Changes In Reservoir Volume Research Articles

The effects of thermomechanical heterogeneities in island arc crust on time‐dependent preeruptive stresses and the failure of an andesitic reservoir

AbstractUsing ground deformation data from Soufrière Hills volcano (SHV), we present results from numerical modeling of the temperature‐ and time‐dependent stress evolution in a mechanically heterogeneous crust prior to reservoir failure and renewed eruptive activity. The best fit models do not allow us to discriminate between a magmatic plumbing system consisting of either a single vertically elongated reservoir or a series of stacked reservoirs. A prolate reservoir geometry with volumes between 50 and 100 km3, reservoir pressure changes between 4 and 7 MPa, and reservoir volume changes between 0.03 and 0.04 km3 with magma compressibility between 4 × 10−11 and 1 × 10−9 Pa−1 provide plausible thermomechanical model parameters to explain the deformation time series; around an order of magnitude less overpressure than is generally inferred from homogeneous, elastic crustal models. Reservoir failure is predicted to occur at the crest of the reservoir except for reservoirs with highly compressible magma ( Pa) for which subhorizontal sill formation is predicted upon reservoir failure. Introducing a deep‐crustal hot zone modulates the partitioning of strains into the hotter underlying crust and results in a further reduction in overpressure estimates to values of around 1–2 MPa upon reservoir failure. Deduced volume fluxes are consistent with constraints from thermal modeling of active subvolcanic systems and imply dynamic failure of a compressible magma mush column feeding eruptions at SHV. Our interpretation of the results is that the combined thermomechanical effects of a deep‐crustal hot zone and hot encasing rocks around a midcrustal andesitic reservoir fundamentally alter the time‐dependent subsurface stress and strain partitioning upon reservoir priming. These effects substantially influence surface strains recorded by volcano geodetic monitoring.

Residual Gastric Volume Estimated with a New Radiological Volumetric Model: Relationship with Weight Loss After Laparoscopic Sleeve Gastrectomy

The volume of the gastric reservoir appears crucial to explain the success of laparoscopic sleeve gastrectomy (LSG) in the treatment of morbid obesity. The aims of this study were to describe a new, easy model to estimate gastric reservoir volume after LSG; to evaluate the volumetric changes 1 year after surgery; and to analyze their relationship with weight loss. This is a prospective observational study of all patients undergoing LSG in the Department of Surgery at our institution. The gastric reservoir was evaluated radiologically considering the image as a complex geometrical shape with two components: a cylinder (gastric body) and a truncated cone (antrum). Radiologic assessment using this new model was performed at 1 and 12 months after surgery. Moreover, body mass index and percentage of excess weight loss (%EWL) were evaluated at 3, 6, 12, and 18 months after LSG. Forty-five patients (34 F/11 M) with a mean age of 46.9 years were included. A significant increase in total gastric reservoir volume (124.8 ± 58.7 and 188.6 ± 76.4 mL at 1 and 12 months, respectively; p = 0.001) was observed. No statistically significant differences were observed comparing volume of the two components at the two time points. The %EWL at 18 months was inversely correlated with reservoir volume changes at 12 months after LSG (p = 0.006). We describe an easy volumetric model to estimate the size of the gastric reservoir after LSG. Moreover, a direct relationship between an increase in gastric reservoir volume and a lower weight loss after surgery was documented.

A Method to Consider Whether Dams Mitigate Climate Change Effects on Stream Temperatures

AbstractThis article provides a method for examining mesoscale water quality objectives downstream of dams with anticipated climate change using a multimodel approach. Coldwater habitat for species such as trout and salmon has been reduced by water regulation, dam building, and land use change that alter stream temperatures. Climate change is an additional threat. Changing hydroclimatic conditions will likely impact water temperatures below dams and affect downstream ecology. We model reservoir thermal dynamics and release operations (assuming that operations remain unchanged through time) of hypothetical reservoirs of different sizes, elevations, and latitudes with climate‐forced inflow hydrologies to examine the potential to manage water temperatures for coldwater habitat. All models are one dimensional and operate on a weekly timestep. Results are presented as water temperature change from the historical time period and indicate that reservoirs release water that is cooler than upstream conditions, although the absolute temperatures of reaches below dams warm with climate change. Stream temperatures are sensitive to changes in reservoir volume, elevation, and latitude. Our approach is presented as a proof of concept study to evaluate reservoir regulation effects on stream temperatures and coldwater habitat with climate change.

