The primary outcome was a composite of VTE, which included lower limb deep vein thrombosis (DVT), pulmonary embolism (PE), and thrombosis at the central catheter insertion site(s). The secondary outcomes were the cumulative incidences of each individual VTE event, as well as plasma concentrations of D-dimers, platelets, and fibrinogen. The presence of DVT was defined by the absence of complete compression of the investigated vessel, regardless of whether hyperechogenic clots were visualized. Pulmonary computed tomography angiography (CTA) was ordered at the discretion of the attending physician, and PE was defined as arterial occlusion with failure to enhance the entire lumen due to a large filling defect or a partial filling defect surrounded by contrast agent. A thoracic radiologist independently reviewed the diagnosis of PE, and the cases were classified according to the approximate central location as main artery, lobar, segmental, or subsegmental. Scans were considered negative if adequate contrast and no filling defects were observed within the pulmonary artery.

Arterial thrombotic events (myocardial infarction or stroke) were considered exploratory outcomes and were adjudicated by two researchers (ACTO and THR). Myocardial infarction was defined by the presence of elevated troponin I ultrasensitive and at a least one of the following: new ischemic electrocardiogram, imaging evidence of new loss of viable myocardium or new wall abnormality in a pattern consistent with ischemic etiology, or the identification of a coronary thrombus by angiography or autopsy. Since all patients were mechanically ventilated at the study entry, stroke was defined based on the findings of the head CT (new hypoattenuation described by an independent radiologist). We considered only findings of acute or subacute stroke as events.

The required sample size was calculated to be 68 patients using G*Power software (version 3.1) to detect a difference of 27% between the groups, with 90% power and an alpha error of 5%. Data were presented as mean ± standard deviation, median (interquartile range [IQR]), or number (percentage). Inter-group comparisons were performed using one-way analysis of variance, the Mann–Whitney test, or the Fisher test, as appropriate. Missing data were not imputed. To assess the diagnostic accuracy of D-dimer concentrations for predicting the composite outcome, we constructed a receiver operating characteristic curve using the nearest D-dimer concentration before the event of interest. Plots with smoothed lines were created to visually assess the changes in the biochemical measurements over time.

Logistic regression was performed with VTE as the outcome and covariates were included in the multivariate model based on univariate p-values of  < 0.1. Results were presented as odds ratios (ORs) and 95% confidence intervals (CIs). A Cox regression analysis with mortality as outcome was used to calculate hazard ratios (HR). Firth’s penalization method was used to reduce the small-sample bias (“nonevents” in the control group) of the maximum likelihood coefficient [13]. Statistical analyses were performed using R software (version 3.5.1, The R Foundation) and differences were considered statistically significant at p-values of  < 0.05.

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