Principles of Anaesthesia: Premedication

PRINCIPLES OF ANAESTHESIA

(see following article), the essence of pain management is to preempt pain if possible by giving analgesics before nociceptors are activated, by giving drugs as a premedication, or by performing local or regional anaesthetic techniques before surgery. The use of regional anaesthetic techniques such as peripheral nerve blocks and epidural anaesthesia requires considerable personal expertise. It is an important part of the perioperative plan and may require some time to institute before surgery. Postoperative analgesic agents and their route and method of administration are chosen, with a combination of regular mild or moderate strength, longer-acting analgesics for background pain, and strong, fast-acting analgesics for breakthrough pain. A plan should be made for regular assessment of adequacy of analgesia, using a telephone help line for day surgery patients or specialist pain nurses for ward patients. An acute pain team is available in many hospitals to monitor patient-controlled analgesia or epidural infusions and provide advice on complex problems with postoperative analgesia.

Principles of Anaesthesia: Premedication Nicola Barber Mark C Blunt

Any drug administered shortly before surgery is premedication. It may be one of the patient’s normal drugs, or prescribed to achieve one of the following objectives: • to decrease anxiety • to cause amnesia • as pre-emptive analgesia • as pre-emptive treatment for postoperative nausea and vomiting • to enhance gastric emptying • to increase the pH of the gastric contents • to attenuate autonomic reflexes • to reduce secretions • as prophylaxis against potential disease

Perioperative fluids and nutrition Perioperative fluid therapy needs to take account of continuing fluid requirements, compensate for haemorrhage and other pathological losses, and restore and/or maintain normal clinical chemistry. It is essential to ensure that adequate nutritional support is provided for patients undergoing major abdominal or cancer surgery, and in those who become critically ill. Addressing these issues may significantly alter the perioperative plan. For example, central venous access may be required for titrating fluid therapy or providing parenteral nutrition, a jejunal feeding tube introduced at laparotomy may accelerate postoperative enteral feeding, and urinary catheterization is required when careful monitoring of fluid balance is a priority.

Examples of premedication are given in Figure 1. Many anaesthetists do not routinely prescribe premedication. Anxiolysis and amnesia Most patients are nervous and frightened before surgery. The anaesthetist’s preoperative visit has been shown to be the most effective way of allaying fear. There is strong evidence that building a rapport with the patient, explaining planned procedures and answering questions preoperatively is as effective as anxiolytic premedication. However, some patients, especially children, benefit from these drugs, most commonly benzodiazepines, which also cause a degree of anterograde amnesia. Benzodiazepines cause respiratory depression, which may result in hypoxia and hypercapnoea. Many have long half-lives so that these adverse effects can persist well into the postoperative period. Paediatric anaesthetists sometimes prescribe midazolam to effect short-term anxiolysis and sleep; it can be administered intranasally, or orally in a flavoured drink. However, absorption via the latter route is unpredictable.

Long-term follow-up Occasionally a patient may need to see an anaesthetist after discharge. This may be for investigation of suspected anaphylactic reaction, malignant hyperpyrexia or pseudocholinesterase deficiency, in conjunction with immunology and genetics services. A patient who has suffered awareness during anaesthesia may develop post-traumatic stress disorder and require psychiatric referral. Intensive care follow-up clinics are under development, and are detecting and treating morbidity that had previously been unrecognized in patients recovering from critical illness. u

Nicola Barber is a Specialist Registrar in Anaesthesia at the Queen Elizabeth Hospital, Kings Lynn, UK. She qualified from St Hugh's and Green Colleges, Oxford, and has trained in Oxford and East Anglia. Her research interests include paediatric anaesthesia and research on ferret brain stem chemistry.

FURTHER READING Boyd O, Hayes M. The oxygen trail: the goal. Brit Med Bulletin 1999; 55(1): 125–39. Shoemaker W C. Oxygen transport and oxygen metabolism in shock and critical illness. Invasive and noninvasive monitoring of circulatory dysfunction and shock. Critical Care Clinics 1996; 12(4): 939–69. Shoemaker W C, Appel P L, Kram H B et al. Hemodynamic and oxygen transport monitoring to titrate therapy in septic shock. New Horizons 1993; 1(1): 145–59.