Estimation of left ventricular stroke volume by impedance cardiography: its relation to the aortic reservoir

Impedance cardiography is a non-invasive technique used to estimate left ventricular (LV) stroke volume (SV) using the change in thoracic impedance (ΔZ). It remains controversial, partly because impedance cardiographic parameters have not been successfully related to haemodynamic events. We hypothesized that the change in ΔZ may be proportional to the variation in thoracic (primarily aortic) blood volumes. Nine anaesthetized and ventilated dogs were divided into the following two groups: the 'aortic volume group' (n = 5), in which aortic and IVC (inferior vena caval) dimensions were measured ultrasonically; and the 'reservoir volume group', in which aortic and IVC reservoir volumes were calculated using the reservoir-wave model. Measurements were made in control conditions, in the presence of nitroprusside and methoxamine and after volume loading. In both the aortic volume group and the reservoir volume group, the maximal rate of increase in ΔZ [(dZ/dt)max] strongly correlated with the maximal rate of change in aortic/reservoir blood volume (R(2) = 0.85 and 0.95, respectively), which in turn was proportional to the LV SV. The LV and IVC contributions to ΔZ were small in control conditions (∼5 and 1%, respectively), but the LV contribution increased slightly (to 7%) with administration of methoxamine and after volume loading (to 10%). It is concluded that the change in thoracic impedance (ΔZ) during the cardiac cycle is proportional to the change in aortic reservoir (i.e. Windkessel) volume, which provides a mechanistic explanation for previously demonstrated good correlations with standard measures of cardiac output.

Geomechanical modeling for InSAR-derived surface deformation at steam-injection oil sand fields

To estimate the distribution of reservoir deformation and reservoir volume change in an oil sand reservoir undergoing steam injection, we applied geomechanical inversion to surface uplift data derived from a differential interferometric synthetic-aperture radar (InSAR) stacking technique. We tested a two-step inversion method based on a tensional rectangular dislocation model. The first step of the inversion used genetic algorithms to estimate the depth of the reservoir and roughly model its deformation. The estimated depth of the reservoir was consistent with the depth of the injection point. The second step used a least-squares inversion with a penalty function and smoothing factor to efficiently invert the distribution of reservoir deformation and volume change from the surface uplift data. The distribution of reservoir deformation can be accurately estimated from InSAR-derived ground surface deformations using our proposed inversion techniques.

Poroelastic model for induced stresses and deformations in hydrocarbon and geothermal reservoirs

Fluid migration and heat transport result in changes in pore pressure and temperature within a reservoir, which can induce stresses and deformations in the reservoir and the bounding rock system through poroelastic and thermoelastic couplings. Analytical and semi-analytical solutions for investigating the induced stresses and deformations are therefore extremely useful when applied to both predicting and monitoring the reservoir volume changes and associated subsurface and surface deformation. The poroelastic and thermoelastic eigenstrains are used here to characterize the pore pressure change and temperature variation, respectively, in the reservoir. The induced stresses and deformations are then obtained by using the single- and double-inclusion models, and are expressed in terms of the Galerkin vector stress function, which is related to the corresponding eigenstrains in a straightforward way. The difference in mechanical properties between the reservoir and the bounding rocks is accounted for using the theory of inhomogeneous inclusions. The effect of the reservoir shape, elastic properties, and the distribution of pore pressure change within the reservoir on the surface deformation has been investigated. The magnitude and pattern of the induced surface tilt have been compared with those produced by a hydraulic fracture. The analytical expressions obtained here for the displacement and tilt fields can serve as a useful forward model for monitoring and mapping hydraulic fractures, subsurface fluid migration and heat transport associated with injection or production of fluid into or from a reservoir by surface deformation-based monitoring techniques, such as tiltmeter monitoring.

