Sudden cardiac death kills more than 300,000 Americans each year. Ordinarily,
electrical impulses cause the heart's muscle fibers to contract. In
a healthy heart, these impulses pass through cardiac tissue as a smooth
wave of electricity. However, sometimes these waves get stuck and form
troublesome, whirlpool-like spirals of electrical activity in the heart.
Investigating these spiral waves, Canadian scientists (Leon Glass,
McGill University, 514-398-4338, email@example.com) have studied chick-embryo
cardiac cells grown as a sheet of tissue. Such arrangements of cells
often exhibit spiral waves in their first two days. When the cardiac
tissue was subjected to a drug that impairs communication between the
cells, rotating spiral waves broke up into multiple rotating spirals
(see figures and movies).
The spiral wave breakup is believed to be similar to the processes
that lead to ventricular fibrillation, a potentially fatal cardiac rhythm
that often occurs when communication between cells is impaired. Reduced
intercellular communication may also be caused by a heart attack or
by other cardiac diseases.
To explain the experimental findings, the researchers devised a simplified
computer model consisting of cells irregularly distributed in space.
As their model shows, the spread of electrical activity in cardiac tissue
is similar to the spread of a fire in a forest: Cells become active
if enough neighboring cells are active (however, active cells are "refractory,"
or electrically inactive, for some time afterwards).
When the neighboring cells interact strongly, electrical waves pass
through the tissue at a high velocity; when weak, wave propagation is
completely blocked. At intermediate levels of interaction, electrical
waves break up into multiple spiral waves.
These observations should help to explain the appearance of multiple
spiral waves in a host of other physical and biological systems, including
corrosion on metal surfaces, aggregation of slime molds, and "Belousov-Zhabotinsky"
chemical reactions that exhibit oscillating spatial patterns. (Bub,
Shrier, and Glass, Physical Review Letters, 4 February 2002.)