Is Ultrasonography a Better Method of Endotracheal Tube Size Estimation in Pediatric Age Group than the Conventional Physical Indices-Based Formulae?

BACKGROUND: Providing safe anaesthesia to paediatric patients is a challenging task. This requires a thorough knowledge of the soft and pliable paediatric airway. Owing to the vulnerability of the anatomical structures involved, choosing an appropiate sized endotracheal tube (ETT) is important in these cases. A larger sized ETT may lead to trauma and a smaller one would result in leakage and risk of aspiration. Both situations demand an immediate tube change, thereby complicating the condition. The physical indices- based formulae have often failed to justify the purpose leading to repeated laryngoscopy and tube change during intubation. The increase in availability of the modern ultrasound devices have shown promise in these cases. AIMS AND
OBJECTIVES: In this study we examine the accuracy of ultrasonography (USG)to assess the appropriate ETT size, comparing it with physical indices based formulae suggested ETT size so that repeated attempts on intubation can be minimized.
MATERIALS AND METHODS: The study group included 100 patients of 1-5 years, ASA I- II, requiring orotracheal intubation under general anaesthesia. The tracheal sub-glottic diameter was estimated by pre-anaesthetic USG to determine the ETT size, both cuffed and uncuffed. ETT data obtained by these methods were compared by Pearson's correlation coefficient and t-test.
RESULTS: USG predicted ETT size were significantly more consistent than the physical indices based formulae. Also the age based formulae were found to be more precise than the height based ones. Seven patients required change of tube once.
CONCLUSION: Ultrasonography is an effective tool in predicting paediatric ETT size. Copyright:

INTRODUCTION

Providing safe anesthesia to pediatric patients is a challenging task for the practitioners. This summons not only for experienced hands but also for a thorough knowledge of the pediatric airway which gradually tranforms with advancing age. In the neonatal period, the tracheal resembles a funnel, tapered downward, maximally narrowed at the level of cricoid cartilage.[12] However, in the later stages, it takes the shape of a cylinder. Hence, the use of uncuffed endotracheal tube (ETT) is advocated in neonates aging <8 months, as a measure to avoid injury and subsequent complication to the soft and pliable pediatric airway.

Choosing an appropriate sized ETT is equally significant in these patients. A large-sized ETT leads to trauma of the surrounding structures and subsequent airway edema, postextubation stridor, and subglottic stenosis in few. On the other hand, a smaller ETT would result in leakage, inadequate ventilation, increased resistance flow, and risk of aspiration.[13] Both the cases demand an immediate tube change, complicating the situation as these patients having lower lung volume reserves tend to desaturate fast.

A list of physical indices-based formulae such as age based, height based, Modified Cole's formula for uncuffed tubes and Khine's formula for cuffed tube, diameter of the distal digit of the little finger, have been used traditionally to determine the most suited size of the ETT. However, these assessments have failed to justify the purpose many a times leading to repeated laryngoscopy and tube change at time of intubation.[45]

With the increase in availability and acquaintance of the modern ultrasound devices to the anesthesiologists, the complication rate is expected to see a downward trend with a promise toward better outcome.

The present study was undertaken primarily with the aim to estimate the correlation of the ETT size used clinically on the operating table with the ETT size estimated by ultrasonography (USG), age-based formula, and height-based formula in the pediatric age. On the other hand, the number of ETTs predicted by USG that was exchanged for a larger or smaller ETT on the basis of the air leak test was also noted.

METHODOLOGY

Keeping the above considerations in mind, a prospective study was designed to be undertaken during a course of 1-year duration. The present study aimed to assess the narrowest transverse tracheal diameter at the subglottic region by USG, to calculate the size of ETT and its subsequent comparison with physical-based indices. The Institutional Ethical Committee approved the study, following which 100 pediatric patients of the age group of 1–5 years were enrolled for the study. The sample size was estimated based on the power of the study kept as 80%, confidence interval of 95%, and standard deviation 1. Patients belonging to ASA classes I and II, posted for elective surgeries under general anesthesia, requiring endotracheal intubation were included in the study. The exclusion criteria were airway pathology, anticipated difficult airway, delayed milestones, cardiovascular instability, or any neck masses.

