420 Capt. H. C. Bazett. Changes in Blood Pressure and 



So that the output of the heart per minute (MV) may be taken as being 

 proportional to PP/(SP)^xPE with a normal value of 0'225, while the 

 average resistance of the arterioles is represented by the formula 



77x[PP/(SP)2xPEp ' 



where A is the resistance in the arterioles (a high figure indicating an 

 increased contraction), MP is the mean pressure, r) is the viscosity of the 

 blood in dynes/ 1000, and the other figures are as before. The normal figure 

 for A is about 60. 



J.A.S modified in this way the formula gives results which usually corre- 

 sponds with the blood volume changes actually observed, an increased vaso- 

 motor tone as estimated by the blood pressure changes being followed by 

 a blood concentration. The interrelationship of .the blood pressure and 

 hfemoglobin percentage curves can then be explained. It will also be seen 

 that in the case of diurnal variations, or patients with only slight shock 

 (see Cases II, III, and IV), the value of A may give a graph nearly parallel 

 with the diastolic pressure changes. On -the other hand, the relationship 

 between the value of A and blood volume changes is not obvious in certain 

 cases, especially in cases with nitrous oxide and oxygen anaesthesia, and in 

 cases where the blood pressure falls to a very low level, where the changes 

 cover a big range. Probably changes in venous tone are also of great 

 importance, since venous contraction is often induced in septic patients under 

 nitrous oxide and oxygen anaesthesia.* It is possible, too, that the curve for 

 the value of pulse pressures does not hold good for very low pressures. With 

 high blood pressures the elasticity of the arteries may depend largely on the 

 fibrous and muscular coats, while with low pressures, the elasticity may 

 resemble much more that of an elastic rubber tube. So that, while the 

 formula seems to hold well for pressures above 100 mm., it is very possible 

 that the pulse pressures should be divided by (100)^ and not by the (systolic 

 pressure)^, whenever "the systolic pressure is below 100 mm. of mercury. 

 Also, with very rapid pulse rates, it may prove necessary to take into con- 

 sideration the shortening of the duration of systole. 



None the less the formula seems to hold good for most conditions. It 

 demonstrates the fact that under circumstances such as exercise and excite- 

 ment, the rise of systolic pressure is accompanied by an actual lowering of 



*, It would seem probable tliat changes ia the blood volanie may be associated with 

 alterations in the capacity of the arterial system, when parallel changes will be seen in 

 the blood pressure, or with alterations in the capacity of the veins, when the blood 

 pressure is only indirectly affected. 



