Interrater Agreement in the Measurement of QRS Interval in Tricyclic Antidepressant Overdose: Implications for Monitoring and Research☆,☆☆,★

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Abstract

Study objective: To assess interobserver agreement when experienced clinicians measure QRS-interval duration in tricyclic antidepressant (TCA) overdose. Methods: We studied the admission ECGs of 231 patients with TCA poisoning. Three of the authors, with experience in the management of TCA poisoning, independently measured QRS intervals manually. Each rater was blinded to patient outcome and the measurements made by the other raters. Our main outcome measure was agreement among raters, particularly as it applies to clinically used QRS cutoff points for the determination of treatment and disposition. Results: Agreement on the measurement of QRS intervals was good (intraclass correlation coefficient, .60; 95% confidence interval [CI], .53 to .66) for transformed data and .82 (95% CI, .78 to .85) for raw data. When assigning patients to categories of QRS interval, the raters agreed on 169 of 231 patients (73%) (weighted κ=.83; P<.0001). However, the raters did not agree unanimously on whether the QRS interval was less than 100 milliseconds or 100 milliseconds or greater in 45 of 231 of patients (19.5%) (κ=.69; P<.0001). Conclusion: Reliance on the manually measured QRS interval to determine management in TCA poisoning is not justified because of substantial observer variation in the determination of whether the interval falls below the clinical cutoff point of 100 milliseconds. Agreement is sufficient to make the measurements useful as part of the overall assessment of toxicity.

[Buckley NA, O'Connell DL, Whyte IM, Dawson AH: Interrater agreement in the measurement of QRS interval in tricyclic antidepressant overdose: Implications for monitoring and research. Ann Emerg Med November 1996;28:515-519.]

Section snippets

INTRODUCTION

Measurement of the QRS interval is widely used to predict the risk of seizures and arrhythmias in tricyclic antidepressant (TCA) overdose and thereby indicate the need for specific treatment.1, 2, 3 For example, alkalinization has been recommended in cases of TCA poisoning involving QRS intervals greater than 160 milliseconds,2 and in the past prophylactic phenytoin and a temporary pacemaker have been recommended if the QRS interval exceeds 120 milliseconds.3 Boehnert and Lovejoy4 reported,

MATERIALS AND METHODS

Because this study consisted of an internal audit, it was exempt from institutional ethics committee approval. The study data were collected as part of a study of dothiepin toxicity in overdose.18 The ECGs were those of consecutive patients presenting between January 1987 and August 1992 with TCA ingestion to our department, which provides a comprehensive poison information and management service covering a population of around 500,000. Three raters with clinical experience in the management of

RESULTS

During the study period, 302 patients with TCA poisoning were admitted. Fifteen were excluded from the original study18 because they had ingested more than one TCA; in 56 others, electrocardiography was not performed on admission or the original ECG had been lost. The admission ECGs of the remaining 231 patients were used for this study.

The distribution of QRS duration was highly skewed; most patients had QRS intervals of 60 to 100 milliseconds (Table). When QRS intervals were compared as

DISCUSSION

We found that experienced clinical toxicologists disagreed in their classification of QRS interval as to whether the QRS interval was less than 100 milliseconds or 100 milliseconds or greater in a fifth of patients. This finding has clinical and research implications. Reliance on QRS interval alone in the determination of need for seizure and arrhythmia monitoring does not appear justified. This is particularly pertinent to phone consultations about poisoning, in which one person, often

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    ☆

    From the Discipline of Clinical Pharmacology, University of Newcastle*; and the Department of Clinical Toxicology and Pharmacology, newcastle Mater Misericordiae Hospital‡, Newcastle Australia

    ☆☆

    Address for reprints: Nicholas A Buckley, BMed, Mater Misericordiae Hospital, Newcastle, NSW 2298, Australia

    ★

    Reprint no. 47/1/77014

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