Since 64-year-old Daniel Perrier
retired last year, he's been taking it easy in the mornings.
His usual routine includes a couple cups of coffee while
attempting the crossword puzzle. A week ago, his peaceful
existence was rudely interrupted by a heart attack. It's
a known fact that heart attacks are most likely to occur
in the morning hours after waking. It seems that blood
pressure and forearm vascular resistance are increased
in the morning, but the underlying cause of the morning
spike has remained as cryptic as Daniel's crossword.
Research published in the
June 1 issue of Circulation examined the role
of endothelium -- the ubiquitous lining of blood vessels
-- in early morning heart attacks. A reduction in the
endothelium's activity would likely decrease the diameter
of blood vessels and increase the risk of thrombus.
"The human body maintains a balance of blood pressure
and blood flow by expanding or contracting blood vessels,"
said Dr Virend Somers, Mayo Clinic cardiologist and
leader of the study. "Much of this expansion is
directed by the endothelium, which lines the blood vessels.
Risks of heart attack and stroke are 30-50% higher in
the early morning hours, so we wanted to determine whether
the endothelium's effects are diminished in the morning."
Mayo Clinic researchers recruited 30 healthy nonsmoking
volunteers whose mean age was 41 years and tested them
for blood vessel expansion caused by the endothelium
at various points of their day.
All of the subjects abstained from alcohol and caffeine
for 24 hours before the study. The subjects underwent
complete overnight polysomnography to ensure adequate
sleep quality. They were woken at 6am and had measurements
of flow-mediated endothelium-dependent vasodilation
(FMD) of the brachial artery taken at 9pm, 6am and 11am.
In both men and women, endothelial function was reduced
by more than 40% in the early morning reading. FMD at
6am was markedly decreased compared with the measurements
at 9am and 11am. In fact, the percent change in brachial
artery diameter was 7.5 at 9pm and 7.7 at 11am, but
only 4.4 at 6am.
"We cannot identify the specific mechanisms responsible
for the morning-related endothelial impairment,"
said the authors. "Potential candidates may include
increases in sympathetic activation and heightened adrenergic
receptor sensitivity." Sympathetic activation in
the early morning appeared to be linked to rapid eye
movement sleep.
The researchers noted that the pathological processes
linking decreased endothelial function to a cardiovascular
event are probably gradual, perhaps extending over several
hours and eventually leading to overt schema.
"We are not sure yet how these changes relate to
people with cardiovascular disease, but this reduced
morning function of the endothelium may be among triggers
that could cause an event. We'll continue to explore
the underlying mechanisms," Dr Somers concluded.
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