Beating Hearts Issues


IT HAPPENED to a 66-year old woman, known in her workout classes as the “cardio queen” for exerting herself more strenuously than the others.  And to a 52-year old man who has participated in sprint- and Olympic-distance marathons, working up to the half-Ironman. Both began fainting suddenly, and then both had pacemakers implanted in their hearts.

Serious aerobic and endurance exercise, while good for the heart, can over time cause the heart rate to drop below 30 beats/minute, called bradycardia—compared to a typical healthy resting heart rate of 60-100 BPM.  A very fit individual’s heart has a larger stroke volume—meaning that each heartbeat delivers more oxygen to the heart—so the resting heart can beat more slowly.

Research on rodents has shown that endurance exercise training causes molecular changes in the heart’s pacemaker (rather than changes in the autonomic nervous system, which was previously assumed).  These changes “may help us to understand the more frequent occurrence of heart rhythm disturbances or even loss of consciousness in athletes,” said one of the study’s researchers, based at the University of Manchester in England.

“Elderly athletes with a lifelong training history are more likely to need an artificial electronic pacemaker,” the study’s lead researcher said.  (Because the study dealt with rodents, what he had in mind by “elderly” wasn’t clear.)

Of two kinds of exercise, endurance training involves sustained elevation of heart output, while strength training involves short bursts of intense heart contractions.  Some sports like cycling combine both: more than half of 286 Tour de France cyclists (from 1995 and 1998) had abnormally low resting heart rates—measured as ejection fractions, the percent of blood ejected from the heart at each beat.

Fainting has several causes, and only the most common—called cardioinhibitory syncope, with syncope meaning loss of consciousness—is relieved by implanting a pacemaker.  It can occur when the heart rate slows to less than 40 beats per minute with a resulting drop in blood pressure.   With another cause called vasovagal syncope, falling blood pressure is the instigator, the result of blood vessels failing to get the blood flowing back up to the heart, so that blood pools in the legs.  The third cause is a combination of the two.

What’s important is identifying the primary cause—slow heart rate or low blood pressure—which can be determined by a tilt table test: when the head is elevated above the feet, heartbeats should increase to pump more blood to the head: when that doesn’t happen, it means the heart could use some help, such as from a pacemaker, to work more efficiently.

For some with cardioinhibitory syncope, the heart can stop completely—called asystole.  A dual-chamber pacemaker detects a heart rate falling below 40 BPM, or an abrupt drop of 20 BPM, and paces the heart at a faster rate for a fixed period of time.  In one study, pacemakers reduced fainting episodes by more than 50% in those who had experienced asystole lasting six seconds or more.

While fainting can be dangerous, merely feeling faint or lightheaded (called presyncope) might not be a good reason for a pacemaker.  A man in his 70s who was scheduled for pacemaker surgery sought a second opinion only to learn that the low heart rate causing his faintness was due at least in part to medication he was taking for a different heart problem.  Switching medications eased the problem.

Skeptics, including some cardiologists, think pacemakers are prescribed too readily because physicians don’t want to be responsible for dangerous fainting episodes.  Others point out that pacemakers provide an income stream because the devices must be checked every six months.

But the first randomized controlled study of pacemakers for vasovagal syncope, published in 1999, was terminated early because of the “vastly significant success” of pacemaker trials in the first 54 patients, each of whom had suffered more than six episodes of fainting.  Although mild lightheadedness persisted even in those who received pacemakers, their chances of recurrent episodes  were reduced 85%.

—Mary Carpenter
Read more posts from  Well-Being EditorMary Carpenter. 

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