Novel Pressure Wave Separation Analysis for Cardiovascular Function Assessment Highlights Major Role of Aortic Root

Abstract

Objective: A novel method was presented to separate the central blood pressure wave (CBPW) into five components with different biophysical and temporal origins. It includes a time-varying emission coefficient ( γ) that quantifies pulse wave generation and reflection at the aortic root.

Methods: The method was applied to normotensive subjects with modulated physiology by inotropic/vasoactive drugs (n = 13), hypertensive subjects (n = 158), and virtual subjects (n = 4,374).

Results: γ is directly proportional to aortic flow throughout the cardiac cycle. Mean peak γ increased with increasing pulse pressure (from <30 to >70 mmHg) in the hypertensive (from 1.6 to 2.5, P < 0.001) and in silico (from 1.4 to 2.8, P < 0.001) groups, dobutamine dose (from baseline to 7.5 μg/kg/min) in the normotensive group (from 2.1 to 2.7, P < 0.05), and remained unchanged when peripheral wave reflections were suppressed in silico. This was accompanied by an increase in the percentage contribution of the cardiac-aortic-coupling component of CBPW in systole: from 11% to 23% (P < 0.001) in the hypertensive group, 9% to 21% (P < 0.001) in the in silico group, and 17% to 23% (P < 0.01) in the normotensive group.

Conclusion: These results suggest that the aortic root is a major reflection site in the systemic arterial network and ventricular-aortic coupling is the main determinant in the elevation of pulsatile pulse pressure.

Significance: Ventricular-aortic coupling is a prime therapeutic target for preventing/treating systolic hypertension.

Fig. 1

(A) Comparison between (left) traditional decomposition of central blood pressure into forward (P_f, dashed) and backward (P_b, dotted) waves and (right) our new decomposition into five components with different biophysical and temporal origins: (i) microcirculation (grey), history (green), cardiac-aortic coupling (red), downstream reflections (blue) and aortic re-reflections (yellow). (B) Relation of emission coefficient, γ (dashed), computed as the ratio of P_f to P_b, to aortic flow wave (solid).

Fig. 2. Relation of emission coefficient, γ(t), to aortic flow, Q(t), waves.

(A) Relative root-mean-square error (RMSE) between normalized γ and Q for three different time spans (early systole until peak flow, late systole from peak flow to valve closure, and whole cardiac cycle) in the in silico, in vivo normotensive, and in vivo hypertensive groups. (B) Relative RMSE between measured and approximated γ calculated using Eqs. (2) and (4), respectively. (C) Scatter plot for the in vivo groups showing the direct relationship between the times of peak aortic γ, γ_peak, and peak aortic flow, Q_peak (coefficient of determination, R²=0.69). (D) Scatter plot for the in vivo groups comparing measured and approximated γ_peak, the latter calculated by Eq. (5) (R²=0.91).

Fig. 3

Variation of central pressure components and emission coefficient γ across 10-mmHg spans of pulse pressure (PP) in the in vivo hypertensive (left) and in silico (right) groups. (A, B) Pressure wave components obtained by the new WSA. (C, D) Relative contribution of each pressure component up to valve closure (straight vertical lines) expressed as a percentage of the area under the pressure wave during systole. (E, F) Mean emission coefficients (solid lines) and their standard deviations (shaded areas) calculated using Eq (2).

Fig. 4

Variation of central pressure components and emission coefficient γ in a subset of the in vivo normotensive group (n = 10) with increasing doses of dobutamine, with the same format as Fig. 3.

Fig. 5

Effect of peripheral reflections on central pressure components and emission coefficient γ(t) for the 25 y.o. baseline subject of the in silico group. (A, B) Pressure wave components obtained by the new WSA applied to the baseline subject with normal (A) and fully absorbent (B) terminal boundary conditions (BCs). (C) Relative contribution of each pressure component up to valve closure (straight vertical lines) for the normal and absorbent cases, expressed as a percentage of the area under the pressure wave during systole of the model with normal terminal BCs. (D) Emission coefficient calculated using Eq. (2) with normal (blue) and fully absorbent (red) terminal BCs, with the relative root man square error (RMSE) between the two curves in systole shown in the legend.