Characteristics of endotracheal tube design of different brands are related to proper endotracheal tube position in pediatrics: a descriptive study

Introduction

Endotracheal intubation is a procedure performed to establish a definitive airway in various settings, including elective intubation in the operating room, life-saving procedures in the emergency department, or emergent intubation in the intensive care unit for critically ill patients. Ideally, the appropriate depth of the endotracheal tube (ETT) is achieved when its tip is placed at the midtrachea level to avoid risks such as subglottic placement of the cuff, ETT dislodgement, or endobronchial intubation (1). The position of the ETT tip can be visualized and confirmed using chest X-ray. However, radiographic confirmation may not be immediately available in some circumstances, such as in the operating room or during cardiopulmonary resuscitation.

Typically, the vocal cord marking (VCmarking) on the ETT serves as a guide for proper intubation depth (1). It is generally accepted that the ETT is appropriately positioned when the VCmarking aligns with the vocal cord level. Nevertheless, complications such as endobronchial intubation and subglottic cuff placement have been reported in both adult (2,3) and pediatric populations (4,5). In pediatric patients, the incidence of endobronchial intubation has been reported at 18% (4), while the incidence of subglottic cuff placement ranges from 24% to 91% (4,5). Variations in pediatric ETT designs among different brands have been documented, even for ETTs of the same size (4-8). These variations include the presence and number of VCmarkings, as well as the distances from the VCmarking to the tip and the cuff. Therefore, selecting the proper ETT can be challenging due to these variations.

In pediatric patients, the chosen ETT size should also be large enough to provide adequate ventilation without causing an increase in inspiratory pressure (9). Conversely, an ETT that is too large can compromise mucosal perfusion pressure, increasing the risk of laryngeal trauma and subglottic stenosis (10,11). Variations in the outer diameter (OD) of the inflated cuff have also been reported in pediatric ETTs (9), which can also affect the selection of an ETT, as the OD corresponds to the patient’s airway diameter (12).

While ETT selection should be considered based on several factors such as age, height, airway diameter, or clinical condition (13,14), the current Pediatric Advance Life Support (PALS) guideline still recommends ETT selection based solely on inner diameter (ID) using an age-based or length-based formula (15), despite variations in designs among the same ETT size. This study aims to examine the characteristics—such as length and OD—of various ETTs across different sizes and brands, including VCmarking designs, cuff locations and ODs. We present this article in accordance with the MDAR reporting checklist (available at https://tp.amegroups.com/article/view/10.21037/tp-2025-71/rc).

Methods

This descriptive study examined characteristics of ETTs available in Thailand. The study was approved from the Siriraj Institutional Review Board (approval No. Si 906/2021). This study did not involve human subject and informed consent was not required.

We purchased uncuffed and cuffed ETTs, with IDs ranging from 3.0 to 8.5 mm, from seven brands available in the market through local distributors in Thailand between March and August 2022. The brands included: 1, Ruschelit, 2; Shiley; 3, Curity; 4, Portex; 5, Unomedical; 6, Fornia; 7, Microcuff. Two samples of each ETT were obtained, and measurements were performed independently by two investigators (J.W. and P.K.), thereby resulting in four measurements per size for each study number. The four measurements were carried out as technical replicates. Each ETT was assigned a study number based on its brand and cuff type, with “U” indicating uncuffed and “C” indicating cuffed. For example, the study number “1U” represents a Ruschelit uncuffed ETT. Distances were measured using an electronic sliding caliper (Digimatic Caliper; Model No. CD-6 ASX, Mitutoyo Corporation, Kawasakishi, Japan).

Distances on the ETT were measured from the midpoint of the VCmarking. For wider VCmarking, the distances were calculated as the average value of the upper and lower edges of the VCmarking. The Mark-Tip distance was defined as the length from the VCmarking to the tip of the ETT. The Mark-Cuff distance was defined as the length from the VCmarking to the upper edge of the cuff. The Cuff-Tip distance was defined as the length from the upper edge of the cuff to the tip of the ETT. ID, OD, and inflated cuff OD were recorded as specified by the manufacturer. For cuffed ETT, the OD and circumference at the deflated cuff level were also measured. Circumference measurements were marked on cotton tape and recorded using an electronic sliding caliper. The materials and the presence of a Murphy’s eye were noted for each ETT. Measurements are illustrated in Figure 1.

Figure 1 Measurement of distances and diameters on ETT. Created in BioRender. Chaikittisilpa N. 2025. Available online: https://BioRender.com/x29a151. ETT, endotracheal tube; VC-midTrachea, the length from vocal cord to mid-trachea.

