Definition of arterial compliance. Re: Hardt et al., "Aortic pressure-diameter relationship assessed by intravascular ultrasound: experimental validation in dogs."

The influence of aortic stiffness on carotid stiffness: computational simulations using a human aorta carotid model

Increased aortic and carotid stiffness are independent predictors of adverse cardiovascular events. Arterial stiffness is not uniform across the arterial tree and its accurate assessment is challenging. The complex interactions and influence of aortic stiffness on carotid stiffness have not been investigated. The aim of this study was to evaluate the effect of aortic stiffness on carotid stiffness under physiological pressure conditions. A realistic patient-specific geometry was used based on magnetic resonance images obtained from the OsiriX library. The luminal aortic-carotid model was reconstructed from magnetic resonance images using 3D Slicer. A series of aortic stiffness simulations were performed at different regional aortic areas (levels). By applying variable Young's modulus to the aortic wall under two pulse pressure conditions, one could examine the deformation, compliance and von Mises stress between the aorta and carotid arteries. An increase of Young's modulus in an aortic area resulted in a notable difference in the mechanical properties of the aortic tree. Regional deformation, compliance and von Mises stress changes across the aorta and carotid arteries were noted with an increase of the aortic Young's modulus. Our results indicate that increased carotid stiffness may be associated with increased aortic stiffness. Large-scale clinical validation is warranted to examine the influence of aortic stiffness on carotid stiffness.

Computational modeling of orthostatic intolerance for travel to Mars

Astronauts in a microgravity environment will experience significant changes in their cardiopulmonary system. Up until now, there has always been the reassurance that they have real-time contact with experts on Earth. Mars crew however will have gaps in their communication of 20 min or more. In silico experiments are therefore needed to assess fitness to fly for those on future space flights to Mars. In this study, we present an open-source controlled lumped mathematical model of the cardiopulmonary system that is able simulate the short-term adaptations of key hemodynamic parameters to an active stand test after being exposed to microgravity. The presented model is capable of adequately simulating key cardiovascular hemodynamic changes—over a short time frame—during a stand test after prolonged spaceflight under different gravitational conditions and fluid loading conditions. This model can form the basis for further exploration of the ability of the human cardiovascular system to withstand long-duration space flight and life on Mars.

Measurement of normal retinal vascular pulse wave attenuation using modified photoplethysmography

Pulse wave attenuation characteristics reflect compliance and resistance properties of the vessel wall as well as initial pulse generation factors. Recently, it has become possible to measure and map the retinal vessel wall pulse wave amplitudes. Predictable pulse wave amplitude distribution may allow inferences to be made concerning vascular compliance and resistance. Twenty-eight eyes from sixteen subjects (8 male and 8 female) were examined using modified retinal photoplethysmography with simultaneous ophthalmodynamometry. This allowed the assessment of vessel wall pulsation amplitudes under a dynamic range of intraocular pressures. Pulse amplitudes were calculated using harmonic regression analysis. The pulse wave attenuation was measured under different ranges of ophthalmodynamometric force (ODF) as a function of distance along the vessel (V_Dist), which in turn was calculated in disc diameters (DD) from the center of the optic disc. A linear mixed-effects model with randomized slopes and intercepts was used to estimate the correlations between the logarithmically transformed harmonic regression wave amplitude (HRW_a) and the Fourier trigonometric coefficients with the predictors (V_Dist and ODF). The retinal venous harmonic regression wave attenuation (coefficient value±standard error) -0.40±0.065/DD, (p-value < 0.00001, 95% confidence interval (CI) -0.53 to -0.27), which was approximately twice that of the arterial -0.17±0.048/DD, (p-value < 0.0004, 95% CI = -0.27 to -0.08). There was a positive correlation between attenuation of the harmonic regression wave and ophthalmodynamometric force in both vascular systems. The attenuation of all but the sine coefficient of the second Fourier harmonic (b_n2) achieved statistical significance in the correlation with V_Dist. The cosine coefficient of the first Fourier harmonic a_n1 was the only coefficient to achieve statistical significance in the correlation with the predictors V_Dist and ODF in both vascular systems. The a_n1 coefficient value in the correlation with V_Dist was -3.79±0.78 and -1.269±0.37 (p < 0.0006), while this coefficient value in the correlation with ODF was 0.026±0.0099 and 0.009±0.04 (p < 0.01) in both the retinal veins and arteries respectively. The predictable attenuation characteristics in normal subjects suggest that this technique may allow the non-invasive quantification of retinal vascular compliance and other hemodynamic parameters.

The Implementation of an Adjustable Afterload Module for Ex Situ Heart Perfusion

PURPOSE

Windkessel impedance analysis has proven to be an effective technique for instituting artificial afterload on ex situ hearts. Traditional fixed parameter afterload modules, however, are unable to handle the changing contractile conditions associated with prolonged ex situ heart perfusion. In this paper, an adjustable afterload module is described comprising of three fully adjustable sub-components: a systemic resistor, a proximal resistor and a compliance chamber.

