To the Editor:
Bioelectrical impedance analysis is a noninvasive and painless technology developed to estimate the bioimpedance of the human body at the bedside by measuring body resistance and reactance to electrical current. The relationship between those two raw electrical components is illustrated by the phase angle, reflecting cell membrane integrity and water distribution in tissues. In cardiac surgery, it was reported that a low preoperative phase angle is associated with postoperative mortality at 1 and 12 months,1 and that a postoperative decline of the phase angle is associated with intraoperative risk factors, such as cardiopulmonary bypass duration or fluid balance.2 However, as its clinical utility is not demonstrated, we aimed to explore whether the phase angle alone or combined with hemodynamic and myocardial damage parameters could be used to predict patient outcomes.
We conducted a single-center historical cohort study of 204 patients admitted in the intensive care unit (ICU) of the Louis Pradel teaching university hospital (Hospices Civils de Lyon, France) between March 23 and September 27, 2021. The primary objective was to evaluate the discrimination of the phase angle to predict a composite endpoint comprising in-hospital mortality and prolonged length of stay. The secondary objectives were to compare the phase angle prognostic performance to current severity scoring systems before and after cardiac surgery, and to evaluate the added value of combining phase angle with anthropometric data,3 vasoactive inotropic score4 at the time of the bioelectrical impedance analysis, and cardiac troponin I at day 1.5 Patients older than 18 yr and who underwent a bioelectrical impedance analysis within 6 h after ICU admission were included. The exclusion criteria were cardiac transplantation, invalid bioelectrical impedance analysis measurements, limitations due to life-sustaining therapies with restriction to only palliative care, and patient refusal regarding the use of health data. The section regarding the method of bioelectrical impedance analysis measurement, the ethics statement, a description of the data collection, and the detailed statistical analysis are provided in Supplemental Digital Content 1 (https://links.lww.com/ALN/D543). The study protocol was registered in clinicaltrials.gov (NCT05229159).
A detailed flow chart of the patients analyzed is reported in supplemental figure 1 in Supplemental Digital Content 2 (https://links.lww.com/ALN/D544). The median [25th to 75th percentile] hospital length of stay was 9 days [7 to 10 days]; hence, a prolonged length of stay was defined as a postoperative hospital length of stay greater than 10 days. The composite primary endpoint was found in 60 patients (30%). The baseline characteristics of the population according to phase angle tertiles are reported in table 1, and the severity scoring systems as well as patient outcomes are reported in supplemental table 1 in Supplemental Digital Content 2 (https://links.lww.com/ALN/D544). The median [25th to 75th percentile] time between ICU admission and bioelectrical impedance analysis was 97 min [46 to 174 min]. The mean ± SD phase angle was 5.1° ± 1°; considered as a continuous variable, it was significantly associated with in-hospital mortality or prolonged hospital length of stay, before (odds ratio, 0.54; 95% CI, 0.38 to 0.76) and after adjustment on sex, age, and body mass index (odds ratio, 0.48; 95% CI, 0.29 to 0.76), or on European System for Cardiac Operative Risk Evaluation (EuroSCORE II; odds ratio, 0.66; 95% CI, 0.45 to 0.94). The prognostic performances of the phase angle as well as those of the other severity scoring systems are reported in table 2. The calibration curves to predict the primary endpoint are reported in supplemental figure 2 in Supplemental Digital Content 2 (https://links.lww.com/ALN/D544). In the parsimonious multiparametric model used to predict the primary endpoint, the vasoactive inotropic score, the body mass index, and the cardiac troponin at day 1 were associated with the phase angle (supplemental table 2, Supplemental Digital Content 2, https://links.lww.com/ALN/D544). The area under the receiver operating characteristic curve of the multiparametric model was significantly higher than that of the phase angle alone to predict in-hospital mortality or prolonged hospital length of stay. The likelihood ratio chi-square was 45 (degrees of freedom, 3; P < 0.01) for the comparison between the phase angle alone and the multiparametric model to predict in-hospital mortality or prolonged hospital length of stay, and was 37 (degrees of freedom, 3; P < 0.01) to predict in-hospital mortality. Decision curves analysis of the phase angle alone and of the multiparametric model are provided in supplemental figure 3 in Supplemental Digital Content 2 (https://links.lww.com/ALN/D544).
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Table 1.: Baseline Characteristics According to Phase Angle Tertiles
Table 2.: Prognostic Performance of the Phase Angle and Different Severity Scoring Systems to Predict In-hospital Mortality or Prolonged Length of Stay
The current study found that a lower phase angle, measured within the 6 h after ICU admission for postoperative care after cardiac surgery with cardiopulmonary bypass, was significantly associated with in-hospital mortality or prolonged hospital length of stay. Alone, its prognostic performance was poor and lower than those of the current severity scoring systems. However, combining the phase angle with the vasoactive inotropic score, the body mass index, and the cardiac troponin at day 1 significantly improved the performance of the model. When compared to other severity scoring systems, the overall prognostic performance, analyzed using Nagelkerke’s R2, was better for the multiparametric model, even though it is simpler. The current study has several limitations, such as its single-center design, the small sample size, and the choice of the outcomes to predict. Although a strict statistical analysis plan was followed with an internal validation using bootstrap resampling, it was not designed to provide a multiparametric prediction model based on a large selection of candidate variables, with a data split in a training, and a validation set. In the ICU, a consistent association of the phase angle with the outcomes, independent of the current severity scoring systems, was reported in the literature. In our opinion, this supports its use as a candidate variable7 for future severity scoring systems. The current study suggests the relevance of an approach combining the phase angle and hemodynamic and myocardial damage parameters to predict outcomes in postoperative care after cardiac surgery. However, a validation in an external multicenter cohort is required.
Research Support
Inbody (Seoul, Korea) kindly issued the Inbody S10 but did not participate in the study or the redaction of the manuscript. This research did not receive any specific grant from any funding agency in the public, commercial, or not-for-profit sectors.
Competing Interests
The authors declare no competing interests.
Supplemental Digital Content
Supplemental Digital Content 1: Methods, https://links.lww.com/ALN/D543
Supplemental Digital Content 2: Supplemental tables and figures, https://links.lww.com/ALN/D544
References
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