The variability of blood pressure due to small changes of hematocrit

Association between Hematocrit and Acute Kidney Injury in Patients with Acute Myocardial Infarction

Backgrounds

Hematocrit is found an independent risk factor for acute kidney injury (AKI) in certain patients, but this effect in patients with acute myocardial infarction (AMI) is unclear. We aim to identify the relationship between hematocrit and AKI in patients with AMI.

Methods

The patient data for the discovery and validation cohorts were extracted from the electronic Intensive Care Unit database and the Medical Information Mart for Intensive Care III database, respectively, to identify the relationship between hematocrit and AKI. With normal hematocrit as the reference, patients were divided into five groups based on the initial hematocrit value. The primary outcome was AKI during hospitalization. A multivariable logistic regression and a marginal effect analysis were used to evaluate the relationship between hematocrit and AKI.

Results

In this study, a total of 9692 patients diagnosed with AMI were included, with 7712 patients in the discovery cohort and 1980 patients in the validation cohort. In the discovery cohort, hematocrit in 30-33%, 27-30% or < 27% were independent risk factors for AKI in the multivariate logistic analysis, with odds ratio (OR) of 1.774 (95% confidence interval [CI]: 1.203-2.617, p = 0.004), 1.834 (95% CI: 1.136-2.961, p = 0.013) and 2.577 (95% CI: 1.510-4.397, p < 0.001), respectively. Additionally, in the validation cohort, low hematocrit levels independently contributed to an increased risk of AKI among patients with AMI. During the analysis of marginal effects, a significant negative linear relationship between hematocrit levels and AKI was observed.

Conclusions

Decreased hematocrit was an independent risk factor for AKI in patients with AMI. The relationship between hematocrit and AKI was negative linear.

Recovery Sleep After Sleep Restriction Is Insufficient to Return Elevated Daytime Heart Rate and Systolic Blood Pressure to Baseline Levels

OBJECTIVE

Sleep restriction alters daytime cardiac activity, including elevating heart rate (HR) and blood pressure (BP). There is minimal research on the cumulative effects of sleep loss and the response after subsequent recovery sleep on HR and BP. This study examined patterns of HR and BP across baseline, sleep restriction, and recovery conditions using multiple daytime cardiac measurements.

METHODS

Participants (15 healthy men, mean [standard deviation] = 22.3 [2.8] years) completed an 11-day inpatient protocol with three nights of 10 hours/night baseline sleep opportunity, five sleep restriction nights (5-hour/night sleep opportunity), and two recovery nights (10-hour/night sleep opportunity). Resting HR and BP were measured every 2 hours during wake. Multilevel models with random effects for individuals examined daytime HR and BP across study conditions and days into the study.

RESULTS

Mean daytime HR was 1.2 (0.5) beats/min lower during sleep restriction compared with baseline ( p < .001). During recovery, HR was 5.5 (1.0) beats/min higher ( p < .001), and systolic BP (SBP) was 2.9 (1.1) mm Hg higher ( p = .009). When accounting for days into the study (irrespective of condition) and measurement timing across the day, HR increased by 7.6 beats/min and SBP increased by 3.4 mm Hg across the study period ( p < .001).

CONCLUSIONS

Our findings suggest that daytime HR and SBP increase after successive nights of sleep restriction, even after accounting for measurement time of day. HR and SBP did not recover to baseline levels after two recovery nights of sleep, suggesting that longer recovery sleep may be necessary to recover from multiple, consecutive nights of moderate sleep restriction.

