Annals of Biomedical Engineering

Sun, Hun

doi: 10.1007/BF02584239pmid: N/A

McInnis, B.; Deng, L.

doi: 10.1007/BF02584240pmid: 4037454

Control of arterial blood pressure has been successfully achieved by infusing a single vasoactive drug. However, in clinical practice, blood pressures are frequently controlled using multiple drug infusions. This paper presents a computer based adaptive control algorithm for simultaneous infusions of both an inotropic agent and a vasoactive agent to maintain the blood pressures at desired levels. We present the dynamics of the system with a bilinear two-input, two-output model. A least-squares parameter estimation algorithm has been employed using an output error method. Results of computer simulations of an electrical analog model of the heart and circulatory system are presented.

Larnard, D.; Koivo, A.

doi: 10.1007/BF02584241pmid: 3929652

A discrete-time model describing intracranial pressure (ICP) dynamics is extended to account for changes in blood osmolarity due to an i.v. injection of mannitol. It includes the effect of a blood osmolarity change on cerebrospinal fluid (CSF) formation and absorption. In order to maintain the ICP at a desired lowered level, a control algorithm is constructed. The controller is designed by minimizing a performance criterion, which consists of the mean squared error between the measured ICP and desired ICP, and the squared input. Minimizing the performance criterion specifies the controller. For on-line implementation, an approximately optimal controller is suggested. Simulation results show the feasibility of using this controller to regulate ICP.

Adams, Thomas; Spielman, William; Holmes, Kenneth; Heisey, S.; Chen, Michael

doi: 10.1007/BF02584242pmid: 3898927

The kidney, with its heterogeneous regional perfusion in the two anatomically and functionally distinct vascular beds of the renal cortex and medulla, and with its nonuniform blood vessel geometries, presents a unique challenge for measuring intrarenal blood flow distribution. Determining whole organ perfusion, on the other hand, is comparatively simple for the kidney, but it provides relatively little information about the suspected dependency of renal excretory function on local perfusion rate. Among the variety of methods proposed for gauging regional renal blood flow, some depend on measuring one or more of the tissue's thermal properties. The most straightforward, but least reliable, involve measurements either of focal tissue temperature alone, or of regional tissue thermal gradients. Simply using heat as a diffusible indicator, however, is unreliable as a measure of blood flow, for many of the same reasons that using an inert gas in a dilution technique is unreliable. Recently developed thermal analytical methods, though, hold promise for measuring local tissue blood flow with accuracy and precision. Two of them are reviewed here. One depends on measurement of the effective thermal conductivity of a small mass of tissue by evaluating the steady state ratio between regional unidirectional heat flux across it and the associated temperature gradient in one vector along a segment of it through an imposed spheroidal heat field. The other depends on analyses of tissue temperature decay subsequent to a controlled pulse of heat delivered through a small inserted thermistor bead. Both techniques use bioheat transfer equations to deduce regional blood flow

Iwasaki, Tsutomu; Ritman, Erik; Fiksen-Olsen, Mary; Romero, Juan; Knox, Franklyn

doi: 10.1007/BF02584243pmid: 4037455

The three-dimensional image data generated by the Dynamic Spatial Reconstructor (DSR) enables measurement of the three-dimensional distribution of blood supply in organs. We have applied this imaging technique to evaluate renal cortical blood flow distribution and compare it with distribution of radiolabeled microspheres. The DSR, a high temporal resolution volumetric roentgenographic computed tomographic scanner, was used to scan the volume containing a kidney in 0.13–0.26 s and repeating this scan 8–4 times per s for six s during a renal arteriogram. Five anesthetized dogs were studied in the prone position with the left kidney exteriorized through a flank incision. An electromagnetic flowmeter was placed around the renal artery and a needle placed retrograde into the artery for injection of a 2 cc bolus of contrast agent. During the scan the contrast agent was injected over a four s period during which radioactively labelled microspheres were injected into the left atrium. The tomographic images of approximately 10 parallel, 5 mm thick sagittal slices corresponding to the slices of the kidney used for counting microspheres in the cortical layers were displayed and analyzed. The time point chosen for analysis was the one in which peak brightness (i.e., concentration of contrast agent) was detected in the cortex. The spatial distribution of peak brightness values was compared to the number of microspheres at the same sampling locations. The microsphere-based value of regional cortical blood flow fell below the regression line for the juxtamedullary cortex and above for the outer cortex. This result is consistent with the preferential distribution of microspheres to the outer cortex whereas the contrast agent distributed more uniformly throughout the cortex.

Jamison, Rex; Zimmerhackl, Bernd; Robertson, Channing

doi: 10.1007/BF02584244pmid: 4037456

We adapted the technique of videomicroscopy for direct determination of blood flow in individual capillaries of the papilla of the kidney, the ascending vasa recta (AVR) and descending vasa recta (DVR). The papilla was exposed in anesthetized rats and positioned under a video-camera-microscope and viewed under epi-illumination. The intravenous infusion of fluorescein-isothiocyanate (FITC)-labeled gamma globulin was combined with fluorescence microscopy to enhance the contrast among plasma, red blood cells and capillary walls. On the television monitor, the walls were clearly outlined, enabling the measurement of capillary diameter. The velocity of red cells (V rbc ) in individual vasa recta was measured using the dual slit technique. From the videotape recorded microscopic image of a vas rectum, two photometric signals were obtained by integrating the light intensity from two electronic “windows” positioned closely together over the same capillary. Red cell velocity was calculated by dividing the distance between the two windows by the time delay between signals. The delay was determined using analog correlation tracking or digital cross correlation techniques. Single vasa recta blood flow was calculated from capillary diameter, V rbc , and F (Fahraeus factor), which converts V rbc to average whole blood velocity, V blood . In quartz capillaries the same size as vasa recta, the ratio F=V rbc /V blood =1.42±0.06. Total papillary blood inflow and outflow was calculated by multiplying the total number of DVR or AVR times the mean single capillary blood flow for DVR or AVR, respectively.

