Management of difficult airway in a pediatric patient can be a stressful situation for all involved. Rose and Cohen in 1994, stated that different methods of airway assessment help in identifying more than 98% of difficult airway [1]. Unlike adults, in the pediatric population, no fixed classifications exists which would help in determination of patients at risk of difficult intubation. There are several external measurements such as thyromental distance which help to identify difficult airways in adults. Correlation of external measurements with laryngoscopic grade may help predict intubation difficulty [2]. The ratio of height-to-thiopental distance (RHTMD) introduced by Schmitt et al. has been found to have improved predictive value in adults. [3-5]. in children there are no such standard measurements because of the changing height and parameters with growth. Since the growth of a child occurs in all aspects, a ratio would be a better predictor of difficult airway than absolute values which change with age. This study was undertaken to find a correlation between ratio of height and thyromental distance in children of different ages.

The study was conducted in the Department of Anaesthesiology, Maulana Azad Medical College and Lok Nayak Hospital on patients undergoing elective surgeries under general anaesthesia requiring endotracheal intubation. Ethical clearance was taken from the institutional ethics committee. Written informed consent was taken from the patient’s parents. An Observational pilot study was conducted on 200 children undergoing any surgery under general anesthesia. Children of 1-12 yrs. posted for any surgery requiring endotracheal intubation irrespective of the ASA grading were included in our study. Study population were divided into 2 groups.

Group A: 0-6 years

Group B: 6-12 years

Pre-anaesthetic evaluation

A detailed pre- anesthetic check-up (PAC) was carried out in all cases. Thyromental distance was measured as the straight distance between the thyroid notch and the lower border of the mental prominence, with the head fully extended and the mouth closed using a rigid ruler. Height of the patient was measured in centimeters with the child standing erect. In younger children or those unable to stand erect the recumbent length was measured with the child lying supine with the vertex or top of the head snugly touching a fixed vertical plank. The legs were fully extended by pressing over the knees, and the feet were kept vertical at 90 degrees. The ratio of thyromental distance and the patient’s height was then calculated.

Anaesthetic technique

In the operating room and standard monitoring devices (electrocardiogram, pulse oximetry and non-invasive blood pressure) were attached to the patient. Anaesthesia was induced by 6% - 8% Sevoflurane with O₂:N_2O: 50:50. After securing intravenous access injection fentanyl was administered (2mcg/ kg) followed by intravenous vecuronium (0.1mg/kg) to facilitate endotracheal intubation. The size of the head ring used was standardised as follows.

1month- 6years: No ring was used during intubation.

6years- 12 years: 5cm ring was be used.

Laryngoscopy was performed with the patient in sniffing position after a period of 3 minutes following vecuronium injection. An appropriate Macintosh blade was used for the same.

Size 1 for children aged 1 month- 2 years

Size 2: for children aged 2 - 12 years

The anesthesiologist performed the laryngoscopy was blinded to the preoperative airway assessment of the patient. Laryngoscopic grading of the best possible view with or without optimal external laryngeal manipulation was made according to Cormack and Lehane’s classification. Grade 3 or 4 laryngoscope view was grouped as difficult laryngoscopy. A back up plan for management of difficult airway, along with the difficult airway cart was readily available in all cases. Our study ended here and the airway was intubated with special manoeuvres if required.

Parameters assessed were

1. The ratio of height-to-thyromental distance (RHTMD) was calculated for each child
2. Direct laryngoscopic view was graded using the Cormack and Lehane’s grading system.

Data was analysed using receiver operating characteristic curve (ROC) and the area under the curve AUC for each was computed and the correlation was sought between the ratio of height of the patient and the thyromental distance to the laryngoscopic view. The results were analysed for both the groups, group A: 0 years – 6 years, group B: 6 years -12 years.

