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updated 4/27/2011 7:57:23 PM ET 2011-04-27T23:57:23

Heart attacks that occur in the early morning may be more serious than those that occur later in the day, a new study suggests.

The results show that people who have heart attacks between 6 a.m. and noon have about 20 percent more damage to their heart tissue than those who have heart attacks at other times of the day.

Patients with more tissue damage after a heart attack are known to have a worse prognosis, including a greater risk of death.

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The findings suggest hospitals should be well-staffed in the early morning hours to attend to heart attack patients, said study researcher Borja Ibanez, of the National Centre for Cardiovascular Research (CNIC) in Spain. A larger staff will help ensure the patient's coronary artery is opened as soon as possible, which will lead to a better prognosis for the patient, Ibanez said.

Coincidentally, previous research has also shown heart attacks are more likely to occur in the morning. The findings suggest "those individuals are also suffering from the worst amount of injury," said Martin Young, of the Baylor College of Medicine in Houston, who was not involved in the study.
The link between heart attack intensity and time of day suggests the body's internal clock, or circadian rhythm, is at play.

Morning heart attacks
Ibanez and his colleagues analyzed data from 811 heart attack patients admitted to the Hospital Clinico San Carlos in Madrid between 2003 and 2009. These patients suffered from what is known as ST segment elevation myocardial infarction (STEMI) -- a type of heart attack in which the blood supply to the heart is blocked for prolonged period.

The researchers divided patients into four categories depending on when the heart attack had occurred.

Two hundred sixty-nine patients had their heart attacks between 6 a.m. and noon; 240 between noon and 6 p.m.; 161 occurred from 6 p.m. to midnight period; and 141 between midnight and 6 a.m.

The researchers examined the levels of certain enzymes released by the body during a heart attack to determine the extent of the tissue damage, or tissue that has died during the attack, also known as the infarct size.

Patients with the largest infarct size had their attack between 6 a.m. and noon. These patients had 21 percent higher levels of enzymes in their bodies than those whose attacks occurred at other times of the day.

Body clock
Every cell in the human body has its own internal clock (called circadian clocks ) that allows it to tell what time of day it is, letting cells anticipate what's next, Young said.

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"If these mechanisms go awry … the cells in the body might not necessarily be telling the right time of day," Young said.

If this happens, individuals may be more susceptible to stresses at certain times of the day and suffer from diseases, such as heart attack.

This may explain why night-shift workers are at an increased risk for a number of diseases, including cardiovascular disease. "Their clocks are out of sync with the environment because of the way they lead their lives," Young said.

In addition, proteins in the heart muscle that follow a circadian rhythm may play a role in how the body responds to a heart attack. For instance, some proteins may be present in higher concentrations at certain times of the day, providing a protective effect against heart attacks, Ibanez said. At other times, the concentration of these proteins may be lower and may leave patients vulnerable. If researchers find out what these proteins are, they may be able to use them in therapies to protect against heart attacks, Ibanez said.

The results will be published in the journal Heart.

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