Reactive hyperemia in the human liver

Compensatory hemodynamic changes in response to central hypovolemia in humans: lower body negative pressure: updates and perspectives

Central hypovolemia is accompanied by hemodynamic compensatory responses. Understanding the complex systemic compensatory responses to altered hemodynamic patterns during conditions of central hypovolemia-as induced by standing up and/or lower body negative pressure (LBNP)-in humans are important. LBNP has been widely used to understand the integrated physiological responses, which occur during sit to stand tests (orthostasis), different levels of hemorrhages (different levels of LBNP simulate different amount of blood loss) as well as a countermeasure against the cephalad fluid shifts which are seen during spaceflight. Additionally, LBNP application (used singly or together with head up tilt, HUT) is useful in understanding the physiology of orthostatic intolerance. The role seasonal variations in hormonal, autonomic and circulatory state play in LBNP-induced hemodynamic responses and LBNP tolerance as well as sex-based differences during central hypovolemia and the adaptations to exercise training have been investigated using LBNP. The data generated from LBNP studies have been useful in developing better models for prediction of orthostatic tolerance and/or for developing countermeasures. This review examines how LBNP application influences coagulatory parameters and outlines the effects of temperature changes on LBNP responses. Finally, the review outlines how LBNP can be used as innovative teaching tool and for developing research capacities and interests of medical students and students from other disciplines such as mathematics and computational biology.

Effect of ingesting a meal and orthostasis on the regulation of splanchnic and systemic hemodynamics and the responsiveness of cardiovascular α1-adrenoceptors

Postprandial orthostasis activates mechanisms of cardiovascular homeostasis to maintain normal blood pressure (BP) and adequate blood flow to vital organs. The underlying mechanisms of cardiovascular homeostasis in postprandial orthostasis still require elucidation. Fourteen healthy volunteers were recruited to investigate the effect of an orthostatic challenge (60°-head-up-tilt for 20 min) on splanchnic and systemic hemodynamics before and after ingesting an 800-kcal composite meal. The splanchnic circulation was assessed by ultrasonography of the superior mesenteric and hepatic arteries and portal vein. Systemic hemodynamics were assessed noninvasively by continuous monitoring of BP, heart rate (HR), cardiac output (CO), and the pressor response to an intravenous infusion on increasing doses of phenylephrine, an α₁-adrenoceptor agonist. Neurohumoral regulation was assessed by spectral analysis of HR and BP, plasma catecholamine and aldosterone levels and plasma renin activity. Postprandial mesenteric hyperemia was associated with an increase in CO, a decrease in SVR and cardiac vagal tone, and reduction in baroreflex sensitivity with no change in sympathetic tone. Arterial α₁-adrenoceptor responsiveness was preserved and reduced in hepatic sinusoids. Postprandial orthostasis was associated with a shift of 500 mL of blood from mesenteric to systemic circulation with preserved sympathetic-mediated vasoconstriction. Meal ingestion provokes cardiovascular hyperdynamism, cardiac vagolysis, and resetting of the baroreflex without activation of the sympathetic nervous system. Meal ingestion also alters α₁-adrenoceptor responsiveness in the hepatic sinusoids and participates in the redistribution of blood volume from the mesenteric to the systemic circulation to maintain a normal BP during orthostasis.NEW & NOTEWORTHY A unique integrated investigation on the effect of meal on neurohumoral mechanisms and blood flow redistribution of the mesenteric circulation during orthostasis was investigated. Food ingestion results in cardiovascular hyperdynamism, reduction in cardiac vagal tone, and baroreflex sensitivity and causes a decrease in α₁-adrenoceptor responsiveness only in the venous intrahepatic sinusoids. About 500-mL blood shifts from the mesenteric to the systemic circulation during orthostasis. Accordingly, the orthostatic homeostatic mechanisms are better understood.

