Capillary refill time (CRT) has emerged as a validated resuscitation target in acute circulatory failure following the ANDROMEDA-SHOCK trials and the 2025 ESICM guidelines, yet the physiological mechanisms underlying its clinical utility remain incompletely understood. The vascular waterfall (VW), defined as the difference between critical closing pressure (Pcc) and mean systemic filling pressure (Pmsf), represents the pressure gradient driving microcirculatory flow. We hypothesized that CRT changes reflect VW changes during hemodynamic treatment in post-cardiac surgery patients.

This was a secondary analysis of a prospective observational study conducted in post-cardiac surgery patients with acute circulatory failure (n = 74). Patients were classified into vasoplegic (n = 30), preload-dependent (n = 33), or cardiogenic (n = 11) phenotypes and treated with norepinephrine, fluid expansion, or dobutamine, respectively. Hemodynamic parameters that comprise mean arterial pressure (MAP), indexed systemic arterial resistances (SARi), indexed venous resistances (VRi) were measured before and after treatment. VW was measured using the inspiratory hold technique before and after treatment. CRT was assessed using standardized methodology. Associations between CRT and VW changes were evaluated using Spearman correlations and multivariable logistic regression.

Patients with prolonged baseline CRT (>3 seconds, n = 55) had significantly lower VW compared to those with normal CRT (2.8 vs 18.3 mmHg, p = 0.014). Changes in CRT correlated significantly with changes in VW across the entire cohort (ρ = −0.40, p < 0.001). Subgroup analysis showed a significant correlation in preload-dependent patients (ρ = −0.37, p = 0.032), with similar trends in vasoplegic (ρ = −0.34, p = 0.065) and cardiogenic groups (ρ = −0.20, p = 0.555). Notably, ΔCRT correlated with ΔPcc (ρ = −0.42, p < 0.001) but not with ΔPmsf (ρ = −0.03, p = 0.81), indicating that CRT changes were more closely associated with the arterial component of vascular waterfall. In multivariable logistic regression, ΔCRT independently predicted VW response (OR 0.31 per second, 95% CI 0.12−0.84, p = 0.021), alongside ΔMAP, ΔSARi, and ΔVRi. Hemodynamic phenotype did not modify this association (p = 0.90).

CRT changes are associated with vascular waterfall changes in post-cardiac surgery patients, primarily through the relationship with critical closing pressure rather than mean systemic filling pressure. These findings may provide a physiological framework for CRT-guided resuscitation and support its potential role as a bedside marker for assessing microcirculatory perfusion pressure.