Electrical System of the Heart | Cleveland Clinic
Electrical System of the Heart

The normal heart rate

The heart’s rhythm is coordinated by its own electrical system. With each heartbeat, the electrical impulse begins at the sinus (or sinoatrial, SA) node, also called the heart’s natural pacemaker. The SA node is a cluster of specialized cells, located in the right atrium. The SA node produces the electrical impulses that set the rate and rhythm of your heartbeat. The impulse spreads through the walls of the right and left atria, causing them to contract, forcing blood into the ventricles.

The Heart's Electrical System

The impulse then reaches the atrioventricular (AV) node, which acts as an electrical bridge allowing impulses to travel from the atria to the ventricles. There is a short delay before the impulse travels on to the ventricles. From the AV node, the impulse travels through a pathway of fibers called the HIS-Purkinje network. This network sends the impulse into the ventricles and causes them to contract. The contraction forces blood out of the heart to the lungs and body.

The SA node fires another impulse and the cycle begins again.

The heartbeat is triggered by electrical impulses that travel down a special pathway through your heart muscle.

Heart rate and rhythm differences between men and women

Women and men are similar when it comes to the basic heart rate and rhythm. However, while the basic electrical system is the same (impulses originating in the SA node, traveling to the AV node, through the HIS-Purkinje, and then starting over), there are differences:

  • Women tend to have a faster baseline heart rate
  • Women’s ECG readings may be different

Women Have a Faster Baseline Heart Rate Than Men

First of all, on average, women tend to have a faster baseline heart rate than men. This difference is seen in girls, on an average, as young as five years old. There is also a shorter sinus node refractory time – this means that it takes a shorter time for the SA node to recover and become ready to fire an impulse again 1.

Women’s ECG Readings may be Different

The ECG (also called EKG or electrocardiogram) is a test used to record on graph paper the electrical activity of the heart. The picture, drawn by a computer from information supplied by electrodes placed on the skin of the chest, arms and legs, shows the timing of the different phases of the heart rhythm.

Normal ECG | Cleveland Clinic

The p wave represents the electrical activity of the upper chambers of the heart (atria). There is a short pause followed by the QRS complex - the electrical activity of the lower chambers (ventricles) - and ends with a small T wave, the recovery phase of the ventricles. The QT interval is the distance from the beginning of the QRS to the end of the T wave and represents the time it takes for the heart muscle to contract and then recover, or for the electrical impulse to fire and then recharge.

On average, the QT interval is shorter in men than in women, beginning after puberty with a linear increase through the major part of adulthood to at least age 55. This period corresponds to the time period when androgen levels are highest in men. Therefore, androgen and estrogen levels may explain the gender differences in QT interval 2.

  1. Taneja T, Mahnert BW, Passman R, Goldberger J, Kadish A. Effects of sex and age on electrocardiographic and cardiac electrophysiological properties in adults. Pacing Clin Electrophysiol. 2001 Jan;24(1):16-21.
  2. Rautaharju PM, Zhou SH, Wong S, Calhoun HP, Berenson GS, Prineas R, Davignon A. Sex differences in the evolution of the electrocardiographic QT interval with age. Can J Cardiol. 1992 Sep;8(7):690-5

Certain types of arrhythmias are more prevalent in women than in men.

These include:

