Measurement uncertainty in pulmonary vascular input impedance and characteristic impedance estimated from pulsed-wave Doppler ultrasound and pressure: clinical studies on 57 pediatric patients

Abstract

Pulmonary vascular input impedance better characterizes right ventricular (RV) afterload and disease outcomes in pulmonary hypertension compared to the standard clinical diagnostic, pulmonary vascular resistance (PVR). Early efforts to measure impedance were not routine, involving open-chest measurement. Recently, the use of pulsed-wave (PW) Doppler-measured velocity to non-invasively estimate instantaneous flow has made impedance measurement more practical. One critical concern remains with clinical use: the measurement uncertainty, especially since previous studies only incorporated random error. This study utilized data from a large pediatric patient population to comprehensively examine the systematic and random error contributions to the total impedance uncertainty and determined the least error prone methodology to compute impedance from among four different methods. We found that the systematic error contributes greatly to the total uncertainty and that one of the four methods had significantly smaller propagated uncertainty; however, even when this best method is used, the uncertainty can be large for input impedance at high harmonics and for the characteristic impedance modulus. Finally, we found that uncertainty in impedance between normotensive and hypertensive patient groups displays no significant difference. It is concluded that clinical impedance measurement would be most improved by advancements in instrumentation, and the best computation method is proposed for future clinical use of the input impedance.

Figure 1

The percent systematic uncertainty in (a) pressure and (b) flow spectra moduli.

Figure 2

The percentage contribution of the systematic error to the total uncertainty in impedance: (a) modulus and (b) phase as computed by the four methods. Bars represent the sample standard deviation.

Figure 3

Representative plots of input impedance: (a) modulus and (b) phase and the associated total uncertainties calculated from the four methods. Bars represent the total uncertainty.

Figure 4

Percent total uncertainty in input impedance: (a) modulus and (b) phase for the four methods. Bars represent the sample standard deviation.

Figure 5

Percent total uncertainty in characteristic impedance modulus calculated from the four methods.

Figure 6

The percent systematic uncertainty in pressure spectrum modulus for (a) normotensive and (b) hypertensive groups.

Figure 7

The percentage contribution of the systematic error to the total uncertainty in input impedance (a) modulus and (b) phase for normotensive and hypertensive groups calculated from method 3 (see section 2.4 for details). Bars represent the sample standard deviation.

Figure 8

Percent total uncertainty in input impedance: (a)modulus and (b) phase for normotensive and hypertensive groups calculated from method 3 (see section 2.4 for details). Bars represent the sample standard deviation.