846 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



T~ig """TH 



table i . Ascending Aorta Flow During Exercise 



ID 



I — * — zz — ~~ 



r_^^i"_ T~U I 



o 

 a 



O'S 



x. 

 a 

 5 



<n II 



■S £ 



_ a, 



o X! 



g I 



™ C 



o 



be 



r • - 



O . 





* 40% above control. 



heart rate and stroke volume. Further effort increased 

 output primarily by increasing the stroke volume 

 with little change in heart rate. The increase in 

 stroke volume was evidenced by a change in the 

 contour of the ejection pulse from its normal tri- 

 angular shape toward that of a square wave. The 

 failure to increase greatly the peak velocity may have 

 been due to turbulence impedance or to a limit in the 

 rate of ventricular contraction. As a result of the 

 limiting factor on peak velocity, the animal may have 

 been near his maximum cardiac output. Table 1 gives 

 the flow and pressure values for figure 8. 



Pressure-Flow Relationship in the Ascending Aorta 



Figure 9 illustrates the instantaneous pressure 

 difference (AP) along 3 cm of the ascending aorta with 

 the axial flow through the same segment. AP deflec- 

 tions lead the flow deflections by approximately 90 

 degrees, which suggests that mass-acceleration laws 

 dominate. A very close approximation of actual flow 

 may be computed from &P using equations 5 and 9. In 

 this situation radial flow is small compared to axial 

 flow, thus absence of the radial flow term seems unim- 

 portant especially when only the net axial flow is 

 desired. 



Fry (13) originally pointed out this relationship and 

 its practical use in a catheter-tip flowmeter. Assuming 

 that the flow profile in the ascending aorta is flat, 

 one would theoretically need only the cross-sectional 

 area of the ascending aorta to compute cardiac output 

 from the pressure gradient. The application of this 

 principle using a double lumen catheter is at the 

 moment fraught with many practical difficulties, 

 primarily concerning Pitot effects at the catheter tip 

 and accurate measurement of the lumen cross section. 



Ventricular Ejection Gradients 



Because of mass-acceleration effects, ventricular ejec- 

 tion during the deceleration phases takes place against 



