Can tissue dielectric constant measurements assess circulating blood volume changes in patients undergoing haemodialysis?

BACKGROUND

The tissue dielectric constant (TDC) method uses an open-ended coaxial probe to achieve non-invasive measurement of water content in skin. The aim of our study was to test the hypothesis that the changes in circulating blood volume would be associated with the changes in TDC values in patients undergoing haemodialysis.

METHODS

In this prospective descriptive study, TDC measurements were performed for three parts of the body (the face, shin and hand) before and after patients underwent haemodialysis (N = 83). The primary outcome measure was the correlation between the amount of water removal and ΔTDC at each body site measured (ΔTDC = posthaemodialysis TDC-prehaemodialysis TDC). The secondary outcome measure was the mean difference in TDC value before and after haemodialysis.

RESULTS

The TDC values measured at each part of the body were significantly reduced after haemodialysis, but the percentage difference between pre- haemodialysis and posthaemodialysis was small for the face, shin and hand, with %mean ± SE values of -4.4 ± 0.70, -3.2 ± 0.98 and -6.0 ± 1.6; 95 per cent confidence intervals (lower bound to upper bound) of 3.0-5.8, 1.3-5.2 and 2.7-9.2; and P values of P = 0.000, P = 0.000 and P = 0.000, respectively. The inverse correlation between ΔTDC and the amount of water removal was also weak (correlation at the face, r = -0.25, P = 0.028; at the shin, r = -0.26, P = 0.018; and at the hand, no significant correlation).

CONCLUSION

Our results indicate that TDC measurement can be used to assess the changes in local oedema, but may be unlikely to evaluate real-time changes in the circulating blood volume in a clinical setting.