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Objectives:  Excessive oxygen (O 2 ) administration may have a negative impact on tissue perfusion by inducing vasoconstriction and oxidative stress. We aimed to evaluate the effects of different inhaled oxygen fractions (FiO 2 ) on macro-hemodynamics and microvascular perfusion in a rat model.   Methods:  Isoflurane-anesthetised spontaneously breathing male Wistar rats were equipped with arterial (carotid artery) and venous (jugular vein) catheters and tracheotomy, and randomized into three groups: normoxia (FiO 2  21%,  n  = 6), hyperoxia (FiO 2  100%,  n  = 6) and mild hypoxia (FiO 2  15%,  n  = 6). Euvolemia was maintained by infusing Lactate Ringer solution at 10 ml/kg/h. At hourly intervals for 4 h we collected measurements of: mean arterial pressure (MAP); stroke volume index (SVI), heart rate (HR), respiratory rate (by means of echocardiography); arterial and venous blood gases; microvascular density, and flow quality (by means of sidestream dark field videomicroscopy on the hindlimb skeletal muscle).   Results:  MAP and systemic vascular resistance index increased with hyperoxia and decreased with mild hypoxia ( p  < 0.001 in both cases, two-way analysis of variance). Hyperoxia induced a reduction in SVI, while this was increased in mild hypoxia ( p  = 0.002). The HR increased under hyperoxia ( p  < 0.05 vs. normoxia at 3 h). Cardiax index, as well as systemic O 2  delivery, did not significantly vary in the three groups ( p  = 0.546 and  p  = 0.691, respectively). At 4 h, microvascular vessel surface (i.e., the percentage of tissue surface occupied by vessels) decreased by 29 ± 4% in the hyperoxia group and increased by 19 ± 7 % in mild hypoxia group ( p  < 0.001). Total vessel density and perfused vessel density showed similar tendencies ( p  = 0.003 and  p  = 0.005, respectively). Parameters of flow quality (microvascular flow index, percentage of perfused vessels, and flow heterogeneity index) remained stable and similar in the three groups.   Conclusions:  Hyperoxia induces vasoconstriction and reduction in skeletal muscle microvascular density, while mild hypoxia has an opposite effect.

Effects of Normoxia, Hyperoxia, and Mild Hypoxia on Macro-Hemodynamics and the Skeletal Muscle Microcirculation in Anesthetised Rats