Prediction of Endotracheal Tube Size in Pediatric Population Using Ultrasonographic Subglottic Diameter and Age-Related Formulas: A Comparative Study.

Background: Choosing the correct Endotracheal tube (ETT) size is important in paediatric patients because an inappropriately large and small sized tube has its own disadvantages and chances of re-intubation with different size tube is high. The currently available modalities do not reflect the actual tracheal diameter for selection of endotracheal tube. Ultrasonography (USG) guided evaluation of transverse diameter at subglottic region may be helpful to estimate the proper size of ETT. We tested the hypothesis that Ultrasound guided subglottic diameter better predicts optimal ETT size than existing methods. Aims: To predict the appropriate size of ETT in pediatric patients to avoid multiple attempts of intubation and airway edema using simple noninvasive USG-guided methods. Settings and Design: Design: Prospective study. Settings: Tertiary care hospital. Materials and
Methods: Institutional ethical committee clearance no. BMCRI/PS/138/2020-21 was taken. Laryngoscopy and endotracheal intubation were done in 27 pediatric patients of age 2-15 years using a predetermined-sized ETT, estimated by ultrasonography. ETT size was considered optimal when the cuff leak test was negative. If there was resistance to ETT passage into the trachea, the tube was exchanged with one that was 0.5 mm smaller. If the cuff leak test was positive, then the ETT was exchanged for one with the 0.5-mm larger tube. The comparison was done between the size of ETT calculated by USG-guided subglottic diameter, age, height-related formulas, and clinical methods to look for accuracy of prediction for proper ETT size after cuff leak test. Statistical Analysis: The data collected were entered into Microsoft Excel and analyzed using SPSS version 22. IBM Corp. Released 2013. IBM SPSS Statistics for Windows, Version 22.0. Armonk, NY: IBM Corp. Appropriate parametric and nonparametric tests were applied wherever necessary. Categorical data: Represented in the form of frequencies and proportions. Continuous data: Represented as mean and standard deviation. Paired sample test: Test to identify the mean difference between two quantitative variables. Kappa statistics: To measure the level of agreement.
Results: A total of 27 children of age between 2 and 15 years were selected, out of which 15 were male and 12 were female. The accuracy of predicting appropriate ETT size is greater in ultrasound-guided subglottic diameter when compared to conventional clinical assessment and age- and height-related formula and almost the same accuracy as the clinical method.
Conclusion: Ultrasound-guided subglottic diameter is an easy, effective, and noninvasive way of predicting accurate ETT size in the pediatric population. Copyright:

INTRODUCTION

Choosing the correct endotracheal tube (ETT) size is important in pediatric patients because an inappropriately large-sized tube may cause damage to the airway, postextubation stridor, and subglottic stenosis. On the other hand, a smaller tube will increase the resistance to gas flow, risk of aspiration, insufficient ventilation, poor monitoring of end-tidal gases, and the need to reintubate with a different size of the tracheal tube.[1]

The cervical radiograph and age-based physical indices do not reflect the actual tracheal diameter for the selection of ETT, and repeated laryngoscopies are often necessary to identify the proper ETT for intubation. To avoid excessive airway instrumentation and minimizing the risk of trauma, the preanesthetic assessment of tracheal diameter is essential to select the appropriate ETT.[234] Reintubation rates are higher with uncuffed ETTs.[5] A high tube exchange rate and prolonged intubation time may increase the possibility of hypoxia and aspiration.[6]

Ultrasonography (USG) is a reliable, safe, noninvasive, pain-free, and real-time modality for evaluation of the upper airway's narrowest transverse diameter at the subglottic region and may be helpful to estimate the proper size of ETT.[27]

However, the extent to which ultrasonography can predict optimal ETT size in pediatric patients remains to be determined. Therefore, we tested the hypothesis that subglottic diameter, as determined by ultrasonography, better predicts optimal ETT size than existing methods.[8]

MATERIALS AND METHODS

Source of data

Patients undergoing elective surgery under general anesthesia at a tertiary care hospital during the period of October 2020 to December 2020.

Methods of collection of data

Study design: Prospective study

Study period: October 2020 to December 2020

Place of study: Tertiary care hospital

Sample size: Based on a previous study, the mean ETT outer diameter by physical indices of age-based formulas is 6.5 ± 0.5.

