Biomedical subjects

Christopher M Quick

Publications and source records attributed to Christopher M Quick.

10 recordsLinked to original sources

PubMed2006

Intrinsic pump-conduit behavior of lymphangions.

Lymphangions, segments of lymphatic vessels bounded by valves, have characteristics of both ventricles and arteries. They can act primarily like pumps when actively transporting lymph against a pressure gradient. They also can act as conduit vessels when passively transporting lymph down a pressure gradient. This duality has implications for clinical treatment of several types of edema, since the strategy to optimize lymph flow may depend on whether it is most beneficial for lymphangions to act as pumps or conduits. To address this duality, we employed a simple computational model of a contracting lymphangion, predicted the flows at both positive and negative axial pressure gradients, and validated the results with in vitro experiments on bovine mesenteric vessels. This model illustrates that contraction increases flow for normal axial pressure gradients. With edema, limb elevation, or external compression, however, the pressure gradient might reverse, and lymph may flow passively down a pressure gradient. In such cases, the valves may be forced open during the entire contraction cycle. The vessel thus acts as a conduit, and contraction has the effect of increasing resistance to passive flow, thus inhibiting flow rather than promoting it. This analysis may explain a possible physiological benefit of the observed flow-mediated inhibition of the lymphatic pump at high flow rates.

Animals↗

PubMed2006

Pulmonary air embolization inhibits lung lymph flow by increasing lymphatic outflow pressure.

BACKGROUND: Air embolization of the pulmonary vascular tree increases pulmonary microvascular filtration and induces pulmonary edema formation. Flow from cannulated pulmonary lymphatic vessels increases significantly following air embolization. However, in the intact animal, lymph flows into the venous system and the magnitude of lymph flow is directly affected by systemic venous pressure. We hypothesized that pulmonary air embolization would lead to systemic venous hypertension and that this increase in lymphatic outflow pressure would prevent an increase in pulmonary lymph flow. METHODS AND RESULTS: Pulmonary air embolization was induced in dogs under general anesthesia. Flow from cannulated pulmonary lymphatic vessels was recorded for lymphatic outflow pressure set equal to atmospheric pressure (Q(LA)) and for outflow pressure set equal to systemic venous pressure (Q(LV)) both before and after embolization. Air embolization resulted in significant increases in systemic venous pressure from 6.4 +/- 0.3 to 12.4 +/- 1.2 mm Hg and in QLA from 48 +/- 9 to 175 +/- 29 microL . min(1). However, embolization did not increase Q(LV) (10 +/- 2 vs. 3 +/- 3 microl . min(1)). CONCLUSIONS: Pulmonary air embolization impedes pulmonary lymph flow by increasing systemic venous pressure and, thereby, contributes to pulmonary edema formation.

Animals↗

PubMed2006

Resolving the hemodynamic inverse problem.

The "hemodynamic inverse problem" is the determination of arterial system properties from pressures and flows measured at the entrance of an arterial system. Conventionally, investigators fit reduced arterial system models to data, and the resulting model parameters represent putative arterial properties. However, no unique solution to the inverse problem exists-an infinite number of arterial system topologies result in the same input impedance (Zin) and, therefore, the same pressure and flow. Nevertheless, there are exceptions to this theoretical limitation; total peripheral resistance (Rtot), total arterial compliance (Ctot), and characteristic impedance (ZO) can be uniquely determined from input pressure and flow. Zin is determined completely by Ctot and Rtot at low frequencies, Zo at high frequencies, and arterial topology and reflection effects at intermediate frequencies. We present a novel method to determine the relative contribution of Zo, Ctot, Rtot and arterial topology/reflection to Zin without assuming a particular reduced model. This method is tested with a large-scale distributed model of the arterial system, and is applied to illustrative cases of measured pressure and flow. This work, thus, lays the theoretical foundation for determining the arterial properties responsible for increased pulse pressure with age and various arterial system pathologies.

Animals↗

PubMed2005

The arterial system pressure-volume loop.

