[1] Sgarbossa criteria (review) - have to know these, and check on every LBBB and paced EKG that they are not present
A large trial of thrombolytic therapy for acute MI (GUSTO-1) provided an opportunity to revisit the issue of the electrocardiographic diagnosis of evolving acute MI in the presence of LBBB [3]. Of 26,003 North American patients who had a myocardial infarction confirmed by enzyme studies, 131 (0.5 percent) had LBBB. A scoring system was developed from the coefficients assigned by a logistic model for each independent criterion, on a scale of 0 to 5. The three ECG criteria with an independent value in the diagnosis of acute infarction and the score for each were:
Rule #1. ST segment elevation of 1 mm or more that was in the same direction (concordant) as the QRS complex — score 5.
Rule #2. ST segment depression of 1 mm or more in lead V1, V2, or V3 — score 3.
Rule #3. ST segment elevation of 5 mm or more that was discordant with the QRS complex (ie, associated with a QS or rS complex) — score 2. **The finding requires further validation, since a high take-off of the ST segment in leads V1 to V3 has been described with uncomplicated LBBB, particularly if there is underlying left ventricular hypertrophy. Importance of the Smith rule.
A minimal score of three was required for a specificity of 90 percent.
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[2] Cabrera and Chapman Signs
-- Cabrera’s sign is a prominent notching (>/= 0.04 sec) in the in the ascending limb of the S wave in lead V3 or V4.
-- Chapman’s sign is a notching >/= 0.05 sec in the ascending limb of the R wave in lead I, aVl or V6. Both are taken as features of myocardial infarction in the presence of LBBB.
-- These signs have a specificity that approaches 90 percent. However, there may be a high degree of interobserver variability in accurate identification and their sensitivity is quite low.
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Studies of acute MI in LBBB — Several studies have systematically evaluated the value of different ECG findings of acute MI in LBBB. One study by Wackers, for example, correlated ECG changes in LBBB with localization of the infarct by thallium scintigraphy [2]. The most useful ECG criteria were:
• Serial ECG changes — 67 percent sensitivity
• ST segment elevation — 54 percent sensitivity
• Abnormal Q waves — 31 percent sensitivity
• Cabrera's sign — 27 percent sensitivity, 47 percent for anteroseptal MI
• Initial positivity in V1 with a Q wave in V6 — 20 percent sensitivity but 100 percent specificity for anteroseptal MI
Other important points that I stumbled across regarding EKG signs of myocardial infarction in LBBB.
• A QS pattern, poor R wave progression, or loss of R waves in the anterior precordial leads or a QS pattern in II, III, aVF, or aVL can occur with uncomplicated LBBB.
• LBBB characteristically masks the Q waves of pure lateral and free wall infarction; it may also mask the Q waves of inferior or anteroseptal infarction.
• ST segment elevation with tall positive T waves are frequently seen in the right precordial leads with uncomplicated LBBB. Secondary T wave inversions are characteristically seen in the lateral precordial leads. However, the appearance of ST elevations in the lateral leads or deep T wave inversions in leads V1-V3 suggests underlying ischemia. Close attention, therefore, should be paid to serial ST-T changes.
• The presence of QR complexes in leads I, V5, or V6, or in II, III, and aVF with LBBB strongly suggests underlying infarction.
• An anterolateral MI should be suspected if new S waves appear in leftward leads (I, aVL, and V6) in a patient with preexisting common LBBB.
EKG showing uncomplicated LBBB
[3] Smith-modified Sgarbossa Criteria
1) at least one lead with concordant STE (Sgarbossa criterion 1) or
2) at least one lead of V1-V3 with concordant ST depression (Sgarbossa criterion 2) or
3) proportionally excessively discordant ST elevation in V1-V4, as defined by an ST/S ratio of equal to or more than 0.20 and at least 2 mm of STE. (this replaces Sgarbossa criterion 3 which uses an absolute of 5mm)
Case 1 - False positive if using Sgarbossa critiera, stressing the importance of using the ST/S ratio > 0.20. No ischemia on this EKG.
> 5 mm STE in V2-V3, which would only give you 2 points if using Sgarbossa criteria, but 0 if using the Smith-modified rule
Case #2 Employing the ratio, LBBB with ischemia
There is sinus tach with LBBB. There is no concordant ST elevation. V4 has 2 mm of discordant ST elevation (at the J-point, relative to the PR segment) following a 5 mm S-wave. The ST/S ratio is 0.40 in this lead. Lead II has proportionally excessively discordant ST depression, with 1.25 mm STD and only 4.0 mm R-wave, for a ratio of 0.31. This is also a sign if ischemia (reciprocal inferior ST depression). Also, look at V3: complexes vary slightly: 2nd complex has approx 2.5-3.0 mm STE following a 14 mm S-wave; complex 4 has 2-2.5 mm STE following a 10.5 mm S-wave. Lastly, this EKG also displays the Cabrera sign in lead V3, as discussed in the prior pearl.
This material is taken from the following blog site, good stuff.
http://hqmeded-ecg.blogspot.com/2011/05/lbbb-is-there-stemi.html
References:
1. Goldberger, AL. Myocardial infarction: Electrocardiographic differential diagnosis, 4th ed. Mosby Year Book, St Louis, 1991.
2. Wackers, FJ. The diagnosis of myocardial infarction in the presence of left bundle branch block. Cardiol Clin 1987; 5:393.
3. Sgarbossa, EB, Pinski, SL, Barbagelata, A, et al. Electrocardiographic diagnosis of evolving acute myocardial infarction in the presence of left bundle branch block. N Engl J Med 1996; 334:481.
4. Wellens, HJJ. Acute myocardial infarction and left bundle branch block — can we lift the veil? N Engl J Med 1996; 334:528.
5. Laham, CL, Hammill, SC, Gibbons, RJ. New criteria for the diagnosis of healed inferior wall myocardial infarction in patients with left bundle branch block. Am J Cardiol 1997; 79:19.
6. Sgarbossa, EB, Pinski, SL, Gates, KB, et al for the GUSTO-I Investigators. Early electrocardiographic diagnosis of acute myocardial infarction in the presence of ventricular paced rhythm. Am J Cardiol 1996; 77:423.
7. Dodd KW. Aramburo L. Broberg E. Smith SW. For Diagnosis of Acute Anterior Myocardial Infarction Due to Left Anterior Descending Artery Occlusion in Left Bundle Branch Block, High ST/S Ratio Is More Accurate than Convex ST Segment Morphology (Abstract 583). Academic Emergency Medicine 17(s1):S196; May 2010.
8. Dodd KW. Aramburo L. Henry TD. Smith SW. Ratio of Discordant ST Segment Elevation or Depression to QRS Complex Amplitude is an Accurate Diagnostic Criterion of Acute Myocardial Infarction in the Presence of Left Bundle Branch Block (Abstract 551). Circulation October 2008;118 (18 Supplement):S578.