Assessment of cerebral and renal autoregulation using near-infrared spectroscopy under normal, hypovolaemic and postfluid resuscitation conditions in a swine model

BACKGROUND The impact of blood pressure changes on tissue oxygenation differs between vital organs and with blood volume conditions. OBJECTIVE To assess cerebral and renal autoregulation simultaneously and compare the impact of blood pressure, hypovolaemia and fluid resuscitation on tissue oxygenation using near-infrared spectroscopy. DESIGN Animal observational study. SETTING An animal laboratory in Hamamatsu University School of Medicine, Hamamatsu, Japan, from April 2018 to August 2018. ANIMALS Fifteen pigs, (mean ± SD) 25.2 ± 0.4 kg. INTERVENTIONS The pigs were anaesthetised with 2.5% isoflurane and phenylephrine 0.5, 1, 2 and 5 μg kg−1 min−1 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 MEASURES Average of the relation between mean arterial pressure (MAP) and cerebral or renal tissue oxygenation index (TOI) and individual TOI response during vasoactive drug infusions. RESULTS The 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. CONCLUSION Renal 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.