Acute kidney injury (AKI) emerged as a significant complication in critically ill COVID-19 patients, presenting as a major risk factor for mortality with a 13-fold increased likelihood of adverse outcomes (16, 17). Early studies have reported a wide range of incidences of AKI, varying from 20–90%, among patients admitted to the intensive care unit (ICU) or requiring mechanical ventilation in the context of COVID-19 (7, 11–13) Furthermore, AKI has been shown to necessitate renal replacement therapy in up to one-third of cases(14, 15) .
Doppler-based measurement of renal resistive index (RRI) offers a non-invasive and rapid approach for early diagnosis and evaluation of tissue oxygenation and hypoperfusion in ICU patients. In adults, normal RRI levels are typically below 0.7. Proposed mechanisms like renal microangiopathy and thromboembolism underline acute kidney failure in COVID-19 patients(18). Thus, renal artery indices should be examined potential as valuable tools for identifying and managing kidney failure in these patients(19).
According to our findings the RRI (Renal Arterial Resistive Index) was a crucial factor in predicting the occurrence of acute kidney injury (AKI) in COVID-19 patients. The multivariate logistic regression analysis revealed that patients with RRI ≥ 0.7 in at least one kidney (case group) were significantly more likely to develop AKI compared to those with RRI < 0.7 in both kidneys (control group). The odds of AKI were notably higher in the case group, with an odds ratio of 34.91 (p < 0.001). This suggests that an elevated RRI level, particularly when it exceeds 0.7, is a strong and independent predictor of AKI in COVID-19 patients. Patients with higher RRI values may have compromised renal blood flow and increased vascular resistance, which could be linked to a higher risk of developing AKI during the course of COVID-19 infection.
The results suggested that RRI is a reliable predictor of the occurrence of acute kidney injury, with patients having RRI values greater than 0.7 showing increased susceptibility to AKI. In line with our findings, a cross-sectional study conducted in 2020 across COVID-19 ICUs in Sweden included 51 patients, 23 of whom had AKI at the time of renal artery RI measurement. The mean renal artery RI was 0.76, with AKI patients showing a mean RI of 0.8, and non-AKI patients having a mean RI of 0.72. This study also highlighted that COVID-19 patients in the ICU with AKI exhibited significantly higher RI values compared to other ICU patients, further supporting the role of RI as a potential indicator for AKI in COVID-19 patients (20). In another study conducted in Mexico, the relationship between renal artery resistive index and urinary output was investigated in 65 COVID-19 patients admitted to the CCU. Among them, 22 patients (33.9%) developed acute kidney injury (AKI), with 15 of them having RI values greater than 0.7 while the others had RI values below 0.7. Similar to our findings, a significant difference was observed, indicating that patients with RI values above 0.7 had a higher incidence of AKI. Additionally, out of the 65 patients, 23 died, and the mortality rate was significantly higher in patients with RI > 0.7 compared to those with RI < 0.7 (21). A systematic review conducted in 2020, similar to our research, demonstrated a significant difference in renal artery RI between the group with acute kidney failure and the group without kidney failure. The review indicated that RI had a sensitivity of 79% and specificity of 72% in predicting acute kidney failure. It also identified the optimal time for RI evaluation to be immediately after major surgery, with a cutoff value above 0.716 serving as a prognostic indicator for the occurrence of kidney failure (22).
According to our findings, the Pulsatility Index (PI) of the kidneys was another significant predictor of AKI in the study population. The multivariate logistic regression analysis showed that for every unit increase in the mean of kidneys' PI (left/right), the odds of developing AKI decreased significantly. This means that patients with lower PI values were less likely to experience AKI compared to those with higher PI values. The odds ratio for PI was 0.16 (p < 0.001), indicating a strong inverse relationship between PI and AKI risk. A lower PI may reflect better renal perfusion and reduced vascular resistance, which could be associated with a decreased risk of AKI in COVID-19 patients.
The pulsatility index (PI), introduced by Gosling and King in 1974, is a measure of vascular resistance using Doppler ultrasound. It is also known as the Gosling index (24, 25). The pulsatility of a cardiac system is closely related to its elasticity, which is a fundamental characteristic (23). While PI has been investigated in several diseases such as atherosclerotic diseases, vascular malformations, diabetes, and neurological disorders, its role as a prognostic indicator of heart failure in all patients with COVID-19 has not been studied (26). A study conducted in 1995 on 21 patients with chronic kidney failure demonstrated that those with a pulsatility index (PI) greater than 1.55 or resistive index (RI) higher than 0.75 experienced a faster decline in kidney function (27). In a 2018 study involving 80 patients with diabetes, the researchers evaluated the renal artery's RI and PI indices. The mean RI was found to be 0.06 ± 0.72, and the mean PI was 0.24 ± 1.36. The study revealed a direct correlation between RI and PI indices with serum albuminuria and creatinine levels, while they were inversely related to glomerular filtration rate (GFR). However, the study did not analyze the period of acute kidney failure (28).
PI has been utilized as a predictor of complications in patients with hypertension and has shown a relationship with disease duration. Cho et al. investigated the PI of the middle cerebral artery (MCA) and internal carotid arteries in 94 patients with hypertension. Their findings indicated an increase in PI among patients with a history of hypertension longer than five years, whereas a shorter duration did not yield a significant difference (29). Bardelli et al. conducted a study investigating the role of renal artery PI in patients with high blood pressure and renal artery stenosis. The results showed that this index identified renal artery stenosis in 84% of cases, while angiography confirmed it in 94% of patients (30). To the best of our knowledge based on the database search, our study is the first to examine the role of PI in patients with COVID-19.
Despite the valuable insights gained from this study, there are several limitations to consider. Firstly, the lack of similar studies limited the comparison of our results, highlighting the need for further research with a larger and more diverse population to enhance statistical validity. Secondly, the observational nature of the study might be susceptible to confounding variables and cannot establish causal relationships. Thirdly, some patients initially examined by sonography had to be withdrawn from the study due to mortality during the follow-up period, which could have affected the completeness of the data. Despite these limitations, our findings provide valuable insights into the predictive potential of renal sonographic indices for AKI in critically ill patients. Further research is warranted to corroborate and expand upon our results.