Cardiac output = Stroke Volume x Heart rate
Electrocardiograms (ECG) show voltage against time. A normal ECG will show five distinct peaks
P Wave – excitation of the atria
QRS complex – excitation of the ventricles
T wave – repolarisation of the ventricles
- The sinoatrial node is responsible for the coordination of the heart beat and is found in the wall of the right atrium
- The sinoatrial node sends out a wave of depolarisation that spreads over the left and right atria. This stimulates the myocardium (heart muscle) of both atria to contract.
- The wave of excitation then reaches the atrioventricular node (found in the right atrium) which then sends a wave of depolarisation to the bundle of His and purkyne fibres (located in the septum)
- The wave of excitation continues along the purkyne fibres to the apex of the heart and then up through each ventricle. This causes the ventricle to contract, forcing blood out of the heart.
- Note there is a slight delay after the atria contract to ensure the ventricles are full of blood and to insure that the atrioventricular valves are closed.
Using ECGs to Diagnose Heart Disease
- If the heart behaves abnormally, the normal electrical activity of the heart is disrupted, causing the rhythm to change. Different rhythms are known as arrhythmias and can be detected using ECGs.
- If the heart is starved of oxygen (ishaemic) then it cannot empty or fill
- Atrial fibrillation : atria beat too fast, increasing chance of a clot
- Tachycardia: heart beats too quickly, can be caused by stress
- Ventrical fibrillation: ventricles contract weakly and erratically, hence little blood is pumped out of the heart. It is often a rapid drop in blood pressure, so the brain, body and heart are rapidly starved of oxygen and glucose. This can lead to a heart attack.
In some cases, an abnormal heart rate can be corrected by giving the heart a large electrical shock with a defibrillator.