A total of 200 children aged between 0 and 12 years were included in this study. The study population was further divided into two age groups, with 90 children in age group 0–6 years and 110 children in age group 6–12 years. The RHTMD ranged between 13.1 and 23.3 with mean and standard deviation of 17.7 ± 2.1. There were no Grade 3or 4 laryngoscopic views in the entire study population. A Grade 2b view being considered a restricted view was used for analytical purpose. The incidence of restricted view was 10.1%. Age group 0-6 years had an incidence of 13.1% (8 out of 90) restricted views, while age group 6-12 years had an incidence of 7.8% (6 out of 110). Areceiver operating characteristic (ROC) curve analysis for the study parameters (RHTMD) for predicting a restricted view was computed. The ROC curve for RHTMD for predicting a restricted view between 1 and 12 years of ageis shown in Figure 1. The area under the curve (AUC) was 0.792 with a 95% confidence interval (CI) between 0.679 and 0.905. An optimal cutoff point of RHTMD >17.95 had a sensitivity of 95% and specificity of 60% (Table 1).

Figure 1: The ROC curve for RHTMD.

Table 1: Sensitivity and specificity of AUC (Area under the curve) for the two age groups.

Few studies exist in pediatric population and to detect a difficult airway. This may be a result of low incidence of a difficult airway, lack of cooperation for routine airway assessment, and difficulty in standardization based on anatomical and physical variations within the pediatric population. Our study aimed to see if RHTMD could predict poor laryngoscopicviewsin children aged between 0 and 12 years. We assumed a poor laryngoscopic view to correlate with a difficult intubation and used Cook’s modification of the Cormack-Lehane Grading System. [6] This system considers the use of three discriminators of laryngoscopic difficulty, easy (Cormack–Lehane Grade 1 and 2a), restricted (Cormack–Lehane Grade 2b and 3a), and difficult (Cormack–Lehane Grade 3b and 4). In our study population of 200 children aged between 0 and 12 years, there were no difficult (Grade 3 or 4) laryngoscopic views, and only 20 (10.1%) children had restricted (Grade 2b) views. The absence of difficult laryngoscopic views in our study may be explained by findings of Heinrich et al. [7]. they observed in a large retrospective study that overall incidence of difficult laryngoscopy was 1.6%, of which incidence was 4.7% in children below 1 year compared to 0.7% children above 1 year of age. Most of difficult laryngoscopies were in the oro-maxillo-facial surgery and pediatric cardiac surgery department, with a possible explanation being high proportion of cleft palate interventions in oral surgery as well as syndromic children in pediatric cardiac surgery department. [7] This may account for the absence of difficult views in our study which excluded children with known airway difficulties. We found RHTMD to be a better predictor of difficult laryngoscopy in children between 0 and 12 years. It had a higher AUC and a superior sensitivity and specificity. An optimal cutoff of ≥17.95 for RHTMD had a sensitivity of 95% and specificity of 60%. RHTMD has variably been shown to be a good individual predictor of difficult laryngoscopic grade in adults [3-4]. RHTMD, introduced by Schmitt et al., has better predictive value for predicting difficult laryngoscopy than TMD as it accounts for individual’s body proportions unlike TMD.[3] This may be useful in pediatric population who present a range of dimensions wherein use of a ratio would better account for these differences. Furthermore, ease of obtaining such measurements may be useful with younger children who may be noncompliant to other routine airway tests. The limitations in our study were that the age group of 1–12 years is a large range being nonhomogenous and with varying anatomical, physical, and developmental characteristics. A child aged in the lower end of group may have a completely different upper airway anatomy and body proportions compared to a prepubertal child in upper end of the study group. In view of this, we divided study population further into two groups to look for any difference within study population. Secondly we did not standardize the height of the head ring used in our study as there has been no literature regarding the same.

Ratio of patients height to thyromental proved to be a good predictor for difficult laryngoscopy in paediatric population firstly as it has a better predictive value for difficult laryngoscopy than thyromental distance as it accounts for individuals body proportion. Secondly, this may be useful in paediatric population who presents a range of dimensions wherein use of a ratio would better account for these differences. Thirdly, ease of obtaining such measurements maybe useful with younger children who are non-compliant to other routine airway tests.

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