Monitoring the anaesthetised patient is one of the most important tasks commonly performed by the veterinary nurse. This lecture will help you to optimise your anaesthesia monitoring. Good monitoring is more than 'has the patient still got a pulse and is it breathing!' Patient monitoring should commence from the administration of pre-anaesthetic medication through to the end of the recovery period.

Anaesthesia is defined as 'the reversible depression of the central nervous system with drugs which result in unconsciousness and either the absence or reduced response to noxious stimuli, of which the patient has no memory'.

When considering anaesthesia, it is helpful to refer to the classic 'triad of anaesthesia':

 Hypnosis

 Muscle relaxation

 Analgesia

This triad refers to the positive attributes of balanced anaesthesia. Unfortunately, general anaesthesia also has negative effects on the cardiovascular and respiratory systems and impairs homeostatic mechanisms. Cardiovascular depression results in reduced cardiac output and consequently can cause a reduction in organ blood flow and tissue damage. Respiratory depression results in reduced or inefficient respiration, and can result in reduced oxygenation, hypoxia and retention of carbon dioxide (CO₂). An example of a homeostatic mechanism affected by anaesthesia is thermoregulation; anaesthesia commonly results in hypothermia.

By monitoring the patient closely and identifying any changes that occur, we can take appropriate measures to maintain organ function and prevent damage to tissues and vital organs.

Record Keeping

Apart from forming a legal document, the use of an anaesthetic record assists the anaesthetist in seeing trends that are developing, i.e., a slowly decreasing heart rate. The use of a graphical format record is most useful as it is a visual aid. It also allows a reflective approach, helping to improve future case management and clinical judgment.

Optimising Your Basic Clinical Monitoring

Diligent monitoring of mucous membrane colour, capillary refill time and pulse quality can be extremely informative, particularly if trends are noted. Careful palpation of a peripheral pulse can give important information about the peripheral circulation. It is important to appreciate that an animal may have an easily palpated pulse but be hypotensive. This is because a person's finger is most able to feel the difference between systolic and diastolic pressures (the pulse pressure). Consequently, it is vital to appreciate how easy it is to occlude the pulse (vascular tone) and therefore better assess blood pressure.

Mucous membrane colour is an indicator of peripheral perfusion. Vasoconstriction leads to pale mucous membranes. This may be due to surgical stimulation and lack of analgesia, e.g., the pale bitch spay during traction on the ovary, or severe blood loss--an important differential! Vasoconstriction can also be as a result of drugs--e.g., alpha₂-agonists, medetomidine (Domitor, Pfizer). Brick red mucous membranes as a result of peripheral vasodilatation can be due to hypercapnia (high blood CO₂ levels) or sepsis. Cyanosis, blue-tinged mucous membranes, indicates hypoxaemia; however, the anaemic patient will often be hypoxaemic before showing signs of cyanosis.

Monitoring patient breathing should also be more than just about the rate. It is vital to appreciate tidal volume in combination with rate, e.g., a patient with a low rate but large tidal volume may be fine; however, a patient with a low rate but small tidal volume may be of concern. Patients with poor thoracic compliance, such as geriatrics or those with abdominal distension, e.g., pregnant or obese patients, may require ventilatory support to prevent CO₂ accumulation.

Changes in respiratory effort along with reservoir bag movement are important to note. A patient with a kinked endotracheal tube will have increased respiratory effort characterised by abdominal effort whilst there is reduced reservoir bag movement.

Close observation of eye position, palpebral reflex (blink reflex) and jaw tone provide information about the depth of anaesthesia. Different depths of anaesthesia are appropriate for different procedures; for example, in a patient anaesthetised for preoperative radiography a palpebral reflex and slight jaw tone would be appropriate; however, when surgery commenced you would deepen the anaesthesia to virtually abolish the palpebral reflex and further reduce the jaw tone. Clinical signs may be altered by the use of dissociative anaesthetic agents, e.g., ketamine

As previously mentioned, general anaesthesia results in a reduced ability to autoregulate body temperature. Hypothermia is a particular problem in small, young, thin-coated patients and those undergoing abdominal or thoracic surgery.