A Mathematical Model for Blood Flow Accounting for the Hematological Disorders

This paper considers a mathematical model that accounts for the hematological disorders of blood in its flow in human arteries. Blood is described as a Newtonian fluid but with its viscosity as a function of the hematocrit, plasma viscosity, and shape factor of the red blood cells. The artery is modeled as a flexible circular pipe with the blood flow as oscillatory. This model is solved using HAM (Homotopy Analysis Method), an approximate analytical method, and we computed expressions for wall shear stress (WSS) and volumetric flow rate. With the help of publicly available data, blood flow in the human femoral artery for male and female populations aged 19 to 60 and above years is simulated for healthy, anemia, and polycythemia cases. The model projected a significant difference in the mean volumetric flow rates in the conditions mentioned above. Results also indicated that the mean WSS of healthy and anemic populations are not significantly different. However, a significant difference in the mean has been observed in healthy and polycythemic conditions. Furthermore, a 33.3% decrease in hematocrit value from that in the normal range (taken as 0.45) of a healthy population has increased the flow rate by 33.5%. For a value 33.3% above 0.45, there is a decrease of 42.7% in the flow rate.

Association between hemoglobin variability and incidence of hypertension over 40 years: a Korean national cohort study

Hemoglobin level determines blood viscosity and as hemoglobin level rises, blood pressure rises. However, hemoglobin level in individuals is not fixed and change in hemoglobin is affected by various clinical conditions. The purpose of this study is to investigate whether the hemoglobin variability affects the development of hypertension using Korean cohort database. This study was conducted with 94,798 adults (age ≥ 40 years) who visited the health screening in 2006 or 2007 (index year) and had at least 3 health screenings from 2002 to 2007. Hemoglobin variability was assessed by 3 indices of coefficient of variation (CV), standard deviation, and variability independent of the mean. Cox proportional hazard regression analysis was performed for each index of quartile groups (Q1–Q4). A total of 29,145 participants (30.7%) had the incidence of hypertension during a median follow-up of 7.4 ± 2.5 years. In the multivariable adjusted model, the hazard ratio and 95% confidence interval for incidence of hypertension of Q2, Q3, and Q4 compared with Q1 of hemoglobin variability CV were 1.014 [0.981–1.047], 1.064 [1.030–1.099] and 1.094 [1.059–1.131] respectively. The results were consistent in various sensitivity and subgroup analyses. This study showed that hemoglobin variability could be associated with hypertension development.

High Hematocrit Levels Are Associated with Risk of Cardiovascular Mortality among Middle-Aged Japanese Women: The Ibaraki Prefectural Health Study (IPHS)

In Japan, heart disease and cerebrovascular disease were the second and fourth leading cause of death, respectively in 2014. Hematocrit, the ratio of the red blood cells to the total volume of blood is known to play a role in cardiovascular diseases. However, the relationship between elevated hematocrit and the risk of cardiovascular disease based on sex has not been examined in Asian countries. We analyzed data from the Ibaraki Prefectural Health Study, a community-based large cohort study, which included 87,533 individuals, aged 40 to 79 years living in 38 municipalities of the Ibaraki Prefecture, who had completed an annual health checkup in 1993, and were followed until 2013. The hematocrit levels of the subjects were categorized into 5 quintiles (Q1-Q5), and hazard ratios for cause-specific mortality were calculated using the Cox proportional hazards regression models. Age and other cardiovascular risk factors were the covariates in the study. During the follow-up for a mean of 17.9 years, 1,207 deaths (615 men and 592 women) due to acute myocardial infarction (AMI) were reported in this population. The p values for the trend in the Q3-Q5 groups were 0.661 for men and 0.020 for women. Additionally, these values were significant in younger (40 to 59 years) women but not in older (60 to 79 years) women. This study is the first one to demonstrate an association between high hematocrit level and risk of AMI mortality in younger Japanese women, but not in men and older women.

Exercise for Prevention and Relief of Cardiovascular Disease: Prognoses, Mechanisms, and Approaches

This review is aimed at summarizing the new findings about the multiple benefits of exercise on cardiovascular disease (CVD). We pay attention to the prevalence and risk factors of CVD and mechanisms and recommendations of physical activity. Physical activity can improve insulin sensitivity, alleviate plasma dyslipidemia, normalize elevated blood pressure, decrease blood viscosity, promote endothelial nitric oxide production, and improve leptin sensitivity to protect the heart and vessels. Besides, the protective role of exercise on the body involves not only animal models in the laboratory but also clinical studies which is demonstrated by WHO recommendations. The general exercise intensity for humans recommended by the American Heart Association to prevent CVD is moderate exercise of 30 minutes, 5 times a week. However, even the easiest activity is better than nothing. What is more, owing to the different physical fitness of individuals, a standard exercise training cannot provide the exact treatment for everyone. So personalization of exercise will be an irresistible trend and bring more beneficial effects with less inefficient physical activities. This paper reviews the benefits of exercise contributing to the body especially in CVD through the recent mechanism studies.

