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Time of day of acute MI shown influential on infarct size
Suárez-Barrientos A. Heart. 2011;doi:10.1136/hrt.2010.212621.
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Researchers have found that the time of day of acute MI had a significant effect on infarct size in patients with STEMI, with 6 a.m. to noon associated with the largest infarct size.
All patients (n=811) in the study were admitted to the coronary care unit of Hospital Clínico San Carlos in Madrid, Spain, for STEMI between March 2003 and September 2009. Investigators split patients into one of four groups depending on occurrence of infarction — midnight to 6 a.m. (n=141), 6 a.m. to noon (n=269), noon to 6 p.m. (n=240) and 6 p.m. to midnight (n=161) — and estimated infarct size by peak enzyme release.
According to their results, there was a circadian variation in infarct size across time of day as determined by concentrations of troponin I (P=.012) and peak creatine kinase (P=.015). Similar curves were found with both biomarkers and showed a global maximum from the 6 a.m. to noon and a local minimum from noon to 6 p.m.
To further support this, researchers conducted additional analysis and found the largest infarct size in those who experienced STEMI during the dark-to-light transition period (6 a.m.-noon), as evidenced by increases in concentrations of peak troponin I of 24.6% (P=.033) and peak creatine kinase of 18.3% (P=.031) compared with the three other time periods.
“Overall, there is an expected increase of about 20% in infarct size in patients with STEMI onset in the dark-to-light transition period compared with any other time of day,” the researchers wrote, adding that these results “advocate the inclusion of ‘time of acute MI onset’ as an important variable in clinical trials testing novel cardioprotective strategies with MI size as an endpoint.” – by Brian Ellis
Circadian rhythms are biologic clocks that regulate body processes over a 24-hour cycle in response to daylight or darkness. In recent years light stimulated alterations in gene expression and protein translation have been identified that cause neurohumoral alterations highly relevant to human physiology and pathophysiology. Some of these are relevant to CVDs. These include circadian variations in proteins critical to fibrinolysis, regulation of BP and heart rate, and modulation of lipid and glycogen metabolism.
It has been known for many years that MIs are most common in the early morning hours. This is likely due to a predisposition due to circadian changes. Results of a recent study done in mice exposed to ischemia and reperfusion have supported the concept that myocardial infarct size may be dependent on time dependent light and darkness alterations.
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The clinical study by Suarez-Barrientos et al in Heart this year retrospectively examines infarct size, estimated by cardiac enzyme levels, according to time-of-day in 811 patients, most of whom were treated with percutaneous coronary intervention. They concluded that the largest infarcts occurred between 6 AM and noon, which is often termed the “light-to-dark transition period.” There was a relatively small difference in infarct size between 6 a.m. to noon and 6 p.m. to midnight periods, but infarcts during the other two time periods were substantially less. There was some inconsistency between the estimates of infarct size and measurements of left ventricular ejection fraction among groups. These discrepancies; the many other factors that may influence infarct size; the limitations of the enzyme technique for estimating infarct size; and the relatively small population examined in this study limit the strength of its conclusions.
Nevertheless, it offers some support to the concept that in addition to the well-established circadian variation in the timing of MIs, there may be a similar variation in the severity of the infarctions. The existence of such variations raise the issue of whether preventive measures focused on the timing of neurohumoral changes could decrease incidence or improve outcomes of MIs. For example, could certain drugs be more effective given in the evening than in the morning, or could drugs that alter the photosensitivity of the biological clock decrease time-of-day dependent increases in the incidence or severity of MI?
– Lawrence Horwitz, MD
Cardiology Today Editorial Board member
Disclosure: Dr. Horwitz reported no relevant financial disclosures.
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