560 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



TABLE 2. Net Volume Uptake per m- Body Surface of 

 Arterial Tree at Various Pressures 



The smaller of the two pulse curves illustrates a very 

 different set of pressure relations. At the time of semi- 

 lunar closure the distending pulse pressure in the 

 arch is effectively zero (diastolic pressure), that in the 

 "head" system about 18 mm Hg, and that in the 

 "visceral" system about 3 mm Hg. The pulse wave 

 transmission at this low diastolic pressure is so slow 

 and systole is so short that the pulse wave does not 

 arrive at the legs before closure of the aortic vaK^es. 

 The uptake then is 8 ml (head) plus 2 ml per m- 

 (viscera) and there is no effective uptake in the arch 

 and legs. This means that whatever uptake may have 

 occurred early in systole in the arch is measured else- 

 where and that changes in the size of the leg arteries 

 occur effectively during diastole. 



The transmission times used in the above calcula- 

 tion are dependent on diastolic pressure and were 

 measured in average-size dogs as the means of numer- 

 ous observations. These are given in table 3. In addi- 

 tion there is a column headed Tw or transmission time 

 weighted for drainage or arteriolar outflow. This is 

 the average transmission time from the arch to the 

 arterioles of the several arterial beds, each weighted 

 for the amount of arteriolar outflow in the particular 

 bed (see above). The "head arteries" are given the 

 most weight because most of the drainage occurs 

 from these arteries. The visceral arteries are given 

 next most weight and the leg arteries the least. In 

 using this concept in a stroke volume calculation the 

 weighted transmission time is measured forward from 

 the incisura. Those parts of the pulse curve occurring 

 before Ts—Tw indicate the pressures controlling 

 systolic drainage and that afterwards (during 

 Td + Tw) or Tc — Ts + Tw control diastolic drain- 

 age (Tc = time of cycle). 



The stroke index is the sum of arterial uptake and 

 arteriolar outflow, or drainage during systole. Svstolic 

 drainage is evaluated according to the principle 

 introduced by Bazett (6). Arteriolar drainage during 

 diastole is equal to uptake, or long as end diastolic 

 arterial pressure does not change from beat to beat. 

 The findings of VVhittaker & Winton (144) are ac- 

 cepted so that .systolic drainage can be related to 

 diastolic drainage in proportion to the effective pres- 

 sure (i.e., the pressure aljove 20 mm Hg) of systole 

 and diastole multiplied by the effective duration of 

 these parts of the cycle. The mean effective pressure 

 (Ps) can be computed by adding the pressures minus 

 20 mm Hg at regular and small intervals during 

 effective systole (Ts — Tw) divided by the number of 

 such measurements. The mean effective diastolic 

 pressure {Pd) can be measured in an analogous man- 

 ner. 



The stroke index (SI) then would be 



SI = U + 



PsjTs - Tw) 

 Pd(Td -\- Tw) 



U 



The drainage volume, which is added to the uptake, 

 is rarely more than 20 per cent of the stroke index and 

 usually much less. It is questionable whether it is 

 needful to calculate Ps and Pd meticulously. Analysis 

 of many curves (108) has shown that the mean pres- 

 sure {Ps) during effective systole {Ts — Tw) rarely 

 exceeds that during effective diastole ( Td -\- Tw) by 

 more than i mm or so. Thus for all practical purposes 

 Ps = Pd and the systolic drainage {Ds) calculations 

 can be simplified. 



Ds = U 



Ts — Tw 



Td + Tw 



= U 



(Ts - Tw) 



Tc 



Ts -\- Tw 



SI = u + u 



(Ts - Tw) 

 Tc - Ts -{■ Tw 



Since flow per second is SI Tc then 



U 



Tc - Ts -}- Tw 



The use of tliis formula in\olves estimating U, the 

 arterial uptake per square meter, measuring the 

 durations of .systole and cycle, and the diastolic pres- 

 sure. Knowing the diastolic pressure, the transinission 

 times for the thoracic, visceral, and leg segments and 

 for Tzv can be obtained from table 2 or preferably 

 from a plot made from the figures therein. These 

 times can then be laid off from the incisura and pres- 

 sures read at the incisura and at the moment of each 



