Introduction

Anaesthetic techniques have improved markedly since chloroform was first administered over 100 years ago. New drugs, better techniques and more advanced anaesthetic equipment allow us to perform anaesthesia for longer periods and in more compromised patients than ever before. However, despite these advances, deaths, disasters and minor mishaps continue to occur during the anaesthetic period.

Anaesthesia does not provide any therapeutic benefit so it should not pose any risk to the patient in its own right. However, many of the mishaps that occur during surgery result from an adverse anaesthetic reaction rather than from the surgical technique itself. In the USA it is estimated that about 2000 people a year suffer cardiac arrest during anaesthesia and many thousands more develop other complications as a direct result of undergoing anaesthesia.

In human studies of adverse anaesthetic reactions almost all cases result from errors with only a small percentage of instances due to underlying disease. Would the veterinary world be any different? Unlikely.

Faulty technique accounts for almost 50% of all cases of anaesthetic related deaths in people. These include hypoxia, inadequate ventilation, airway obstruction and dislodgement of the ET tube to an endobronchial position. Other common causes of mishaps include 1) failure to adequately assess the patient pre-operatively or monitor them appropriately postoperatively, 2) distraction during anaesthesia resulting in inadequate intra-operative support and monitoring, 3) fatigue occurring in emergency cases and long anaesthetics, 4) failure to monitor blood loss, 5) failure to provide adequate fluid replacement, and 6) anaesthetic overdose, mostly commonly of intravenous agents.

Overall, in human anaesthesia, lack of careful monitoring and accidental overdose are the two most common causes of deaths. Drug reactions (i.e., unusual sensitivity or allergy) are rare. No similar studies have been carried out in small animals but do you think that it would be different for us?

Avoiding Adverse Reactions

Adequately Assess the Patient Pre-operatively

Weigh the Patient

Ideally, every animal should be weighed before anaesthesia so that drug dosages can be accurately calculated. This is especially important if the patient is compromised, as a small overdose of drugs can produce devastating results. If you don't routinely weigh animals, test yourself on the next 10 animals by estimating their weight and then checking to see how close you are. Soft coated wheaten terriers have a bad reputation when it comes to adverse reactions during anaesthesia and my theory is that their long tresses make them look much bigger dogs than they actually are, so their weight is over-estimated.

Undertake an Adequate Physical Examination

Underlying disease in a healthy animal and greater severity of disease than anticipated in a compromised patient can markedly increase morbidity rate. Similarly, inadequate pre-operative support and subsequent anaesthesia of an unstable patient can result in complications during anaesthesia. There are few patients that need to go to surgery immediately even when they present with urgent conditions. There are perhaps two exceptions. The animal that presents with a diaphragmatic hernia in which the stomach is trapped in the chest as a GDV is one exception. The expanding stomach impinges on lung function causing rapid and extensive respiratory compromise. The second is the animal with mesenteric volvulus. Torsion of the mesenteric vessels causes rapid tissue hypoxia, necrosis, shock and death unless rapidly resolved. For every other condition stabilisation with fluids, oxygen, abdominal wrapping etc should be carried out before surgery. Anaesthesia should not be induced until cardiovascular and respiratory stability have been achieved because depressant effects of the anaesthetic agents in combination with unstable disease can combine to produce rapid decompensation. Even the 'safe' anaesthetics will cause cardiovascular compromise and so it is imperative that the patient is as stable as possible before induction. In patients in which there is any suspicion of underlying disease obtain appropriate pre-anaesthetic blood work.

Pay Attention to the Anaesthetic Equipment

Before induction, check the machine to ensure that there is sufficient agent in the vaporiser and oxygen in the cylinder to last the duration of the procedure and ensure that no leaks are present in the anaesthetic circuitry. There is nothing more dangerous or frustrating than finding that you have an apnoeic patient and on attempting ventilation discover that there are so many leaks in the anaesthetic machine that more oxygen moves into the atmosphere than into the animal. Check for leaks by closing the pop-off valve, blocking the patient port with a thumb or finger and inflating the reservoir bag until there are no wrinkles present. The oxygen is turned off and the bag observed for reappearance of wrinkles. Alternatively, if there is a pressure gauge on the anaesthetic machine, increase pressure to 20 mmHg, turn the oxygen off and see if the pressure gauge falls. A fall in pressure or reappearance of wrinkles in the bag indicate a leak somewhere in the machine. Leaks most commonly occur at the neck of the reservoir bag where the rubber perishes, around connections (especially soda lime) and within the corrugated tubing.

Collect a range of ET tubes (1-2 either side of the anticipated size) so that in case of a difficult intubation a small tube is available. Check ET cuff integrity by filling with air and gently squeezing.

Pre-Oxygenate

Critical patients or those with cardiorespiratory disease (this includes the brachycephalics breeds) benefit from pre-oxygenation for 5 to 10 minutes before induction. Most animals become apnoeic at induction and those with low cardiopulmonary reserves will become hypoxic within a short space of time (less than 1 minute) unless oxygen has been provided beforehand.

Use the Correct Tube

Choose an ET tube that is the correct length for the patient by selecting a tube that extends from the tip of the nose to the thoracic inlet. Longer tubes may result in endobronchial intubation or excessive dead-space. Endobronchial intubation can induce hypoxia and an unstable plane of anaesthesia with the patient frequently becoming light during the procedure.

Estimate the tube diameter and then gather a selection of tubes one to two sizes bigger and smaller. Brachycephalic breeds frequently have airways that are smaller than anticipated. Often in cats and sometimes in dogs an ET tube that is too small is introduced and leaks either cannot be prevented or excessive airway wall pressure is exerted in an attempt to inflate the cuff to a point where no leaks occur.

