ECG rates along the conduction pathway for nurses & nursing students

What is the cardiac conduction pathway?

Electricity and plumbing. These are the two major systems working synchronously within the heart. Let’s take a look at the conduction pathway: the path an impulse follows through the heart to tell the heart muscle when to beat. I like to think of this conduction pathway as a high-speed train travelling along the tracks from top to bottom.

The electrical impulse is the brains behind the operation and the cardiac muscle as just that, the muscle. All brawn and no brain, only contracting when they’re told to do so by the specialized pacemaker cells. Let’s compare.

What does this mean?

Myocardial cells: the machinery that performs the work of contraction resulting in systole. The myocardial cells take orders from the pacemaker cells. Calcium is needed within the cell to activate special sites allowing for what’s known as excitation-contraction-coupling to occur. This produces the squeeze that propels blood forward. With less available calcium this occurrence is slowed. This is one way that some (not all) calcium channel blockers work to decrease heart rate and blood pressure.   

Pacemaker cells: the electrical power source of the heart. Inherent rates differ among these special cells based upon location (sinus, junctional, or ventricular) and can be influenced by neuro-hormonal input. Think adrenergic stimulation versus vagal stimulation. Adrenergic (adrenaline receptors) stimulation increases chronotropy (speed of contraction; chrono = time) and inotropy (strength of contraction; ino = I know my own strength). Beta adrenergic receptors in the heart elicit this response when stimulated by catecholamines. Beta blocker medications work to lessen this effect and decrease blood pressure as a result. 

Vagal nerve stimulation increases gastro-intestinal motility but has the opposite effect on the heart when stimulated. For this reason, "bearing-down" or performing a vagal maneuver is sometimes used to terminate supraventricular tachycardias (SVT), or to help differentiate certain arrhythmias from one another by slowing the heart rate.

Automaticity is a characteristic of cardiac cells, meaning they can depolarize free of any outside stimulation. In fact, when the heart is removed from the chest (i.e. with transplant) the heart still beats.

Now, the pacemaker cells of the SA node are the leakiest (most permeable) to the ions that cause depolarization. Due to this, they set the rate and they like to beat 60 - 100 times per minute. If they fail, or die (think right coronary artery occlusion; the RCA feeds the SA node) the AV junction is next in line at 40 - 60 BPM inherently, followed by ventricular cells at 20 - 40 BPM. With each progression down the line, we run the risk of decreased cardiac output partly due to a decrease in heart rate. Remember, cardiac output = HR x SV.

Shown in the image above, the natural pathway of cardiac conduction should look like this: 

1.       The impulse begins with the SA node setting the pace

2.       From here the impulse travels along the inter-nodal pathways shown between 1 & 2 above, to the AV junction and AV node. There is a slight delay here which is the reason the P-R segment is flat on the ECG. This pause allows for complete ventricular filling. As a side note: this is one danger of sustained rapid heart rates. Without the pause to fill, the ventricles eject less blood resulting in decreases in cardiac output

3.       From the AV node, we head to the bundle of His followed by

4.       The left and right bundle branches, finally giving way to

5.       The purkinje fibers, located near the terminal parts of the ventricles at the apex

Because the ventricles are thick and slow to conduct relative to the rest of the heart, the impulse actually speeds up quite a bit in the purkinje fibers allowing for complete and simultaneous ventricular contraction. The end result is maximum blood expulsion from the ventricles. 

Why should we care?

Thankfully the back-up sites listed above (AV and ventricular) exist if the SA node were to fail. These sites may keep us beating long enough to have an artificial pacemaker put in or other treatment performed to keep things ticking along if needed. Additionally, stating whether a rhythm from a particular origin site is accelerated, versus normal, versus tachycardic begins with an inherent understanding of the natural intrinsic rate for the associated pacemaker site. Hope this helps!

TAKE HOME POINTS     

• Think of the electrical impulse as the brains behind the operation and the cardiac muscle as just that, the muscle

• Myocardial cells are the machinery performing the work of contraction resulting in systole

• Pacemaker cells set the rate of contraction

• Rates are typically between 60 – 100 BPM, 40 – 60 BPM, and 20 – 40 BPM for sinus, junctional, and ventricular sites, respectively

• Normal cardiac conduction begins with the SA node and terminates in the purkinje fibers