Non-invasive venous waveform analysis (NIVA) for volume assessment in patients undergoing hemodialysis: an observational study

Abstract

Background: Accurate assessment of volume status to direct dialysis remains a clinical challenge. Despite current attempts at volume-directed dialysis, inadequate dialysis and intradialytic hypotension (IDH) are common occurrences. Peripheral venous waveform analysis has recently been developed as a method to accurately determine intravascular volume status through algorithmic quantification of changes in the waveform that occur at different volume states. A noninvasive method to capture peripheral venous signals is described (Non-Invasive Venous waveform Analysis, NIVA). The objective of this proof-of-concept study was to characterize changes in NIVA signal with dialysis. We hypothesized that there would be a change in signal after dialysis and that the rate of intradialytic change in signal would be predictive of IDH.

Methods: Fifty subjects undergoing inpatient hemodialysis were enrolled. A 10-mm piezoelectric sensor was secured to the middle volar aspect of the wrist on the extremity opposite to the access site. Signals were obtained fifteen minutes before, throughout, and up to fifteen minutes after hemodialysis. Waveforms were analyzed after a fast Fourier transformation and identification of the frequencies corresponding to the cardiac rate, with a NIVA value generated based on the weighted powers of these frequencies.

Results: Adequate quality (signal to noise ratio > 20) signals pre- and post- dialysis were obtained in 38 patients (76%). NIVA values were significantly lower at the end of dialysis compared to pre-dialysis levels (1.203 vs 0.868, p < 0.05, n = 38). Only 16 patients had adequate signals for analysis throughout dialysis, but in this small cohort the rate of change in NIVA value was predictive of IDH with a sensitivity of 80% and specificity of 100%.

Conclusions: This observational, proof-of-concept study using a NIVA prototype device suggests that NIVA represents a novel and non-invasive technique that with further development and improvements in signal quality may provide static and continuous measures of volume status to assist with volume directed dialysis and prevent intradialytic hypotension.

Keywords: Dialysis; Monitoring; Venous waveform analysis.

Fig. 1

NIVA prototype. (a) The NIVA device consists of a piezo electric crystal sensor and housing control box that is (b) applied to the surface of the skin over the venous plexus at the volar aspect of the wrist. The current from the crystal is transferred via wire to the control box where the signal is amplified and converted to a digital signal and transferred via USB port to a computer for analysis

Fig. 2

Representative waveforms in the time and frequency domains. Raw waveform (top) and fast Fourier transform (bottom) of signals taken (a) prior to and (b) post-dialysis. (c) Description of technique for calculation of a NIVA value. c_1–3 are weighted constants, p_f0-f2 represent the powers of f₀-f₂

Fig. 3

Average NIVA value pre- and post- HD. NIVA values decreased significantly after dialysis compared to pre-dialysis levels (*p < 0.05, n = 38)

Fig. 4

Prediction of IDH with changes in NIVA values and heart rate. (a) Slopes of the least-squares model for NIVA over time were significantly steeper in patients with IDH. (b) Slopes of the least-squares model for NIVA over time were significantly steeper prior to the onset of IDH in patients with IDH. IDH occurred at a mean dialysis time of 68 min (range 15–210 min) (c) Slopes of the least-squares model for NIVA over time were significantly steeper prior to the onset of IDH in patients with IDH compared to patients without IDH (D) These slopes were able to predict the onset of IDH with an AUC = 0.87, sensitivity of 80% and specificity of 100% (n = 16)