Something that is often overlooked in cardiac rhythm interpretation are the heart blocks. No one really pays much attention to these in their march on towards VF, VT, and ACLS in school. As a result, they are quickly forgotten as soon as a student passes NREMT or a state exam, and since we rarely ever see these rhythms in regular practice they can be easily missed when a problem arises.
In order to talk about the cardiac conduction system we should probably do a quick review of how it works and the QRS complex. At rest, the cardiac cells are negatively charged, with a positive charge on the exterior of the cell. Depolarization is activated by an inflow of sodium triggered by the action potential of surrounding cells. This is called rapid depolarization. This is accompanied by an inflow of calcium, which causes the muscle contraction. This balances the outflow of potassium. For a brief time the charge of cell is depolarized, after which potassium is released at a higher rate and restoring the cell’s negative charge.
The impulse is caused by an electrical signal from the sinoatrial (SA) node (for issues with sinoatrial node, see one of my earlier posts on atrial fibrillation). Now this impulse is not lost to the surrounding cells. It travels to both atria through the cells via intracalated discs, and down an intranodal pathway to the atrioventricular (AV) node. After another short delay, the impulse is transmitted to the Bundle of His through the septum of the heart by the bundle branches, and finally down to the Purkinje fibers, where the impulse terminates. The process repeats itself over 3 billion times in the average human life span.
Now we can visualize this electrical impulse via an electrocardiogram (ECG). The ECG allows us to view abnormalities in the electrical conduction system by viewing the wave impulse as it is transmitted throughout body tissues and finally to the skin. The ECG is one of the most valuable tools in the paramedic’s rolling arsenal against death. Combined with 12 lead capability we can diagnose and treat a wide variety of cardiac abnormalities. The theory we know and use today was developed by Willem Einthoven in the early 1900’s. Check out this article over at Wikipedia about electrocardiography to refresh yourself on the basic components and theory of ECG’s, or revisit any standard textbook.
There are 5 different waves that appear in a standard ECG. The first wave is the “P” wave, lasting about 80 ms, it shows the SA node’s depolarization wave and how long it takes to travel across the atria and to the AV node. The second set of waves is the QRS complex itself, it shows the impulse rapidly depolarizing the ventricles. This can last as long as 120 ms, but should not last any longer. The last wave is the T wave. This shows the repolarization of the ventricles. In all the process should last about 1.2 seconds.
Next week we will start examining the different basic heart blocks individually and talk about some of the ECG findings and physiologic causes, as well as ACLS treatment options.
EDIT: Thanks Christopher for the correction!