Developing a "dry lab" activity using lower body negative pressure to teach physiology

In this paper we assessed how lower body negative pressure (LBNP) can be used to teach students the physiological effects of central hypovolemia in the absence of the LBNP and/or a medical monitor using a "dry lab" activity using LBNP data that have been previously collected. This activity was performed using published LBNP papers, with which students could explore LBNP as an important tool to study physiological responses to central hypovolemia as well as consider issues in performing an LBNP experiment and interpreting experimental results. The activity was performed at the All India Institute of Medical Sciences, New Delhi, with 31 graduate students and 4 teachers of physiology. Both students and teachers were provided with a set of questionnaires that inquired about aspects related to the structure of the activity and how this activity integrated research and knowledge, as well as aspects related to motivation of the students and teachers to perform the activity. Our results from student and teacher surveys suggest that a "dry lab" activity using LBNP to teach physiology can be an important tool to expose students to the basics of systems physiology as well as to provide useful insights into how research is performed. Providing insight into research includes formulating a research question and then designing (including taking into account confounding variables), implementing, conducting, and interpreting research studies. Finally, developing such an activity using LBNP can also serve as a basis for developing research capacities and interests of students even early in their medical studies.

Fluid Shifts Induced by Physical Therapy in Lower Limb Lymphedema Patients

Complete decongestive therapy (CDT), a physical therapy including manual lymphatic drainage (MLD) and compression bandaging, is aimed at mobilizing fluid and reducing limb volume in lymphedema patients. Details of fluid shifts occurring in response to CDT are currently not well studied. Therefore, we investigated fluid shifts before, during and after CDT. Thirteen patients (3 males and 10 females, aged 57 ± 8.0 years, 167.2 ± 8.3 cm height, 91.0 ± 23.4 kg weight) diagnosed with stage II leg lymphedema participated. Leg volume, limb and whole-body fluid composition (total body water (limbTBW/%TBW), extracellular (limbECF/%ECF) and intracellular (limbICF/%ICF fluid), as well as ECF/ICF and limbECF/limbICF ratios were determined using perometry and bioelectrical impedance spectroscopy. Plasma volume, proteins, osmolality, oncotic pressure and electrolytes were assessed. Leg volume (p < 0.001), limbECF (p = 0.041), limbICF (p = 0.005) and limbECF/limbICF decreased over CDT. Total leg volume and limbTBW were correlated (r = 0.635). %TBW (p = 0.001) and %ECF (p = 0.007) decreased over time. The maximum effects were seen within one week of CDT. LimbICF (p = 0.017), %TBW (p = 0.009) and %ICF (p = 0.003) increased post-MLD, whereas ECF/ICF decreased due to MLD. Plasma volume increased by 1.5% post-MLD, as well as albumin and the albumin-to-globulin ratio (p = 0.005 and p = 0.049, respectively). Our results indicate that physical therapy leads to fluid shifts in lymphedema patients, with the greatest effects occurring within one week of therapy. Fluid shifts due to physical therapy were also reflected in increased plasma volume and plasma protein concentrations. Perometry, in contrast to bioelectrical impedance analysis, does not seem to be sensitive enough to detect small fluid changes caused by manual lymphatic drainage.

Coagulation Changes during Central Hypovolemia across Seasons

Lower body negative pressure (LBNP) application simulates hemorrhage. We investigated how seasons affect coagulation values at rest and during LBNP. Healthy participants were tested in cold (November–April) and warm (May–October) months. Following a 30-min supine period, LBNP was started at −10 mmHg and increased by −10 mmHg every five minutes until a maximum of −40 mmHg. Recovery was for 10 min. Blood was collected at baseline, end of LBNP, and end of recovery. Hemostatic profiling included standard coagulation tests, calibrated automated thrombogram, thrombelastometry, impedance aggregometry, and thrombin formation markers. Seven men (25.0 ± 3.6 years, 79.7 ± 7.8 kg weight, 182.4 ± 3.3 cm height, and 23.8 ± 2.3 kg/m² BMI) and six women (25.0 ± 2.4 years, 61.0 ± 8.4 kg weight, 167 ± 4.7 cm height, and 21.8 ± 2.4 kg/m² BMI) participated. Baseline levels of prothrombin (FII), tissue factor (TF) and markers for thrombin generation F1+2 and the thrombin/antithrombin complex (TAT) were higher during summer. Factor VIII, prothrombin fragment 1+2 (F1+2), TAT and the coagulation time showed significant increases during LBNP in both seasons. Some calibrated automated thrombography variables (Calibrated automated thrombography (CAT): lag, time to peak (ttPeak), peak) shifted in a procoagulant direction during LBNP in summer. Red blood cell counts (RBC), hemoglobin and white blood cell counts (WBC) decreased during LBNP. LBNP application reduced prothrombin time in winter and activated partial thromboplastin time in summer. Greater levels of FII, TF, F1+2, and TAT—a more pronounced LBNP-induced procoagulative effect, especially in CAT parameters (lag time (LT), Peak, ttPeak, Velindex)—were seen in summer. These results could have substantial medical implications.

