Image: Sgarbossa's rules. Courtesy of EMS12lead.com
Primer: In patients presenting with an acute ST Elevated Myocardial Infarction (STEMI) in the presence of a Left Bundle Branch Block (LBBB), infarct diagnosis based on the ECG can be difficult as ST segments and T waves tend to be shifted. The Sgarbossa rule has been previously proposed as a clinical tool to aid in rapid diagnosis in this scenario. The rule, published in 1996 in the NEJM consists of 3 criteria: 1. Concordant ST elevation > 1mm in leads with a positive QRS (score 5) 2. Concordant ST depression > 1mm in V1-V3 (score 3) 3. Excessively discordant ST elevation > 5mm in leads with a negative QRS (score 2). A total score of ≥ 3 has a specificity of 90% for an acute MI. A score of ≥3 points is interpreted as a positive result using the unweighted rule, while a score of ≥2 points is positive using the weighted rule (any of the three criteria tested positive).
This [retrospective] study: Published on Sept 3rd in the Annals of Emergency Medicine, evaluated the ECGs of 162 subjects admitted for suspected STEMI with LBBB. The study compared the original rule with a modified rule whereby the 3rd component is replaced with an ST/S ratio of less than -0.25 in any one lead. Sensitivity of the revised rule (~91%) was significantly greater than either the weighted (~52%) or unweighted (~67%) Sgarbossa rule (p<.001 and <.008, respectively). The revised rule was significantly more accurate than both the weighted and unweighted Sgarbossa rules. Specificity of the revised rule (~90%) was lower than the weighted rule (~98%) but similar to that of the unweighted rule (~90%).
In sum: Replacement of the 3rd component of the Sgarbossa rule (absolute ST elevation greater than or equal to 5 mm) with an ST/S ratio of less than −0.25 greatly improves the rule's sensitivity and accuracy while only slightly decreasing specificity. As a result, a modified version of the Sgarbossa rule has an improved diagnostic utility for acute STEMI. If conﬁrmed by an independent validation study, the modified criteria could potentially be used as a tool to guide the need for reperfusion therapy in patients with ischemic symptoms and a LBBB. The study’s main limitations included: limited power to detect small changes in sensitivity, the possibility of measurement bias, and the need for validation of the ST/S ratio cut-off point.
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