Once intubated and connected to the anaesthetic machine, leaks around the ET tube are eliminated by inflating the cuff until air no longer escapes around the tube. Excessive pressure on the tracheal wall may occur if the ET tube cuff is randomly inflated without checking.

It is impossible to inflate ET cuffs to a point that prevent fluid leaking into the airway (dental scaling, oesophageal regurgitation etc) without potentially producing tracheal wall necrosis so an ET tube alone should not be relied on to protect the airway.

Place an IV Catheter

Ideally, every patient should have an IV catheter placed during anaesthesia so that emergency drugs can be administered easily if needed. If this can't be justified, consider catheterising all patients undergoing procedures >30 minutes and/or compromised patients (age, disease, difficult surgery).

Adequate Intra-Operative Support and Monitoring

Be a Pessimist

It pays to think ahead and be a pessimist! Before inducing anaesthesia in a critical patient make a list of all the possible emergency drugs that might be needed and calculate the volume of drug that should be drawn up for the particular patient (Table 1). This list goes into surgery with the drugs still in their bottles and an assortment of syringes. If emergency drugs are required then all you have to do is look at the chart, draw the volume and administer. No mathematical gymnastics required at a time when your mind is on saving a life.

Table 1. Emergency drugs--example list.

Don't Allow Distraction to Reduce Patient Care

It is very easy to become distracted by surgery and forget to monitor the patient during anaesthesia. Similarly, a nurse may occasionally come into the surgery to assess the patient but may become waylaid by other duties so that the animal can go for long periods without appropriate monitoring and support. It may seem pedantic to check the patient every 5 minutes but the reason for this is that irreversible changes in respiratory and cardiovascular function can occur within 10 minutes. By monitoring every 5 minutes detrimental changes in blood pressure, heart rate or respiratory pattern can be noted and corrective procedures put in place before it becomes impossible to do so. Parameters that should be monitored include heart rate and rhythm, respiratory rate, depth and rhythm and blood pressure. Haemoglobin saturation is useful in patients in which hypoxia is a potential problem, although the 'beeping' of the pulse oximeter can be used as a reassuring guide to heart rate and rhythm in any patient.

Monitoring anaesthetic depth by assessing eye position, jaw tone and response to surgery is also vital because it is very easy for a patient to become deeply anaesthetised during surgery.

Blood Pressure

Blood pressure is constantly under threat during anaesthesia especially in critical patients. These animals have underlying disease that disrupts cardiovascular function and in addition we ply them with depressant anaesthetic drugs. Blood pressure requires constant monitoring and often, constant support during anaesthesia if we are not to face the consequences of hypotension. Severe hypotension during surgery may result in shock at the time whereas a less profound fall in blood pressure may induce renal failure (transient or irreversible) in the post-operative period. Systolic blood pressure should be over 100 mmHg for long surgeries whereas 90 mmHg is probably OK for shorter procedures.

There are a number of things that can be done to improve blood pressure. Firstly check whether the inhalation agent can be reduced. Over time, anaesthesia deepens if the agent is not reduced so depth must be checked periodically. If hypotension is present, decrease depth if possible. In our practice we reduce the concentration of agent and provide additional analgesia by giving either intravenous morphine (0.1-0.2 mg/kg) or fentanyl (2-5 μg/kg). The opioids don't provide muscle relaxation so an increasingly tight jaw tone can be anticipated. In addition to reducing anaesthesia, a bolus of intravenous fluids should be considered. Lactated Ringer's Solution (LRS: 10-20 ml/kg over 10-20 minutes) should be considered provided that there are no contra-indications to rapid fluid infusion. An alternative is to provide a 1 ml/kg bolus of 7% hypertonic saline while continuing LRS at the usual rate of 10 ml/kg/hr. Occasionally, a second bolus of 1 ml/kg may be required. If this is unsuccessful at increasing blood pressure then a positive inotrope must be considered. A dopamine or dobutamine CRI (5 μg/kg/min) is started and adjusted as necessary to maintain an appropriate blood pressure.

If blood loss has occurred and is contributing to the hypotension, then LRS is increased to provide approximately 3 ml of fluid for every ml of blood lost. This is because only about 1/3 of the fluid provided remains in the intravascular space. A blood or plasma transfusion may also need to be considered.

Use the Correct Drug Dosage

Many anaesthetic disasters occur at induction when a large bolus of drugs is given to enable intubation. A relative overdose can result in profound hypotension, bradycardia and arrhythmias. Compromised patients are a special risk because they do not require as much agent as healthier counterparts. The calculated dosage should be drawn up and depending upon the level of CNS depression due to pre-existing disease and the effects of the sedative, ¼ to ½ a dose is administered and intubation attempted. Because all intravenous induction agents can induce cardiovascular changes the aim is to provide as little agent as it necessary to obtain intubating conditions.

Agents

You will note that agents come near the bottom of the list and really they are the least important component of a 'safe' anaesthetic. No matter which agents are used, complications can be anticipated if the patient is not appropriately assessed, monitored and supported throughout the entire anaesthetic period.

Always consider providing a sedative agent to a compromised patient even if they have pre-existing CNS depression. This is because sedation will decrease the amount of injectable agent and concentration of inhalational agent needed to maintain anaesthesia. Select an agent with minimal cardiovascular effects such as subcutaneous pethidine or butorphanol. Inadequately sedated, excitable patients may be overdosed at induction because they are so difficult to handle. Refer to any recently published anaesthetic text book for information on anaesthetic agents.