Friday, November 14, 2008

Phun Physiology: Thought of a Heart Cramp Shouldn't Cramp Your Cycling

If you’ve ever experienced a leg cramp, you may have wondered if you could get a heart cramp. A leg cramp is one thing. But a heart cramp would be disastrous.

A search of “heart cramp” on the Internet reveals a few self-reporting individuals who are convinced they’ve experienced one. Even more amazing—they’ve lived to tell. We need to remind ourselves that self-diagnosis is tricky, hence the time-honored admonitions of seeking second opinions and physicians not treating themselves.

Repeating the Internet search, substituting highfalutin synonyms for heart cramp, “cardiac tetanus” and “cardiac tetany,” one is introduced to a world of medicalese.

The result? Great news for cyclists! The heart does not normally cramp unless one considers the administration of electrical shock or hypoparathyroidism normal. At least that is what biology and medical students are taught nowadays in the halls of academe.

Danesh and Dao at the UC-Davis School of Medicine assert: “Tetany is not possible in cardiac muscle because of the prolonged refractory period.” In the textbook I use in my human anatomy and physiology class, the authors Marieb and Hoehn write: “The long cardiac refractory period normally prevents tetanic contraction, which would stop the heart’s pumping action.”

For non-physiologists, refractory period is the time after a muscle cell is stimulated and contracts before it can be stimulated to contract again. Because skeletal muscle cells have a relatively short refractory period, constant or rapid-fire stimulation can cause nonstop whole-muscle contraction known variously as a cramp, tetanus, or tetany. But the long refractory period of cardiac muscle discourages tetany.

True—cardiac muscle can undergo tetany with the application of electrical shock, a phenomenon reported by Burridge in 1920 in an article entitled “Cardiac Tetanus” in the Journal of Physiology. But as long as cyclists don’t get zapped by lightning or step on downed electrical wires this shouldn’t be cause for worry.

Extremely low blood calcium levels can also cause muscle tetany. This is the connection with hypoparathyroidism and cardiac tetany. The parathyroid glands are stuck to the back of the larger thyroid gland. When blood calcium levels are too low to maintain proper nerve and muscle function, parathyroid hormone release restores calcium levels.

But hypoparathyroidism, or too little parathyroid hormone release, results in chronic low blood calcium levels, or hypocalcemia. This puts skeletal and cardiac muscle at increased risk for tetany. However, someone is more likely to experience hypocalcemic-induced neurological problems before experiencing muscle tetany.

Halpern and Wang make this point writing on the various health ailments associated with hypothyroidism: “Cardiac effects are limited because cardiac Tetany [sic] occurs at levels of hypocalcemia lower than those causing neurologic effects; therefore, the patient usually comes to medical attention prior to the onset of cardiac disturbances.” In other words, those suffering from hypothyroidism are likely to have non-cardiac-related health problems that would tend to curtail if not prevent cycling.

One hypocalcemic malady for example is the bone disease known as rickets. I am aware of articles in British Medical Journal and Indian Pediatrics which have reported the rare occurrence of cardiac tetany in children suffering from rickets.

It is important to note that heart cramps are not the same thing as either angina or heart attack. Any cyclist who senses some sort of chest discomfort should get checked posthaste by a medical expert competent to distinguish between heart burn and a true heart malady.

My purpose here is to assure cyclists that a leg cramp does not necessarily portend anything more sinister relating to the heart. Rather, leg cramps prevent further physical exertion. Leg cramps may also motivate cyclists to try to determine their possible cause.

My advice to cyclists? Don’t let the thought of a heart cramp slow you down.

No comments: