Adaptation of tissue to a chronic heat load

Determination of the chronic effect of heat on tissue is one of the important issues facing mechanically actuated total artificial heart (TAH) development. In an effort to characterize this effect, implantations of heating devices producing constant heat fluxes of 0.04 watts/cm², 0.06 W/cm², and 0.08 W/cm² were performed in 11 calves (H-series). Heated disks were implanted adjacent to lung and muscle tissue for a period of 7 weeks. Temperature sensors were placed at the surface as part of the heater assemblies. The results showed that initially, temperature elevations above body temperature (ΔT) were 6.4 ± 0.6° C, 4.5 ± 0.2° C, and 1.8 ± 0.5° C at the muscle heater surface for 0.08, 0.06, and 0.04 W/cm², respectively. At 2 weeks after implant ΔT values changed to 5.5 ± 0.6° C, 3.4 ± 0.2° C, and 1.8 ± 0.2° C, respectively. Seven weeks after implant, ΔT values decreased to 3.7 ± 1.2° C, 2.8 ± 0.1° C, and 0.8° C for 0.08, 0.06, and 0.04 W/cm², respectively. The authors think this change is attributable to an adaptive response of the tissue to increase heat dissipation through angiogenesis. Results from three TAH cases indicated that at two measured tissue interfaces, ΔT decreased by 1° C during a 15 day period. At the same time, the waste heat (volts x current in-flow x afterload to the blood) remained constant at 11.1 ± 0.5 W during this period. This decrease in ΔT corresponded to that observed for the H-series experiments at the higher heat fluxes. Thus, it appears that adaptation observed in the H-series experiments also is seen for tissues surrounding heat sources such as the TAH.