Ultrasound-guided hyomental distance versus weight-based method for ProSeal LMA™ size selection: A randomised controlled study

ABSTRACT

Background and Aims

Conventional weight-based laryngeal mask airway (LMA) size selection does not consistently correlate with individual upper airway anatomy, leading to suboptimal device fit. Ultrasound-guided measurement of hyomental distance (HMD), an indicator of submandibular space, has emerged as a promising anatomy-based alternative. This study aimed to compare the efficacy and safety of ultrasound-guided HMD-based sizing with conventional weight-based sizing of the ProSeal LMA™ (Teleflex Inc., Wayne, PA, USA) (PLMA) in adult patients undergoing elective surgery.

Methods:

This prospective, double-blinded randomised controlled trial included 116 American Society of Anesthesiologists (ASA) physical status I–II adults aged 18–60 years scheduled for elective surgery under general anaesthesia. Patients were randomised into two groups: Group A underwent PLMA size selection based on body weight, while Group B underwent ultrasound-guided HMD-based size selection. The primary outcome was first-attempt insertion success. Secondary outcomes included insertion time, number of attempts, need for reinsertion, and postoperative oral injury.

Results:

Demographic characteristics were comparable between groups. First-attempt insertion success was similar in both groups (93.1% in weight-based group vs. 96.6% in HMD group; P = 0.67). The mean insertion time was comparable (18.1 ± 4.8 s vs. 18.5 ± 4.6 s; P = 0.60). The HMD group predominantly received smaller PLMA sizes (P < 0.001) without increased reinsertion rates or complications. Postoperative oral injury was rare and comparable between groups.

Conclusion:

Ultrasound-guided HMD-based PLMA sizing provides an individualised, anatomy-based alternative to weight-based selection, achieving comparable insertion success, efficiency, and safety while favouring anatomically appropriate device sizing.

INTRODUCTION

Effective airway management is fundamental to safe anaesthetic and critical care practice. Since its introduction by Dr. Archie Brain in 1983, the laryngeal mask airway (LMA) has transformed airway management by providing a secure, less invasive alternative to face mask ventilation and tracheal intubation. Its ease of insertion, minimal need for laryngoscopy or muscle relaxation, favourable safety profile, and reduced postoperative airway complications—such as sore throat, hoarseness, and dental injury—have led to its widespread acceptance.[1]

The LMA is a supraglottic airway device that forms a seal over the laryngeal inlet without entering the trachea. Its use has expanded beyond the operating room to include difficult airway management, emergency resuscitation, cardiopulmonary arrest, and pre-hospital care, particularly in day-care surgeries where rapid recovery is essential.[2] Traditionally, LMA size selection has been based on patient weight;[3] however, poor correlation between body weight and upper airway anatomy often leads to suboptimal device selection.

To address this limitation, ultrasound-guided, anatomy-based techniques are gaining attention. Hyomental distance (HMD)—the distance between the hyoid bone and mentum—serves as an indirect measure of the submandibular space critical for LMA accommodation. Shorter HMD values are associated with difficult airway anatomy.[4] Ultrasonography allows accurate, real-time HMD measurement in neutral and extended positions, improving reliability.[5] Emerging evidence suggests HMD-guided ProSeal LMA™ (Teleflex Inc., Wayne, PA, USA) (PLMA) selection may improve sealing pressures and reduce complications, especially in obese or difficult airway patients. However, direct comparisons between weight-based and HMD-based LMA sizing remain scarce, providing the rationale for the present randomised controlled trial.

METHODS

The prospective double-blinded randomised controlled trial was started after obtaining the institutional ethical committee approval [vide approval number TP (MD/MS) 89/2024/IEC/ABVIMS/RMLH/102] in a tertiary healthcare institute. The study was prospectively registered with the Clinical Trials Registry-India (CTRI) (vide approval number CTRI/2024/03/064875). The first patient was recruited on 22/05/24, and the study was completed on 30/06/2025. This study included adult patients aged 18–60 years of either gender classified as American Society of Anesthesiologists (ASA) physical status I or II, scheduled for elective surgery under general anaesthesia. Patients weighing less than 30 kg or more than 70 kg, pregnant women, and those with achalasia cardia, diabetes mellitus, or clinically predicted difficult airways were excluded from participation. Randomisation was performed using a computer-generated sequence (Red Pill – Sealed Envelope software). Group names were placed in sequentially numbered, opaque envelopes opened immediately before group allocations. The anaesthesiologist performing LMA insertion was not involved in patient randomisation or ultrasound measurement, maintaining double blinding of procedural execution. The primary outcome was the rate of successful first-attempt insertion of the predicted PLMA size. Secondary outcomes included the number of insertion attempts, time required for optimal LMA placement (from device insertion to effective ventilation), and incidence of postoperative oral injury, defined as visible blood staining of the device on removal. The principles of the Declaration of Helsinki (2013) and Good Clinical Practice guidelines were followed in the study.