Statistical analysis

Data were collected and managed using electronic data capture tools (REDCap; Research Electronic Data Capture) (16,17) hosted at Mahidol University (HSRI 64-143). Statistical analyses were conducted using SPSS program version 18.0.0 (PASW Statistics 18, Chicago, IL, USA). Cross-sectional area (CSA) was calculated from the OD for uncuffed ETTs and from the circumference at the deflated cuff level for cuffed ETTs. Descriptive statistics were presented as the mean, standard deviation (SD), minimum, maximum, and range (maximum − minimum). Outliers and missing data were identified and rechecked through repeated measurements.

Results

A total of 98 ETTs were included in the study, comprising 27 uncuffed ETTs (ID 3.0–6.0 mm) from four brands and 71 cuffed ETTs (ID 3.0–8.5 mm) from seven brands (Table 1). VCmarkings were present in 79 (80.6%) ETTs, of which 42 ETTs had a single VCmarking and 37 ETTs had multiple VCmarkings. Notably, 19 (19.4%) ETTs lacked VCmarkings. These includes pediatric Ruschelit uncuffed ETTs (ID 3.0–6.0 mm), Shiley pediatric cuffed ETTs (ID 3.0–6.0 mm), and Fornia pediatric cuffed ETTs (ID 3.0–5.5 mm). Examples of variations in VCmarkings and ETT designs are illustrated in Figure 2. A Murphy’s eye was absent in 14 (14.3%) ETTs [Portex uncuffed ETTs (ID 3.0–6.0 mm), and Microcuff pediatric ETT (ID 3.0–6.0 mm)]. Details on the presence of Murphy’s eye are provided in Table S1. Cuffs for all brands are made from polyvinyl chloride, except the Microcuff, which is made from polyurethane.

Figure 2 Examples of ETT designs among different brands for the corresponding age. Study number: 1, Ruschelit; 2; Shiley, 3, Curity; 4, Portex; 5, Unomedical; 6, Fornia; 7, Microcuff. U refers to uncuffed ETT. C refers to cuffed ETT. The maximum (blue line), minimum (pink line), and differences in Mark-Tip distances for age-specific ETT sizes are presented. Created in BioRender. Chaikittisilpa N. 2025. Available online: https://BioRender.com/t56v416. ETT, endotracheal tube; ID, inner diameter; max, maximum; min, minimum.

The Mark-Tip distances of ETTs varied among the same-sized ETTs from different brands, with greater variation observed as the ETT size increased. Distances ranged from 10.0 to 40.3 mm, with the highest variation occurring ETTs with an ID of 8.5 mm (Figure 3A and Table S2). The Mark-Cuff distances also varied among same-sized ETTs, ranging from 1.2 to 30.4 mm. The greatest variation was observed in ETTs with an ID of 6.0 mm (Figure 3B and Table S3). Only 22 (22.4%) ETT had Mark-Tip distances provided by the manufacturer, and only 3 (3.1%) ETTs had Mark-Cuff distances provided (Table S4). The variation of the Cuff-Tip distances among the same-sized ETTs ranged from 10.7 to 21.2 mm, with the highest variation observed in ETTs with an ID of 6.0 mm (Figure 3C and Table S5).

Figure 3 Differences in distances on the ETT related to proper ETT position among different brands. (A) Mark-Tip distance; (B) Mark-Cuff distance; (C) Cuff-Tip distance. Created in BioRender. Chaikittisilpa N. 2025. Available online: https://BioRender.com/j02r856. ETT, endotracheal tube; VCmarking, vocal cord marking.

The ODs reported by manufacturers exhibited minimal variation, with a maximum range of 1.0 mm (Figure 4A and Table S6). However, the OD measured at the deflated cuff area was consistently larger than the manufacturer-reported OD (Figure 4B and Table S7). The differences between the two parameters, referred to as the OD difference, ranged from 0.1 to 2.6 mm. Microcuff ETTs demonstrated the smallest OD difference compared to other brands (Figure 4C and Table S8). The CSA at the deflated cuff level and the OD of the inflated cuff were reported in Tables S9,S10, respectively.

Figure 4 Differences in the OD of ETT among different brands. (A) OD provided from manufacturers; (B) OD measured at the deflated cuff level; (C) the difference of OD between non-cuffed area and at the deflated cuff level. Created in BioRender. Chaikittisilpa N. 2025. Available online: https://BioRender.com/w48h968. ETT, endotracheal tube; OD, outer diameter.

Discussion

This study revealed significant variations in Mark-Tip, Mark-Cuff, and Cuff-Tip distances among ETTs of the same ID from different brands, likely due to the differences in VCmarking designs and the cuff locations. Overall, the Mark-Tip distances generally increased with larger ETT sizes; however, the Mark-Cuff distances did not consistently follow this trend. Our findings showed greater variation in Mark-Tip and Cuff-Tip distances compared to those previously reported by Weiss et al. (7). These variations can lead to differences in ETT depth when the VCmarking is placed at the vocal cord level. Such variations, while seemingly minor, could significantly impact clinical outcomes, especially in pediatric patients with relatively short total airway lengths (18).

