| Spaceflight Vol 45:171, 2003
Twenty-one months after the Apollo 15 mission in 1971, James Irwin, the lunar module pilot, suffered a heart attack. I postulated that both Jim Irwin and Dave Scott, the commander, probably experienced severe spasm of the peripheral blood vessels to account for the pain and swelling of the fingertips they encountered on the moon (The Apollo 15 Space Syndrome) probably complicating primarily magnesium ion deficits .
The heart rhythm disturbances both astronauts experienced could also have been a complication of a magnesium deficit, secondary to exposure to "intense summer heat" in training with space suits before lift-off  and aggravated by further loss of magnesium from atrophy in space, of the storage sites in skeletal muscle and bone.
But why would Irwin's heart rate reach 167 per minute in comparison to Scott's rate of only 130 per minute, while performing similar work and after just two hours on the first of three lunar excursions? 
Even though the in-suit water devices didn't function at all in Irwin's case and only partially in the case of Scott, I believe the heart rate particularly in Irwin's case was too high, as a complication of dehydration alone .
Irwin's widow may have provided an explanation recently: "Jim's cooling system was working in his suit (confirmed). He did not select enough cooling temperature for the suit. [He] said he wished he had selected a little cooler [temperature]. But then Jim always liked the heat and the direct sunshine." (Personal communication, Irwin's widow 11-15-02)
A heat-induced magnesium deficit complicated by a vicious cycle with high levels of adrenaline can indeed be responsible for a high heart rate (tachycardia) . Irwin describes in his autobiography his plight on the lunar surface encountering "heavy perspiration" conducive at least partially to magnesium deficits, and even in the command module, he states, "The temperature was so high when we were on the sunlit side of the Moon that we were uncomfortable even in our underwear" (134°C).
Maybe there were predisposing genetic influences in Irwin's case, but the insults to the cardiac vessels possibly just prior and during this brief 12-day lunar mission, could have been entirely responsible for Irwin's subsequent heart attack. Furthermore, his heart rate of 167 per minute, i.e. above 85 percent of his maximum predicted rate at age 41, was dangerous because it was conducive to an increased risk of sudden death complicating a fatal exercise-induced rhythm disturbance (ventricular fibrillation) .
By William J. Rowe MD
1, W.J. Rowe, The Apollo 15 space syndrome. Circulation 97,1998, pp.1
2. G.A. Beller, J.T. Maher, L.H. Hartley, et al, Changes in serum and sweat magnesium levels during work in the heat. Aviat Space Environ Med 46, 1975, pp.709-712.
3. Apollo 15 Mission Report, NASA Manned Spacecraft Center, Houston, Texas, 1971, 10-1-15 NASA MSC 05161.
4. C.V. Apte, M.B. Dikshit, B.A. Prasad, Beta-adrenoreceptor blockade attenuates heat-induced tachycardia but not the tolerance to the stress. Indian J Physiol Pharmacol 36, 1992, pp.149-154.
5 . J.B. Irwin, W.A. Emerson Jr, To rule the night. The discovery voyage of astronaut Jim Irwin. A.J. Hohman, J.B. Lippincott. Philadelphia, 1973, pp.73, 94.
6. A. Bayes deluna, P. Coumel, i.F. Leclereq. Ambulatory sudden cardiac death: Mechanisms of production of fatal arrhythmia on the basis of data from 157 cases. Am Heart J 117, 1989, pp.151 -159.