Correlation of tracheal size with body mass index – Retrospective computerised tomographic evaluation

Kalyani Anand Sathe; Hrishikesh Kale; Harshal Wagh; Barton Branstetter

doi:10.4103/arwy.arwy_55_21

ORIGINAL ARTICLE

Year : 2022 | Volume : 5 | Issue : 1 | Page : 25-29

Correlation of tracheal size with body mass index – Retrospective computerised tomographic evaluation

Kalyani Anand Sathe¹, Hrishikesh Kale², Harshal Wagh³, Barton Branstetter⁴
¹ Deenanath Mangeshkar Hospital and Research Centre, Pune, India
² Department of Radiology, Kokilaben Dhirubhai Ambani Hospital, Mumbai, Maharashtra, India
³ Department of Anesthesiology, Kokilaben Dhirubhai Ambani Hospital and Research Centre, Mumbai, Maharashtra, India
⁴ Department of Radiology, University of Pittsburgh Medical Centre, Pittsburgh, Pennsylvania, USA

Date of Submission	24-Sep-2021
Date of Acceptance	15-Jan-2022
Date of Web Publication	11-Mar-2022

Correspondence Address:
Dr. Kalyani Anand Sathe
Atharva, 257/2/3, Green Park Society, Lane No. 3, Opp Anandban Health Club, Aundh, Pune - 411 007, Maharashtra
India

Source of Support: None, Conflict of Interest: None

DOI: 10.4103/arwy.arwy_55_21

Abstract

Introduction: A question often asked in the anaesthetic room is 'What size endotracheal tube (ETT) should be used for this patient?' In the recent past, it has become common for anaesthesiologists to use ETTs 1–2 mm smaller than the expected tracheal size. However, it is difficult to gauge the appropriate size of ETT in obese patients. Aim: This study aimed to establish the baseline dimensions of the normal adult trachea and determine whether body mass index (BMI) affects cervical tracheal size. Patients and Methods: A total of 179 patients were included in the study. All imaging was performed on a 64-slice Lightspeed scanner (GE Healthcare) using collimation of 1.25 mm or 2.5 mm. Two axial levels were identified: the first tracheal ring and the most superior segment of the substernal trachea (i.e., the thoracic inlet). The diameter of the trachea in the anteroposterior (AP) and transverse (Trans) dimensions, as well as the cross-sectional area (using freehand region of interest tool) were measured at both the identified levels. The BMI was calculated from weight and height or taken directly from the clinical notes when available. To test the null hypothesis of no association between BMI and tracheal size, Pearson correlation coefficients along with 95% confidence interval were computed. Results: No trends or statistically significant associations were found between BMI and tracheal size on computerised tomography using AP, transverse and cross-sectional area measurements. Conclusion: Our study suggests that there is no link between BMI and tracheal size.

Keywords: Body mass index, endotracheal tube size, obesity, tracheal diameter

How to cite this article:
Sathe KA, Kale H, Wagh H, Branstetter B. Correlation of tracheal size with body mass index – Retrospective computerised tomographic evaluation. Airway 2022;5:25-9

How to cite this URL:
Sathe KA, Kale H, Wagh H, Branstetter B. Correlation of tracheal size with body mass index – Retrospective computerised tomographic evaluation. Airway [serial online] 2022 [cited 2023 Mar 30];5:25-9. Available from: https://www.arwy.org/text.asp?2022/5/1/25/339388

Introduction

In 1928, Magill suggested that 'the largest endotracheal tube which the larynx will comfortably accommodate' should be used for intubating a patient.^[1] Choosing the correct size of an endotracheal tube (ETT) or tracheostomy tube is a challenging task for most anaesthesiologists. The general trend is to use an ETT or tracheostomy tube 1 or 2 mm smaller than the expected tracheal size. However, this is especially challenging in obese patients. One study demonstrated that obese patients are more likely to be intubated with larger sized ETTs when compared with non-obese patients in order to overcome the work of breathing which is believed to be less with a larger tube.^[2] Another study showed that with an increase in body mass index (BMI), there is a decrease of tracheal width, with the consequent need to avoid the tendency to use a larger tube to intubate an obese patient.^[3]

The aim of this study was to establish the baseline dimensions of the normal adult trachea and determine whether BMI affects cervical tracheal size. We believe the outcome of this study can help anaesthesiologists in selecting the appropriate size of ETT.