Thus, the sample size is calculated using the formula,

n = Z2 σ2/d2

where Zα = Standard table value for 95% confidence interval

Σ = Standard deviation (SD) = 0.5

d = precision = 3% relative precision = 0.195

n = Z2 σ2/d2

n ~ 27

Total sample size: 27.

Inclusion criteria

Patients willing to give informed written consent

American Society of Anesthesiologists physical Status I and II patients

Patients of age group 2–15 years of either sex

Patients undergoing elective surgery with endotracheal intubation under general anesthesia.

Exclusion criteria

Patient refusal

Emergency surgery

The patients suffering from any respiratory diseases causing airway narrowing

Preexisting laryngeal or tracheal pathology, any lesion that could cause airway deformity due to fibrosis

Anticipated difficult airway.

Methodology

After obtaining approval from the institutional ethical committee and written informed consent from the patient and doing routine preanesthetic evaluation, patients with normal airway were enrolled in the study.

Airway assessment like mouth opening, Mallampati grading, neck flexion and extension, and dentition of all patients was done. General and systemic examination was done.

The patients were kept nil per oral for 4 h for liquids and 6 h for solids prior to surgery. A pro forma was used to enter the data which includes the patient's particulars, indication for surgery, the anesthetic details, and intraoperative monitoring.

Technique of ultrasonography

The subglottic diameter was estimated with high-resolution B-mode ultrasonography with a linear probe of small footprint (40-mm length, frequencies 7–15 MHz) [Figure 1], positioned on the midline with their head extended and neck flexed (the sniffing position) [Figure 2].

Figure 1

High-resolution B-mode USG with a linear probe. USG: Ultrasonography

Figure 2

The sniffing position

The standard scanning plane was predetermined to prevent any examination bias and artifacts. Ultrasonography, performed by an experienced person, began with the location of the true vocal cords, seen as paired hyperechoic linear structures, and then moved caudally to visualize the cricoid arch to avoid any confusion between the cricoid cartilage and the tracheal ring. The transverse air column diameter was measured at the cephalic half of the cricoid cartilage which is narrower than the caudal part and was considered as tracheal diameter. The larynx is situated below the hyoid bone, and the ring-shaped trachea, located inferior to the cricoid cartilage, can be easily visualized by ultrasound in a vertical or transverse section [Figure 3].

Figure 3

Transverse section-subglottic diameter

Laryngoscopy and endotracheal intubation were done using predetermined-sized ETTs, estimated by ultrasonography. ETT size was considered optimal when the tracheal leak was not detected at an inflation pressure of 20–30 cm H2O. The cuff leak test was done. If there was resistance to ETT passage into the trachea or there was an audible leak when the lungs were inflated to a pressure of <10 cm H2O, the tube was exchanged with one that was 0.5 mm smaller.

However, if a leak occurred at an inflation pressure of 20–30 cm H2O, then the ETT was exchanged for one with the 0.5-mm larger tube. The relationship between calculated external diameters of the ETT from physical indices of age-based formulas predetermined by ultrasound and clinically used ETT for intubation during general anesthesia was done.

Statistical analysis

The data collected were entered into Microsoft Excel and analyzed using SPSS version 22 IBM Corp. Released 2013. IBM SPSS Statistics for Windows, Version 22.0. Armonk, NY: IBM Corp.

Appropriate parametric and nonparametric tests were applied wherever necessary. Categorical data: Represented in the form of frequencies and proportions.

Continuous data: Represented as mean and SD.

Paired sample test: Test to identify the mean difference between two quantitative variables.

Kappa statistics: To measure the level of agreement.

RESULTS

The descriptive data analysed showed no significance in age, weight and height in the respective groups [Table 1]. Paired sample t-test shows that there is no significant difference (P > 0.057) between the mean value of ETT size measured according to subglottic diameter and the actual ETT size after the cuff leak test [Table 2a].

