Number 713, December 27, 2004
by Phil Schewe and Ben Stein
Why Do Heart Attacks Occur Most Frequently Between 9 And 11 Am?
Studying five healthy volunteers for 10-day periods in pioneering efforts
to ultimately answer this question, a collaboration of Boston University
physicists and Harvard physiologists has found evidence that the body's
circadian clock (a part of the brain that regulates daily biological
activities) influences patterns in the heart's "interbeat intervals,"
the lengths of time between successive heartbeats. At around 10AM for
all the healthy individuals, the values of successive interbeat intervals
displayed increased signs of randomness, statistically resembling that
seen in previous studies of individuals with heart disease.
In their studies, the researchers took special care to isolate the effects
of a person's internal circadian clock (which has a 24.2-hour rhythm,
marked by a regular rise and fall of body temperature) from the effects
of behavior (such as physical activity and a person's wake/sleep time)
or external stimuli (such as the rising or setting of the sun). Towards
these ends, the researchers made sure to "desynchronize" the individuals'
internal body clocks from these other factors by keeping the volunteers
in a dimly lit room and by varying their sleep and wake times from day
to day while keeping activity levels relatively constant.
The researchers next plan to explore how an individual's behavior may
interact with the circadian clock to influence the correlations in interbeat
intervals. The researchers have not yet studied patients with heart
disease and are far from being able to make clinical recommendations.
However, their further research may obtain insights into the underlying
causes of increased cardiac risk and could lead to improved therapy,
such as more appropriately timed medication to coincide with phases
of the body clock. (Hu et al., Proceedings
of the National Academy of Sciences, December 28, 2004; contact
Plamen Ch. Ivanov, Boston University, 617-353-3891, plamen@argento.bu.edu;
Steven Shea, Harvard Medical School, 617-732-5013, sshea@hms.harvard.edu)
A Pea-Sized Magnetometer
A pea-sized magnetometer can do the job of much bigger units, and measure
magnetic fields with a sensitivity of 50 pico-tesla. Researchers at
NIST exploit the fact that rubidium atoms possess quantum levels whose
energies will depend on the ambient magnetic field. By encapsulating
a tiny portion of atoms in a cell and making precision measurements
of laser light traveling through the atoms, a field reading can be made.
All of this is packaged in only about 12 cubic millimeters. Furthermore,
the device can be manufactured in large batches through lithographic
means. For geophysical applications, such as for detecting underwater
or underground iron objects such as pipelines, tanks, and shipwrecks,
the device’s tiny power consumption, compact size, and low price should
move it ahead of several existing magnetometer designs with a few more
years of development work. (Schwindt
et al., Applied Physics Letters, 27 December 2004; contact Peter
Schwindt, schwindt@boulder.nist.gov, 303-497-7969; lab website at www.boulder.nist.gov/timefreq/ofm/smallclock/CSAM.htm
)
DNA Stretching Cross-Stream
A new experiment shows that in specially engineered fluid flows typical
of coating processes, single DNA molecules can sometimes enter into
a kind of flow instability in which the DNA orients itself perpendicular
to the plane of the flow. The experiment, conducted at Rice University
by Matteo Pasquali and Rajat Duggal, was part of a broader study of
how polymer molecules behave in moving fluids, a subject pertinent to
many biological and technological research areas, such as inkjet printing,
paper coating, the movement of air in lung alveoli, and DNA arrays.
Studying polymers in complex fluid flows is difficult because single
polymers are hard to resolve (being typically only 10-100 nm in size)
and because polymers can influence each other and the flow itself even
at very low concentration (down to few parts per million). That's why
DNA (above 10 microns in contour length) was chosen and why the DNA
was kept "ultradilute," so that it would not influence the flow and
that only one DNA molecule is visible at a time. In the Rice experiment,
a dilute suspension of DNA in water thickened by sugar is taken up by
a rotating drum which moves past a glass knife edge. In this way a thin
slice of solution can be moved as if on a conveyor belt past a lens.
The lens focuses a blue-green light on the DNA and picks up green-yellow
light emitted by the previously fluorescently-stained DNA molecules.
The resulting 30-frame-per-second film clearly can image individual
DNAs at a time with a spatial resolution of 250 nm (the thickness of
the molecule cannot be resolved but its length can be). The researchers
had expected that in the complex flow (a flow in which the velocity
of the fluid varies across the width of the channel) the DNA would deploy
itself with the flow rather than at right angles. Indeed, this happened
at the lowest drum rotation speeds; the direction of stretching changed
once the drum speed became high enough to induce ripples on the surface
of the liquid moving past the glass knife. (Journal
of Rheology, July/August 2004)
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