A treatment for heart attack may be dangerously underused
A thrombolytic agent can save your life if you suffer a heart attack. l But in the U.S., if you are old or slow in getting to the hospital, your chances of getting one may be disturbingly worse than you'd like. Surveys show that only about a third of all heart attack patients receive a throm-bolytic—roughly half of those who may be eligible and far below the 85 percent mark attained in parts of the U.K.
Moreover, even patients who do get a thrombolytic must often wait almost an hour and a half for it, a delay that significantly reduces the drug's effectiveness. By one estimate, 14,000 more lives might be saved annually if physicians used thrombolytics sooner and more liberally. "I think the situation is improving, but it's woefully inadequate," remarks Andrew J. Doorey of the Medical Center of Delaware.
Streptokinase, tissue plasminogen activator (TPA) and other thrombolyt-ics work by dissolving the blood clots that block coronary arteries and cause heart attacks. At least one study found that administering these agents within an hour of the onset of chest pain cut mortality by 90 percent, although most estimates put the benefit at a more modest 50 percent. Unfortunately, that gain decreases when treatment is postponed, and most patients do not reach an emergency room until at least four hours after their heart attack begins. Still, thrombolytics reduce mortality by 30 percent when given within the first six hours and by about 15 percent between the sixth and 12 th hours.
The drawback of the drugs is that they promote bleeding and raise the odds of a potentially fatal stroke from an intracranial hemorrhage. Physicians have therefore tended to prescribe clot-busters only for the minority of patients who offered the best ratio of benefits to risks. "Interfering with the body's blood-clotting mechanism is a serious business," cautions H. Vernon Anderson of the University of Texas Health Science Center. "You want to be very, very careful."
Last fall in the New England Journal of Medicine, Anderson and James T. Willerson of the Texas Heart Institute phenomenon the existence of which Aleksander F. Andreev of the Institute for Physical Problems in Moscow proposed in 1964. At the super-semi interface, an electron from the well enters a superconductor to form a Cooper pair. As it does so, it leaves behind a positively charged "hole" in the sea of electrons in the well. The hole is a kind of mirror image of the electron. According to theory, the hole moves along a time-reversed path of the original electron—that is, the hole travels to the other side of the well.
Once the hole reaches the other interface, it breaks up a Cooper pair in the other superconducting contact. One of the Cooper electrons destroys the hole; the other takes up this annihilation energy and shoots across the well back to the other side. The process can repeat once this electron moves across the interface and forms a Cooper pair. In theory, the cycle can go on forever.
More startling was the effect's dependence on an external magnetic field. Kroemer found that a rising magnetic field caused resistance to increase episodically instead of smoothly. The jerkiness or bumpiness of the increasing resistance should involve a fundamental parameter—the flux quantum. The flux quantum dictates that bundles of magnetic-field lines penetrating a sample must take on a particular, discrete value. Instead, Kroemer reports, the measured value is smaller than the predicted one by a factor of four to five.
So far no good explanation exists for the oscillations. One speculation is that the magnetic-flux lines assume the form of a lattice as they penetrate the semiconductor. When the magnetic field is increased, the entire lattice shifts suddenly to accommodate the new flux bundles. Kroemer plans to look for the effect in new samples before submitting his results for publication.
Multiple Andreev reflections may be more common than previously observed. For instance, Alan W. Kleinsasser of the IBM Thomas J. Watson Research Center and his colleagues will report their observations of the reflections in a quantum structure known as a tunnel junction. So whereas the birth of superconducting computers remains distant, investigators are finding plenty of excitement during the courtship period. Kroemer explains: "The physics for now takes precedence over the hypothetical applications." —Philip Yam
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