METHODS

Using a centrifugal pump, the systemic resistor and compliance chamber were subjected to testing across their operating ranges, whereby the predictability of resistance and compliance values was evaluated. The components were then assembled, and the full module tested on three separate porcine hearts perfused for 6 h with success defined by the ability to maintain physiological systolic and diastolic aortic pressures across flow rate variability.

RESULTS

For both the systemic resistor and compliance chamber, experimental measurements agreed with their theoretical equivalents, with coefficients of determination of 0.99 and 0.97 for the systemic resistor and compliance chamber, respectively. During ex situ perfusion, overall 95% confidence intervals demonstrate that physiological systolic (95-96.21 mmHg) and diastolic (26.8-28.8 mmHg) pressures were successfully maintained, despite large variability in aortic flow. Left ventricular contractile parameters, were found to be in line with those in previous studies, suggesting the afterload module has no detrimental impact on functional preservation.

CONCLUSIONS

We conclude that due to the demonstrable control of our afterload module, we can maintain physiological aortic pressures in a passive afterload working mode across prolonged perfusion periods, enabling effective perfusion regardless of contractile performance.

Mechanical factors in the prediction of integrity of the gastrojejunal anastomosis in ex-vivo RYGB models

BACKGROUND

Surgical staplers represent one of the important instruments in modern surgery. Laparoscopic Roux-en-Y gastric bypass is one of the most commonly performed bariatric procedures. Various techniques have been described for performing gastrojejunal (GJ) anastomosis, including linear stapled anastomosis (LSA), circular stapled anastomosis (CSA) and hand-sewn anastomosis (HSA).

OBJECTIVES

An ex-vivo porcine-based experiment was designed to compare the mechanical integrity of the GJ anastomosis among the 3 different techniques by measuring burst pressure (BP).

SETTING

Laboratory-based study conducted at the clinical skills laboratory at Birmingham Heartlands Hospitals, Birmingham, United Kingdom.

METHODS

Porcine stomachs and small bowels were used to create a GJ model. Four GJ anastomosis models each were created using circular stapler (CSA group) and hand-sewn techniques (HSA group). Stomach and small bowel thickness were recorded. BP was measured by sequential injections of methylene-blue diluted saline until a leak was detected. Total volume until leak is recorded. Compliance (C) was calculated using the formula C = ΔP/ΔV.

RESULTS

Results from our previous experiment for the LSA group are included. One model was excluded from the CSA and the HSA groups due to technical errors. Results were presented as mean ± standard deviation. Total volume in LSA, CSA, and HSA groups was 60 ± 4.08 mL, 73.67 ± 3.22 mL, and 51.67 ± 20.21 mL, respectively. BP in LSA, CSA, and HSA groups was 18 ± 4.69 mm Hg, 20.33 ± 5.77 mm Hg, and 9.67 ± 3.79 mm Hg, respectively. There was a statistically significant difference in BP among the 3 groups (P = .033; Kruskal-Wallis test). C in LSA, CSA, and HSA were 3.50 ± .88 mm Hg/mL, 3.78 ± .85 mm Hg/mL, and 5.39 ± 1.34 mm Hg/mL, respectively (P = .064).

CONCLUSION

BP was higher in CSA and LSA groups compared with the HSA group, suggesting a mechanically stronger anastomosis. Despite the lack of statistical significance, higher BP recorded in the CSA group than in the LSA group suggests better anastomotic integrity.

Roux-en-Y Gastric Bypass: Does the Direction of Staples Matter?

PURPOSE

Mechanical gastro-intestinal anastomosis using stapler is a critical step in laparoscopic Roux-en-Y gastric bypass (RYGB). To date the effect of the direction of staples on anastomotic leak has not been investigated. The study aim was to investigate the impact of the direction of staples on the integrity of the gastro-jejunal anastomosis.

MATERIALS AND METHODS

Eight gastro-jejunostomy (GJ) models were performed using porcine stomach and small intestine. Specimens were divided into group A where the cartridge was in the gastric lumen with the anvil in the jejunal lumen and vice versa in group B. Enterotomy was closed and gastric and jejunal ends were stapled off. Burst pressure (BP) was measured by infusion of methylene blue saline into the GJ model until leak occurred. Site of leak, BP, and total volume (TV) infused were recorded. Compliance (C) was calculated from the equation ΔTV/ΔBP.

RESULTS

The BP was greater in group A compared to group B (18 mmHg (range 15-25) versus 11 mmHg (range 8-15); p = 0.045) despite similar TV between the groups 60.00 mL (range 55.00-65.00) versus 51.25 mL (range 40.00-60.00); p = 0.11). The compliance did not significantly differ between groups A and B (6.38 mL/mmHg (range 4.34-8.59) versus 5.61 mL/mmHg (range 3.93-7.21); p = 0.48).

CONCLUSION

BP recorded when the stapler cartridge was introduced in the gastric lumen was higher than when it was introduced from the jejunal lumen. The lack of difference in compliance between groups suggests that the difference in BP was due to the difference in the direction of staples.