A pragmatic method for estimating seepage losses for small reservoirs with application in rural India

The informal construction of small dams to capture runoff and artificially recharge ground water is a widespread strategy for dealing with water scarcity. A lack of technical capacity for the formal characterization of these systems, however, is often an impediment to the implementation of effective watershed management practices. Monitoring changes in reservoir storage provides a conceptually simple approach to quantify seepage, but does not account for the losses occurring when seepage is balanced by inflows to the reservoir and the stage remains approximately constant. To overcome this problem we evaluate whether a physically-based volume balance model that explicitly represents watershed processes, including reservoir inflows, can be constrained by a limited set of data readily collected by non-experts, specifically records of reservoir stage, rainfall, and evaporation. To assess the impact of parameter non-uniqueness associated with the calibration of the non-linear model, we perform a Monte Carlo analysis to quantify uncertainty in the total volume of water contributed to the subsurface by the 2007 monsoon for a dam located in the Deccan basalts near the village of Salri in Madhya Pradesh, India. The Monte Carlo analysis demonstrated that subsurface losses from the reservoir could be constrained with the available data, but additional measurements are required to constrain reservoir inflows. Our estimate of seepage from the reservoir (7.0 ± 0.6 × 104 m3) is 3.5 times greater than the recharge volume estimated by considering reservoir volume changes alone. This result suggests that artificial recharge could be significantly underestimated when reservoir inflows are not explicitly included in models. Our seepage estimate also accounts for about 11% of rainfall occurring upstream of the dam and is comparable in magnitude to natural ground water recharge, thereby indicating that the reservoir plays a significant role in the hydrology of this small watershed.

Monitoring and Modelling in Coupled Geomechanics Processes

Abstract Geomechanics issues are vital in all reservoir processes, but particularly so in weak, unconsolidated sandstones. Coupled stress-flow simulation is necessary to analyze and understand effects such as changes in reservoir volume that arise from heating and pressurization. Also, non-linear plasticity models incorporating shear dilatancy are needed to simulate the dilation effects that are observed in thermal extraction processes in unconsolidated sands. Stress-flow coupling is based on the volume changes that arise with pressure and temperature changes (?p, ?T). Incorporating shear dilation is based on computation of effective stresses from ?T and ?p, then assessing the state of the rock to see if it is shearing and by how much it must dilate. These processes are poorly quantified at present, so it is necessary to monitor the process to calibrate simulation models. The two monitoring domains of greatest interest to coupled geomechanics simulation are the deformation field and the seismic attributes field. How these fields evolve in space and with time are the key factors to tracking processes, to calibrate geomechanics models and to successfully optimize complex in situ processes. A general geomechanics view of how to achieve process monitoring and optimization goals is presented here. Though recent developments have been promising, further progress in monitoring, inversion and coupled geomechanics simulation is needed. Introduction Conventional monitoring in petroleum engineering addresses pressure, temperature and rate measurements, as well as data collected by wellbore logs such as temperature or rate surveys (e.g. spinner surveys). Oil, gas and water production and injection rates are required for regulatory purposes and to help calculate saturations and recovery factors (RF). Changes in reservoir response were commonly assessed using classical well tests and analyses(1, 2). Because classic reservoir simulation in conventional low viscosity cases deals only with mass and heat transport (Darcy and Fourier diffusion processes) combined with saturation changes and relative permeability calculations, these measures were deemed sufficient for reservoir management. Flow rates (Q), well test data and facilities capability analyses are also used for production optimization(3). These measures are considered insufficient for heavy oil (HO) thermal extraction, HPHT reservoir management, high compaction cases and gravitationally-dominated production technologies. In such cases, we are more interested in measures such as the reservoir pore volume change (?V), gas saturation changes in situ (?Sg), swept volume distribution, and so on. These cannot be measured by conventional p-T-Q methods or geophysical wellbore logging, nor are they easily amenable to calculation. When shear dilation, compaction or induced fracturing take place, major changes in rock mass properties occur; understanding what is happening and where it is taking place requires different monitoring and simulation methods. To make monitoring data more useful, flow-stress coupled modelling, also referred to as coupled geomechanical modelling, is carried out. Changes in p and T are analyzed in terms of effective stress changes (? s'ij) through their effect on rock and pore volumes. The links between ?V and the changes ?p, ?T and ? s'ij are established through p-, T-, s'-compressibilities of the bulk rock and of the mineral matter comprising the matrix.

