BACKGROUND: Markedly reduced urine have been commonly observed during mastoidectomy under general anesthesia. The aim of study was to evaluate the surgery-related mechanism of reduced urine during mastoidectomy.

METHODS: 11 patients undergoing mastoidectomy were studied. Blood samples were drawn from CVP line inserted through right internal jugular vein just before drilling (Pre-D); at 15 min after drilling (D-15); at the time closest to the inner ear (CHP-1); at 15 min (CHP-2) and 30 min (CHP-3) after CHP-1 and just before emergence (End). MAP, HR, temperature, CVP, and urine output (UO) were recorded at each period. 0.9% normal saline with room temperature was used to irrigate surgical field. Serum ADH, and the osmolalities (serum and urine) were measured. In 6 patients, serum TSH and FSH were measured, simultaneously.

RESULTS: There were no significant hemodynamic changes during procedure. UO was decreased in 50% during and after the drilling. No differences showed in UO between before and after the drilling of CHP. Plasma ADH level after the CHP was increased 2-3 fold than before. Serum osmolality was unchanged throughout the periods, while the urine osmolality was significantly increased after the period of CHP. FSH was not changed during the periods and TSH was decreased slightly than in Pre-D.

CONCLUSIONS: The reduced urine output during mastoidectomy may be influenced by the drilling-related vibration, which may affect the supraoptico-hypophyseal tract, resulting in ADH release. The irrigated isotonic saline with higher osmolality (308 mOsm/kgH₂O) than plasma osmolality may partly contributed to the increased ADH.