18. INDUCTION OF ANAESTHESIA

The aim of general anaesthesia is to maintain the health and safety of the patient, produce amnesia and analgesia, and provide optimal surgical conditions.

Induction of anaesthesia produces an unconscious patient. Reflexes are depressed. The patient is entirely dependent on the anaesthetist for their safety. Most complications occur during induction and extubation. Problems include hypotension, arrhythmias, hypoventilation, apnoea, hypoxia, aspiration, laryngospasm and adverse drug reactions.

Preoperative Assessment

All patients must have a complete preoperative assessment (history, health, airway, fasting, premedication and consent). Even emergency cases can be assessed while resuscitating the patient and preparing for the anaesthetic.

The choice of anaesthesia will depend on the patient’s medical condition, patient’s preference, surgery required, drugs and equipment available, and the experience and preference of the anaesthetist.

Anaesthetic Plan

After assessing the patient, the anaesthetist must decide on a plan for the anaesthetic. The anaesthetist must also plan for anticipated problems with the anaesthetic (secondary plan). For example, if a nerve block fails the anaesthetist must be prepared to provide general anaesthesia. Finally, the anaesthetist must be prepared for any major complication that is not anticipated such as failed intubation, anaphylaxis, severe hypotension or arrhythmias.

Before inducing anaesthesia the anaesthetist must check equipment, drugs, staff and monitoring. Anaesthesia must never be started before all preparation is complete.

Operating Room

The operating room should be warm and quiet. The patient should be positioned lying down on a firm operating table at a height that is comfortable for the anaesthetist. It should be possible to quickly tilt the operating table head down if required. The patient’s head should be resting on a low firm pillow. The anaesthetist should have an assistant who is trained to help. The assistant should have no other duties. The assistant’s only job should be to assist the anaesthetist.

Monitoring

Before inducing anaesthesia the anaesthetist should attach any monitoring. All patients must have their respiratory rate, blood pressure and pulse recorded by the anaesthetist at least every five minutes. The anaesthetist can also monitor the skin colour (cyanosis, anaemia), sweating and dilatation of the eyes, peripheral blood supply (capillary return), temperature, blood loss and urine output. Monitoring machines (e.g. ECG, pulse oximeter) should be used when available. Standard monitoring in a major teaching hospital would include the continuous presence of the anaesthetist, ECG, non-invasive blood pressure, pulse oximeter, end tidal carbon dioxide, temperature, inspired and expired oxygen concentration, inspired and expired inhalation agent concentration, MAC, degree of neuromuscular blockade, respiratory rate, airway pressure as well as ventilator settings. More complicated patients may have central venous pressure and invasive arterial pressure or other monitors.

Drugs

All drugs that the anaesthetist plans to use should be drawn up before inducing anaesthesia and carefully labelled. For difficult cases, or if the assistant is inexperienced, the anaesthetist may draw up some emergency drugs such as suxamethonium, atropine and a vasopressor before starting the anaesthetic. These emergency drugs must be very clearly labelled and kept away from the anaesthetic drugs. It may be wise to label emergency drugs with a different colour (e.g. red).

Intravenous Induction

Intravenous induction of anaesthesia is fast, pleasant for the patient and easy. After pre-oxygenation the induction agent should be given slowly until the patient can no longer keep his/her eyes open and the eyelash reflex is lost. (The dose of intravenous induction agent may be greatly reduced for patients who have lost large amounts of blood. The dose should also be reduced in the elderly). The patient will become apnoeic and their airway will become obstructed. The anaesthetist must have airway equipment and be skilled in airway management. People who are not skilled with airway management must not use intravenous induction agents.

Inhalational Induction

Inhalational induction of anaesthesia will maintain spontaneous ventilation. Patients with a difficult or unstable airway may be safely managed with inhalation induction of anaesthesia. If the patient’s airway starts to become obstructed, the anaesthetist can decide if they need to stop the anaesthetic and give 100% oxygen before complete obstruction of the airway occurs.

Inhalational induction of anaesthesia does not protect the patient against the risk of aspiration of gastric contents.

Inhalational induction can be performed by having the patient breathe a low concentration of the inhalation agent (e.g. halothane, sevoflurane, ether) and then increasing the concentration by 0.5% every 4 to 5 breaths until the required induction concentration is reached. Once the patient is asleep the anaesthetist must remember to turn the inhalation agent concentration down to the correct maintenance concentration.

