European Journal of Cardiovascular Medicine

The estimation of optimal endotracheal tube (ETT) size in pediatric patients is fundamental to airway safety and effective ventilation. Improper sizing can cause serious complications—oversized tubes can lead to mucosal injury, subglottic stenosis, and post- extubation stridor, while undersized tubes can result in air leaks and inadequate ventilation1,2

Traditionally, pediatric anesthesiologists rely on formulae based on age (Cole's formula), weight, or height. However, these approaches may not account for individual anatomical variability, especially in children with growth abnormalities, malnutrition, or obesity3,4 Cole’s formula, developed in the 1950s, remains the most widely used but demonstrates significant inter-individual variability5

Recent advancements in ultrasonography have enabled real-time visualization and measurement of airway structures. Ultrasound-based measurement of the subglottic diameter—identified as the narrowest part of the pediatric airway—has shown strong correlation with optimal ETT size6,7 It offers non-invasive, point-of-care assessment, making it an attractive alternative for pediatric airway planning8

This study was designed to compare three ETT size prediction methods—age-based (Cole’s), height-based, and ultrasound-guided subglottic diameter—and assess their correlation with actual tube sizes confirmed intraoperatively.

Study Design and Participants

This prospective observational study was conducted at Gayatri Vidya Parishad Institute of Health Care and Medical Technology over a 12-month period following approval from the Institutional Ethics Committee. A total of 150 children aged 1 to 10 years, classified as ASA physical status I or II, scheduled for elective surgery under general anesthesia with tracheal intubation, were included.

Exclusion Criteria

• Known upper airway anomalies
• History of recent intubation (<6 months)
• Emergency procedures
• Need for cuffed ETTs
• Uncooperative for preoperative ultrasound

Sample Size

The sample size was calculated to detect a minimum correlation coefficient of 0.85 for ultrasound prediction with 95% confidence and 5% margin of error, yielding a required sample of 150 patients.

ETT Size Estimation Methods

1. Age-Based (Cole’s) Formula:

The Cole formula is expressed as:

ETT ID (mm) = (Age in years / 4) + 4

Where:

𝐼𝐷   = Inner Diameter of the uncuffed ETT (in millimeters)

𝐴𝑔𝑒 = Child's age in years

2. Height-Based Formula:

ETT ID (mm) = Height in cm /4 + 3.5 

Where:

𝐼𝐷   = Inner Diameter of the uncuffed ETT (in millimeters)

Height = Child's Height in cms

3. Ultrasound Method:

A high-frequency (7–15 MHz) linear probe was used to measure the subglottic transverse diameter at the level of the cricoid cartilage in the transverse view.

Predicted ETT ID (mm) = Subglottic diameter (mm) − 0.5

4. Actual ETT Size:

An uncuffed ETT was inserted and the correct size was confirmed by the minimal leak technique during positive pressure ventilation.

Statistical Analysis

Data were analyzed using SPSS v25. Pearson correlation coefficients were used to compare estimated versus actual ETT sizes. Accuracy was defined as the percentage of cases where the predicted size matched the inserted ETT. ANOVA was used to test differences between methods, with significance set at p < 0.05.

Demographics

• Total participants: 150

• Mean age: 3 ± 2.7 years

• Mean weight: 1 ± 4.6 kg

• Mean height: 2 ± 13.4 cm

• No adverse airway events or complications were

Prediction Accuracy and Correlation

Table 1: Correlation and prediction accuracy by method.

Figure 1: Correlation scatter plots comparing estimated and actual ETT sizes using three methods.

Ultrasound demonstrated significantly higher prediction accuracy (p < 0.001).

This study affirms that ultrasound-guided subglottic diameter measurement offers superior accuracy in determining optimal ETT size in pediatric patients compared to age- or height-based formulas. The high correlation coefficient (r = 0.91) and 90.7% prediction success rate underscore its clinical utility.    