Table 1

Descriptive data analysis

Descriptive statistics
	n*	Range	Minimum	Maximum	Mean±SD
Age	27	14	2	16	8.00±4.169
Weight	27	47.0	8.0	55.0	24.404±12.2774
Height	27	67	88	155	124.81±20.799
Age-based formula	27	4	4	8	6.00±1.190
Height-based formula	27	2.0	5.0	7.0	6.033±0.6674
Clinically	27	3.5	4.0	7.5	5.611±1.0127
ET tube according to subglottic diameter	27	4	4	8	5.80±0.766
After cuff leak test	27	3.5	4.0	7.5	5.909±0.8257

*n=Sample size. SD=Standard deviation, ET=Endotracheal

Table 2

Comparison of endotracheal tube size according to subglottic diameter and endotracheal tube size after cuff leak test

(a) Paired samples test - subglottic diameter - after cuff leak test
Pair 1	Paired differences				t*	df	P
	Mean±SD	SEM	95% CI of the difference
			Lower	Upper
ET tube according to subglottic diameter - after cuff leak test	−1136±0.2642	0.0563	−0.2308	0.0035	−2.017	21	0.057
(b) Symmetric measures
	Value	Asymptomatic SEa	Approximate Tb		Approximate significance
Measure of agreement (kappa)	0.777	0.101	7.927		0.000
Number of valid cases	27

*T=Value. df=Degrees of freedom, SE=Standard error, SEM=SE mean, SD=Standard deviation, ET=Endotracheal, CI=Confidence interval

Kappa statistic calculation shows that there is a good amount of agreement (value = 0.777) to say that the mean value of ETT size was measured according to subglottic diameter and the actual ETT size after the cuff leak test does not show the difference [Table 2b].

This shows that ultrasound-guided subglottic diameter accurately predicts the ETT size as compared to ETT inserted after cuff leak test.

Paired sample t-test shows that there is no significant difference (P > 1.000) between the mean value of ETT size measured according to clinical examination and the actual ETT size after the cuff leak test [Table 3a].

Table 3

Comparison of endotracheal tube size according to clinical examination and endotracheal tube size after cuff leak test

(a) Paired samples test - clinically - after cuff leak test
Pair 1	Paired differences				t*	df	P
	Mean±SD	SEM	95% CI of the difference
			Lower	Upper
ET tube clinically - after cuff leak test	−0000±0.3780	0.0806	−0.1676	0.1676	−0.000	21	1.000
(b) Symmetric measures
	Value	Asymptomatic SEa		Approximate Tb	Approximate significance
Measure of agreement (kappa)	0.348	0.125		3.818	0.000
Number of valid cases	27

*T=Value. df=Degrees of freedom, SE=Standard error, SEM=SE mean, SD=Standard deviation, ET=Endotracheal, CI=Confidence interval

Kappa statistic calculation shows that there is a good amount of agreement (value = 0.348) to say that the mean value of ETT size that was measured according to clinical examination and the actual ETT size after the cuff leak test does not show the difference [Table 3b].

This shows that the size of the patient's little finger accurately predicts the ETT size as compared to ETT inserted after the cuff leak test.

Paired sample t-test shows that there is a significant difference (P < 0.001) between the mean value of ETT size measured according to the age-based formula and the actual ETT size after the cuff leak test [Table 4a].

Table 4

Comparison of endotracheal tube size according to age-based formula and endotracheal tube size after cuff leak test

(a) Paired samples test – age-based formula - after cuff leak test
Pair 1	Paired differences				t*	Df	P
	Mean±SD	SEM	95% CI of the difference
			Lower	Upper
ET tube according to age-based formula - after cuff leak test	−0.3409±0.3232	0.0689	−0.1976	0.4842	−4.948	21	0.000
(b) Symmetric measures
	Value	Asymptomatic SEa		Approximate Tb		Approximate significance
Measure of agreement (kappa)	0.150	0.102		1.797		0.072
Number of valid cases	27

*T=Value. df=Degrees of freedom, SE=Standard error, SEM=SE mean, SD=Standard deviation, ET=Endotracheal, CI=Confidence interval

Kappa statistic calculation shows that there is no good amount of agreement (=0.72) to say that the mean value of ETT size that was measured according to the age-based formula and the actual ETT size after the cuff leak test does not show the difference [Table 4b].

This shows that the size of the ETT calculated according to the age-based formula does not accurately predict the ETT as compared to ETT inserted after the cuff leak test.

Paired sample t-test shows that there is a significant difference (P < 0.001) between the mean value of ETT size measured according to the age-based formula and the actual ETT size after the cuff leak test [Table 5a].