Patients and Methods

Approval for the study was obtained from the Institutional Review Board (PRO13050139 dated 01 May 2013). A total of 202 patients were sequentially imaged over a period of 5 months with computerised tomographic (CT) angiography or CT of the cervical spine in the emergency department of a tertiary care hospital. The images were retrospectively evaluated by a single fellowship-trained neuroradiologist. After applying the exclusion criteria mentioned below, a total of 179 patients were included in the study.

Patients who were already intubated; those aged between 18 and 70 years; those with thyroid masses, significant cervical or upper thoracic trauma with fractures; neck surgery or advanced respiratory disease; patients with a tortuous trachea and those whose images where the thoracic inlet was not completely included or was degraded by movement artefact were excluded from the study.

All imaging was performed on a 64-slice Lightspeed scanner (GE Healthcare) using collimation of 1.25 mm or 2.5 mm. The scans were typically performed at a tube potential of 120 kVp, maximum tube current of 350 mA and display field of view 21 cm adjusted to patient size. Intravenous contrast (Isovue 370) was administered for patients undergoing CT angiography. The scans were performed during quiet respiration. Axial images were reconstructed using a soft-tissue algorithm and viewed with a window width/level of 350/50. Two axial levels were identified: the first tracheal ring and the most superior segment of the substernal trachea (i.e., the thoracic inlet). The diameter of the trachea in the anteroposterior (AP) and transverse (Trans) dimensions, as well as the cross-sectional area (using freehand region of interest tool) were measured at both of the identified levels [Figure 1]a and [Figure 1]b.

Figure 1: Axial images from enhanced computerised tomography of the neck at the levels of the first tracheal ring (a), and the thoracic inlet (b). At each level, the anteroposterior and lateral dimensions of the trachea were measured, and the cross-sectional area was measured by freehand region of interest

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A review of the electronic medical records provided demographic data (age, gender, weight, height and BMI). The BMI was calculated from weight and height or taken directly from the clinical notes when available.

To test the null hypothesis of no association between BMI and tracheal size, Pearson correlation coefficients along with 95% confidence interval were computed. Two-sample t-tests were conducted to test the null hypothesis of no differences between males and females with respect to each variable. All statistical tests were two tailed and deemed statistically significant when P < 0.05. If the test of equality of variances was rejected, the Satterthwaite approximation was used. Satterthwaite approximation is an alternative method to the pooled variance t-test. It is used when the assumption of the equality of variances cannot be confirmed. Statistical analysis was performed using SAS 9.3 Statistical Software (SAS Institute, Cary, NC, USA).

Results

A total of 179 patients were studied of whom 83 (46%) were female and 96 (54%) were male. The mean (range) of BMI for these patients was 28.9 kg/m² (15.2–56.3 kg/m²). The mean age of the patients was 51.6 years (range 20–70 years) [Table 1] and [Table 2].

Table 1: Correlation coefficients for tracheal measurements at the first tracheal ring and thoracic inlet with body mass index

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Table 2: Comparison of tracheal dimensions and gender

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The range of measurements at the first tracheal ring (AP diameter, transverse diameter and cross-sectional area) was 1.1–2.6 cm, 1.2–2.4 cm and 1.4 to 4.6 cm². The range of measurements at the thoracic inlet (AP diameter, transverse diameter and cross-sectional area) was 1–3.3 cm, 1.1–2.6 cm, 1.4–5 cm² [Table 2].

No trends or statistically significant associations were found between BMI and any of the six radiologic measurements [Table 1].

Statistically significant appreciable differences were observed between male and female patients for the means of all the three radiologic tracheal measurements [Table 2].

All P values were < 0.0001, with males having higher mean values for each of the six tracheal measurements [Table 2].

