PURPOSE: With lithotripters today the shock waves are typically transmitted into the body via water filled bellows using coupling gel to make contact with the skin. Usually the coupling zone is not visible to the operator. We investigated coupling quality during routine clinical shock wave lithotripsy and the associated effect on shock wave disintegration efficiency.
MATERIALS AND METHODS: During 30 routine shock wave lithotripsy treatments the coupling zone was continuously monitored by a video camera integrated into a DoLi SII lithotripter (Dornier MedTech, Wessling, Germany). However, it was not shown to the blinded operator to resemble the standard clinical situation. We used 3 coupling gels, including LithoClear®, Sonogel® and a custom-made gel of low viscosity. The ratio of air in the relevant coupling area was measured. Lithotripter disintegration efficiency was evaluated by in vitro model stone tests at an air ratio of 0%, 5%, 10% and 20%.
RESULTS: Only in 10 of 30 treatments was good coupling achieved with an air ratio of less than 5%. In 8 treatments the ratio was greater than 20%. The best coupling conditions were achieved with low viscosity gel. The mean ± SD number of shock waves needed for complete fragmentation in the model stone tests was 100 ± 4 for bubble-free coupling, and 126 ± 3 for 5%, 151 ± 8 for 10% and 287 ± 5 for 20% air bubbles.
CONCLUSIONS: At 20 of 30 shock wave lithotripsy sessions there was imperfect coupling, accompanied by significant loss of disintegration capability. A surveillance camera is useful to monitor and improve coupling.
Copyright © 2012 American Urological Association Education and Research, Inc. Published by Elsevier Inc. All rights reserved.
J Urol. 2012 Jan;187(1):157-63. doi: 10.1016/j.juro.2011.09.039. Epub 2011 Nov 17
PMID: 22100005 [PubMed - in process]
In this highly interesting report attention is paid to the coupling between the shockwave source and the patient. By visualizing the areas of air in the coupling medium, model experiments showed that that the required number of shockwaves increased dramatically in the presence of 20% air compared with no bubbles at all. It was also shown that low viscosity gels had a lower propensity to bind air than had high viscosity gels.
It is noteworthy that the influence of air bubbles, because of the geometry, was most pronounced when the skin-to-stone distance was long.
These findings clearly emphasize the need of careful coupling with sufficient volumes of gel appropriately applied on the shockwave head. Inappropriate coupling is probably one of the most common factors in explanation of ESWL treatment failures.