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Guide to Electrocardiogram

An electrocardiogram (ECG or EKG from the German “Elektrocardiogramm”) is the recording produced by an electrocardiography procedure designed to monitor and interpret the heart’s electrical activity over a given length of time. The procedure is noninvasive and gathers information by measuring the rate and regularity of heartbeats. It can also record information about the position and size of the heart’s chambers. Its main function is to determine whether any damage is present or whether particular drugs or devices are having an effect on the heart.

Operation

During different stages of heartbeat, the muscle is charged either negatively or positively. When at rest, the cells of the heart muscle are charged negatively. Through a process called depolarization, positively charged sodium (Na+) and calcium (Ca++) cations enter the heart causing the charge to move toward zero. This activates the muscle which contract thereby completing a heartbeat. The ventricles then repolarize and the cycle begins again.

An ECG device is designed to monitor the efficacy of this process of depolarization. Two electrodes are placed on either side of the heart, and these electrodes are able to detect small changes in voltage. The information detected is relayed to a display screen or piece of paper, producing a graph that looks like a wavy line, called the QRS complex. This is the primary part of the tracing on the graph. It monitors the rhythm and general health of the heart and can provide specific information about the health of particular parts of the heart.

While the minimum number of electrodes needed is two, most ECG procedures utilize several pairs of electrodes which can each monitor different activities in the heart. For example, by placing  electrodes on the left leg (LL), left arm (LA) and right arm (RA) a doctor or specialist can create different pairs of electrodes which monitor the heart from multiple positions. The output recorded from each of these pairs is called a “lead”, each of which provides a different angle. Often times an ECG is referred to by how many leads it has. A 12-lead, for example, monitors the heart from twelve different positions on the body

An ECG is the best way to diagnose any abnormal heart rhythms, usually ones caused by damage to the conductive tissue which carries the electrical signals or from an electrolyte imbalance. When someone has suffered a myocardial infarction, an ECG can be used to determine which particular areas of the heart have been damaged. However, not all areas can be adequately measured, and the ECG device cannot properly monitor or determine the ability of the heart to pump. For that an ultrasound or nuclear medicine test is necessary.

Interpretation

There is a variety of information produced by an ECG device, and being able to accurately interpret this information is vital to proper health care. Generally there are seven sets of information which an ECG provides for the doctor to interpret. Sometimes, when investigating certain types of ECG abnormalities, not all the steps are necessary. If the heart is not sinus, or normally beating, the algorithm for interpreting arrhythmias, or non-normal beating, should be followed rather than moving on to the next step.

  • Rhythm: Normal sinus rhythm means that the heart is depolarizing normally. The sinus node will generate an electrical pulse causing the right atrium to depolarize. The left atrium follows causing the ventricles to be stimulated. An ECG devices monitors this activity, and when the heart is healthy a P Wave precedes every QRS complex. The P Wave created by the monitor reaches a maximum height of 2.5mm and the heart rate remains between 60-100 bpm.
  • Rate: Determining the heart rate of a patient means being able to interpret the time between two QRS complexes on an ECG display. An ECG device produces a P Wave line on a piece of graph paper with thick lines that are 5mm apart, representing 0.2 seconds, and thin lines 1mm apart which represent 0.04 seconds.
  • Conduction: This part of interpretation is determining the intervals between the different parts of the P Wave. This is broken into three parts: the PQ interval, the QRS duration and the QT interval. The PQ begins with start of the atrial contraction and ends with the beginning of the ventricular contraction and is normally within 0.12 and 0.2 seconds. The QRS duration monitors how fast the ventricles depolarize and is normally less than 0.10 seconds. The QT interval display how fast the ventricles repolarize, preparing the heart for a new cycle. This interval normally lasts for 460ms in women and 450ms for men.
  • Heart Axis: Also called the QRS axis, the electrical heart axis averages all the depolarizations in the heart. Normally the direction of the signals from the heart can be indicated with an arrow, or vector. A change in the heart axis or some deviation can indicate pathology.
  • P Wave Morphology: Used to determine the presence of left or right hypertrophy or atrial arrhythmias, the P Wave Morphology has normal characteristics that are represented on the display or graph paper.
  • QRS Morphology: If a heart has had previous myocardial infarctions, the QRS Morphology can be helpful in determining this. Pathologic Q waves indicate the absence of some electrical activity which should be present in a normal heartbeat.
  • ST Morphology: The ST segment of the ECG display represents the ventricular repolarization. In this process the heart prepares for the next heartbeat and usually takes a much longer time than the depolarization.

Further resources