After obtaining written informed consent, eligible patients were randomly allocated into two groups using a computer-generated sequence concealed in sequentially numbered opaque envelopes. Group A underwent PLMA size selection based on conventional weight-based criteria, whereas in group B, PLMA size was selected according to the ultrasound-measured HMD. For Group B, patients with HMD > 4.4 cm received a size 4 PLMA, while those with HMD < 4.4 cm received size 3, based on previously published anatomical correlations. This 4.4 cm cut-off corresponds to the optimal threshold identified by Wang et al.[6] on receiver operating characteristic curve analysis for predicting appropriate LMA size from the HMD, and it was adopted in the present study to enable comparability with this existing evidence. All participants adhered to standard preoperative fasting guidelines. Patients assigned to the ultrasound group underwent airway scanning in the preoperative holding area. HMD was measured in the supine, neutral head position using a low-frequency curvilinear probe placed in the mid-sagittal plane of the submandibular region [Figure 1]. The distance between the anterior border of the hyoid bone and the posterior aspect of the mandibular symphysis was recorded [Figure 2]. All measurements were performed by an experienced anaesthesiologist proficient in airway ultrasonography.

Figure 1.

Ultrasound measurements of hyomental distance: ultrasound probe in mid-sagittal position with head in neutral position. (Anterior view and Lateral view)

Figure 2.

Ultrasound measurement of hyomental distance

In the operating theatre, routine monitoring was instituted, including non-invasive blood pressure, five-lead electrocardiography, pulse oximetry, and heart rate. Anaesthesia was induced with intravenous midazolam (1 mg) and fentanyl (2 µg/kg), followed by propofol (2 mg/kg) for induction. After confirming adequate mask ventilation, vecuronium (0.1 mg/kg) was administered to facilitate muscle relaxation. Patients were manually ventilated for 3 min with a mixture of oxygen, nitrous oxide, and volatile anaesthetic [sevoflurane or desflurane at 1.0 minimum alveolar concentration (MAC)]. PLMA insertion was performed by a faculty anaesthesiologist experienced with the device. The device was inserted using the standard digital technique, and the cuff was inflated with 20 mL of air for size 3 and 30 mL for size 4, maintaining an intracuff pressure of 60 cm H₂O using a manometer. Correct placement was confirmed by visible chest expansion without air leak, a square-wave capnographic trace, and an exhaled tidal volume within ±50 mL of the set tidal volume on the ventilator. All patients were ventilated with a tidal volume of 8 mL/kg. In cases of inadequate ventilation, the PLMA was reinserted (up to three attempts). If effective ventilation could not be achieved, tracheal intubation was performed, and the case was recorded as a failure. The number of attempts, time to successful insertion, and evidence of oral injury (blood staining on the device) were documented. Based on prior data,[6] the success rate of ultrasound-guided HMD selection was 77.14% compared to 51.5% with the weight-based method. Assuming a two-tailed α = 0.05, power = 80%, and using the formula for comparing two proportions, the required sample size was calculated as 52 patients per group. To account for potential exclusions, the final sample size was increased to 58 per group, yielding a total of 116 participants. Data were entered into Microsoft Excel and analysed using International Business Machines (IBM) Statistical Package for the Social Sciences Statistics for Windows, Version 25.0 (IBM Corp., Armonk, NY, USA). The normality of continuous variables was assessed using the Kolmogorov–Smirnov test. Quantitative variables were summarised as mean ± standard deviation (SD) for normally distributed data and as median with interquartile range (IQR) for non-normally distributed data. Comparisons of continuous variables between the two groups were performed using the independent samples t-test for parametric data and the Mann–Whitney U test for non-parametric data. Categorical variables were expressed as absolute numbers and percentages. Intergroup comparisons of categorical data were analysed using the Chi-square test or Fisher’s exact test where appropriate. All statistical tests were two-tailed. A P value <0.05 was considered statistically significant.

