Ratio of Height to Interincisor Distance (RHIID): Preoperative Assessment of Difficult Laryngeal Exposure

The ratio of height to interincisor distance (RHIID) is a new metric proposed for assessing laryngeal exposure. Anatomic ratios controlling for height, such as the ratio of height to thyromental distance (RHTMD), are associated with difficult laryngeal exposure among anesthesiologists. This study examines the correlation between RHIID, RHTMD, and force metrics, as measured by the laryngeal force sensor (LFS), during suspension microlaryngoscopy (SML).

Retrospective analysis of prospectively collected data.

Tertiary medical center.

Data from 476 adult patients (2017-2024) were analyzed to examine force metrics during suspension laryngoscopy. Height, thyromental distance (TMD), and interincisor distance (IID) were measured, and RHTMD and RHIID were calculated by dividing height (cm) by the respective variables (cm). Average and maximum forces (N) during direct laryngoscopy were recorded using the LFS. Multiple regression models were developed, and several linear models were selected as the best fit for identifying associations between these variables.

RHIID's associations with force metrics were twice as strong as those observed with individual anatomical variables, including height, TMD, and IID. Patients with higher RHIID values exhibited significantly greater average and maximum forces during SML (P < .001). Although RHIID and RHTMD displayed comparable predictive strength for average force, RHIID consistently showed stronger correlations with maximum forces.

RHIID is a new ratio that strongly correlates with force metrics, outperforming individual variables such as height, TMD, and IID. Incorporating these anatomic ratios into preoperative assessments provides clinicians with another tool for predicting laryngeal exposure during direct laryngoscopy.