1. CIRCULATORY SYSTEM: ELECTRICAL ACTIVITY OF THE HEART
AND ELECTROCARDIOGRAPHY (ECG)
CIRCULATORY
SYSTEM L 2
CONTENTS
1. Introduction
2.Conductive system
3.Basis of excitability
4.ECG
Dr Phiri S B
DR S B PHIRI
2. CIRCULATORY SYSTEM: LECTURE 2 OBJECTIVES
Electrical properties of the heart
1. Describe the conductive system of the heart
2. Describe the basis and principles of excitability of the heart
3. Describe the normal ECG pattern
Dr Phiri S B
3. EXCITABILITY AND CONDUCTIVE SYSTEM OF THE
HEART
If the heart is removed from the body and all neural innervations are severed
it will still continue to beat as long as the myocardial cells remain alive.
The automatic nature of the heartbeat is referred to as automaticity.
Many regions within the heart have been shown to be capable of originating
action potentials and functioning as pacemakers.
In a normal heart, however, only one region demonstrates spontaneous
electrical activity and by this means functions as a pacemaker.
This pacemaker region is called the sinoatrial node, or SA node.
The SA node is located in the right atrium, near the opening of the superior vena
cava.
Dr Phiri S B
Pacemaker Potential
The cells of the SA node do not maintain a resting membrane potential in the manner of resting neurons or skeletal muscle cells.
Instead, during the period of diastole, the SA node exhibits a slow spontaneous depolarization called the pacemaker potential.
The membrane potential begins at about –60 mV and gradually depolarizes to –40 mV, which is the threshold for producing an
action potential in these cells.
This spontaneous depolarization is produced by the diffusion of Ca2+ through openings in the membrane called slow calcium
channels.
At the threshold level of depolarization, other channels, called fast calcium channels, open, and Ca2+ rapidly diffuses into the
cells.
The opening of voltage-regulated Na+ gates, and the inward diffusion of Na + that results, may also contribute to the upshoot
phase of the action potential in pacemaker cells.
Repolarization is produced by the opening of K+ gates and outward diffusion of K +, as in the other excitable tissues.
Once repolarization to –60 mV has been achieved, a new pacemaker potential begins, again culminating with a new action
potential at the end of diastole.
Conducting Tissues of the Heart
Action potentials that originate in the SA node spread to adjacent myocardial cells of the right and left atria through the gap junctions between
these cells.
Since the myocardium of the atria is separated from the myocardium of the ventricles by the fibrous skeleton of the heart,
however, the impulse cannot be conducted directly from the atria to the ventricles.
Specialized conducting tissue, composed of modified myocardial cells, is thus required.
These specialized myocardial cells form the AV node, bundle of His, and Purkinje fibers.
Once the impulse has spread through the atria, it passes to the atrioventricular node (AV node), which is located on the inferior portion of the
interatrial septum
From here, the impulse continues through the atrioventricular bundle, or bundle of His beginning at the top of the interventricular septum.
This conducting tissue pierces the fibrous skeleton of the heart and continues to descend along the interventricular septum.
The atrioventricular bundle divides into right and left bundle branches, which are continuous with the Purkinje fibers within the ventricular walls.
Stimulation of the Purkinje fibers causes both ventricles to contract simultaneously and eject blood into the pulmonary and systemic circulation.
Conduction of the Impulse
Action potentials from the SA node spread very quickly at a rate of 0.8
to 1.0 meter per second (m/sec) across the myocardial cells of both
atria.
The conduction rate then slows considerably as the impulse passes
into the AV node.
Slow conduction of impulses (0.03 to 0.05 m/sec) through the AV
node accounts for over half of the time delay between excitation of
the atria and ventricles.
After the impulses spread through the AV node, the conduction rate
increases greatly in the atrioventricular bundle and reaches very high
velocities (5 m/sec) in the Purkinje fibers.
As a result of this rapid conduction of impulses, ventricular
contraction begins 0.1 to 0.2 second after the contraction of the atria.
4. VENTRICULAR/ATRIAL MUSCLE ACTION POTENTION
Depolarization (phase 0)
- Opening of fast voltage-gated Na+ channels.
-Rapid Influx of Sodium ions leading to rapid depolarization.
-AP via gap junction from adjacent cells
Small Repolarization (phase 1)
-Opening of a subclass of Potassium channels which are fast channels.
-Rapid Potassium Efflux.
Plateau phase (phase 2)
- 250 msec duration (while it is only 1msec in neuron)
- Opening of the L-type voltage-gated slow Calcium channels & Closure of the
Fast K+ channels.
