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Open Access
P2.11 Estimation of Lv Stroke Volume by Impedance Cardiography: Its Relation to the Aortic Reservoir
(2012-11-17) Wang, J. J.; de Vries, G.; Tyberg, J. V.
Abstract Impedance cardiography (ICG) is a noninvasive technique used to estimate left ventricular stroke volume (SVLV) using thoracic impedance (ΔZ). It remains controversial, partly because ICG parameters have not been successfully related to hemodynamic events.We hypothesized that the change in ΔZ may be proportional to the variation in thoracic blood volumes, primarily aortic blood volume. Nine anaesthetized dogs were divided into two groups: the “Aortic Volume Group” (n=5), where aortic and IVC (inferior vena cava) dimensions were measured ultrasonically, and the “Reservoir Volume Group”, where aortic and IVC reservoir volumes were calculated using the reservoir-wave approach. Measurements were made under control conditions, with nitroprusside, with methoxamine (Mtx), and after volume loading. In both groups, the maximum rate of increase in ΔZ, (dZ/dt)max, strongly correlated with the maximum rate of change in aortic/reservoir blood volume (r2 = 0.85 and 0.95, respectively), which in turn were proportional to SVLV. As shown in the figure, the aortic reservoir volume (VAo reservoir) explains SVLV in that measured aortic flow (QAo) equals the sum of dVAo reservoir/dt and calculated Qout. The LV and IVC contributions to ΔZ were small under control conditions (~5 and 1%, respectively), but the LV contribution increased slightly with administration of Mtx and after volume loading (to 7 and 10%, respectively). We conclude that the change in thoracic impedance (ΔZ) during the cardiac cycle is proportional to the change in aortic reservoir volume, which mechanistically explains why ICG and standard measures of cardiac output have sometimes been shown to correlate well.
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Open Access
P3.02 Describing Waves in the Pulmonary Veins: Application of a Reservoir-Wave Model
(2012-11-17) Bouwmeester, J. C.; Shrive, N. G.; Tyberg, J. V.
Abstract Background The pulmonary venous pressure waveform is typically described by the downstream events in the left atrium and ventricle. These downstream events create waves that contribute to the overall waveform. Methods In anesthetised open-chest dogs, measurements of pressure and flow were made in the pulmonary artery and vein. Experiments involved increases to blood volume and the application of 10 cm H2O positive end-expiratory pressure (PEEP). The reservoir-wave model describes the reservoir pressure, which is subtracted from measured pressure, to result in the excess pressure (Pexcess). Excess velocity (Uexcess) is similarly formulated. Pexcess and Uexcess are used in wave intensity analysis to calculate wave speed and separate the contributions of waves originating upstream (forward waves) and downstream (backward waves). Results Separated waves are shown in the bottom panel of Figure 1. The effect of PEEP resulted in larger decreases to Pbackward (p < 0.001) after the mitral valve opened. As a result, y was lower than x by ~2.0 mmHg. With PEEP, the delay between arterial and venous forward waves increased from 155 ± 4 ms to 183 ± 4 ms (mean ± SE, p < 0.001). Conclusion The majority of pulmonary venous pressure landmarks can be attributed to the actions of the left atrium and ventricle but the v wave has substantial contributions from waves originating in the pulmonary artery. Diastolic suction has a larger effect with PEEP, presumably from some external constraint applied to the heart and consequently lowered end-systolic left ventricular volume. Figure 1 Common venous markers related to measured pressures (top panel) and the separation of Pexcess into forward and backward components (bottom panel) at control conditions.
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Open Access
P3.21 Conductance and Capacitance Effects of Acute, Electrical, Carotid Baroreflex Stimulation
(2012-11-17) Burgoyne, S.; Tyberg, J. V.; Belenkie, I.; Georgakopoulos, D.