Menstrual Phase Affects Coagulation and Hematological Parameters during Central Hypovolemia

Background: It has been reported that women have a higher number of heart attacks in the “follicular phase” of the menstrual cycle. We, therefore, tested the hypothesis that women in the follicular phase exhibit higher coagulability. As lower body negative pressure (LBNP) has been used previously to assess coagulation changes in whole blood (WB) samples in men and women, effects of menstrual phase on coagulation was assessed during LBNP. Methods: Seven women, all healthy young participants, with no histories of thrombotic disorders and not on medications, were tested in two phases of the menstrual cycle (early follicular (EF) and mid-luteal (ML)). LBNP was commenced at −10 mmHg and increased by −10 mmHg every 5 min until a maximum of −40 mmHg. Recovery up to 10 min was also monitored. Blood samples were collected at baseline, at end of LBNP, and at end of recovery. Hemostatic profiling included comparing the effects of LBNP on coagulation values in both phases of the menstrual cycle using standard coagulation tests, calibrated automated thrombogram, thrombelastometry, impedance aggregometry, and markers of thrombin formation. Results: LBNP led to coagulation activation determined in both plasma and WB samples. During both phases, coagulation was affected during LBNP, as reflected in their decreased partial thromboplastin time (PTT) and elevated coagulation factor VIII FVIII, F1 + 2, and thrombin-antithrombin (TAT) levels. Additionally, during the ML phase, greater PT [%] and shorter time to peak (ttPeak) values (implying faster maximum thrombin formation) suggest that women in the ML phase are relatively hypercoagulable compared to the early follicular phase. Conclusions: These results suggest that thrombosis occurs more during the midluteal phase, a finding with substantial medical implications.

Effect of novel short-arm human centrifugation-induced gravitational gradients upon cardiovascular responses, cerebral perfusion and g-tolerance

KEY POINTS

The aim of this study was to determine the effect of rotational axis position (RAP and thus g-gradient) during short-arm human centrifugation (SAHC) upon cardiovascular responses, cerebral perfusion and g-tolerance. In 10 male and 10 female participants, 10 min passive SAHC runs were performed with the RAP above the head (P1), at the apex of the head (P2), or at heart level (P3), with foot-level Gz at 1.0 g, 1.7 g and 2.4 g. We hypothesized that movement of the RAP from above the head (the conventional position) towards the heart might reduce central hypovolaemia, limit cardiovascular responses, aid cerebral perfusion, and thus promote g-tolerance. Moving the RAP footward towards the heart decreased the cerebral tissue saturation index, calf circumference and heart rate responses to SAHC, thereby promoting g-tolerance. Our results also suggest that RAP, and thus g-gradient, warrants further investigation as it may support use as a holistic spaceflight countermeasure.