Discussion

The trachea continuously enlarges from the postnatal period to adulthood. Tracheal growth is the greatest in infancy, with the trachea reaching its maximum size around puberty (in some males up to 20 years). Tracheal intubation is a common procedure to secure the airway in the operation theatre. The debate surrounding appropriate ETT size for adults is still ongoing. An oversized ETT can lead to significant complications ranging from self-limiting localised injuries to more severe conditions such as glottic and subglottic stenosis, vocal cord immobility and arytenoid dislocation. These complications may eventually need complex airway surgeries, temporary or even permanent tracheostomy. By contrast, undersized ETTs may compromise ventilation and hinder pulmonary hygiene or fibreoptic bronchoscopy, particularly in intensive care settings.^[4]

In 1928, Magill suggested that 'the largest endotracheal tube which the larynx will comfortably accommodate' should be used for intubating a patient.^[1] Around 30 years ago, it was a common practice to use a 9 mm or 10 mm ID tube for males and an 8 mm ID tube for females. However, in recent years, it has become common for anaesthesiologists to select tubes 1–2 mm smaller. This evolution in practice was initially driven by the observation that there is little impact on ventilator pressures during anaesthesia, and also by a perceived reduction in sore throat and hoarseness. The reported incidence of postoperative sore throat and hoarseness after tracheal intubation varies from 14% to 50%.^[5],[6],[7] Contributory factors include tube size, cuff design and pressure, variation in skills and techniques between anaesthesiologists and the subjectivity of the symptom of sore throat in individual patients.^{[5],[8],[9],[10],[11],[12]}

One study showed that smaller ETTs produce less sore throat (with the incidence being halved with use of smaller ETTs). Smaller tubes may also be easier to insert, including using fibreoptic endoscopy, as the view of the larynx during passage of the tube is subjectively better.^[12] A smaller tube is also associated with less tracheal damage. There is a reported association between larger tubes and glottic/tracheal damage, particularly in women.^[13],[14] In an imaging-based study, CT imaging 6 months after intubation demonstrated some degree of laryngeal abnormality, including tears, scars and laryngocoeles in 86 of 100 patients.^[15] All the above-described factors favour the use of a smaller tube.

There are other studies which have shown that a smaller size ETT may increase ventilatory pressures. Simultaneous measurements of the pressures in the trachea and the ventilator have shown high pressures at the machine end of the tubes at high volumes of ventilation when small ETTs were used. Shapiro et al. found that in normal volunteers breathing through tracheal tubes (6 mm to 9 mm ID) at a constant tidal volume of 500 mL, work of breathing and tension-time index increased as tube diameter decreased.^[15]

Overall, the above studies show the importance of choosing an appropriate size of the ETT that is neither too large nor too small. Selection of an appropriately sized ETT in obese patients is more difficult than is readily apparent. One study showed that obese patients do not have larger airways and indicated a trend toward smaller airways as the BMI increased. Specifically, as BMI increases, the tracheal width appears to decrease. This study points towards the need to avoid the use of a larger ETT to secure the airway in obese patients.^[16]

In another study, BMI showed a negative association with the transverse diameter of the trachea with an increase of 1 kg/m² predicting a decrease of 0.05 mm. This supports the findings of D'Anza et al.^[3]

Another study demonstrated that obese patients are more likely to receive a larger sized ETT compared with non-obese patients, in order to overcome the work of breathing which is believed to be less with a larger tube.^[2]

Such conflicting literature makes our study important to understand the relationship between BMI and tracheal size. In our study, no trends or statistically significant associations were found between BMI and tracheal diameters/cross-sectional area at either the thoracic inlet or the first tracheal ring. Our findings indicate that tracheal size is not associated with BMI. There is, however, a statistically significant appreciable difference between tracheal size in males and females, with the trachea being larger in males. This study also provides baseline measurements of the trachea at the first tracheal ring and at the thoracic inlet in a given set of patients studied.

A potential limitation of our study is the lack of patient medical history and evaluation, and the fact that CT images were used for excluding patients. However, the large number of patients included in our study truly depicts the day-to-day variations which would be encountered in practice by a clinical anaesthesiologist.

Conclusion

Tracheal size is not dependent on BMI. Based on our study, we suggest the use of regular-sized tubes in patients with higher BMI. Anaesthesiologists should refrain from the tendency to use either larger or smaller sized tubes in obese patients.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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