Is there a component of coronary collateral flow which cannot be detected by radiolabelled microspheres?

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Abstract
The possibility of a component of collateral flow to the ischaemic myocardium that cannot be detected by the radiolabelled microsphere technique was examined in the cat, rat, and rabbit in vivo. Animals were anaesthetised and a coronary artery ligated. The regional distribution of blood flow was assessed by the simultaneous injection of 141Ce labelled microspheres and 86Rb. Blue dye was injected into the circulation to delineate the perfused tissue and the heart removed, frozen, and lyophilised, after which tissue was separated into ischaemic and non-ischaemic fractions and the radioactivity of each assessed. Flow to the ischaemic zone, expressed as a percentage of that in the non-ischaemic tissue, for 86Rb based assessments and 141Ce measurements was: for the cat 21.6(3.9)% and 12.4(1.4)% (n=12 hearts); the rat 13.2(2.6)% and 5.2(1.8)% (n=9); and the rabbit 7.2(0.8)% and 1.9(0.8)% (n=5) respectively. In all species there was a statistically significant difference (p86Rb data were corrected for possible artefacts due to the flow rate dependent extraction of the isotope. The new 86Rb values for collateral flow in the cat, rat, and rabbit were 15.2(2.9)% (NS vs microspheres), 9.5(1.9)% (p141Ce microsphere values, significant differences between the two results still remained, possibly owing to other limitations of the 86Rb method. In an attempt to overcome this, additional studies were carried out with 3H desmethylimipramine (DMI), a marker of flow with more complete uptake characteristics than 86Rb. This was examined in the in vitro rabbit heart with regional ischaemia. Collateral flow, as a percentage of the radioactivity in the non-ischaemic myocardium, was 1.9(0.4)% for 3H desmethylimipramine and 1.1(0.3)% for 141Ce labelled microspheres (n=5 hearts; NS). In conclusion, these results do not support the concept of a biologically significant microsphere insensitive component of collateral flow.

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