby revealing the condition of the crucial life support mechanisms. If any component of these systems is compromised, life itself is at risk.
The vital signs of life include: Heart rate, pulse rate and strength, quality and rate of breathing, colour of the mucous membranes (e.g. the lining of the mouth), capillary refill time (also known as CRT), and core body temperature.
INITIAL ASSESSMENT OF A TIGER AFTER GIVING AN ANAESTHETIC:
After a tiger has been injected with an anaesthetic and appears to be anaesthetised it is very important to check that it is safely immobilized and that its vital signs are good.
Checking that a tiger is safely immobilized: Approach very carefully from behind and be ready for any sudden arousal. It should always be remembered that with most induction agents, apparent immobilization occurs well in advance of true anaesthesia. Be aware that it can take 15 – 20 minutes for drugs to be fully absorbed from the site of an intramuscular injection – longer if the injection was subcutaneous or into a fat deposit. If an animal responds to your approach or other stimuli and 20 minutes have not yet passed since darting, consider waiting. (For example, if an animal responds after 10 minutes, it is tempting to give more anaesthetic drugs. This may be unnecessary and may increase the eventual depth and length of the anaesthetic. In extreme cases, a dangerously high dose of drug may be given inadvertently).
Test the tiger’s responsiveness by throwing stones or sticks from a few metres away to land lightly on the animal’s body, breaking sticks, and/or clapping your hands to create sudden noise. Finally, tap the animal with a long stick from behind starting at the tail and finishing with the ears. It is better to know whether it will react to these minor stimuli than discover it is reactive when you touch it! Consider using a forked pole or metal fork pressed lightly on the neck to increase safety whilst the initial checks are made. Prodding the animal with such poles can also be used to establish how responsive it may be to minor stimuli without getting too close. If the dart is still in the animal, check that it has completely injected its contents. Partial injections will lead to partially anaesthetised animals!
Although there are many methods to assess depth of anaesthesia, one of the most dependable methods is to assess whether the jaws can be opened, and the tongue exteriorised with little or no resistance. Check the tiger’s responses to stimuli such as pressing hard across the tail (essential in cats anaesthetised with medetomidine and ketamine), pinching between toes (try hind leg first), pinching the ear, etc.. Lightly brushing a finger across the eyelashes should elicit a slow blink of the eyelids (DO NOT touch the surface of the eye).
Once it has been established that the animal is safely immobilized, correct its position if necessary to allow ease of breathing etc, and then check its vital signs. A tiger should be placed on its side, in a position that allows it to breathe easily. The head and neck is extended to facilitate breathing. The head should be slightly higher than the thorax with the nose pointing down so that any excess saliva or fluids can flow easily out of the mouth thus avoiding aspiration. Soft rope ties or commercially available hobbles can be applied to the front and back legs at this stage for additional security. Some operators like to cover the eyes with a soft cloth at this stage.
If the tiger has been caught in an Aldrich snare remove the snare from the animal’s foot and place it on the other front foot (an extra snare should be carried in the capture pack in case the capture snare has to be cut from the animal’s foot or is structurally damaged).
NB: Chest movement during breathing should appear smooth and easy. An airway obstruction may cause a gasping, exaggerated pattern of breathing which is a sign that immediate action to clear the airway is needed. Particular attention to this should be paid when observing from a distance (during recovery for example).
Checking initial vital signs: The easiest way to remember what to check at this stage is to adopt an “ABC” approach
Airway: Open the mouth and check that nothing is obstructing the airway using a laryngoscope or torch.
Breathing: Check that the breathing is regular, paying attention to rate, depth and quality of breaths.
Circulation: Check the heart with a stethoscope, pulse rate & quality, colour of mucous membranes and CRT*
* (CRT or capillary refill time = the time taken for the colour to return to the skin or mucous membrane in an area which has been blanched by pressure. In very cold conditions, or when medetomidine + ketamine has been used to induce anaesthesia, it can be difficult to determine the CRT due to peripheral vasoconstriction.)
Finally: Take a rectal temperature
ONCE SAFE IMMOBILISATION IS ESTABLISHED AND THE VITAL SIGNS ASSESSED:
Move the tiger to a safe place, e.g., far from water in which it may drown or high features from which it may fall when awakening from anaesthesia. Do not wait until the end of the procedure to do this as arousal from anaesthesia may prevent moving the tiger at that stage. In cold conditions place the tiger on a protective (thermal) blanket and cover with a similar material to reduce heat loss. Space blankets or “Bubble Wrap” are also suitable. Place a small amount of an ophthalmic lubricant onto the surface of the eyes to prevent drying of the cornea, and protect the eyes from exposure to UV light. If a tiger is to be released immediately after anaesthesia, use only ophthalmic lubricants with an aqueous base, such as “Gel-Tears” or “Hypromellose Eye Drops”. Oily preparations leave a film on the surface of the eye which can impair vision for several hours. Do not allow the sun to shine directly into the eyes at any stage during anaesthesia – this may cause serious damage to the retina. Although covering the eyes with a cloth will prevent this and may also reduce stress and prevent dirt and debris getting onto the eyes, it will also make accurate monitoring of an anaesthetic slightly more difficult. (Increased eye movement is one of the indicators of an anaesthetic wearing off). A cloth placed under the head will help keep dirt and leaf litter from the lower eye and mouth.
