Hemodynamic physiology and thermoregulation in liposuction

Little is known about the physiology of large-volume liposuction. Patients are exposed to prolonged procedures, general anesthesia, fluid shifts, and infusion of high doses of epinephrine and lidocaine. Consequently, the authors examined the thermoregulatory and cardiovascular responses to liposuction by assessing multiple physiologic factors. The aims of their study were to serially determine hemodynamic parameters perioperatively, to quantify perioperative and postoperative plasma epinephrine levels, and to chronologically document fluctuations in core body temperature. Five female volunteers with American Society of Anesthesiologists' physical status I and II underwent moderate- to large-volume liposuction. Heart rate, blood pressure, mean pulmonary arterial pressure, cardiac index, and central venous pressure were monitored. Serum epinephrine levels and core body temperature were assessed perioperatively. The hemodynamic responses to liposuction were characterized by an increase in cardiac index (57 percent), heart rate (47 percent), and mean pulmonary arterial pressure (44 percent) (p < 0.05). Central venous pressure was not significantly altered. Maximum epinephrine levels were observed 5 to 6 hours after induction. Significant correlations between cardiac index and epinephrine concentrations were shown intraoperatively (r = 0.75). All patients developed intraoperative low body temperatures (mean 35.5 degrees C). An overall enhanced cardiac function was observed in patients subsequent to large-volume liposuction. The etiology of the altered cardiac parameters was multifactorial but may have been attributable in part to the administration of epinephrine, which counters the effects of general anesthesia and operative hypothermia. Additional explanations for raised cardiac output may be hemodilution or emergence from general anesthesia. Elevated mean pulmonary arterial pressure may be a result of subclinical fat embolism demonstrated in previous porcine studies, although fat was not observed in urine. The unchanged central venous pressure levels indicate that young healthy patients with compliant right ventricles can accommodate the fluid loads of large-volume liposuction. Overall hemodynamic parameters remained within safe limits. Within these surgical parameters, patients should be clinically screened for cardiovascular and blood pressure disorders before liposuction is undertaken, and preventative measures should be taken to limit intraoperative hypothermia.     

Renal medullary blood flow: its measurement and physiology

The vasculature of the mammalian renal medulla is complex, having neither discrete input nor output. There is also efficient countercurrent exchange between ascending and descending vasa recta in the vascular bundles. These considerations have hampered measurement of medullary blood flow since they impose pronounced constraints on methods used to assess flow. Three main strategies have been used: (i) indicator extraction; (ii) erythrocyte velocity tracking; and (iii) indicator dilution. These are discussed with respect to their assumptions, requirements, and limitations. There is a consensus that medullary blood flow is autoregulated, albeit over a narrower pressure range than is total renal blood flow. When normalized to gram tissue weight, medullary blood flow in the dog is similar to that in the rat, on the order of 1 to 1.5 mL X min-1 X g-1. This is considerably greater than estimated by the radioiodinated albumin uptake method which has severe conceptual and practical problems. From both theoretical and experimental evidence it seems that urinary concentrating ability is considerably less sensitive to changes in medullary blood flow than is often assumed.     

Kinetic modeling in physiology

The author discusses the construction of model biochemical/physiological systems to fit experimental data which is always incomplete. He suggests that the first experiments on any system should be first-order (usually tracer) perturbations of steady-state systems. The proposed model can then be given a preliminary fit to the data; criteria for best fit, consistency, and uniqueness of fit are suggested. More data may then be obtained by perturbations at another steady-state, and finally by studying transient situations.References are included to some of the author's own publications in which these matters are discussed in detail.     

Role of skin blood flow and sweating rate in exercise thermoregulation after bed rest.

