What is most suggestive of a myocardial infarction (MI), development of Q waves, right bundle branch block, new cardiac arrhythmia, or peaked T waves?

Diagnostic Findings Most Suggestive of Myocardial Infarction

Among the options of Q waves, right bundle branch block, new cardiac arrhythmia, or peaked T waves, the development of Q waves is most suggestive of myocardial infarction (MI). 1, 2

Q Waves as Diagnostic Indicators

• Q waves or QS complexes in the absence of QRS confounders are pathognomonic of a prior MI in patients with ischemic heart disease, regardless of symptoms 1, 2
• The specificity of the ECG diagnosis for MI is greatest when Q waves occur in several leads or lead groupings 1
• Pathologic Q waves are defined by specific duration and amplitude criteria (≥0.03 sec and ≥0.1 mV deep) and indicate myocardial necrosis from prior myocardial infarction 2
• Even minor Q waves (0.02-0.03 sec that are 0.1 mV deep) are suggestive of prior MI if accompanied by inverted T waves in the same lead group 1

Comparison with Other ECG Findings

Right Bundle Branch Block (RBBB)

• In patients with RBBB, ST-T abnormalities in leads V1–V3 are common, making it difficult to assess the presence of ischemia in these leads 1
• While RBBB can complicate interpretation, it is only when new ST elevation or Q waves are found alongside RBBB that myocardial ischemia or infarction should be strongly considered 1
• RBBB alone is not pathognomonic for MI and can occur in various cardiac and non-cardiac conditions 1

New Cardiac Arrhythmia

• While arrhythmias may occur during acute MI, they are not specific diagnostic indicators of MI 1
• Arrhythmias are not included in the primary diagnostic criteria for MI according to the universal definition 1
• New arrhythmias can result from many causes besides MI, including electrolyte abnormalities, medications, and structural heart disease 1

Peaked T Waves

• Peaked T waves may be an early and transient finding in acute myocardial ischemia, but they are not specific for MI 1
• Pseudo-normalization of previously inverted T waves during an episode of acute chest discomfort may indicate acute myocardial ischemia, but this finding is not pathognomonic 1
• Peaked T waves can also occur in hyperkalemia, early repolarization, and other non-ischemic conditions 1

Clinical Significance of Q Waves

• Q waves reliably predict MI location, size, and transmural extent, particularly in anterior infarctions 3
• The number of anterior Q waves correlates strongly with anterior MI size (r=0.70) 3
• Larger infarct size (typically >6.2% of left ventricular mass) is associated with pathologic Q waves 2
• Silent Q wave MI has been associated with significantly increased mortality risk, accounting for 9-37% of all non-fatal MI events 1

Important Caveats and Pitfalls

• Not all Q waves are pathologic - normal Q waves can occur in specific leads:
  • QS complex in lead V1 is normal 1, 2
  • Q wave <25% of R wave amplitude in lead III is normal if frontal QRS axis is between 30° and 0° 1, 2
  • Q wave may be normal in aVL if frontal QRS axis is between 60° and 90° 1, 2
  • Septal Q waves (small, non-pathological Q waves <0.03 sec and <25% of R-wave amplitude) in leads I, aVL, aVF, and V4–V6 are normal 1, 2
• Other conditions can mimic Q waves, including pre-excitation, cardiomyopathies, cardiac amyloidosis, LBBB, LVH, and myocarditis 1, 2
• The positive predictive value of asymptomatic Q-waves to diagnose Q-wave unrecognized MI is only 29.2% in the general population 4

In conclusion, while all four findings can be present in MI, pathologic Q waves have the highest specificity for diagnosing MI and are considered pathognomonic when present in the appropriate clinical context and in the absence of confounding factors.