The _Journal of Pharmacology and Experimental Therapeutics_, for
November, 1915, contains a report of a research by Means and Newburgh in
which they report experiments upon the blood-flow of two normal subjects
during rest, and of one subject during muscular work.
The action of caffeine on the blood-flow was studied in both subjects
while at rest, and in one during work.
The average blood-flow of the two subjects at rest was 4.5 and 4.0
liters per minute; the systolic outputs were 61 and 57 cc.; the
coefficients of utilization of the oxygen-carrying capacity of the blood
were 31 per cent and 41 per cent.
With increasing work a steady rise in blood-flow, oxygen absorption, and
pulmonary ventilation was found. The increase in blood-flow was produced
first by an increase in systolic output until a maximum of 118 cc. was
reached, beyond that by an increase in pulse-rate. This suggested that
the supply of venous blood in this subject becomes “adequate” at about
640 kg. meters of work per minute. The coefficient of utilization showed
a slight rise during work, indicating a slightly greater economy of the
circulation.
After giving caffeine during rest, or when the supply of venous blood is
“inadequate,” evidence of drug action was found with both subjects. This
action consisted in an increase in total blood-flow without a
corresponding increase in oxygen absorption, and hence a decreased
coefficient of utilization of the oxygen-carrying capacity of the blood.
The pulse-rate was unchanged. Consequently the systolic output was
increased.
During work probably no other action was obtained from caffeine than
possibly an increase in pulse-rate, and consequently slight diminution
in systolic output.
It is suggested that during rest when the supply of blood to the right
heart is “inadequate”, caffeine increases the blood-flow by increasing
the venous supply through an action upon some mechanism outside the
heart. When the supply becomes “adequate” or approaches adequacy, no
such action is obtained.—_The Therapeutic Gazette._