Two potential mechanisms, reduced skin blood flow (SBF) and sweating rate (SR), may be responsible for elevated intestinal temperature (T(in)) during exercise after bed rest and spaceflight. Seven men underwent 13 days of 6 degrees head-down bed rest. Pre- and post-bed rest, subjects completed supine submaximal cycle ergometry (20 min at 40% and 20 min at 65% of pre-bed rest supine peak exercise capacity) in a thermoneutral room. After bed rest, T(in) was elevated at rest (+0.31 +/- 0.12 degrees C) and at the end of exercise (+0.33 +/- 0.07 degrees C). Percent increase in SBF during exercise was less after bed rest (211 +/- 53 vs. 96 +/- 31%; P < or = 0.05), SBF/T(in) threshold was greater (37.09 +/- 0.16 vs. 37.33 +/- 0.13 degrees C; P < or = 0.05), and slope of SBF/T(in) tended to be reduced (536 +/- 184 vs. 201 +/- 46%/ degrees C; P = 0.08). SR/T(in) threshold was delayed (37.06 +/- 0.11 vs. 37.34 +/- 0.06 degrees C; P < or = 0.05), but the slope of SR/T(in) (3.45 +/- 1.22 vs. 2.58 +/- 0.71 mg x min-1 x cm-2 x degrees C-1) and total sweat loss (0.42 +/- 0.06 vs. 0.44 +/- 0.08 kg) were not changed. The higher resting and exercise T(in) and delayed onset of SBF and SR suggest a centrally mediated elevation in the thermoregulatory set point during bed rest exposure.     

Multiscale modeling of blood flow: from single cells to blood rheology

Mesoscale simulations of blood flow, where the red blood cells are described as deformable closed shells with a membrane characterized by bending rigidity and stretching elasticity, have made much progress in recent years to predict the flow behavior of blood cells and other components in various flows. To numerically investigate blood flow and blood-related processes in complex geometries, a highly efficient simulation technique for the plasma and solutes is essential. In this review, we focus on the behavior of single and several cells in shear and microcapillary flows, the shear-thinning behavior of blood and its relation to the blood cell structure and interactions, margination of white blood cells and platelets, and modeling hematologic diseases and disorders. Comparisons of the simulation predictions with existing experimental results are made whenever possible, and generally very satisfactory agreement is obtained.     

Multiscale modeling of red blood cell mechanics and blood flow in malaria

Red blood cells (RBCs) infected by a Plasmodium parasite in malaria may lose their membrane deformability with a relative membrane stiffening more than ten-fold in comparison with healthy RBCs leading to potential capillary occlusions. Moreover, infected RBCs are able to adhere to other healthy and parasitized cells and to the vascular endothelium resulting in a substantial disruption of normal blood circulation. In the present work, we simulate infected RBCs in malaria using a multiscale RBC model based on the dissipative particle dynamics method, coupling scales at the sub-cellular level with scales at the vessel size. Our objective is to conduct a full validation of the RBC model with a diverse set of experimental data, including temperature dependence, and to identify the limitations of this purely mechanistic model. The simulated elastic deformations of parasitized RBCs match those obtained in optical-tweezers experiments for different stages of intra-erythrocytic parasite development. The rheological properties of RBCs in malaria are compared with those obtained by optical magnetic twisting cytometry and by monitoring membrane fluctuations at room, physiological, and febrile temperatures. We also study the dynamics of infected RBCs in Poiseuille flow in comparison with healthy cells and present validated bulk viscosity predictions of malaria-infected blood for a wide range of parasitemia levels (percentage of infected RBCs with respect to the total number of cells in a unit volume).     

Cold physiology: postprandial blood flow dynamics and metabolism in the Antarctic fish Pagothenia borchgrevinki

Previous studies on metabolic responses to feeding (i.e. the specific dynamic action, SDA) in Antarctic fishes living at temperatures below zero have reported long-lasting increases and small peak responses. We therefore hypothesized that the postprandial hyperemia also would be limited in the Antarctic fish Pagothenia borchgrevinki. The proportion of cardiac output directed to the splanchnic circulation in unfed fish was 18%, which is similar to temperate fish species. Contrary to our prediction, however, gastrointestinal blood flow had increased by 88% at twenty four hours after feeding due to a significant increase in cardiac output and a significant decrease in gastrointestinal vascular resistance. While gastric evacuation time appeared to be longer than in comparable temperate species, digestion had clearly commenced twenty four hours after feeding as judged by a reduction in mass of the administered feed. Even so, oxygen consumption did not increase suggesting an unusually slowly developing SDA. Adrenaline and angiotensin II was injected into unfed fish to investigate neuro-humoral control mechanisms of gastrointestinal blood flow. Both agonists increased gastrointestinal vascular resistance and arterial blood pressure, while systemic vascular resistance was largely unaffected. The hypertension was mainly due to increased cardiac output revealing that the heart and the gastrointestinal vasculature, but not the somatic vasculature, are important targets for these agonists. It is suggested that the apparently reduced SDA in P. borchgrevinki is due to a depressant effect of the low temperature on protein assimilation processes occurring outside of the gastrointestinal tract, while the gastrointestinal blood flow responses to feeding and vasoactive substances resemble those previously observed in temperate species.     

