Anaesthesia explained

In the first of a new series on anaesthesia Nina Ruth Lewis and Jo Fitz-Henry explain why preoperative investigations are so important to anaesthetists

During your time at medical school most of you will spend only a few days studying the glorious art that is anaesthesia. Preoperative investigations are the constant source of anguish in a house surgeon's life. Please take heart, we fully appreciate that you will never win. A thorough history and examination will give us most of the answers, but investigations, guided by our clinical skills, will help us to complete the picture. There is no place for "routine" preoperative investigations - they are unpleasant for the patient (venepuncture) or involve unnecessary risk (x ray examinations), but they are also a waste of money.

What do we do with the information?

When the anaesthetic assessment is complete, the anaesthetist will grade the patient according to his or her physiological reserve and the attendant risk from both anaesthesia and surgery (the other thing that upsets the patient's physiology). The most common scoring system in use was devised by the American Society of Anesthesiologists (ASA) (see box 1).

The ASA grade has been shown to be a gross predictor of overall perioperative outcome. A patient of ASA I is your friend, on whom you don't inflict any tortures of preoperative investigations. If your patient falls into any of the other categories you must think of how his or her general health may be improved. If it can be improved this will increase the patient's physiological reserve and make him or her less at risk of serious harm. This is an indication for postponing surgery. The time taken for improvement may be anything from a few hours (intravenous fluids for someone about to undergo laparotomy) to several months (weight loss in the morbidly obese patient). At the end of this "physiology improvement period" the patient is reassessed and if the desired improvement has been achieved the operation can go ahead.

What investigations do we need and when?

Here are some of the more common investigations, their abbreviations, and why we do them.

Full blood count (FBC)

Ordering an FBC should not be a reflex action if there is no clear indication from the history and examination that the cellular component may be abnormally high or (more commonly) low. An FBC should, however, be done when large intraoperative blood losses are expected - for example, trauma or major surgery. We need to know what haemoglobin level we are starting from to judge the need for cross- matched blood and how early we will need it during the operation (see box 2).

A low haemoglobin (Hb) of whatever cause means a reduction in the oxygen carrying capacity of the blood. This is com- pensated for by an increase in the cardiac output and oxygen consumption. This anaemia should concern you because it increases the risk of perioperative ischaemia. The lowest limit of Hb that an anaesthetist is prepared to tolerate will vary depending on the patient's starting physiological state. A fit young person with a Hb of 80 g/l is likely to tolerate an anaesthetic better than a patient with critical myocardial ischaemia and a Hb of 120 g/l. The patient with new or worsening angina and declining haemoglobin concentration may be a candidate for a preoperative blood transfusion, whatever the absolute concentration is.

Conversely, and much more rarely, a high Hb (>180 g/l) carries a high risk of thrombosis and needs appropriate prophylaxis.The more common secondary polycythaemia is usually a response to chronic hypoxia, and the patient should be assumed to have chest disease until proved otherwise. Exercise tolerance is a good measure of respiratory reserve, though electrocardiography, a chest x ray examination, and a test of blood gases are mandatory. Check with the anaesthetist whether a venesection is indicated as part of the preparation.

The anaesthetist is also interested in the platelet levels. If they are low, because of underproduction, increased consumption, or malignancy, increased perioperative blood losses may be increased. If the platelet levels are less than 100*10⁹/ml seek advice from the anaesthetist or the haematologist, or both.

Coagulation screen (clotting)

This is indicated when the patient is being given anticoagulants or where the ability to stop bleeding rapidly after the patient has been cut or bruised is in doubt. Such bleeding tendencies may occur because the production of clotting factors is decreased due to liver failure, alcoholic liver disease, hepatitis, malignancy, or inherited factor deficiencies. You should involve the patient's own haematologist early in the preoperative management, particularly if the current coagulation state needs altering.

