ECG Activity - Part 2

ECG Activity - Part 2

HASPI Medical Anatomy & Physiology 13c

Cardiac Conduction

The heart has its own system in place to create nerve impulses and does not actually require the brain to make it beat.  This electrical system is called the cardiac conduction system.  It is made up of specialized tissues and cells embedded in the heart that send impulses directly to the atria and ventricles, causing them to contract.  In the normal cardiac cycle, the left and right atria contract together first followed by the contraction of the ventricles together.

The cardiac conduction system is complex, but there are three crucial parts:

• The Sinoatrial (SA) Node

Also known as the pacemaker; this is a small bundle of cells capable of starting the electrical impulse that will cause the heart to beat.

• The Atrioventricular (AV) Node

A small bundle of cells that can receive the electrical impulse from the SA node, and continue the electrical signal down into the ventricles.

• His-Purkinje System

These fibers actually carry the electrical impulse to the ventricles, causing a contraction.  The impulse starts at the His bundle fibers, continues into the right bundle branch and left bundle branch, and ends in Purkinje fibers spread throughout the ventricles.

The Heartbeat

The sinoatrial (SA) node begins the contraction by sending an impulse through both atria, causing them to contract and reach the atrioventricular (AV) node.  The AV node continues the impulse, sending it down the Bundle of His, right and left bundle branches, and into Purkinje fibers within the ventricles causing both ventricles to contract. 

As the impulse moves through the heart, cardiac muscle tissue contracts as the cells depolarize.  Depolarization is caused by a flood of ions (sodium and potassium) across the cell membrane and drives the cells, and therefore cardiac muscle tissues, to contract. Once the impulse has passed, the cardiac cells must reset to be able to send another impulse.  This is called repolarization and occurs when the ions move back through the cell membrane, causing the cardiac muscle tissue to relax.

The SA node will create anywhere from 60-100 electrical impulses per minute.  Each of these impulses causes your heart to beat and translates to your heart rate.  The electrical impulse causes the heart to depolarize and contract, followed by repolarization and relaxation, at a constant rate.  The normal rhythm and rate is called the sinus rhythm.  The cardiac conduction system is also capable of increasing or decreasing the number of electrical impulses depending on the amount of oxygen needed by the body.  For example, when you run a mile your need for energy increases, causing the need for more oxygen, and resulting in an increase in the heartbeat and respiratory rate to bring more oxygen into the body. 

Electrocardiograms

An electrocardiogram is a measurement of the electrical activity and impulses created by the heart. It is most often referred to as an EKG or ECG.  Electrodes are placed on the chest and each electrical impulse is translated into a wave pattern on a grid.  The grid allows for the calculation of the duration and amplitude of each impulse, as well as whether the impulses are abnormal or irregular.  Flat lines on the grid represent no electrical activity, upward spikes represent electrical activity or depolarization, and downward spikes represent repolarization.

Specific waves on the ECG grid represent depolarization and repolarization in different parts of the heart.  The diagram below shows how each of these waves should normally appear on an ECG grid.

• P wave – depolarization and contraction of the right and left atria
• QRS complex – depolarization and contraction of the right and left ventricles
• T wave – repolarization and relaxation of the ventricles
• PR interval – time between the depolarization of the atria and the ventricles
• QT interval – time for the ventricles to depolarize and repolarize, or contract and relax
• RR interval – time for the entire cardiac contraction cycle, based on the ventricles
• PP interval – time for the entire cardiac contraction cycle, based on the atria

Information from the questions comes from the following website.  

Practical Clinical Skills - https://www.practicalclinicalskills.com