Anaesthesia can also be induced by using a single breath. The anaesthetist needs to fill the breathing system and reservoir bag with 4 to 5% halothane or 5 to 8% sevoflurane. Ideally the anaesthetist needs a 5-litre reservoir bag. The patient is instructed to blow all the air out of their lungs. The mask is placed over the patient’s face and they inhale as deeply as possible and hold their breath for as long as possible. (It is wise to practice a few times with the patient using oxygen to ensure the patient understands).

Once unconscious, from whatever cause (e.g. anaesthesia, head injury, critical illness or trauma) the anaesthetist is responsible for the total care of the patient.

Positioning

Position the patient gently and carefully. Joints, tendons and muscle may be damaged by over extension. Be very careful with the elderly who may have osteoporotic bones and less muscle bulk. Patients with rheumatoid arthitis may have cervical spine instability. Aggressive movement of the cervical spine may cause spinal cord injury.

The table and arm boards must be padded to prevent pressure sores.

Tourniquets can cause nerve damage. Always record the application time of tourniquets. Two hours is a common maximum recommendation. Arm tourniquets should be inflated to the systolic blood pressure plus 50 mmHg (plus 100 mmHg for intravenous regional anaesthesia) and leg tourniquets to 2 times systolic blood pressure. Be careful in the elderly, diabetic and patients with peripheral vascular disease.

Incorrect positioning and overextension can cause peripheral nerve damage. Nerves can be stretched or compressed. The damage may be temporary (recovery in 6 weeks) or permanent.

Supine position

The brachial plexus is usually stretched by shoulder abduction and extension with supination. The stretch is made worse if both shoulders are abducted. The shoulders should not be abducted more than 90 degrees and not extended. The soft tissues in the axilla should be loose. If both arms are abducted keep the head facing forwards. If only one arm is abducted face the head towards that arm. The ulnar nerve may be compressed between the humerus and the operating table or trapped in the cubital tunnel by acute flexion of the elbow. The radial nerve can be damaged if the patient’s arm is hanging over the side of the operating table. The legs should lie flat and uncrossed in the supine position. Pressure on the eye can cause arterial haemorrhage and retinal ischaemia. The eyes should be gently taped shut to avoid corneal abrasions. The supraorbital nerve can be compressed by a tight facemask. This will cause photophobia, pain in the eye and numbness of the forehead.

Head Down (Trendelenberg)

The patient is supine with a head down tilt. This has a number of physiological effects including increased venous return, increased intracranial pressure and increased intraocular pressure. The contents of the abdomen displace the diaphagm reducing lung compliance and functional residual capacity, especially in obese patients. This may cause hypoxia. There is increased intragastric pressure and an increased risk of regurgitation of gastric contents

Head Up (Reverse Trendelenburg)

Reduced venous return may lead to a fall in cardiac output and blood pressure.

Face Down (Prone)

This position can be especially dangerous. Disconnections or accidents can occur while turning the patient and the anaesthetist has limited access to the airway. A sufficient number of people are required to turn a patient prone. The anaesthetist must control the head and co-ordinate the turning team. It is usually easier to anaesthetise the patient on their bed and then turn them prone onto the operating table. At the end of the operation the patient is turned supine onto their bed then extubated. A well secured armoured endotracheal tube is most suitable. Chest wall and abdominal movement during respiration may be reduced. Supports (e.g. pillows) should be placed under the iliac crests and shoulders. The face and eyes must be carefully padded. Be certain that the neck is not over-extended or rotated. The axillary nerve may be stretched when the shoulders are extended and arms placed above the head.

Legs Up (lithotomy)

The common peroneal nerve may be compressed between the lithotomy pole and the fibular head. Both legs should be moved together to avoid strain on the pelvic ligaments. Two people should move the legs. One hand should be behind the knee to prevent hyperextension injuries. The lithotomy position is often combined with the Trendelenburg position.

Anaesthetised patients cannot protect themselves from trauma or burns. Objects should not be unnecessarily passed over the patient. Hot liquid and equipment must be kept away. The diathermy plate must be correctly applied and the site checked after surgery.