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WHAT makes the rhythmic beating of a heart go haywire? Patches of unevenness
in cardiac tissue could turn smooth waves of contraction into spiralling eddies
that render the heart useless, say researchers in the US. They hope their
finding will lead the way to new drugs for treating 鈥渁rrhythmias鈥� such as
fibrillation, when the heart quivers erratically.

Each heart contraction originates from an electrical impulse that spreads as
a smooth wave. To investigate what makes these electrical waves go awry, Leon
Glass and his colleagues from McGill University in Montreal looked at the effect
of blocking gap junctions鈥攖iny channels that connect neighbouring
cells鈥攊n heart cells from chick embryos.

When they blocked the junctions with a drug called heptanol, what they saw
took them by surprise. Instead of the waves simply spreading more slowly as the
connections between the cells were blocked, the signal split into smaller,
spiralling waves.

The researchers created a computer model of a beating heart and found the
effect was due to the natural unevenness of heart tissue. As the number of
connections between the cells was reduced, the waves slowed, but when numbers
fell to a critical level, the waves broke up into lots of small spirals.

Even healthy hearts contain uneven clumps of cells, but diseases such as
arteriosclerosis can make the unevenness worse, making the heart much more
susceptible to any breakdown in connections. 鈥淭his break-up into a spiral
movement may be what is happening when a heart goes into fibrillation,鈥� says
Glass.

The finding could suggest ways for researchers to investigate how connections
between heart cells can be restored and maintained.