Circulatory model studies of external cardiac assist by counterpulsation

A hydraulic model of the circulatory system was used to study external cardiac assist (ECA) as a noninvasive, synchronous, counterpulsative aid to the failing heart. Four different modes of external pressurisation of the legs were compared. (1) In uniform compression, a spatially-uniform pressure is applied simultaneously to the entire length of the leg. (2) In graded compression, a decreasing pressure is applied simultaneously from foot to groin. (3) In sequential compression, a wave of compression is applied, at uniform pressure level, from foot to groin. (4) In graded-sequential compression, the gradation of pressure and wave-like application are combined. Depressurisation was either to atmospheric pressure or to a vacuum. For each mode tests were made with broad ranges of pressures and timing parameters. 'Optimal' combinations of these were selected for comparisons among the modes. The effectiveness of cardiac assist was assessed in terms of (i) the augmentation of mean diastolic pressure, signifying increased coronary perfusion; and (ii) the lowering of mean systolic pressure, signifying reduced left ventricular oxygen requirement. Measurements of regional volume flow rates and of cross-sectional area of the leg artery provided additional insights. All four external cardiac assist modes produced substantial augmentation of mean diastolic pressure. The simplest mode, namely, uniform compression, was however the least effective in this respect. The graded, sequential, and graded-sequential modes were all better, in ascending order. These results are explained haemodynamically in terms of the tendency of a uniformly-pressurised collapsible vessel to form an occlusive flow-limiting throat at its downstream (proximal) end. Non-uniform compression is the means by which this phenomenon may be avoided. Application of vacuum during depressurization further increased the diastolic augmentation for every external cardiac assist mode. The changes in mean systolic pressure produced by ECA were much smaller than the changes in mean diastolic pressure. In general, the modes with greater diastolic augmentation also had relatively higher mean systolic pressures. With depressurization to atmosphere, application of external cardiac assist increased the mean systolic pressure slightly, relative to the unassisted condition. However, depressurization to 6.7 or 13.3 kPa (50 or 100 mmHg) vacuum resulted in either a reduction or virtually no change of mean systolic pressure, relative to the unassisted condition.