Table 2.

Measurements of Collateral Growth

Measurement	Principle/assumption	Limitations
Blood flow (48, 59, 102, 114)	Collaterals represent a series-coupled  resistance to upstream conduit vessels  and downstream microvessels/resistance  vessels in the area of risk Because native  collateral resistance is substantially  higher than the microvascular or  conduit resistance, flow to the  collateral-dependent zone is dependent  primarily-to-exclusively (depending on  the organ system) on the growth  of collateral vessels	Empiric measurements of  collateral resistance are difficult
Retrograde flow (88–90)	Measurement of flow through the collateral  circulation without influences  of the downstream microvascular  resistance	Terminal experiment
Perfusion ratio  (collateral-dependent  flow/normal flow) (35, 82)	Same principle as for blood flow,  but absolute measurements of volume  flow are not necessary	No empiric measurements of  collateral flow. Ratio can be  influenced by changes in flow  to the normal zone or region  and/or flow to the  collateral-dependent region
Distal pressure/pressure ratio  (68, 72, 110, 123)	During an occlusion, pressure measured  downstream from the occlusion reflects  pressure inputs from the downstream  microcirculation and the collateral vessels.  As collateral growth is stimulated,  downstream pressures increase, which  reflects increased transmission of pressure  through these vessels because of lower  resistance	Distal pressure (that below the  occlusion is a function of both  collateral (indirect) and  microvascular (direct) resistances.  An increase in distal pressure  may be related to higher  microvascular resistance,  (e.g., embolization recovery of  tone), in addition to lower  collateral resistance
Rentrop score/angiography  (49, 50, 71, 81, 98, 110	Characteristics of retrograde “filling” of  conduit vessels in the collateral-dependent  region during angiography, where retrograde  filling is equated to collateral-dependent flow	Collateral-dependent flow is  poorly correlated with Rentrop  scores. Somewhat subjective  categories of results. Not  quantitative
Imaging/tomography  (8, 19, 57, 75, 86, 116	Visualization of collateral vessels by using  angiographic imaging, computerized  tomography, etc. Allows measurement  and quantification of collateral vessels	With fluorescence angiography,  collaterals below the surface  are not visualized. Accurate  measurements of diameter are  difficult. Improper or  inadequate filling also corrupts  the measurement. Difficult to  ascertain total cross-sectional  area of the collateral bed, which  affects total collateral resistance
Function/pathology  (15, 24, 49)	Function in most organ systems (e.g., cardiac  contraction, limb movement) dependent  on perfusion. If perfusion is too low (as after  an occlusion), severe pathology (e.g., infarction,  gangrene) may ensue. Growth of the  collateral circulation allows more perfusion  to the area at risk, thereby improving function  and ameliorating the pathology	Not an empiric measurement of  collateral flow, except in the  myocardium, where flow and  function are associated in a  quantitative manner. Scoring  of the pathology is often not  quantitative