EFFICIENCY OF THE HEART 371 



capacity is limited. By a reflex mechanism the heart rate is 

 increased and so the output per minute is augmented. The 

 table on p. 370 shows approximately the share of the burden 

 of increasing the output borne by increased distension of the 

 ventricular walls and by increased pidse rate. 



It will be seen that at first the pulse rate is practically unaltered 

 although the amount of work done has been increased from 270 to 

 735 kilogram-metres while the output per beat has increased from 

 75 to 120 c.c. After this, the output per beat is not materially 

 changed, if anything it tends to decrease, while there is a marked 

 increase in the pulse rate. It is interesting to note the increase 

 in the rhythm of the heart when work has just been started, viz. 

 from 70 to 100. This is associated with the initial changes 

 originated by the acts of volition and attention. The mere 

 caution, " Are you ready ? " is sufficient to cause a rise in the 

 pulse rate due, in part, to the increase of muscular tone in the 

 act of attention, and, in part, to psychological causes. 



A fair day's muscular work may be taken at 100,000 kilogram- 

 metres. We have seen that the work done by the heart is, at 

 least, 10,000 kilogram-metres per day. Hence the work done 

 by the heart is always more than one-tenth of that done by the 

 skeletal muscles. 



Efficiency of the Heart under Various Conditions 



The efficiency of the heart may be taken as the percentage 

 amount of the energy taken in as fuel that is converted into w^ork. 

 Workers in this field are agreed that it is extremely probable that 

 the sole normal source of cardiac energy is the glucose taken to 

 the heart by the blood and in part stored as glycogen in the 

 heart substance. This storage of glycogen renders difficult the 

 interpretation of the results of estimations of the amount of 

 glucose in the blood before and after passing through the coronary 

 vessels. More accurate calculations of the energy generated 

 during the cardiac cycle can be made from the oxygen consump- 

 tion and carbon dioxide production during bodily rest and during 

 measured work. The table on p. 372 from a paper by Evans 

 and Matsuoka demonstrates this method for obtaining a value 

 for the efficiency of the heart. The total output of blood from 

 the ventricle is fairly constant — averaging about 16 litres per 

 hour. The resistance to outflow was increased by steps of 40 mm. 

 Hg from 80 to 160 nmi. Hg, corresponding to an increase in 

 cardiac work of about 10 kilogram-metres a time. 



To free the energy necessary for this increased work the heart 

 uses up more oxygen. The amount of oxygen (in cubic centimetres) 



24—2 