SURGERY

Mark C Blunt is a Consultant in Anaesthesia and Intensive Care at the Queen Elizabeth Hospital, King's Lynn, UK. He qualified from Sidney Sussex College, Cambridge, and the London Hospital College, and trained in London and East Anglia. He has written a number of books for anaesthesia trainees. His research interests include regional anaesthesia and outcome prediction in critical illness.

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PRINCIPLES OF ANAESTHESIA

Examples of commonly prescribed premedications Indication

Drug

Class

Comments

Anxiety, fear

Temazepam

Benzodiazepine

8-hour half-life

Pain

Lorazepam

12-hour half-life. Said to give better amnesia than other benzodiazepines

Diazepam

20–70-hour half-life. May be given rectally

Midazolam

2-hour half-life. Mainly used in children. Intranasal route described but stings

Paracetamol

(Acetaminophen)

Rectal route available

Diclofenac

NSAID

Slow-release or rectal preparations have long duration of action

Morphine

Opioid

Routinely given before cardiac surgery

Cyclizine

Antihistamine/anticholinergic

An effective antiemetic. Causes dry mouth, confusion in elderly

Metoclopramide

Similar to phenothiazine

Accelerates gastric emptying. Can cause oculogyric crisis

Ischaemic heart disease

Atenolol

β-adrenoceptor blocker

Decreases risk of perioperative myocardial ischaemia. Also used to attenuate sympathetic reflexes

Intraoperative bradycardia

Atropine

Anticholinergic (tertiary ammonium)

Some antiemetic effect. Tachyarrhythmias. Dry mouth. Sedation, especially in the elderly

Hyoscine

Anticholinergic (tertiary ammonium)

Antiemetic, amnesic, less action on heart rate than atropine

Glycopyrronium

Anticholinergic (quaternary ammonium)

Does not cross blood–brain barrier, not sedative or amnesic

Atropine, hyoscine, glycopyrronium

See above

See above

Ranitidine

H2-receptor antagonist

Well absorbed orally. Can be given intravenously

Sodium citrate

Antacid

Non-particulate so less harmful if inhaled

Metoclopramide

See above

See above

Nausea and vomiting

Antisialogogues Risk of acid reflux and aspiration

1

Pre-emptive analgesia The sensation of pain is transmitted from the peripheries by Aδ- and C-neurons, which terminate in the dorsal horn of the spinal cord and synapse with second-order neurons which ascend to higher centres in the contralateral spinothalamic and spinoparabrachial pathways. In the dorsal horn, other neurons act at the synapse to influence pain transmission. There is thought to be a positive feedback system (‘wind up’) responsible for maintaining pain sensation after peripheral nociceptors have returned to their resting state. Furthermore, by administering analgesics before the painful insult, it is thought that this phenomenon can be inhibited. There is strong evidence that analgesic premedication decreases postoperative analgesic requirements. Long-acting analgesic premedications are most suitable to provide background analgesia in the immediate postoperative period. A similar effect is obtained by performing peripheral nerve blocks or regional anaesthesia before surgery.

SURGERY

Antiemesis Nausea and vomiting are very common after anaesthesia. Women, patients with a history of motion sickness or those who are undergoing bowel or gynaecological surgery are most at risk. However, pre-emptive oral administration of antiemetics is less effective than intraoperative intravenous administration. Drugs acting on the stomach Patients with a full stomach, hiatus hernia or with a gravid uterus are at risk of vomiting or regurgitation at induction of anaesthesia. Subsequent aspiration of acid gastric contents causes pneumonitis, acute respiratory distress syndrome and hypoxia. Ideally, the stomach should be emptied, but if this is not possible or practical, premedication may help. As Wheeler describes (page 55), metoclopramide enhances gastric emptying and increases lower oesophageal sphincter tone. Ranitidine and sodium citrate act by different mechanisms to raise gastric pH,

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PRINCIPLES OF ANAESTHESIA

thus making the stomach contents less irritant. These drugs are routinely prescribed before Caesarean section.

