In order to properly understand the 12 lead ECG as well as the many drugs we use to affect our patients heart rate and blood pressure it is important to understand the electrical activity that goes on within the heart itself.
So this quick blog post aims to do just that!
We start by discussing the cardiac pacemaker cells which are-
- Sino-Atrial Node
- Atrio-Ventricular Node
- Bundle of His and Purkinje fibres
These differ from the other cells in the heart in that possess what is known as automaticity- that is they will continue to beat by themselves with not stimulation from anywhere else.
It is the movement of ions- charged molecules that make this possible.
The above image represents the charge difference across the membrane. You can see that the charge goes from negative to slightly positive and then back again.
How does this happen? Lets go from points 1-4 to understand.
Sodium is flowing into the cell making the membrane more positive as it does so.
The charge at its lowest is about -60 mV. As the sodium flows in this this moves towards a charge of -40 mV.
Once enough sodium has flowed into the cell with its positive charge the membrane potential will reach -40 mV. This is called the threshold potential.
At this point voltage gated calcium channels will open. These will not be open until this charge level is reached.
Calcium ions then rush into the cell. They too have positive charge and so the move from negative to positive will become quicker.
Now, with the influx of both the positively charged calcium and sodium, the membrane quickly reaches +10 mV.
At this point the calcium channels are shut, no longer allowing the passage of calcium into the cell.
This is also the moment that voltage gated potassium channels open allowing the passage of potassium out of the cell.
As the calcium has stopped coming in and the potassium is leaving the membrane starts to become very quickly negative again.
Once the membrane potential reaches -60 mV the voltage gated potassium cells also shut.
Potassium can no longer leave the cells.
However sodium is still entering the cell gradually making the cell more positive again.
And so it all begins again!
Quite simple really! And if your heart rate is about 60 bpm then all of this takes place in just one second. Isn't the body just brilliant.