INTRODUCTION TO THE HEART’S ELECTRICAL CONDUCTION SYSTEM
The heart lies in the space between the lungs, which called the mediastinum. It is approximately the size of a person’s fist and consists of four muscular chambers, two atria, and two ventricles. The role of the heart is to pump blood around the body, enabling oxygen, nutrients, and waste products transported to and from the different cells and organs. ECG represents the electrical activity of the heart. ECG helps in the diagnosis of heart disease and further treatment.
Blood travels through the heart in the following way— oxygen-depleted blood returning from the body cells enters the right atrium, and at the same time, oxygenated blood in the lungs returns to the left atrium.
The increase in the volume of blood in the atria then starts to flow into both ventricles. That results in the contraction of the atrial walls. Once the ventricles are full of blood, they in turn contract and eject their contents. Blood from the right ventricle travels to the lungs to pick up oxygen, and blood that has already been oxygenated in the left ventricle travels to the rest of the body’s cells and organs.
for the effective way to pump the blood out the atria and ventricles need to contract in sequence. And it is the role of the heart’s electrical conduction system to control this. the specialized cells embedded within the heart’s muscular walls which help in conduction
These cells are able to generate and conduct electrical impulses, which in turn stimulate the heart muscle to contract. The conduction system works as follows. Firstly, the sinus node generates an electrical impulse that sweeps across both atria in a wave-like fashion. the wave of depolarisation means the spread of impulse.
The atria respond by contracting and this ejects blood into the ventricles. Meanwhile, the impulse reaches the atrioventricular node, which lies between the atria and the ventricles, before traveling on through the bundle of His and right and left bundle branches. The ventricles, which have now had sufficient time to fill with blood, contract in response to the impulse.
impulses generation again and repeat then. As each cycle originates at the sinus node, referred to as a sinus beat. And where this cycle occurs repeatedly, called sinus rhythm. This represents the normal sequence of events leading to controlled electrical activation and contraction of the heart. We will revisit the concept of sinus rhythm in week two of the course.
WHAT IS AN ECG?
an electrocardiograph or ECG is a graphic representation of two electrical events that show depolarisation and repolarisation. Depolarisation is the spread of the electrical impulse across the heart, and repolarisation is the recovery stage following this.
electrodes or conductors detect current which placed on the skin and ECG graph showing paper as positive and negative deflections called waves and complexes. When current not detected, you can see a flat line, called a baseline.
We now need to look in more detail at the ECG graph paper itself.
you can see a magnified image of the paper. But on an ECG printout, one small square measures one millimeter in width and one millimeter in height. Larger squares comprise five x five small squares and are identified by dark outlines.
In a graph, we have a vertical and a horizontal axis. On the vertical axis, we measure amplitude, which is a generation of electrical force. This means that the greater the force is, the taller or deeper the deflections. On the vertical axis, 10 millimeters, or 10 small squares in height are equivalent to one millivolt of electrical current.
On the horizontal axis, we measure time in seconds, which means the longer it takes for the electrical forces to move across the heart tissue, the wider the waveforms will appear. One small square or one millimeter on the horizontal axis is equivalent to 0.04 of a second, and this corresponds to a standard paper speed of 25 millimeters a second.
The ECG shows 12 leads, or viewpoints, of the heart. There are six limb leads, which are I, II, III, aVR, aVL, and aVF, and six chest leads, C1 to C6.
Although the leads are looking at the same electrical events, they are viewing these events from different angles, so the waveforms in each lead will look slightly different. Having multiple viewpoints is useful because it provides more detailed information about what may be happening in different areas of the heart.
You might compare this to watching a football match, where in order to get to good overall picture of the action, you would need to have cameras positioned in different locations around the pitch.
Some of the 12 leads have similar views, so specific areas or territories can group together. For example, leads I, aVL, C5, and C6 all view the left side of the heart, which is the lateral territory. Leads II, III, and aVF view the bottom of the heart, known as the inferior territory. Finally, leads C1 to C4 all broadly view the front or anterior territory of the heart.