On the previous day of surgery, sizes predicted by the physical indices formulae and USG were calculated preoperatively and recorded. High-resolution B-mode USG (Philips, IU-22, United States of America) with a linear probe (5–12 MHz and 7–15 MHz) was used for USG for all the patients by an experienced radiologist to eliminate undue bias. Patients were placed in a sniffing position. The probe was placed in the midline; paired hyper-echoic linear structures were identified as true vocal cords. The probe was further moved caudally to visualize the cricoid arch. The transverse air column at the level of cricoid cartilage was measured and taken to be the tracheal diameter [Figure 1]. The tracheal diameter corresponded to the outer diameter of the ETT, and subsequently, the inner diameter of chosen ETT was noted.

Figure 1

Transverse ultrasound view at the subglottic level showing the trachea (TR), esophagus (O), and the measurement of the diameter of the trachea

The height of the child was estimated in centimeters using a standard measuring tape keeping the surface levelled and against an even wall. The height-based formula used was inner diameter of ETT (in mm.) = (height [in cm]/30) +2.[5]

The age-based formulae used for uncuffed ETT: Inner diameter (Modified Cole's formula) = (Age [in years]/4 + 4) and for cuffed ETT: Inner diameter (Khine's formula) = (age [in years]/4 + 3).[6]

On the morning of surgery, these cases were kept as priority on the OT list owing to their age. Fasting guidelines were considered. Premedication was done with injectable midazolam 0.05 mg.kg−1, glycopyrrolate 0.01 mg.kg−1, and fentanyl 1 mcg.kg−1. Preoxygenation was done with 100% oxygen for 3 min. For general anesthesia, intravenous propofol 2 mg.kg−1 and intravenous succinylcholine 2 mg.kg−1 were used. Intubation was done under direct laryngoscopy by an experienced anesthesiologist with polyvinyl chloride ETT, cuffed or uncuffed, depending on the anesthesiologist's choice, and size that was predetermined by USG. If cuffed ETT was chosen by anesthesiologist, the ETT size used was 1 mm external diameter less than that estimated by USG. On the contrary, if uncuffed ETT was chosen, then the size used was the same as estimated by USG.

If the intubating anesthesiologist faced any resistance in passing the ETT through the trachea, or if no leak was audible when the lungs were inflated to a pressure of 20 cm of water (measured from ventilator), the ETT was exchanged with 0.5 mm smaller ETT. ETT size was considered optimal when a tracheal leak was detected at an inflation pressure between 10 and 20 cm of water.[16] The final ETT that was intubated and found optimal on the OT was considered as clinically used ETT.

Following this, a comparison between ETT size estimated by USG, age-based formula, height-based formula, and that used clinically was drawn.

The data were analyzed using Statistical Package For Social Science (SPSS) International Business Machine (IBM) Version 20, SPSS South Asia Pvt. Ltd., Bengaluru, India. The results were represented as mean ± standard deviation. One-way analysis of variance test was used for the comparisons of age groups with different methods followed by post hoc Tukey's test. Comparisons of various methods were done using dependent-t test, whereas independent t-test was used for the comparison of gender with different methods. P value < 0.05 was considered statistically significant.

RESULTS

No dropouts were noted in our study. This study enrolled 46 male and 54 female patients, out of which 18 patients belonged to the age group of 1–2 years, 39 patients in the group of 2–4 years, and the remaining 43 between 4 and 5 years of age. The mean age of the study group was 3.47 years. Cuffed ETT was used in 78 patients and uncuffed in the rest 22.

In our study, the mean ETT size for clinically used was 3.92 ± 0.23, USG was 3.83 ± 0.69, age-based formula was 4.32 ± 0.44, and height-based formula was 5.09 ± 0.81. Comparing the ETT size estimated by USG with the ETT used clinically did not show any statistical significance (P = 1.000) [Table 1]. Out of the total patients studied, USG predicted the ETT size optimally in 88 cases. The rest 12 required change of ETT either with a larger or smaller tube with a margin of 0.5 mm.