Recruitment of non-perfused sublingual capillaries increases microcirculatory oxygen extraction capacity throughout ascent to 7126 m

KEY POINTS

A physiological response to increase microcirculatory oxygen extraction capacity at high altitude is to recruit capillaries. In the present study, we report that high altitude-induced sublingual capillary recruitment is an intrinsic mechanism of the sublingual microcirculation that is independent of changes in cardiac output, arterial blood pressure or systemic vascular hindrance. Using a topical nitroglycerin challenge to the sublingual microcirculation, we show that high altitude-related capillary recruitment is a functional response of the sublingual microcirculation as opposed to an anatomical response associated with angiogenesis. The concurrent presence of a low capillary density and high microvascular reactivity to topical nitroglycerin at sea level was found to be associated with a failure to reach the summit, whereas the presence of a high baseline capillary density with the ability to further increase maximum recruitable capillary density upon ascent to an extreme altitude was associated with summit success.

ABSTRACT

A high altitude (HA) stay is associated with an increase in sublingual capillary total vessel density (TVD), suggesting microvascular recruitment. We hypothesized that microvascular recruitment occurs independent of cardiac output changes, that it relies on haemodynamic changes within the microcirculation as opposed to structural changes and that microcirculatory function is related to individual performance at HA. In 41 healthy subjects, sublingual handheld vital microscopy and echocardiography were performed at sea level (SL), as well as at 6022 m (C2) and 7042 m (C3), during ascent to 7126 m within 21 days. Sublingual topical nitroglycerin was applied to measure microvascular reactivity and maximum recruitable TVD (TVD_NG ). HA exposure decreased resting cardiac output, whereas TVD (mean ± SD) increased from 18.81 ± 3.92 to 20.92 ± 3.66 and 21.25 ± 2.27 mm mm^-2 (P < 0.01). The difference between TVD and TVD_NG was 2.28 ± 4.59 mm mm^-2 at SL (P < 0.01) but remained undetectable at HA. Maximal TVD_NG was observed at C3. Those who reached the summit (n = 15) demonstrated higher TVD at SL (P < 0.01), comparable to TVD_NG in non-summiters (n = 21) at SL and in both groups at C2. Recruitment of sublingual capillary TVD to increase microcirculatory oxygen extraction capacity at HA was found to be an intrinsic mechanism of the microcirculation independent of cardiac output changes. Microvascular reactivity to topical nitroglycerin demonstrated that HA-related capillary recruitment is a functional response as opposed to a structural change. The performance of the vascular microcirculation needed to reach the summit was found to be associated with a higher TVD at SL and the ability to further increase TVD_NG upon ascent to extreme altitude.

Prevalence and risk factors for hypertensive crisis in a predominantly African American inner-city community

PURPOSE

A 3-year case control study was conducted to determine the prevalence of hypertensive crisis and its subtypes, hypertensive emergency and hypertensive urgency. The secondary objectives were to identify risk factors for hypertensive emergencies and to determine the odds of developing acute target organ damage among predominantly African American patients with a confirmed diagnosis of hypertension.

MATERIALS AND METHODS

Using emergency department medical records, patients with a confirmed diagnosis of hypertension were identified. From the pool of hypertensive patients, cases and controls were selected and matched 1:1 for age, gender and race. Cases were hypertensive patients with hypertensive crisis, defined as BP ≥ 200/120 mmHg. Controls had a diagnosis of hypertension and BP < 200/120 mmHg. Cases and controls, as well as cases with hypertensive emergencies and hypertensive urgencies were compared based on important demographic and clinical variables.

