Summer Lowers Heart Attack Threat

New study strengthens link between our biological clocks and our health

By Janice Billingsley HealthSCOUT Reporter SATURDAY, Aug. 5 (HealthSCOUT) -- Summertime heart attacks are less likely to cause as much damage to the heart than those suffered at any other time of year, a recent study by a Los Angeles cardiologist concludes. Dr. Robert A. Kloner, director of research at The Heart Institute at Good Samaritan Hospital in Los Angeles, studied heart-muscle damage in more than 1,200 patients in two nationwide clinical trials and found 15 percent to 22 percent less damage in summer attacks.

"To the best of our knowledge, this is the first study to show that the size of a heart attack varies by season and is smallest in the summer," Kloner says.

Part of the reason, he surmises, could be temperature-related: Heat widens blood vessels, which permits blood to flow more easily and allows more oxygen to reach the heart. Warmer temperatures also could lower clotting factors, a risk in heart attacks.

But temperature alone doesn't seem to account for the severity of heart attacks or their seasonal fluctuations, Kloner says.

"It could be that the heart-attack rate is related to the number of hours of daylight," he says, adding that fewer hours of daylight may be conducive to an increase in the number of heart attacks.

"We know that daylight can modulate a whole lot of physiologic effects on the body," agrees Dr. Douglas P. Zipes, head of cardiology at the Indiana University School of Medicine, who has reviewed a number of other heart-attack studies that echo Kloner's latest findings.

Zipes points out that a small organ above the eye called the suprachiasmatic nucleus (SCN) processes light and other external signals to regulate a variety of body functions, including temperature, the sleep cycle and secretion of hormones.

"This is absolutely speculative," Zipes says, "but the SCN regulates body function in response to the length of the day ... and could alter it in a way that might predispose a person with heart disease toward death in the winter and not in the summer months."

Kloner's latest study follows one he published last year in which he found there were one-third more heart attacks in Los Angeles during winter months, even though the city's seasonal temperature fluctuates only by about 30 degrees. Conversely, cities like Boston or Chicago can have temperature swings in excess of 100 degrees. Kloner's earlier findings were based on a review of more than 200,000 death certificates over a 12-year period in Los Angeles.

Mondays, mornings prime heart-attack time

A number of other links between body cycles and heart attacks have already been established. A 1980s study by Dr. James Muller, director of clinical research in cardiology at Massachusetts General Hospital and Harvard Medical School, found that heart attacks occur more often during the morning than in the afternoon. Another study identified Mondays as the most common day of the week to suffer an attack. The reason: the stress associated with the start of a new work week. Many doctors now prescribe heart medicine with these cycles in mind.

Morning attacks are due to the rise in blood pressure when people get out of bed, which increases the workload on the heart, doctors say. Morning also induces an increase in the production of stress hormones like adrenaline, which helps you get ready to meet the day. That increase causes the arteries to narrow and raises the clotting tendency in blood. In people already at risk for heart attacks, these factors could raise the likelihood of an attack.

"About 10 percent of non-fatal heart attacks are attributable to circadian [24-hour cycles] triggers," Muller says, but adds that the percentages are small enough that "by and large triggers are not worth worrying about."

Adherents of chronobiology, the study of body rhythms on daily, weekly, monthly and annual cycles, disagree with Muller's assessment, arguing that body cycles can have a major impact on health.

When people are healthy, their biological clocks -- meaning the variations in their blood pressure, body temperature or hormone levels -- can adapt to and tolerate outside stresses like exercise or temperature change without problems, says University of Texas chronobiologist Michael Smolensky. But, he adds, people who've been diagnosed with illnesses can be at high risk for health complications if they don't pay attention to their bodies' rhythms.

In his new book, The Body Clock Guide to Better Health, Smolensky cites 35 medical conditions, including heart disease, which he says can be improved by everything from timing diagnostic tests to certain times of the day, to prescribing amounts of medicine to coincide with daily fluctuations in the body's temperature, blood pressure and hormone levels.

When it comes to the overall treatment of heart disease, however, cardiologists fall back on traditional approaches.

While acknowledging that chronobiology research is provocative, Zipes says, "I would recommend that heart patients stick to the basics, be certain to take their medication with regularity, be certain to do what their physicians tell them to do."

What To Do

There is growing interest in the field of chronobiology as it affects health. For an interesting overview on the subject, visit Northwestern University's Center for Circadian Biology.

However, the bottom line on preventing heart disease, according to doctors, remains basic: reduce known risk factors by exercising, keeping your weight down, watching your cholesterol, eating properly and not smoking.

-- cin (cin@cinn.cin), August 05, 2000