Bevan, John; Hwa, Joyce

doi: 10.1007/BF02584245pmid: 4037457

Cerebral vessels of many mammalian species exhibit stretch-dependent or myogenic tone which would be expected to contribute to autoregulation. This tone is dependent on extracellular calcium and probably involves mechanisms other than those associated with agonist and potassium-induced tone. This tone may be contingent upon the redistribution of strategic substances within the cell membrane.

Halpern, Wiliam; Osol, George

doi: 10.1007/BF02584246pmid: 4037458

We examined the diameter responses of isolated and pressurized posterior cerebral artery branches to various static and dynamic pressure alterations. These vessels, dissected from an anatomically identifiable location in the rat brain, developed tone when placed in a normal calcium physiological salt solution (1.6 mM Ca-PSS). Following a series of transmural pressure steps (Δp) of 25 or 50 mm Hg completed in 1–2 s and made every 5 min, they attained additional tone resulting in a mean luminal diameter of 139 μm at 100 mm Hg which was 35% less than their relaxed size measured in 1 mM EGTA-PSS. Continuous measurements of wall thickness and lumen diameter were obtained using a video electronic system in 1–2 mm long arterial segments, and autoregulatory gain factors calculated. Myogenic responses were obtained from each of 6 vessels taken from 6 WKY rats. Diameters following the step pressure changes were usually stable within 2–4 min. The data defined a myogenic regulatory pressure range from 49–145 mm Hg. Gain values averaged about 17% of that necessary for these arteries to maintain perfect flow autoregulation. Our results for myogenicity are comparable with the pressure range for blood flow autoregulation reported by others for the rat. We conclude that myogenic mechanisms, at least in this size artery, are partly responsible for flow autoregulation, and that they are supplemented by metabolic mechanisms operative in the intact rat brain.

Rusch, Nancy; Hermsmeyer, Kent

doi: 10.1007/BF02584247pmid: 4037459

The effects of vasopressin on membrane potential and tension were studied in isolated segments of basilar arteries from the University of Iowa colonies of normotensive inbred Kyoto-Wistar rats (WKY) and stroke-prone spontaneously hypertensive rats (SP-SHR). In the presence of vasopressin (0.01–0.3 IU/ml), basilar arteries from WKY, but not from SP-SHR, developed rhythmic contractions. These contractions were recorded in 13 of 14 WKY basilar arteries, were unaffected by pretreatment with 6-hydroxydopamine, and were characterized by 20–100 dyne oscillations in tension, occurring 1–3 cycles/min, and superimposed on the vasopressin-induced contraction (averaging 60 dynes at 0.01 IU/ml or 160 dynes at 0.3 IU/ml). However, resting membrane potentials were not different in SP-SHR vs. WKY at 37°C, and both strains showed about the same (11 mV) depolarization by 0.1 IU/ml of vasopressin. The rhythmic contractions were enhanced by K+-free solution, and abolished in the presence of high K+ solution (30 mM), suggesting that active Na+−K+ transport may be involved in modulating the rhythmic activity. These findings are consistent with the hypothesis that the vasopressin-induced rhythmic contractions in WKY basilar arteries are at least partly dependent on a reduced activity of electrogenic Na+−K+ active transport in WKY as compared to SP-SHR.

Baumbach, Gary; Heistad, Donald

doi: 10.1007/BF02584248pmid: 3898928

Autoregulation of cerebral blood flow is heterogeneous in several ways: regional, segmental, and temporal. We have found regional heterogeneity of the autoregulatory response during both acute reductions and increases in systemic arterial presure. Changes in blood flow are less in brain stem than in cerebrum during decreases and increases in cerebral perfusion pressure. Segmental heterogeneity of autoregulation has been demonstrated in two ways. Direct determination of segmental cerebral vascular resistance indicates that, while small cerebral vessels (<200 μm in diameter) make a major contribution to autoregulation during acute increases in pressure between 80 and 100 mm Hg, the role of large cerebral arteries (>200 μm) becomes increasingly important to the autoregulatory response at pressures above 100 mm Hg. Measurement of changes in diameter of pial vessels has shown that, during acute hypotension, autoregulation occurs predominantly in small resistance vessels (<100 μm). Finally, there is temporal heterogeneity of autoregulation. Sudden increases in arterial pressure produce transient increases in blood flow, which are not observed under steady-state conditions. In addition, the blood-brain barrier is more susceptible to hypertensive disruption after rapid, compared to step-wise, increases in arterial pressure. Thus, when investigating cerebral vascular autoregulation, regional, segmental, and temporal differences in the autoregulatory response must be taken into consideration.