Monitoring in anaesthesia is a vital aspect of safe anaesthesia. The goals of monitoring are to provide an appropriate depth of anaesthesia, maintaining normal physiological function of the patient. Close regular attention to the patient allows rapid intervention if the patient becomes too light, too deep or physiologically compromised. Good monitoring and rapid correction of developing problems prevents minor disturbances becoming major complications.

Anaesthetic Equipment

Always check the anaesthetic machine, monitoring and resuscitation equipment before each anaesthetic. Monitors allow many parameters to be measured simultaneously. During critical periods constant update on several parameters cannot be done manually. This will ensure that technical errors do not occur during anaesthesia which could put the patient at risk.

Monitoring Gas Flows and Vaporiser Settings

Oxygen cylinder contents must be continuously assessed. Flowmeters may drift with time and should be constantly checked. Vaporiser fluid levels and settings should also be monitored.

Monitoring the Breathing System

The breathing system must be continuously monitored for disconnection, carbon dioxide absorber, rebreathing and scavenge valves action.

Clinical Monitoring

The veterinary surgeon must make full use of their senses to collect clinical information, on which decisions can be made as to whether intervention is necessary.

Monitoring Central Nervous System Function

Depression of the cranial nerve reflexes (palpebral and corneal reflex, eye position, pupillary diameter and jaw tone) gives valuable information as to the depth of the patient. Additional changes in the cardiopulmonary parameters give further indirect information on depth of anaesthesia. More advanced monitoring using electroencephalogram records can be used to assess anaesthetic depth in referral centers.

Monitoring Cardiovascular Function

Assessing cardiovascular variables continuously provides information on the depth of anaesthesia, on the effects of surgery and the cardiovascular function itself. The primary goal is to ensure adequacy of tissue perfusion. In practice there is no one parameter that indicates perfusion absolutely, but rather the surgeon must assess cardiovascular function. Monitoring the heart rate, rhythm and pulse characteristics is combined with the evaluation of mucous membranes, blood pressure and other parameters to form an overall assessment of the cardiovascular system of the patient. The regular direct assessment of the peripheral pulse character, strength and rate should form the central component of the cardiovascular monitoring around which other aspects and electronic monitoring (monitoring of arterial blood pressure (direct/indirect), central venous blood pressure and electrocardiogram) can be added.

Monitoring of Respiratory Function

The veterinary surgeon must monitor the respiratory function, continuously watching chest excursions and reservoir bag inflation and deflation. The rate and depth of respiration gives some adequacy of ventilation and depth of anaesthesia. The pattern of respiration can also provide valuable information. Irregular ventilation may indicate pathology, dangerously 'deep' anaesthesia or, with very shallow rapid breathing, may be indicative of light plane of anaesthesia. Electronic monitors (respiratory rate counters, apnoea detectors, airway pressure gauges and spirometers) can be used to scrutinise ventilatory function and these can provide valuable information on the respiratory function.

Pulse Oximetry

Pulse oximetry is a non-invasive method of measuring arterial oxygen saturation and gives valuable information about gas exchange and arterial oxygenation that has become increasingly popular in practice. It rapidly responds to falling arterial oxygen saturation and can be life-saving in situations such as during airway obstruction and hypoxia. It does not however allow accurate assessment of the adequacy of ventilation as it does not tell the clinician anything about arterial carbon dioxide tension, which is what animals regulate their ventilation to. The pulse oximeter alerts to hypoxaemia. It can be placed anywhere on the patient where there is an area of non-pigmented skin where light can easily be transmitted through tissues.

Capnography

Capnography is based on the measurement of exhaled carbon dioxide (end-tidal carbon dioxide) and provides valuable information on respiratory as well as cardiovascular function. Usually it is measured from gas withdrawn from a small tube. End-tidal carbon dioxide approximates to alveolar and arterial carbon dioxide and so gives an excellent indicator of ventilatory adequacy; high carbon dioxide suggests inadequate ventilation whereas low values could indicate overventilation. The end-tidal carbon dioxide level is also related to how much carbon dioxide is produced in the tissues and how effectively it is delivered to the lungs. Capnography is becoming affordable and increasingly popular in practice. It is a valuable aid to patient assessment under anaesthesia.

Blood Gas Analysis

Blood gas analysis of a blood sample withdrawn from an artery allows the definitive assessment of respiratory gas exchange. The blood gas analyser measures the partial pressure of carbon dioxide (PCO₂) and oxygen (PO₂) in the sample and pH. It allows the assessment of adequacy of ventilation (PCO₂), arterial oxygenation (PO₂) and acid-base status of the patient (pH). However blood gas machines are expensive and require regular maintenance, although there is increasing popularity of handheld monitors in practice.

Body Temperature

General anaesthesia interferes with the animal's ability to regulate its temperature. Hypothermia is common, delays recovery, increases morbidity and can result in mortality. Measuring of core body temperature is useful. It can be estimated by measuring thoracic oesophageal temperature. Rectal temperature is less accurate, but should be monitored at regular intervals during longer operations and in recovery if oesophageal temperature is not available.

Urine Output

Kidney function can be measured by catheterising the bladder and measuring the collected urine. A value in excess of 1 ml/kg represents adequate renal perfusion.

Monitoring Fluid Administration

If intravenous fluids are administered, the rates should be checked periodically. Calibrated burettes and infusion pumps can be used to aid accurate fluid therapy.

Summary

Good patient monitoring is essential for safe anaesthesia. By carefully monitoring the patient, prevention of complications and assessment of intraoperative therapy can be achieved. Monitoring depth of anaesthesia aims at ensuring appropriate depth for the intended procedure, and relies primarily on assessment of indirect signs suggestive of appropriate anaesthesia. Monitoring cardiovascular and respiratory function is used to ensure physiological well-being of the patient as well as providing indirect indicators of depth of anaesthesia. Monitoring should continue into the recovery period, as a high proportion of perianaesthetic deaths occur at this stage.