Effect of influenza-induced fever on human bioimpedance values

Abstract

Background and aims: Bioelectrical impedance analysis (BIA) is a widely used technique to assess body composition and nutritional status. While bioelectrical values are affected by diverse variables, there has been little research on validation of BIA in acute illness, especially to understand prognostic significance. Here we report the use of BIA in acute febrile states induced by influenza.

Methods: Bioimpedance studies were conducted during an H1N1 influenza A outbreak in Venezuelan Amerindian villages from the Amazonas. Measurements were performed on 52 subjects between 1 and 40 years of age, and 7 children were re-examined after starting Oseltamivir treatment. Bioelectrical Impedance Vector Analysis (BIVA) and permutation tests were applied.

Results: For the entire sample, febrile individuals showed a tendency toward greater reactance (p=0.058) and phase angle (p=0.037) than afebrile individuals, while resistance and impedance were similar in the two groups. Individuals with repeated measurements showed significant differences in bioimpedance values associated with fever, including increased reactance (p<0.001) and phase angle (p=0.007), and decreased resistance (p=0.007) and impedance (p<0.001).

Conclusions: There are bioelectrical variations induced by influenza that can be related to dehydration, with lower extracellular to intracellular water ratio in febrile individuals, or a direct thermal effect. Caution is recommended when interpreting bioimpedance results in febrile states.

Fig 1. Mean bioelectrical values of subjects with fever (red; N = 27) and without (black; N = 25).

R: resistance (Ohm); Xc: reactance (Ohm); H: height (m); Z(R/H) and Z(Xc/H): R/H and Xc/H standardized for sex and age using bioelectrical Italian standards.

Fig 2. Permutation tests for the subjects with (f) or without (wf) fever.

Sex and age observed standardized resistance, Z(R/H), and reactance, Z(Xc/H), box plots (panels a, b) and frequency distribution of permuted mean differences (panels c, d). Sex and age observed standardized phase and impedance box plots (panels e, f) and frequency distribution of permuted mean differences (panels g, h). The vertical dashed lines in the frequency graphs (panels c, d, g and h) show the corresponding observed mean difference. P-values for the corresponding permutation tests are reported under each histogram. R: resistance (Ohm); Xc: reactance (Ohm); H: height (m); phase: phase angle of the impedance vector (degrees); impedance: length of the impedance vector (Ohm).

Fig 3. Changes in body temperature after 24h of Oseltamivir treatment, and bioelectrical impedance vector analysis (BIVA).

Panel a: Body temperature in seven children at the time of initial BIVA (red bars) and then 24 hours after receiving the first dose of Oseltamivir (blue bars). "T 1^st" and "T 2nd” refer to the corresponding temperatures. Children’s ages are: 1, 10 y; 2, 10 y; 3, 12 y; 4, 9 y; 5, 16 y; 6, 11 y; 7, 5 y. Panel b: BIVA (Z score = standardized data of paired data from children with or without fever (T≥37.5°C, red dots; 37.1°C≤ T<37.5°C, yellow dots; T≤37°C, black dots), with direction of arrows showing the vector migration from the initial measurement.