Estimating permeability from quasi-static deformation: Temporal variations and arrival-time inversion

Transient pressure variations within a reservoir can be treated as a propagating front and analyzed using an asymptotic formulation. From this perspective, one can define a pressure “arrival time” and formulate solutions along trajectories, in the manner of ray theory. We combine this methodology and a technique for mapping overburden deformation into reservoir volume change as a means to estimate reservoir flow properties, such as permeability. Given the entire “travel time” or phase field obtained from the deformation data, we can construct the trajectories directly, thereby linearizing the inverse problem. A numerical study indicates that, using this approach, we can infer large-scale variations in flow properties. In an application to Interferometric Synthetic Aperture Radar (InSAR) observations associated with a [Formula: see text] injection at the Krechba field, Algeria, we image pressure propagation to the northwest. An inversion for flow properties indicates a linear trend of high permeability. The high permeability correlates with a northwest trending fault on the flank of the anticline that defines the field.

An investigation of the effect of powder reservoir volume on the consistency of alumina powder flow rates in dental air-abrasion devices

Objectives To investigate the effect of powder reservoir fill volume on the emitted alumina powder flow rate of four dental air-abrasion units. The null hypothesis examined was that powder reservoir fill volume has no effect on α-alumina powder flow rate. Methods The spent alumina powder from the four air-abrasion units tested (Abradent, Crystalmark, Clendale, CA, USA; Aquacut, Velopex, Horesham, UK; MicroPrep Associate Lares, Chico, CA, USA; Rondoflex 360, KaVo, Lake Zurich, IL, USA) was gathered in a collecting chamber over a 60 s running period. The difference in the weight pre- and post-collection was noted and flow rate calculated for different reservoir fill volumes (100%, 75%, 50% and 25%). Results The powder flow rate of each machine was significantly affected by a change in powder reservoir volume ( p < 0.05). In all devices a partially empty reservoir caused a significant decrease in powder flow rate, compared to that at maximum fill volume. The maximum percentage difference seen using the Rondoflex was similar for 27 μm and 50 μm abrasive (70% and 69%, respectively), whereas, smaller differences were seen with the Abradent (59%), Aquacut 29 μm and 53 μm abrasive (62% and 30%) and MicroPrep (35%) machines. Conclusion It was shown that the fill of the reservoir affects the powder flow rate of air-abrasion units, thus disproving the null hypothesis. In a clinical setting and in future research studies, the powder reservoir should be filled to a set level to achieve a constant and reproducible cutting performance of the tested units.

Mapping Reservoir Volume Changes During Cyclic Steam Stimulation Using Tiltmeter-Based Surface-Deformation Measurements

Surface-deformation measurements have been used for years in oil fields to monitor production, waterflooding, waste injection, steam flooding, and cyclic steam stimulation (CSS). They have been proved to be a very effective way to monitor field operations and save money for operators wishing to avoid unwanted surface breaches, casing failures, and excessive subsidence because of production. This paper demonstrates that more information can be extracted from surface-deformation measurements by inverting the surface deformation for the volumetric deformation at the reservoir level using the inversion techniques from the literature, so that the areal distribution of volumetric deformation can be identified. This leads to a better understanding of reservoir behavior and also provides additional data for integration into coupled reservoir simulation modeling. This paper shows the results of mapped reservoir volume changes from two cyclic steam injection projects using tiltmeter-based surface deformation measurements.

On the use of quasi-static deformation to understand reservoir fluid flow

Deformation above a producing reservoir provides a valuable source of information concerning fluid flow and flow properties. Quasi-static deformation occurs when the displacements are so slow that we may neglect inertial terms in the equations of motion. We present a method for inferring reservoir volume change and flow properties, such as permeability, from observations of quasi-static deformation. Such displacements may represent surface deformation such as tilt, leveling, interferometric synthetic aperture radar (InSAR), or bathymetry observations or subsurface deformation, as inferred from time-lapse seismic surveys. In our approach, the equation for fluid flow in a deforming reservoir provides a mapping from estimated fractional volume changes to reservoir permeability variations. If the reservoir behaves poroelastically over the interval of interest, all the steps in this approach are linear. Thus, the inference of reservoir permeability from deformation data becomes a linear inverse problem. In an application to the Wilmington oil field in California, we find that observed surface displacements, obtained by leveling and InSAR, are indeed compatible with measured reservoir volume fluxes. We find that the permeability variations in certain layers coincide with fault-block boundaries suggesting that, in some cases, faults are controlling fluid flow at depth.

