EFFICIENCY OF THE HEART 285 



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

 Hg from 80 to 160 mm. 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 c.c.) so used 

 multiplied by 2-07 gives, in kilogram-met res, the energy developed. 

 It is clear that, with a moderate increase in arterial resistance, the 

 mechanical efficiency of the heart improves but tends to decrease 

 when the resistance is doubled. In other words, when the arterial 

 pressure is raised, the oxygen intake is increased, and more tension 

 developed in the cardiac muscle. The mechanical efficiency is 

 raised to a certain limit, beyond which it again diminishes. The 

 venous pressure in the experiment quoted, and in most others, 

 runs parallel with the oxygen usage. In the series of observations 

 tabulated as B, the arterial pressure was kept constant at about 

 80 mm. Hg, while the output per hour was increased roughly 

 as 1 : 2 : 3. This was done by varying the inflow of blood to the 

 heart. The increase in oxygen usage is not quite proportional 

 to the increase in work done, but is if anything, less. The efficiency 

 values therefore tend to increase with increasing outputs up to a 

 certain limit. Beyond this point, the amount of oxygen used 

 increases very suddenly. In the example given, for a little less 

 than double the output, almost two and a half times as much 

 oxygen is required. As this involves the liberation of enough 

 energy to lift 1343 kilograms to the height of a metre, and as only 

 126-3 kilogram-metres of work are done, the increased work is not 

 done so economically and therefore the efficiency value falls. 

 How can this primary increase in efficiency and subsequent 

 decrease be explained and what factors are brought into play to 

 settle the critical point at which maximal efficiency will be found ? 

 If output is to be increased, intake must first be increased and the 

 ventricle must be distended to hold the extra amount of blood. 

 That is, the muscle fibres of the ventricular wall will be stretched. 

 We have already mentioned, in connection with skeletal muscle, 

 that a stretched muscle develops more tension during the iso- 

 metric phase. The heart responds to increased work by such a 

 lengthening of its fibres. If the lengthening process is carried too 

 far. the muscle fibres per unit of area will become fewer, so that 

 the larger the ventricular volume, the more strongly will each 

 fibre have to contract in order to produce a given tension. At 

 this greater length they also use up more potential energy just 

 as skeletal muscle does. 



