Effect of induction on the airway
The induction of general anaesthesia or sedation tends to result in hypoxia because of:
Airway obstruction. Maintenance of a patent upper airway is a complex physiological mechanism. Even minor depression of consciousness can result in obstruction.
Decreased cardiac output.
Mechanism of airway obstruction
It is a usual practice to induce anaesthesia with the patient supine and nearly horizontal, which is the least favourable position for airway patency, but the most convenient for intervention. Relaxation of the musculature of the pharynx, face and neck allows the soft palate, tongue and glottic opening to approach the posterior nasopharyngeal wall. The greatest contribution to obstruction is believed to be at glottic and velo-palatine level.
Signs of airway obstruction
An obstructed upper airway results in increases in negative intrathoracic pressure during inspiration. This causes tracheal ‘tug’, intercostal and supra-clavicular recession and paradoxical or ‘seesaw’ breathing (if the airway is obstructed the chest sinks in and the abdomen rises – the opposite of normal).
Turbulent flow may occur in the upper airway, resulting in noise. The noise is worse on inspiration. The characteristic timbre of snoring (palatal, tongue and peri-glottic level obstruction) and stridor (glottic and tracheal level) are readily recognized with experience. Absence of noise does not mean that the airway is not critically narrowed and a completely obstructed airway is silent. Auscultation of the neck to assess airway patency is a valuable examination.
Secretions or foreign bodies in the airway may cause obstruction. The noise of secretions is characteristic. Foreign bodies in anaesthetic circuits (introduced by accident or malicious design) have resulted in tragedies. If airway obstruction occurs, the anaesthetist must ensure that the circuit and any artificial airway is patent.