Table 1

Summary of age, height, and weight of patients involved in the study

Summary	Age (year)	BMI (mg/kg)	Height (cm)	Weight (kg)
Minimum	1.00	10.20	70.00	8.00
Maximum	5.00	25.00	109.00	14.00
Mean	3.47	16.16	92.47	12.80
SD	1.49	3.65	8.36	5.89
SE	0.22	0.43	1.63	1.40

BMI=Body mass index, SD=Standard deviation, SE=Standard error

On comparing the estimated ETT size by age-based formulae with clinically used one, no statistical significance could be appreciated (P = 0.678). Of the total patients, cuffed tube was used in 80 and uncuffed in the remaining 20. Out of these 80 patients, correlation was found in 62 of them between ETT size predicted by both the methods, i.e., age-based formula and clinically used ones. In the remaining 18 patients, age-based formulae predicted smaller tube size 12 times and larger tube size 6 times, when compared to the clinically used ones.

In 20 patients where uncuffed ETT was used, the age-based formulae calculated the tube size accurately in 4, underestimated the size in 2 and overestimated the size in 14 patients, as compared to the clinically used ETT size. Thereby, it can be derived that age-based formula predicted the ETT size better for the cuffed tubes, whereas for uncuffed ETT, it predicted a larger size when compared with the clinically used tubes.

When the comparison was drawn between the height-based formulae and the clinically used ETT size, it was found to be statistically significant (P = 0.0002) [Table 2]. Out of the 80 patients in whom cuffed ETT was used, the ETT size as determined by the height-based formula correlated with the clinically used ETT size in only 32 patients. In 20 patients where uncuffed tube was used, the ETT size determined by the height-based formula correlated with clinically used ETT size in four patients, determined larger ETT size than clinically used ETT size in 16 patients.

Table 2

Comparison of endotracheal tube size (mm) estimated by ultrasonography, height-based formula, and age-based formula with endotracheal tube size used clinically on the operating table by dependent t-test

Techniques	Mean	SD	SE	t	P
ETT size estimated by USG (mm)	3.83	0.69	0.08	0.0000	1.0000
ETT size used clinically on OT table (mm)	3.92	0.23	0.08
ETT size derived by height-based formulae (mm)	5.09	0.44	0.06	3.8331	0.0002*
ETT size used clinically on OT table (mm)	3.92	0.23	0.08
ETT size derived by age-based formulae (mm)	4.32	0.56	0.07	0.5527	0.4325
ETT size used clinically on OT table (mm)	3.92	0.23	0.08

*P value <0.05=Significant. SD=Standard deviation, SE=Standard error, OT=Operating table, ETT=Endotracheal tube, USG=Ultrasonography

The ETT size estimated by USG and age-based formula correlated with the clinically used ETT size, no statistical difference was noted on comparing these two methods (P = 0.2196). However, there was a highly significant statistical difference on comparing ETT size estimated by height-based formula with age-based formula (P < 0.001) as well as on comparing ETT size estimated by height-based formula and USG (P < 0.001) [Table 3].

Table 3

Comparison of endotracheal tube size (in mm) estimated by ultrasonography, height-based formula, and age-based formula by dependent t-test

Techniques	Mean	SD	Mean difference	SD difference	Paired-t	P
ETT size estimated by USG (mm)	3.83	0.69	−0.39	0.44	0.5734	<0.001*
ETT size derived by height-based formulae (mm)	5.09	0.44
ETT size estimated by USG (mm)	3.83	0.69	0.06	0.41	1.5332	0.143
ETT size derived by age-based formulae (mm)	4.32	0.56
ETT size derived by height-based formulae (mm)	5.09	0.44	0.45	0.27	10.768	<0.001*
ETT size derived by age-based formulae (mm)	4.32	0.56

*P<0.05=Significant. SD=Standard deviation, ETT=Endotracheal tube, USG=Ultrasonography

DISCUSSION

The key to provide safe anesthesia to the pediatric age group lies in a successful intubation in the very first attempt. This avoids any undue handling of the nascent airway. Several formulae and method have been suggested in the past to accurately calculate the ETT size for these patients. Age-based formulae, namely Modified Cole's and Khine's have been used to estimate optimal ETT size more commonly. Under emergency conditions where age and weight parameters are unavailable, body length is the easiest parameter to be obtained.

