EKG Tutorial: Cardiac contraction

Electrocardiography

Cardiac contraction and ECG

Action Potentials - the Key to Contraction

This gets a little tough, so take it slowly...

I spoke of automaticity earlier and mentioned that all myocardial cells possess this wonderful property. Automaticity is the direct result of a cleverly designed "leaky membrane" which regulates the exchange of Sodium, Potassium, and Calcium ions to change the polarization of the cells. The sequence is as follows:

1. Sodium ions enter the cell and begin the depolarization.
2. Calcium ions follow and extend the depolarization even further.
3. Once Calcium stops moving inward, Potassium ions move out and repolarization begins.

In a nutshell, sodium starts the cells' stimulation. Calcium extends the stimulation thereby allowing the entire muscle to contract before Potassium finally comes along and tells the muscle to relax for a moment and prepare for the next cycle.

The important part of this cycle is the period where the cells reset and prepare for the next wave. This is called the refractory period because the cells are refractory to (or unaffected by) further stimulation.

Actually, there are two portions of the refractory period:

• Absolute refractory period - during this period, absolutely no stimulation can cause another action potential. This is the first part of the refractory period.
• Relative refractory period - during this portion, it is possible to cause another action potential, but the intensity of the contraction will be relative to the time in this period. So, the further into the period, the better the contraction.
  • An impulse during the relative refractory period may cause a premature contraction. In this situation, the chambers are not filled completely. According to the Frank-Starling Law, this decreased preload will cause cardiac output to decrease.
  • Additionally, serious and life-threatening dysrhythmias can arise if the R-wave of the next beat falls in certain portions of the previous T-wave. It didn't take a genious to call this the "R on T phenomena." -- Just remember... R on T... bad...

    

If you're still scratching your head we still have one more chance... (an old paramedic once told this story around a campfire)

Imagine, if you will, a toilet. Please bear with me... I swear this will work!

When you pull the handle, (initiate an impulse) water floods the bowl(kinda' like contraction!). This event takes a couple of seconds and you cannot stop it in the middle. Once the bowl empties, the flush is complete. Now the upper tank is empty. If you try pulling the handle at this point, nothing happens (absolute refractory?) Wait for the upper tank to begin refilling (Potassium moves back). You can now flush again, but the intensity of the flushes increases as the upper tank refills (relative refractory...)

Physiology will never be the same again...

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