Numerical models of auto-regulation and blood flow in the cerebral circulation

A two-dimensional time-dependent computational fluid dynamics model of the Circle of Willis has been developed. To simulate, not only the peripheral resistance of the cerebrovascular tree but also its auto-regulation function, a new "active" boundary condition has been defined and developed using control theory to provide a model of the feedback mechanism. The model was then used to simulate different common abnormalities of the Circle of Willis while a pressure drop, simulating a rapid compression of the right internal carotid artery, was imposed. Test results using a simple tube compared excellently with experiment. The total time-dependent flux for each efferent artery was tabulated and showed the important relationship between geometrical variations in the Circle of Willis and the auto-regulation of blood flow by vascular vaso-dilation and contraction. From this study, it was found that the worst case seemed to be that of a missing or dysfunctional right A1 segment of the anterior cerebral artery. The use of valid physiological models of the peripheral resistance allows for more realistic models of the blood flow in the Circle whilst allowing an easy extension to 3D patient specific simulations.     

Human muscle length-dependent changes in blood flow

Although a multitude of factors that influence skeletal muscle blood flow have been extensively investigated, the influence of muscle length on limb blood flow has received little attention. Thus the purpose of this investigation was to determine if cyclic changes in muscle length influence resting blood flow. Nine healthy men (28 ± 4 yr of age) underwent a passive knee extension protocol during which the subjects' knee joint was passively extended and flexed through 100-180° knee joint angle at a rate of 1 cycle per 30 s. Femoral blood flow, cardiac output (CO), heart rate (HR), stroke volume (SV), and mean arterial pressure (MAP) were continuously recorded during the entire protocol. These measurements revealed that slow passive changes in knee joint angle did not have a significant influence on HR, SV, MAP, or CO; however, net femoral blood flow demonstrated a curvilinear increase with knee joint angle (r(2) = 0.98) such that blood flow increased by ～90% (125 ml/min) across the 80° range of motion. This net change in blood flow was due to a constant antegrade blood flow across knee joint angle and negative relationship between retrograde blood flow and knee joint angle (r(2) = 0.98). Thus, despite the absence of central hemodynamic changes and local metabolic factors, blood flow to the leg was altered by changes in muscle length. Therefore, when designing research protocols, researchers need to be cognizant of the fact that joint angle, and ultimately muscle length, influence limb blood flow.     

Fractal modeling of pulmonary blood flow heterogeneity

The heterogeneity of pulmonary blood flow is not adequately described by gravitational forces alone. We investigated the flow distributions predicted by two fractally branching vascular models to determine how well such networks could explain the observed heterogeneity. The distribution of flow was modeled with a dichotomously branching tree in which the fraction of blood flow from the parent to the daughter branches was gamma and 1-gamma repeatedly at each generation. In one model gamma was held constant throughout the network, and in the other model gamma varied about a mean of 0.5 with a standard deviation of sigma. Both gamma and sigma were optimized in each model for the best fit to pulmonary blood flow data from experimental animals. The predicted relative dispersion of flow from the two model fractal networks produced an excellent fit to the observed data. These fractally branching models relate structure and function of the pulmonary vascular tree and provide a mechanism to describe the spatially correlated distribution of flow and the gravity-independent heterogeneity of blood flow.     