RESULTS

Almost 90% of study population were African Americans. The prevalence of hypertensive crisis was 11.4% and hypertensive emergencies was 3.2%. Hypertensive emergencies accounted for 28% of patients with crisis. The predictors for hypertensive emergencies were older age (p = .002), male gender (p < .007), anemia (p < .0001), history of coronary artery disease (p < .001), congestive heart failure (p < .001) and chronic renal insufficiency (p < .001). Having healthcare insurance and access to medical care did not reduce the odds of developing hypertensive emergencies. Race was not a significant risk factor in the progression from hypertensive crisis to hypertensive emergencies (p = .47).

CONCLUSIONS

The study highlights the high prevalence of hypertensive crisis and hypertensive emergencies in the predominantly African American urban population, which is 5 times the United States average. However, race is not a predictor of development of hypertensive emergencies and acute target organ damage in patients with already severely elevated blood pressure.

Understanding hypertension in the light of Ayurveda

Different theories have been proposed to explain hypertension from an Ayurvedic perspective, but there is no consensus amongst the experts. A better understanding of the applied physiology and etio-pathogenesis of hypertension in the light of Ayurvedic principles is being attempted to fill this gap. A detailed review of available Ayurvedic literature was carried out to understand the physiology of blood pressure and etio-pathogenesis of hypertension from the perspective of Ayurveda. Many parallels were drawn from the concepts such as Shad Kriyakala (six stages of Dosha imbalance) and Avarana of Doshas (occlusion in the normal functioning of the Doshas) to the modern pathogenesis of hypertension to gain a deeper understanding of it. Hypertension without specific symptoms in its mild and moderate stages cannot be considered as a disease in Ayurveda. It appears to be an early stage of pathogenesis and a risk factor for development of diseases affecting the heart, brain, kidneys and eyes etc. Improper food habits and modern sedentary lifestyle with or without genetic predisposition provokes and vitiates all the Tridoshas to trigger the pathogenesis of hypertension. It is proposed that hypertension is to be understood as the Prasara-Avastha which means spread of vitiated Doshas from their specific sites, specifically of Vyana Vata, Prana Vata, Sadhaka Pitta and Avalambaka Kapha along with Rakta in their disturbed states. The Avarana (occlusion of normal functioning) of Vata Dosha by Pitta and Kapha can be seen in the Rasa-Rakta Dhathus, which in turn hampers the functioning of the respective Srotas (micro-channels) of circulation.

Blood rheology as a mirror of endocrine and metabolic homeostasis in health and disease1

Rheological properties of plasma and blood cells are markedly influenced by the surrounding milieu: physicochemical factors, metabolism and hormones. Acid/base status, osmolality, lipid status, plasma protein pattern, oxidative stress induced by increased free radicals production, endothelium-derived factors such as nitric oxide (NO), achidonic acid derivatives modulate both red blood cell (RBC) and white cell mechanics. Therefore, regulatory axes involving liver, endothelium, kidney, pancreas, adrenal gland, endocrine heart, adipose tissue, pituitary gland, and surely other tissues play important roles in the regulation of blood fluidity. A comprehensive picture of all this complex network of regulatory loops is still unavailable but current progress of knowledge suggest that some attempts can currently be made.

Renal autoregulation and blood pressure management in circulatory shock

The importance of personalized blood pressure management is well recognized. Because renal pressure–flow relationships may vary among patients, understanding how renal autoregulation may influence blood pressure control is essential. However, much remains uncertain regarding the determinants of renal autoregulation in circulatory shock, including the influence of comorbidities and the effects of vasopressor treatment. We review published studies on renal autoregulation relevant to the management of acutely ill patients with shock. We delineate the main signaling pathways of renal autoregulation, discuss how it can be assessed, and describe the renal autoregulatory alterations associated with chronic disease and with shock.

Hematocrit and hematocrit viscosity ratio during exercise in athletes: Even closer to predicted optimal values?