Effect of phosphodiesterase III inhibitor on contractility, afterload, and vascular capacitance during right heart bypass preparation.

Phosphodiesterase III inhibitors, which have both positive inotropic and vasodilatory effects, occasionally cause hypotension due to afterload reduction and possibly due to preload reduction caused by the increase in vascular capacitance. Six open-chest adult mongrel dogs were used to compare the effects on left ventricular contractility, afterload, and vascular capacitance of the phosphodiesterase III inhibitor, olprinone, with those of dobutamine using a right-heart-bypass model. Contractility and afterload were evaluated by the left ventricular pressure-volume relations with the use of a conductance catheter to derive the end-systolic elastance (Ees) and the effective arterial elastance (Ea). Vascular capacitance change was evaluated by reservoir volume change under a constant bypass flow (80 ml/kg per minute). Ees increased significantly both with dobutamine (7.6 +/- 2.8 to 14.3 +/- 4.8 mmHg/ml, p < 0.05) and with olprinone (7.6 +/- 2.9 to 11.5 +/- 4.2 mmHg/ml, p < 0.05). Ea did not change with dobutamine (14.4 +/- 3.5 to 14.5 +/- 3.6 mmHg/ml, p = 0.9), whereas it decreased with olprinone (14.0 +/- 4.1 to 11.4 +/- 3.8 mmHg/ml, p = 0.093). Reservoir volume increased after the infusion of dobutamine (-94.0 +/- 39.8 ml), and decreased after the infusion of olprinone (-114.0 +/- 62.3 ml). The difference was statistically significant (p = 0.007). The reservoir volume change indicated that vascular capacitance decreased with dobutamine, and increased with olprinone. Pre- and afterload reduction of olprinone combined with the positive inotropic effect are useful in treating congestive heart failure and managing low cardiac output syndrome after cardiac surgery.

Hindlimb unweighting affects rat vascular capacitance function.

Microgravity is associated with an impaired stroke volume and, therefore, cardiac output response to orthostatic stress. We hypothesized that a decreased venous filling pressure due to increased venous compliance may be an important contributing factor in this response. We used a constant flow, constant right atrial pressure cardiopulmonary bypass procedure to measure total systemic vascular compliance (C(T)), arterial compliance (C(A)), and venous compliance (C(V)) in seven control and seven 21-day hindlimb unweighted (HLU) rats. These compliance values were calculated under baseline conditions and during an infusion of 0.2 microg*kg(-1)*min(-1) norepinephrine (NE). The change in reservoir volume, which reflects changes in unstressed vascular volume (DeltaV(0)) that occurred upon infusion of NE, was also measured. C(T) and C(V) were larger in HLU rats both at baseline and during the NE infusion (P < 0.05). Infusion of NE decreased C(T) and C(V) by ~20% in both HLU and control rats (P < 0.01). C(A) was also significantly decreased in both groups of rats by NE (P < 0.01), but values of C(A) were similar between HLU and control rats both at baseline and during the NE infusion. Additionally, the NE-induced DeltaV(0) was attenuated by 53% in HLU rats compared with control rats (P < 0.05). The larger C(V) and attenuated DeltaV(0) in HLU rats could contribute to a decreased filling pressure during orthostasis and thus may partially underlie the mechanism leading to the exaggerated fall in stroke volume and cardiac output seen in astronauts during an orthostatic stress after exposure to microgravity.

