Abstract
Introduction:
Acute kidney injury (AKI) is frequently observed in patients with COVID-19 admitted to intensive care units (ICUs). Observational studies suggest that cardiovascular comorbidities and mechanical ventilation (MV) are the most important risk factors for AKI. However, no studies have investigated the renal impact of longitudinal covariates such as drug treatments, biological variations, and/or MV parameters.
Methods:
We performed a monocentric, prospective, longitudinal analysis to identify the dynamic risk factors for AKI in ICU patients with severe COVID-19.
Results:
Seventy-seven patients were included in our study (median age: 63 [interquartile range, IQR: 53-73] years; 58 (75%) men). Acute kidney injury was detected in 28 (36.3%) patients and occurred at a median time of 3 [IQR: 2-6] days after ICU admission. Multivariate Cox cause-specific time-dependent analysis identified a history of hypertension (cause-specific hazard (CSH) = 2.46 [95% confidence interval, CI: 1.04-5.84]; P = .04), a high hemodynamic Sequential Organ Failure Assessment score (CSH = 1.63 [95% CI: 1.23-2.16]; P < .001), and elevated Pa
Conclusion:
Acute kidney injury is frequent in patients with severe COVID-19 and is associated with a history of hypertension, the presence of hemodynamic failure, and increased P
Introduction
Acute kidney injury (AKI) is one of the most frequent organ dysfunction encountered in intensive care units (ICUs), particularly in patients admitted for acute respiratory distress syndrome (ARDS).1,2 Resulting from various mechanisms (inflammatory, hemodynamic, ischemic, etc), the occurrence of AKI affects short-term and long-term outcomes. 3
Since December 2019, ICUs around the world have faced a significant increase in admissions for a particular form of ARDS: severe COVID-19. It is estimated that AKI is frequently observed in patients with COVID-19. If several risk factors for AKI identified are mainly related to patients’ comorbidities (hypertension, diabetes, chronic kidney disease), mechanical ventilation (MV) seems to be the most affected. 4
The kidney-lung interaction concept is not new, and several animal models had already reported kidney alteration occurring after MV initiation through hemodynamic, immunoinflammatory, and/or neurohormonal mechanisms. 5 In humans, recent data published by Geri et al 6 suggested the relationship between increased positive end-expiratory pressure (PEEP) and the onset of AKI. However, although using an adapted model for longitudinal data, the absence of adjustment on other biomarkers and/or nephrotoxic drugs limited the interpretation.
We aimed to identify risk factors for AKI in a cohort of ICU patients admitted for severe SARS-CoV-2 infection.
Patients and Methods
Patients
All adult patients admitted to our ICU between March 1, 2020, and June 1, 2020, for acute respiratory failure with a positive SARS-CoV2 reverse transcription-polymerase chain reaction (RT-PCR) by nasopharyngeal swab or bronchoalveolar lavage (protocol based on the RdRp gene [nCoV_IP2 and nCoV_IP4] developed by the National Reference Center for Respiratory Viruses, Institut Pasteur, Paris) or SARS-CoV-2 serology were included (Figure 1).

Flowchart.
Included patients presented at least several characteristics suggesting onset of an ARDS, with chest radiograph showing bilateral opacities not fully explained by effusions, lobar/lung collapse, or nodules; clinical respiratory failure with polypnea or struggle to breath; and need for high levels of oxygen (MV, noninvasive ventilation [NIV], high-flow nasal oxygen [HFNO], or standard oxygen >10 L/min). Baseline characteristics of the patients are listed in Table 1.
Baseline Characteristics of the Study Population.
Note. IQR = interquartile range; BMI = body mass index; ICU = intensive care unit; RASS = rennin-angiotensin-aldosterone system; SOFA = Sequential Organ Failure Assessment; RAAS = RAAS = renin-angiotensin-aldosterone system.
RT-PCR Protocol
Because our cohort study is composed of patients admitted during first wave, no commercial RT-PCR kit for SARS-CoV-2 diagnosis was available. We used the protocol based on the RdRp gene (nCoV_IP2 and nCoV_IP4) developed by the National Reference Center for Respiratory Viruses, Institut Pasteur, Paris. This protocol is available on the World Health Organization Web site (https://www.who.int/docs/default-source/coronaviruse/real-time-rt-pcr-assays-for-the-detection-of-sars-cov-2-institut-pasteur-paris.pdf). Real-time RT-PCR was performed with the SuperScript III Platinum One-Step Quantitative RT-PCR System (Invitrogen, Waltham, MA) on the Roche LightCycler 480 Real-Time PCR Detection System. Primers and probes were provided by Eurogentec (Liege, Belgium).
