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EKG
Tutorial: AV Blocks
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For various reasons, conduction through the AV node may become impeded even more than usual (remember that the AV node is supposed to slow conduction down). This may be entirely harmless, or depending on the degree of resistance, could prove to be a fatal dysrhythmia. AV-blocks are rated with any of three degrees of severity.
1st Degree Heart Block
1st degree blocks are generally benign. They are characterized
by a constant PR interval greater than 0.2 seconds. The rhythm is otherwise
normal. Rates may range from bradycardias to tachycardias with a full degree
of variation in between. Ordinarily, there will be no symptoms associated with
a 1st degree block.
2nd Degree Heart Block
2nd degree blocks are subdivided into two types:
This rhythm is distinguished by a repeating cycle of increasing
PR intervals. As the interval get longer, a P-wave is either not conducted
(ie; there is no QRS) or the P-wave is simply dropped. The cycle then repeats
again. Typically, RR intervals become shorter until the dropped beat.
Type I blocks are generally not dangerous. The patient may complain of palpitations, or skipped beats.
This rhythm can be recognized by a consistent PR interval and frequently non-conductive P waves. QRS complexes may appear widened depending on the location of the block. Wide QRS complexes indicate that the ventricles are depolarizing from an action potention in the ventricular tissue, rather than from or above the AV junction.
Generally speaking, Type II blocks are not a good sign. They have a tendency to worsen, leading to 3rd degree blocks.
3rd Degree Heart Block
The 3rd degree block is by far the most dangerous. There is absolutely no conduction through the AV node. Due to the automaticity of each region of the heart, the atria beat at there intrinsic rate (60-80 bpm) and the ventricles, which are completely isolated from the atria beat at their slower rate of 20-40 bpm.
The QRS complexes will often be wide, but depending
on the origin the ventricular action potential, they may remain narrow.
The P-P interval and R-R interval will each be regular and consistent. The P-P interval will be faster than the R-R and there will be no relation between the two.
A 3rd degree block is also called Atrioventricular dissociation.
The danger of these high-degree blocks should be obvious. Ventricular contraction will not always be preceeded by an atrial contraction. Hence, the ventricles are not guaranteed to contain enough blood for a detectable contraction.
Clinical treatment for high degree heart blocks can be pharmacological, or invasive. Autonomic drugs, such as Atropine, can be used to inhibit vagal stimulation and increase the bradycardic rates typically associated with heart blocks. If conduction is not improved with medication, artificial pacemakers can be installed to stimulate either the atria, ventricles, or both, in a synchronized rhythm.
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