Using surface deformation to image reservoir dynamics

The inversion of surface deformation data such as tilt, displacement, or strain provides a noninvasive method for monitoring subsurface volume change. Reservoir volume change is related directly to processes such as pressure variations induced by injection and withdrawal. The inversion procedure is illustrated by an application to tiltmeter data from the Hijiori test site in Japan. An inversion of surface tilt data allows us to image flow processes in a fractured granodiorite. Approximately 650 barrels of water, injected 2 km below the surface, produces a peak surface tilt of the order of 0.8 microradians. We find that the pattern of volume change in the granodiorite is very asymmetrical, elongated in a north‐northwesterly direction, and the maximum volume change is offset by more than 0.7 km to the east of the pumping well. The inversion of a suite of leveling data from the Wilmington oil field in Long Beach, California, images large‐scale reservoir volume changes in 12 one‐ to two‐year increments from 1976 to 1996. The influence of various production strategies is seen in the reservoir volume changes. In particular, a steam flood in fault block II in the northwest portion of the field produced a sudden decrease in reservoir volume.

Assessment of pulmonary venous and transmitral flow in closed-chest dogs under various loading conditions by transesophageal Doppler echocardiography.

Pulmonary venous flow velocity (PVFVe), pulmonary venous dimension (PVD) and transmitral flow (TMF) velocity were examined under various loading conditions in 15 anesthetized closed-chest dogs by transesophageal Doppler echocardiography (TEE). We also compared PVFVe with pulmonary venous flow volume (PVFVo) simultaneously in open-chest dogs using an ultrasonic flow probe. PVFVo decreased by more than 50% and PVD also decreased significantly during preload reduction, while there was no change in PVFVe. This discrepancy between PVFVo and PVFVe was apparently due to the collapse of pulmonary veins. TMF consisted of both rapid-filling flow velocity and atrial flow velocity components (R and A), while PVF consisted of systolic and diastolic forward flow velocity components (S and D). The peak values of R,A,S and D and the time-velocity integrals of each wave (RI, AI, SI and DI, respectively) were measured. There was a significant correlation between the changes in RI and SI/DI during preload reduction (r = 0.82, p < 0.001) and during after-load increase (r = -0.59, p < 0.05). These results suggest that changes in RI with different loading conditions might be attributable to changes in atrial reservoir volume and conduit volume.

Extravascular fluid uptake during cardiopulmonary bypass in hypertensive dogs

We investigated the effects of increases in central venous pressure (CVP) and carotid baroreceptor-induced vasodilation on the rate of extravascular fluid uptake during cardiopulmonary bypass in normotensive and Goldblatt hypertensive dogs. Carotid sinus baroreceptors were selectively perfused to control the level of vasodilation. Central venous pressure was controlled by changing the height of the venous outflow cannula. Extravascular fluid uptake was determined from the rate of change in reservoir volume. After 3 hours of bypass, total fluid accumulation was 56.11 ± 14.16 mL/kg in normotensive dogs, significantly less than in hypertensive dogs (110.90 ± 23.20 mL/kg) ( p < 0.05). Raising CVP from 1 to 5 mm Hg increased the rate of extravascular fluid uptake in both normotensive (from 0.05 ± 0.25 to 0.85 ± 0.22 mL · kg −1 · min −1; p < 0.05) and hypertensive dogs (from 0.68 ± 0.28 to 2.57 ± 0.46 mL · kg −1 · min −1; p < 0.01)). At a constant CVP, baroreceptor-induced vasodilation increased the rate of extravascular fluid uptake in normotensive (from 0.25 ± .15 to 0.81 ± .22 mL · kg −1 · min −1) and in hypertensive dogs (from 0.84 ± .12 to 1.72 ± .32 mL · kg −1 · min −1; p < 0.05). Hypertensive dogs were more sensitive to changes in CVP and to baroreceptorinduced vasodilation. The results of this study imply that elevations in CVP or the use of vasodilators may lead to increased extravascular fluid uptake during bypass; this effect may be exacerbated in the hypertensive state.

