MI IN BUNDLE BRANCH BLOCKS

DIAGNOSIS OF MI IN BUNDLE BRANCH BLOCKS

Dr.I.Tammi Raju

GENERAL PRINCIPLES 

•  The ECG diagnosis of MI is more difficult when the baseline ECG shows a bundle branch block pattern that may precede or be a complication of the infarct or the patient has a paced rhythm .

• The frequency of bundle branch block was best assessed in a review of almost 300,000 infarctions from the National Registry of Myocardial Infarction investigators.

• Right bundle branch block was present in approximately 6 percent and left bundle branch block in 7 percent of infarctions.

• RIGHT BUNDLE BRANCH BLOCK WITH MI — • The effect of right bundle branch block (RBBB)

must be considered in both Q wave (ST elevation) and non-Q wave (non-ST elevation) infarctions.

• Q wave MI — • Does not usually interfere with the diagnosis of a Q wave

MI. • Myocardial infarction most often involves the left ventricle

and therefore affects the initial phase of ventricular depolarization, sometimes producing abnormal Q waves.

• In contrast, RBBB primarily affects the terminal phase of ventricular depolarization, producing a wide R' wave in the right chest leads and a wide S wave in the left chest leads.

• These changes are due to delayed depolarization of the right ventricle, while depolarization of the left ventricle is not affected.

• The net effect is that the ECG patterns are combined when complete RBBB and a Q wave infarct occur together, and the criteria for the diagnosis of a Q wave MI are the same as in patients with normal conduction:– Due to the bundle

branch block, the QRS complex will be abnormally wide (0.12 sec or more), lead V1 will show a terminal positive deflection, and lead V6 will show a terminal negative deflection (wide S wave)

– If the infarction is anterior, there will be a loss of R wave progression with abnormal Q waves in the anterior leads and characteristic ST-T changes;

– if the infarction is inferior, Q waves will appear in leads II, III and aVF

• There are, however, problems that can occur with interpretation of the ECG in patients with RBBB and acute MI:

• Large clinical trials in which serial ECGs are performed have shown both false-positive and false-negative diagnoses of MI in the presence of RBBB

• After revascularization, for example, Q wave durations in patients who develop RBBB can shorten significantly, primarily in the inferior leads, suggesting that the initial orientation of wavefronts can change and that false-negative results may be obtained in patients with inferior infarction

• There has been a case report of a woman with an acute anterior MI in whom the initial ECG revealed a small R wave in leads V1 and V4.

• These R waves were replaced by Q waves after the development of RBBB associated with PR-interval prolongation, suggesting extension of the infarction. However, the Q waves disappeared with resolution of the RBBB.

• The coexistence of left anterior fascicular block with or without RBBB can be associated with Q waves suggestive of an anterior MI; in this setting, the altered initial vector is attributed to the left anterior fascicular block .– These Q waves can often be distinguished from

pathologic Q waves in an acute MI by their short duration (0.02 sec versus 0.04 to 0.05 sec with an infarction) and their presence in only leads V2 and/or V3.

• A new posterior wall infarct in the presence of RBBB might be expected to increase the anterior forces in the right precordial leads but this has not been systematically studied.

• RBBB-Non-Q wave MI — • There may be some diagnostic difficulties in interpreting

the ECG in patients with RBBB who have a non-Q wave MI. – RBBB is typically associated with secondary ST-T

changes due to abnormal right ventricular repolarization.

– Thus, leads with an R' wave (leads V1, V2, and sometimes V3) will show T wave inversions.

– In contrast, ST depressions or T wave inversions in leads with a terminal S wave (leads V5 and V6) cannot be attributed to the RBBB alone.

– Such ST-T changes may be due to ischemia, or to other factors such as drug effects or electrolyte abnormalities

• LBBB-MI

• The proportion of patients with LBBB and acute chest pain having an acute MI in different studies has been between 13 to 32 percent.

• As a result, inaccurate diagnosis can lead to both undertreatment and unnecessary overtreatment of patients.

• In one report, for example, thrombolysis was given to only 73 percent with LBBB and an acute MI and to 48 percent of patients with LBBB and chest pain but no biochemical evidence of infarction.

• In addition to difficulties in ECG interpretation, approximately one-half of patients with LBBB and an acute MI do not have chest pain .

• These patients are much less likely to receive appropriate medical therapy (eg, aspirin, beta blockers) or reperfusion therapy than LBBB patients with chest pain.

LBBB-MI STATISTICAL ANALYSIS

• LEFT BUNDLE BRANCH BLOCK WITH MI — • Left bundle branch block (LBBB) is present in

approximately 7 percent of acute infarctions . • The diagnosis of MI in the presence of LBBB is

considerably more complicated and confusing than that of RBBB.

• The reason is that LBBB alters both the early and the late phases of ventricular depolarization and produces secondary ST-T changes.

