Sir, I believe that with a five-pronged approach, the high heart rates in space can be reduced. This would involve the use of subcutaneous magnesium, far more aggressive hydration and exercise programmes, and a considerable change in the dietary approach. The high heart rate, experienced by Gibbons of 150 beats per minute and by Irwin up to the extraordinary heart rate of 180, during the manned test of an Apollo space craft in a fully pressurized oxygen atmosphere on 27 May 1968 (Spaceflight, December 2003, page 509) are considerably higher than would be expected with such conditions as 'altitude sickness' and suggests that high oxidative stress, may have played a prominent role.
The discrepancy between the heart rate of Irwin and on this occasion Scott, occurred again on the actual Apollo 15 mission, in 1971. On the first of three lunar excursions Irwin's heart rate rose to 167, whereas Scott's was much lower, rising to only 130, while again performing similar activities. An improper setting of Irwin's space suit, to correct it for the lunar heat at that particular time of the day, might have been partially responsible for this tachycardia. 
A heat-induced magnesium deficit, with in turn increased release of adrenaline and in turn tachycardia, might have been partially responsible, for the episodes, both in May 1968 and during the Apollo missions. This is because training occurred in 'intense summer heat' in southern United States, conducive to a magnesium deficit. This would be compounded by a reduction in magnesium storage sites complicating even the brief (12 day) Apollo mission. 
Tachycardia is not uncommon, particularly during space walk activities. During an EVA, Wolf and Sellers experienced heart rates "to some 170 beats per minute" . Those heart rates, particularly above 85 percent of one's maximum predicted rate, based on age are dangerous because they predispose to sudden death, complicating a fatal rhythm disturbance. 
Pharmaceuticals, except for symptomatic treatment and emergency use, should not be utilized on space flights. This is due to unpredictable side effects, complicating invariable malabsorption and the potential for hepatic and renal dysfunction, as a result of diffuse impairment of the blood supply of these organs. A strong case can be made however for reducing the potential for such high heart rates, by providing subcutaneous magnesium and a delivery device, to reduce oxidative stress and high adrenaline levels, which probably played a role in the 1968 vacuum chamber test and can occur on space missions. 
Because of invariable dehydration, contributing to the high heart rates, secondary to increased adrenaline levels, water must be taken conscientiously despite space flight-related thirst reduction; this will serve also as an antioxidant. 
There must be a far more aggressive exercise programme i.e. 15 minutes out of every waking hour, which would more than double the existing exercise duration of up to two hours a day. This could ultimately lower the resting and activity-related heart rates with increased stroke volume, but at the same time maintain a similar cardiac output. 
There is the necessity for
an appreciable change in the dietary concept. There is new evidence in support
of the cardiovascular benefit of long-chain omega 3 fatty acids by ingesting
oily fish such as tuna, herring, salmon and mackeral
(Eskimo diet). 
Finally, since adrenaline
levels and in turn heart rates are higher in the morning, between 6 am and
noon, with a significant increased cardiovascular risk,  it would be advantageous
to avoid space walks during that period.
William J. Rowe MD
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