Background: Velocity/pressure (Vel/P) loops are obtained by combining aortic blood velocity (measured by esophageal Doppler-ED-, CombiQ™, Deltex Medical, Chichester, UK) and arterial pressure signals. They represent a tool to estimate afterload of the heart and arterial stiffness with at least two remarkable angles: β and γ. Pressure is usually measured in the radial artery (PRad) rather than in the descending thoracic aorta (PAoDesc) where ED measures blood flow. Our aims were to assess the influence of the site of pressure recording on the values of β and γ and to develop a mathematical transfer function (TF) to estimate PAoDesc from PRad and then reconstruct Vel/PTFAoDesc loops.

Methods: After institutional review board approval (CE SRLF n#17611-356), 15 patients scheduled for elective endovascular neuroradiology were included. Pressures were recorded simultaneously in the radial artery and in the aorta. Vel/PRad and Vel/PAoDesc loops were constructed and compared. A transfer function was estimated using an autoregressive-exogenous (ARX)[1] model to obtain a simulated descending thoracic aorta pressure waveform (PTFAoDesc). The estimation was quantified by the normalized root mean squared error (NRMSE).Vel/PTFAoDesc loops were constructed and compared to Vel/PAoDesc loops.

Results: 153 loops were analysed. β and γ angles were systematically lower in the Vel/PRad compared to the Vel/PAoDesc loops (36 º [34 º – 40 º] vs. 43 º [38 º – 48 º] for β, 11 º [3 º −15 º] vs 25 º [13 º −30 º] for γ, p < 0.0001). The ARX model simulated PTFAoDesc with a NRMSE of 93% [77 – 96]. β and γ obtained with Vel/PAoDesc and Vel/PTFAoDesc were similar and strongly correlated ρ = 0.96, p < 0.0001) (Fig 1&2)

Conclusions: The location where the arterial pressure is monitored has a huge influence on the Vel/P loop parameters. Using a transfer function improves the estimation of the pressure waveform at the site of the Doppler signal.