In 2008, Shih et al. conducted a study on Chinese children which suggested that the length-based formula (for internal diameter in mm) =2+ (body length in cm/30), successfully predicted the appropriate ETT tube size in them.[5] In this study, we tried to draw similar comparison in the Indian population. In the similar context, a Broselow tape has been invented with suggests an appropriate tube size in accordance with the height of the children. Not many studies have been undertaken which compares uncuffed ETT size and cuffed ETT estimation done by USG. To name, few of them are Shibasaki et al.[7] and Sutagatti et al.[8]

Gupta et al. drew a comparison between the ETT size derived by USG and that by age-based formula with the clinically used ETT. Their research showed that there was a high correlation between clinically used ETT and predetermined ETT by USG than that predicted by age-based formula. Our study results were comparable with their findings. However, their study did not clarify the fact whether cuffed or uncuffed ETT was used.

In a study conducted by Bae et al., it was concluded that USG estimated ETT size had better prediction than the age-based ones. Nonetheless, like our study where both cuffed and uncuffed tube was used, in this study, only the uncuffed tubes were used. Furthermore, the USG was done on the operation table after induction of anesthesia, contrary to ours. In our study, USG was done in awake patient, but by a single experienced radiologist to eliminate undue bias. Khine et al. found that modified Khine's formula predicted 99% of cuffed ETTs and that Cole's formula predicted 77% of uncuffed tubes as appropriate in young children.[6]

Raphael et al. found that the determination of cuffed ETT size by ultrasound was a good predictor of appropriate sized ETT in pediatric patients when compared with age-based formula. They performed USG-guided estimation of tracheal diameter after induction of anesthesia during mask ventilation of patient.[9] In two studies, ETT size was estimated using the length-based method using Broselow tape that is used in pediatric resuscitation. However, validation of Broselow tape for ETT size estimation is done for children from the United States.[10]

From this study, we could conclude that, that age-based formula for cuffed tubes better predicted the ETT size, whereas for uncuffed ETT, the formula predicted the ETT size larger than used clinically. Furthermore, the height-based formula not only overestimated the ETT size tube for both cuffed and uncuffed tubes but was also highly inaccurate. This was not in accordance to the results derived by Khine et al., and this difference could be attributed to difference in ethnicity and variability of age group of the population studied.

USG does not require strict immobility, especially in infants and children, as opposed to MRI or computed tomography scan;[3] nevertheless, USG depends on the skill of the operator, requires training and yet is relatively simple to learn.

Few limitations of this study can be listed as: USG in nonsedated children who were uncooperative at times; the subjective nature of the air leak test; factors affecting air leak test such as head position and degree of neuromuscular blockade; nonuniformity of children chosen in terms of height for weight.

This study determines the potentiality of USG in the estimation of ETT size, both cuffed and uncuffed. Therefore, it can be recommended that USG is an effective tool for the assessment of ETT size in pediatric patients. As mentioned by some authors, ultrasound may be useful to evaluate patients with subglottic stenosis, a common complication in neonatal or pediatric anesthesia.[9]

CONCLUSION

USG is effective in estimating the appropriate sized ETT both for cuffed and uncuffed tubes. Comparability was seen in both age-based formulae and USG for cuffed ETTs; however, age-based formula overestimated the tube size in case of uncuffed ETT. Be it for the cuffed or the uncuffed tubes, the height-based formula proved to be ineffective in both the cases. Furthermore, exchange of the tube was required once in seven patients.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.