Minimal static excess pressure minimises the effect of extracorporeal shock waves on cells and reduces it on gallstones.

The effects of extracorporeal shock waves on haemoglobin release, membrane permeabilisation and stone fragmentation were examined at minimal static excess pressures. Shock waves from an electrohydraulic lithotripter were applied at 15, 20 and 25 kV to red blood cells in plastic pipettes pressurised with 0, 30, 50, 75, 100, 200, 300 and 400 kPa of static excess pressure; the freed haemoglobin was determined as a marker of cell destruction. Using 15-kV discharges, 30-kPa excess pressure reduced the freed haemoglobin from 1.21 g/L at ambient pressure to 0.39 g/L, 20-kV discharges reduced it from 2.01 g/L to 0.92 g/ L and 25-kV discharges from 2.56 g/L to 1.61 g/L. Haemoglobin values at 400-kPa excess pressure were reduced to 0.03 g/L (15-kV discharges), 0.07 g/L (20-kV discharges) and 0.09 g/L (25-kV discharges), which is a 95%-97% reduction of the values obtained at ambient pressure. There was a steep initial drop from 30-100 kPa excess pressure followed by a plateau at low level. Propidium iodide uptake by L1210 tumour cells, a marker for transient membrane permeabilisation by shock waves, was reduced by 90% at these slight excess pressures. Stone fragmentation was also suppressed by excess pressure yet not as markedly as at cells; 100 kPa reduced the amount of gallstone fragments by 20%, and 400 kPa reduced it by 65%. A further reduction, by 93%, was obtained when 1-MPa excess pressure was applied to gallstones in a Plexiglas cylinder. Shock wave-gas bubble interaction has been previously proposed to mediate the shock wave action. It is suggested that the excess pressure reduced the size or number of the bubbles, thus reducing this interaction, at least in the case of the cellular effects. The reduced effect of shock waves on cells, in contrast to the effect on stones, might open up a new approach to the design of lithotripters that would reduce tissue damage yet keep fragmentation up at a similar level.