BACKGROUND Ultrasound measurement of the radial resistance index (RRI) in the anatomical snuffbox has been proposed as a useful method for assessing the systemic vascular resistance index (SVRI). This study aims to establish the correlation between SVRI measured by pulmonary artery catheter (PAC) and RRI. METHODS A cross-sectional study included all consecutive patients undergoing postoperative (POP) cardiac surgery with hemodynamic monitoring using PAC. Hemodynamic assessment was performed using PAC, and RRI was measured with ultrasound in the anatomical snuffbox. The Pearson correlation test was used to establish the correlation between RRI and SVRI measured using PAC. Hemodynamic behavior concerning RRI with a cutoff point of 1.1 (described to estimate under SVRI) was examined. Additionally, consistency between two evaluators was assessed for RRI using the intraclass correlation coefficient and Bland-Altman analysis. RESULTS A total of 35 measurements were obtained. The average cardiac index (CI) was 2.73 ± 0.64 L/min/m², and the average SVRI was 1967.47 ± 478.33 dyn·s·m²/cm5. The correlation between RRI and SVRI measured using PAC was 0.37 [95% CI 0.045-0.62]. The average RRI was 0.94 ± 0.11. RRI measurements > 1.1 had a mean SVRI of 2120.79 ± 673.48 dyn·s·m²/cm5, while RRI measurements ≤ 1.1 had a mean SVRI of 1953.1 ± 468.17 dyn·s·m²/cm5 (p = 0.62). The consistency between evaluators showed an intraclass correlation coefficient of 0.88 [95% CI 0.78-0.93], and Bland-Altman analysis illustrated adequate agreement of RRI evaluators. CONCLUSIONS For patients in cardiac surgery POP, the correlation between the SVRI measured using PAC and the RRI measured in the anatomical snuffbox is low. Using the RRI as a SVRI estimator for patients is not recommended in this clinical scenario.