RESULTS

A total of 132 patients were assessed for eligibility, of whom 16 were excluded. The remaining 116 adult patients were enroled and randomised equally into two groups: the conventional weight-based PLMA size selection group (group A, n = 58) and the ultrasound-guided HMD group (group B, n = 58), as illustrated in the Consolidated Standards of Reporting Trials (CONSORT) flow diagram [Figure 3]. Baseline demographic and airway characteristics, including age, gender, body weight, American Society of Anesthesiologists (ASA) physical status, and Mallampati classification, were comparable between the groups (P > 0.05) [Table 1]. The mean age was 40.5 ± 12.6 years in group A and 38.1 ± 12.0 years in group B (P = 0.30), while the mean body weight was 56.3 ± 8.3 kg and 56.9 ± 6.3 kg, respectively (P = 0.08). Approximately 85% of patients in both groups underwent laparoscopic cholecystectomy, with similar surgical profiles. A statistically significant difference was observed in the distribution of PLMA sizes between the two groups (χ²[1] = 14.06, P < 0.001) [Table 2]. In group A, size 4 PLMA was used in 74.1% of patients, whereas in group B, size 3 was used in 60.3% and size 4 in 39.7% of patients. Despite this difference, first-attempt insertion success rates were similarly high in both groups (93.1% in group A vs. 96.6% in group B; P = 0.67). Because the study was not powered on the basis of a peer-reviewed external estimate, we added a post hoc power analysis for the primary binary outcome. The achieved power was low (12–15%), suggesting that the study was underpowered to detect small differences between groups. The mean insertion time did not differ significantly between groups (18.1 ± 4.8 s vs. 18.5 ± 4.6 s; P = 0.60). The requirement for device reinsertion or change was low and comparable (5.2% in Group A vs. 1.7% in Group B; P = 0.61). No episodes of ventilation failure, persistent airway leak, or intraoperative PLMA displacement were observed. Postoperative oral injury occurred in one patient in each group (1.7%; P = 1.00). Overall, ultrasound-guided HMD–based PLMA size selection demonstrated comparable ease of insertion and safety to conventional weight-based sizing, with a tendency towards better anatomical fit.

Figure 3.

Consolidated Standards of Reporting Trials (CONSORT) flow diagram. USG: Ultrasonography; HMD: Hyomental distance; n: Number of patients

Table 1.

Baseline demographic and airway characteristics

Variable	Group A (n=58)	Group B (n=58)	P
Age (years), mean±SD	40.48±12.61	38.10±12.03	0.30
Weight (kg), mean±SD	56.29±8.32	56.91±6.33	0.08
Gender—Male, n (%)	18 (31.0%)	12 (20.7%)	0.20
Gender—Female, n (%)	40 (69.0%)	46 (79.3%)	0.20
ASA I, n (%)	28 (48.3%)	28 (48.3%)	1.00
ASA II, n (%)	30 (51.7%)	30 (51.7%)	1.00
Mallampati (MPG) grade 1, n (%)	7 (12.1%)	8 (13.8%)	0.67
MPG grade 2, n (%)	48 (82.8%)	44 (75.9%)	0.67
MPG grade 3, n (%)	2 (3.4%)	5 (8.6%)	0.67
MPG grade 4, n (%)	1 (1.7%)	1 (1.7%)	0.67
Laparoscopic cholecystectomy, n (%)	50 (86.2%)	49 (84.5%)	0.79

Data are mean±SD or n (%). P values are from Chi-square/Fisher/Mann–Whitney as appropriate. MPG: Mallampati grade; ASA: American Society of Anesthesiologists; SD: Standard deviation; Group A: Weight-based group; Group B: Hyomental distance group

Table 2.

