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Figure 13-5

Prolonged P-R interval caused by first degree A-V heart block (lead II).

different types of myocarditis, caused, for example, by diphtheria or rheumatic fever. 4. Extreme stimulation of the heart by the vagus nerves in rare instances blocks impulse conduction through the A-V node. Such vagal excitation occasionally results from strong stimulation of the baroreceptors in people with carotid sinus syndrome, discussed earlier in relation to bradycardia.

Incomplete Atrioventricular Heart Block

Prolonged P-R (or P-Q) Interval—First Degree Block. The usual lapse of time between beginning of the P wave and beginning of the QRS complex is about 0.16 second when the heart is beating at a normal rate. This so-called P-R interval usually decreases in length with faster heartbeat and increases with slower heartbeat. In general, when the P-R interval increases to greater than 0.20 second, the P-R interval is said to be prolonged, and the patient is said to have first degree incomplete heart block.

Figure 13-5 shows an electrocardiogram with prolonged P-R interval; the interval in this instance is about 0.30 second instead of the normal 0.20 or less. Thus, first degree block is defined as a delay of conduction from the atria to the ventricles but not actual blockage of conduction. The P-R interval seldom increases above 0.35 to 0.45 second because, by that time, conduction through the A-V bundle is depressed so much that conduction stops entirely. One means for determining the severity of some heart diseases—acute rheumatic heart disease, for instance—is to measure the P-R interval.

Second Degree Block. When conduction through the A-V bundle is slowed enough to increase the P-R interval to 0.25 to 0.45 second, the action potential sometimes is strong enough to pass through the bundle into the ventricles and sometimes is not strong enough. In this instance, there will be an atrial P wave but no QRS-T wave, and it is said that there are "dropped beats" of the ventricles. This condition is called second degree heart block.

Figure 13-6 shows P-R intervals of 0.30 second, as well as one dropped ventricular beat as a result of failure of conduction from the atria to the ventricles.

At times, every other beat of the ventricles is dropped, so that a "2:1 rhythm" develops, with the atria beating twice for every single beat of the ventricles. At other times, rhythms of 3:2 or 3:1 also develop.

Complete A-V Block (Third Degree Block). When the condition causing poor conduction in the A-V node or A-V bundle becomes severe, complete block of the impulse

Dropped beat

Dropped beat

Figure 13-6

Second degree A-V block, showing occasional failure of the ventricles to receive the excitatory signals (lead V3).

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