Mechanical haemolysis in shock wave lithotripsy (SWL): I. Analysis of cell deformation due to SWL flow-fields

• Published in: Physics in medicine & biology Vol. 46; no. 2; pp. 413 - 437
• Main Authors: Lokhandwalla, Murtuza, Sturtevant, Bradford
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 01.02.2001; Institute of Physics
• Subjects: Biological and medical sciences; Biomechanical Phenomena; Biomechanics. Biorheology; Biophysical Phenomena; Biophysics; Erythrocyte Deformability; Fundamental and applied biological sciences. Psychology; Hemolysis; Humans; In Vitro Techniques; Lithotripsy - adverse effects; Medical sciences; Membrane Fluidity; Miscellaneous; Models, Biological; Radiotherapy. Instrumental treatment. Physiotherapy. Reeducation. Rehabilitation, orthophony, crenotherapy. Diet therapy and various other treatments (general aspects); Tissues, organs and organisms biophysics; Viscosity; Human; Hemolysis; Pathophysiology; Instrumentation therapy; Cavitation bubble; Red blood cell; Experimental study; In vitro; Shock wave; Mechanical characteristic; Lithotripsy; Plasma membrane; Complication; Mechanical deformation
• ISSN: 0031-9155; 1361-6560
• DOI: 10.1088/0031-9155/46/2/310

This work analyses the interaction of red blood cells (RBCs) with shock-induced and bubble-induced flows in shock wave lithotripsy (SWL), and calculates, in vitro, the lytic effects of these two flows. A well known experimentally observed fact about RBC membranes is that the lipid bilayer disrupts when subjected to an areal strain (deltaA/A)c of 3%, and a corresponding, critical, isotropic tension, Tc, of 10 mN m(-1) (1 mN m(-1) = 1 dyne cm(-1)). RBCs suspended in a fluid medium tend to deform in accordance with the deformation of the surrounding fluid medium. The fluid flow-field is lytically effective if the membrane deformation exceeds the above threshold value. From kinematic analysis, motion of an elementary fluid particle can always be decomposed into a uniform translation, an extensional flow (e.g. -->uinfinity(x, y, z) = (k(t)x, -k(t)y, 0)) along three mutually perpendicular axes, and a rigid rotation of these axes. However, only an extensional flow causes deformation of a fluid particle, and consequently deforms the RBC membrane. In SWL, a fluid flow-field, induced by a non-uniform shock wave, as well as radial expansion/implosion of a bubble, has been hypothesized to cause lysis of cells. Both the above flow-fields constitute an unsteady, extensional flow, which exerts inertial as well as viscous forces on the RBC membrane. The transient inertial force (expressed as a tension, or force/length), is given by Tiner approximately rhor(c)3k/tau, where tau is a timescale of the transient flow and r(c) is a characteristic cell size. When the membrane is deformed due to inertial effects, membrane strain is given by deltaA/A approximately ktau. The transient viscous force is given by Tvisc approximately rho(nu/tau)1/2r(c)2k, where rho and nu are the fluid density and kinematic viscosity. For the non-uniform shock, the extensional flow exerts an inertial force, Tiner approximately 64 mN m(-1), for a duration of 3 ns, sufficient to induce pores in the RBC membrane. For a radial flow-field, induced by bubble expansion/implosion, the inertial forces are of a magnitude 100 mN m(-1), which last for a duration of 1 micros, sufficient to cause rupture. Bubble-induced radial flow is predicted to be lytically more effective than shock-induced flow in typical in vitro experimental conditions.