Contractile function and reserve during acute ischaemia in the canine lateral border zone

The hypothesis that there is a lateral border zone with function intermediate to adjacent ischaemic and non-ischaemic tissue was tested in 10 open chest anaesthetised dogs. Four pairs of segment length crystals were placed in parallel so as to span the ischaemic and non-ischaemic zones. Graded occlusion was produced with a screw clamp applied to a carotid to left anterior descending artery cannulation system. Contractile reserve was assessed using postextrasystolic potentiation. A balloon perfusion labelling system was used to label negatively the potentially ischaemic zone and quantify the admixture of ischaemic and non-ischaemic tissue in the lateral border zone, defined by the fraction of normal zone tissue. When the 40 crystal pairs from the 10 dogs were grouped according to fraction of normal zone tissue (FNZT), 13 were in the central ischaemic zone(FNZT<0.1), seven were in the border ischaemic zone (FNZT 0.1−0.5), five were in the border non-ischaemic zone (FNZT 0.5−1.0), and 15 were in the non-ischaemic zone (FNZT 1.0). When the lateral border zone is predominantly non-ischaemic tissue, the tissue behaves as though it is non-ischaemic. Segmental shortening before and after postextrasystolic potentiation in the border non-ischaemic zone and non-ischaemic zone did not change with ischaemia. When tissue in the lateral border zone is predominantly ischaemic, it behaves as though it is ischaemic. Segmental shortening decreased in parallel with progressive ischaemia in the border ischaemic zone and ischaemic zone. At total occlusion, segmental shortening in the border ischaemic zone was −2.3(5.9)% and in the ischaemic zone −3.5(3.6)% (NS). Segmental shortening after postextrasystolic potentiation also decreased in parallel with ischaemia in the border ischaemic zone and ischaemic zone and was 7.0(3.9)% and 8.4(4.5)% respectively at total occlusion (NS). Thus in the heterogeneously perfused lateral border zone function was not intermediate but was determined by the proportions of ischaemic and non-ischaemic tissue.