The predictive value of screening tests for difficult larygoscopy: interincisor gap,thyromental distance,Mallampati classification,ratio of height to thyromental distance in Chiang Rai Hospital

Abstract

Background : Preoperative evaluation is important in the detection of high risk patient for difficult tracheal intubation.Several predictive test for assessing a patient ’s airway for difficult larngoscopy,hower,predictive reliability is unclear. Interincisor gap,Thyromental distance, Mallmpati classification , ratio of patient ’s height to TMD (ratio of height to TMD= RHTMD) are simple bed-side tests for assessing airway of patients.

Objective : To evaluate and compare the accuracies of interincisor gap,Thyromental distance,Mallampati classification and ratio of height to TMD for prediction the difficult tracheal intubation.

Materials and Methods : This retrospective study in the anesthetized patients in the year 2007.Data of 340 patients were collected. Inclusion criteria is age> 18 yrs,elective case,generalize anesthesia, ASA class 1,2 and exclusion criteria is emergency case,rapid sequence induction,C-spine problem, BMI > 35 kg/m2,pregnancy GA >20 wks, no teeth. By using computer analyzided and medical records,age,sex,height,weight, ASA class, interincisor gap, TMD ,Mallampati classification,RHTMD were collected.Difficult intubation was defined in the present study by Cormack and Lehane grade 3 0r 4.The sensitivity,specificity, and positive and
negative predictive values of each of the predictive tests were calculated according to standard formulae.The odd ratio(95 percent confidence interval), Chi-square test and P <0.05 is the significant analysis statistics.

Result : The four predictive test were significant for predictive diffucult intubation . The study show as follows:Interincisor gap ≤3.5cm (sensitivity 70 percent,specificity 81 percent),TMD ≤6.5cm (sensitivity 48 percent,specificity 91percent), Mallampati class 3 or 4 (sensitivity 52 percent, specificity 97 percent), RHTMD ≥23.5 (sensitivity 52 percent, specificity 89 percent). The odds ratio (95 percent confidence interval) of the interincisor gap ≤3.5cm, TMD ≤6.5 cm, Mallampati class 3 or 4, RHTMD ≥23.5 were 10.44 (4.36-25.02), 9.09 (3.90-21.19), 32.52 (12.16-86.94) and 8.55 (3.72-19.67) respectively. Mallampati class 3 or 4 is high sensitivity, high NPV and highest of specificity and PPV may the best useful screening test for difficult laryngoscopy.

Keywords : difficult intubation, assessing airway