Thermal injury prediction during cryoplasty through in vitro characterization of smooth muscle cell biophysics and viability

Abstract

Restenosis in peripheral arteries is a major health care problem in the United States. Typically, 30-40% of angioplasties result in restenosis and hence alternative treatment techniques are being actively investigated. Cryoplasty, a novel technique involving simultaneous stretching and freezing of the peripheral arteries (e.g., femoral, iliac, popliteal) using a cryogen-filled balloon catheter, has shown the potential to combat restenosis. However, evaluation of the thermal and biophysical mechanisms that affect cellular survival during cryoplasty is lacking. To achieve this, the thermal history in arteries was predicted for different balloon temperatures using a thermal model. Cellular biophysical responses (water transport (WT) and intracellular ice formation (IIF)) were then characterized, using in vitro model systems, based on the thermal model predictions. The thermal and biophysical effects on cell survival were eventually determined. For this study, smooth muscle cells (SMC) isolated from porcine femoral arteries were used in suspensions and attached in vitro systems (monolayer and fibrin gel). Results showed that for different balloon temperatures, the thermal model predicted cooling rates from 2200 to 5 degrees C/min in the artery. Biophysical parameters (WT & IIF) were higher for SMCs in attached systems as compared to suspensions. The "combined" fit WT parameters for SMCs in suspension (at 5, 10, and 25 degrees C/min) are L (pg) = 0.12 microm/(min atm) and E (Lp) = 24.1 kcal/mol. Individual WT parameters for SMCs in attached cell systems at higher cooling rates are approximately an order of magnitude higher compared to suspensions (e.g., at 130 degrees C/min, WT parameters in monolayer and fibrin TE systems are L (pg) = 18.6, 19.4 microm/(min atm) and E (Lp) = 112, 127 kcal/mol, respectively). Similarly, IIF parameters assessed at 130 degrees C/min are higher for SMCs in attached systems than suspensions (Omega 0 = 1.1, 354, 378 (x 10(8) (1/m(2) s)) and kappa(o) = 1.6, 1.8, 2.1 (x 10(9) K(5)) for suspensions, monolayer, and fibrin TE, respectively). One possible reason for the differences in IIF kinetics was verified to be the presence of gap junctions, which facilitate cell-cell connections through which ice can propagate. This is reflected by the change in the predicted IIF parameters when a gap junction inhibitor was added and tested in monolayer (Omega 0 (1/m(2) s)); kappa(o) = 2.1 x 10(9) K(5)). SMC viability was affected by the model system (lower viability in attached systems), the thermal conditions and the biophysics. For e.g., IIF is lethal to cells and SMC viability was verified to be the least in fibrin TE (most % IIF) and the most in suspensions (least % IIF) at all cooling rates. Using the results from the fibrin TE (suggested as the best in vitro system to mimic a restenosis environment), conservative estimates of injury regimes in the artery during cryoplasty is predicted. The results can be used to suggest future optimizations and modifications during cryoplasty and also to design future in vivo studies.