Care and Monitoring of the Anaesthetized Patient Including the Prevention of Injuries

Attenuation of autonomic reflexes Traction on the epiglottis at laryngoscopy, rectus medialis muscle during squint surgery (known as the oculocardiac reflex), or peritoneum can cause vagal bradycardia or even cardiac standstill. Children have high parasympathetic tone and so are at particular risk. Some anaesthetists prescribe oral or intramuscular atropine as a premedication for its antimuscarinic effects; others administer other, shorter acting drugs intraoperatively. Laryngoscopy, intubation and surgical incision can cause marked sympathoadrenal activity and release of catecholamines, with detrimental effects in patients with coronary artery disease or raised intracranial or intraocular pressure. Perioperative administration of a β-adrenoceptor antagonist or an opioid (with supplemental oxygen) reduces the risk of myocardial ischaemia.

Mark R Stoker

Effective anaesthetic care aims to reduce the morbidity, mortality and complication rates from surgery. General anaesthesia renders the patient in a suitable state for surgery but, by interfering with control of vegetative functions, has many side effects. Sedation is often accomplished using lower doses of the same drugs used for general anaesthesia and forms part of a smooth continuum of increasing depth of general anaesthesia. The sedated patient should therefore be monitored just as diligently. Control of the airway is lost in general anaesthesia or ‘deep’ sedation by abolition of the protective reflexes which normally prevent aspiration. Control of breathing is impaired, leading to hypoventilation. Circulatory control is similarly blunted, with a tendency to vasodilatation whether general or regional anaesthesia is employed. Under general or regional anaesthesia, reflexes protecting the patient from noxious events are abolished (e.g. blinking, limb withdrawal, movement to relieve uncomfortable pressure points). Control of body temperature is impaired by both regional and general anaesthesia.

Antisialogogues Antimuscarinic drugs also reduce secretions. With modern anaesthetic drugs secretions are rarely a problem, but they can be troublesome in children, during awake fibre-optic intubation, or during anaesthesia with ketamine. Prophylaxis against disease Prophylactic antibiotics are normally given intraoperatively, except for patients with structural cardiac or valvular defects or mechanical valve prostheses who should receive antibiotics as a premedication. The patient’s normal drugs Patients should almost always take their normal drugs preoperatively. Omitting treatment for angina or hypertension can cause rebound effects causing the patient pain and distress, and possibly leading to postponement of their operation. Most drugs can be given with a sip of water during the preoperative fasting period. Otherwise, the rectal, intramuscular or intravenous route should be considered, with any appropriate dose adjustment to account for the difference in formulation and bioavailability of the alternative. Some treatments may require significant modification, for instance diabetic, steroid or anticoagulant regimes.

Role of the anaesthetist The anaesthetist is primarily a ‘physiology policeman’, a role that encompasses five broad functions (Figure 1). Good anaesthetic care should maximize patient safety, prevent harm from factors other than surgery and minimize the perturbation of patient physiology by treating heat loss and haemorrhage. By treating pain with analgesics and regional anaesthetic techniques, the metabolic response to surgery can be reduced.

Preoperative preparation for induction of anaesthesia

Why are so few pre-meds prescribed? The number of patients receiving premedication is declining. Anaesthetists are reluctant to prescribe drugs that can cause respiratory depression and hypoxia on the ward, especially with the increased incidence of cardiovascular co-morbidity amongst surgical patients. Most of the objectives of premedication can be achieved in the anaesthetic room by giving drugs intravenously. This route is more reliable and predictable than the oral route, and drugs with short half-lives can be given so that there are no hangover effects postoperatively. From a practical point of view, list order is often changed and patients cancelled, so patients may receive premedication then not be called to theatre, or vice versa. In modern anaesthetic practice, premedication is most usually for anxiolysis in children, or analgesia before cardiac and day surgery. u

SURGERY

The process of anaesthetic care starts at the preoperative visit. While anaesthesia and surgery are day-to-day activities for surgeons and anaesthetists, the prospect of these same procedures may be terrifying for the patient. Surveys have shown that a large proportion of UK patients fear both awareness and death during anaesthesia, both of which are now extremely rare events. Determination of the patient’s expectations is vital if misunderstanding and the litigation arising from this is to be avoided. In particular, it is probably unrealistic for the majority

Mark R Stoker is a Consultant Anaesthetist to Peterborough Hospitals NHS Trust, Peterborough, UK. He qualified from Oxford University and trained in anaesthesia in High Wycombe, Oxford, Cambridge, Peterborough and Norwich. His main interest is education and training.

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