1. The Heart and Circulation

2. The ElectroCardioGram (E.C.G)
The invasion of the cardiac muscle by the contraction wave is associated with electrical changes.
These potentials may be recorded at points remote from the heart and after electronic amplification may be displayed on a pen recorder or a cathode ray oscilloscope.
The wave form obtained is termed ECG.
The various waves present in the waveform are lettered starting with the letter P.

3. The P wave corresponds to the spread of excitation from the S. A
The P wave corresponds to the spread of excitation from the S.A. node over the atrial muscle and therefore represents atrial systole.

4. The propagation of the contraction wave down the bundle of His does not produce any detectable external electrical change. It is denoted on the E.C.G. by the iso-electric P-Q interval where there is no deflection.
The QRS complex at the commencement of the ventricular excitation, and the T wave at the end are all that remain of this algebraic summation. The relaxation phase T-P is iso-electric.

5. Leads I, II, Ill, aVR, aVL and aVF
The detailed waveform depends on the site of the recording electrodes.
To record the standard limb leads, electrodes are applied to the left arm, right arm, and left leg. These electrodes usually consist of a metal plate which is strapped over the flat part of the limb.
Electrode fluid or jelly is used between the electrode and the skin to ensure a good electrical connexion.

6. The right leg is not used for recording the ECG, but a further electrode is often applied to this limb to 'earth' the subject, and thus minimize interference from the mains and electrical apparatus.
In order to make a recording, two input connexions must be made to the amplifier and there is usually a switch on the electrocardiograph apparatus which makes the appropriate connexions internally when a particular lead is chosen.

7. When the switch is set at Lead I, the electrodes on the right and left arms are connected to the amplifier. The amplifier then records the potential difference between the right arm and the left arm.
When the switch is set to Lead II, the potential difference between the right arm and left leg is recorded.
When set to Lead III, the potential difference between the left arm and left leg is recorded.

8. An alternative way of recording the ECG is to connect one input of the amplifier to a limb electrode, and to connect the other input of the amplifier to the other two limbs through two resistances.
Such an arrangement records the difference between the potential in one limb and the mean potential in the other two.
If the single limb is the right arm, the lead is termed 'aVR', if the left arm, it is termed 'aVL', and if the left leg (or foot), it is termed 'aVF'.
Leads I, II III, aVR, aVL and aVF are the six standard limb leads which are recorded clinically.

9. Cardiac Vector
The figure shows the ECG recorded using six limb leads.
They have been arranged to correspond to the figure below.

10. Chest Leads
The six limb leads (so far discussed) give information about the 'electrical axis of the heart' in the frontal plane.
To study the activity in a horizontal plane, chest leads are used.
These usually take the form of a single chest electrode which will adhere to the chest wall by suction, and which is connected to one input of the amplifier.
The other input of the amplifier is connected to an electrically neutral point V which is obtained by joining the three limb leads together through resistances.

11. It is found that the point V does not change its potential during the cardiac cycle and this potential may thus be used as a reference.
As the search electrode is moved across the chest, the ECG shows a dominant S-wave in chest positions 1 and 2, and a dominant R-wave in chest positions 5 and 6.

13. Heart Block
The duration of the P-Q, or as it is often called the P-R, interval is a measure of the conduction time in the bundle. It is normally ( ) seconds.
Any longer time denotes delay in the transmission along the bundle and maybe the forerunner of complete failure of conduction along the bundle known as heart block.

14. Complete heart block occurs when the ventricles will be cut off from the pacemaker and will stop beating.
However, they may restart at a slow independent rate due to the inherent rhythmicity of the ventricular muscle.
The rate (about 30 beats/minute) is often too slow for an efficient circulation.

15. In all probability the S. A
In all probability the S.A. node and atria will continue at their original rate of 70 beats/minute.
Atrial contractions will cease to be effective in aiding the circulation as many will occur whilst the A-V valves are shut.
The ECG will show numerous P waves but few abnormal QRS complexes and there will be no time correlation between them.

16. In such a case an electrical stimulator may be implanted in the axilla or abdominal wall and connected to electrodes in the ventricle.
Such a device acts as an artificial pacemaker and the ventricular contractions follow at the rate set by the electronic circuits. About 20 x 10-6 joules of energy are needed for each pulse.

17. If a ventricular beat originates at some ectopic focus, an abnormal QRST complex will appear on the E.C.G.

18. This is a common occurrence in young adults and is known as an extra-systole or dropped beat.
The term dropped beat is used because the ectopic beat may prevent the next pacemaker beat from contracting the ventricular muscle.

19. There is therefore a pause until the next successive beat arrives.

20. Atrial Fibrillation
Atrial fibrillation occurs when the increased excitability of the atrial muscle leads to abnormal beats arising from Ectopic Foci in the atria.
As a result, different parts of the atria are contracting and relaxing out of phase with one another.

21. The general appearance of the atria during fibrillation has been described as resembling a bag of wriggling worms.
The atria never empty and the atrial wall becomes a quivering sheet of muscle.

22. In atrial fibrillation the atria cease to pump blood into the ventricles.
In addition, the beat originated by the pacemaker is not transmitted to the ventricles.
Instead, impulses down the bundle of His in a haphazard fashion at a rate too fast for the ventricle to respond to each impulse.

23. The ventricular contractions which do occur are irregular both in rate and amplitude.
A proportion of these beats fail to develop sufficient pressure to open the aortic valve.
As a result the radial pulse is "irregularly irregular" and the heart rate measured at the heart may be higher than if it is measured by the pulse at the wrist. This is known as a pulse deficit.

24. Ventricular Fibrillation
Should the ventricular muscle fibrillate, the circulation stops immediately. Unless immediate cardiac massage is carried out, life will cease.
Under suitable conditions ventricular fibrillation may be stopped by passing an electric current through the heart.
The apparatus is termed a cardiac defibrillator.