Although the ventricular P-V loop has become a popular tool to characterize aspects of the performance of the heart, an arterial system P-V loop has not yet been described. In principle, the volume stored in the arterial system (V) could be calculated by integrating the difference between inflow and outflow. In practice, however, flow out of the innumerable arterioles cannot be measured directly. To overcome this obstacle, it has been shown that outflow can be approximated by input pressure divided by total peripheral resistance. Recently, the classical Windkessel model was generalized with the concept of apparent arterial compliance (C(app)), the transfer function relating pressure and volume expressed in the frequency domain. The arterial system P-V loop serves as a time-domain representation of C(app). This simple technique provides the first known characterization of an arterial system P-V loop.

Animals↗

PubMed2003

Computational approach to quantifying hemodynamic forces in giant cerebral aneurysms.

BACKGROUND AND PURPOSE: The options for treating giant fusiform basilar aneurysms are limited, and the potential impact of planned interventions is difficult to assess. We developed a computational framework to evaluate the impact that interventions might have on hemodynamic conditions. METHODS: A computational fluid dynamics approach was used to determine the velocity field, wall shear stress, and pressure distribution within a model of a basilar artery before and after a simulated occlusion of one vertebral artery. The vascular geometry in a patient with a giant fusiform basilar artery aneurysm was determined by using contrast-enhanced MR angiography, and the numerical simulation approach was used to calculate the flow fields in the presenting geometry and to predict the flow field that would occur if a vertebral artery were occluded. RESULTS: In the model geometry, computational fluid dynamics indicated that there would be a symmetric flow pattern with a strong central stream and large recirculation zones at the walls. After simulated occlusion of one vertebral artery, the primary stream was diverted to one side, resulting in high pressure and increased wall shear stress. For the patient-specific geometry, flow patterns were shown to depend strongly on how much flow there was in each vertebral artery. CONCLUSION: Contrast-enhanced MR angiography is an effective tool for demonstrating the luminal boundaries of large intracranial aneurysms. Computational fluid dynamics is a powerful tool for determining the prevailing flow conditions in vascular territories and for modeling the possible alterations of the flow field that would result from interventional treatments.

Basilar Artery↗

PubMed2002

Adaptation of cerebral circulation to brain arteriovenous malformations increases feeding artery pressure and decreases regional hypotension.

PURPOSE: To determine how the adaptation of extranidal cerebral vessels affects feeding artery pressure, draining vein pressure, and regional hypotension due to the presence of brain arteriovenous malformations (BAVMs). CONCEPT: BAVMs cause high flows in feeding arteries and draining veins and can induce profound hypotension in the neighboring vasculature. Despite the large difference in flow, endothelial shear stress (tau) observed in vessels ipsilateral to the BAVM is similar to tau in vessels contralateral to the BAVM, suggesting that the conductance vessels successfully adapt to keep tau constant. However, because BAVMs are discovered only after they are well developed, the natural history of the adaptation process in extranidal vessels is unknown. RATIONALE: Currently, no way exists to determine experimentally the effects of adaptation of extranidal vessels in human patients. Therefore, a mathematical model of the cerebral vasculature is used to study adaptation in response to BAVMs. By comparing pressures and flows calculated before and after adaptation, the effect of adaptation of the conductance vessels on regional hemodynamics can be evaluated. DISCUSSION: Structural adaptation of the extranidal circulation seems not only to reset tau, but also to ameliorate regional hypotension induced by BAVMs. However, this compensatory mechanism also increases feeding artery pressure and thus may increase the risk of hemorrhagic stroke.

Blood Flow Velocity↗

PubMed2002

Increased cerebral blood flow after brain arteriovenous malformation resection is substantially independent of changes in cardiac output.

Brain arteriovenous malformation (BAVM) resection can result in an acute increase in cerebral blood flow (CBF) of unclear etiology. This observational study investigated the relationship between changes in CBF and cardiac output (CO) in patients undergoing microsurgical resection of BAVMs. In 20 patients undergoing a BAVM resection during an isoflurane-based anesthesia, we measured CBF and systemic cardiovascular parameters immediately before and after BAVM resection. CBF was measured on the hemisphere ipsilateral to the lesions and on the contralateral side, using intravenous cold 133Xe washout. Cardiac output was measured using thermodilution technique via a pulmonary artery catheter. There was an increase in global CBF after resection (25 +/- 8 versus 31 +/- 13 mL/100 g/min, preresection versus postresection, mean +/- SD, P =.002), ipsilateral CBF (25 +/- 8 versus 31 +/- 13 mL/100 g/min, P =.002), and contralateral CBF (24 +/- 7 versus 30 +/- 13 mL/100 g/min, P =.003). There was no change in CO, mean systemic arterial pressure, central venous pressure, or pulmonary artery diastolic pressure. The change in CBFGLOBAL was not correlated with changes in CO (r =.154, P =.517). BAVM resection resulted in global increases in CBF that was not substantially related to changes in CO or other systemic parameters.

