Critical closing pressure in the circulation: Understanding the vascular waterfall phenomenon

The vascular waterfall (VW) concept describes a key mechanism of blood flow autoregulation, linking arterial critical closing pressure (Pcrit) to mean systemic filling pressure (Pmsf). This pressure gradient defines tissue perfusion pressure (Pa - Pcrit) and supports a framework for interpreting circulatory dynamics and blood flow autoregulation in health and disease. This review explores the historical development, physiological foundations, clinical implications, and future directions of the VW model in critical care. Originating from Starling's resistor model, the VW arises when surrounding pressure, whether from vasomotor tone or interstitial forces, exceeds intraluminal arterial pressure, defining a Pcrit, causing flow to become independent of downstream pressure. In such conditions, Pcrit functions as the effective backpressure. The highly compliant capillary bed accommodates low inflow pressures, often 5-10 mmHg above Pmsf. Bedside techniques, such as inspiratory hold maneuvers, enable estimation of Pcrit and Pmsf as global circulatory parameters. In vasoplegia, such as sepsis, reduced vascular tone lowers Pcrit, often approaching Pmsf, which abolishes the VW and impairs autoregulation. Vasopressors may restore perfusion only if Pcrit increases more than Pmsf. Similarly, in pulmonary circulation, VW physiology helps explain how elevated alveolar and pleural pressures during mechanical ventilation, particularly with high PEEP, affect venous return and right ventricular load, with implications for ARDS management. Looking forward, non-invasive measurement of Pcrit, integration with continuous hemodynamic monitoring, and predictive modeling may enable real-time VW-guided therapy. By bridging macro- and microcirculatory physiology, the VW concept offers a promising basis for precision hemodynamic interventions in critically ill patients.