Comparison of six methods for predicting difficult intubation in obstetric patients

Background: Although the use of general anesthesia has been declining in obstetric patients, it may still be required in selected cases. Maintenance of the airway during obstetric anesthesia is difficult to achieve, and still remains the single most important cause of anesthetic related maternal morbidity and mortality. The purpose of this study was to determine the capability of the following preoperative airway predictive tests, in isolation and combination, to predict difficult visualization of larynx (DVL) in obstetric patients: upper lip bite test (ULBT), modified Mallampati test (MMT), sterno-mental distance (SMD), thyro-mental distance (TMD), horizontal length of the mandible (HLM), and inter incisor gap (IIG). Methods: 203 consecutive obstetric patients with ASA grade 1 and 2 undergoing general anesthesia for cesarean delivery were evaluated preoperatively using the ULBT, MMT, SMD, TMD, HLM, and IIG tests. The cut-off points for the airway predictors were ULBT III, Mallampati III and IV, ≤ 13.5 cm, ≤ 6.5 cm, ≤ 9 cm and ≤ 4cm, respectively. During direct laryngoscopy by a blinded expert anesthesiologist, the laryngeal view was graded, using the Cormack and Lehane classification. Laryngoscopic grades III and IV were considered difficult laryngoscopy. The sensitivity, specificity and positive predictive value for each test in isolation and in combination were determined. Results: The difficulty to visualize the larynx was found in 37 (18.2%) parturients. The sensitivity, specifity and positive predictive value for these tests were: ULBT (94.6%, 97.6%, 89.7%), MMT (29.7%, 74.7%, 20.8%), SMD (13.5%, 86.7%, 18.5%), TMD (10%, 99.4%, 10%), HLM (62.2%, 43.4%, 19.7%), and IIG (0%, 98.8%, 0%). Conclusion: The best single predictor was ULBT and the best combination of the tests was ULBT and MMT. Data analysis showed that demographic data, MMT and IIG were independent predictors of DVL. Thus, the ULBT is a useful predictor of difficult intubation in obstetric patients and also the combination of ULBT and MMT is a valuable airway predictor in parturients.