Hypothermia can result in:

 Decreased anaesthetic requirement (increased risk of overdose)

 Prolonged recovery--increased risk of recovery-associated complications

 Bradycardia

 Increased risk of arrhythmias

 Shivering in recovery--increasing the patient's oxygen requirements

 Increased morbidity rate

Consequently, there are great benefits in monitoring temperature and taking measures to prevent hypothermia. Temperature monitoring can be performed using many techniques. Rectal thermometers are easy to use before and after anaesthesia but during surgery drapes may obscure access and only intermittent readings can be gained. Temperature probes provide continuous readings and can be placed either rectally, nasally or down the oesophagus. Temperature monitoring during recovery is vital.

Blood glucose monitoring is easily performed and can make a real difference to clinical outcomes in the following patient groups:

 Young patients

 Patients with hepatic insufficiency

 Diabetics

Glucose monitoring can be done cheaply and rapidly using glucometers bought from pharmacies. Samples can be obtained from the normal blood sampling sites or alternatively from pricking the tongue. The latter is the easiest to perform perioperatively.

Monitoring Equipment

Monitoring equipment should be used as an aid to help you monitor anaesthetised patients. Monitors do not ever replace careful monitoring by a diligent anaesthetist. They can also malfunction, so it is important always to compare the results gained with the clinical picture.

Pulse Oximeters

The pulse oximeter is probably the most commonly used monitor in veterinary practice. It is a non-invasive method of measuring the percentage of oxygen-bound haemoglobin in arterial blood. Monitoring this is important because hypoxia results in cell death and consequently organ damage.

Pulse oximeters emit red and infrared light at different wavelengths. Haemoglobin absorbs different wavelengths of red and infrared light depending on whether it is bound to oxygen or not. The amount absorbed at each wavelength is measured by photodetectors. This is expressed as the percentage of saturated haemoglobin.

Patients under anaesthesia receiving 100% oxygen should have a SpO₂ close to 100%. SpO₂ must remain above 90% to prevent hypoxia and subsequent tissue damage. N.B. Anaemic patients may have a SpO₂ of 100%; however, this does not represent the ability to oxygenate tissues because of the absolute deficit in haemoglobin.

Capnography

Capnography measures the amount of CO₂ inhaled/ exhaled by the patient, examination of the trace provides additional information about respiration. Information on the cardiovascular system is provided as blood flow is required for the transport of CO₂ from cells to the lungs for elimination. A sudden drop in CO₂ may indicate a drop in blood flow suggesting hypotension and impending or actual cardiac arrest. Various details about respiration are provided:

 Respiratory rate

 Allows evaluation of respiratory depression

 Shows the occurrence of rebreathing

 Wave form allows prompt identification of apnoea, breathing system leaks, inspiratory and expiratory obstruction etc.

Normal end-tidal CO₂ should range between 35 and 45 mmHg (4.6 and 6 kPa, 5 and 6%).

Blood Pressure Monitoring

Monitoring of blood pressure provides information on cardiovascular function. Hypotension (mean blood pressure <60 mmHg) leads to short-and long-term complications. Short-term complications include accumulation of lactic acid resulting in a metabolic acidosis, increased oxygen and glucose demand and increased cardiac work. Long-term complications include ischaemic tissue damage due to a lack of oxygen, e.g., renal failure.

There are several different methods of measuring blood pressure. Direct monitoring is an invasive technique which involves the placement of a catheter into an artery--this is unlikely to be used for routine procedures in healthy patients. Alternatively, there are two commonly used non-invasive techniques: oscillometric and Doppler. Many practices have Doppler ultrasound monitors for monitoring blood pressure in conscious cats. The same technique can be used to monitor the anaesthetised patient. The result provided represents the systolic blood pressure in dogs and slightly underestimates systolic blood pressure in cats; however, the trends seen provide important information. Normal blood pressures are:

 Systolic 90-120 mmHg

 Mean 60-85 mmHg

 Diastolic 55-90 mmHg

Electrocardiography

Electrocardiography is a non-invasive technique that only provides information about the electrical activity of the heart and not directly about how well the heart is pumping. It allows identification of arrhythmias. Abnormalities can also indicate if there are certain complications, including hypoxia, acid-base balance abnormalities or electrolyte disturbances (potassium especially).

Electrocardiographs used for monitoring during anaesthesia generally use a three electrode configuration; however, some monitors use a transoesophageal probe.

References

1.  Bilbrough G. A practical guide to capnography. In Practice 2006; 28: 312-319.

2.  Moens Y, Coppens P. Patient monitoring and monitoring equipment. In: Seymour, C; Duke-Novakovski T. eds. Manual of canine and feline anaesthesia and analgesia (second edition). Gloucester: BSAVA Publications, 2007: 30-49.