Explain an electrocardiogram (EKG)?

Understanding the Electrocardiogram (EKG/ECG)

What is an EKG?

An electrocardiogram (EKG or ECG) is a noninvasive recording of the heart's electrical activity displayed as a standard 12-lead tracing that must be interpreted by a qualified physician to be considered diagnostically valid. 1

The EKG is the most widely employed technical procedure for evaluating cardiovascular function and remains a cornerstone for diagnosing both cardiac and noncardiac diseases. 1 It is painless, carries no direct risk to the patient, is reproducible, and is relatively inexpensive compared to other diagnostic procedures. 1

Core Clinical Applications

The EKG is unequaled as a noninvasive method for diagnosing and treating arrhythmias. 1 Beyond rhythm analysis, the EKG serves multiple critical functions:

• Detects acute coronary syndromes - It is indispensable for diagnosing acute coronary syndromes and prompting immediate therapy initiation. 1 A 12-lead ECG should be acquired and interpreted as soon as possible at first medical contact to facilitate early diagnosis and triage. 2

• Identifies life-threatening conditions - ECG abnormalities may be the first indicators of life-threatening drug side effects, severe metabolic or electrolyte disturbances, and occasionally the only sign of myocardial disease such as asymptomatic myocardial infarction in elderly patients. 1

• Diagnoses conduction abnormalities - It is the most accurate means of diagnosing intraventricular conduction disturbances. 1

• Reflects multiple cardiac changes - The EKG may serve as an independent marker of myocardial disease and reflect electrophysiologic, anatomic, metabolic, and hemodynamic alterations. 1

Technical Foundation and Limitations

The EKG records electrical activity, meaning diagnoses of structural changes (myocardial infarction, hypertrophy) or pathophysiologic changes (electrolyte disturbances, drug effects) are made by inference and are therefore subject to error. 1

Standard Technical Specifications

• Paper speed: 25 mm/second 3
• Amplitude calibration: 10 mm/mV (meaning 1 mV produces a 10 mm deflection) 3
• Frequency response: Minimum 150 Hz for adults and adolescents, 250 Hz for children to preserve diagnostic accuracy 3

Critical Interpretation Requirements

Computer-generated ECG interpretations must be verified by a qualified physician - an ECG interpreted by computer alone is not recognized as properly interpreted. 1, 3 This is particularly important because:

• Computer interpretations of rhythm disturbances, ischemia, or infarction require careful physician over-reading 1
• A skilled physician must integrate clinical data, provide differential diagnosis, and compare with previous tracings 1
• The same ECG pattern may be recorded in patients with different structural and pathophysiologic states, explaining the frequent low specificity for determining etiology and anatomy of cardiovascular disease 1

Class I Indications (First-Line Use)

The ACC/AHA guidelines designate the following as Class I indications where electrocardiography is the first choice:

• Patients presenting with chest pain, dizziness, or syncope - symptoms that may predict sudden death or myocardial infarction 1
• Suspected acute coronary syndromes - Serial ECGs should be performed if the initial ECG is equivocal or doesn't support clinical suspicion of myocardial ischemia 2
• Known cardiovascular disease requiring monitoring 4

Common Pitfalls to Avoid

Improper electrode placement is a critical error that alters the frontal plane axis and can mask or create false patterns of infarction. 3 Specific technical pitfalls include:

• Incorrect precordial lead positioning significantly changes QRS morphology and ST-segment interpretation, potentially causing false diagnoses of old MI 3
• Poor frequency response causes smoothing of critical features including pathological Q waves and notched QRS components 3
• Muscle tremor creates baseline artifact that obscures ST-segment and T-wave abnormalities 3
• Over-reliance on computer interpretation without physician verification remains a common error 3

Systematic Interpretation Approach

When interpreting an EKG, assess these parameters systematically:

• Rate: Normal is 60-100 beats per minute 3
• Rhythm: The EKG is the only practical noninvasive method for recording and analyzing cardiac rhythm abnormalities 1, 3
• PR interval: Normal is 120-200 ms (3-5 small squares) for AV conduction assessment 3
• QRS duration: Normal is <120 ms (<3 small squares) for ventricular conduction evaluation 3
• QT interval: Must be corrected for heart rate (QTc; normal <450 ms for men, <460 ms for women) 3

Always compare current ECG tracings with previous tracings using identical calibration settings to detect interval changes. 3, 4 An incorrect ECG diagnosis can have significant undesirable medical and legal consequences. 1

Clinical Context Integration

ECG findings must always be interpreted in the context of cardiac biomarkers, symptoms, and imaging studies. 3 The responsible physician's judgment may transcend specific guideline recommendations in certain clinical situations, such as obtaining an ECG in a patient with acute illness even when no cardiac symptom, physical finding, or risk factor is present. 1