ECG Interpretation

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The ability to accurately perform a standard ECG interpretation is the most fundamental task that anyone working in cardiac care must master. A 12-lead electrocardiogram provides an image of heart functionality and is essential to the process of identifying an abnormality in the rate and rhythm. Those who plan to work in the medical field, as an EKG technician, will spend the majority of their time administering electrocardiograms and might be expected to assess both the exam quality and differentiate between normal and abnormal results.

Waves and Intervals


Proper ECG interpretation requires a basic understanding of the different elements found on a tracing. Every ECG consists of a series of waves and intervals that represent electrical current as it flows through the heart and initiates contractions within the cardiac cells. The ‘P wave’ is seen first on the tracing and it represents the depolarization of the right and left atria. The atria are the chambers of the heart that sit above the ventricles and they are responsible for pumping blood into the lower ventricular chambers. In order for blood to flow through the heart, the atria must contract before the ventricles and; therefore, the ‘P wave’ is seen first on the electrocardiogram tracing.


The ‘QRS complex’ is the large spike that occurs following the ‘P wave’ and it represents the depolarization of the right and left ventricles. The ventricles are the chambers that sit directly below the atria and they are responsible for sending blood to the lungs and the rest of the body. It is absolutely essential that the ventricles contract after the atria; otherwise, the heart would be attempting to pump blood that has not yet entered the ventricles and the body would not receive the oxygen that it needs in order to survive.

Following the ‘QRS complex,’ there is a gradual rise from baseline that is referred to as the ‘ST segment’ and a second small wave that is called the ‘T wave.’ The ‘ST segment’ and ‘T wave’ represent ventricular repolarization during which the ventricles have returned to a resting state and the electrochemical gradient is restored. The amount of time involved in the occurrence of each of these waves as well as the period of time between each wave is referred to as an interval.


Intervals provide important information for a proper ECG interpretation and can be used to identify abnormalities in cardiac functionality. The ‘PR Interval’ represents the time lapse between the onset of atrial depolarization and the subsequent ventricular depolarization, the ‘QRS Interval’ demonstrates the duration of ventricular depolarization, the ‘QT Interval’ is the period during a ventricular depolarization and repolarization, the ‘PP Interval’ is the rate of sinus cycle, and the ‘RR Interval’ is the rate of the ventricular cycle.

Spatial Interpretation

The electrocardiogram tracing presents waves in the way that it does because of how the ECG leads are placed on the chest. Willem Einthoven is credited with assigning the letters P, Q, R, S, and T to the different deflections of the electrocardiogram and he was the creator of the imaginary inverted triangle that is used to represent the standard leads that are placed on the arms and leg. Einthoven’s triangle is commonly used to understand the spatial qualities of the heart’s electrical signal. Each lead in a 12-lead ECG utilizes a positive and negative pole that causes either an upward or downward deflection depending on whether or not electricity is flowing towards or away from it. A thorough understanding of spatial electrical activity is important for proper ECG interpretation.

Precordial lead placement is as follows…

  • V1 – Fourth intercostal space adjacent to the right sternal border
  • V2 – Fourth intercostal space adjacent to the left sternal border
  • V3 – Midway between V2 and V4
  • V4 – Fifth intercostal space along the midclavicular line
  • V5 – Horizontal to V4 along the anterior axillary line
  • V6 – Horizontal to V4 – V5 along the midaxillary line

Interpretive Strategies

Most cardiac specialists use a methodical approach to ECG interpretation from which they never deviate. A consistent routine for examination is essential to ensuring a thorough exam free from error or omission. While not all practitioners and technicians use the same approach, they nearly always have a system for making sure that they do not overlook important details. The fact that many different aspects of an electrocardiogram must be considered during an evaluation means that each should be considered in turn.


One commonly used approach is as follows…

  • Measurements – including Heart Rate, the PR Interval, the QRS Duration, QT Interval, and QRS Axis
  • Rhythm Analysis
  • Conduction Analysis
  • Waveform Description
  • ECG Interpretation
  • Previous ECG Comparison