Adult↗

PubMed2002

Arterial pulse wave reflection as feedback.

Traditionally, input impedance (Z(in)) has been used to characterize the global dynamic properties of an arterial system independent of properties of the heart. Defined as the relationship of pressure and flow at the entrance of an arterial system, it describes the ability of an arterial system to dynamically impede blood flow. Recently, a new description has been developed that also characterizes the arterial system independent of properties of the heart. Apparent arterial compliance (C(app)) is defined as the dynamic relationship of input pressure and volume stored in an arterial system, and describes the ability of the arterial system to dynamically store blood. Both Z(in) and C(app) are influenced by pulse wave propagation and reflection. However, the functional form of Capp lends itself to describing the arterial system in terms of negative feedback. Pulse wave reflection decreases the pulsatile volume stored (gain) at low frequencies, but increases the range of frequencies (bandwidth) in which the pulsatile volume is determined by total arterial compliance. This paper illustrates, by simple analytical formula, large-scale arterial system modeling, and direct analysis of data, how this conceptualization of reflection offers a new means to interpret changes in arterial system dynamics resulting from changes in arterial compliance.

Animals↗

PubMed2002

Relationship of nidal vessel radius and wall thickness to brain arteriovenous malformation hemorrhage.

Cerebral (brain) arteriovenous malformations (BAVMs) are a tangle of disorganized vessels that are a rare cause of hemorrhagic stroke in the general population. Although clinical presentation of hemorrhage may be related to the structure of BAVM vessels, there has been no systematic quantitative analysis of BAVM vessel morphology. Histological sections of excised BAVM lesions were prepared from patients who presented with hemorrhage (n = 14) and from patients with no history of hemorrhage (n = 22). Mean values of radius and wall thickness in each section were determined. BAVM radii were 422+/-136 microm (mean +/- SD), minimum wall thickness (thinnest portion of the wall) was 54+/-14 microm; and the minimum thickness/radius ratio was 0.23+/-0.07. Greater vessel wall thickness was associated with hemorrhagic presentation (OR= 1.1; p = 0.046) after adjusting for feeding artery pressure. Because BAVM vessels from patients presenting with hemorrhage had thicker vessel walls, the search for structural properties predisposing BAVM rupture should be expanded beyond the morphological properties analyzed here.

Adult↗

PubMedDate not supplied

Effect of venous air embolization on pulmonary microvascular protein permeability.

OBJECTIVE: An increase in pulmonary lymph flow and lymph protein clearance following pulmonary air embolization has been interpreted as evidence of increased pulmonary microvascular permeability. The authors hypothesized that air embolization does not alter the pulmonary microvascular permeability to protein and that this could be demonstrated by determining the effect of air embolization on the pulmonary solvent drag reflection coefficient (sigma(f)). METHODS: Anesthetized dogs were instrumented with left atrial balloon-tipped catheters, pulmonary lymphatic cannulae, and inferior vena caval catheters. Values were determined for pulmonary lymph flow (Q(L)) and the lymph-to-plasma protein concentration ratio (C(L)/C(P)) at baseline, after C(L)/C(P) was decreased to a filtration independent value by raising left atrial pressure via progressive balloon inflation and after 2 h of air embolization into the inferior vena cava with continued left atrial hypertension. RESULTS: Q(L) increased and C(L)/C(P) decreased to a filtration-independent value following induction of left atrial hypertension. Air embolization induced during left atrial hypertension resulted in no significant change in C(L)/C(P). The authors were unable to demonstrate that sigma(f) changed following pulmonary air embolization. CONCLUSIONS: Utilizing the washdown technique, the authors could not find any evidence that pulmonary microvascular protein permeability is altered by air embolization.

Animals↗