Eight dogs underwent cardiac denervation by complete thoracic sympathectomy and selective removal of all thoracic cardiac vagal branches. Exercised on the treadmill they were found to possess still some residual ability for increasing their cardiac output; the increase was much less than in the ease of normal exercising dogs.
The increase could not have been due to liberation of epinephrin since the splanchnic nerves had been entirely excised. As about one fifth of the body's total sympathetic system was still intact, it is conceivable, but improbable, that the increased cardiac output was due to the slight generalized liberation of norepinephrin. It was considered more likely that the increased output was due to an intrinsic ability of the hearS which is still unexplained; since the output was due to increase of rate rather than of stroke volume, STARLING'S Law cannot be the explanation.
Since the increase in output was very limited in these denervated hearts, the animals depended more on other reserve mechanisms to obtain the extra energy for their exercising muscles. Observations of cardiac output, arterio-venous oxygen difference and blood lactate (as indicative of oxygen debt) shows wide individual variations. Some animals depend on one of these three mechanisms preferentially; others may use two or even all three mechanisms.