Hypoxaemia during induction of general anaesthesia in pregnant women – a surrogate for overall airway difficulty?

Abstract

Obstetric general anaesthesia continues to present unique challenges as a result of factors that may include: anatomical and physiological changes of pregnancy; indications for surgery based on the wellbeing of the fetus; clinical urgency; and remote locations of obstetric operating rooms. It is clear that the incidence of failed intubation is greater in obstetric than non-obstetric practice, even if the definition of this outcome is surprisingly variable.¹ Difficult intubation is also defined variably² and potentially open to even more subjectivity. Is there an objective marker that can indicate a change in a woman’s physical status resulting from a failure to speedily position a tracheal tube and commence lung ventilation? One answer, when we consider the usual sequence of general anaesthetic induction–muscle paralysis–apnoea, is that of hypoxaemia at induction measured by pulse oximetry. The manoeuvres that are taken to avoid this include: maximising oxygen stored in the lungs; rapidly-acting drugs to allow intubation as soon as possible after anaesthetic induction; and, in some cases, facemask ventilation of the lungs (see below). Although brief mild hypoxaemia is not harmful in itself, it is clear that this can progress to organ damage if it is severe and prolonged. Hypoxaemia (SpO₂ ≤ 95%) has been found to be associated with difficult intubation defined as multiple attempts at intubation.³ Predictors of hypoxaemia (SpO₂ < 90%) have been investigated⁴ as well as the association of hypoxaemia with hypertensive disorders of pregnancy.⁵