Renal Resistive Index on Intensive Care Unit Admission Correlates with Tissue Hypoperfusion Indices and Predicts Clinical Outcome

Background: Recent advancements in the context of shock pathophysiology, support ultrasound assessment of organ perfusion. Renal resistive index (RRI) has been used to evaluate renal blood flow. Our aim was to investigate the relation between RRI, and global tissue hypoperfusion indices, in mechanically ventilated critically ill patients and their association with clinical outcome.Methods: In this prospective observational study, RRI was measured within 24 hours of intensive care unit (ICU) admission. Clinical and laboratory data, routine hemodynamic variables and gas exchange at the time of RRI assessment were recorded. The ratio of central venous-to-arterial carbon dioxide partial pressure difference by arterial-to-central venous oxygen content difference (P(cv-a)CO2/C(a-cv)O2) and lactate were used as global tissue hypoperfusion indices. Results A total of 126 mechanically ventilated patients were included [median age 61 (IQR 28) years, 74% males]. Seventy-seven patients had RRI values >0.7. P(cv-a)CO2/C(a-cv)O2 ratio and arterial lactate, were significantly higher in patients with RRI > 0.7 compared to those with RRI ≤0.7 [2.4 (2.2) versus 1.2 (0.6) and 2.88 (3.39) versus 0.62 (0.57) mmol/l respectively, both p<0.001)]. RRI was significantly correlated with P(cv-a)CO2/C(a-cv)O2 ratio and arterial lactate for the whole patient population (rho=0.64, both p<0.0001) as well as for the subset of patients with shock (rho=0.47, p=0.001; and r=0.64, p<0.0001 respectively).All-cause ICU mortality was 27.8%. Compared to survivors, ICU non-survivors had a higher RRI [0.80 (0.10) versus 0.70 (0.10), p<0.001] and higher P(cv-a)CO2 / C(a-cv)O2 ratio [3.67 (3.8) versus 0.91 (1.4)] and lactate levels [2.80 (2.00) versus 1.50 (1.20)], both p <0.001). Logistic regression models showed a significant association between RRI and P(cv-a)CO2/C(a-cv)O2 ratio with clinical outcome. RRI showed good ability to predict ICU mortality (AUC 74.9% (95% CI 61% - 88.8%). The combination of RRI with P(cv-a)CO2)/(C(a-cv)O2 ratio and lactate better predicted mortality than RRI alone [AUC 84.8% (95% CI 5.1% - 94.4%)] versus 0.74.9%, respectively, p<0.001).Conclusions: In mechanically ventilated patients, renal blood flow impairment, assessed by the RRI on ICU admission, correlates with global tissue hypoperfusion indices. In addition, RRI in combination with tissue perfusion estimation is more valuable in predicting clinical outcome than RRI alone.