• Tadayoshi Kurita, Shingo Kawashima, Koji Morita, and Yoshiki Nakajima.
• From the Department of Anaesthesiology and Intensive Care, Hamamatsu University School of Medicine, Hamamatsu, Japan (TK, SK, KM, YN).
• Eur J Anaesthesiol. 2019 Jul 1; 36 (7): 531-540.

BackgroundThe impact of blood pressure changes on tissue oxygenation differs between vital organs and with blood volume conditions.ObjectiveTo assess cerebral and renal autoregulation simultaneously and compare the impact of blood pressure, hypovolaemia and fluid resuscitation on tissue oxygenation using near-infrared spectroscopy.DesignAnimal observational study.SettingAn animal laboratory in Hamamatsu University School of Medicine, Hamamatsu, Japan, from April 2018 to August 2018.AnimalsFifteen pigs, (mean ± SD) 25.2 ± 0.4 kg.InterventionsThe pigs were anaesthetised with 2.5% isoflurane and phenylephrine 0.5, 1, 2 and 5 μg kg min was administered in a stepwise fashion at 10-min intervals (baseline), followed by similar administration of sodium nitroprusside. Hypovolaemia was induced by a 600-ml bleed (33% of estimated total blood volume). Then phenylephrine was administered again (same protocol). Hypovolaemia was reversed by infusion of 600-ml hydroxyethyl starch. Phenylephrine and sodium nitroprusside were then administered again (same protocol).Main Outcome MeasuresAverage of the relation between mean arterial pressure (MAP) and cerebral or renal tissue oxygenation index (TOI) and individual TOI response during vasoactive drug infusions.ResultsThe average relationship between MAP and cerebral or renal TOI both showed classic autoregulation patterns, whereas the renal TOI was more pressure-dependent than the cerebral TOI. Hypovolaemia shifted the relationship downward, reducing the cerebral and renal TOIs by approximately 5 and 20%, respectively, at similar MAPs. Subsequent fluid resuscitation preserved the autoregulatory pattern in both organs, not changing cerebral TOI but reducing renal TOI to 10% under baseline. TOI responses in both organs included paradoxical changes (tissue oxygenation changed inversely with MAP) in 60% of animals. Animals with paradoxical reactions maintained more stable cerebral and renal oxygenation.ConclusionRenal oxygenation is more pressure-dependent than pressure-tolerant cerebral oxygenation, and autoregulation is not robust. Renal oxygenation decreased four-fold compared with cerebral oxygenation during hypovolaemia and two-fold during isovolaemic anaemia. Thus, paradoxical responses are part of normal autoregulatory function and beneficial for maintaining stable oxygenation.

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