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Adequate volume expansion for hypovolemia should improve cardiac preload and stroke volume leading to improved microvascular blood flow, tissue perfusion and oxygenation. The dynamic recovery slope of tissue oxygen saturation (StO2) during a standardized vascular occlusion test (VOT) is proposed to reflect microvascular reactivity. We aimed at evaluating the dynamic StO2 variations during fluid challenge in high-risk surgical patients.
Thirteen adult Caucasian patients, ASA II to III, undergoing major abdominal surgery, were included. In all, cardiac output (CO), stroke volume (SV), pulse pressure variation (PPV), and standard hemodynamic parameters were continuously recorded. Fluid responsiveness was defined as an increase in SV >15%, after a 500 ml colloid (HEA 130/0.4) infusion over 10 minutes. StO2 measurements and VOT (sphygmomanometer inflated until >50 mmHg above systolic pressure and kept inflated until StO2 decreased to 40%) were performed after anaesthesia induction (baseline), and before (PPV >13%, therefore defining hypovolemia) and after fluid challenge.
At baseline, no patient was preload dependent (mean PPV 7.9 ± 2%), and none required vasopressors during the procedure. Baseline mean StO2 was 85 ± 6%, while CO and SV were 4.9 ± 0.9 l/minute and 68 ± 21 ml, respectively. During hypovolemia, compared with baseline, there was no difference in both StO2 (85 ± 6 to 84 ± 9%, P = 0.76) and StO2 desaturation rate (-9.6 ± 1.6 to -11 ± 2.7%/minute, P = 0.10), while the StO2 recovery slope was lowered (5.1 ± 1.6 to 3.8 ± 1.5%/second, P = 0.04). Patients were all responders to fluid challenge. After volume expansion, SV (84 ± 20 vs 62 ± 12 ml, P < 0.01) and CO (6.5 ± 2 vs 4.8 ± 1.2 l/minute, P < 0.01) were higher, with no significant changes in both StO2 and StO2 desaturation rate. Fluid challenge induced a 46% increase in StO2 recovery slope (P < 0.01; Figure Figure1),1), which was comparable with baseline.
Whilst redistribution of blood flow may occur during circulatory failure (vasoconstriction of lesser vital organs), impairment of microcirculation and tissue hypoperfusion indicators are of particular importance. Both hypovolemia and intravascular volume expansion are associated with significant modifications in the dynamic recovery slope of StO2. Future studies are needed to better clarify its potential clinical applications.