【Anesthesia hot spot】Prediction of difficult mask ventilation

This case illustrates several priorities
of airway management.
Since there is currently no technique for airway examination alone, preoperative airway examination must evaluate several parameters
.
In this patient, difficult mask ventilation should be predicted
.
Mask ventilation can be obtained using the oropharyngeal airway and improves dysventilation after induction of anesthesia
.
In this case, direct laryngoscopic cannula is difficult and requires the use of a flexible fiberoptic laryngoscope
.
Rapid and proficient use of this technique reduces the occurrence of
airway adverse events.
In this example, mask ventilation up to laryngeal mask
insertion was not possible due to premature extubation.
The importance of the airway during anesthesia recovery is often overlooked, even if difficult airways are known
.
To avoid bleeding and excessive choking, extubation
under deep anesthesia is usually desirable for head and neck surgery.
However, depth of anesthesia is difficult to predict, and extubation under deep anesthesia usually requires careful planning to avoid serious consequences
.
If the endotracheal tube is removed before the patient is fully awake, there is a risk of laryngospasm and upper airway obstruction, especially in patients with obstructive sleep apnea
.

Airway management is an important responsibility
reserved for anesthesiologists.
It is also the most common cause
of adverse outcomes in the administration of anesthesia.
In the past 15 years, research on airway management and related technical equipment have developed
considerably.
Management of difficult airways is usually as important
as endotracheal intubation.
Three authoritative publications focusing on difficult airways reported that the incidence of difficult mask ventilation was 0.
07%~1.
4%.

However, research on difficult mask ventilation is rare
.
Mask ventilation is the premier technique for post-anesthesia ventilation, so its importance is self-evident, and it has changed little
over the past few decades.
Management of difficult airways should be strictly divided into mask ventilation and endotracheal intubation, as the two techniques are not the same
in application.

Preoperative airway examination should evaluate mask ventilation and endotracheal intubation
.
Two studies estimated that the proportion of mask ventilation difficulty was between 1.
56%~5%, and the probability of not being able to ventilate with a mask was 1/1500~1/600
.
In both studies, the predictors of dyspnea were consistent: (1) history of sleep apnea; (2) beards; (3) Age greater than 55~57 years old; (4) Body mass index> (26~30) kg/m²
.
Other inconsistent predictors are: (1) mandibular protrusion; (2) Lack of teeth; (3) The distance between the nail and chin is less than 6cm
.
The different findings in the two studies may be secondary variables
in assessing difficult mask ventilation.

The American Society of Anesthesiologists' implementation guidelines for difficult airway management provide for difficult mask ventilation as follows:

1.
For patients with oxygen saturation greater than 90% before induction of anesthesia, anesthesiologists cannot operate
independently if 100% pure oxygen is used to maintain their arterial oxygen saturation greater than 90%.

2.
It is not possible for the anesthesiologist alone to prevent or reverse hypoventilation
during positive pressure ventilation.
Signs of hypoventilation include cyanosis, low end-expiratory carbon dioxide levels, weak breath sounds and thoracic movements, and haemodynamic changes
associated with hypoxia and hypercapnia.
(Table 28.
1)

Table 28.
1 Preoperative airway assessment

British Difficult Airway Association guidelines focus on unpredictable difficult intubation, and there is little
information on mask ventilation.
Following the publication of difficult airway guidelines by the American Society of Anesthesiologists in 1993, subsequent studies of difficult mask ventilation adopted a more reasonable definition of difficult mask ventilation, which increased arterial oxygen saturation to a low limit of 92% and proposed a rating system
for mask ventilation.
A stricter definition of difficult mask ventilation has the advantage of selective early intervention to reduce the risk of adverse outcomes (Table 28.
2).

Table 28.
2 Mask ventilation grading scale

Mask ventilation is only one part of
airway management.
However, mask ventilation is particularly important
because it is the first technique after the patient's loss of consciousness.
If the mask is adequately ventilated, there is time to change airway management techniques
, even if intubation is difficult.
Hypoventilation leads to hypoxaemia, reducing the time available
to change ventilation techniques.
Therefore, airway assessment to predict difficult mask ventilation is important
for obtaining other ventilation devices and techniques.

The supralaryngeal airway, particularly the laryngeal mask, changes the definition of
difficult ventilation.
Because the superior laryngeal airway is particularly effective for ventilation in difficult situations, difficult ventilation is defined as the inability to establish ventilation
with the superior laryngeal airway (rather than a mask).
Anesthesiologists must be highly skilled in the use of laryngeal masks, and anesthesia training programs are responsible for ensuring that every student is proficient in the use of
laryngeal masks.
Learning from laryngeal mask application is divided into two stages
.
The first stage requires the use of a laryngeal mask 50~75 times so that the user can learn the principle of the laryngeal mask and establish ventilation in healthy patients
.
The second stage requires the use of a laryngeal mask hundreds of times, improving skills to manage difficult airways
.