  • Supraventricular Tachycardia (SVT) or Paroxysmal SVT (PSVT) – a rapid heart rate that originates above the AV node, in the atria. SVT is common in both men and women, but more women have AV node reentrant tachycardia and atrial tachycardia 1.
  • Sinus Node Dysfunction (also called sick sinus syndrome) – a slow or irregular heart rhythm that originates in the SA node. The signal starts in the SA node but may be slow or delayed in progressing to the atria, causing a very slow or irregular heart beat.
  • AV Nodal Re-entry Tachycardia (AVNRT) - a type of SVT with a fast heart rate that originates in the AV node. Instead of the AV node sending the impulse down one pathway, there are two pathways through the AV node. The impulses travel through one pathway as well as back up through the second pathway. This allows the impulses to travel around the AV node very quickly in a circular fashion, causing the heart to beat unusually fast.
  • Long QT Syndrome - a QT interval longer than normal. This increases the risk for life-threatening forms of ventricular tachycardia.
  • Postural Orthostatic Tachycardia Syndrome (POTS) - a condition that affects 500,000 Americans, primarily women. Those with POTS have an abnormal response to change in position, related to the autonomic nervous system, causing drop in blood pressure, raise in heart rate and sometimes syncope (passing out), dizziness or lightheadedness 2.

These arrhythmias occur more often in men, but may present differently in women:

  • Atrial fibrillation - one of the most common irregular heart rhythms. It is a rapid irregular heart rhythm originating in the atria. Men have atrial fibrillation more often than women. Atrial fibrillation can be associated with other types of heart disease. Women are more likely to have atrial fibrillation associated with valve disease, while men more often have atrial fibrillation associated with coronary artery disease. The incidence of atrial fibrillation increases in both men and women with age, and when they also have hypertension and diabetes. The Copenhagen Heart Study showed that women with atrial fibrillation had an increased risk for stroke and cardiovascular death as compared to men. This is particularly true in women who have atrial fibrillation and are older than age 75 3,4. Women who have paroxysmal atrial fibrillation, a type of atrial fibrillation that is intermittent (or comes and goes), may have a faster heart rate response than men, and tend to have longer episodes 5.
  • Sudden cardiac death is a sudden, unexpected death caused by loss of heart function (sudden cardiac arrest). Sudden cardiac death (SCD) occurs less frequently in women, but is still related to about 400,000 deaths per year in women. The Nurses’ Health Study showed that while the majority of women who had SCD had no prior history of cardiovascular disease before death, they had at least one cardiac risk factor (smoking, hypertension and diabetes had the greatest impact). Family history also played a role in increased risk if one parent died of heart disease before age 60. The study also showed that as with men, the majority of SCD in women was related to an abnormality of the heart rhythm (88%) 5-7. This reinforces the need for careful screening of heart disease risk factors in women and managing these concerns even without symptoms present.
  • Michael J. Porter, MD, Joseph B. Morton, MBBS, Russell Denman, MBBS, Albert C. Lin, MD, Sean Tierney, MD, Peter A. Santucci, MD, John J. Cai, MD, Nathaniel Madsen, MD, David J. Wilber, MD. Influence of age and gender on the mechanism of supraventricular tachycardia. Heart Rhythm 1:4. October, 2004, pp: 393-396
  • National Dysautonomia Research Foundation, www.ndrf.org/orthostat.htm
  • Kannel WB, Wolf PA, Benjamin EJ, Levy D Prevalence, incidence, prognosis, and predisposing conditions for atrial fibrillation: population-based estimates. Am J Cardiol. 1998 Oct 16;82(8A):2N-9N.
  • Friberg J. Comparison of the impact of atrial fibrillation on the risk of stroke and cardiovascular death in women versus men (The Copenhagen City Heart Study). Am J Cardiol - 1-OCT-2004; 94(7): 889-94.
  • Hnatkova K, Waktare JE, Murgatroyd FD, Guo X, Camm AJ, Malik M. Age and gender influences on rate and duration of paroxysmal atrial fibrillation. Pacing Clin Electrophysiol. 1998 Nov;21(11 Pt 2):2455-8.
  • American Heart Association Heart Disease and Stroke Statistics 2012 Update
  • Christine M. Albert, Claudia U. Chae, Francine Grodstein, Lynda M. Rose, Kathryn M. Rexrode, Jeremy N. Ruskin, Meir J. Stampfer, and JoAnn E. Manson. Prospective Study of Sudden Cardiac Death Among Women in the United States. Circulation, Apr 2003; 107: 2096 – 2101