Airway difficulties continue to pose problems for the attending anesthesiologists and emergency physicians due to risk of hypoxic damage to brain and other vital organs. Most authors have concentrated on prediction of difficult tracheal intubation, but very little has been done to predict difficult mask ventilation. Thus, we undertook a prospective study to find the factors leading to difficult mask ventilation and to prepare a simple prediction criterion.
After approval by the Hospital Ethics Committee, and informed consent, 500 adult patients irrespective of ASA grade requiring general anesthesia were enrolled in the study. Patients with respiratory disease or contraindication for prolonged mask ventilation were excluded. Biometric indices and airway were assessed on preoperative visit. The mandibular protrusion test was assessed as the ability to protrude the lower jaw in front of the upper jaw. A subjective opinion for assessment of anticipated difficult mask ventilation by a consultant anaesthesiologist was also noted. All patients had a standard premedication and induction of anaesthesia. Monitoring comprised heart rate, ECG, SPO2 and blood pressure. After preoxygenation, anaesthesia was induced with a sleep dose of thiopentone. Succinylcholine 1.5 mg kg−1 was administered intravenously for neuromuscular blockade. The lungs were ventilated through a mask, with or without a Guedel's oropharyngeal airway using a Magill's circuit with 100% oxygen and a flow of 10 L min−1 for 3 min. The anaesthesiologist rated the mask ventilation as difficult when the difficulty was considered clinically relevant and could have resulted in potential problems if mask ventilation has to be continued for a prolonged period. Failure to ventilate with the mask was recorded whenever there was inability to ventilate and maintain SPO2 >90% by an anaesthesiologist with the use of emergency oxygen flush and when assistance or an alternative means (laryngeal mask, tracheal intubation or cricothyroidotomy) to ventilate the lungs was required. Then laryngoscopy and intubation were performed with a Macintosh laryngoscope and intubation was considered difficult whenever more than two attempts to intubate the trachea were required by an experienced anaesthesiologist or there was a laryngoscopic view of grade III or IV on Cormack and Lehane classification.
Data was analysed using univariate comparison for various potential factors causing difficult mask ventilation. The factors with higher significance were subjected to multivariate analysis. Diagnostic values of different variables for predicting difficult mask ventilation were calculated. In addition, a Receiver Operator Characteristic curve was used to judge the discrimination ability of various clues to predict difficult mask ventilation.
Difficult mask ventilation was found in 65 patients (13%). In one patient out of these 65, the anaesthesiologist failed to ventilate the lungs with the use of airway adjuncts and assistance however tracheal intubation was successful. It was found that weight, body mass index (BMI), age, Mallampati Class, macroglossia, lack of teeth, beard, waist: hip ratio, mandibular protrusion test, short neck, double chin and snoring history were statistically significant variables associated with difficult mask ventilation (Table 1). In contrast gender, mouth opening and thyromental distance did not gain statistical significance. On multivariate analysis seven variables - weight, BMI, age, Mallampati Class, mandibular protrusion test, short neck and double chin with high Odds Ratios (ranging from 14.74 to 4.18) were found to be independent risk factor for difficult mask ventilation. It was also found that difficult intubation occurred 37 times more frequently in patients with difficult mask ventilation (P = 0.001). The diagnostic value of different variables in combination for prediction of difficult mask ventilation is shown in Table 2.
We found a strikingly high incidence of difficult mask ventilation. In 1989, Benumof reported an incidence of completely failed mask ventilation and tracheal intubation in 0.001-0.02% of cases . Langereron in his pioneering prospective work found overall an incidence of difficult mask ventilation of 5% . The experience of the anaesthetist might have contributed to bias as most of our anaesthetists were in the 3rd year of their residency. Another factor causing this difference is that the previous results had been derived from incidental reports and analysis of hospital records which often miss potentially difficult mask ventilation cases. However, the contribution of the population sample and ethnic variation to difficult mask ventilation cannot be ignored. Our population sample included 17.6% bearded Sikh males. Many previous authors have explored the relation of obesity or obesity related objective indices with difficulties in airway problems and their management. A higher BMI (>26 kg m−2) poses difficulty in ventilation due to the increased force required to ventilate a large or heavy chest . Young and Willet found that 60-90% of patients with obstructive sleep apnoea had a BMI >29 kg m−2 and concluded that indices of obesity strongly correlated with severity of obstructive sleep apnoea. Haemoglobin saturation has been shown to decrease more rapidly during apnoea in patients with a BMI >40 kg m−2 [4,5]. Age has been found to be closely associated with an increased pharyngeal resistance to airflow (from choanae to epiglottis) in men [2,6]. Many of the factors present in univariate analysis indicated a high likelihood of difficult mask ventilation but had a poor false negative predictive value in multivariate analysis. Difficult mask ventilation is a multifactorial problem. A number of fixed and variable patient factors play a role in causing difficult mask ventilation in addition to the anaesthetist's experience and competence, type of mask, etc.
We found the mandibular protrusion test to be an important variable strongly associated with difficult mask ventilation. On calculating the diagnostic value of a combination of factors on the receiver operating characteristics curve it was found that combining the two factors of BMI >26 kg m−2 and mandibular protrusion reliably gave the best overall prediction of difficult mask ventilation. However, when compared with subjective assessment, it was found that subjective assessment had a better sensitivity, specificity and overall predictive value.
Our study has limitations including a small sample size, experience bias and its design to comment upon the difficulties faced during prolonged mask ventilation. Mask ventilation becomes difficult with passage of time. Operator fatigue and gastric insufflation of gases associated with difficult mask ventilation compound the existing problem. While problems with airway maintenance may be obviated during anaesthesia by the use of airway adjuncts or aids, identification of risk and caution are keys to management and the airway should be secured before anaesthesia where doubt exists. If tracheal intubation is needed, spontaneous breathing until intubation is an important principle. Every anaesthetist should have in mind a plan for failed intubation or worse, failed ventilation.
In conclusion, we found the incidence of difficult mask ventilation to be higher than previously reported. Difficult mask ventilation can be predicted quickly and reliably using BMI and the mandibular protrusion test. As there is a strong correlation between difficult mask ventilation and difficult tracheal intubation, we recommend formulating a departmental policy to manage airway in expected difficult mask ventilation candidates and that these subjects should be either intubated awake using fibreoptic assisted tracheal intubation or an alternative means of ventilation such as a supraglottic device be available to prevent untoward accidents.
T. K. Gaul
Department of Anaesthesia and Resuscitation, Dayanand Medical College and Hospital, Tagore Nagar, Ludhiana, India
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