Primary and secondary outcomes

Outcome	Group A (n=58)	Group B (n=58)	P
LMA size 3, n (%)	15 (25.9%)	35 (60.3%)	<0.001
LMA size 4, n (%)	43 (74.1%)	23 (39.7%)	<0.001
First-attempt insertion success, n (%)	54 (93.1%)	56 (96.6%)	0.67*
Number of attempts—1, n (%)	54 (93.1%)	56 (96.6%)	0.67*
Number of attempts—2, n (%)	4 (6.9%)	2 (3.4%)	0.40
Mean insertion time (s), mean±SD	18.09±4.75	18.48±4.61	0.60
Change of LMA—Yes, n (%)	3 (5.2%)	1 (1.7%)	0.61
Reinsertion—Yes, n (%)	4 (6.9%)	1 (1.7%)	0.36
Intra-operative displacement—Yes, n (%)	0 (0%)	0 (0%)	—
Post-op oral injury—Yes, n (%)	1 (1.7%)	1 (1.7%)	1.00

Data are n (%) or mean±SD. *=Fisher’s exact test is applied. LMA: Laryngeal mask airway; SD: Standard deviation; Group A: Weight-based group; Group B: Hyomental distance group. The counts for number of attempts, reinsertion, and change of LMA do not align exactly because these variables captured related but distinct events: A second attempt includes both “reinsertion” of the same LMA size (not counted as change of LMA) after formal removal and “change of LMA”. “Change of LMA” was recorded only when the device was fully withdrawn and reinserted or exchanged for a different size

DISCUSSION

The present study compared ultrasound-guided HMD–based sizing with conventional weight-based selection of the PLMA in adult patients undergoing elective surgery under general anaesthesia. Both approaches demonstrated high overall insertion success; however, ultrasound-guided sizing resulted in a significantly different distribution of PLMA sizes, favouring smaller, anatomically matched devices without compromising insertion ease, ventilation efficacy, or patient safety. These findings underscore the growing importance of individualised, anatomy-based strategies for airway device selection in contemporary anaesthetic practice. Since its introduction by Sir Archie Brain in 1983, the LMA has transformed airway management by offering a reliable supraglottic alternative to face-mask ventilation and endotracheal intubation. Subsequent refinements led to the development of second-generation LMAs, with the PLMA representing a significant advancement over the classic design. The PLMA incorporates a dual-cuff system that improves oropharyngeal seal pressure and an integrated drain tube that facilitates gastric decompression and reduces aspiration risk. These features have made the PLMA particularly suitable for controlled ventilation and laparoscopic procedures, where higher airway pressures are often required. Despite these advantages, optimal PLMA performance depends critically on appropriate size selection. Manufacturer guidelines traditionally recommend sizing based on body weight,[3] a method that is simple and widely adopted. However, body weight alone does not reliably reflect interindividual variability in upper airway anatomy, including mandibular dimensions, neck length, and submandibular compliance.[7,8] Inappropriate sizing may result in air leaks, suboptimal glottic alignment, reduced seal pressures, and increased mucosal trauma or aspiration risk.[9,10] Consequently, increasing attention has been directed towards anatomical parameters that more accurately represent airway geometry. Ultrasonography has emerged as a valuable, non-invasive tool for preoperative airway assessment, providing real-time visualisation of upper airway structures. Measurement of the hyomental distance—the linear distance between the hyoid bone and the mentum—serves as a surrogate for submandibular space capacity, which directly influences supraglottic airway seating and stability. Kristensen et al.[4] demonstrated that ultrasound-derived airway parameters, including HMD and tongue thickness, are useful predictors of airway difficulty, with sensitivities exceeding 80%. More recent studies have extended the application of ultrasound from airway prediction to supraglottic device sizing. Umesh et al.[11] reported improved glottic alignment, higher oropharyngeal leak pressures, and greater first-attempt insertion success with ultrasound-guided sizing compared with conventional weight-based methods. The findings of the present study are consistent with this evolving evidence. Although baseline demographic and airway characteristics were comparable between groups, patients in the ultrasound-guided cohort more frequently received smaller PLMA sizes, with size 3 selected in the majority of cases. This contrasts with the weight-based group, where size 4 predominated. Importantly, this shift towards smaller, anatomically matched devices did not adversely affect clinical performance. First-attempt insertion success remained high in both groups, insertion times were similar, and postoperative airway morbidity was minimal. These results suggest that precise anatomical matching, rather than routine oversizing, is sufficient to achieve effective ventilation and airway stability similar to that reported previously.[12] Previous investigations support the superiority of anatomy-based parameters over body weight alone.[13] Berry et al.[14] demonstrated that weight-based charts correctly predicted optimal LMA size in fewer than 70% of adults, whereas incorporation of mandibular and neck measurements significantly improved accuracy. Gallart et al.[15] reported similar benefits in paediatric populations, with higher first-attempt success and fewer air leaks when anatomical dimensions guided device selection. Weng et al.[16] further showed that thyromental distance–based sizing resulted in improved fibreoptic views and seal pressures compared with weight-based methods. Collectively, these studies reinforce the principle that airway geometry, rather than generalised anthropometric indices, should guide supraglottic airway sizing.[17] Accurate PLMA sizing is particularly relevant for optimising oropharyngeal seal pressure, a key determinant of ventilation safety. Higher seal pressures enable effective positive-pressure ventilation while minimising the risk of gastric insufflation and aspiration.[11] Although seal pressures were not formally measured in the present study, the absence of ventilation failure, intraoperative displacement, or aspiration suggests that adequate sealing was achieved with both sizing strategies. The predominance of laparoscopic procedures in the study population further emphasises the clinical relevance of achieving a reliable airway seal. From a practical standpoint, ultrasound-guided HMD measurement is feasible and time-efficient. The assessment can be completed preoperatively within seconds and does not prolong induction time. By facilitating appropriate initial device selection, ultrasound may reduce the need for reinsertion, airway manipulation, and associated haemodynamic fluctuations.[18] Although overall reinsertion rates were low in both groups, fewer secondary interventions were observed in the ultrasound-guided cohort, consistent with prior studies reporting improved precision with sonographic sizing.[11,19] The transition to second-generation supraglottic airway devices has already enhanced airway safety margins.[20,21] Previous studies have demonstrated that the PLMA achieves significantly higher seal pressures than the classic LMA, while Cook emphasised the reduced aspiration risk and improved performance of these devices in modern anaesthetic practice.[2,22,23] The present study adds to this body of evidence by demonstrating that pairing second-generation devices with ultrasound-guided, anatomy-based sizing may further optimise performance without added complexity.

