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Overview br General anaesthesia is a complex procedure involving br Preanaesthetic assessments br Administration of general anaesthetic drugs br Cardiorespiratory monitoring br Analgesia br Airway management br Fluid management br Postoperative pain relief br Preanaesthetic evaluation br Prior to surgery the anaesthetist interviews the patient to determine the best combination of drugs and dosages and the degree to which monitoring is required to ensure a safe and effective procedure Key factors of this determination are the patient s age weight medical history current medications previous anaesthetics and fasting time Patients are typically required to fill out this information on a separate form during the pre operative evaluation Depending on the existing medical conditions reported the anaesthetist will review this information with the patient either during the pre operative evaluation or on the day of the surgery br Truthful and accurate answering of the questions is important so that the anaesthetist can select the proper anaesthetic drugs and procedures For example a heavy drinker or drug user who does not disclose their chemical uses could be undermedicated which could then lead to anaesthesia awareness or dangerously high blood pressure Commonly used medications such as Viagra can interact with anaesthesia drugs failure to disclose such usage can endanger the patient br An important aspect of this assessment is that of the patient s airway involving inspection of the mouth opening and visualisation of the soft tissues of the pharynx The condition of teeth and location of dental crowns and caps are checked neck flexibility and head extension observed If an endotracheal tube is indicated and airway management is deemed difficult then alternative placement methods such as fibreoptic intubation may be required after induction of anaesthesia br Premedication br Anaesthesiologists may prescribe or administer a sedative pre medication by injection or by mouth anywhere from a couple of hours to a couple of minutes before induction br The most common drugs used for pre medication are narcotics opioids such as fentanyl and sedatives most commonly benzodiazepines such as midazolam br General anaesthesia br General anaesthesia implies loss of consciousness and of protective reflexes General anaesthesia is traditionally described as comprising of 3 components hypnosis relaxation and analgesia br Hypnosis or sleep refers to being deeply asleep unconscious and totally unaware of events br Relaxation implies abolition of reflex muscle tone or specific block of nerve muscle function causing immobility and allowing easy surgical access br Analgesia refers to use of one or more of a wide range of pain reducing drugs from paracetamol to morphine and perhaps local anaesthetics to block pain impulse transmission along nerves in the hope of reducing heart rate and blood pressure responses to surgery br Induction of anaesthesia br The general anaesthetic is administered in either the operating theatre itself or a special ante room br General anaesthesia can be induced by intravenous IV injection or breathing a volatile anaesthetic through a facemask inhalational induction Onset of anaesthesia is faster with IV injection than with inhalation taking about 10 20 seconds to induce total unconsciousness citation needed This has the advantage of avoiding the excitatory phase of anaesthesia see below and thus reduces complications related to induction of anaesthesia An inhalational induction may be chosen by the anesthesiologist where IV access is difficult to obtain where difficulty maintaining the airway is anticipated or due to patient preference e g children Commonly used IV induction agents include propofol sodium thiopental etomidate and ketamine The most commonly used agent for inhalational induction is sevoflurane because it causes less irritation than other inhaled gases br Maintenance br The duration of action of IV induction agents is generally 5 to 10 minutes after which time spontaneous recovery of consciousness will occur In order to prolong anaesthesia for the required duration usually the duration of surgery anaesthesia must be maintained Usually this is achieved by allowing the patient to breathe a carefully controlled mixture of oxygen nitrous oxide and a volatile anaesthetic agent or by having a carefully controlled infusion of medication usually propofol through an IV The inhalation agents are transferred to the patient s brain via the lungs and the bloodstream and the patient remains unconscious Inhaled agents are frequently supplemented by intravenous anaesthetics such as opioids usually fentanyl or a fentanyl derivative and sedative hypnotics usually propofol or midazolam Though for a propofol based anaesthetic supplementation by inhalation agents is not required At the end of surgery the volatile or intravenous anaesthetic is discontinued Recovery of consciousness occurs when the concentration of anaesthetic in the brain drops below a certain level usually within 1 to 30 minutes depending upon the duration of surgery br In the 1990s a novel method of maintaining anaesthesia was developed in Glasgow UK Called TCI target controlled infusion this involves using a computer controlled syringe driver pump to infuse propofol throughout the duration of surgery removing the need for a volatile anaesthetic and allowing pharmacologic principles to more precisely guide amount of infusion of the drug Purported advantages include faster recovery from anaesthesia reduced incidence of post operative nausea and vomiting and absence of a trigger for malignant hyperthermia At present TCI is not permitted in the United States br Other medications will occasionally be given to anaesthetized patients to treat side effects or prevent complications These medications include antihypertensives to treat high blood pressure drugs like ephedrine and phenylephrine to treat low blood pressure drugs like albuterol to treat asthma or laryngospasm bronchospasm and drugs like epinephrine or diphenhydramine to treat allergic reactions Sometimes glucocorticoids or antibiotics are given to prevent inflammation and infection respectively br Muscle relaxation Neuromuscular blockade br Paralysis or temporary muscle relaxation with a neuromuscular blocker is an integral part of modern anaesthesia The first drug used for this purpose was curare introduced in the 1940s which has now been superseded by drugs with fewer side effects and generally shorter duration of action br Muscle relaxation allows surgery within major body cavities eg abdomen and thorax without the need for very deep anaesthesia and is also used to facilitate endotracheal intubation br Acetylcholine the natural neurotransmitter substance at the neuromuscular junction causes muscles to contract when it is released from nerve endings Muscle relaxants work by preventing acetylcholine from attaching to its receptor br Paralysis of the muscles of