ABSTRACT

Artificial gravity (AG) through short-arm human centrifugation (SAHC) has been proposed as a holistic spaceflight countermeasure. Movement of the rotational axis position (RAP) from above the head towards the heart may reduce central hypovolaemia, aid cerebral perfusion, and thus promote g-tolerance. This study determined the effect of RAP upon cardiovascular responses, peripheral blood displacement (i.e. central hypovolaemia), cerebral perfusion and g-tolerance, and their inter-relationships. Twenty (10 male) healthy participants (26.2 ± 4.0 years) underwent nine (following a familiarization run) randomized 10 min passive SAHC runs with RAP set above the head (P1), at the apex of the head (P2), or at heart level (P3) with foot-level Gz at 1.0 g, 1.7 g and 2.4 g. Cerebral tissue saturation index (cTSI, cerebral perfusion surrogate), calf circumference (CC, central hypovolaemia), heart rate (HR) and digital heart-level mean arterial blood pressure (MAP) were continuously recorded, in addition to incidence of pre-syncopal symptoms (PSS). ΔCC and ΔHR increases were attenuated from P1 to P3 (ΔCC: 5.46 ± 0.54 mm to 2.23 ± 0.42 mm; ΔHR: 50 ± 4 bpm to 8 ± 2 bpm, P < 0.05). In addition, ΔcTSI decrements were also attenuated (ΔcTSI: -2.85 ± 0.48% to -0.95 ± 0.34%, P < 0.05) and PSS incidence lower in P3 than P1 (P < 0.05). A positive linear relationship was observed between ΔCC and ΔHR with increasing +Gz, and a negative relationship between ΔCC and ΔcTSI, both independent of RAP. Our data suggest that movement of RAP towards the heart (reduced g-gradient), independent of foot-level Gz, leads to improved g-tolerance. Further investigations are required to assess the effect of differential baroreceptor feedback (i.e. aortic-carotid g-gradient).

Coagulation changes induced by lower-body negative pressure in men and women

We investigated whether lower-body negative pressure (LBNP) application leads to coagulation activation in whole blood (WB) samples in healthy men and women. Twenty-four women and 21 men, all healthy young participants, with no histories of thrombotic disorders and not on medications, were included. LBNP was commenced at −10 mmHg and increased by −10 mmHg every 5 min until a maximum of −40 mmHg. Recovery up to 10 min was also monitored. Blood samples were collected at baseline, at end of LBNP, and end of recovery. Hemostatic profiling included comparing the effects of LBNP on coagulation values in both men and women using standard coagulation tests, calibrated automated thrombogram, thrombelastometry, impedance aggregometry, and markers of thrombin formation. LBNP led to coagulation activation determined in both plasma and WB samples. At baseline, women were hypercoagulable compared with men, as evidenced by their shorter “lag times” and higher thrombin peaks and by shorter “coagulation times” and “clot formation times.” Moreover, men were more susceptible to LBNP, as reflected in their elevated factor VIII levels and decreased lag times following LBNP. LBNP-induced coagulation activation was not accompanied by endothelial activation. Women appear to be relatively hypercoagulable compared with men, but men are more susceptible to coagulation changes during LBNP. The application of LBNP might be a useful future tool to identify individuals with an elevated risk for thrombosis, in subjects with or without history of thrombosis.

NEW & NOTEWORTHY LBNP led to coagulation activation determined in both plasma and whole blood samples. At baseline, women were hypercoagulable compared with men. Men were, however, more susceptible to coagulation changes during LBNP. LBNP-induced coagulation activation was not accompanied by endothelial activation. The application of LBNP might be a useful future tool to identify individuals with an elevated risk for thrombosis, in subjects with or without history of thrombosis.

Lower Body Negative Pressure: Physiological Effects, Applications, and Implementation

This review presents lower body negative pressure (LBNP) as a unique tool to investigate the physiology of integrated systemic compensatory responses to altered hemodynamic patterns during conditions of central hypovolemia in humans. An early review published in Physiological Reviews over 40 yr ago (Wolthuis et al. Physiol Rev 54: 566-595, 1974) focused on the use of LBNP as a tool to study effects of central hypovolemia, while more than a decade ago a review appeared that focused on LBNP as a model of hemorrhagic shock (Cooke et al. J Appl Physiol (1985) 96: 1249-1261, 2004). Since then there has been a great deal of new research that has applied LBNP to investigate complex physiological responses to a variety of challenges including orthostasis, hemorrhage, and other important stressors seen in humans such as microgravity encountered during spaceflight. The LBNP stimulus has provided novel insights into the physiology underlying areas such as intolerance to reduced central blood volume, sex differences concerning blood pressure regulation, autonomic dysfunctions, adaptations to exercise training, and effects of space flight. Furthermore, approaching cardiovascular assessment using prediction models for orthostatic capacity in healthy populations, derived from LBNP tolerance protocols, has provided important insights into the mechanisms of orthostatic hypotension and central hypovolemia, especially in some patient populations as well as in healthy subjects. This review also presents a concise discussion of mathematical modeling regarding compensatory responses induced by LBNP. Given the diverse applications of LBNP, it is to be expected that new and innovative applications of LBNP will be developed to explore the complex physiological mechanisms that underline health and disease.

