The S/Z ratio: A simple and reliable clinical method of evaluating laryngeal function in patients after intubation

Abstract 

Objectives

1. To determine the consequences of prolonged intubation on laryngeal function.

2. To evaluate simple clinical criteria or tests that could alert the clinician to potential laryngeal pathology requiring ear, nose, and throat/otolaryngology (ENT) referral.

Design

A prospective case series.

Setting

A surgical intensive care unit in a tertiary academic hospital in Cape Town, South Africa.

Participants

Thirty-two patients who had undergone a period of translaryngeal intubation for a period greater than 12 hours.

Main outcome measures

1. Patient subjective voice change rating.

2. Clinician assessment of laryngeal function.

3. S/Z ratio.

4. Presence of laryngeal pathology on endoscopic assessment of the larynx.

Results

Upon initial evaluation within 6 hours of extubation, 26 (81%) of patients exhibited symptomatic laryngeal dysfunction. At this stage, 13 (40%) had S/Z ratios greater than 1.4. The degree of dysfunction as described by subjective scoring and the S/Z ratio was proportional to the duration of intubation. After 24 hours, 23 (72%) patients' voices had improved subjectively; and the S/Z ratio exceeded 1.4 in just 6 patients (19%). Of these 6 patients, 4 exhibited laryngeal pathology on flexible nasoendoscopy. These 4 patients were followed up over 1 year, and 1 patient was ultimately offered a vocal cord medialization procedure. The S/Z ratio is 100% sensitive and 93% specific, with an accuracy of 93.75%, in diagnosing laryngeal pathology requiring ENT referral.

Conclusions

1. A period of laryngeal intubation carries signification risk of laryngeal dysfunction. Most, but not all, dysfunction resolves within 24 hours.

2. Residual laryngeal pathology can be reliably and simply screened for by the use of the S/Z ratio. We recommend that patients with an S/Z ratio greater than 1.4 more than 24 hours after extubation require ENT referral for laryngoscopy.

Keywords: Larynx, Voice quality, Intubation, S/Z ratio