Heart attacks occurring between 6 a.m. and noon were associated with the most the damage in the study, reported Wednesday in the journal Heart.
Researchers reviewed data from more than 800 heart attack patients treated at a hospital cardiac center in Madrid, Spain, between 2003 and 2009. Heart attacks that occurred in the morning hours were associated with about 20% more dead heart tissue.
The study is the first to link circadian fluctuations to heart attack severity in humans. If confirmed, the findings could affect treatment and research, study researcher Borja Ibanez, MD, PhD, tells WebMD.
Ibanez is a senior investigator for Spain’s National Center for Cardiovascular Research and an interventional cardiologist at Madrid’s Hospital Clinico San Carlos.
The association was quite robust,” he says. “In our study, events that took place in the morning were associated with more damage.”
Morning Heart Attacks vs. Overnight Heart Attacks
The patients included in the study all had a type of heart attack caused by blockages in the arteries.
Heart muscle damage was calculated by examining peak concentrations of creatine kinase (CK) and troponin-I (TnI), key enzymes released in response to muscle injury.
Heart attack timing was divided into four six-hour time periods over 24 hours.
As expected, the largest number of heart attacks occurred in the morning hours, with 269 patients needing treatment between 6 a.m. and noon. The fewest heart attacks occurred between midnight and 6 a.m., with 141 patients needing treatment.
Patients whose heart attacks occurred between 6 a.m. and noon had 21% higher CK and TnI levels than patients whose heart attacks occurred between midnight and 6 a.m.
Ibanez says the recognition that morning heart attacks may be more severe could have important implications for their treatment.
Early treatment with clot-busting drugs and angioplasty can prevent or limit damage to the heart muscle, but most cardiac catheterization labs are not fully staffed in the early morning hours.
Research suggests that around-the-clock access to a catheterization lab reduces treatment delays, he says.
“It could be that having one or two cath labs open in a city could have a significant impact on outcomes,” Ibanez says.
Searching for Answers
It is not entirely clear why heart attacks are more common in the morning hours, but a recent study from Harvard Medical School suggests that elevated blood pressure does not explain the link.
High blood pressure is a major risk factor for heart attack and stroke, but Harvard researcher Steven A. Shea, PhD, and colleagues found that study participants actually had their lowest blood pressure readings of the day in the late morning hours.
Ibanez’s own research suggests that fluctuations in hormones receptors that bind adrenaline over the course of the day may explain the circadian influence on heart attacks.
“If we are able to identify protective proteins that are elevated later in the day, we might be able to develop drugs to replicate this protection.”