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"Jim's cooling system was working in his suit. 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).

In Irwin's autobiography (2) he describes "heavy perspiration" while working on the lunar surface (extra vehicular activity).

Exposure to excessive heat is conducive to magnesium ion deficits (3) and in turn adrenaline elevations with rapid heart rate (4) and a vicious cycle.( see Fig. 1 Interplanetary Travel and Permanent Injury to Normal Heart ).

This rapid heart rate up to 167 per minute, i.e. above 85% of his maximum predicted heart rate for his age of 41, would increase the risk of sudden death, complicating a fatal exercise- induced rhythm disturbance (ventricular fibrillation). (5)

References:

1. Stendig-Lindberg G, Moran D, Shapiro Y,  How significant is magnesium in thermoregulation ?J  Basic Clin  Physiol   Pharmacol  9: 73-85, 1998.

2. Irwin JB, Emerson WA.To Rule The Night .The Discovery Voyage Of Astronaut Jim Irwin . Holman, Lippincott, Philadelphia. 73,1973.

3. Beller GA, Maher JT, Hartley LH et al. .Changes In Serum And Sweat Magnesium Levels During Work In The Heat . Av Space Environ Med 46:709-712,1975.

4. Apte CV,Dikshit MB, Prasad BA Beta-adrenoreceptor blockade attenuates heat-induced tachycardia but not the tolerance to the stress. Indian J Physiol Pharmacol 36:149-154,1992.

5. Bayes de Luna A ,Coumel P ., Leclereq JF. Ambulatory sudden cardiac death:mechanisms of production of fatal arrhythmia on the basis of data from 157 cases. Am Heart J 117:151-159,1989.

6.  High Noon On The Moon. Rowe WJ (letter) Spaceflight 45:171, 2003.

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