Suspended animation might save lives or permit long-distance space travel. Yet, depriving human cells of oxygen for even a few minutes can cause severe consequences, including death if the supply of oxygen continues to be restricted. But some organisms, ranging from bacteria to hibernating mammals, can radically limit their own supply of oxygen and enter into a state of suspended animation, alive and waiting for the right conditions to reassert themselves. Recent research has shown that otherwise poisonous gases--such as hydrogen sulfide, a constituent of foul-smelling sewer gas--can rapidly induce similar states in mice by limiting their cells' access to oxygen without lowering blood pressure.

Mark Roth of the Fred Hutchinson Cancer Research Center in Seattle and his colleagues demonstrated last year that hydrogen sulfide--poisonous because it effectively replaces oxygen in the basic energy-generating processes at the cellular level--can induce suspended animation in mice. After six hours of radically decreased heart rate and breathing, the mice were revived without any ill effect. The finding raised hopes that similar procedures could be used to induce suspended animation in organs for human transplant or even humans themselves.

Gian Paolo Volpato and Fumito Ichinose of Massachusetts General Hospital and their colleagues repeated the experiment to examine in detail the impacts on the mice's cardiovascular systems. Just as with Roth's team, breathing an 80-parts-per-million (ppm) mixture of hydrogen sulfide and air reduced their breathing and heart rate within 10 minutes. Within two hours, the mice stabilized at a temperature roughly two degrees Celsius warmer than room temperature and remained largely still, though they could move when prodded. In this kind of suspended animation, warm-blooded animals essentially turned into cold-blooded ones.

The mice's heart rate dropped from roughly 600 beats-per-minute to an average of just 134 bpm. Surprisingly, however, their blood pressure did not exhibit a similar drop, remaining the same throughout the induced state, according to the research presented at the American Physiological Society conference on October 8. Kept in the state for as long as eight hours, the mice returned to normal within a few hours of breathing ordinary air and showed no ill effects. "H2S reduces CO2 production, that is metabolic rate, before the reduction of [body] temperature," Ichinose says. "Therefore, the primary effect of H2S appears to be the inhibition of metabolism that leads to reduction of temperature."

Other studies have hinted that hydrogen sulfide might be able to play a similar role in humans; health and safety studies at 10 ppm have shown that the gas decreases oxygen uptake in exercising men and women. Roth, for one, believes that such suspended animation might prove a cellular relic of the distant past when the earth's atmosphere lacked oxygen and sulfur might have played the key energy-producing role for early life. Regardless, the stage is set for further experiments. "The most important next step will be to test the effects of H2S in a larger mammal such as a pig or sheep, that is a nonhibernating species," Ichinose says. "We are also interested in the downstream mechanisms and dose response."