EBQ:Incidence and predictors of difficult and impossible mask ventilation

Complete Journal Club Article

Kheterpal S. et al. "Incidence and predictors of difficult and impossible mask ventilation.". Anesthesiology. 2006. 105(5):885-891.
PubMed Full text PDF

Clinical Question

What patient characteristics are associated with difficult mask ventilation?

Conclusion

• Difficult mask ventilation occurs in approximately 1.4% of patients; impossible mask ventilation in 0.16%
• Five independent predictors of difficult mask ventilation were identified: neck radiation, male sex, sleep apnea, Mallampati III/IV, and beard
• Limited neck extension and BMI >26 were additional risk factors in some analyses

Major Points

• This was one of the first large studies to quantify the incidence and predictors of difficult mask ventilation
• Impossible mask ventilation is rare but carries significant risk if not anticipated
• The MOANS mnemonic (Mask seal, Obesity, Age >55, No teeth, Stiffness/snoring) was subsequently derived from similar risk factors
• Knowledge of these predictors is critical for emergency airway management planning
• Impossible mask ventilation was strongly associated with concurrent difficult intubation

Study Design

• Prospective, observational cohort study
• Single center: University of Michigan
• N = 22,660 general anesthesia cases
• Study period: July 2004 - January 2006
• Primary Outcome: incidence and predictors of difficult and impossible mask ventilation

Population

Patient Demographics

• Mean age: 52 years
• Male: 48%
• Mean BMI: 28.4

Inclusion Criteria

• All adult patients undergoing general anesthesia requiring mask ventilation

Exclusion Criteria

• Pediatric patients
• Cases where mask ventilation was not attempted

Interventions

• No therapeutic intervention; observational study of standard anesthetic practice
• Mask ventilation was graded by the anesthesiologist on a 4-point scale (easy, difficult, impossible, not attempted)
• Difficult was defined as inability of unassisted anesthesiologist to maintain SpO2 >92% or requiring two-person technique

Outcomes

Primary Outcome

• Difficult mask ventilation: 313/22,660 (1.4%)
• Impossible mask ventilation: 37/22,660 (0.16%)

Secondary Outcomes

• Independent predictors of difficult mask ventilation: neck radiation changes (OR 7.1), male sex (OR 2.4), sleep apnea (OR 2.4), Mallampati III/IV (OR 2.0), beard (OR 1.8)
• 25% of patients with impossible mask ventilation also had difficult intubation

Criticisms & Further Discussion

• Single-center study in an anesthesia setting; may not generalize to emergency intubations
• Grading of mask ventilation difficulty is subjective and operator-dependent
• Anesthesiologists are typically more skilled at mask ventilation than emergency physicians, so ED rates may be higher
• Study did not account for variations in patient positioning or adjunct use (oral airways, etc.)

Funding

• Department of Anesthesiology, University of Michigan