• Two issues need to be addressed: • The impact of LBBB on the diagnosis of acute MI;

and • The effect on diagnosis of a prior MI.• There are issues that vary with the site of the

infarct and there are changes that are independent of the site of the infarct, such as the ST-T changes that can occur.

• Because of these difficulties, careful attention to the strength of the clinical history and confirmation of the diagnosis of an acute MI by cardiac enzyme elevations is essential.

• Acute MI — • The sequence of repolarization is altered in LBBB,

with the ST segment and T wave vectors being directed opposite to the QRS complex.

• These changes may mask the ST segment depression and T wave inversion induced by ischemia.

• On the other hand, the diagnosis of an acute MI or ischemia can occasionally be made in a patient with underlying LBBB if certain ST-T changes are seen, particularly if the ST-T vectors are in the same direction as the QRS complex as in the Sgarbossa criteria .

• The presence of deep T wave inversions in leads with a predominantly negative QRS complex (eg, V1-V3) is highly suggestive of evolving ischemia or MI.

• ST elevations in leads with a predominant R wave (as opposed to QS or rS waves) are also strongly suggestive of acute ischemia.

• Pseudonormalization of previously inverted T waves is suggestive but not diagnostic of ischemia.

CONCORDANCE/DISCORDANCE

MI IN BUNDLE BRANCH BLOCKS

RBBB in V1

RBBB in V6

MI IN BUNDLE BRANCH BLOCKS

LBBB in V1

MI IN BUNDLE BRANCH BLOCKS

LBBB-V6

• Sgarbossa criteria —•  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 .

• Among 26,003 North American patients who had a myocardial infarction confirmed by enzyme studies, 131 (0.5 percent) had LBBB. A scoring system, often called the Sgarbossa criteria, was developed from the coefficients assigned by a logistic model for each independent criterion, on a scale of 0 to 5.– Sgarbossa EB, Pinski

SL, Gates KB, Wagner GS. Early electrocardiographic diagnosis of acute myocardial infarction in the presence of ventricular paced rhythm. GUSTO-I investigators. Am J Cardiol 1996; 77:423

MI IN BUNDLE BRANCH BLOCKS

• ST segment elevation of 1 mm or more that was in the same direction (concordant) as the QRS complex in any lead — score 5.

• ST segment depression of 1 mm or more in any lead from V1 to V3 — score 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

A minimal score of 3 was required for a specificity of 90 percent

Sgarbossa criteria 

• The first two criteria are similar to those described above since the ST segment is concordant rather than discordant with the QRS complex.

• However, the third 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.

• In a substudy from the ASSENT 2 and 3 trials, the third criteria added little diagnostic or prognostic value .

• A Sgarbossa score of ≥3 was highly specific (ie, few false positives) but much less sensitive (36 percent) in the validation sample in the original report .

• Similar findings were noted in a subsequent meta-analysis of 10 studies of 1614 patients in which a Sgarbossa score of ≥3 had a sensitivity of 20 percent and a specificity of 98 percent .

• The sensitivity may increase if serial or previous ECGs are available .

• In addition to their utility in diagnosis, the Sgarbossa criteria may also predict prognosis in patients with acute MI.

• Attempts to improve ECG diagnosis — • Several studies have systematically evaluated the value of

different ECG findings of acute MI in LBBB. • An analysis by Wackers correlated ECG changes in LBBB with

localization of the infarct by thallium scintigraphy . • The most useful ECG criteria were:

– Serial ECG changes — 67 percent sensitivity– ST segment elevation — 54 percent sensitivity– Abnormal Q waves — 31 percent sensitivity– Initial positivity in V1 with a Q wave in V6 — 20 percent sensitivity

but 100 percent specificity for anteroseptal MI– Cabrera's sign — 27 percent sensitivity overall, 47 percent for

anteroseptal MI

• Cabrera's sign refers to prominent (0.05 sec) notching in the ascending limb of the S wave in leads V3 and V4;

Chapman's sign- prominent notching of the ascending limb of the R wave in lead V5 or V6

• 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.

• Ventricular pacing —•  A similar problem is the diagnosis of an acute MI in the

presence of a ventricular paced rhythm, which is usually associated with a left bundle branch block pattern.

• Thirty-two patients in the GUSTO-1 trial (0.1 percent of enrolled patients) had a ventricular paced rhythm.

• The only ECG criterion with a high specificity and statistical significance for the diagnosis of an acute MI was ST segment elevation ≥5 mm in leads with a negative QRS complex .

• Two other criteria with acceptable specificity were:– ST elevation ≥1 mm in leads with concordant QRS polarity– ST depression ≥1 mm in leads V1, V2, or, V3

• Prior infarction — • Changes in the sequence of depolarization in

LBBB can also mask typical findings associated with prior transmural (Q wave) infarctions. Certain ECG patterns, however may suggest prior infarction despite LBBB

• Left ventricular free wall — • Infarction of the left ventricular free (or lateral) wall

ordinarily results in abnormal Q waves in the midprecordial to lateral precordial leads (and selected limb leads).