Table 5

Comparison of endotracheal tube size according to height-based formula and endotracheal tube size after cuff leak test

(a) Paired samples test - height-based formula - after cuff leak test
Pair 1	Paired differences				t*	Df	P
	Mean±SD	SEM	95% CI of the difference
			Lower	Upper
ET tube according to height-based formula - After cuff leak test	−0.3136±0.3932	0.0838	−0.1393	0.4879	−3.742	21	0.001
(b) Symmetric measures
	Value	Asymptomatic SEa		Approximate Tb		Approximate significance
Measure of agreement (kappa)	0.109	0.106		1.151		0.250
Number of valid cases	27

*T=Value. Df=Degrees of freedom, SE=Standard error, SEM=SE mean, SD=Standard deviation, ET=Endotracheal, CI=Confidence interval

Kappa statistic calculation shows that there is no good amount of agreement (=250) to say that the mean value of ETT size measured according to the height-based formula and the actual ETT size after the cuff leak test do not show the difference [Table 5b].

This shows that the size of the ETT size calculated according to the height-based formula does not accurately predict the ETT as compared to ETT inserted after the cuff leak test.

DISCUSSION

Shibasaki et al. compared USG-guided subglottic diameter with conventional formulas in predicting the accurate ETT size in children. They concluded that subglottic upper airway diameter was highly correlated with outer ETT diameter (98% for cuffed ETTs and 96% for uncuffed ETTs).[8] In our study, similar results were obtained, i.e., the ETT size was more accurate when it was determined by USG-guided subglottic diameter.

Gupta et al. compared USG-guided subglottic diameter and physical indices in predicting the ETT size in pediatric patients. They concluded that ultrasound-guided selection criterion has estimated the appropriate-sized ETT better than physical index (age or height)-based formulas. The estimated ETT size by ultrasound was significantly correlated with the clinically used ETT.[2] The results obtained in our study also show similar conclusions.

Altun et al., in their study, investigated the first attempt success of ultrasonography (USG) in 150 pediatric patients in predicting an appropriate cuffed ETT size. They found that the success rate of the first attempt with USG was 86%; the ETT was replaced in five patients with a tube one size larger and in two patients with a tube one size smaller. Hence, they concluded that USG was a good predictor of appropriate ETT size in pediatric patients.[9]

Sutagatti et al. aimed at determining the accuracy of ultrasonography to assess the appropriate ETT size and compare it with physical indices-based formulae. They found out that USG predicted the appropriate ETT size (P < 0.05) better than physical index-based formulae for cuffed and uncuffed tubes. The age-based formula predicted the clinically used ETT size well (P = 0.58) and the height-based formula did not correlate with the clinically used tube size (P = 0.0002 – a statistically significant value). They concluded that ultrasonographic estimation of subglottic diameter is useful for optimal pediatric ETT size selection in both cuffed and uncuffed tubes.[1] Observations were similar according to our study.

Singh et al., in their study, estimated the ETT size by USG-guided tracheal diameter in pediatric patients and the obtained values were compared with the values of ETT size calculated by various ages, height, weight-based formulas, and diameter of right and left little finger. They concluded that ultrasonography is a reliable method of estimation of subglottic diameter and for prediction of ETT size in children.[10] The results of our study also suggest that USG is a reliable and safe method to predict the ETT diameter in pediatric patients.

The recent report on the feasibility of ultrasonography to examine the subglottic diameter showed a strong correlation between ultrasonography and magnetic resonance image.

The study showed that direct measurement of subglottic diameter by ultrasonography has significant advantages in predicting the optimal ETT size.

This study has shown the higher correlation between endotracheal tube size determined by clinical method i.e., diameter of patients little finger and predetermined endotracheal tube size by ultrasonography than predicted endotracheal tube size by age and height based formulas which was comparable to above studies.

This study has revealed the potential utility of ultrasound to measure the transverse diameter of the upper airway at the subglottic region for the selection of the appropriate-sized ETT.

Limitation of the study is the small sample size, hence a larger study is to be conducted to estimate the authenticity of the study.

CONCLUSION

Ultrasound-guided subglottic transverse diameter is an easy and effective way of predicting accurate ETT size in pediatric population. It can be used as a routine in selecting the ETT size. It is also found to reduce the number of attempts of intubation and prevent airway trauma. Measurements of the transverse subglottic diameter suggest that ultrasound could predict the subglottic diameter adequately.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.