Kidney perfusion in critical illness: between the macrocirculation and the microcirculation

Purpose of review Acute kidney injury (AKI) remains a major challenge in critical care, with high morbidity and mortality. This review aims to highlight existing and upcoming tools to integrate macrocirculatory and microcirculatory perspectives to better understand and prevent AKI. Recent findings Hemodynamic optimization after initial resuscitation is currently based on central hemodynamic measurement of arterial/venous pressure and cardiac output, although they do not correlate well with kidney hemodynamics. Bedside ultrasound techniques, particularly contrast-enhanced ultrasound (CEUS), have uncovered impaired renal perfusion even when systemic flow appears adequate. Emerging high-frame-rate and super-resolution ultrasound methods promise to visualize renal microvessels at micrometer scales enabling true assessment of the microcirculation. Furthermore, urinary partial oxygen pressure monitoring provides continuous insight into medullary hypoxia. These diagnostics can be combined with biological phenotyping to define treatable AKI sub-phenotypes. Summary The integration of multimodal hemodynamic monitoring holds promise for identifying actionable AKI sub-phenotypes and guiding precision therapies. Future clinical trials should incorporate mechanistic endpoints from both the macrocirculatory and microcirculatory domains to improve our understanding of treatment effects, optimize trial design, and ultimately enhance patient outcomes in this high-risk population.