Patients who undergo heart surgery in the afternoon are less likely to encounter potentially fatal complications than those operated on in the morning, scientists say.
A study in the medical journal The Lancet identifies a link between the body's biological clock and the risk of heart failure and heart attacks after heart surgery.
Experts say the findings are preliminary but could have major implications for how some hospital operations are timetabled.
The study, led by the University of Lille, France, examined what happened to 596 people who had heart valve replacement surgery in the 500 days
after their operation. Half of the patients were operated on in the morning, and the other half during the afternoon.
They identified those who had any major cardiac event such as a heart attack, heart failure, or who died from heart disease.
The scientists found that people undergoing surgery in the afternoon had a 50% lower risk of a major cardiac event than those who had surgery
in the morning.
Afternoon surgery cases accounted for 9.4% of major cardiac events compared with 18.1% among morning cases.
Another part of the investigation involved 88 patients who were randomly assigned to undergo surgery in the morning or afternoon.
None of the patients died and they spent on average 12 days in hospital. But those who had afternoon surgery had lower levels of heart tissue damage
after their operations than patients in the morning surgery group.
A genetic analysis of 30 tissue samples taken from these patients showed that 287 genes linked to the body's biological clock – known as the circadian
clock – were more active in those who had afternoon surgery compared to the morning group.
The scientists say their findings suggest that the heart is subject to the body's circadian clock and is better able to withstand the traumas of surgery in
the afternoon than in the morning.
In a further experiment involving mice, they used drugs to alter activity in a gene linked to the circadian clock. They also looked at what happened to
mice when the gene was deleted and replaced. Both approaches, they say, improved the ability of heart tissue to recover.
If confirmed in humans, development of drugs to alter the circadian clock could make heart surgery safer at times when outcomes are typically worse,
'Heart cells have circadian rhythms'
Dr John O’Neill, from the MRC Laboratory of Molecular Biology, Cambridge, says the findings are in line with what experts would have expected,
"because 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 – i.e. our heart 'expects' to work harder during the day than at night, with the opposite being true in mice".
He continues in a statement: "In healthy individuals, not engaged in shift work, our cardiovascular system has the greatest output around mid/late-afternoon,
and explains why professional athletes usually record their best performances around this time.
"In the morning, just after we’ve woken up, the cardiovascular system is not yet at peak performance, which partly explains why heart attacks and strokes
occur more frequently in the morning.
"This new study may also explain why survival after aortic valve replacement is also poorer in the morning, both in terms of during the surgery and
Dr O'Neill says the only drawback to the study is that only 2 surgeons were involved, each operating in both morning and afternoon, and there was no
way of knowing whether they were 'morning' or 'afternoon' people – larks or owls. "This alternative is quite possible, since we know that hand-eye
co-ordination, concentration and cognitive abilities are also affected by time-of-day," he says.
Dr Tim Chico, a consultant cardiologist at the University of Sheffield, comments: "If it is true that outcomes are worse if someone has their operation
in the morning, this would have major implications for scheduling of operations. However, there are many factors that could explain this finding other
than the circadian rhythms of the patient, or indeed the surgeon."
Dr Mike Knapton, associate medical director at the British Heart Foundation, says in an emailed statement: "If this finding can be replicated in other
hospitals this could be helpful to surgeons planning their operating list, for non-urgent heart surgery.
"The study also suggests that modifying the genes responsible for this phenomenon could lead to the development of new drugs to protect the heart
from damage during open heart surgery."