The hemorheological theory of optimal hematocrit suggests that the best value of hematocrit (hct) should be that which results in the highest value of the hematocrit/viscosity (h/η) ratio. Trained athletes compared to sedentary subjects have a lower hct, but a higher h/η, and endurance training reduces the discrepancy between the actual hct and the ⪡ideal⪢ hct that can be predicted with a theoretical curve of h/η vs hct constructed with Quemada's model. In this study we investigated what becomes this homeostasis of h/η and hct during acute exercise in 19 athletes performing a 25 min exercise test. VO2max is negatively correlated to resting hct and positively correlated to discrepancy between actual and ideal resting hct which is correlated to the maximal rise in hct during exercise. Predicted and actual values of the h/η were fairly correlated (r = 0.970 p < 0.001) but the actual value was lower at rest and this discrepancy vanished at 25 min exercise. Exercise-induced decrease in discrepancy between actual and theoretical h/η was negatively correlated with the score of overtraining. All these findings suggest that h/η is a regulated parameter and that its model-predicted ⪡optimal⪢ values yield a ⪡theoretical optimal⪢ hct which is close to the actual value and even closer when athletes are well trained. In addition, acute exercise sets h/η closer from its predicted ideal value and this adaptation is impaired when athletes quote elevated scores on the overtraining questionnaire.

The optimum hematocrit

The hematocrit (Hct) determines the oxygen carrying capacity of blood, but also increases blood viscosity and thus flow resistance. From this dual role the concept of an optimum Hct for tissue oxygenation has been derived. Viscometric studies using the ratio Hct/blood viscosity at high shear rate showed an optimum Hct of 50-60% for red blood cell (RBC) suspensions in plasma. For the perfusion of an artificial microvascular network with 5-70μm channels the optimum Hct was 60-70% for high driving pressures. With lower shear rates or driving pressures the optimum Hct shifted towards lower values. In healthy, well trained athletes an increase of the Hct to supra-normal levels can increase exercise performance. These data with healthy individuals suggest that the optimum Hct for oxygen transport may be higher than the physiological range (35-40% in women, 39-50% in men). This is in contrast to clinical observations. Large clinical studies have repeatedly shown that a correction of anemia in a variety of disorders such as chronic kidney disease, heart failure, coronary syndrome, oncology, acute gastrointestinal bleeding, critical care, or surgery have better clinical outcomes when restrictive transfusion strategies are applied. Actual guidelines, therefore, recommend a transfusion threshold of 7-8 g/dL hemoglobin (Hct 20-24%) in stable, hospitalized patients. The discrepancy between the optimum Hct in health and disease may be due to factors such as decreased perfusion pressures (low cardiac output, vascular stenoses, change in vascular tone), endothelial cell dysfunction, leukocyte adhesion and others.

Association of hematocrit and pre-hypertension among Chinese adults: the CRC study

Elevated blood pressure is regarded as an independent risk factor for cardiovascular diseases and diabetes. We examined the relation between hematocrit and pre-hypertension as well as the effect of sex, obesity, fasting glucose, and lipids in Chinese adults. The study samples were from a community-based health examination survey in China and included a total of 2,3691 patients with blood pressure in normal range. The odds ratios [ORs, 95 % confidence interval (CI)] of pre-hypertension across increasing quartiles of hematocrit were 1.000, 1.176 (1.050-1.318), 1.213 (1.081-1.363), and 1.364 (1.209-1.540) (P for trend < 0.001), when adjusted for age, sex, body mass index, glutamic-pyruvic transaminase, glutamic-oxalocetie transaminase, serum uric acid, glucose, and lipids. Associations were significant in both men and women, but not in individuals older than 60 years. In addition, low-density lipoprotein cholesterol significantly interacted with hematocrit (P for interaction <0.024). The associations were more evident in patients with low (P < 0.001) and median LDL-C levels (P < 0.013) than those with high glucose levels. Hematocrit was associated with pre-hypertension, and was independent of metabolic risk factors. These associations were not significant in older individuals and low-density lipoprotein cholesterol may modify these associations.

