Electrocardiogram and heart sounds

Student Protocol

Electrocardiogram and Heart Sounds
An introduction to the recording and analysis of electrocardiograms, and the sounds of the heart.

Written by staff of ADInstruments

Introduction
The beating of the heart is accompanied by both electrical activity and sound. The pattern of electrical activity
produced by each heart beat cycle is called the electrocardiogram or ECG.The aim of this session is to record
and analyze an ECG from a student volunteer, and to examine the relationship between the ECG and the
characteristic sounds of the heart.

Background
The heart is a dual pump that pushes blood around the body and through the lungs. Blood enters the atrial
chambers of the heart at a low pressure and leaves the ventricles at a higher pressure; it is thishigh arterial
pressure that provides the energy to force blood through the circulatory system. Figure 1 shows the organization
of the human heart and the circulatory system, in schematic form. Blood returning from the body arrives at the
right side of the heart and is pumped through the lungs to pick up oxygen and release carbon dioxide. This
oxygenated blood then arrives at the left side of theheart, from where it is pumped back to the body.

Figure 1. A schematic diagram of the human heart and circulatory system.

Cardiac contractions are not dependent upon a nerve supply. A group of weak muscle cells (sinoatrial or
sinuatrial node, SA node) acts as the pacemaker for the heart (Figure 2). These cells rhythmically produce action
potentials that spread through the fibers of theatria. The resulting contraction pushes blood into the ventricles.
The only electrical connection between the atria and the ventricles is via the atrioventricular (AV) node. The
action potential spreads slowly through the AV node (thus giving a time delay for ventricular filling) and then
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Student Protocol
rapidly through the AV bundle and Purkinjefibers to excite both ventricles. The large muscle mass of the
ventricles allows powerful contractions.

Figure 2. Components of the human heart involved in conduction.

Each side of the heart is provided with two valves, to convert rhythmic contractions into a unidirectional
pumping. The valves close automatically whenever there is a pressure difference across the valve that would
causebackflow of blood. Closure gives rise to audible vibrations (heart sounds). Atrioventricular (AV) valves
between the atrium and ventricle on each side of the heart prevent backflow from ventricle to atrium. Semilunar
valves are located between the ventricle and the artery on each side of the heart, and prevent backflow of blood
from artery to ventricle. The cardiac cycle involves a sequentialcontraction of the atria and the ventricles. The
combined electrical activity of the different myocardial cells produces electrical currents that spread through the
body fluids. These currents are large enough to be detected by recording electrodes placed on the skin. The
regular pattern of peaks produced by each heart beat cycle is called the electrocardiogram or ECG (Figure 3).

Figure 3. Atypical ECG showing the fundamental parts that make up the signal.

The action potentials recorded from atrial and ventricular fibers are different from those recorded from nerves
and skeletal muscle. The cardiac action potential is composed of three phases: a rapid depolarization, a plateau
depolarization (which is very obvious in ventricular fibers), and a repolarization back to restingmembrane
potential.
The components of the ECG can be correlated with the electrical activity of the atrial and ventricular muscle:
• the P-wave is produced by atrial depolarization
• the QRS complex is produced by ventricular depolarization; atrial repolarization also occurs during this
time
• the T-wave is produced by ventricular repolarization.

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