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EKG
Tutorial: Ventricular rhythms
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We're almost done! Before you go watch "E.R." we'll cover the most serious portion of this tutorial.
The ventricles represent the largest, strongest portions of the heart, and therefore lead to the most serious problems when they fail (what good is a hockey team if nobody can shoot?)
We already learned that the purkinje fibers permit impulses to speed through the ventricular muscle in 0.04 to 0.12 seconds. In a typical atrial or junctional complex, we see narrow QRS complexes for this very reason.
If the fibers become blocked, though, the QRS time will increase to longer than 0.12 seconds.
Premature Ventricular Contractions
On occasion, a ventricular cell may initiate an impulse and cause a contraction. A single occurrence probably won't cause any serious problems. PVCs can be caused by a variety of conditions including respiratory problems and stress (and taxes).
Ordinarily, there will be a compensatory pulse with a PVC. If there is no pulse, we say the PVC is interpolated. If you have a patient with PVCs, be sure to check for a corresponding pulse. This has a large part in deciding whether the PVCs are dangerous or not.
PVCs are classified on the basis of their origin
They all have the same shape, or morphology.
Each focus has a unique morphology. Often, there will be a repeating sequence, indicating several specific focii. Make an effort to note the number and sequence of focii.
PVCs are also classified by their frequency
If each normal contraction is followed by a single PVC, we call this bigeminy. If two normal contractions are followed by a single PVC, we have trigeminy. Be smart... what do you think quadrigeminy is?
Notice in this example, that the PVCs are unifocal... bigeminy tends to arise from a single focus.
Exactly two PVCs in a row is called a couplet.
R on T
Phenomena
We discussed R on T when we spoke about the refractory periods. R on T can lead to a fatal dysrhytmia called ventricular tachycardia. Obviously, we'd like to avod this. R on T can occur with very fast rates, but also with ectopic beats like PACs, PJCs, and PVCs.
Ventricular
Tachycardia
V-Tach is a rapid dysrhytmia in which the ventricles depolarize very quickly and without regard for the atria.V-Tach is actually said to happen whenever three or more PVS occur in a row (which is why we don't name anything beyond a couplet). Regarding pulses, any of the following could happen:
- pulse for every complex - the pulses will be weak and cardiac output low.
- pulse for some beats - this is ominous
- no pulse - there may or may not be any contraction at all, but if there's no pulse, you're patient is in bad shape.
We'll discuss the treatment for this a little later.
Ventricular
Fibrillation
V-fib is the most common fatal dysrhytmia in adult patients. You see it "E.R." every week. V-fib represents a chaotic depolarization of random ventricular cells. A heart in V-fib literally looks like jiggling Jell-o. There is no pulse associated with this rhythm. CPR won't do much good either, nor will most drugs. You'd better hope the defibrillator works!
V-Fib is usually described as coarse (above) or fine (below). Generally, as the tissue dies, the voltage decreases. Hence, coarse is a little better than fine V-Fib.
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