Case Studies in Physiology: Using premature ventricular contractions to understand the regulation of carotid artery longitudinal wall motion

Benign arrhythmias can be a useful tool to probe new hypotheses in physiology. We tested the control of longitudinal motion of the common carotid artery wall using observations from spontaneous premature ventricular contractions in a healthy male. Forwards wall motion remained unchanged despite large deviations in local blood velocity and backwards wall motion mirrored changes in pulse pressure, blood velocity, and cardiac motion, thereby revising our original hypothesis of the control of longitudinal wall motion.

A Hypothesized Mechanistic Model of Longitudinal Wall Motion at the Common Carotid Artery

By adopting a physics-physiology integrative approach, this work hypothesizes a mechanistic model of longitudinal wall motion ux(t) at the common carotid artery (CCA) and explores its antegrade amplitude ux0-ante and retrograde amplitude ux0-retro in systole for their clear implications. By examining the findings on ux(t) and other relevant findings on the cardiovascular (CV) physiology in the context of the physical essence of ux(t) and the CV system, a mechanistic model of ux(t) is hypothesized and formulated as a boundary-value problem of nonhomogeneous wave propagation in a semi-infinite domain. With the aid of findings on ux(t) and other related findings in the CV system, a scaling analysis of the boundary-value problem gives rise to two longitudinal arterial indices based on the two amplitudes: longitudinal elasticity Exx at the CCA being estimated from ux0-ante, and ux0-retro as an inverse indicator of the maximum base rotation of the left ventricle (LV) and a positive indicator of longitudinal elasticity at the ascending aorta (AA). The two arterial indices are qualitatively validated by their consistency with the effect of subclinical atherosclerosis and aging on the related parameters in the CV system. The implications of longitudinal elasticity to the LV function and energy transmission in the arterial tree are also discussed.

Left ventricular longitudinal wall fractional shortening accurately predicts longitudinal strain in critically ill patients with septic shock

Background Left ventricular longitudinal strain (LVLS) may be a sensitive indicator of left ventricular (LV) systolic function in patients with sepsis, but is dependent on high image quality and analysis software. Mitral annular plane systolic excursion (MAPSE) and the novel left ventricular longitudinal wall fractional shortening (LV-LWFS) are bedside echocardiographic indicators of LV systolic function that are less dependent on image quality. Both are sparsely investigated in the critically ill population, and may potentially be used as surrogates for LVLS. We assessed if LVLS may be predicted by LV-LWFS and MAPSE in patients with septic shock. We also assessed the repeatability and inter-rater agreement of LVLS, LV-LWFS and MAPSE measurements. Results 122 TTE studies from 3 echocardiographic data repositories of patients admitted to ICU with septic shock were retrospectively assessed, of which 73 were suitable for LVLS analysis using speckle tracking. The correlations between LVLS vs. LV-LWFS and LVLS vs. MAPSE were 0.89 (p < 0.001) and 0.81 (p < 0.001) with mean squared errors of 5.8% and 9.1%, respectively. Using the generated regression equation, LV-LWFS predicted LVLS with a high degree of accuracy and precision, with bias and limits of agreement of -0.044 ± 4.7% and mean squared prediction error of 5.8%. Interobserver repeatability was good, with high intraclass correlation coefficients (0.96–0.97), small bias and tight limits of agreement (≤ 4.1% for all analyses) between observers for all measurements. Conclusions LV-LWFS may be used to estimate LVLS in patients with septic shock. MAPSE also performed well, but was slightly inferior compared to LV-LWFS in estimating LVLS. Feasibility of MAPSE and LV-LWFS was excellent, as was interobserver repeatability.

