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Electrocardiogram (ECG) (cont.)

ECG Interpretation

Interpreting an ECG requires a fair amount of education and experience. Numerous textbooks are devoted to ECG interpretation. The ECG is just one test to assess the heart. History and physical examination remain the cornerstones for diagnosing heart disease. The doctor-patient discussion may uncover the potential for heart problems even if the ECG is normal.

Most often, the ECG assessment includes the following:

  • determination of the rate,
  • assessment of the rhythm,
  • evaluation of the electrical conduction patterns. Heart muscle that is irritated conducts electricity differently than heart muscle that is normal. Abnormal conduction may be apparent during ventricular contraction and during ventricular recovery.

The ECG records the heart tracing in12 leads: Six limb leads (I, II, III, AVR, AVL, AVF) and six chest leads (V1-V6).

The P wave looks at the atria. The QRS complex looks at the ventricles and the T wave evaluates the recovery stage of the ventricles while they are refilling with blood.

The time it takes for electricity to travel from the SA node to the AV node is measured by the PR interval. The QRS interval measures electrical travel time through the ventricles and the QT interval measures how long it takes for the ventricles to recover and prepare to beat again.

Basic P-QRS-T wave sequence: Strip shows a simple sequence where M equals 1.0 millivolts.

Picture of basic P-QRS-T wave sequence

The computers imbedded in most ECG machines are able to measure the time it takes for the electrical impulse to travel from the SA node to the ventricles. These measurements can help the doctor assess heart rate and some types of heart block.

Some computer programs may also try to interpret the ECG. And as artificial intelligence and programming improves, they are often correct. However, there are enough subtleties in interpretation that the human element is still a very important part of the assessment. The ECG machine is not always correct.

The decision to act upon the results of an ECG depends not only upon the ECG tracing, but also upon the clinical situation. A normal ECG does not exclude heart disease and an abnormal ECG may be the "normal" baseline for that patient.

Other ECG pictures:

Rhythm strip of a person who was cardioverted out of ventricular tachycardia by an electric shock.

Rhythm strip of a person who was cardioverted

A 12-lead electrocardiogram (ECG) of a person with chest pain. It shows heart attack (acute inferior wall myocardial infarction). Image courtesy of Vibhuti N Singh, MD, MPH, FACC.

Picture of a 12-lead electrocardiogram (ECG) of a person with chest pain

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