The perioperative period may be divided into three phases and in each of these a variety of factors will determine the choice of drugs given:
2 Frederick Churchill, a butler from Harley Street, had his leg amputated at University College Hospital, London. After removing the leg in 28 seconds, a skill necessary to compensate for the previous lack of anaesthetics, Robert Liston turned to the watching students, and said "this Yankee dodge, gentlemen, beats mesmerism hollow". That night he anaesthetised his house surgeon in the presence of two ladies. Merrington W R1976 University College Hospital and its Medical School: A History. Heinemann, London.
Before surgery, an assessment is made of:
• the patient's physical and psychological condition
• any intercurrent illness
• the relevance of any existing drug therapy.
All of these may influence the choice of anaesthetic drugs.
During surgery, drugs will be required to provide:
• muscular relaxation when necessary
• control of blood pressure, heart rate, and respiration.
After surgery, drugs will play a part in:
• reversal of neuromuscular block
• relief of pain, and nausea and vomiting
• other aspects of postoperative care, including intensive care.
Patients are often already taking drugs affecting the central nervous and cardiovascular systems and there is considerable potential for interaction with anaesthetic drugs.
The techniques for giving anaesthetic drugs and the control of ventilation and oxygenation are of great importance, but are outside the scope of this book.
Before surgery (premedication)
The principal aims are to provide:
Anxiolysis and amnesia. A patient who is going to have a surgical operation is naturally apprehensive and this anxiety is reduced by reassurance and a clear explanation of what to expect. Very anxious patients will secrete a lot of adrenaline (epinephrine) from the suprarenal medulla and this may make them more liable to cardiac arrhythmias with some anaesthetics. In the past, sedative premedication was given to virtually all patients undergoing surgery. This practice has changed dramatically because of the increasing proportion of operations undertaken as 'day cases' and the recognition that sedative premedication prolongs recovery. Sedative premedication is now reserved for those who are particularly anxious or those undergoing major surgery.
Benzodiazepines, such as temazepam (10-30 mg for an adult), provide anxiolysis and amnesia for the immediate presurgical period.
Analgesia is indicated if the patient is in pain preoperatively or it can be given pre-emptively to prevent postoperative pain. Severe preoperative pain is treated with a parenteral opioid such as morphine. Nonsteroidal anti-inflammatory drugs and paracetamol are commonly given orally preoperatively to prevent postoperative pain after minor surgery. For moderate or major surgery, these drugs are supplemented with an opioid towards the end of the procedure.
Drying of bronchial and salivary secretions using antimuscarinic drugs to inhibit the parasympathetic autonomic system is rarely undertaken these days. The exceptions include those patients who are expected to require an awake fibreoptic intubation or those undergoing bronchoscopy. Glycopyrronium is the antimuscarinic of choice for this purpose and atropine and hyoscine are alternatives.
Timing. Premedication is given about an hour before surgery.
Gastric contents. Pulmonary aspiration of gastric contents can cause severe pneumonitis. Patients at risk of aspiration are those with full stomachs, e.g., bowel obstruction, recently consumed food and drink, third trimester of pregnancy, and those with incompetent gastro-oesophageal sphincters, e.g. hiatus hernia. A single dose of an antacid, e.g. sodium citrate, may be given before a general anaesthetic to neutralise gastric acid in high-risk patients. Alternatively or additionally, a histamine H2-receptor blocker, e.g. ranitidine, or proton-pump inhibitor, e.g. omeprazole, will reduce gastric secretion volume as well as acidity. Metoclopramide usefully hastens gastric emptying, increases the tone of the lower oesophageal sphincter and is an antiemetic.
The aim is to induce unconsciousness, analgesia and muscular relaxation. Total muscular relaxation
(paralysis) is required for some surgical procedures, e.g., intra-abdominal surgery, but most surgery can be undertaken without neuromuscular blockade.
A typical general anaesthetic consists of:
1. Usually intravenous: pre-oxygenation followed by a small dose of an opioid, e.g., fentanyl or alfentanil to provide analgesia and sedation, followed by propofol or, less commonly, thiopental or etomidate to induce anaesthesia. Airway patency is maintained with an oral airway and face-mask, a laryngeal mask airway (LMA), or a tracheal tube. Insertion of a tracheal tube usually requires paralysis with a neuromuscular blocker and is undertaken if there is a risk of pulmonary aspiration from regurgitated gastric contents or from blood.
2. Inhalational induction, usually with sevo-flurane, is undertaken taken less commonly. It is used in children, particularly if intravenous access is difficult, and in patients at risk from upper airway obstruction.
1. Most commonly with nitrous oxide and oxygen, or oxygen and air, plus a volatile agent, e.g., isoflurane or sevoflurane. Additional doses of a neuromuscular blocker or opioid are given as required.
2. A continuous intravenous infusion of propofol can be used to maintain anaesthesia. This technique of total intravenous anaesthesia is becoming more popular because the quality of recovery may be better than after inhalational anaesthesia.
When appropriate, peripheral nerve block with a local anaesthetic, or neural axis block, e.g., spinal or epidural, provides intraoperative analgesia and muscle relaxation. These local anaesthetic techniques provide excellent postoperative analgesia.
The anaesthetist ensures that the effects of neuromuscular blocking agents and opioid-induced respiratory depression have either worn off or have been adequately reversed by an antagonist; the patient is not left alone until conscious, with protective reflexes restored, and a stable circulation.
Relief of pain after surgery can be achieved with a variety of techniques. An epidural infusion of a mixture of local anaesthetic and opioid provides excellent pain relief after major surgery such as laparotomy. Parenteral morphine, given intermittently by a nurse or by a patient-controlled system, will also relieve moderate or severe pain but has the attendant risk of nausea, vomiting, sedation and respiratory depression. The addition of regular paracetamol and a NSAID, given orally or rectally, will provide additional pain relief and reduce the requirement for morphine. NSAIDs are contra-indicated if there is a history of gastrointestinal ulceration of if renal blood flow is compromised.
Postoperative nausea and vomiting (PONV) is common after laparotomy and major gynaecological surgery, e.g., abdominal hysterectomy. The use of propofol, particularly when given to maintain anaesthesia, has dramatically reduced the incidence of PONV. Antiemetics, such as cyclizine, metoclo-pramide, and ondansetron, may be helpful.
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Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...