While Cole’s formula remains popular due to its simplicity, it does not account for individual anatomical variations5,10 Height-based equations provide a slight improvement in prediction but still fall short in diverse populations9,11

Ultrasound bridges this gap by offering direct visualization of the airway anatomy. Several studies have corroborated its high predictive accuracy. Shibasaki et al. reported ultrasound prediction success in over 90% of cases, consistent with our findings^6. Similarly, Gupta et al. observed superior outcomes in Indian pediatric patients using ultrasound7

Lakhal et al. and Bae et al. demonstrated that ultrasound measurement of the transverse diameter at the cricoid level correlates well with actual tube sizes, especially in neonates and infants4,12 Moreover, Bhalotra et al. emphasized that ultrasound could help reduce the number of ETT exchanges, improving patient safety13

Ultrasound is particularly valuable in children with syndromic facies, craniofacial abnormalities, or those with a history of prolonged intubation—situations where formula-based methods are least reliable14,15 It is also non-invasive, portable, and increasingly available in operating rooms and emergency departments16

Our findings reinforce the growing consensus in pediatric anesthesiology that ultrasound- guided subglottic diameter measurement significantly outperforms traditional age- and height-based formulae.

Strength of Evidence from Literature

• A comprehensive systematic review of 16 studies involving 1,633 pediatric patients demonstrated that ultrasound predicted optimal ETT size correctly in 48– 100% of cases, whereas age-based formulae ranged between 24–95% accuracy; correlation coefficients ranged from R² = 0.68 to 0.9817
• A prospective Turkish study of 152 children using cuffed ETTs found that ultrasound matched the best-fit size in 88%, with better agreement than height- or age-based methods via Bland–Altman analysis18
• Gupta et al. validated subglottic ultrasound in 112 Indian children and found significantly higher correlation between predicted and actual ETT sizes compared to physical indices like age or height19
• Another observational study involving children with congenital heart disease reported similar advantages of ultrasound-guided prediction over traditional formulas

Meta-Analysis & Clinical Impact20,21

• A recent meta-analysis by Gupta & Ahluwalia (2022) confirmed ultrasound as a superior predictor of ETT size relative to formula-based methods, with pooled sensitivity and specificity markedly higher in ultrasound cohorts
• The learning curve is manageable: Orhan-Sungur et al. showed that proficiency in measuring subglottic diameter via ultrasound can be achieved after relatively short hands-on training sessions

Clinical Implications22,23

• Reduced tube exchanges & trauma: Multiple studies (including our own and the literature) show fewer ETT repositioning events when ultrasound is used, minimizing airway injury from oversized or undersized tubes
• Relevance in anatomically at-risk populations: In children with syndromic or craniofacial anomalies, formulae are particularly unreliable—ultrasound offers a more individualized and precise approach
• Utility in emergency or resource-limited settings: Liu et al.’s systematic review reported diagnostic accuracy for ultrasound in determining intubation depth and tube size ranging from 90.6% to 100% across pediatric subgroups.

Limitations and Caveats24,25,26

• A few studies (e.g., Ain-Shams Journal of Anesthesiology) reported that although ultrasound measurements correlated strongly with age-based formulae, the differences between methods were not always statistically significant in certain samples.
• Operator dependency remains a concern. Standardization and formal training are essential to ensure consistent measurement techniques; Orhan-Sungur et al. identified measurable improvement after repeated scanning sessions.
• Most studies to date—including ours—have used uncuffed ETTs. More research is needed on cuffed tube selection, with preliminary evidence from Altun et al. and Abdel-Ghaffar et al. (2024) suggesting similar predictive utility in those settings.

Future Directions

• Large multicenter trials are needed for broader validation across diverse pediatric populations and geographic regions.
• Protocol development: A standardized airway ultrasound nomogram may streamline clinical decision-making
• Integration with AI and machine learning: Emerging tools may enhance predictive accuracy by combining biographical data with ultrasound metrics.
• Emergency and critical care settings: Early evidence supports ultrasound utility for confirming intubation depth and tube placement in high-stress situations

Limitations

• Operator-dependent nature of ultrasound requires training and standardization27
• Only uncuffed ETTs were studied; cuffed tubes may require different adjustments28
• Single-center study; results may not be generalizable.
• Future studies should explore:
• Integration into emergency intubation protocols
• Standardized nomograms based on ultrasound data
• Comparison with video-laryngoscopic confirmation
• Pediatric cuffed ETT predictions29,30

Ultrasound-guided subglottic diameter measurement is a reliable, accurate, and reproducible method for selecting ETT size in pediatric patients. It clearly outperforms age- and height-based formulas, particularly in anatomically variable children. Adoption into routine pediatric anesthetic practice can enhance airway safety, minimize complications, and improve outcomes.

Declarations

 Funding: None

Conflict of Interest: None

Ethical Approval: Approved by Institutional Ethics Committee

Consent: Written informed consent obtained from parents or legal guardians.

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