For example, the Mark-Tip distances of the cuffed ETTs with an ID of 4.0 mm, typically recommended for 2-year-old patients, ranged from 35.1-65.6 mm (up to 30.5 mm variation). In comparison, the median total airway length (distance from the vocal cord to the carina) in patients aged 2 to 4 years was reported as 76.2 mm (5). This 30.5 mm variation accounts for approximately 40% of the total airway length for this age group. Similarly, the measured Mark-Cuff distances ranged from 7.0 to 31.8 mm, while distance from the vocal cord to the cricoid outlet in this age group is reported to be a median of 8.0 [interquartile range (IQR), 6.8–9.0] mm (5). This finding suggests that in some patients, an ETT with a Mark-Cuff distance of 7.0 mm could result in the cuff being positioned above the cricoid outlet, potentially leading to glottic injury. According to systematic review and meta-analysis in adults from Brodsky et al., laryngeal injury after intubation during surgery was reported in 20% of patients (19). The ranges of Mark-Cuff, Cuff-Tip, and Mark-Tip distances across different ETTs in relation to age-specific pediatric airway anatomy are detailed in Table 2. Some of the observed variations may arise from the presence of both upper and lower VCmarkings. Traditionally, the intended placement aligns the vocal cords between these two markings. The ETT designs with a long Mark-Tip distance or those positioned using the upper VCmarking are more likely to place the cuff below the subglottis, with the tip positioned closer to the carina. Conversely, ETT designs with a short Mark-Cuff distance or a long Cuff-Tip distance may increase the risk of cuff placement within the subglottis.

This study also identified variations in OD and CSA at the deflated cuff level in cuffed ETTs. The variation of OD for uncuffed ETTs was minimal. Both cuffed and uncuffed ETTs are considered acceptable for use in infants and children undergoing emergency intubation (20), though there is insufficient evidence to recommend the routine use of cuffed ETT in neonates (21). PALS recommends using the following formulas to estimate ETT size in patients older than 2 years (15):

UncuffedETTID(mm)=4+[age(years)/4][1]

CuffedETTID(mm)=3.5+[age(years)/4][2]

While these formulas guide ID selection based on age, the OD of the ETT, which more closely correlates with the patient’s airway diameter, is often overlooked. For a 2-year-old patient, the recommended size of ETT would be a cuffed ETT ID 4.0 mm or an uncuffed ETT ID 4.5 mm. For these sizes, the OD ranges from 5.3 to 6.0 mm for cuffed ETTs and 6.0 to 6.2 mm for uncuffed ETTs. The measured ODs for cuffed ETTs with an ID of 4.0 mm at the deflated cuff level, which were not provided by the manufacturer, ranged from 5.7 to 6.6 mm. Studies using different methods have reported variations in tracheal diameter among children. An international study (Europe, United States, and Australia) reported the median diameters of the proximal trachea in the 2–4 years age group were 7.3 mm in male and 6.5 mm in female (22). A study in Saudi Arabia reported the diameter of cricoid as mean anteroposterior diameter of 7.41 mm (95% confidence interval: 6.22–8.60), and mean transverse diameter of 7.59 mm (95% confidence interval: 6.94–8.23) (23). However, in the Thai population, the median transverse ultrasonographic subglottic diameter for 2-year-old was only 4.1 (IQR, 3.6–5.0) mm (12). For children with smaller airway diameters, an additional thickness of 1.7–2.6 mm from the ID 4.0 mm at the deflated cuff level could exceed the patient’s airway diameter, leading to inappropriate size selection if the recommended cuff ETT was used. Radiological evidence has shown oversized intubation in 60.0% of uncuffed ETTs and 38.3% of cuffed ETTs (P<0.05) (24). The ideal cuff properties should allow sufficient inflation to provide an adequate seal for the respective airway size (9), while remaining small enough when deflated to pass through the narrowest part of the airway. Fischer et al. reported considerable heterogeneity in cuff OD among pediatric ETT brands, noting that many designs lack an age-related anatomical rationale (9). However, these findings in ETT design require further validation in clinical settings to determine the appropriate leak pressure (20–25 cmH₂O) to ensure adequate inspiratory pressure and effective sealing while minimizing the risk of mucosal pressure injury (25).

