General anaesthesia

General anaesthesia

Induction of anaesthesia
Intravenous injection is most common in contemporary practice, although the recent introduction of nonpungent sevoflurane has led to renewed use of Inhalational induction.
Inhalational induction is useful in young (children, or ‘ needle phobic ’ adults, and may also be used in patients who are at risk of airway obstruction) (of, for example, blood) when the patient is put into the lateral position with head down tilt to drain the fluid away from the trachea.

For intravenous induction, propofol with its rapid recovery is replacing the short-standing barbiturate agent, thiopentone. .
Analgesic agents are frequently also injected at the time of anaesthetic induction.

Maintenance of anaesthesia

Following the induction of anaesthesia, inhalational volatile or intravenous (anaesthetic agents) are continuously administered to maintain an adequate depth of anaesthesia.
Adding nitrous oxide contributes analgesic and weak anaesthetic effects, which reduce the concentration of volatile anaesthetic agent required for maintenance, to provide a safety margin, at least 30 per cent oxygen is added to the inspired mixture.

Ether has generally been replaced by halothane, and isoflurane.

Desflurane and sevoflurane are the most recently introduced agents, conferring the advantages of fewer side effects and more rapid recovery. The use of nitrous oxide is slowly waning, as oxygen-enriched air with volatile or intravenous maintenance gains popularity.

Type of ventilation

If compressed sources of oxygen, nitrous oxide or air are scarce, then air may be drawn into the anaesthetic circuit, either by the (unparalysed) patient ’ s own respiratory effort or by a mechanical ventilator.
(Total intravenous anaesthesia), a technique avoids the use of inhalational anaesthetic agents and is claimed to provide enhanced quality and rapidity of recovery.


I.V anesthesia
It is also used when inhalational anaesthesia may be impractical, such as during 1.airway laser surgery or 2.endoscopy, and is popular for 3.cardiopulmonary bypass. It is also indicated in 4.spinal surgery during neurophysiological monitoring of cord integrity, as evoked potential signals are suppressed by inhalational anaesthesia.

Intravenous anaesthesia avoids atmospheric pollution, and is usually conducted by infusing propofol and a short acting opioid analgesic agent, such as fentanyl or alfentanil, in combination with neuromuscular block and pulmonary ventilation with a mixture of air and oxygen.

Management of the airway during anaesthesia

General anaesthesia reduces the tone of the muscles required to preserve airway patency, and hence the need for manual methods (e.g. jaw thrust), or devices such as the Guedel or laryngeal mask airways, or endotracheal tubes. Sir Ivan Magill developed the endotracheal tube during World War I to facilitate plastic surgery around the mouth without a face mask. The addition of a cuff to the tube allowed a seal of the trachea to protect the lungs from aspiration of blood or secretions, and later mechanical positive pressure pulmonary ventilation. The following means of airway control in the anaesthetised or unconscious patient are used

OPEND AIRWAY

Positioning of the tongue and jaw
The anaesthetist thrusts the jaw forward, from behind the temporomandibular joints, thereby elevating the tongue off the posterior pharyngeal wall, which may also be achieved by inserting an artificial oropharyngeal airway such as the ‘ Guedel ’ . The anaesthetic gases are given through a face mask.

The laryngeal mask airway (LMA)

LMA is also inserted via the mouth, and is positioned with the mask over the larynx, sealed by an inflatable cuff. It frees the anaesthetist ’ s hands from holding the patient ’ s jaw or face mask. Its placement is less stimulating than endotracheal intubation. It has proved to be a reliable means of maintaining a patent airway, and is a technique readily taught to non anaesthetists for emergency airway management. It is likely to replace the face mask for immediate care prior to endotracheal intubation.

ENDOTREACHEAL TUBE

The endotracheal tube may be passed into the trachea via either the mouth or the nose. It is usually placed by direct laryngoscopy, using a laryngoscope, but it is occasionally impossible to visualise the larynx. A fibreoptic technique may be used in which the tracheal tube is ‘ rail-roaded ’ over the flexible laryngoscope, once the tip has been steered into the trachea. A cuffed endotracheal tube is used to facilitate artificial ventilation or surgery around the face or airway, and to protect the lungs if there is a risk of pulmonary aspiration. If fluid may collect in the mouth from above (as in nasal surgery), a throat pack is placed in the oropharynx.

Complication of ETT

Although endotracheal intubation is generally straight forward, complications do occur:
accidental and unrecognised oesophageal intubation;
accidental intubation of a main bronchus;
trauma to larynx, trachea or teeth;
aspiration of vomitus during neuromuscular blockade for intubation;
failure to intubate and loss of airway control; •disconnection or blockage of the tube;
delayed tracheal stenosis, in children or after prolonged intubation.


MONITORING THE ENTUBATION
Careful observation of physical signs and constant vigilance, aided by pulse oximetry, capnography of the expiratory gases, inspiratory oxygen concentration measurement and ventilator disconnection alarms are mandatory to minimise these risks.