Urea and electrolytes (U and Es)

Taking blood for urea and electrolytes (U and Es) is going to be useful only when there is an indication that the biochemistry is going to be deranged. This is particularly the case in elderly people, who have diminishing renal function with age; patients with renal or hepatic failure; patients with diabetes; patients taking cardiac drugs - diuretics, &223; blockers, ACE inhibitors, calcium blockers, and digoxin; and those who have disturbed fluid balance, such as dehydration, vomiting, or diarrhoea.

Liver function tests (LFTs)

Liver function tests (LFTs) are needed if the patient is known to have hepatic problems, such as liver failure or hepatitis; is predisposed to such problems - for example, high alcohol intake or malignancy - or is bright yellow when you clerk him or her in. Albumin is a protein normally synthesised by the liver, without which you get oedema and ascites. The first sign of liver disease, however, is a deranged international normalised ratio (INR), so a coagulation screen must be done too. Liver enzymes reflect liver damage, not liver function.

Chest x ray examination (CXR)

A chest x ray examination (CXR) is justified only when there is an acute change or instability in the patient's cardiac or respiratory function. A stable patient with a "bit of bronchitis" does not deserve one; neither does the person labelled "asthmatic" carrying a "blue inhaler in my handbag just in case." The patient who has suddenly become too dyspnoeic to fetch his cigarettes or looks on examination to have gone into acute heart failure does need an examination. Similarly, new infective signs, such as a florid productive cough, are another indication.

Electrocardiograph (ECG)

An ECG is a cheap and useful non-invasive investigation that shows previous damage to the heart. It shows evidence of infarction (bundle branch block or pathological Q waves); dysrhythmias; strain pat- terns (from hypertension or valvular stenosis); or acute critical myocardial ischaemia. This damage is often silent, especially in elderly people, which is a good enough reason for ordering an ECG on age criteria alone if there is no other indication.

You should order an ECG in all patients over 60; in heavy smokers and patients with diabetes over the age of 40; in patients of any age with a previous cardiac history, including hypertension; and in patients with unexplained or new chest pain or shortness of breath.

Echocardiogram ("echo")

Although this is useful, it does not tell us how well the patient's heart is actually functioning and if it has the reserve to work any harder. An echo gives a good indication of the heart's ability to tolerate anaesthesia and surgery, through study of myocardial motility, valvular lesions, and, most importantly, an accurate assessment of the ejection fraction. This is an important test in a patient with limited cardiac reserve since the tachycardia produced by hypovolaemia and the increased afterload induced by surgical manoeuvres, such as crossclamping the aorta, may precipitate car- diovascular collapse very easily. Order an echo if the patient has severe exercise limitation because of cardiac disease and especially in patients requiring major surgery, such as aortic aneurysm repair. As these patients will be at least ASA III, discuss them with the anaesthetist.

Pulmonary function tests (PEFR)

Surgery and anaesthesia reduce lung comPliance - that is, they stiffen the lungs, disturb the mechanisms that would normally match perfusion to ventilation, and induce Hypoventilation - either centrally through the depressant effects of anaesthetic agents and opioids or peripherally because of, for example, pain or sputum retention. All these factors conspire to cause hypoxaemia, and this is much worse in the pres- ence of chronic lung disease. Identify the patient with severe disease (the one who gets breathless rolling up his or her next cigarette) and anticipate the likely problems. Pulmonary function tests should be performed to measure the respiratory reserve and response to bronchodilator treatment.

Baseline arterial blood gases should be obtained as the patient may benefit from controlled oxygen treatment. It is mandatory to exclude an intercurrent chest infection in the patient with chronically bad lungs. Use your respiratory physician to advise you about optimal treatment for the perioperative period. Remember that regional anaesthesia may avoid some problems for certain surgical procedures, but some sick patients do need a general anaesthetic - for example, the patient who cannot stop coughing or lie completely flat.