Association of hemodilution and blood pressure in uncontrolled bleeding

BACKGROUND

Hemodynamic status and coagulation capacity affect blood loss after injury. The most advantageous fluid and blood pressure to optimize resuscitation and minimize perturbation of coagulation are unclear. We investigated interactions of isovolumic hemodilution on hemodynamics, coagulation, and blood loss after injury.

METHODS

Twenty-five male rats were randomized into three groups (Whole Blood Uncontrolled Blood Pressure [WBU], n = 7; Lactated Ringers Uncontrolled Blood Pressure [LRU], n = 10; Whole Blood Controlled Blood Pressure [WBC], n = 8) with isovolumic hemodilution of 50% blood volume, with and without control of pre-injury blood pressure. All rats underwent uniform grade IV liver injury 30 min after serial exchanges. Post-injury blood loss and coagulation function were measured.

RESULTS

Dilution occurred, determined by hematocrit, with LRU having a greater reduction. Pre-injury mean arterial pressure (MAP) decreased compared with baseline (98 ± 7 mmHg) with LRU (62 ± 14 mmHg) and WBC (61 ± 10 mmHg), resulting in WBU (101 ± 13 mmHg) being significantly higher and not changed from baseline. Post-injury, MAP decreased from pre-injury, with LRU significantly lower than the other two groups. No differences were observed in prothrombin time/international normalized ratio or thromboelastography. Bleed volume was significantly different between groups: WBU < WBC < LRU and associated with the pre-injury MAP. Controlling baseline MAP, dilution with Lactated Ringers (LR) resulted in greater blood loss than whole blood (3.0 ± 0.4 versus 1.9 ± 0.3 mL).

CONCLUSIONS

In this rat model of liver injury, blood loss was associated with baseline MAP and type of fluid used for dilution. Hemodilution with LR did not produce coagulopathy based on laboratory values. When controlling baseline MAP, dilution with LR increased bleeding, confirming a functional coagulopathic state.

Autoregulation and mechanotransduction control the arteriolar response to small changes in hematocrit

Here, we present an analytic model of arteriolar mechanics that accounts for key autoregulation mechanisms, including the myogenic response and the vasodilatory effects of nitric oxide (NO) in the vasculature. It couples the fluid mechanics of blood flow in arterioles with solid mechanics of the vessel wall and includes the effects of wall shear stress- and stretch-induced endothelial NO production. The model can be used to describe the regulation of blood flow and NO transport under small changes in hematocrit and to analyze the regulatory response of arterioles to small changes in hematocrit. Our analysis revealed that the experimentally observed paradoxical increase in cardiac output with small increases in hematocrit results from the combination of increased NO production and the effects of a strong myogenic response modulated by elevated levels of WSS. Our findings support the hypothesis that vascular resistance varies inversely with blood viscosity for small changes in hematocrit in a healthy circulation that responds to shear stress stimuli. They also suggest beneficial effects independent of changes in O(2) carrying capacity associated with the postsurgical transfusion of one or two units of blood.

Hematocrit predicts long-term mortality in a nonlinear and sex-specific manner in hypertensive adults

