Stages of Anaesthesia

Four stages and planes of anaesthesia have been classified. The use of intravenous anaesthetic agents plunge the animal through the initial stages so rapidly that they may not be recognised.

To achieve surgical anaesthesia we must maintain the animal at Stage 3 between Planes 2 and 3. The deeper Plane 3 will be needed for painful procedures that require good muscle relaxation—e.g., orthopaedic and ophthalmic operations. If the patient is being maintained at Stage 3, Plane 3 careful monitoring and I/V fluid support, if the procedure is prolonged will be required.

Monitoring of Anaesthesia

Early detection of adverse events and evaluation of corrective intervention’s the aim of the game. Various pieces of equipment are of use but reliance should not be placed on just one item. For example, the apnoea alert. By the time this alarm sounds to signal that the animal is no longer breathing, irreversible damage has occurred to the heart and brain. You are too late!

Remember these important rules:

• Monitor the patient not the equipment
• Check the patient before checking the equipment
• A machine should not replace a skilled anesthetist

A machine is primarily directed at the body systems that are essential for the maintenance of life—the central nervous system and the cardiovascular system. Thus we need to monitor the following:

• Central nervous system—reflexes, depth of anaesthesia
• Cardiovascular system—heart rate, perfusion
• Pulmonary system & airway
• Oxygenation
• Temperature—this is difficult to maintain and subnormal temperatures will slow metabolic rate and lengthen recovery and post-anaesthetic complications

Note: Never overinterpret one piece of information. The aim of anaesthetic monitoring is to obtain data from a number of different body systems that will allow an overall picture of the patient to emerge. The most important monitor is the operator’s senses—sight, hearing, touch etc.

Monitoring of Central Nervous System Reflexes

Basic reflexes and muscle tone are the method of monitoring whether the brain still functions. Under anaesthesia most of the central nervous system undergoes a selective, dose-dependent, reversible depression caused by the anaesthetic drugs at their site of action. Basic reflexes are:

• Spontaneous motor activity
• Jaw tone
• Palpebral reflex
• Corneal reflex
• Lacrimation
• Pupil position
• Respiratory effort (intercostal/diaphragmatic)

Summary of Central Nervous Monitoring

Monitoring the Cardiovascular System

The aim of the cardiovascular system is the maintenance of adequate tissue perfusion with oxygenated blood. The aim of intraoperative monitoring of the cardiovascular system is to ensure that it is achieving this and intervene if it is not.

Mucous Membrane Colour

The colour of the mucous membranes (gums, conjunctiva, vagina, etc.) provides information on the adequacy of oxygenation of blood in the lungs, adequacy of blood volume, cardiac output and the delivery of oxygenated blood to the tissues.

• Pink—good perfusion and oxygenation.
• Pale—poor perfusion. This may be the result of (a) poor perfusion from low cardiac output—cardiac problem, low venous return; (b) poor perfusion from vasoconstriction—pain; (c) low blood volume—haemorrhage.
• White—severe blood loss, hypovolaemia etc.—immediate action required.
• Cyanotic—slight blue/grey tinge to mucosa—immediate action required.
• Muddy—this indicates both poor oxygenation and poor perfusion. Beware, this animal is in trouble and the oxygen concentration in the blood is below normal!

Capillary Refill Time

Place firm digital pressure on a mucous membrane. This will push all blood from the capillary bed. The speed at which it returns is relative to blood pressure and blood volume. A normal rate is less than 2 seconds.

If it is longer than 2 seconds and the colour is muddy, then the animal is shunting blood away from the periphery of the body because of a drop in blood pressure—alert the veterinarian immediately.

Heart Rate

Although mechanical monitoring is often implemented by the use of a pulse oximeter or similar, it is always best to listen to the heart rate. This can be achieved by using a stethoscope (as long as it does not interfere with the surgical site) or by use of an oesophageal stethoscope.

External Auscultation

Place the stethoscope on the left-hand side where the elbow lies against the chest. Reposition slightly to give best hearing of heart sounds. The apex of the heart points downwards and lies near the sternum at the level of rib 7.

Listen to the heart sounds. The normal sound is described as being a “lub, dup” sound. The sound that we hear is caused by the snapping closed of certain heart valves as the chambers contract and start to eject blood.

The heart should have a regular beat, if you are concerned that the beat is irregular ask the veterinarian to listen.

Listen to the rhythm. Normally the heartbeats will have the same interval between them or has a sinus arrhythmia. This means there is a regular irregularity. The heart speeds up slightly as the animal inhales and slows down slightly as it exhales.