The primary limitation of this study is that oropharyngeal leak pressure and fibreoptic position scores were not measured; consequently, statements in this study regarding “better anatomical fit” and “sealing efficacy” of HMD-guided sizing reflect indirect clinical inference from insertion success, insertion time, and complication rates, rather than objectively measured seal quality or glottic alignment, and should be interpreted with this caveat in mind. Several additional limitations of the current study need to be acknowledged. The study population was restricted to ASA class I–II patients, which may limit generalisability to higher-risk groups. Ultrasound measurements are operator-dependent, although all assessments were performed by an experienced operator to minimise variability. Future multicentre studies incorporating larger and more diverse populations, objective seal measurements, and fibreoptic evaluation would strengthen the evidence base.

CONCLUSION

In conclusion, this randomised study demonstrates that ultrasound-guided HMD measurement provides a simple, accurate, and individualised method for selecting PLMA size. Compared with traditional weight-based selection, HMD-guided sizing favours smaller devices without compromising insertion success, procedural efficiency, or safety. As anaesthesia continues to evolve towards image-guided and patient-specific care, routine incorporation of airway ultrasonography may enhance the safety, efficiency, and predictability of supraglottic airway management.

Author contributions

SY: Designing the research, proofreading the manuscript, clinical data collection. DV: Guiding the ultrasound techniques and revising the manuscript text. SR: Manuscript writing. RR: Data collection, obtaining patient consent, protocol and manuscript writing and statistical analysis. NSC: Guidance for manuscript preparation and data analysis. All authors participated in the review, drafting and final approval of the manuscript.

Presentation at conferences/CMEs and abstract publication

The author(s) have no conference/CME presentations or abstract publications to declare.

Study data availability

De-identified data may be requested with reasonable justification from the authors (email to the corresponding author) and shall be shared after approval as per the authors’ institutional policy.

Disclosure of use of artificial intelligence (AI)-assistive or generative tools

The authors declare that no AI-assistive or generative tools were used in the preparation of this manuscript.

Declaration of use of permitted tools

The authors declare that no permitted tools, including AI-assisted or generative tools, were used in the preparation, writing, editing, or analysis of this manuscript.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.