respiration ie the diaphragm and intercostal muscles of the chest requires that some form of artificial respiration be implemented As the muscles of the larynx are also paralysed the airway usually needs to be protected by means of an endotracheal tube br Monitoring of paralysis is most easily provided by means of a peripheral nerve stimulator This device intermittently sends short electrical pulses through the skin over a peripheral nerve while the contraction of a muscle supplied by that nerve is observed br The effects of muscle relaxants are commonly reversed at the termination of surgery by anticholinesterase drugs br Examples of skeletal muscle relaxants in use today are pancuronium rocuronium vecuronium atracurium mivacurium and succinylcholine br Airway management br With the loss of consciousness caused by general anaesthesia there is loss of protective airway reflexes such as coughing loss of airway patency and sometimes loss of a regular breathing pattern due to the effect of anaesthetics opioids or muscle relaxants To maintain an open airway and regulate breathing within acceptable parameters some form of breathing tube is inserted in the airway after the patient is unconscious To enable mechanical ventilation an endotracheal tube is often used intubation although there are alternative devices such as face masks or laryngeal mask airways br Monitoring br Monitoring involves the use of several technologies to allow for a controlled induction of maintenance of and emergence from general anaesthesia br 1 Continuous Electrocardiography ECG The placement of electrodes which monitor heart rate and rhythm This may also help the anaesthetist to identify early signs of heart ischemia br 2 Continuous pulse oximetry SpO2 The placement of this device usually on one of the fingers allows for early detection of a fall in a patient s haemoglobin saturation with oxygen hypoxemia br 3 Blood Pressure Monitoring NIBP or IBP There are two methods of measuring the patient s blood pressure The first and most common is called non invasive blood pressure NIBP monitoring This involves placing a blood pressure cuff around the patient s arm forearm or leg A blood pressure machine takes blood pressure readings at regular preset intervals throughout the surgery The second method is called invasive blood pressure IBP monitoring This method is reserved for patients with significant heart or lung disease the critically ill major surgery such as cardiac or transplant surgery or when large blood losses are expected The invasive blood pressure monitoring technique involves placing a special type of plastic cannula in the patient s artery usually at the wrist or in the groin br 4 Agent concentration measurement Common anaesthetic machines have meters to measure the percent of inhalational anaesthetic agent used e g sevoflurane isoflurane desflurane halothane etc br 5 Low oxygen alarm Almost all circuits have a backup alarm in case the oxygen delivery to the patient becomes compromised This warns if the fraction of inspired oxygen drops lower than room air 21 and allows the anaesthetist to take immediate remedial action br 6 Circuit disconnect alarm indicates failure of circuit to achieve a given pressure during mechanical ventilation br 7 Carbon dioxide measurement capnography measures the amount of carbon dioxide expired by the patient s lungs It allows the anaesthetist to assess the adequacy of ventilation br 8 Temperature measurement to discern hypothermia or fever and to aid early detection of malignant hyperthermia br 9 EEG or other system to verify depth of anaesthesia may also be used This reduces the likelihood that a patient will be mentally awake although unable to move because of the paralytic agents It also reduces the likelihood of a patient receiving significantly more amnesic drugs than actually necessary to do the job br Stages of anaesthesia br The four stages of anaesthesia were described in 1937 Despite newer anaesthetic agents and delivery techniques which have led to more rapid onset and recovery from anaesthesia with greater safety margins the principles remain br Stage 1 br Stage 1 anaesthesia also known as the induction is the period between the initial administration of the induction medications and loss of consciousness During this stage the patient progresses from analgesia without amnesia to analgesia with amnesia Patients can carry on a conversation at this time br Stage 2 br Stage 2 anaesthesia also known as the excitement stage is the period following loss of consciousness and marked by excited and delirious activity During this stage respirations and heart rate may become irregular In addition there may be uncontrolled movements vomiting breath holding and pupillary dilation Since the combination of spastic movements vomiting and irregular respirations may lead to airway compromise rapidly acting drugs are used to minimize time in this stage and reach stage 3 as fast as possible br Stage 3 br Stage 3 surgical anesthesia During this stage the skeletal muscles relax and the patient s breathing becomes regular Eye movements slow then stop and surgery can begin br It has been divided into 4 planes br rolling eye balls ending with fixed eyeballs br loss of corneal and laryngeal reflexes br pupils dilate and loss of light reflex br intercostal paralysis shallow abdominal respiration dilated pupils br Stage 4 br Stage 4 anesthesia also known as overdose is the stage where too much medication has been given and the patient has severe brain stem or medullary depression This results in a cessation of respiration and potential cardiovascular collapse This stage is lethal without cardiovascular and respiratory support br Postoperative care br Post operative analgesia br The anaesthesia should conclude with a pain free awakening and a management plan for postoperative pain relief This may be in the form of regional analgesia oral transdermal or parenteral medication Minor surgical procedures are amenable to oral pain relief medications such as paracetamol and NSAIDs such as ibuprofen Moderate levels of pain require the addition of mild opiates such as tramadol br Major surgical procedures may require a combination of modalities to confer adequate pain relief Parenteral methods include patient controlled analgesia PCA involving a strong opiate such as morphine fentanyl or oxycodone Here to activate a syringe device the patient presses a button and receives a preset dose or bolus of the drug e g one milligram of morphine The PCA device then locks out for a preset period e g 5 minutes to allow the drug to take effect If the patient becomes too sleepy or sedated they make no more morphine requests This confers a fail safe aspect which is lacking in continuous opiate infusion techniques br Shivering br Shivering is a frequent occurrence in the post operative period Apart from causing discomfort and exacerbating post operative pain shivering has been shown to increase oxygen co

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