title: Effect of air bubbles in the coupling medium on efficacy of extracorporeal shock wave lithotripsy.
abstract: OBJECTIVE
  Replacement of the water bath by a water cushion in newer lithotriptors introduces an acoustic interface and an ideal coupling agent is required to prevent energy loss at this interface. We aim to study the effect of bubbles in the coupling media on efficacy of extracorporeal shock wave lithotripsy (ESWL) by an in vitro experiment.
  
  
  METHODS
  Using a standardised in vitro model 40 artificial stones were randomly treated on Modulith SLK lithotriptor using either conventional ultrasound gel (high bubble content) before and after displacing visible bubbles, a thin ultrasound gel (Therasonic) or silicon oil (both with negligible bubbles). Percentage area covered by bubbles in each case and the diameters and depth of crater created in each stone were measured by two blinded observers to determine the correlation between the bubble contents and disintegration efficacy. In vivo effect of two ultrasound gels was compared in terms of pain scores and stone fragmentation rates in ten patients treated with both gels.
  
  
  RESULTS
  Volume of the craters was significantly greater with the Therasonic gel (102.4+/-33.4 mm3) or silicon oil (98.8+/-9.8 mm3) than the conventional ultrasound gel (49.2+/-32.6 mm3). But it was greatest (p<0.001) with ultrasound gel without bubbles (163.5+/-22.6 mm3). Depth and volume of the stone crater increased significantly with decreasing bubble contents of gel (p<0.001). Compared to standard ultrasound gel, patients treated with Therasonic gel reported significantly higher pain scores (median 3.5 vs. 8.0; p<0.001).
  
  
  CONCLUSIONS
  Efficacy of ESWL is significantly correlated to air bubbles within the coupling gel and can be improved significantly by eliminating the bubbles from the coupling medium.
authors.affiliation: Department of Urology, Bradford Teaching Hospitals NHS Foundation Trust, Bradford Royal Infirmary, Bradford, United Kingdom. a.jain@hotmail.co.uk
authors.forename: Arun
authors.initials: A
authors.lastname: Jain
authors.forename: Tariq K
authors.initials: TK
authors.lastname: Shah
mesh_terms: D000388:Air; D002137:Calculi / Q000628:therapy; D004867:Equipment Design; D005782:Gels; D006801:Humans; D008096:Lithotripsy / Q000295:instrumentation*; D045423:Microbubbles; D012827:Silicone Oils
chemical_list: D005782:Gels; D012827:Silicone Oils