Orthostatic Challenge Shifts the Hemostatic System of Patients Recovered from Stroke toward Hypercoagulability

Aims: The objective of our study was to assess the effects of orthostatic challenge on the coagulation system in patients with a history of thromboembolic events and to assess how they compared with age-matched healthy controls. Methods: Twenty-two patients with histories of ischemic stroke and 22 healthy age-matched controls performed a sit-to-stand test. Blood was collected prior to- and at the end of- standing in the upright position for 6 min. Hemostatic profiling was performed by determining thrombelastometry and calibrated automated thrombogram values, indices of thrombin generation, standard coagulation times, markers of endothelial activation, plasma levels of coagulation factors and copeptin, and hematocrit. Results: Orthostatic challenge caused a significant endothelial and coagulation activation in patients (Group 1) and healthy controls (Group 2): Plasma levels of prothrombin fragment F1+2 were increased by approximately 35% and thrombin/antithrombin-complex (TAT) increased 5-fold. Several coagulation variables were significantly altered in Group 1 but not in Group 2: Coagulation times (CTs) were significantly shortened and alpha angles, peak rate of thrombin generation (VELINDEX), tissue factor (TF) and copeptin plasma levels were significantly increased (comparison between standing and baseline). Moreover, the shortening of CTs and the rise of copeptin plasma levels were significantly higher in Group 1 vs. Group 2 (comparison between groups). Conclusion: The coagulation system of patients with a history of ischemic stroke can be more easily shifted toward a hypercoagulable state than that of healthy controls. Attentive and long-term anticoagulant treatment is essential to keep patients from recurrence of vascular events.

Increased Hepato-Splanchnic Vasoconstriction in Diabetics during Regular Hemodialysis

Background and Objectives

Ultrafiltration (UF) of excess fluid activates numerous compensatory mechanisms during hemodialysis (HD). The increase of both total peripheral and splanchnic vascular resistance is considered essential in maintaining hemodynamic stability. The aim of this study was to evaluate the extent of UF-induced changes in hepato-splanchnic blood flow and resistance in a group of maintenance HD patients during regular dialysis.

Design, Setting, Participants, & Measurements

Hepato-splanchnic flow resistance index (RI) and hepato-splanchnic perfusion index (QI) were measured in 12 chronic HD patients using a modified, non-invasive Indocyaningreen (ICG) dilution method. During a midweek dialysis session we determined RI, QI, ICG disappearance rate (k_ICG), plasma volume (V_p), hematocrit (Hct), mean arterial blood pressure (MAP) and heart rate (HR) at four times in hourly intervals (t₁ to t₄). Dialysis settings were standardized and all patient studies were done in duplicate.

Results

In the whole study group mean UF volume was 1.86 ± 0.46 L, V_p dropped from 3.65 ± 0.77L at t₁ to 3.40 ± 0.78L at t₄, and all patients remained hemodynamically stable. In all patients RI significantly increased from 12.40 ± 4.21 mmHg∙s∙m²/mL at t₁ to 14.94 ± 6.36 mmHg∙s∙m²/mL at t₄ while QI significantly decreased from 0.61 ± 0.22 at t₁ to 0.52 ± 0.20 L/min/m² at t₄, indicating active vasoconstriction. In diabetic subjects, however, RI was significantly larger than in non-diabetics at all time points. QI was lower in diabetic subjects.

