The risk of serious heart problems after open heart surgery nearly doubles when the operation is performed in the morning rather than the afternoon, researchers say.

Experiments and lab tests pointed to our biological clock as the primary cause of the startling difference in outcomes, they report in medical journal The Lancet.

“Our study found that postsurgery heart damage is more common among people who have heart surgery in the morning,” says lead author David Montaigne, a cardiologist at the University of Lille. “The time of day — that is, the biological clock or circadian rhythm — influences the patient’s reaction to this kind of operation.”

The circadian clock governs the body’s day-night cycles, thus influencing sleep patterns, the release of hormones and even body temperature.

When disrupted — as with jet lag — repeatedly over long periods, it can aggravate depression, bipolar disorder, cognitive function and memory formation, research has shown. Last month, the Nobel Prize for medicine was awarded to three US scientists who pioneered our understanding of how the circadian clock ticks.

The new study unfolded in four steps. To start, scientists examined the medical records of nearly 600 people who had surgery to replace heart valves, half in the morning, half in the afternoon. Fifty-four of 298 morning patients experienced heart attacks or other major cardiac events in the 500 days after the operation, compared with 28 of the 298 afternoon patients.

Then, in a year-long clinical trial, 88 patients were randomly scheduled for morning or afternoon valve replacement surgery. Not only did tissue from the afternoon group show less damage, it also regained the ability to contract more quickly in lab tests that mimicked the heart refilling with blood.

A genetic analysis of the same tissues showed hundreds of genes linked to circadian rhythms were more active in the afternoon group, suggesting that the heart, too, is influenced by our biological clock.

Montaigne and his team deleted and replaced the corresponding genes in mice to study the impact on the transition from sleep to wakefulness, and vice versa.

Finally, they identified candidate drugs that might modulate these genes in such a way as to protect the heart during surgery.

“The authors have clearly shown that circadian rhythm is of clinical importance,” comments Michel Ovize, a cardiologist from Louis Pradel Hospital, outside Lyon, France. High-risk patients might be given preference for afternoon surgery in the light of the findings, he suggests.

John O’Neill from the Medical Research Council Laboratory of Molecular Biology in Cambridge, England says the results were solid but “not hugely surprising”. “Just like every other cell in the body, heart cells have circadian rhythms that orchestrate their activity to anticipate the external rhythm of night and day. Our heart ‘expects’ to work harder during the day than at night.”

But the discrepancy in outcomes between morning and afternoon operations might also be explained by variance in the bio logical clocks of the surgeons, he adds. “We know that handeye coordination, concentration and cognitive abilities are also affected by the time of day. The circadian clock may affect outcomes from heart surgery, but understanding why — and how to leverage this information — requires more research.”

Humans have been shown to be either “owls” or “larks”, corresponding to so-called genetic chronotypes that determine whether we function better at night or during the day. — AFP Relaxnews

This article appeared in Issue 804 (Nov 6) of The Edge Singapore.

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