• However, the initial septal depolarization forces with LBBB are directed from right to left.

• These leftward forces produce an initial R wave in the mid- to lateral precordial leads, usually masking the loss of potential (Q waves) caused by the infarction.

• As a result, acute or chronic left ventricular free wall infarction by itself will not usually produce diagnostic Q waves in the presence of LBBB.

• If, however, the loss of lateral force is sufficiently large, late rightward forces generated by other portions of the left ventricle may predominate, possibly resulting in S waves in I, aVL and V6.

• Thus, an anterolateral MI should be suspected in the appropriate clinical setting if new S waves appear in leftward leads in a patient with preexisting LBBB.

• Anteroseptal —•  The presence of LBBB has a variable effect on the

ECG changes that can occur with anteroseptal MI. • Perhaps most important, the leftward shift in the

initial vector in LBBB causes the loss of normal septal q waves in I, aVL, and V6.

• Furthermore, the leftward and posterior orientation of the initial vector often results in a QS pattern in the anterior leads, V1 and sometimes in V2.

• These changes can mask the presence of an anteroseptal MI.

• There are, however, other changes that can occur that may suggest the presence of an anteroseptal or septal MI.

• The infarct may cause the leftwardly directed initial vector of LBBB to shift to the right, resulting in "pseudonormalization" of the initial vector and the reappearance of q waves in I, aVL and V6.

• If enough of the septum is infarcted, abnormal QR, QRS, or qrS types of complexes may appear in the mid- to lateral precordial leads in conjunction with the LBBB pattern

• Free wall and septal —•  Acute or chronic infarction involving both the free wall and

the septum may produce abnormal Q waves (usually as part of QRS or QrS types of complexes) in leads V4 to V6.

• These initial Q waves probably reflect posterior and superior forces from the spared basal portion of the septum.

• Small q waves (0.03 sec or less) may be seen in leads I and V5 to V6 with uncomplicated LBBB.

• Thus, wide Q waves (0.04 sec) in one or more of these leads are a more reliable sign of underlying infarction.

• As an example, wide Q waves (as part of QR complexes) in V6, particularly with an R wave in V1, appear to be a specific, although relatively insensitive, marker of anterior infarction .

• Inferior wall — • In a retrospective analysis, 35 patients with LBBB

and an unequivocal inferior MI on thallium imaging were compared to 131 patients with LBBB without an inferior wall MI .

• Two ECG findings were most useful for the diagnosis of an inferior wall MI:– Q or QS wave in lead aVF, found in 29 percent of those

with a documented MI versus only 3 percent of those without an inferior MI

– Diagnostic T wave inversion (compete or biphasic with an initial negative deflection), present in 66 percent with and 6 percent without a MI.

• The presence of either finding was 86 percent sensitive and 91 percent specific for the diagnosis of an inferior wall MI

• Summary — The following points summarize the electrocardiographic signs of myocardial infarction in LBBB.

• The Sgarbossa criteria have high specificity but low sensitivity ; thus, their presence is highly suggestive of acute infarction but their absence has little value.

• 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 concordant ST elevations in the lateral leads or ST depression or deep T wave inversions in leads V1-V3 suggests underlying ischemia . Thus, close attention 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.

• Underlying MI is also suggested by notching of the ascending part of a wide S waves in the mid-precordial leads (Cabrera's sign), or of the ascending limb of a wide R wave in V5 or V6 (Chapman's sign).

TMT in RBBB• The resting ECG in patients with right bundle branch

block (RBBB) is frequently associated with T wave and ST-segment changes in the early anterior precordial leads (V1 to V3).

• Exercise-induced ST-segment depression in leads V1 to V4 is a common finding in patients with RBBB and is nondiagnostic

• The new development of exercise-induced ST-segment depression in leads V5 and V6 or leads 2 and aVF, reduced exercise capacity, and inability to increase systolic blood pressure adequately are useful for detecting patients who have CAD and a high clinical pretest risk of disease

• The presence of RBBB decreases the sensitivity of the test

• The presence of this finding in leads V1 through V4 is not diagnostic of obstructive coronary disease; however, if ischemic changes are seen in lead II or aVF, or in leads V5 or V6, the specificity for coronary disease is improved.

TMT in LBBB

• Exercise-induced ST segment depression is found in most patients with LBBB and cannot be used as a diagnostic or prognostic indicator, regardless of the degree of ST-segment abnormality.

• The relative risk of death or other major cardiac events in patients with exercise-induced LBBB is increased approximately threefold over the risk in patients without this abnormality

• The development of ischemic ST-segment depression before the LBBB pattern appears or in the recovery phase after the LBBB has resolved does not attenuate the diagnostic yield of the ST segment shift.