If there is an abnormal arrhythmia there will be irregular gaps between heat beats. This irregularity can be heard.

To calculate the heart rate, count the beats for 15 seconds then multiply by four to gain heart per minute.

Oesophageal Stethoscope

The oesophageal stethoscope is passed down the oesophagus to the level of the heart. It enables constant monitoring of the heart beat either via the ear pieces or can be amended to attach to small speakers.

Heart Rate and Anaesthetic Depth

General indicators:

• Increased heart rate—tachycardiac—decreasing depth of anaesthesia—becoming lighter
• Decreased heart rate—bradycardia—increasing depth of anaesthesia—becoming too deep

Arterial Blood Pressure

Arterial blood pressure is useful in providing continuous information on the performance of the cardiovascular system and indirectly on the depth of the anaesthetic. However, it is important to remember that good blood pressure during anaesthesia does not always correlate with good perfusion. That is, increased blood pressure > vasoconstriction > decreased blood flow.

Direct measurement of blood pressure involves catheterisation of a peripheral artery—dorsal metatarsal, femoral, lingual and carotid. Indirect measurement is commonly done using a Doppler ultrasound unit to detect arterial blood flow and a pressure cuff.

Using the Doppler

• Cuff should be placed above hock for dorsal metatarsal or above carpus for dorsal pedal artery.
• Follow the cuff manufacturer’s instruction on choosing a size that is appropriate for the limb. In general, the width of the cuff should measure 30–50% the circumference of the limb.
• Ensure snug fit.
• Measurement should be performed several times to ensure correct.
• Clip area over artery to be used (dorsal metatarsal, dorsal pedal).
• Apply liberal amount of gel to area probe is to be placed.
• Position probe perpendicular to the artery.
• Tape in place.
• When pulse audible—increase pressure until no longer audible.
• Slowly decrease pressure until pulse just becomes audible once more. This is the systolic blood pressure reading.
• This should be repeated at least 3 times to ensure correct reading.

Pressure readings are expressed in mm Hg (millimetres of mercury) e.g., 120 mm Hg.

The pressure (or tension) of the blood within the arteries is maintained by the contraction of the left ventricle, the resistance of arterioles & capillaries, the elasticity of arterial walls and viscosity and volume of blood. Changes to any one of these factors will affect the blood pressure.

Systolic blood pressure results from systolic contraction of the cardiac chamber. The highest arterial blood pressure reached during any given ventricular cycle.

Diastolic blood pressure results from diastolic relaxation of the cardiac chamber. The lowest arterial blood pressure reached during any given ventricular cycle.

Blood pressure ranges can vary depending on referring text.

A mean arterial blood pressure of greater than 60 mm of mercury (mm Hg) is required to ensure adequate perfusion of vital organs such as the brain and kidney. To be cautious, it is best that the anaesthetised patient maintain a mean arterial blood pressure of greater than 70 mm Hg. Because the Doppler ultrasound measures only systolic pressure and because the systolic pressure is about 20 to 30 mm Hg higher than mean pressure, Doppler readings of greater than 90 mm Hg are desirable.

In the dog it is time for action if the Doppler readings fall below 80 mm Hg. In the cat this time for action occurs at 65 mm Hg (there is an overestimation of mean arterial pressure in cats with this device).

Monitoring of the Respiratory System

It is not just good enough to have blood being delivered at an adequate rate and pressure to the body, there must be oxygen in that blood for any good to be done.

Pulse Oximetry

Use a pulse oximeter to monitor that the patient is oxygenating sufficiently. The pulse oximeter measures the pulse rate and the degree of oxygen saturation of arterial blood, by measuring the absorbency of particular wavelengths of red and infrared light.

Most instruments require a probe placed on non-pigmented skin or mucous membrane (most frequently the tongue). They are susceptible to movement, and interference to blood flow by the clamp resulting in erroneous measurements. Also, saturation measurements may be inaccurate in cases of hypovolaemia.

Pulse oximeter readings of SpO₂:

• 98–99% Normal
• <95% Hypoxemia
• <90% Severe hypoxemia
• <75% Lethal hypoxemia

Areas where a probe may be placed include:

• Tongue
• Lip
• Ear
• Between toes
• Prepuce/vulva

If placing the probe on the tongue, place a damp/wet swab over the tongue to keep it moist, then place probe on over this.

Respiration

The rate, rhythm and nature of breathing effort should be consciously observed. See previous notes in ‘Summary of Central Nervous Monitoring’ table.

