Correlation between impedance cardiography and b-type natriuretic peptide levels in dyspneic patients

Background: Both serum measurements of B-type natriuretic peptide (BNP) and impedance cardiography (ICG) are used to evaluate patients for congestive heart failure (CHF) in the Emergency Department (ED), but the agreement between the data obtained by these two methods remains uncertain. Study Objectives: We sought to measure the correlation between BNP levels and ICG measurements in patients presenting to the ED with dyspnea, and compare the diagnostic accuracy of each method in diagnosing CHF. Methods: We performed a prospective observational study of a convenience sample of patients presenting to the ED with dyspnea and being evaluated for CHF as a cause of their symptoms. An ICG measurement was obtained on each patient and the correlation between BNP level and the ICG parameters cardiac index (CI), systemic vascular resistance (SVR), and thoracic fluid content (TFC) was measured. To further quantify the diagnostic accuracy of ICG and BNP, we then constructed receiver operating characteristic curves based on discharge diagnosis of CHF and compared the area under the curve (AUC) of each test. Results: Out of 54 patients enrolled, correlation was poor between BNP and CI (Spearman's ρ = -.07, p = 0.64) and between BNP and SVR (Spearman's ρ = -.10, p = 0.46), but moderate between BNP and TFC (Spearman's ρ = .32, p = 0.02); a wide degree of scatter was seen in all correlations. BNP levels showed the best diagnostic accuracy for a discharge diagnosis of CHF, with an AUC of .77 (95% confidence interval .59.95), whereas CI had an AUC of .72 (95% confidence interval .55.88). Conclusion: We found limited correlation between BNP levels and ICG parameters, suggesting that, in our population, the two tests may not consistently give similar information. BNP level and CI both provided only fair diagnostic accuracy for discharge diagnosis of CHF.