Data Collected
Data collected at admission to the ICU were extracted: medical and surgical history, demographic characteristics, date of symptoms’ onset, date of PCR or serology positivity, and biological results. To these admission data were added the longitudinal follow-up data composed of hemodynamic monitoring with the need for vasoactive support, fluid load and fluid balance, mechanical ventilation parameters including volume and pressure parameters, prone positioning, administration of curare or nitrous oxide, nature and posology of nephrotoxic drugs administered, the need for kidney replacement therapy, all biological values, and the occurrence of ventilator-associated pneumonia and bacteremia over a maximum period of 14 days after admission. Vital status and discharge serum creatinine (SCr) levels were collected. Ethics discussions with decision of therapeutic limitation were also recorded.
Outcome
The primary outcome was the occurrence of AKI defined as any AKI ≥2 according to the KDIGO (Kidney Disease Improving Global Outcomes) classification (urine output [UO] and/or SCr). We limited our analysis to KDIGO stage 2 because the impact of KDIGO stage 1 on mortality is still on debate. 2 In case of discordance between UO and SCr, the worst staging was retained.
Baseline SCr is defined using the median value of all SCr measurements recorded in the 6 months to 1 year prior to admission. In the case of prior SCr not available, either a back-calculated value or ICU admission SCr was used as the baseline SCr. 7 Because France public health policies do not allow ethnic record information, back-calculated SCr was performed using the assumption of low-normal estimated glomerular filtration rate (eGFR) of 75 mL/min/1.73m2 and back-calculation of the associated SCr using the CKD Epidemiology Collaboration (CKD-EPI) without race equation 8 as follows:
with
Statistical Analysis
Categorical data were described as absolute numbers (%) and continuous data as medians [IQR]. Given the competing nature of the occurrence of AKI in the ICU with discharge from the ICU (alive or dead), a survival analysis adapted to the situation of competing risks (Cox cause-specific model regression) with a modeling of the cumulative incidence of AKI was performed. Data previously reported in the literature as a risk factor for AKI and those with a P < .20 in the univariate analysis were used to create a final multivariate model. A boostrap procedure according Austin et al 9 was used for multivariate selection process. This method uses bootstrap to assess the distribution of an indicator variable denoting the inclusion of a specific variable. Basically, if one covariate is selected frequently in models derived from bootstrap samples using the same selection method, it will be included in the final model. One hundred bootstrap replicates were used, and the choice of the stepwise method was backward stepwise selection. The cutoff for when to include or exclude a variable used was 50%. 10
Covariates daily collected and subjected to longitudinal variation were analyzed as time-dependent variables. In the case of nonlinearity of the observed continuous variables, a transformation of the continuous variables using Splines function was applied. Proportionality assumption was checked by the Schoenfeld residual analysis. Missing data were imputed using a multiple imputation chained equation methodology.
Two sensitive analyses were performed. We first realized the same analysis in the population study using the Fine-Gray model. 11 We next performed analysis in the subgroups of patients intubated and excluded patients treated by only noninvasive ventilation.
All statistical tests were performed with a risk α of 5%. All analyses were performed using R and R Studio software.
Results
Ninety-five patients were admitted to our ICU for severe COVID-19. Of these patients, 11 were excluded due to transfer to another ICU in the first 24 hours and 7 patients were excluded because of missing kidney status.
The median age was 63 [53-73] years, and male sex was predominant with 58 (75%) patients. Arterial hypertension was present in 36 (47%) patients and treated in 17 (47%) of them with a renin-angiotensin system blocker (angiotensin-converting enzyme inhibitor or angiotensin II receptor blocker). Diabetes was present in 20 patients (26%) (Table 1). Median length of stay in the ICU was 11 [6-23] days. More than three-quarters of the included patients used MV during treatment (77.9%). Introduction of neuromuscular blockade was present in more than 90% of ventilated patients, and the use of prone positioning was observed in 17% of cases. Initial MV parameters were characterized by PEEP levels of 10 [8-12] cm H2O, a plateau pressure of 24 [20-26] cm H2O, and a motor pressure of 14 [11-16] cm H2O. Intensive care unit admission gas exchange analysis showed a Pa
Acute kidney injury was observed in 28 (36.3%) (Supplemental Figure 1) patients and occurred at a median time of 3 [2-6] days (Supplemental Table 1).
