BACKGROUND: In hemodynamically unstable patients with spontaneous breathing activity, predicting volume responsivenss is a difficult challenge. Our objective was to test whether the respiratory changes in pulse oxymetry plethysmographic waveform amplitude (POP) and in stroke volume (deltaSV) could predict fluid responsiveness to passive leg raising (PLR) in normal volunteers. METHODS: We investigated 25 normal volunteers. We assessed hemodynamic status (HR, SBP, MAP, CI and SVI) and calculated the respiratory variation in pulse oximetry plethysmographic waveform amplitude at supine and after PLR. We attached a pulse oximeter of 25 spontaneously breathing volunteers as several time points: after 1 min and 5 min in supine position and during PLR at 60degrees. Heart rate, non-invasive blood pressures (mean arterial pressure, systolic blood pressure), maximal POP (POPmax), minimal POP (POPmin) and deltaPOP defined as (POPmax-POPmin)/[(POPmax+POPmin)/2] were recorded using monitor. RESULTS: Comparing to supine and PLR, systolic blood pressure and mean arterial pressure were not different, but the change in cardiac index, stroke volume and respiratory variation in POP were significant different. In response group (> or =10% in deltaCI), the change in cardiac index, stroke volume and respiratory variation in POP were significant greater. CONCLUSION: PLR induces a significant decrement of variation in POP amplitude among spontaneouely breathing volunteers. We suppose that the changes in stroke volume and the respiratory variation in pulse oximetry plethysmographic waveform amplitude induced by PLR predict fluid responsiveness in spontaneous breathing patients.