Yves P.S. Moens, Dr.med.vet., Dr habil., PhD, DECVAA
Read the French translation: Les Meilleures Réponses aux Problèmes Anesthésiques les Plus Fréquents
The list of possible complications during anaesthesia of companion animals is long. Documented information about their incidence is difficult to compare and is different for first line practices and referral centres. The definition of a complication is variable while the ultimate complication is anaesthetic death. The most recent figures mention an anaesthesia related mortality of 0.1 to 0.2% in healthy dogs and cats respectively and 0.2 to 2% in sick animals. Rabbits have a higher mortality up to7%. The major cause for this mortality are cardiovascular and respiratory complications. This mortality rate might be considered as relatively low but is much higher than in human medicine. One of the reasons is the different standards used in veterinary medicine for anaesthesia management. It is clear that unfortunately a lot of complications are still self made and can be avoided by simple organisational measures such as dedicating a person to anaesthesia monitoring, using anaesthetic monitors, proper preanaesthetic assessment with risk estimation and stabilisation, defined workflow and responsibilities, presence of check procedures for equipment, acknowledgment of increased risk of complications by urgent and out- of- working hours procedures. It is also now clear that the immediate post anaesthetic phase (3hrs) carries a high incidence of complications: 50% of anaesthetic deaths were recorded in this phase for dogs and >60% for cats and rabbits. Complications may happen unnoticed when there is insufficient anaesthetic management and lack of monitoring until fatal hypoxemia and cardiac arrest occur. When anaesthesia monitors are used one is able to detect (and quantify) e.g., hypercapnia, hypoxemia, hypotension, arrhythmias, hyperthermia, etc.
Common problems in practice with inhalation anaesthesia are e.g., unexpected awakening with movements of the animal or the impossibility to reach an insufficient anaesthetic depth despite high vaporiser settings. This situation needs immediate check of reflexes, colour of mucous membrane (hypoxemia?), oxygen flow, vaporizer filling and the connection of the patient with the anaesthetic system. Actions include manual compression of the breathing bag, observing thorax movement, listening for leaks, observing pressure build up (manometer). Oesophageal intubation, leaks in the system (cave the breathing bag), insufficient seal by the cuff of the endotracheal are common causes. Panting during anaesthesia following intravenous induction is sometimes present (opioid premedication) and makes deepening anaesthesia difficult. This panting increases dead space ventilation at the cost of decreased alveolar ventilation making uptake of volatile agent to low. The patient must be ventilated with intermittent positive pressure ventilation manually until a sufficient depth is reached and a more normal pattern returns. The addition of different connectors and sensors between the endotracheal tube and the anaesthetic system in very small animals has the same effect of increased dead space.
Paradoxically the use of intubation has been associated with increased complications and mortality in cats. Traumatic intubation due to inappropriate anaesthetic depth or large tubes are a likely reason. Increasing the dose of the induction agent and titrating properly until atraumatic intubation is possible is indicated. In cats progressive obstruction of the endotracheal tube often goes unnoticed until almost total respiratory obstruction occurs. Inspection of the tube before intubation (use clear tubes) and careful observation of the respiratory pattern and its changes and the tube itself is indicated. In case of doubt a short disconnection and aspiration from the tube must be done. A cat should never be extubated and left immediately alone to recover. Careful inspection of respiratory pattern and mucous membrane colour will indicate upper respiratory airway obstruction (laryngeal spasm). It has been recently demonstrated that pulse monitoring and monitoring of oxygenation in cats with a pulse oximeter reduces mortality. Similarly upper airway obstruction is likely to occur in brachycephalic breeds. When blood pressure is monitored it is clear that hypotension (mean blood pressure < 60 mmHg) regularly occurs. The measurement obtained by non-invasive methods must always be regarded cautiously and supported by clinical confirmation. Hypotension should be treated first by checking anaesthetic depth and if necessary by reducing its concentration, second by administering fluids (saline, colloids) and in the last place by using inotropic or vasopressor drugs. A physiologically normal lower blood pressure in very young animals must be acknowledged. Bradycardia regularly occurs during anaesthesia not in the last place caused by opioids and alpha2 agonists (premedication). The degree of bradycardia that can be tolerated depends on the expected normal rate, acknowledgment of the fact that the patient is immobile, the age (very young tolerate bradycardia less), the degree of noxious stimulation, the possibility of acute vagal reflex. Atropine administration can cause undesired side effects (tachycardia, gastrointestinal) and should not be used unnecessarily. In case of alpha2 agonists severe hypertension will be the result. Clinical observation of good pulse quality, peripheral perfusion (CRT) and colour suggests not treating in the first place. A normal end tidal CO2 (capnography) and blood pressure supports this strategy.
1. Brodbelt, et al. The Veterinary Journal 2009; 182:152
2. Brodbelt, et al. British Journal of Anaesthesia 2007; 99:617
3. Egger C. BSVA Manual of Canine and Feline Anaesthesia and Analgesia, 2007, 2nd ed.