Hematocrit has been inconsistently reported to be a risk marker of cardiovascular morbidity and mortality. The Glasgow Blood Pressure Clinic Study cohort included 10951 hypertensive patients, who had hematocrit measured at their initial clinic visit and followed for ≤35 years. Cox proportional hazards models were used to estimate hazard ratios for all-cause, cardiovascular, ischemic heart disease, stroke, and noncardiovascular mortality. There were 3484 deaths over a follow-up period of 173245 person-years. Hematocrit was higher in men (median, 0.44; interquartile range, 0.42-0.47) than in women (median, 0.41; interquartile range, 0.38-0.43). The lowest risk for all-cause mortality was seen in quartile 2 for men (range, 0.421-0.440) and women (range, 0.381-0.400). Compared with quartile 2, the adjusted hazard ratios for quartiles 1, 3, and 4 were, respectively, 1.11 (range, 0.97-1.28), 1.19 (range, 1.04-1.37), and 1.22 (range, 1.06-1.39) in men and 1.17 (range, 1.01-1.36), 0.97 (range, 0.83-1.13), and 1.19 (range, 1.04-1.37) in women. Men showed a J-shaped pattern for cardiovascular mortality and a linear pattern for noncardiovascular mortality in cause-specific analysis, whereas in women a U-shaped pattern was observed for noncardiovascular mortality only. Higher baseline hematocrit was associated with higher on-treatment blood pressure during follow-up. Baseline hematocrit did not affect the time to reach target blood pressure. The increased risk of death attributed to higher hematocrit was seen in men and women irrespective of their achievement of target blood pressure, indicating that the risk is independent of the effect of hematocrit on blood pressure. Hypertensive patients with hematocrit levels outside of the sex-specific reference ranges identified in this study should be targeted for more aggressive blood pressure and cardiovascular risk reduction treatment.

High-altitude diving in river otters: coping with combined hypoxic stresses

River otters (Lontra canadensis) are highly active, semi-aquatic mammals indigenous to a range of elevations and represent an appropriate model for assessing the physiological responses to diving at altitude. In this study, we performed blood gas analyses and compared blood chemistry of river otters from a high-elevation (2357 m) population at Yellowstone Lake with a sea-level population along the Pacific coast. Comparisons of oxygen dissociation curves (ODC) revealed no significant difference in hemoglobin-oxygen (Hb-O(2)) binding affinity between the two populations - potentially because of demands for tissue oxygenation. Instead, high-elevation otters had greater Hb concentrations (18.7 g dl(-1)) than sea-level otters (15.6 g dl(-1)). Yellowstone otters displayed higher levels of the vasodilator nitric oxide (NO), and half the concentration of the serum protein albumin, possibly to compensate for increased blood viscosity. Despite compensation in several hematological and serological parameters, theoretical aerobic dive limits (ADL) were similar between high-elevation and sea-level otters because of the lower availability of O(2) at altitude. Our results suggest that recent disruptions to the Yellowstone Lake food web could be detrimental to otters because at this high elevation, constraints on diving may limit their ability to switch to prey in a deep-water environment.

Prolonged hypervolemic hemodilution decreases functional capillary density of ileal mucosa in pigs revealed by sidestream dark-field imaging

OBJECTIVE

Hemodilution changes the physical properties of blood by reducing its hematocrit and blood viscosity. We tested whether prolonged hypervolemic hemodilution (HHD) impairs functional capillary density (FCD) of ileal mucosa in healthy mechanically-ventilated pigs and if there is any correlation between changes in FCD of ileal and sublingual mucosas during HHD.

METHODS

Sixteen domestic female pigs were anesthetized, mechanically-ventilated, and randomly assigned to the HHD (20 ml/(kg∙h) Hartmann's solution for 3 h) or fluid restrictive (5 ml/(kg∙h) Hartmann's solution for 3 h) group. Microcirculations of sublingual and ileal mucosas via ileostomy were visualized using sidestream dark-field (SDF) imaging at baseline conditions (t=0 h) and at selected time intervals of fluid therapy (t=1, 2, and 3 h).

RESULTS

A significant decrease of ileal FCD (285 (278-292) cm/cm(2)) in the HHD group was observed after the third hour of HHD when compared to the baseline (360 (350-370) cm/cm(2)) (P<0.01). This trend was not observed in the restrictive group, where the ileal mucosa FCD was significantly higher after the third hour of fluid therapy as compared to the HHD group (P<0.01). No correlation between microhemodynamic parameters obtained from sublingual and ileal mucosas was found throughout the study.

CONCLUSIONS

Prolonged HHD established by crystalloid solution significantly decreased ileal villus FCD when compared to restrictive fluid regimen. An inappropriate degree of HHD can be harmful during uncomplicated abdominal surgery.