Abstract Introduction Chronic baroreflex activation is a therapy for resistant hypertension and has potential as a therapy in heart failure. We hypothesized that acute baroreflex activation therapy (CVRx, Inc.) would increase both the capacity of the abdominal venous circulation (lowering “preload”) and aortic conductance (reducing “afterload”). Methods Six 20-kg mongrel dogs were anaesthetized and ventilated. Arterial blood pressure (BP) and diaphragmatic aortic and caval flow (ultrasonic) were measured. Venous capacity changes were evaluated using a modified Brooksby-Donald technique*. A CVRx electrode was affixed to the right carotid sinus. BP and flow data were collected under control conditions and during device activation and drug infusions. Angiotensin II (ANG II) was infused to raise BP to hypertensive levels; the current from the device was then increased. Results Device activation decreased mean aortic BP 22.5±1.3 mmHg, decreased heart rate 14.7±3.4% and cardiac output 10.8±3.9%, increased aortic conductance 16.2±4.9%, and increased abdominal blood volume at a rate of 2.2±0.6 mL/kg/min (5-minute activations). ANG II infusion increased BP 40.3±3.4 mmHg and reduced venous capacitance at a rate of 1.1±0.5 mL/ kg/min. Subsequent electrical stimulation restored BP to baseline while aortic conductance only returned to 82.3±3.3% of control. Venous capacitance increased at a rate of 3.4±0.7 mL/kg/min, reversing the ANG II effects. Conclusions Acute electrical activation of the carotid baroreflex increases arterial conductance, decreases BP, and increases venous capacitance. Modulation of venous capacitance may be an important effect of barore-ceptor activation in hypertension and heart failure.
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Open Access
1.3 Is it Time to Question the Validity of Impedance Analysis?
(2010-12-02) Tyberg, J.; Shrive, N.; Burrowes, L.; Sridharan, S.; Bouwmeester, C.; Wang, J.-J.
Abstract Although the 3-element Windkessel (Wk) is still a useful analogue of arterial hemodynamics, can the validity of the frequency-domain analysis continue to be assumed? Our alternative time-domain approach holds that measured pressure is the sum of a Wk (PWk) and an “excess” pressure (Pexcess). “Characteristic impedance” (Z0) is critical. Originally called characteristic resistance by Westerhof, Z0 was simulated like peripheral resistance in a hydraulic model but recently has been interpreted only in the frequency domain. We have shown that Pexcess varies linearly with aortic inflow with a slope of Z0. Bench-top experiments with canine peak flows and aortic dimensions yielded pressure drops equal to those measured physiologically, and a proximal resistance approximating Z0. A bench-top experiment simulating Westerhof’s hydraulic circuit demonstrated a PWk waveform. We calculated the frequency-dependent impedance of measured pressure, PWk and Pexcess, under the influence of nitroprusside (NP) and methoxamine (Mtx). With NP, there was no impedance minimum and the modulus of Pexcess was frequency-independent. With Mtx, an impedance minimum was demonstrated but was due entirely to PWk. Thus, the impedance minimum appears to be due only to the PWk and may not also be essentially related to wave reflection. Finally, we used our approach to demonstrate positive and negative wave reflection in the canine aorta. However, if PWk was not initially subtracted, backward waves appeared first in the ascending aorta and they appeared to be propagated forward (figure). These profoundly paradoxical results above seem to undermine the fundamental presuppositions of the frequency-domain analysis.
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Open Access
Some effects of domain size and boundary conditions on the accuracy of airfoil simulations
(2024-03-01) Golmirzaee, Narges; Wood, David H.
Abstract This paper investigates a specific case of one of the most popular fluid dynamic simulations, the incompressible flow around an airfoil (NACA 0012 here) at a high Reynolds number ( $$6 \times 10^6$$ 6 × 10 6 ). OpenFOAM software was used to study the effect of domain size and four common choices of boundary conditions on airfoil lift, drag, surface friction, and pressure. We also examine the relation between boundary conditions and the velocity, pressure, and vorticity distributions throughout the domain. In addition to the common boundary conditions, we implement the “point vortex” boundary condition that was introduced many years ago but is now rarely used. We also applied the point vortex condition for the outlet pressure instead of using the traditional Neumann condition. With the airfoil generating significant lift at incidence angles of $$5^\circ , 10^\circ$$ 5 ∘ , 10 ∘ , and $$14^\circ$$ 14 ∘ , we confirm a previous finding that the boundary conditions combine with domain size to produce an induced (pressure) drag. The change in the pressure drag with domain size is significant for the commonly-used boundary conditions but is much smaller for the point vortex alternative. The point vortex boundary condition increases the execution time, but this is more than offset by the reduction in domain size needed to achieve a specified accuracy in the lift and drag. This study also estimates the error in total drag and lift due to domain size and shows it can be almost eliminated using the point vortex boundary condition. We also used the impulse form of the momentum equations to study the relation between drag and lift and spurious vorticity, which is generated as a result of using non-exact boundary conditions. These equations reveal that the spurious vorticity throughout the domain is associated with cancelling circulation around the domain boundaries.