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Molecular and particulate depositions for regional myocardial flows in sheep
J B Bassingthwaighte1, M A Malone, T C Moffett
1Department of Radiology, University of Washington, Seattle 98195.
Abstract:
The deposition of microspheres in small tissue regions is not strictly flow dependent. In comparison with the soluble flow marker 2-iododesmethylimipramine (IDMI), deposition of 16.5-microns microspheres was mildly but systematically biased into high flow regions of rabbit hearts (Bassingthwaighte JB, Malone MA, Moffett T-C, King RB, Little SE, Link JM, Krohn KA. Am J Physiol 1987;253 (Heart Circ Physiol 22):H184-H193). To examine the possibility of bias in larger hearts, a similar study was undertaken in sheep. 141Ce- and 103Ru-labeled 16.5-microns microspheres in one syringe and 125I- and 131I-DMI in another syringe were injected simultaneously into the left atrium of five open-chest sheep while obtaining reference blood samples from the femoral artery. In six other sheep, one microsphere type and one IDMI were used. Hearts were removed 1 minute after injection, cut into approximately 254 pieces averaging 217 mg, and regional deposition densities calculated for each tracer from the isotopic counts. Correlations in the five animals between the two differently labeled IDMIs and between the two microspheres were both greater than or equal to 0.98. In all 11 sheep, scatter plots of microsphere deposition densities versus IDMI densities showed that differences between microspheres and IDMI had substantially more scatter (0.84 less than r less than 0.98) but were not random. Microsphere depositions tended to be lower than IDMI depositions in low flow regions and higher in high flow regions, in accord with the expected bias that at a bifurcation a microsphere is most likely to enter the branch with higher flow. There was less bias ascribable to endomyocardial/epicardial maldistribution. Thus, while microsphere depositions appear to err systematically with respect to flow when the regions of interest are small enough that the diameters of their arterioles are only a few times those of the microspheres, microspheres are, in sheep as in rabbits, adequate for estimating regional flows.
Insights
Microsphere deposition in small tissue regions shows a slight bias towards high-flow areas, differing from soluble markers. However, microspheres remain adequate for estimating regional blood flow in sheep hearts.
Area of Science:
- Cardiovascular Physiology
- Biomedical Engineering
- Pharmacokinetics
Background:
- Microsphere deposition in tissues is a common method for assessing regional blood flow.
- Previous studies in rabbits suggested microsphere deposition is not strictly flow-dependent and can be biased in small tissue regions.
- The reliability of microspheres as flow markers in larger organs like the sheep heart required further investigation.
Purpose of the Study:
- To investigate the regional deposition patterns of microspheres compared to a soluble flow marker in sheep hearts.
- To determine if microsphere deposition exhibits flow-dependent bias in larger animal models.
- To assess the adequacy of microspheres for estimating regional blood flow in sheep.
Main Methods:
- Simultaneous injection of radiolabeled microspheres and a soluble flow marker (2-iododesmethylimipramine, IDMI) into the left atrium of sheep.
- Collection of reference blood samples and subsequent removal and dissection of hearts into small tissue pieces.
- Calculation of regional deposition densities for each tracer using isotopic counts and correlation analysis.
Main Results:
- High correlations (≥0.98) were observed between differently labeled IDMI and microsphere tracers within animals.
- Scatter plots revealed systematic differences between microsphere and IDMI deposition densities (0.84 ≤ r ≤ 0.98).
- Microsphere deposition was lower in low-flow regions and higher in high-flow regions, indicating a mild flow bias, particularly in small vascular beds.
Conclusions:
- Microsphere deposition in sheep hearts exhibits a mild, systematic bias towards high-flow regions, especially when vascular diameters are close to microsphere size.
- Despite this bias, microspheres are considered adequate for estimating regional blood flow in sheep, similar to findings in rabbits.
- The study highlights the importance of considering microsphere size relative to vessel diameter for accurate flow estimations.