RESISTANCE AND CAPACITANCE PHENOMENA IN VASCULAR BEDS 



943 



PRESSURE 



fig. 9. Sequential flow readings following a series of square 

 wave changes of perfusion pressure in a vascular bed showing 

 autoregulation. The pressure-flow points are numbered suc- 

 cessively As,, Ai, At, A 3 , and A, for the first square wave change 

 of pressure (on both insert and graph). The points for the 

 second pressure change are numbered similarly B n -B t and for 

 the third, C -C a . Light solid line represents the pressure-flow 

 relationship which would be found if vasomotor tone remained 

 constant at the level which existed during perfusion at a 

 pressure corresponding to Aq for a period of time sufficient to 

 establish a steady-state flow at this pressure. Dash-dot line 

 represents the corresponding pressure-flow plot which would 

 be found if the vasomotor tone were to remain constant at a 

 level corresponding to that found when steady-state flow was 

 established at a perfusion pressure corresponding to point B 2 - 

 Dash-dash line represents the pressure-flow plot which would 

 correspond with the steady-state flow established for perfusion 

 pressure A? assuming no change in vasomotor tone were to 

 occur with subsequent change of perfusion pressure. These 

 three plots then represent three levels of vasomotor tone. Heavy 

 solid line represents the actual "steady state" pressure-flow rela- 

 tionship after reactive changes have occurred in the vasomotor 

 tone following each change of perfusion pressure; it is the plot 

 characteristic of autoregulation. 



decreased with further lowering of perfusion pressure 

 down to about 50 mm Hg. This progressively de- 

 creasing resistance tended to maintain flow relatively 

 constant over the range of arterial pressure from 90 

 down to approximately 50 mm Hg. Below 50 mm 

 Hg arterial pressure, flow dropped rapidly and 

 approached zero at a pressure of 10 to 20 mm Hg. 



When the pressures and flows in figure 1 oA are 

 plotted on log-log graphs (fig. 10B), the portion 

 corresponding to the pressures above 90 mm Hg plots 

 as a straight line with a slope greater than 1 ; this por- 

 tion of the curve corresponds to a "passive" relation- 

 ship between pressure and flow (see above). On the 

 other hand, in the range between 50 and 90 mm Hg 

 the slope is less than 1. In the equation F = cP n , 

 the corresponding values of n are greater than and 

 less than 1, respectively. Values for n of less than 1 are 

 characteristic of autoregulation (28, 29). 



ARTIFACTS INDUCED IN AUTOREGULATION STUDIES BY 



pump perfusion schemas. Failure to detect auto- 

 regulation in vascular beds has been attributed to 

 occurrence of some alteration in the vascular bed as a 

 result of changes in the blood due to contact with 

 artificial structures or to traumatization of the blood 

 by perfusion pumps (28, 29, 66). This effect is notice- 

 able particularly in the cerebral vascular bed, as 

 shown in figure 8 (76 and unpublished data). 



The heavy line in figure 8 represents the stable 

 pressure-flow relationship in a cerebral vascular bed 

 during an initial study when the pressure was regu- 

 lated by compressing the arteries supplying the brain. 

 The light line gives the stable pressure-flow relation- 

 ship during a subsequent period when a perfusion 

 pump was inserted in the arterial inflow circuit. With 

 the perfusion pump in operation, flow at all levels of 

 pressure was significantly above that with the brain 

 perfused directly from the heart. Furthermore, the 

 flow did not remain constant but increased regularly 



fig. 10. Plots of the relationship between 

 blood flow in a skeletal muscle vascular bed in 

 the dog and the arterial perfusion pressure 

 (arterial pressure minus venous pressure). A: 

 linear plot; B: log-log plot. Pressures were 

 varied from the control, at approximately 

 160 mm Hg, to successively lower or higher 

 pressures and returned to the control following 

 each determination at the experimental 

 pressure. Each point represents the data after 

 the flow had stabilized at the new pressure. 



100 140 180 30 50 70 100 



ARTERIAL PERFUSION PRESSURE mm Hg 



200 



