Fig. 45.—Electrocardiogram showing auricular extrasystoles (P). (Courtesy of Dr. G. C. Robinson.)
Fig. 46.—Electrocardiogram showing ventricular extrasystole. Heart rate 56-60 beats per minute. Note that diastolic pause in which extrasystole occurs is practically equal to two normal diastolic pauses. (Courtesy of Dr. G. C. Robinson.)
When one listens over the chest to a heart when extrasystoles are occurring, one suddenly hears a weak beat which has taken place rather too early after the previous systole to be strong enough to effect the opening of the aortic valves. Consequently there is no pulse, the blood does not move, and that beat is lost to the circulation. Moreover, when the next regular stimulus comes from the s-a node it finds the ventricle in a refractory condition, having just ceased a contraction, and it is not until the next sinus impulse that the ventricle responds normally. (Fig. 46.)
Patients who have occasional extrasystoles will say that all of a sudden the heart turns upside down in the chest. Sometimes there is slight sharp twinge of pain. Patients are at times quite alarmed about their condition. Provided there is no evidence of gross myocardial lesion, the extrasystole itself is of no great significance.
While many cases showing pathologic causes for extrasystoles have more or less marked arteriosclerosis, there are other states in which no arteriosclerosis is found where the extrasystole is present.
Heart Block
As heart block occurs frequently in cases characterized by extensive arteriosclerosis, a brief discussion of the essential features will be given. It is, however, probable that arteriosclerosis is not the cause of any of the cases of heart block directly, but it is only a result of the same etiological conditions which produce the lesion or lesions which result in heart block. We may define heart block as the condition in which the auricles and ventricles beat independently of each other. There may be delayed conduction (Fig. 47), partial (Fig. 48), or complete heart block (Fig. 49). In the former there are ventricular silences, during which the auricles beat two, three, four, five, even up to nine times, with only one ventricular contraction. It is believed by most physiologists that the essential factor in the production of heart block is an interference in the conduction of impulses from the auricles to the ventricles through the band of tissue known as the auriculoventricular bundle.