Crossmatching (X match) versus group and save (G and S)

"Group and save" involves determining what major blood group the patient is - for example, rhesus plus blood group A - and screening the serum for the presence of common red cell antigens that can cause transfusion reactions. Crossmatching not only involves determination of the blood group and detection of reactive red cell antigens but the mixing of potential donor blood with the patient's blood sample to see if any untoward reaction occurs.

You order a G and S if the patient is unlikely to require a blood transfusion. If the need arises he or she can be quickly crossmatched with a few more tests performed on the original G and S sample. On the other hand, if you have crossmatched a patient's blood you believe that the patient will need a blood transfusion and the blood bank will be working hard to prepare the blood for transfusion. Each hospital will have its own crossmatching policy; table 2 gives a rough guide.

Pacemaker check

This is essential if the pacemaker has not been tested within the week before surgery. The check can be arranged by the cardiology department. The check makes sure that "on demand" pacemakers will still kick in if necessary - this is useful when general anaesthesia depresses cardiac function - and the "fixed rate" threshold is not too high.

Talking it over

Problems are most likely to arise when patients with pre-existing diseases are being prepared. It is here where omissions most commonly occur and cause operations to be cancelled. The first rule for you to learn as a house surgeon is to anticipate the "difficult" patient and ask the advice of the anaesthetist who is to perform the anaesthetic about further investigations and preparation. Do not hesitate to contact the medical team if you think that an improvement can be made to a patient's medical condition. The physician's role is not to provide an opinion on fitness for surgery or anaesthesia.

Patients will often have questions about the anaesthetic. Do not try to guess what the anaesthetic technique will involve or what risks the patient will face. Arrange for the anaesthetist to meet the patient.

The important bits

• Anaesthesia is a drug induced coma and adversely affects the whole body.
• The more unhealthy a patient is, the less well he or she will tolerate an anaesthetic.
• Investigations should be done to answer a question about a patient's physiological reserve. That answer should affect the patient's treatment.
• If you have any doubts about which investigations to order or the patient has important disease, discuss the case with the anaesthetist as far in advance of the planned surgery as possible.

Questions

(1) What is the ASA grade of a 20 year old female athlete who needs a general anaesthetic for arthroscopy of her knee? What preoperative investigations would you order?
(2) What investigations would you order preoperatively for a 65 year old patient with mild asthma for a transurethral resection of the prostate?
(3) Which of the following patients would you tell the anaesthetist about in advance of the planned surgery: (a) an ASA I woman for removal of breast lump; (b) a 60 year old male smoker with a history of two previous myocardial infarctions and angina at 200 metres who is to have an open cholecystectomy; and (c) an ASA I patient whose sister died "as a result of an anaesthetic reaction?"

Answers

(1) ASA grade I. No other investigations are necessary for this patient.
(2) Full blood count, urea and electrolytes, electrocardiography, and two units of packed red cells crossmatched; he does not need a chest x ray.
(3) (a) It is not necessary to discuss this patient with the anaesthetist in advance on the basis of her ASA grade alone. However, the woman may have other concerns about the anaesthetic which she may wish to discuss before surgery. (b) This patient is at least ASA grade III. His physiological status may be improved by optimising his cardiac drugs and getting him to stop smoking. The anaesthetist may also wish to order more extensive investigations, such as an echocardiogram or respiratory function tests. (c) It is important to see such patients well in advance of surgery so that a full assessment of the problem can be made. The death may have been totally unconnected with the anaesthetic or have been due to the poor physiological state of the patient. Members of families with known genetically related anaesthetic problems, such as malignant hyperpyrexia, are usually well informed and carry appropriate medical warning cards or MedicAlert bracelets.

Nina Ruth Lewis, medical house officer, Nottingham City Hospital

Jo Fitz-Henry, consultant anaesthetist, King's Mill Hospital, Mansfield
Email: jo@fitz-henry.demon.co.uk


studentBMJ 2000;08:395-434 November ISSN 0966-6494