Interpreting Cardiac Arrhythmias: How Do Doctors Decide Which Arrhythmia You Have and Where It Starts?

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About Arrhythmias

There are a number of abnormal heart rhythms, or cardiac arrhythmias, which physicians may diagnose. Some have few symptoms, while others are life-threatening and may require immediate surgical intervention. Patients can wear Holter monitors or event monitors to aid in the diagnosis of less severe or sporadic arrhythmias. However, in hospitals, physicians diagnose cardiac arrhythmias using measurements of the electrical functioning of a patient’s heart.

When the heart beats, an electrical impulse propagates through the heart muscle from the sinoatrial node (SA node) in the wall of the right atrium.. This impulse travels through the heart causing the muscle to contract and, then, relax. This allows for the ejection of blood from the heart chamber and the subsequent filling. Arrhythmias interrupt this normal rhythm by speeding up, slowing down or causing parts of the heart to contract out of sync with the rest of the heart. When interpreting cardiac arrhythmias, physicians measure the electrical impulse that causes the heart to beat.


Within hospitals, physicians use a non-invasive test to determine arrhythmic cardiac function. The electrocardiogram (EKG or ECG) measures a number of aspects of the electrical activity in a patient’s heart, such as pulse, electrical function and interval between beats. Physicians must use all of this information when interpreting cardiac arrhythmias. Electrocardiograms may appear to be a nonsensical mess of scribbles, but each section means something different. The first wave is the P wave, and it measures the electrical function of the atria. The next wave, the largest, is the QRS complex, and it measures the impulses in the ventricles. The third wave, the T wave, represents the ventricles resetting for the next heartbeat.

Physicians read the results of the EKG and determine from which arrhythmia a patient is suffering. For example, if a patient’s EKG displays a “sawtooth” set of P waves, the doctor may diagnose atrial flutter, as this is characteristic of that particular arrhythmia. Long intervals between P waves most likely indicates bradycardia, which is a heart rate of less than 60 beats per minute (bpm). Conversely, short intervals may indicate tachycardia, a resting heart rate of greater than 100 bpm.

Electrophysiology Studies

As another option to study the electrical function of a patient’s heart, physicians use electrophysiology studies (EPS), which is much more invasive than EKG. In EPS, a doctor will perform a cardiac catheterization. In this procedure, doctors insert a long, thin tube, or catheter, into a vein in the leg or neck and run it to the heart. Once there, the catheter sends electrical signals to the heart in order to record where in the heart the arrhythmia begins. Doctors may also use drugs to stimulate the arrhythmia or to determine which drugs cause cessation of the arrhythmia. If possible, the physician may perform a procedure called ablation in which, using the catheter, electricity is used to kill cells that are causing the arrhythmia.


Mayo Clinic: Heart Arrhythmias: Tests and Diagnosis

University of Rochester Medical Center: Arrhythmia

University of Utah Spencer S. Eccles Health Sciences Library: Lesson 1: The Standard 12 Lead ECG

Louisiana State University Health Sciences Center-Shreveport: 12-Lead ECG Heart Function-How the Heart Works

Harvard Health Publication: Electrophysiological Testing of the Heart

Texas Heart Institute: Electrophysiology Studies