Pulse wave imaging using coherent compounding in a phantom and in vivo

Pulse wave velocity (PWV) is a surrogate marker of arterial stiffness linked to cardiovascular morbidity. Pulse wave imaging (PWI) is a technique developed by our group for imaging the pulse wave propagation in vivo. PWI requires high temporal and spatial resolution, which conventional ultrasonic imaging is unable to simultaneously provide. Coherent compounding is known to address this tradeoff and provides full aperture images at high frame rates. This study aims to implement PWI using coherent compounding within a GPU-accelerated framework. The results of the implemented method were validated using a silicone phantom against static mechanical testing. Reproducibility of the measured PWVs was assessed in the right common carotid of six healthy subjects (n  =  6) approximately 10-15 mm before the bifurcation during two cardiac cycles over the course of 1-3 d. Good agreement of the measured PWVs (3.97  ±  1.21 m s^-1, 4.08  ±  1.15 m s^-1, p  =  0.74) was obtained. The effects of frame rate, transmission angle and number of compounded plane waves on PWI performance were investigated in the six healthy volunteers. Performance metrics such as the reproducibility of the PWVs, the coefficient of determination (r ²), the SNR of the PWI axial wall velocities ([Formula: see text]) and the percentage of lateral positions where the pulse wave appears to arrive at the same time-point, indicating inadequacy of the temporal resolution (i.e. temporal resolution misses) were used to evaluate the effect of each parameter. Compounding plane waves transmitted at 1° increments with a linear array yielded optimal performance, generating significantly higher r ² and [Formula: see text] values (p  ⩽  0.05). Higher frame rates (⩾1667 Hz) produced improvements with significant gains in the r ² coefficient (p  ⩽  0.05) and significant increase in both r ² and [Formula: see text] from single plane wave imaging to 3-plane wave compounding (p  ⩽  0.05). Optimal performance was established at 2778 Hz with 3 plane waves and at 1667 Hz with 5 plane waves.

Swept-Source Anatomic Optical Coherence Elastography of Porcine Trachea

Quantitative endoscopic imaging is at the vanguard of novel techniques in the assessment upper airway obstruction. Anatomic optical coherence tomography (aOCT) has the potential to provide the geometry of the airway lumen with high-resolution and in 4 dimensions. By coupling aOCT with measurements of pressure, optical coherence elastography (OCE) can be performed to characterize airway wall stiffness. This can aid in identifying regions of dynamic collapse as well as informing computational fluid dynamics modeling to aid in surgical decision-making. Toward this end, here we report on an anatomic optical coherence tomography (aOCT) system powered by a wavelength-swept laser source. The system employs a fiber-optic catheter with outer diameter of 0.82 mm deployed via the bore of a commercial, flexible bronchoscope. Helical scans are performed to measure the airway geometry and to quantify the cross-sectional-area (CSA) of the airway. We report on a preliminary validation of aOCT for elastography, in which aOCT-derived CSA was obtained as a function of pressure to estimate airway wall compliance. Experiments performed on a Latex rubber tube resulted in a compliance measurement of 0.68±0.02 mm2/cmH2O, with R2=0.98 over the pressure range from 10 to 40 cmH2O. Next, ex vivo porcine trachea was studied, resulting in a measured compliance from 1.06±0.12 to 3.34±0.44 mm2/cmH2O, (R2>0.81). The linearity of the data confirms the elastic nature of the airway. The compliance values are within the same order-of-magnitude as previous measurements of human upper airways, suggesting that this system is capable of assessing airway wall compliance in future human studies.

Age-related vascular stiffening: causes and consequences

Arterial stiffening occurs with age and is closely associated with the progression of cardiovascular disease. Stiffening is most often studied at the level of the whole vessel because increased stiffness of the large arteries can impose increased strain on the heart leading to heart failure. Interestingly, however, recent evidence suggests that the impact of increased vessel stiffening extends beyond the tissue scale and can also have deleterious microscale effects on cellular function. Altered extracellular matrix (ECM) architecture has been recognized as a key component of the pre-atherogenic state. Here, the underlying causes of age-related vessel stiffening are discussed, focusing on age-related crosslinking of the ECM proteins as well as through increased matrix deposition. Methods to measure vessel stiffening at both the macro- and microscale are described, spanning from the pulse wave velocity measurements performed clinically to microscale measurements performed largely in research laboratories. Additionally, recent work investigating how arterial stiffness and the changes in the ECM associated with stiffening contributed to endothelial dysfunction will be reviewed. We will highlight how changes in ECM protein composition contribute to atherosclerosis in the vessel wall. Lastly, we will discuss very recent work that demonstrates endothelial cells (ECs) are mechano-sensitive to arterial stiffening, where changes in stiffness can directly impact EC health. Overall, recent studies suggest that stiffening is an important clinical target not only because of potential deleterious effects on the heart but also because it promotes cellular level dysfunction in the vessel wall, contributing to a pathological atherosclerotic state.