Study area, study design and period

This multicenter hospital-based cross-sectional study was conducted in three governmental hospitals of Addis Ababa, Ethiopia: Tikur Anbessa Specialized Hospital (TASH), Yekatit 12 Hospital Medical College and Menelik II Specialized Hospital. They were selected purposely because the majority of pediatric surgical procedures are performed in these hospitals. The study was conducted from December 1, 2021, to April 30, 2022.

Population, eligibility criteria, and variables

The source population was all pediatric elective surgical patients aged 5–12 years who underwent surgery under general anesthesia with endotracheal intubation at selected hospitals. The study population was pediatric patients aged 5–12 years who underwent elective surgery under general anesthesia with endotracheal intubation in the selected hospitals who met the inclusion criteria and selected for the sample during the study period.

ASA I and II pediatrics 5–12 years scheduled for elective surgery under general anesthesia with endotracheal intubation during the study period were included. Pediatric patients 5–12 years of age with congenital upper airway malformation, swelling in the neck region, oral mass, restrictive mouth opening due to pathological condition, maxillofacial trauma, protruded teeth, temporomandibular joint ankylosis, burn contracture on the neck, limitation of cervical mobility, and uncooperative pediatric patients were excluded. The dependent variables of this study were difficult laryngoscopy (YES/No) and the predictive values of the four airway parameters in predicting DL, and the independent variables were MMT, TMD, ULBT, and RHTMD.

This study has been registered on the Research Registry and has received a unique identification number (UIN) “researchregistry8122’’ which can be accessed through https://www.researchregistry.com/browse-the-registry#home/. This study is reported according to the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) criteria [20].

Sample size and sampling technique

The sample size was determined using a single population proportion formula, with a prevalence of 0.5 and a margin of error of 5% at the 95% confidence interval.

n = \(\frac}_}}})}^\mathbf\mathbf(1-\mathbf)}}^}\)  = 384 where n = sample size, Z = 1.96, p = 0.5 & w = 0.05. Since the target population who underwent surgery in elective bases under general anesthesia with endotracheal intubation during the past three months in the study hospitals was (N = 185), from log book review, a finite population correction formula was used for sample size adjustment (nf) = \(\frac}}}}}})}\)=128. Then, by adding 10% for the nonresponse rate, a total of 141 elective pediatric surgical patients were included in this study.

Therefore, study subjects for each hospital were selected using proportion allocation by dividing the number of cases during situational analysis in each hospital multiplied by the sample size (n = 141) by the total number of patients (N = 185). Therefore, 69 at TASH, 34 at Yekatit 12 Hospital Medical College and 38 at Menelik II Specialized Hospital were involved in the sample. Study participants were selected using a systematic random sampling technique using a sampling interval (K): K = N/n; 185/141≈2. Therefore, the first study participant was selected using the lottery method from the daily operation schedule list. Then, every second case was included from each study hospital, using the daily operation elective schedule as a sampling frame (Fig. 1).

Fig. 1

Flow diagram of proportional allocation and sampling technique of the participants. Data collection procedures and quality assurance

Before the study, an ethical approval letter was written from the Institution Review Board (IRB) of Addis Ababa University College of Health Sciences (reference number: Anes 15/2021/2022). Then, a legal letter was submitted to the study hospitals, and ethical clearance was obtained. Data collectors and supervisors were trained on the study objective and the assessment tool for one day before data collection. Before the data collection began, the study participants’ families were informed about the study’s purpose. Then, after obtaining written informed assent, data were collected using a pretested structured questionnaire. Age, sex, and ASA status were collected from the chart. The height (in centimeters) and weight of the children (in kilograms) were measured using a flexible meter and weight balance, respectively. The MMT was graded by visualizing orophargeal structure, while the patient was seated, head in a neutral position, mouth wide open, and tongue protruding maximally. The ULBT was assessed with patient sitting, head in neutral position. and instructed to bite his/her upper lip and graded accordingly. The thyromental distance was measured in the sitting position, with the head completely stretched on the neck and the mouth closed, using a rigid ruler by trained data collectors before the patients entered the operation room. RHTMD was determined by dividing the patient's height by the TMD. After applying standard monitors, preoxygenation was carried out followed by intravenous anesthesia induction and muscle relaxation using Suxamethonium based on milligrams per kilogram of the children. Then, direct laryngoscopy was performed using a Macintosh number 2 blade in 108 cases and a Miller number 2 blade in 33 cases. The patient's Cormack and Lehane laryngoscopic grades were determined by the anesthetist who performed the laryngoscopy by providing diagrammatic representation of the larynx after direct laryngoscopy and completion of intubation. All 141 direct laryngoscopies and endotracheal intubations were performed by anesthetists with a minimum of four years of experience in this study. The supervisors kept an eye on the data gathering process. Each questionnaire was checked for completeness by the principal investigator and the supervisors daily.