A new simplified bioheat equation for the effect of blood flow on local average tissue temperature

A new simplified three-dimensional bioheat equation is derived to describe the effect of blood flow on blood-tissue heat transfer. In two recent theoretical and experimental studies [1, 2] the authors have demonstrated that the so-called isotropic blood perfusion term in the existing bioheat equation is negligible because of the microvascular organization, and that the primary mechanism for blood-tissue energy exchange is incomplete countercurrent exchange in the thermally significant microvessels. The new theory to describe this basic mechanism shows that the vascularization of tissue causes it to behave as an anisotropic heat transfer medium. A remarkably simple expression is derived for the tensor conductivity of the tissue as a function of the local vascular geometry and flow velocity in the thermally significant countercurrent vessels. It is also shown that directed as opposed to isotropic blood perfusion between the countercurrent vessels can have a significant influence on heat transfer in regions where the countercurrent vessels are under 70-micron diameter. The new bioheat equation also describes this mechanism.     

Platelet physiology: in cold blood

One of the impediments precluding long-term storage of platelets for blood transfusion is that refrigerated platelets are rapidly cleared from the circulation upon transfusion. New evidence suggests that this clearance is mediated in the liver by the Mac-1 integrin on Kupffer cells recognizing clustered GPIb receptor on platelets, leading to platelet phagocytosis.     

Mathematical modeling of gravitational effects on the circulation: importance of the time course of venous pooling and blood volume changes in the lungs

A dip in blood pressure (BP) in response to head-up tilt (HUT) or active standing might be due to rapid pooling in the veins below the heart (preload) or muscle activation-induced drop in systemic vascular resistance (afterload). We hypothesized that, in the cardiovascular response to passive HUT, where, in contrast to active standing, little BP dip is observed, features affecting the preload play a key role. We developed a baroreflex model combined with a lumped-parameter model of the circulation, including viscoelastic stress-relaxation of the systemic veins. Cardiac contraction is modeled using the varying-elastance concept. Gravity affects not only the systemic, but also the pulmonary, circulation. In accordance with the experimental results, model simulations do not show a BP dip on HUT; the tilt-back response is also realistic. If it is assumed that venous capacities are steady-state values, the introduction of stress-relaxation initially reduces venous pooling. The resulting time course of venous pooling is comparable to measured impedance changes. When venous pressure-volume dynamics are neglected, rapid (completed within 30 s) venous pooling leads to a drop in BP. The direct effect of gravity on the pulmonary circulation influences the BP response in the first approximately 5 s after HUT and tilt back. In conclusion, the initial BP response to HUT is mainly determined by the response of the venous system. The time course of lower body pooling is essential in understanding the response to passive HUT.     

Starvation-induced changes in metabolic rate, blood flow, and regional energy expenditure in rats

Starvation results in an energy-conserving reduction in metabolic rate that has features of an adaptive response. Tissue and organ sites of this response were investigated by examining the effects of starvation for 5 d on tissue blood flow (microsphere method) and regional arteriovenous O2 differences ((a-v)O2) in conscious rats resting quietly at 28 degrees C. Comparison was with fed and overnight-fasted animals. Whole body resting metabolic rates (MR), colonic temperatures (Tc), and tissue weights were also determined. Quantitative changes in energy expenditure (as O2 consumption) were obtained for two regions: the portal-drained viscera (PDV) and the hindquarters (HQ). Fasting overnight resulted in increased blood flow to white adipose tissue (WAT) and decreased flow to the brain, PDV, testes, and skin; however, MR, Tc, the two regional ((a-v)O2, and the weights of most tissues were not significantly altered. In comparison with overnight fasting, starvation for 5 d resulted in a 13% reduction in body weight, weight loss in many tissues and organs, a 26% reduction in MR, a decline of 0.5 degree C in Tc, decreased (a-v)O2 across both the PDV and HQ, reduced cardiac output, and decreased blood flow to the heart, PDV, skin, WAT, leg muscle, HQ, and the musculoskeletal body as a whole. Utilization of O2 by the PDV and HQ (flow X (a-v)O2) declined by amounts that accounted for 22 and 18%, respectively, of the reduction in MR. The reductions in cardiac output (18%) and heart blood flow (36%) indicate that the heart also made a contribution to energy conservation (roughly estimated as 5%).(ABSTRACT TRUNCATED AT 250 WORDS)