Blood pressure and blood viscosity are not correlated in normal healthy subjects

The relationship between blood viscosity, hematocrit (Hct), and mean arterial blood pressure (MAP) was studied in a healthy population of 91 men and 66 women with an average age of 30.6 ± 8.0 years, from the city of Victoria de Durango (1800 m elevation). In women and men, Hct values were 42.4% ± 2.9% and 47.2% ± 2.3%, blood viscosities were 4.5 ± 0.7 and 6.1 ± 1.0 cP, and MAP was 83.0 ± 6.8 and 88.0 ± 6.1 mmHg, respectively. The correlation between blood viscosity and Hct was linear and positive (r² = 0.48) and identical to that of previous studies reported in the literature when men and women are taken as a single group. Separating the data by gender yielded positive, linear correlations (r² = 0.18 and 0.10, respectively) with identical slopes, however blood viscosity for men was 1.2 cP greater than in women (P = 0.02). MAP and blood viscosity (and Hct) were not statistically associated when men and women were analyzed separately and were weakly positively correlated (r² = 0.08, P < 0.02) when treated as a group. The present results suggest that studies that show a positive correlation between MAP and blood viscosity (and Hct) do not differentiate data according to gender, or involve populations that do not compensate for increased blood viscosity and potentially increased shear stress.

Integration of cardiovascular regulation by the blood/endothelium cell-free layer

The cell-free layer (CFL) width separating red blood cells in flowing blood from the endothelial cell membrane is shown to be a regulator of the balance between nitric oxide (NO) production by the endothelium and NO scavenging by blood hemoglobin. The CFL width is determined by hematocrit (Hct) and the vessel wall flow velocity gradient. These factors and blood and plasma viscosity determine vessel wall shear stress which regulates the production of NO in the vascular wall. Mathematical modeling and experimental findings show that vessel wall NO concentration is a strong nonlinear function of Hct and that small Hct variations have comparatively large effects on blood pressure regulation. Furthermore, NO concentration is a regulator of inflammation and oxygen metabolism. Therefore, small, sustained perturbations of Hct may have long-term effects that can promote pro-hypertensive and pro-inflammatory conditions. In this context, Hct and its variability are directly related to vascular tone, peripheral vascular resistance, oxygen transport and delivery, and inflammation. These effects are relevant to the analysis and understanding of blood pressure regulation, as NO bioavailability regulates the contractile state of blood vessels. Furthermore, regulation of the CFL is a direct function of blood composition therefore understanding of its physiology relates to the design and management of fluid resuscitation fluids. From a medical perspective, these studies propose that it should be of clinical interest to note small variations in patient's Hct levels given their importance in modulating the CFL width and therefore NO bioavailability. WIREs Syst Biol Med 2011 3 458-470 DOI: 10.1002/wsbm.150

Temporal variations of the cell-free layer width may enhance NO bioavailability in small arterioles: Effects of erythrocyte aggregation

Recently, we have shown that temporal variations in the cell-free layer width can potentially enhance nitric oxide (NO) bioavailability in small arterioles. Since the layer width variations can be augmented by red blood cell aggregation, we tested the hypothesis that an increase in the layer width variations due to red blood cell aggregation could provide an underlying mechanism to improve NO bioavailability in the endothelium and promote vasodilatory effects. Utilizing cell-free layer width data acquired from arterioles of the rat cremaster muscle before and after dextran infusion in reduced flow conditions (wall shear stress=0.13-0.24Pa), our computational model predicted exponential enhancements of NO bioavailability in the endothelium and soluble guanylyl cyclase (sGC) activation in the smooth muscle layer with increasing temporal variability of the layer width. These effects were mediated primarily by the transient responses of wall shear stress and NO production rate to the layer width variations. The temporal variations in the layer width were significantly enhanced (P<0.05) by aggregation, leading to significant improvements (P<0.05) in NO bioavailability and sGC activation. As a result, the significant reduction (P<0.05) of sGC activation due to the increased width of the layer after aggregation induction was diminished by the opposing effect of the layer variations. These findings highlighted the possible enhancement of NO bioavailability and vascular tone in the arteriole by the augmented layer width variations due to the aggregation.