BLOOD SUPPLY TO THE HEART 



[ 539 



Reactive Hyperemia 



Reactive hyperemia is considered to be the excess 

 blood flow (over the control flow that normally would 

 have occurred) following release of a coronary artery 

 occlusion. The coronary bed is extremely reactive to 

 the stimulus of anoxia. After release of temporary 

 occlusion of a coronary artery, even for as short a 

 time as 2 to 3 sec, left coronary arterial flow increases 

 almost immediately in the isolated heart, heart- 

 lung preparation, the anesthetized open-chest dog, 

 and the unanesthetized dog (66, 67, 147, 301). The 

 augmented flow exists throughout systole and diastole. 

 The flow response occurs without necessarily any 

 change in blood pressure or heart rate and before any 

 impairment of myocardial contraction occurs in the 

 area rendered ischemic. Beyond 30 to 60 sec of occlu- 

 sion, the area bulges during systole (153)- Depending 

 upon the duration of the occlusion, the peak flow 

 response (100-300% of control flow) does not usually 



occur immediately upon release of the occluded 

 coronary artery, but reaches a maximum some time 

 during the first half minute of reactive hyperemia and 

 may last up to 4 min (fig. 8). The volume of reactive 

 hyperemia blood flow, its duration and peak flow 

 values increase with lengthening periods of left 

 circumflex arterial occlusion up to 120 sec. The 

 theoretical blood flow "debt" (control blood flow X 

 duration of occlusion) seems to be always greatly over- 

 paid (average 2 1 9 c '< ) in the presence of periods of 

 occlusion lasting from 5 sec to 180 sec. The reactive 

 hyperemia responses in skeletal muscle vascular beds 

 are similar (222) except that the blood flow debt is 

 variably over- or underpaid (410). In other vascular 

 beds, such as the superior mesenteric artery, this re- 

 sponse is much smaller than in the myocardium; in 

 the renal (150), it is essentially nonexistent. 



The presence of reactive hyperemia has not been 

 satisfactorily explained. Its purpose must be to supply 



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 £89 



HEART RATE = 162 



90 



82 



88 



<o 

 ouj 60 



30 SEC 

 "OCCLUSION ]/ R H.B.F 



43 7 



□ z 24 



O Ld 



3? 12 



m x 



° 



tA 





24r 



16 



6 2 



DEBT 31 cc 



11.8 cc 



6 2 



SECONDS 



30 60 90 120 150 180 



fig. 8. Diagrammatic redrawing of arterial blood pressure (upper tracing), left coronary blood 

 How (next lower tracing), and coronary sinus oxygen saturation (third tracing down), before, during, 

 and after release of 30 sec of left coronary artery occlusion in the open-chest dog. Lowest curve repre- 

 sents Oj consumption of left myocardium (flow times A-V O2) in ml/min, calculated from above 

 experimental data. Arrows A and B represent, respectively, beginning and end of measurements of 

 reactive hyperemic blood flow (RHBF) and its average A-V 2 difference used in calculation. [From 

 Coffman & Gregg (67).] 



