Cardiorenal syndrome: pathophysiological crosstalk between heart and kidney

Cardiorenal syndrome (CRS) describes the bidirectional deterioration of cardiac and renal function, in which dysfunction of one organ induces or perpetuates dysfunction of the other. Since Robert Bright's 1836 observations and the formal five-subtype classification proposed by Ronco and colleagues in 2008, understanding of CRS has evolved from organ-sequential taxonomy toward a mechanism-driven, phenotype-based framework. This narrative review synthesizes contemporary evidence across the molecular, diagnostic, and therapeutic dimensions of CRS. We examine the convergent pathophysiological pathways underlying the syndrome - hemodynamic derangement, renin-angiotensin-aldosterone and sympathetic nervous system activation, systemic inflammation, oxidative stress, mitochondrial and endothelial dysfunction, and the recently characterized gut-heart-kidney axis - with particular emphasis on venous congestion as a unifying mechanistic driver. We review the global epidemiological burden of cardiorenal overlap, now estimated to affect over half of hospitalized heart failure patients, and appraise the diagnostic evolution from creatinine-dependent assessment toward multi-marker biomarker panels and point-of-care venous congestion ultrasonography (VExUS). On the therapeutic front, we highlight the shift toward combination pharmacotherapy - particularly finerenone-SGLT2 inhibitor co-administration - alongside device-based decongestive strategies and early applications of artificial intelligence in cardiorenal phenotyping. We argue that the traditional five-subtype classification, while pedagogically useful, insufficiently captures the overlapping, transitional phenotypes seen in clinical practice, and that congestion-quantified, mechanism-specific management increasingly supersedes rigid subtype-based decision-making. This review concludes by identifying priority areas for future investigation, including randomized congestion-guided decongestion trials and prospective validation of artificial intelligence-derived cardiorenal phenotypes, positioning CRS within the broader emerging construct of cardiovascular-kidney-metabolic syndrome.