The Shulchan Aruch, Yoreh De'ah 370, reveals the understanding of our sages that halachic death does not necessarily coincide with total cessation of all muscular activity. The section's subtitle reads: "Who is to be considered dead even though he is still alive?" This siman of the Shulchan Aruch rules that decapitation is an absolute cause of death even if there is still residual muscular movement. The Shulchan Aruch adds two other conditions, both involving massive traumatic injury—hemicorpectomy (horizontal severing of the body) and a sword blow that severs the individual longitudinally along his back. In all these cases there are residual signs of life which are without halachic import because the key sign of life, the ability to breathe independently, has been lost.
The Rambam, in Laws of Mourning 4:5, expresses what surely was a concern of our sages in attempting to determine death in an absolutely error-free way. As the Rambam puts it, they feared that "perhaps he had fainted." Due to concern that the patient might be in a deep coma and not dead, they required the passage of time before anything could be done to him.
The patient was in the state of goses until the legal fact of his death could be absolutely determined by the prolonged persistence of apnea. Today, we have no difficulty in determining whether the absence of breathing is temporary or permanent. This can be done by a stress-free test known as the nuclide scan.
This test utilizes an innocuous amount of radioactive material of the kind that is used routinely in studying heart action in patients who have complained of chest pain, and even in pediatric medicine. A quantity of this radioactive substance, considered to be perfectly safe, is injected through one of the intravenous tubes. A few minutes later a "portable camera" takes a picture of the patient. If it is revealed that no blood flow enters the base of the brain, the condition is certainly irreversible. Brain cells are extremely sensitive to anoxia (absence of oxygen) and die within minutes after they are deprived of their oxygen supply, which is provided by the circulatory system. Many of those who oppose Rav Moshe's teshuvah on brain death rely on the responsum of the Chasam Sofer (Yoreh De'ah 338), although it is cited by both Rav Moshe and the Chief Rabbinate of Israel as proof that brain-stem death is halachically valid death. At the time of the Chasam Sofer, the interval between brain-stem death, where the heart still maintains its contractions, and "total death," was a matter of minutes, since there was no ventilator to keep the blood oxygenated in the absence of an independent ability to breath.
Hence, within minutes after the organism died, the organs, including the heart, ceased functioning. With the advent of today's technologically sophisticated ventilators, it is possible, even in a decapitated individual, to maintain the organs, such as heart, lungs, kidney, liver, and pancreas, in a viable state. However, we have no ability to maintain the brain in a viable state once the brain cells have been injured by anoxia. Many transplant surgeons are considering using organs from recently dead donors. Although such organs are less than desirable, the shortage of organs and the certainty that the patient will die unless he receives a new organ has made this a terrible dilemma. How much risk should be taken if the new organ fails to function? In the case of a kidney, it is possible to put the patient back on dialysis. But if a transplanted lung or liver fails, and a new donor is not immediately available, the transplant will cause the death of the recipient.
A second source, cited by those opposed "in the past" to Rav Feinstein's careful proof that brain-stem death is halachic death, is the Chacham Tzvi, siman 77. In this lengthy teshuvah, the Chacham Tzvi unequivocally affirms that respiratory death is the only halachically acceptable death. It is an interesting if confusing teshuvah, because the Chacham Tzvi did not have our present knowledge of the circulatory system, and in his scheme of things the heart was a respiratory organ, involved in the warming and cooling of the air. In this teshuvah, however, the Chacham Tzvi clearly emphasizes that motion per se is not a sign of life when the patient has been decapitated, and he refers to an execution by decapitation in which a headless corpse took several steps. Likewise, proof that the brain is no longer in any physiological association with the rest of the body (since it has been cut off from the circulation) would be a proper analogy to the Chacham Tzvi's insistence that only the "breath of life" determines life and death. The Chacham Tzvi's emphasis on the importance of the heart is grossly misunderstood by those who do not give this teshuvah a careful reading. According to the biological facts available to the Chacham Tzvi, the heart was a respiratory organ; its importance to him in determining halachic death was precisely due to its presumed vital function in promoting respiration.