What are the key steps in interpreting an electrocardiogram (EKG)?

Key Steps in Interpreting an Electrocardiogram (EKG)

A systematic approach to EKG interpretation is essential for accurate diagnosis and appropriate clinical decision-making, focusing on rate, rhythm, intervals, axis, and morphology analysis.

Initial Assessment

• Begin by calculating the heart rate by counting the number of QRS complexes in a 6-second strip and multiplying by 10, or using the formula 300 divided by the number of large boxes between consecutive R waves 1
• Identify the underlying rhythm, looking for P waves before each QRS complex with consistent PR intervals to confirm sinus rhythm 1
• Note any irregularities in rhythm, such as premature beats, pauses, or completely irregular patterns that might suggest atrial fibrillation 1
• Evaluate the quality of the EKG recording and identify any technical artifacts that may affect interpretation 2

Interval and Conduction Analysis

• Measure the PR interval (normal: 120-200 ms) to assess AV conduction 1
• Evaluate QRS duration (normal: <120 ms) to identify ventricular conduction delays or bundle branch blocks 1
• Calculate the QT interval corrected for heart rate (QTc) using Bazett's formula, with normal values being <450 ms for men and <460 ms for women 1
• Identify any conduction abnormalities such as AV blocks, bundle branch blocks, or pre-excitation patterns 1

Axis Determination

• Determine the electrical axis by examining leads I and aVF, with normal axis between +90° to -30° (positive in both leads I and aVF) 1
• Identify left axis deviation: -30° to -90° (positive in lead I, negative in aVF) 1
• Identify right axis deviation: +90° to +180° (negative in lead I, positive in aVF) 1
• Note extreme axis deviation: +180° to -90° (negative in both leads I and aVF) 1

Chamber Enlargement Assessment

• Look for voltage criteria for left ventricular hypertrophy: S in V1 + R in V5 or V6 >3.5 mV 1
• Assess for atrial enlargement patterns in the P wave morphology 1
• Consider these findings in the context of the patient's clinical presentation, as they may indicate underlying structural heart disease 3

Ischemia, Injury, and Infarction Evaluation

• Examine for ST-segment deviations, with ST elevation (>0.1 mV in limb leads or >0.15-0.2 mV in precordial leads) potentially indicating acute injury 1
• Look for T-wave abnormalities (inversion, hyperacute, flattening) that may suggest ischemia 1, 4
• Identify pathological Q waves (>0.04 seconds or >25% of the R wave amplitude) suggesting myocardial infarction 1
• Note the location of abnormalities to determine the affected coronary territory 5
• Recognize that up-sloping ST-segment depression with positive T waves may indicate regional subendocardial ischemia associated with severe left anterior descending coronary artery obstruction 5

Clinical Context Integration

• Always interpret the EKG in the context of the patient's clinical presentation, as the same EKG finding may have different implications depending on symptoms 2
• Compare with previous EKGs when available to identify significant changes 1
• Recognize that noncardiologists are more influenced by patient history in interpreting EKGs than cardiologists, making clinical context particularly important 2
• Consider computer interpretations as helpful adjuncts but not substitutes for physician interpretation in clinical decision making 2

Common Pitfalls to Avoid

• Avoid misplacement of electrodes, particularly precordial leads, which can significantly alter EKG interpretation 1
• Avoid over-reliance on computer interpretation without physician verification, as errors in computer interpretation are still common 2
• Be aware that many patients may have ST elevation due to non-ischemic causes, requiring careful clinical correlation 4
• Recognize that interpretation of EKGs varies greatly, even among expert electrocardiographers 2
• Avoid interpreting EKG findings in isolation without considering clinical context, which may lead to inappropriate management decisions 1

Maintaining Competency

• Continuing medical education through seminars or self-assessment programs is encouraged to keep EKG interpretation skills current, especially for physicians who read EKGs infrequently 2
• Consider periodic self-study or continuing medical education to ensure skills remain current 2
• Recognize that successful completion of residency, board certification, and Advanced Cardiac Life Support training provides a reasonable foundation for bedside EKG interpretation in routine and emergency situations 2