Effects of assessment and processing techniques on the shape of arterial pressure-distension loops

The dynamic mechanical characteristics of the arterial wall, especially its alinearity and viscoelastic behaviour, are revealed by pressure-distension loops, where the change in diameter (distension) of a lumen is plotted as a function of the change in transmural pressure. For linearity, the pressure-distension loop should be closed and straight, while viscous behavior results in an open loop. For a correct interpretation of the loops, it is essential that both distension and pressure are recorded simultaneously at the same location and processed by circuitries having the same frequency characteristics. The present paper aims at quantifying the effect of misalignment in recording position and of mismatch in frequency characteristics by analyzing the distension and pressure waveforms obtained at known locations in a phantom rig under pulsatile conditions with equipment with a validated frequency response. It is concluded that misalignment of only a few millimeters or mismatch in frequency response, even if the cutoff frequencies are far beyond the signal frequencies of interest, strongly affects the shape of the pressure-distension loop. If the forward and reflected pressure wave partially coincide, position misalignment cannot adequately be corrected for. The frequency mismatch, however, can be corrected for by crosswise preprocessing a waveform by a filter with the frequency response of the registration equipment used for the recording of the other waveform. It is important to consider these artifacts in interpreting pressure-distension loops in the clinical situation.