Monitoring a tiger during anaesthesia: After a tiger has been safely immobilized and the initial assessment of vital signs has been made, the anaesthetist’s job has only just begun! It is necessary to constantly assess the well being of an animal and the depth of anaesthesia throughout the entire period of anaesthesia as problems are much easier to correct if detected early. The most likely problems to arise and their correction are discussed in Anaesthetic emergencies.
Every 5 minutes during an anaesthetic the following parameters should be assessed and recorded (see example of Anaesthetic record sheet):
- Breathing – number of breaths per minute, depth of breathing, any abnormal sounds. Note any changes in the breathing rate and depth. As an animal arouses from anaesthesia, it will likely breathe more rapidly and more deeply.
- Pulse rate – number of pulses per minute. Generally this will be the same as the heart rate, and although it is worth checking that this is the case, it is more significant to detect a pulse than a heartbeat. If a pulse cannot be detected, immediately check the heart with a stethoscope. The strength of the pulse should also be noted. A convenient and reliable place to feel a pulse in any animal is in the groin (femoral artery), although many other arteries are accessible in most individuals. Note changes in the pulse rate over time. Slowing of the pulse rate may indicate deterioration in cardiac or respiratory function and conversely, acceleration of the pulse rate may indicate that an animal is waking up.
- Mucous membrane colour. The colour of the inside of the mouth in a healthy animal is pink. Note any change over time. Pale or even white mucous membranes may indicate that the circulation in the capillaries is inadequate and a blue colour (“cyanosed”) that respiration is ineffective.
- Capillary refill time (CRT). This should be between 1.5 and 3 seconds. Significantly longer CRT’s may indicate inadequate cardiac function. (In very cold conditions or when medetomidine + ketamine has been used to induce anaesthesia the CRT may be very difficult to determine due to peripheral vasoconstriction).
- Rectal temperature. Most anaesthetic drugs interfere with an animal’s ability to regulate its body temperature. When working in cold conditions a dangerous loss of heat can occur (hypothermia) which can be fatal if left uncorrected. Conversely when working in hot conditions overheating can occur (hyperthermia) which is equally dangerous to the animal. Hyperthermia can also result from excessive pre-anaesthetic excitement or seizures at any stage. The normal rectal temperature of tigers is between 37.8 degrees C and 39.4 degrees C. Ambient temperature should also be recorded. Rectal temperature can be taken with a conventional thermometer but it is far easier to use an electronic thermometer that constantly monitors both rectal and ambient temperatures simultaneously. Ensure that the thermometer probe is actually in contact with the wall of the rectum. Core body temperature is generally a little higher than that measured from the rectum.
- Responses to simple stimuli. Monitor response to pinching between toes, jaw muscle tone, ear pinching, brushing eyelashes, etc. Increasing responsiveness usually indicates that an anaesthetic is wearing off.
- General muscle tone. This can be assessed by flexing and extending a leg. Any increase in resistance may indicate that an anaesthetic is wearing off.
- Position of the eye. The position of the eyes can provide useful indicators of the depth of anaesthesia. During deep anaesthesia the eyes tend to roll downwards in the sockets. This changes to a more normal position when anaesthesia is lightening or wearing off.
- Spontaneous eye and leg movements. Movement of the eyes and/or legs may indicate that an anaesthetic is wearing off. While checking the eyes for movement, note whether more lubricant is required to prevent drying of the surface (cornea) and apply if necessary.
- Blood oxygen levels. Generally, the use of sophisticated electronic anaesthetic monitoring equipment is inappropriate in field situations. However, battery-operated pulse oximeters which provide information on respiratory function and pulse rates are relatively cheap and simple to use. Pulse oximeters measure the percent oxygen saturation of haemoglobin in the blood (SpO2). The sensor clip should be applied to areas of the body that are not heavily pigmented. The tongue usually provides a good signal, although ears, toes, nipples, nasal septum, scrotum, and tail, can all be used. Rectal (reflectance) sensors are also widely available. The sensor probe should be relocated every 10 minutes to obtain consistently reliable SpO2 readings. Local movement of the sensor will prevent a signal being detected, as will bright sunlight shining directly onto the sensor. SpO2 values of above 95% are desirable, but values of 90-95% still indicate adequate haemoglobin oxygenation. When potent alpha-2 agonists such as medetomidine have been included in the anaesthetic induction values may be significantly lower. The trend in SpO2 values in these cases is generally more informative than absolute values. Steadily decreasing SpO2 values indicate a serious respiratory problem and the protocol to investigate and reverse respiratory failure should be triggered.
- When breathing air (i.e. not supplemented with oxygen), if SpO2 is > 90% it can be assumed that respiration is adequate to blow off C02 effectively. Prolonged high C02 levels can cause acidosis, headache, cardiac dysrhythmias and a poor recovery.
Pulse oximeter sensor placing:
Signs that a tiger may be waking up:
For safety reasons it is very important to be able to recognise the signs of a tiger waking up or recovering from an anaesthetic. Although somewhat inconsistent, these include:
- Increased eye movement and greater sensitivity to lightly brushing eyelashes. (more vigorous blinking).
- Increased rate and depth of breathing.
- Increased heart rate (except in the case of cats given medetomidine + ketamine when a decreased heart rate may be an indicator of wakening).
- Increased reactions to stimuli such as pinching the ears, tail, between toes etc.
- Increased muscle tone – legs and jaw.
- Spontaneous movements of the tongue, lips, ears, head, paws, legs and tail.
- Reacting to sounds.