Along the bottom of the ECG, you will usually see a continuous strip of lead II. This makes it easier to assess the heart rhythm, without the distraction of the intermittently changing leads above.
HOW TO PLACE CHEST ELECTRODES FOR AN ECG
the first thing you’re going to do is look for a sternal notch at the top of the sternum. and then you’ll run your finger down until you feel a ridge, which is the angle of Louis.
It should be around about 3 and 1/2 to 4 centimeters down from the top. And then when you feel that ridge, you should just let your finger slide to the right size and slightly down. then you should find that your finger just drops into the second intercostal space. after that, you can just feel down to the third intercostal space, and then the fourth intercostal space. And that is the location of the V1 electrode.
So it’s in the fourth intercostal space to the right of the sternum. You would do exactly the same procedure to find the V2 location. So you start at the top, let your finger drop until you feel the ridge, and then you will go to the left side, in this case, and down. And your finger should drop into the second intercostal space. After that, you feel down to the third and the fourth. And then that will be your location for the V2 electrode.
So that’s the fourth intercostal space to the left of the sternum. And then I’m going to move on to looking at how to locate V4. So that will be in the midclavicular line, which is in the middle of the collarbone. And then also the fifth intercostal space. So I’m just going to bring my hand down, and I’m looking for the fifth intercostal space and the midclavicular line.
So I’m going to just pop my electrode on there. And then V3 can literally just go halfway between V2 and V4. So, I’m just going to put that electrode halfway between. To find the position of V5, we’re going to use the same horizontal line as we’ve got now for V4. And I’m looking for the front of the armpit,
the anterior axillary line. So I’m just going to put my electrode there. So that’s in the same horizontal line as before and in the anterior axillary line. And then for V6, it’s the middle of the armpit or the midaxillary line. And also, in the same horizontal plane as we’ve got for V4 and V5. So I’m just going to put the electrode around about there. And there are the locations for the chest electrodes.
EGC Assessment: An introduction for Healthcare Providers Limb electrode and lead placements:
Label Position of electrode / lead on body Colour
AVR Right wrist Red
AVL Left wrist Yellow
AVF Left ankle Green
Right ankle Black
REMEMBER: • Ride • Your • Green
Chest electrode and lead placements:
Reference Sheet Label Position of electrode / lead on body Colour
V1 Fourth intercostal space at the right border of the sternum Red
V2 Fourth intercostal space at the left border of the sternum Yellow
V3 Midway between placement of V2 and V4 Green
V4 Fifth intercostal space at the midclavicular line Brown
V5 Anterior axillary line on the same horizontal level as V4 Black
V6 Mid-axillary line on the same horizontal level as V4 and V5 Purple
RELATIONSHIP BETWEEN SINUS RHYTHM AND THE ECG WAVEFORMS
sinus rhythm which is normal electrical conduction of the heart, and how this is links, coordinates contraction and movement of blood. ECG as waves and complexes represent the electrical activity of the heart.
depolarisation across the atria represents a normal sinus beat that starts with the generation of an electrical impulse at the sinus node and the spread of this impulse This event represented on the ECG as a smooth, rounded waveform called a p-wave.
The second step in the process is the impulse moving through the atrioventricular node and the bundle of his. This does not produce a deflection on the ECG, so it represents a gap between the end of the p-wave and the beginning of the next waveform.
Following this, the impulse travels down both bundle branches and depolarises the ventricles. This represented by the QRS complex, which can vary in its shape but is often of two or three deflections. After depolarisation, repolarisation of the ventricles occurs. And this represented by an asymmetric t-wave.
Parameters for normal sinus rhythm. In an earlier step, we saw how the electrical activity of the heart is represented on an ECG. The presence of a P wave, followed by a QRS complex and a T wave, represents what we call a normal sinus beat, indicating that generation of the impulse at the sinus node and then spread across the atria and the ventricles.
There are a wide variety of different measurements which can be taken to examine the wave forms and their relationships in greater detail; however, we’re just going to briefly look at two of these, which are of particular significance.
