We investigated the effects of hemidiaphragmatic paresis caused by interscalene brachial plexus block on breathing patterns, chest wall mechanics, and arterial blood gas tensions using respiratory inductive plethysmography. Ten healthy patients received interscalene block with 20-40 mL 1.5% lidocaine with epinephrine. Rib cage contribution to tidal volume (%RC) increased from 28.9% +/- 9.7% to 50.0% +/- 8.3% (P < 0.01), respiratory frequency (f) increased from 14.6 +/- 3.2/min to 16.3 +/- 2.4/min (P < 0.05), and PaO2 decreased from 84.7 +/- 7.3 mm Hg to 78.0 +/- 9.5 mm Hg (P < 0.05). No significant changes were observed in tidal volume (VT), minute volume (VE), or PaCO2. These results indicated that VT, VE, and PaCO2 were maintained after interscalene block, apparently by increases in f and %RC to compensate for hemidiaphragmatic paresis caused by interscalene block. Nevertheless, PaO2 was reduced, presumably due to increased ventilation-perfusion mismatching. Recognizing that we studied healthy patients, the decrease in PaO2 may be more in patients with cardiopulmonary disease.