From the Guidelines
Yes, a normal electrocardiogram (ECG) result can still indicate a myocardial infarction (heart attack).
Key Points to Consider
- A normal ECG does not rule out the diagnosis of MI, as not all patients who develop myocardial necrosis exhibit ECG changes 1.
- The ECG is an integral part of the diagnostic work-up of patients with suspected MI and should be acquired and interpreted promptly, with serial recordings in symptomatic patients with an initial non-diagnostic ECG 2.
- Other conditions, such as acute pericarditis, left ventricular hypertrophy (LVH), left bundle branch block (LBBB), Brugada syndrome, stress cardiomyopathy, and early repolarization patterns, can also cause ST deviation, making ECG interpretation challenging 2.
- The diagnosis of MI is more difficult in the presence of LBBB, but concordant ST-segment elevation or a previous ECG may be helpful to determine the presence of acute MI in this setting 3.
- New or presumed new ST segment depression or T wave abnormalities, or both, should be observed in two or more contiguous leads on two consecutive ECGs at least several hours apart to support the diagnosis of MI 1.
Important ECG Findings
- Q waves >0.03 sec and 0.1 mV deep or QS complex in leads I, II, aVL, aVF or V1–V6 in any two leads of a contiguous lead grouping may indicate prior myocardial infarction 3.
- ST elevation >0.05 mV in leads V7–V9 may indicate acute circumflex occlusion, especially in patients with high clinical suspicion 3.
- ST depression in leads V1–V3 may be suggestive of inferobasal myocardial ischemia (posterior infarction), especially when the terminal T wave is positive (ST elevation equivalent) 3.
From the Research
Myocardial Infarction with Normal ECG
- A study published in 1995 4 found that myocardial infarction with an initially normal electrocardiogram (ECG) is uncommon, occurring in 3.7% of patients with acute myocardial infarction (AMI).
- The study identified three distinct groups of patients with normal initial ECGs: those who developed ST elevation or Q waves, those who developed ST depression or T-wave inversion, and those whose ECG remained normal.
- Patients with persistently normal ECGs had less myocardial damage and fewer complications compared to the other two groups.
Limitations of ECG in Diagnosing Myocardial Infarction
- A study published in 1995 5 compared the findings of ECG and two-dimensional echocardiogram (2-D echo) in patients with single vessel coronary artery disease and found that ECG has limitations in diagnosing infero-posterior myocardial infarction, especially during the acute phase.
- The study found that 2-D echo is an additional useful diagnostic procedure in diagnosing AMI.
Clinical Significance of ECG Changes
- A study published in 2013 6 highlighted the importance of considering other clinical conditions that may result in ECG changes mimicking AMI, in order to avoid incorrect diagnosis and unnecessary tests or therapeutic procedures.
- The study found that both cardiac and non-cardiac clinical syndromes may mimic AMI, and comprehensive clinical examination and medical history are crucial for making the correct diagnosis.
Predictive Value of Serial ECGs
- A study published in 2020 7 analyzed the predictive value of ischemic ECG signs in patients with suspected AMI and found that persistent ischemic signs are predictive of a higher rate of all-cause-mortality, AMI, and revascularization.
- The study found that patients with persistent ischemic signs had significantly worse outcomes compared to those without ischemic signs.
ECG in Acute Myocardial Infarction
- A study published in 1988 8 discussed the importance of the initial ECG in the evaluation of patients presenting with suspected myocardial infarction and found that the initial ECG is diagnostic of acute myocardial infarction in only a minority of patients.