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Introduction  Adequate volume expansion (VE) in patients with evidence of hypoperfusion should be aimed not only at achieving an increase in stroke volume (SV) and cardiac index (CI) but also at improved tissue perfusion and oxygenation. Our aim in this study was to assess the dynamic changes in muscle tissue oxygen saturation (StO 2 ) during hypovolaemia and in response to VE.    Methods  We conducted a prospective study of 42 fluid challenges in patients undergoing major abdominal surgery with evidence of hypovolaemia, defined as pulse pressure variation (PPV) >13% and SV variation (SVV) >12%. CI, SV, SVV (FloTrac/Vigileo) and PPV were measured before and after VE. Fluid responsiveness was defined as an increase of SV >15% after a 500-mL colloid infusion over 15 minutes. In all patients, the muscle StO 2  and its changes during a standardised vascular occlusion test were analysed using a near-infrared spectroscopy device after anaesthesia induction (which defined the baseline state) and before and after each VE.    Results  No patients were preload-responsive after anaesthesia induction. Twenty-nine of forty-two fluid challenges (69%) were positive for VE, with a statistically significant ( P  < 0.001) difference in SV changes between positive and negative responses to VE. There was a statistically significant difference in PPV and SVV values before VE in the positive and negative fluid responses [PPV: 16% (15% to 18%) vs. 14% (13% to 15%),  P  = 0.001; and SVV: 14% (13% to 16%) vs. 16% (15% to 16%),  P  = 0.03 or positive and negative fluid responses, respectively]. Data are presented as medians and 25th and 75th percentiles Before VE there was no significant difference in StO 2  values relative to baseline [86% (78% to 88%) vs. 84% (77% to 91%),  P  = 0.83], without a significant difference ( P  = 0.36) between positive and negative fluid challenges. Hypovolaemia was associated with a significant reduction ( P  = 0.004) in StO 2  recovery slope, with a significant difference ( P  = 0.02) between positive and negative fluid challenges. The VE-induced increase in the StO 2  recovery slope was 62 ± 49% ( P  < 0.001) for positive fluid challenges and 26 ± 34% ( P  = 0.04) for negative fluid challenges.    Conclusions  Hypovolaemia significantly affects the muscle StO 2  recovery slope. Restoring effective intravascular volume with fluid loading significantly improves the StO 2  recovery slope, despite apparently ineffective changes in systemic haemodynamics.

Use of near-infrared spectroscopy during a vascular occlusion test to assess the microcirculatory response during fluid challenge