Mechanism of HRT
Everyone agrees that HRT is essentially a baroreflex phenomenon. That is, a PVC interrupts the normal cardiac cycle, so the ventricles of the heart haven't had time to fill up to their normal level, before contracting and pumping their contents out. This results in a pulse (blood pressure) weaker than expected and triggers normal homeostatic mechanisms that try to compensate by constricting arteries and increasing heart rate (the turbulence onset part of HRT). This is accomplished by the brain reflexively withdrawing the parasympathetic nerve signals and increasing the sympathetic nerve signals it sends to the heart. The compensatory constriction of the arteries and increased heart rate frequently cause blood pressure to oveshoot normal values (overcompensates), and activate the baroreflex in reverse. This time, the brain reinstates parasympathetic nerve signals and decreases sympathetic nerve signals, which cause the heart rate to slow (the turbulence slope part of HRT).
The exact quantitative contribution of the parasympathetic and sympathetic nervous flow to the heart to HRT is unknown. The simplistic view assumes that HRT is solely dependent upon parasympathetic activity because atropine, a parasympathetic activity blocker abolishes HRT while a beta-blocker (sympathetic blocker) has no effect on HRT. The contribution of the compensatory pause, the pause between the PVC and the next normal beat, to HRT is also unknown. Some investigators believe that the compensatory pause produces a single beat blood pressure increase beyond normal only in compromised hearts, whereas other investigators have shown that the single beat surge occurs in normal hearts as well. To date, no physiological parameter has been linked in a quantitative manner to turbulence slope, whereas turbulence onset was shown by researchers in Calgary, Canada to be linearly dependent upon duration of subnormal blood pressure in a well designed experiment.
Finally, the reason the size of HRT after a PVC predicts cardiac death is suggested by its mechanism. Parasympathetic nervous activity to the heart is believed to be protective and sympathetic nervous activity, deleterious, to the heart. Especially after a heart attack, sympathetic nervous activity tends to be increased. A healthy HRT indicates the presence of a healthy amount of parasympathetic activity, countering sympathetic activity. To take a wider view, however, it may be that a healthy HRT is also an indication of a healthy brain, and is the reason small HRT also predicts a likelihood of death from non- cardiac causes as well as from cardiac causes.
Read more about this topic: Heart Rate Turbulence
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