Read the French translation: Anesthésie--Les Bases du Monitoring

Introduction

Ideally monitoring in veterinary practice is done by a person dedicated almost entirely to this task and using clinical skills. This person can also make use of different devices that monitor physiological variables useful for anaesthesia monitoring. Such devices are increasingly compact, affordable and where necessary adapted to veterinary needs. They come as stand-alone devices or in various combinations as multi-parameter anaesthetic monitors. The latter is practical but limits the use to one animal at the same time.

An excellent combination that covers a broad range including advanced indications would be the monitoring of the electrocardiogram, the haemoglobin saturation of arterial blood (pulse oximetry), the concentration (or partial pressure) of CO₂ in the respiratory gases (capnometry), blood pressure and body temperature. Regularly the measurement of the respiratory gas concentration of oxygen and of volatile agent is also incorporated. In case a choice has to be made between parameters that will be monitored the ranking in importance is a matter of debate, personal preferences, the nature of the patient and the type of anaesthetic/surgical procedures. Respiratory gas analysis in non intubated patients is in principle not reliable. Inspiratory Oxygen concentration measurement is less mandatory when pulse oximetry is used and volatile agent concentration measurement is not a must.

Various Parameters to be Monitored

The capnometer measures CO₂% (or partial pressure in mmHg) in respiratory gas. Stand- alone devices can be extremely compact. A system which displays the capnographic curves (capnogram, capnograph) is worth the extra cost. The capnograph is considered a unique monitor by many because of the fact that its measurement reflects the interaction of three important physiological mechanisms simultaneously: metabolism, ventilation and pulmonary circulation. On top several aspects of the proper functioning of the anaesthetic apparatus and the connection with the patient can be checked. It offers a continuous non-invasive way to estimate the partial pressure of CO₂ in arterial blood (PaCO₂) which is directly determined by alveolar ventilation. Therefore it is particularly valuable when intermittent positive pressure ventilation is used. Also capnography allows rapid detection of cardiac arrest and is used to monitor efficiency of cardiac massage during resuscitation.

They are available as "side stream" or "mainstream" analysers. Both systems have advantages and disadvantages with miniaturised mainstream being more adapted for very small tidal volumes and high respiratory rates and side stream being susceptible for artefacts due to problems with the sampling.

Normal ventilation and PaCO₂ can exist with a low PaO₂ (hypoxemia). Hypoventilation and high PaCO₂ (hypercapnia) can exist with high PaO₂ when the animal breathes oxygen enriched air. The oxygenation of the patient can be estimated by using the relationship between saturation of arterial blood with oxygen (SaO₂) and PaO₂ which is given by the sigmoid shaped oxyhemoglobin dissociation curve. A pulse oximeter calculates the O₂-saturation of haemoglobin (SpO2) using this relationship It is a monitor of the efficiency of the cardio-respiratory system and indicates if the blood is sufficiently oxygenated (SpO₂ must be >95%). A device which displays the curves is worth the extra cost. As the principle relies on the presence of pulsating arterioles the incidence of erroneous readings increases with vasoconstriction (shock, alpha-₂ agonists).The latest technologies perform better in vasoconstrictive states and have less motion artefacts. The pulse oximeter also displays pulse rate (special veterinary editions allow rates of >250 bpm) and is more reliable than an e.c.g.-derived heart rate because it is based on the existence of real pulsations in a peripheral vascular bed. It also will indicate the existence of rhythm disturbances. The pulse oximeter is compact and can contribute to increased safety by monitoring oxygenation during transport. It is clear that capnograph and pulse oximeter give excellent and complementary information. Used together they have the potential to prevent >90% of the complications during anaesthesia. The e.c.g. remains an essential tool for the differential diagnosis of arrhythmias and cardiovascular collapse.

Arterial blood pressure is one of the most useful measures of cardiovascular function available to the anaesthetist. However one must realise that invasive blood pressure is less straightforward to measure in practice conditions on one hand and that NIBP might not fulfill the expectations of reliability and accuracy. Modern developments in NIBP like high definition oscillometry are likely to improve the accuracy but are yet not included in multi-parameter devices. An alternative and versatile non-invasive method remains the Doppler ultrasound method which indicates systolic blood pressure. It seems to have varying popularity but offers a lot for a modest price. It also allows continuous audible monitoring of heart rate and to some extent myocardial contractility (also useful in some exotic species).

Body temperature monitoring is too often neglected. Especially in the smallest patients profound hypothermia can develop with subsequent heart rhythm disturbances and prolonged recovery. An oesophageal temperature measurement is to be preferred to rectal measurement. New trends are inclusion of spirometry and airway pressure in the multi-parameter devices. The new parameters for estimating anaesthetic depth introduced in human anaesthesia like e.g., bispectral index (BIS) are not yet validated for veterinary use.

Practical Conclusion

Although especially a capnograph (inhalation anaesthesia) and a pulse oximeter (parenteral anaesthesia) are worthwhile to purchase for every practitioner, many still have to monitor anaesthesia without these devices. The "presence" of electronic monitoring is not enough for this can never totally replace the "clinical" supervision by a skilled person. Of uttermost importance is to understand the meaning of measured values and the nature and effect of possible mishaps in data collection. It is not proven that use of electronic monitoring devices in veterinary medicine has increased safety. However in a multicentre survey of anaesthetic mortality in cats there was strong evidence that monitoring of the pulse and use of pulse oximetry increased anaesthetic safety. If one has to perform well with minimal equipment the standard use of the oesophageal stethoscope remains unsurpassed and simple measures such as arranging draping in such a way that the tongue or mucous membranes are always visible during anaesthesia (anaesthesia screen) are very effective. It should also be emphasised that a part of anaesthetic mortality occurs during the induction phase when monitoring devices are not yet attached to the patient on one hand, and during the recovery phase when the animal is left unattended without further monitoring on the other hand.

References

1.  Brodbelt, et al. British Journal of Anaesthesia 2007; 99:617

2.  Moens, Coppens. BSVA Manual of Canine and Feline Anaesthesia and Analgesia, 2007, 2^nd Ed.