Introduction

From the earliest days of anaesthesia, the anaesthetist has monitored the patient's pulse rate, pattern of breathing and general condition. Advances in electronic technology have made reasonably reliable, easily attached, non-invasive monitoring devices available for clinical practice. Observations and measurements of certain parameters before, during, and after anaesthesia provide important data to support the clinical assessment of the animal's condition and improve the chances of survival of the very ill by indicating what treatment is needed, as well as the response to treatment already given.

It is necessary to know what to measure as well as how to measure it and not all anaesthetists may agree on the priority ranking of the monitoring devices available. However, for major surgery, for anaesthesia and surgery of poor-risk patients, and for equine anaesthesia, it would be difficult to defend the failure to use monitoring equipment, especially if it were available. Recommendations for monitoring of anaesthetized patients are available on the web sites for the Association of Veterinary Anaesthetists, American College of Veterinary Anesthesia and Analgesia, and the American Animal Hospital Association.

General considerations relating to monitoring

Complications, including death, may occur in healthy patients at all stages of anaesthesia and monitors provide early warning of life-threatening developments. Anaesthetic mishaps may be caused by mechanical malfunction, disconnection of equipment, or human error. Judgemental error frequently occurs when the anaesthetist is in a hurry and circumvents basic practices and procedures, or when a decision must be made in an emergency. The prevalence of complications may also be associated with inadequate training or experience of the anaesthetist. Knowledge and experience are a function of the nature of the training received and the years of practice, but proper vigilance at all times can only be generated by self-motivation.

Routines should be developed to ensure that each aspect of apparatus function is checked before use. Failure to follow a simple checklist in every case features high on the list of causes of anaesthetic disasters. All anaesthetic equipment, including monitoring devices, should be maintained in good functioning order. It should be a matter of course to maintain monitors with a battery back up fully charged in case of need in an area without a convenient electricity outlet nearby, failure of electricity supply, or the need to disconnect from the main supply to minimize electrical interference with other monitoring equipment.

Proficiency with methods of electronic surveillance must be acquired during minor procedures so that they can be applied properly in circumstances where their use is mandatory (e.g. during major surgery or a cardiovascular crisis). Routine use ensures that probes, sensors, electrodes, etc. can be applied quickly to the animal and increases the likelihood that the information obtained is reliable.

Although current practice is to establish monitoring only after the animal has been anaesthetized, it must be recognized that many complications occur during induction of anaesthesia. Ideally, especially for poor-risk patients, monitoring should begin when the drugs for premedication are administered. Dogs and cats may vomit after administration of an opioid and the quantity and content of the vomit may warn that the animal was fed recently and so may be at risk for regurgitation and pulmonary aspiration of gastric material. Brachycephalic breeds and animals with respiratory problems should always be observed after administration of preanaesthetic drugs because sedation may cause partial or complete airway obstruction or serious respiratory depression and hypoxaemia. In any animal, evaluation of the degree of sedation produced by premedicant drugs may indicate that the anaesthetic plan should be reassessed and either drug doses reduced or additional agents included.

Careful observation of the patient during induction of anaesthesia may allow precise titration of drugs to achieve the desired depth of anaesthesia and ensure early recognition of a complication that requires immediate specific treatment, such as cyanosis, anaphylaxis, or cardiac arrest. Where possible, patients at risk for complications may be attached to specific monitoring equipment before induction of anaesthesia. Appropriate equipment for this would be the electrocardiograph (ECG), a device for measurement of blood pressure, or a pulse oximeter.

Recording drugs, dosages and responses for each patient is essential and provides valuable information for any subsequent time that anaesthesia may be needed. Noting all measurements on an anaesthetic record provides a pictorial description of changes that can be used to predict complications and plan treatment (Fig. 2.1). Retrospective evaluation of difficult cases and of series of records, perhaps of patients with similar surgical procedures, or to compare different anaesthetic protocols, can be used to monitor the anaesthetist's performance and identify difficult situations that require further thought and improved management. For research purposes, data can be acquired into a computer for accurate data summaries.

Monitoring animals during anaesthesia must include observation of behaviour and reflexes and measurement of various physiological parameters at regular intervals to accomplish two objectives. The first objective is to ensure that the animal survives anaesthesia and surgery. The second objective is to obtain information that can be used to adjust anaesthetic administration and management to minimize physiological abnormalities, which is especially important for animals that have already compromised organ systems. The goal is to prevent development of preventable adverse consequences 1 hour, 12 hours, or even 3 days after anaesthesia.

Monitoring should continue into the recovery period to determine the need for additional analgesic drugs, adequacy of ventilation, and to record serious deviations in body temperature. Mucous membrane colour should be checked for at least 20 minutes after the animal has been disconnected from oxygen as it may take that long for hypoxaemia to develop in animals that are moderately hypoventilating and breathing air.

A variety of methods using inexpensive or expensive equipment can be used to monitor parameters determined by the species of animal to be anaesthetized and by the abnormalities already present in the patient. Not all monitoring techniques need to be applied to every patient. A recommendation for three levels of monitoring is presented in Table 2.1; level 1 monitoring information should be obtained from all anaesthetized animals, level 2 monitors are affordable and recommended for routine use in some groups of patients, and level 3 monitors individually offer improved monitoring for patients with specific problems.

This chapter will describe the techniques of monitoring using a systems approach, and offer guidelines for interpretation of the information obtained. Further recommendations are given in the chapters devoted to species anaesthesia and the chapter on management of complications.

Clinical assessment of the patient