The ideal position for ETT placement is achieved when the VCmarking is aligned with the vocal cord level, the ETT tip should reside at the mid-trachea, with the cuff is located below the subglottis. To accomplish this positioning, the key ETT characteristics include: (I) an appropriate Mark-Tip distance; (II) a sufficient Mark-Cuff distance that exceeds the length from the vocal cord to the cricoid outlet; and (III) a short Cuff-Tip distance. However, data on these specific distances has only been investigated for a limited number of manufacturers. Weiss et al. investigated the intubation depth markings of the pediatric Microcuff ETT, demonstrating effective placement with a cuff-free subglottic zone, thereby minimizing the risk for endobronchial intubation (26,27).

In our study, among all ETT brands, the Microcuff ETTs exhibited not only the shortest Cuff-Tip distance (Figure 3C), but were also made of polyurethane, resulting in the smallest OD difference at the deflated cuff level (Figure 4B). Nevertheless, the short Cuff-Tip distance carries an inevitable disadvantage, as it eliminates space for Murphy’s eye (28). The Murphy’s eye, a lateral hole on the side of an ETT, serves as a secondary vent to prevent complete airway obstruction in case the primary distal opening becomes occluded (29). The importance of Murphy’s eye was not clearly stated in pediatric literatures. Our findings suggest that variations in ETT designs may not consistently ensure safe intubation depth markings, potentially compromising optimal tube placement. A pediatric cadaveric study reported that, despite placement based on the VCmarking, the upper edge of the cuff frequently rested within or close to the cricoid outlet (30). Previous clinical studies reported an incidence of endobronchial intubation of 18% (4), and reported that cuff placement within the subglottis in pediatric patients ranged from 24% to 91% (4,5). Theoretically, a large OD combined with a high cuff location may contribute to an increased risk of laryngeal injury following intubation (10,11).

After evaluating the available ETT brands, we observed variations in design that may potentially position the cuff within the subglottic region, particularly for tubes with a long Cuff-Tip distance. Among all brands examined, the Microcuff ETT consistently exhibited shorter Cuff-Tip distances across all sizes. Therefore, the Microcuff design reduces the likelihood of the cuff being positioned at the glottic level when the ETT marking is aligned with the vocal cords. Further research is required to develop a safer ETT designs that address these risks while ensuring effective and accurate placement.

There are several limitations in this study. First, as a descriptive study of ETTs, the findings require validation in clinical settings, which future research should compare pediatric airway anatomy with current ETT designs when the ETTs are placed in situ. This pediatric ETT study does not provide guidance on appropriate ETT selection based on diameter. Optimal ETT sizing should consider not only airway diameter but also factors such as sealing pressure or cuff pressure, which depend on the interaction between the airway dimensions and the cuff or the OD of the ETT. Second, the measurements of ODs and circumferences at the deflated cuff level may not be entirely precise due to the variability in the shapes of folded cuffs. Lastly, although ETT brands examined are marketed globally, the samples were limited to those available in Thailand in 2022, which may limit the generalizability of our findings to other pediatric ETT types.

Conclusions

Variations in the Mark-Tip, Mark-Cuff, and Cuff-Tip distances were observed across all sizes of ETTs. Overall, the Mark-Tip distances generally increased with larger ETT sizes; however, the Mark-Cuff distances did not consistently follow this trend. In addition to selecting ETT size based on age-appropriate ID, the ETT designs for optimal Mark-Tip and Mark-Cuff distances for the patient’s age should also be considered. Preferred characteristics of cuffed ETTs include a short Cuff-Tip distance and a thin cuff material or design to minimize additional thickness to the OD as much as possible. However, short Cuff-Tip distance and the absence of Murphy’s eye must be considered.

Acknowledgments

This manuscript was accepted for abstract presentation in the 101^st Annual Scientific Meeting of the Korean Society of Anesthesiologists on 9^th November 2024, Incheon, Korea. We acknowledge Miss Arporn Pimtong from Department of Anesthesiology for her administrative support.

Footnote

Reporting Checklist: The authors have completed the MDAR reporting checklist. Available at https://tp.amegroups.com/article/view/10.21037/tp-2025-71/rc

Data Sharing Statement: Available at https://tp.amegroups.com/article/view/10.21037/tp-2025-71/dss

Peer Review File: Available at https://tp.amegroups.com/article/view/10.21037/tp-2025-71/prf

Funding: This study was supported by Siriraj research fund, Faculty of Medicine Siriraj Hospital, Mahidol University [grant number (IO) R016533022].

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://tp.amegroups.com/article/view/10.21037/tp-2025-71/coif). T.K. received honorarium from Teleflex Medical Asia Pte Ltd. as a consultant for delivery of clinical skill expertise in hands on workshop for ultrasound-guided peripheral-inserted central catheter (PICC) placement on 10^th September 2023. The other authors declare no conflicts of interest.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was approved by the Siriraj Institutional Review Board (approval No. Si 906/2021). This study did not involve human subject and informed consent was not required.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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