Book Reviews; August 1993; Scientific American Magazine; by Philip Morrison; 3 Page(s)
An adept at fluid biomechanics--from airflow through prairie dog burrows to nutrient seawater past arrays of hungry clams--wideranging Duke physiologist Steven Vogel found his mind wonderfully concentrated not long ago. He was loading hand-sawed firewood into his pickup one autumn day; he strained to lift and hurl that last unsplit chunk to the top of the load. The effort cost him 10 days in the hospital, another repentant from the Valsalva maneuver, a few stressful seconds of exhaling against a tightclosed throat.
In this maneuver the air pressure inside the thorax rises substantially as the body stiffens like an inflated tire. The action does take load off the backbone, so that it comes naturally enough. But since 1707 it has been recognized as risky, even dangerous. The increased internal pressure must be matched by that of the venous blood, so that for some seconds the flow of blood out and back to the lungs dwindles. The working heart, "that consummately aerobic muscle," is starved of oxygen just at the peak of its demand for power. "If you're prone to heart trouble, the stunt is likely to bring it on." Most urban snowfalls offer plausible epidemiological evidence, a flurry of heart attacks among middle-aged shovelers, their breath too often held during quick, unusual exertions. Cheerfully devoted to his own rehabilitation, Vogel is no longer the pudgy meat eater he was. No longer does he hand-saw his own wood; the fun has gone out of it.