Data analysis and interpretation

Data with complete information were entered and coded into SPSS version 26 (IBM corporate) for analysis. Categorical variables were presented as a number and percentage. Mean and standard deviation were used to express continuous demographic data. Cross tabulation was performed to obtain the numbers of DL in relation to the MMC, ULBT, TMD, and RHTMD. Chi-square and Fisher’s exact tests were used to compare categorical variables, as appropriate. Receiver operating characteristic curve analysis was used to compare the accuracy of MMC, ULBT, TMD, and RHTMD against DL. Cut-off values of TMD and RHTMD were calculated by receiver operating curve analysis. Sensitivity, specificity, positive and negative predictive values with 95% confidence intervals were calculated using cross tabulation to assess the association between the outcome and exposure variables. Statistical significance was defined at a P value of less than 0.05.

Operational definitionsDirect laryngoscopy

The procedure performed to visualize the vocal cords using laryngoscope.

Difficult laryngoscopy

Inability to visualize the vocal cord during direct laryngoscopy after induction and muscle relaxation (CLG III and/or IV) [21, 22].

Cormack and Lehane grade (CLG)

Methods of classifying the degree of visualization of the vocal cord (grade I-IV) during direct laryngoscopy after induction of anesthesia and during direct laryngoscopy [22].

Grade I: Visualization of the entire laryngeal aperture.

Grade II: Only the posterior commissure of the laryngeal opening is seen.

Grade III: Only the epiglottis is visible.

Grade IV: Any portion of the laryngeal structure is invisible

Difficult intubation

Tracheal intubation requires > three attempts or > ten minutes for an experienced anesthetist utilizing direct laryngoscopy [21].

American Society of Anesthesiologists (ASA) physical status

A method of classifying patients' physical status, which categorizes patients into six categories based on their systemic wellbeing.

Modified Mallampati Test (MMT)

A simple airway assessment widely used to predict difficult laryngoscopy/intubation by viewing oropharyngeal structure, while the patient is seated, head in a neutral position, mouth wide open, and tongue protruding maximally. Based on this test, there are five classes viz: 0, I, II, III and IV [22].

Class 0: part of the epiglottis is visible

Class I: hard palate, soft palate, uvula, tonsillar fauces and pillars are visible

class II: hard palate, soft palate, fauces and uvula-tip are visible

Class III: the base of the uvula and the soft palate are observable

Class IV: Only the hard palate is visible, and the soft palate is not visible at all.

MM Class III and IV are considered difficult laryngoscopy.

Thyromental distance (TMD)

The distance between the mentum and the thyroid notch measured in centimeters (cm) when the patient's neck is fully extended.

Upper lip bite test (ULBT)

Ability of patients to bite the top lip with the lower jaw incisors [18].

Class I: Lower incisors have the ability to bite the upper lip above the vermilion line

Class II: Lower incisors have the ability to bite the upper lip below the vermilion line

Class III: Lower incisors unable to bite the upper lip (difficult laryngoscopy can occur)

Ratio of height to thyromental distance (RHTMD)

The patient's height in centimeters (cm) divided by the TMD (in cm).

Sensitivity

The conditional likelihood of screening tools properly identifying the presence of difficult laryngoscopy \(\frac\).

Specificity

The chance of screening tools successfully diagnosing laryngoscopy is not difficult \(\frac\).

Positive predictive value

The likelihood of being difficult laryngoscopy for screening tools predicted difficult laryngoscopy \(\frac\).

Negative predictive value

The likelihood of not being difficult laryngoscopy for screening tools predicted not being difficult laryngoscopy \(\frac\).

Limitation of cervical mobility

It is defined as the limitation of cervical extension secondary to cervical injury or fixed atlantooccipital joint.

Apparent difficult airway indicator

Any mass in the mouth, large anterior neck mass, short neck, fixed atlantooccipital joint and cervical vertebrae, maxillofacial trauma, protruded teeth, temporomandibular joint ankylosis, burn contracture on the neck.