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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 (1) he describes "heavy perspiration " while working on the lunar surface (extra vehicular activity).

Exposure to excessive heat is conducive to magnesium ion deficits (2) and in turn catecholamine elevations with tachycardia (3) and a vicious cycle.( see Fig. 1 Interplanetary Travel and Permanent Injury to Normal Heart ).

This tachycardia 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). (4)

References:

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

2.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.

3.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.

4. 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.

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