THE NUTRITION OF THE HEART 



169 



convenient method for changing the hydrogen-ion concentration of such 

 fluids is by altering their tension of carbon dioxide (see page 371). In 

 a heart-lung preparation (page 163), such alteration in carbon-dioxide 

 tension can very readily be brought about by altering the percentage of 

 this gas in the air with which the lungs are ventilated. To measure the 

 efficiency of the heartbeat in such an experiment, it is convenient to enclose 

 the organ in a cardioplethysmograph, the tracing of which will tell us the 

 degree to which the heart is contracted or relaxed, as well as the output 

 of blood per minute. By increasing the tension of carbon dioxide, it 

 has been found in such experiments that the dilatation of the ventricle 

 is encouraged, so that the heart with each beat discharges a larger quan- 

 tity of blood (Fig. 39). When defibrinated blood is used the optimum 

 pressure or tension of carbon dioxide has been found to lie between 5 

 and 10 per cent of an atmosphere. 



Fig. 39. Volume curve of ventricles of cat (lower curve) in a heart-lung perfusion prepara- 

 tion. The air used to ventilate the lungs was replaced between the arrows by a mixture con- 

 taining 20% CO 2 and 25% O 2 . This caused dilatation of the ventricles along with feebler beats 

 and a tendency for the arterial pressure to fall (upper curve). The after effect was an im- 

 provement of the beat. (From Starling.) 



That the effect of carbon dioxide in encouraging the relaxation of the 

 heart between beats is in part at least dependent upon the change in 

 hydrogen-ion concentration of the perfusion fluid has been shown by 

 securing similar results in experiments with perfusion fluids to which 

 different quantities of weak nonvolatile acids have been added. These 

 observations are of practical importance because of the light which they 

 throw on the cause of cardiac failure following upon conditions in which 

 there has been excessive removal of carbon dioxide from the blood, as 

 in forced ventilation of the lungs. Yandell Henderson has suggested 

 that surgical shock may be, partly at least, due to cardiac failure fol- 

 lowing the "washing out" of carbon dioxide from the blood by the 

 dyspnea so often incident to the administration of anesthetics in surgical 

 operations. 



