General Anaesthesia PDF

GENERAL ANAESTHESIA ‫ صالح جاسم‬.‫د‬ General anesthesia is commonly described as the triad of unconsciousness (loss of awareness), analgesia (pain relieve) and muscle relaxation. Induction of general anesthesia is most frequently done by intrav...

GENERAL ANAESTHESIA ‫ صالح جاسم‬.‫د‬ General anesthesia is commonly described as the triad of unconsciousness (loss of awareness), analgesia (pain relieve) and muscle relaxation. Induction of general anesthesia is most frequently done by intravenous agents. Propofol considered as the most widely used induction agent and can be used for maintenance of anesthesia. Other infrequently used intravenous agents include ketamine. Newer agents based on benzodiazepine receptor agonist, etomidate derivatives and fospropofol are still in the experimental stage. Inhalational induction using agents such as non-pungent sevoflurane is useful in children, needle-phobic adults and those in whom a difficult airway is anticipated. These patients will have a higher risk of developing airway obstruction Another method used in the induction of the General anesthesia is the Rapid sequence induction (RSI) is a technique that allows the airway to be rapidly secured. It is used when there is a high risk of regurgitation that may lead to pulmonary aspiration by using a predetermined dose of intravenous an aesthetic agent together with rapidly acting muscle relaxant is used in those with high risk of regurgitation in order to secure the airway quickly. Commonly needed in emergency surgery, it is also a technique of choice in any non-emergency surgery in a patient with delayed emptying of stomach. 1  However, nowadays the Total intravenous anesthesia (TIVA) is becoming popular following the introduction of propofol and ultra-short acting opioid remifentanil due to 1- The lack of a cumulative effect, 2- better hemodynamic stability 3- excellent recovery profile and concerns over the environmental effects of inhalational agents Total intravenous anesthesia is routinely used in neuro-surgery, in airway laser surgery, during cardiopulmonary bypass and for day-case anesthesia. Management of airway during anesthesia Loss of muscle tone as a result of general anesthesia means that the patient can no longer keep their airway open. Therefore, the patients need their airway maintained for them. The use of muscle relaxants will mean that they will also be unable to breathe for themselves and so will require artificial ventilation. Different approaches used to manage the airway during anesthesia including 1- Chin lift and jaw thrust: suitable for short term when no aid available 2- Guedel airway: holds tongue forward but does not prevent aspiration 3- Laryngeal mask: easy insertion, reliable airway, allows ventilation 4- Endotracheal intubation: secure and protected airway Laryngeal mask airway or endotracheal tube is then inserted and the patient is allowed to breathe spontaneously or is ventilated during the procedure. The addition of a cuff to the endotracheal tube facilitates positive pressure ventilation and protects the lungs from aspiration of regurgitated gastric contents. 2 Laryngeal mask airway (LMA). Developed by Dr Archie Brain in the UK, the mask with an inflatable cuff is inserted via the mouth and produces a seal around the glottic opening, providing a very reliable means of maintaining the airway. Its placement is less irritating and less traumatic to a patient’s airway than endotracheal intubation. The technique can be easily taught to non-anesthetists and paramedics and can be used as an emergency airway management tool. Several varieties of LMA are available including reinforced, I-gel and an intubating LMA that aids endotracheal intubation. Difficult intubation: Endotracheal intubation is feasible in most patients, but in a certain proportion of patients this may be difficult or impossible; if compounded by inability to ventilate the patient by bag-mask, the consequences can be catastrophic hypoxia. Many devices have been developed to aid intubation if difficulty is anticipated and protocols have been created by specialized societies to deal with such situations. The gold standard for intubation in difficult situations is the use of the fiberoptic intubating bronchoscope, facilitated by topical local anesthetic in awake patients or using general anesthesia. The anesthetist places the endotracheal tube in the trachea by threading the tube over the bronchoscope and so places the tube in the trachea under direct bronchoscopic vision. 3 Muscle relaxation and artificial ventilation Pharmacological blockade of neuromuscular transmission provides relaxation of muscles allowing easy surgical access, but the patient requires artificial ventilation. Neuromuscular blocking agents are broadly classified into depolarizing and non-depolarizing groups according to their mode of action. Suxamethonium is the most commonly used depolarizing agent. It binds to the nicotinic acetylcholine receptors resulting in opening of the cation channel leading to depolarization and rapid relaxation of muscles. Despite its adverse effects, such as hyperkalemia, muscle pain, anaphylaxis and potentially lifethreatening malignant hyperthermia, suxamethonium is still widely used because of its quick onset and short duration of action. These properties are useful where rapid endotracheal intubation is necessary to protect the patient’s airway or short duration surgery is performed. Ventilation during anesthesia Mechanical ventilation is required when the patient’s spontaneous ventilation is inadequate or when the patient is not breathing because of the effects of the anesthetic, analgesic agents or muscle relaxants. In volume control ventilation, a preset volume is delivered by the machine irrespective of the airway pressure. The pressure generated will be in part dependent on the resistance and compliance of the airway. In laparoscopic surgery requiring the Trendelenburg position (the patient is positioned head down), and in morbidly obese patients and those with lung disease, this may result in excessive pressures being developed, which may lead to barotrauma (pneumothorax). In pressure control mode, the ventilator generates flow until a preset pressure is reached. The actual tidal volume delivered is variable and depends on airway resistance, intra-abdominal pressure and the degree of relaxation. -The End- 4