Influence of Dornier HM3 system on respiration during extracorporeal shock-wave lithotripsy

Patients undergoing extracorporeal shock-wave lithotripsy (ESWL) for renal stones in the Dornier HM3 apparatus are immersed to the clavicles in water and anchored by abdominal and leg restraining straps. The respiratory effects of this system and sedation with midazolam were studied in 70 awake patients anesthetized with epidural blockade. On immersion, surface abdominal pressure and respiratory frequency (f) increased. Tidal volume (VT) and arterial saturation (SaO2) decreased. After intravenous midazolam 0.045 mg/kg body weight, VT decreased further to 35% below pre-immersion values and stone excursions measured by fluoroscopy diminished by more than 50%; reduction of VT and stone excursions were linearly correlated (R = 0.45). SaO2 declined sharply to 6.1% (range -1.5-13.8%) below pre-immersion values, whereas end-tidal CO2 (PETCO2) increased more slowly and f remained above the pre-immersion rate. In three patients, SaO2 decreased to below 85% but was rapidly restored to 98-100% by oxygen administration. The respiratory effects of sedative drugs are exaggerated by the Dornier HM3 system, and continuous monitoring of gas exchange and appropriate oxygen administration are recommended in patients receiving regional anesthesia and sedation.