Univariate analysis showed a significant statistical association between the use of invasive MV and the occurrence of AKI (cause-specific hazard (CSH) = 3.94 [1.18-13.2], P = .02). Concerning MV parameters, the level of PEEP (CSH = 1.11 [1.01-1.23] per 1 cm H2O increase, P = .04) and the use of neuromuscular blockade (CSH = 2.96 [1.22-7.18], P = .02) were associated with kidney outcome. No statistical association was observed with plateau pressure (CSH = 1.04 [0.99-1.10] per 1 cm increase in H2O, P = .08) and motor pressure (CSH = 1.04 [0.99-1.08] per 1 cm increase in H2O, P = .06). In multivariate Cox cause-specific time-dependent model, risk factors of AKI identified were a history of hypertension (CSH = 2.46 [1.04-5.84], P = .04), a high hemodynamic Sequential Organ Failure Assessment (SOFA) score (CSH = 1.63 [1.23-2.16], P < .001) and an elevated Pa
Time-Dependent Cox Cause-Specific Analysis of Acute Kidney Injury.
Note. BMI = body mass index; RAAS = renin-angiotensin-aldosterone system; PEEP = positive expiratory end pressure; SOFA = Sequential Organ Failure Assessment; CSH = cause-specific hazard.
All covariates are considered time-dependent.
Several variables were significantly associated with AKI with the Fine-Gray model: Pa
Discussion
After respiratory failure, AKI is the second most common organ dysfunction in patients with COVID-19. Although the mechanisms involved are still controversial, ischemia seems to be the most important one, given the high prevalence of acute tubular necrosis in ICU patients. 12
In our cohort, we observed that severe AKI is frequently observed and associated with 3 risk factors: a history of hypertension, a greater hemodynamic instability, and a more important hypercapnia.
The first 2 risk factors identified are widely described in the literature and can be linked to kidney hemodynamics. Hemodynamic management and optimal mean arterial pressure target have been advocated in the literature to prevent AKI. Our findings are consistent with the Acute Kidney Injury-Epidemiologic Prospective Investigation (AKI-EPI) study which observed arterial hypertension and/or shock at ICU admission as AKI risk factors. 2 These results are also consistent with the Sepsis and Mean Arterial Pressure (SEPSISPAM) trial that evidenced a significantly lower proportion of severe AKI and rate of kidney replacement therapy in patients with chronic hypertension when a higher blood pressure was targeted in ICU. 13
Our work highlights the impact of Pa

Boxplots of mean P
This study has several strengths, the first being the statistical methodology used. Study of complications in the ICU being by its nature dependent on the occurrence of competing events, failure to take this competition into account, as is the case with classical regression models, may be at the origin of a major bias. Furthermore, due to the longitudinal design of our study, it was important to be able to assess all the information occurring over time. Application of a cause-specific survival model with time-dependent covariates allows these 2 major features to be considered.
However, this study also has several limitations. The monocentric nature and relatively small size limit the statistical power and external validity of our results. Among our population, 17 patients (22 %) did not require invasive MV during their ICU stay and only 1 developed AKI. Sensitive analysis realized in the subgroup of invasive MV patients found similar results. Finally, we have chosen a particular ARDS study population: the severe forms of COVID-19. This could be limiting the extrapolation of our results. However, the hypothesis of a direct kidney involvement of SARS-CoV-2 is still debated. Moreover, the similarity of the incidences of kidney damage found in patients with ARDS or severe forms of COVID-19 pleads in the direction of an indirect physiopathological pathway of kidney lesions.1,4 Finally, an important limitation for the validation of our results is the absence of a control group (due to the retrospective design and the impossibility of matching because of the small size of our cohort).