Abstract 16886: Wall Stress Profiles in Tricuspid Aortic Valve Associated Ascending Thoracic Aortic Aneurysms: The Effect of Surgical Threshold Diameter ≥5.5cm

Introduction: Ascending thoracic aortic aneurysms (aTAAs) carry a risk of dissection. Elective repair guidelines are designed around size thresholds, but the one-dimensional parameter is insufficient to predict acute events in small aneurysms. Biomechanically, aortic events can occur when wall stress exceeds wall strength. Patient-specific aTAA wall stresses may be a better predictor of complications. Our aim was to compare wall stresses in aTAAs associated with a tricuspid aortic valve (TAV) based on diameter. Methods: Patients with TAV-aTAA and diameter >4.0cm (n=448) were divided into groups by 0.5 cm diameter increments. Pre-stress three-dimensional aneurysm geometries were reconstructed from ECG-gated computer tomography images. A fiber-embedded hyperelastic material model was applied to obtain longitudinal and circumferential wall stress distributions under systolic pressure. Medians with interquartile ranges are reported. The Kruskal-Wallis test is used for comparisons between size groups. Results: Peak longitudinal wall stresses for TAV-aTAA were 308[282-338] kPa for size 4.0-4.4cm vs 341[309-362] kPa for 4.5-4.9cm vs 339[289-370] kPa for 5.0-5.4cm vs 319[297-355] kPa for 5.5-5.9cm vs 373[364-449] for 6.0cm (p=0.003). Peak circumferential wall stresses were 487[448-579] kPa for size 4.0-4.4cm vs 516[473-619] kPa for 4.5-4.9cm vs 506[422-580] kPa for 5.0-5.4cm vs 540[468-591] kPa for 5.5-5.9cm vs 565[506-634] for >6.0cm (p=0.19) (figure). 95th-percentile longitudinal peak stress for TAV-aTAA <5.5cm vs ≥5.5cm is 408 vs 465 kPa. Conclusions: Longitudinal wall stresses are higher as diameter increases. The 95% percentile longitudinal peak stress for diameter ≥5.5cm is ~450 kPa, which correlates with established ~5% dissection risk for size ≥5.5cm. Wall stress thresholds may be a better predictor of patient-specific risk of dissection than diameter and require testing in clinical trials.

Unsteady Motion of Viscous Electrically Conductive Fluid Rotating in Half-Space Bounded by a Wall in the Presence of Medium Injection (Suction)

The study is devoted to studying motion of a viscous electrically conductive incompressible fluid, which initially rotates as a solid body with constant angular velocity together with a porous wall bounding it under the influence of suddenly appearing longitudinal oscillations of the wall. The wall forms an arbitrary angle with the axis of rotation. Unsteady flow is induced by longitudinal wall oscillations, injection (suction) of the medium directed perpendicular to the porous plate surface and by suddenly activated constant magnetic field directed on the normal to the plate. Solutions were constructed for velocity fields and fluid pressure. Induced magnetic field in the flow of electrically conductive fluid was determined. A number of particular cases of the wall motion were considered. Based on the results obtained, separate structures of the boundary layers adjacent to the wall were examined.

A Hypothesized Mechanistic Model of Longitudinal Wall Motion at the Common Carotid Artery

In light of recently recognized independent clinical values of longitudinal wall motion ux(t) at the common carotid artery (CCA) and the struggle on appropriate arterial indices for interpreting ux(t), this paper hypothesizes a mechanistic model of ux(t) and explores clear implications of the antegrade amplitude ux0-ante and retrograde amplitude ux0-retro of ux(t) in systole to the cardiovascular (CV) system. By examining findings on ux(t) and other relevant findings through the lens of the engineering essence of ux(t), a mechanistic model of ux(t) is hypothesized: the left ventricle (LV) base rotation is the excitation source for initiating the longitudinal elastic wave propagating along the arterial tree; wall shear stress at an artery serves as a local external source for supplying energy to the longitudinal elastic wave; and longitudinal elasticity at the arterial wall dictates the wave propagation velocity. Integrating the mechanistic model with findings on ux(t) gives rise to interpretation of ux0-ante and ux0-retro for their clear implications: longitudinal elasticity Exx at the common carotid artery (CCA) is estimated from ux0-ante, and ux0-retro is an inverse indicator of the maximum base rotation of the LV and a positive indicator of longitudinal elasticity at the ascending aorta (AA). For the first time, this model reveals the mechanisms underlying those statistical-based findings on ux(t).