Airway Assessment

INCIDENCE OF IMPOSSIBLE MASK VENTILATION
Impossible or grade 4 mask ventilation is a rare occurrence with the incidence reported
between 0.07 per cent and 1.4 per cent 10,12,13 . Kheterpal and colleagues reported an incidence
of 0.15 per cent (one in 690 patients) 11 . This included 77 cases of impossible mask ventilation
in 53,041 patients. They identified five independent predictors of impossible mask ventilation
(Table 10).
Table 10. Predictors of impossible mask ventilation as identified by Kheterpal and
colleagues.
Predictors of impossible mask ventilation
1 Neck radiation changes
2 Male gender
3 Obstructive sleep apnoea
4 Mallampati 3 or 4
5 Presence of a beard
Of the 77 patients who were impossible to bag-mask ventilate, 19 (25 per cent) were also
reportedly difficult to intubate. Of these, 15 were successfully intubated. Two patients required
surgical airways; two were woken up and had an awake, fibreoptic intubation. In six cases an
attempt was made to insert a non-intubating LMA. All but four of the 77 received neuromuscular
blockade (65 patients receiving succinylcholine and a non-depolarising neuromuscular junction
blocker used in the remaining eight cases).
Again, many of the patients predicted to be difficult would have been excluded from this study.
They would have had an awake fibreoptic intubation, a regional procedure or surgical procedure
under sedation with monitored anaesthesia care. Thus the incidence of 0.15 per cent could be
described as conservative. This highlights the fact that even with a relatively low threshold for
managing the airway by an AFOI technique, unanticipated difficult facemask ventilation remains
a clinical problem.
Kheterpal and colleagues highlighted the importance of reduced mandibular protrusion as an
important predictor of both difficult mask ventilation and difficult intubation 18 .
DIFFICULT LARYNGEAL MASK VENTILATION
The laryngeal mask airway (LMA) is a well-established means by which the airway is managed
in anaesthesia practice for both spontaneous and controlled ventilation. It has been shown to
have a high success rate and low complication rate. The LMA has also been shown to be a
useful adjuvant in cases of failed intubation 21 . The “classic” LMA has also been reported to be
useful in managing the unanticipated difficult airway in obese patients 14,22 .
DEFINITION
Difficult laryngeal mask ventilation has been defined as the inability to, within three insertions,
place the laryngeal mask in a satisfactory position to allow clinically adequate ventilation and
airway patency. Clinically adequate ventilation was defined as greater than 7ml.kg -1 with a leak
pressure no greater than 15-20cm H 2
O pressure.
INCIDENCE
The incidence of difficult LMA insertion has been reported to be between 0.16 per cent and
0.9 per cent 21 .
13 Airway Assessment