Baroreflex control of arterial and venous compliances and vascular capacity in hypertensive dogs

The ability of the carotid sinus baroreflex to elicit reflex changes in vascular capacity was studied in chronically hypertensive (one-kidney, one-clip) and sham-operated normotensive dogs under pentobarbital sodium anesthesia. Vascular compliances and reflex-induced changes in external reservoir volume were measured in response to changes in isolated carotid sinus pressure (CSP). The mean arterial pressure-CSP reflex characteristic curve was shifted to a higher arterial pressure with hypertension with no change in maximum reflex gains. Arterial compliance in both groups increased significantly (P < 0.01) when CSP was increased from 50 to 200 mmHg. Total, arterial, and venous vascular compliances were not different in normotensive and hypertensive groups. Changes in CSP caused no significant changes in either total systemic vascular or calculated venous compliances. A decrease in CSP from 250 to 50 mmHg resulted in an increase in external reservoir volume of 8.02 +/- 1.03 and 7.44 +/- 1.33 ml/kg in normotensive and hypertensive groups, respectively, with changes in venous volume of 11.99 +/- 1.39 and 12.58 +/- 1.52 ml/kg (NS). We conclude that despite the increase in arterial resistance, Goldblatt hypertensive dogs retain the ability to make short-term reflex adjustments in both arterial pressure and venous blood volume.

Modifications by Halothane of Responses to Acute Hypoxia in Systemic Vascular Capacitance, Resistance, and Sympathetic Nerve Activity in Dogs

To examine the effects of halothane on segmental vascular responses to hypoxia, we used cardiopulmonary bypass with venous outflow divided into three compartments (splanchnic, coronary, and "other") in dogs anesthetized with pentobarbital sodium. The reservoir volume changes represented the inverted changes in systemic blood volume (SBV). In addition, sympathetic efferent nerve activity (SENA) was simultaneously recorded from the ventral ansa subclavian nerve. Experiments were done in two groups: severe hypoxia (PO2 of 19 mm Hg) and moderate hypoxia (PO2 of 50 mm Hg). Hypoxia provoked a significant decrease in SBV of 22.3 +/- 3.1 mL/kg and 10.5 +/- 1.6 mL/kg during severe and moderate hypoxia, respectively. Two percent end-tidal halothane attenuated the decrease in SBV to 10.3 +/- 1.3 mL/kg during severe hypoxia, and 1% halothane attenuated the decrease to 3.7 +/- 1.4 mL/kg during moderate hypoxia. Subsequent chemoreceptor denervation in the presence of 1% halothane completely abolished the moderate hypoxia-induced decrease in SBV. In the presence of halothane, vascular resistance during hypoxia was significantly less than that during control conditions. Sympathetic efferent nerve activity increased significantly during severe and moderate hypoxia by about 180% and 55%, respectively. During severe hypoxia, halothane did not cause any change in the response of SENA, whereas during moderate hypoxia, halothane tended to decrease SENA, but not significantly, and subsequent chemoreceptor denervation completely abolished the increase in SENA. Coronary resistance showed a hypoxia-induced reduction that was not influenced by halothane. These results suggest that acute hypoxia causes a decrease in SBV dependent on the severity of hypoxia.(ABSTRACT TRUNCATED AT 250 WORDS)

Whole body vascular capacitance response to vasopressin is mediated by autonomic function

Effects of intravenous infusions of arginine vasopressin (AVP) on whole body vascular capacitance were determined in anesthetized cats when autonomic nervous system function was intact and, in other cats, when reflexes were blocked by the ganglionic blocking agent pentolinium. With the use of the constant cardiac output-reservoir technique, changes in reservoir volume were assumed to reflect reciprocal changes in whole body vascular capacitance. Relationships between the dose of AVP and the plasma concentration of the peptide achieved during infusions were not significantly different in the two groups of animals. Blood pressure responses to AVP were greater, whereas heart rate responses to the peptide were abolished in ganglion-blocked cats. In cats with intact autonomic function, reservoir volume decreased by 1.6, 4.2, and 7.8 ml/kg at AVP doses of 1, 10, and 100 ng.kg-1.min-1, respectively. In contrast, in ganglion-blocked cats, reservoir volume did not change significantly at 1.0 and 10 ng.kg-1.min-1 of AVP, and the highest dose caused a much smaller change in volume (3 ml/kg) than that observed in cats with intact autonomic function (7.8 ml/kg). Systemic compliance was unchanged by AVP in both groups of animals, suggesting that the increases in whole body vascular capacitance were likely due to changes in unstressed volume. The results suggest that reflexively mediated changes in autonomic function increase whole body vascular capacitance during elevations in the circulating levels of AVP to plasma concentrations that are biologically relevant. These findings may explain how AVP decreases cardiac output in animals with an intact autonomic nervous system.