The first is called the PR interval, which is the distance from the beginning of the P wave to the beginning of the QRS complex. It, therefore, includes the whole of the P wave and the gap between the P and the QRS. This interval tells us whether or not there is normal conduction between the atria and the ventricles.
And a normal PR interval is between three and five small squares in width. The second measurement is the QRS duration, which should be no more than 2.5 small squares in width. A normal QRS duration tells us that the ventricles are depolarising normally.
SINUS RHYTHEM ECG
A run of continuous sinus beats, as you see here, is called sinus rhythm. When assessing ECG for the presence of sinus rhythm,
you should look for the following features: Distinct P waves should be present.
Every P wave should be followed by a QRS complex and a T wave. The P waves, QRS complexes, and T waves should look similar across the rhythm strip. And the PR interval should be constant and within the normal range of three to five small squares.
This indicates that the same electrical events are happening over and over again, and there is a normal connection between the atria and the ventricles. The heart rate is also usually regular. And this can be confirmed by looking for consistent intervals between the peaks of neighbouring QRS complexes
SUPRAVENTRICULAR TACHYCARDIA ECG
Supraventricular arrhythmias, an introduction. Abnormal heart rhythms occur when the electrical impulse does not originate at the sinus node and follow the normal path through the atria and ventricles. There are many causes of arrhythmias, including coronary heart disease, hypoxia and congenital heart disease. Broadly speaking, arrhythmias can be divided into two main types, supraventricular and ventricular, depending on their origin.
Supraventricular arrhythmias arise from a problem within the atria, the AV node, or the bundle of His. Examples include atrial fibrillation, atrial flutter, and supraventricular tachycardia. All these arrhythmias are characterized by the absence of normal P waves and are typically associated with fast heart rates.
However, because they affect atrial rather than ventricular depolarisation, QRS complexes are usually normal. Atrial fibrillation is a common supraventricular arrhythmia in which multiple areas within the atrial muscle tissue generate impulses randomly and at a very fast rate. Some of these impulses will travel through the atrial ventricular node and depolarise the ventricles in the normal fashion.
On the ECG, P waves will be absent, although you may be able to see some small non-uniform waveforms called fibrillation waves. These represent the chaotic atrial activity. As the impulses are generated randomly within the atria, the overall heart rate is also irregular. QRS complexes appear normal.
One of the risks of atrial fibrillation is that the uncoordinated depolarisation and contraction within the atria can lead to pooling of blood and clot formation. If part of the clot then breaks off or embolises, it can potentially travel through the bloodstream to the brain and cause a stroke. Because of this, people with chronic atrial fibrillation may require long term anticoagulation therapy.
VENTRICUAR TACHYCARDIA ECG
Ventricular arrhythmias– an introduction. Ventricular arrhythmias is an umbrella term used to describe abnormal rhythms originating from the ventricles themselves. On the ECG, they’re characterized by abnormally wide QRS complexes, fast heart rates, and typically, the absence of visible P waves. Examples include premature ventricular contractions, ventricular tachycardia, and ventricular fibrillation.
Ventricular tachycardia arises when an area within the ventricles generates an impulse in a regular fashion and at a very fast rate. Because the impulses are derived from outside of the normal conduction pathway, they take longer to depolarise the ventricles, and therefore the QRS complexes are wider. The abnormal, rapid depolarizations can significantly impair the heart’s ability to pump blood. So emergency treatment may be required, depending on the presentation.
On the ECG, you will see wide, bizarre-looking QRS complexes. The rate is fast and regular, and typically, no P waves are seen.
Ventricular fibrillation arises when multiple areas within the ventricular muscle generate electrical impulses in a very rapid and random fashion. The chaotic activity leads to uncoordinated ventricular contraction, and a failure in the heart’s ability to pump blood. Someone with this arrhythmia will therefore be unconscious with no pulse, and will require emergency advanced life support.
The chaotic ventricular activity is represented on the ECG as irregular waveforms with no discernible pattern.
THANKS FOR READING
DR.MANISH KHOKHAR MD MEDICINE