The effects of variations in thoracic blood volume and intravenous sodium loading on plasma levels of NTproANP and NTproBNP in healthy volunteers

Background

The N-terminal prohormones of the natriuretic peptides ANP and BNP (NTproANP and NTproBNP) are accepted markers of myocardial dysfunction [1] and are increasingly used in critically ill patients. While it is well recognized that plasma levels of ANP are altered by variations in thoracic blood volume and during intravenous sodium loading [2], only incomplete data are available on the course of BNP during these interventions [3].

Materials and methods

Two groups of eight healthy subjects were randomly tilted into a 15°-feet-down (FD) or a 15°-head-down (HD) position. Ten volunteers were crossover subjected to an infusion of 15 ml/kg NaCl 0.9% (over 60 min) or control during an observation period of 10 hours. Blood was sampled at timed intervals. NTproANP and NTproBNP were determined by radiochemiluminescence and electrochemoluminiscence immunoassays, respectively.

Results

NTproANP levels (in % of baseline levels) were higher (P < 0.05) during HD (124 ± 13%) than during FD (82 ± 6%), while NTproBNP levels were not affected by tilting. Following sodium loading (Fig. 1), plasma NTproANP levels increased immediately and returned back to baseline after 8 hours. In contrast, NTproBNP levels increased 3 hours after infusion and had doubled at the end of the observation period.

Figure 1

The course of plasma levels of NTproANP and NTproBNP in 10 healthy volunteers during the control protocol (○) and the sodium loading protocol (□). Data are mean ± SEM. *P < 0.05, between-group difference. ^§P < 0.05, versus baseline value. Friedman's test and Wilcoxon's matched pairs test as appropriate.

Conclusions

Besides relevant physiological implications – a sequential increase of NTproANP and NTproBNP after sodium loading has not been shown previously in humans – our data show that NTproBNP levels are influenced by sodium infusion. This may be relevant for the interpretation of NTproBNP levels in critically ill patients.