Changes in the respiratory rate and effort may indicate a change in areas such as anaesthetic depth

• Increased respiratory rate—becoming lighter
• Decreased respiratory rate—becoming deeper
• Increased or ‘jerky’ respiration may indicate that the patient is feeling pain stimuli

It is also important to monitor both the rebreathing bag and the patient’s thorax to detect:

• Anaesthetic circuit disconnection
• Inconsistency between breathing excursions of the chest and the amount of air moving in and out of the bag
• Airway obstruction problems (kinked endotracheal tube)

End Tidal CO₂ Monitor

Capnometry is the measurement of carbon dioxide in exhaled gas of a patient. The basic physiology behind capnometry is that tissues generate carbon dioxide that is delivered to the lungs, via the blood and then exhaled. The exhaled carbon dioxide is sampled from the distal end of the endotracheal tube, analysed by a capnometer and displayed on a capnograph.

Capnographs generally work on the principle that CO₂ absorbs infra-red radiation; a beam of infra-red light is passed across the gas sample to fall onto the sensor, the presence of CO₂ leads to a reduction in the amount of light falling onto the sensor which in turn changes the voltage in the circuit and the reading is portrayed.

Phases of a Capnogram

There are 4 distinct phases of the capnogram.

Phase I

The initial flat portion. The animal is inspiring and because inspired gas is 100% O₂ the CO₂ reading is 0.

Phase II

Ascending portion of the graph that represents the first appearance of carbon dioxide during exhalation. As the animal exhales, the first air out represents what was in the trachea. This is relatively low in CO₂. Next the bronchial and bronchiolar air comes out with increasing levels of CO₂.

Phase III

A plateau representing the exhalation of alveolar gas which contains the highest level of CO₂. The very last part of phase III is the air that was deep in the alveoli and is referred to end tidal CO₂.

Phase IV

The downward slope that corresponds to the initial inspiration. Because there is some CO₂ present in the mechanical dead space of the anaesthetic machine the phase initially shows some CO₂. This rapidly falls to 0 once dead space gas is inhaled only pure O₂ is passing the sampling port.

Normal end tidal CO₂ = 35–45 mm Hg

Analysing Alteration of the Normal Graph

An elevated baseline Phase I (inhalation) is caused by the patient rebreathing CO₂.

Causes include:

• Exhausted soda lime
• Incompetent one-way valves in circle system
• Inadequate fresh gas flow rates in a non-rebreathing system

A slanted or prolonged expiratory upstroke Phase II indicates an obstruction of airflow. The exhalation of the anatomical dead space air is prolonged giving a gentler slope to that part of the graph.

Causes include:

• Kinked endotracheal tube
• Secretions that may be partially blocking endotracheal tube or patient airways
• Patient bronchospasm

Lack of expiratory plateau Phase III indicates lack of good alveolar sample. The animal is not completely exhaling the alveolar air.

Causes include:

• Patients taking small shallow breaths
• High fresh gas flow rates in non-rebreathing systems

Terminology

Hypercapnia

From the Greek hyper = ‘above’ and kapnos = ‘smoke’, a condition where there is too much carbon dioxide in the blood. Hypercapnia will generally trigger reflexes that increase breathing and access to oxygen.

Hypocapnia

A state of reduced carbon dioxide in the blood and often results from hyperventilation—deep or rapid breathing.

The Electrocardiogram

The ECG, or electrocardiogram, measures the electrical activity of the heart. In a normal animal, electrical impulses pass through the cardiac tissue in an orderly and regular manner and cause the heart muscle (myocardium) to contract. If this electrical activity is abnormal, then this will result in abnormal muscular contractions.

There are two main types of ECG machines. More modern machines will display the image obtained on a video screen, while the type of ECG machines found more commonly in veterinary practice, record the electrical activity on heat sensitive paper. In both cases, the resulting image is referred to as a trace.

The use of ECG can be a useful aid in the diagnosis of heart conditions when used in conjunction with other diagnostic tools, such as ultrasound (echocardiography) or radiography.

In addition, ECG is also commonly used to monitor the heart rate and rhythm of patients undergoing general anaesthesia and also to monitor ‘at risk’ patients (i.e., those patients who may have an increased risk of developing cardiac arrhythmias such as post GDV surgery patients).

The P wave corresponds to atrial depolarisation (i.e., electrical activity), which results in the atrial myocardium contracting (i.e., muscular activity).

The QRS waves are usually considered together and are referred to as the QRS complex. This complex relates to ventricular depolarisation (i.e., electrical activity), which results in ventricular myocardium contraction (i.e., muscular activity).

The final component of the trace is the T wave, which relates to repolarisation of the ventricles (electrical activity) and resultant relaxation of the myocardium (muscular activity).