The x axis on the graph is over time. All this means is that the activity on the far left of the graph happened first and then the activity on the far right happened last. Everything in between happened in time order.
This ECG ‘rhythm strip’ shows four complete heart beats
, bear with me and you will soon understand exactly why 🙂
Lets rewind just briefly to the previous chapter. What happens first in the Hearts contraction? The S.A. Node fires and causes the top of the heart to contract cell by cell. This electrical energy is the first to be recorded by an ECG and is called the P Wave.
With the electrical signal having propagated through the top chambers of the heart it then reaches the A.V. Node where the signal is kept alive but delayed, during this period there is very very little electrical activity in the heart and so no activity is picked up by the ECG machine and therefore none appears on the graph.
This is none as the PR interval…
As the heart beat continues the signal is eventually passed on to the Ventricles. Now because of the network of special conductive tissue (Bundles of His and Pirkinje Fibres) a large amount of cellular activity happens relatively quickly. All these Ventricular heart cells contracting are picked up by the ECG machine as 3 ‘waves’ Q,R’ and S. Together these waves are quite predictable called the the QRS Complex and they represent the contraction of the bottom chambers of the heart – the Ventricles.
Note that the waves are bigger than the P Wave and this is because there are many more cells firing! The more electrical energy picked up by the ECG machine the bigger the wave it draws.
So that is a heart beat traveling through your heart and picked up on an ECG. There is however one more wave that we always see called the T Wave.
The T Wave shows us the Ventricles recharging ready to contract again in the very near future.
Think about how your mobile phone recharges? there is a transfer of electricity back into your phone battery.
This is not dissimilar to how the heart cells recharge. As the Ventricles recharge (repolarise), this activity is picked up by the ECG and drawn on the graph as the T Wave.
Those of you who are on the ball may be asking where the ‘wave’ showing the Atria recharging is? Well you are correct in that you would expect to see a recharging for the Atria too. However due to the small size of this process and the fact it may well be hidden by the other waves, it does not appear on the ECG.
Now if we look back at the initial strip I am sure you can all see that there are four clear heart beats… and you can probably name the ‘waves’ too?
The individual who named these ‘waves’ (Willem Einthoven) allegedly used P-T because he was a fan of Descartes who used P and Q to label the curves of his geometry graphs. I also believe he had an ECG inventing rival at the time who had already used A and B for different waves! Grrr!!!
Thanks for Reading
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