Clinical meaning of brain natriuretic peptide in the intensive care unit

Introduction

The aim of this study is to evaluate differences in brain natriuretic peptide (BNP) dosage (vn < 100 pg/ml) during the weaning procedure, in patients with heart disease (HD) vs patients without HD, admitted to the ICU.

Methods

Ten patients with HD (group A) and 33 patients without HD (group B) were studied by BNP dosage in three specific steps: (1) admission to the ICU; (2) before the extubation (performed if patients, after the end of mechanical ventilation and 1 hour of connection to T-tube, were considered suitable for extubation); and (3) 24 hours after extubation. Necessity of noninvasive ventilation or reintubation after the extubation was considered as weaning failure. Data are shown as the mean ± standard error of the mean; nominal variables were analyzed with the chi-square test and the risk ratio (RR) with 95% confidence interval (IC95) was performed; intragroup ordinal variables were analyzed with the Wilcoxon test (W), intergroup ordinal variables were analyzed with the Mann–Whitney test (MW). The receiver operative characteristic test was used to discriminate the BNP cutoff value with sensibility, specificity and respective IC95, between group A and group B. P < 0.01 is taken as statistically significant.

Results

Weaning failure in group A occurred in 50% of patients vs 12% of group B patients (P < 0.01 chi-square test; RR = 4.13, IC95 = 1.36–12.49). BNP value differences in group A are not significant (step 1: 662 ± 147 pg/ml; step 2: 769 ± 171 pg/ml; step 3: 843 ± 167 pg/ml), while BNP value differences in group B are statistically significant (W) (step 1: 130 ± 21 pg/ml; step 2: 236 ± 41 pg/ml, P < 0.01 vs step 1; step 3: 375 ± 75 pg/ml, P < 0.001 vs step 1 and P < 0.01 vs step 2). There are statistically significant differences between group A and group B in every step (P < 0.01 MW). The BNP cutoff value to discriminate group A from group B is 274 pg/ml with sensibility 90 (IC95 = 55–98) and specificity 79 (IC95 = 61–91).

Conclusion

Risk of weaning failure is increased four times in patients with HD. BNP values of group B patients are higher than normal people probably because the heart of ICU patients is submitted to different kinds of stress; therefore the BNP cutoff value to consider for discrimination of patients with HD from patients without HD in the ICU should be higher. BNP production in ICU patients with good performance of the heart is the right protective response to stress performed by therapy adopted during the ICU stay, this response is absent in patients with HD because their hearts already work in safety mode.