Conclusion
Acute kidney injury in patients admitted to the ICU for COVID-19 is frequent and associated with the presence of 3 risk factors: a history of hypertension, the presence of hemodynamic failure, and increased P
Supplemental Material
sj-docx-1-cjk-10.1177_20543581221145073 – Supplemental material for Risk Factors of AKI in Acute Respiratory Distress Syndrome: A Time-Dependent Competing Risk Analysis on Severe COVID-19 Patients
Supplemental material, sj-docx-1-cjk-10.1177_20543581221145073 for Risk Factors of AKI in Acute Respiratory Distress Syndrome: A Time-Dependent Competing Risk Analysis on Severe COVID-19 Patients by Antoine Marchiset, Valerie Serazin, Omar Ben Hadj Salem, Claire Pichereau, Lionel Lima Da Silva, Siu-Ming Au, Christophe Barbier, Yann Loubieres, Jan Hayon, Julia Gross, Herve Outin and Matthieu Jamme in Canadian Journal of Kidney Health and Disease
Supplemental Material
sj-docx-2-cjk-10.1177_20543581221145073 – Supplemental material for Risk Factors of AKI in Acute Respiratory Distress Syndrome: A Time-Dependent Competing Risk Analysis on Severe COVID-19 Patients
Supplemental material, sj-docx-2-cjk-10.1177_20543581221145073 for Risk Factors of AKI in Acute Respiratory Distress Syndrome: A Time-Dependent Competing Risk Analysis on Severe COVID-19 Patients by Antoine Marchiset, Valerie Serazin, Omar Ben Hadj Salem, Claire Pichereau, Lionel Lima Da Silva, Siu-Ming Au, Christophe Barbier, Yann Loubieres, Jan Hayon, Julia Gross, Herve Outin and Matthieu Jamme in Canadian Journal of Kidney Health and Disease
Supplemental Material
sj-docx-3-cjk-10.1177_20543581221145073 – Supplemental material for Risk Factors of AKI in Acute Respiratory Distress Syndrome: A Time-Dependent Competing Risk Analysis on Severe COVID-19 Patients
Supplemental material, sj-docx-3-cjk-10.1177_20543581221145073 for Risk Factors of AKI in Acute Respiratory Distress Syndrome: A Time-Dependent Competing Risk Analysis on Severe COVID-19 Patients by Antoine Marchiset, Valerie Serazin, Omar Ben Hadj Salem, Claire Pichereau, Lionel Lima Da Silva, Siu-Ming Au, Christophe Barbier, Yann Loubieres, Jan Hayon, Julia Gross, Herve Outin and Matthieu Jamme in Canadian Journal of Kidney Health and Disease
Supplemental Material
sj-jpg-4-cjk-10.1177_20543581221145073 – Supplemental material for Risk Factors of AKI in Acute Respiratory Distress Syndrome: A Time-Dependent Competing Risk Analysis on Severe COVID-19 Patients
Supplemental material, sj-jpg-4-cjk-10.1177_20543581221145073 for Risk Factors of AKI in Acute Respiratory Distress Syndrome: A Time-Dependent Competing Risk Analysis on Severe COVID-19 Patients by Antoine Marchiset, Valerie Serazin, Omar Ben Hadj Salem, Claire Pichereau, Lionel Lima Da Silva, Siu-Ming Au, Christophe Barbier, Yann Loubieres, Jan Hayon, Julia Gross, Herve Outin and Matthieu Jamme in Canadian Journal of Kidney Health and Disease
Footnotes
Ethics Approval and Consent to Participate
This study was approved by the ethic committee of the french society of anesthesia and intensive care (Comission d’éthique pour la recherche en anesthésie et réanimation - CERAR) (IRB number : 00010254 - 2022 - 022).
Consent for Publication
All authors agreed to the publication of this manuscript.
Availability of Data and Materials
The data for this study is publicly available and the models used for the analysis are available upon request.
Authors’ Note
Presented at the 2020 Live Digital Congress of the European Society of Intensive Care Medicine (ESICM) and the 2021 Congress of the French Society of Intensive Care Medicine (SRLF).
Author Contributions
AM designed the study, collected the data, and drafted the manuscript. VZ, OBHS, CP, LLDS, SMA, CB, YL, JH, HO, and JG collected the data; contributed to data interpretation and analysis; and revised the manuscript for important intellectual content. MJ designed the study, performed the statistical analysis, contributed to data interpretation and analysis, and revised the manuscript for important intellectual content.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.
Supplemental Material
Supplemental material for this article is available online.
References
Supplementary Material
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