- Large Calcium influx
- K+
Efflux is very small as K+
permeability decreases & only few K channels are
open.
Repolarization (phase 3)
- Opening of the typical, slow, voltage-gated Potassium channels.
- Closure of the L-type, voltage-gated Calcium channels.
- Calcium Influx STOPS
- Potassium Efflux takes place.
RMP (phase 4)
- Na K ATPase
- Other channels responsible for RMP
L-type channel Ca++
channel
acts as voltage gated
channel
Ca++
enters cytosol from T
tubules
Ca++
from T tubules
stimulates opening of
ryanodine receptor Ca++
channel
Ca++
enters cytosol from
sarcoplasmic reticulum
contraction
5. THE ELECTROCARDIOGRAM (ECG OR EKG)
Dr Phiri S B
The body is a good conductor of electricity because tissue fluids have a high concentration of ions that move (creating a current) in response to
potential differences.
When the cardiac impulse passes through the heart, electrical current also spreads from the heart into the adjacent tissues surrounding the
heart
Potential differences generated by the heart are thus conducted to the body surface, where they can be recorded by surface electrodes placed
on the skin.
The recording is known as an electrocardiogram.
The recording device is called an electrocardiograph.
Note that the ECG is not a recording of action potentials, but it does result from the production and conduction of action potentials in the heart.
A pair of surface electrodes placed directly on the heart will record a repeating pattern of potential changes.
As action potentials spread from the atria to the ventricles, the voltage measured between these two electrodes will vary in a way that provides
a “picture” of the electrical activity of the heart.
By changing the position of the ECG recording electrodes on the body surface, a more complete picture of the electrical events can be
obtained.
Normal Cardiac Rhythm
1. Heart rate ( 60 to 100 bpm)
In a normal functioning human heart (Adults),the heart should beat within this range
2. Originate from sinoatrial node
The SA node generates action potential at a rate of 60-100/min
AV node generates AP at 40-60/min
Purkinje fibers within the ventricles generate at 15-30/min
SA node is the leader of electrical activity hence it is the pacemaker.
Its rate of discharge determines the rate at which the heart beats
3. Propagation of cardiac impulse through the normal conduction pathway
4. Normal cardiac impulse velocity in the pathway.
Characteristics of the Normal Electrocardiogram
The normal electrocardiogram is composed of a P wave, a QRS complex, and a T wave.
The QRS complex is often, but not always, three separate waves: the Q wave, the R wave,
and the S wave.
P WAVE
Depolarization of the atria
Normal origin is SA node (sinus origin)
Normally, all the same shape, all followed by QRS complex
QRS COMPLEX
Q-wave: Septal depolarization
R-wave: Major ventricular depolarization
S-wave: Basal ventricular depolarization
0.04 – 0.10s
If prolonged relates to abnormalities of conduction,
most commonly bundle branch block
T WAVE : Ventricular recovery ( repolarization)
U wave
Delayed repolarization of Purkinje fibers
Prolonged repolarization of mid-myocardial M-cells
After-potentials resulting from mechanical forces in the
ventricular wall
The repolarization of the papillary muscle
PR segment
is the flat line between the end of the P-wave and the start of the QRS
complex. (0.1 – 0.2s)
reflects the time delay between atrial and ventricular activation.
AV node delay and conduction of via bundles to purkinje fibers
serves as the baseline (reference line or isoelectric line) of the ECG curve
ST SEGMENT
Time period between end of ventricular depolarization and the
beginning of ventricular repolarization
Isoelectric
Baseline elevation or depression is indication of abnomality in
ventricular recovery usually because of injury to the heart muscle
PQ or PR INTERVAL
Measured from the beginning of P wave to the beginning of QRS complex
Is the interval between the beginning of electrical excitation of the atria and the beginning
of excitation of the ventricles.
Often this interval is called the P-R interval because the Q wave is likely to be absent
0.12 - 0.20s
The time from the beginning of atrial contraction to the beginning of ventricular
contraction
If prolonged relates to atrioventricular problem
QT INTERVAL
Normal range 0.30 – 0.46s
Total time of ventricular contraction and recovery (normally varies according to
individual heart rate and age)
Short or long QT interval may reflect drug effect or electrolyte abnormalities
measured from beginning of QRS complex to end of T wave
Calculating Heart Rate From ECG
Paper speed 25mm/s
1 small square = 0.04s = 1mm
1 big square = 0.2s = 5 mm