Conclusions

In chronic HD-patients hepato-splanchnic blood flow substantially decreases during moderate UF as a result of an active splanchnic vasoconstriction. Our data indicate that diabetic HD-patients are particularly prone to splanchnic ischemia and might therefore have an increased risk for bacterial translocation, endotoxemia and systemic inflammation.

Effects of individualized centrifugation training on orthostatic tolerance in men and women

Aims

Exposure to artificial gravity (AG) at different G loads and durations on human centrifuges has been shown to improve orthostatic tolerance in men. However, the effects on women and of an individual-specific AG training protocol on tolerance are not known.

Methods

We examined the effects of 90 minutes of AG vs. 90 minutes of supine rest on the orthostatic tolerance limit (OTL), using head up tilt and lower body negative pressure until presyncope of 7 men and 5 women. Subjects were placed in the centrifuge nacelle while instrumented and after one-hour they underwent either: 1) AG exposure (90 minutes) in supine position [protocol 1, artificial gravity exposure], or 2) lay supine on the centrifuge for 90 minutes in supine position without AG exposure [protocol 2, control]. The AG training protocol was individualized, by first determining each subject’s maximum tolerable G load, and then exposing them to 45 minutes of ramp training at sub-presyncopal levels.

Results

Both sexes had improved OTL (14 minutes vs 11 minutes, p < 0.0019) following AG exposure. When cardiovascular (CV) variables at presyncope in the control test were compared with the CV variables at the same tilt-test time (isotime) during post-centrifuge, higher blood pressure, stroke volume and cardiac output and similar heart rates and peripheral resistance were found post-centrifuge.

Conclusions

These data suggest a better-maintained central circulating blood volume post-centrifugation across gender and provide an integrated insight into mechanisms of blood pressure regulation and the possible implementation of in-flight AG countermeasure profiles during spaceflights.

Volume regulation and renal function at high altitude across gender

Aims

We investigated changes in volume regulating hormones and renal function at high altitudes and across gender.

Methodology

Included in this study were 28 subjects (n = 20 males; n = 8 females. ages: 19 – 65 yrs), who ascended to a height of 3440m (HA1), on the 3^rd day and to 5050m (HA2), on the 14^th day. Plasma and urinary creatinine and urinary osmolality as well as plasma levels of plasma renin activity (PRA), Aldosterone, antidiuretic hormone (ADH), and atrial natriuretic peptide (ANP) were measured. The plasma volume loss (PVL) was estimated from plasma density and hematocrit. Glomerular filtration rate (GFR) was measured based on nocturnal (9 hour) creatinine clearance; this was compared with various methods for estimation of GFR.

Results

The mean 24-hour urine production increased significantly in both sexes across the expedition. But PVL reached significance only in males. No changes in Na⁺ in plasma, urine or its fractional excretion were seen at both altitudes. Urinary osmolality decreased upon ascent to the higher altitudes. ADH and PRA decreased significantly at both altitudes in males but only at HA2 in females. However, no changes in aldosterone were seen across the sexes and at different altitudes. ANP increased significantly only in males during the expedition. GFR, derived from 9-h creatinine clearance (CreaCl), decreased in both sexes at HA1 but remained stable at HA2. Conventional Crea[p]-based GFR estimates (eGFR) showed only poor correlation to CreaCl.

Conclusions

We report details of changes in hormonal patterns across high altitude sojourn. To our knowledge we are not aware of any study that has examined these hormones in same subjects and across gender during high altitude sojourn. Our results also suggest that depending on the estimation formula used, eGFR underestimated the observed decrease in renal function measured by CreaCl, thus opening the debate regarding the use of estimated glomerular filtration rates at high altitudes.

Internal filtration in a high-flux dialyzer quantified by mean transit time of an albumin-bound indicator

Internal filtration in high-flux (HF) dialyzers significantly contributes to convective solute removal of molecules with poor diffusibility, but it is difficult to quantify. The aim of this study was to present the theory and to develop a method for measuring internal filtration and backfiltration in HF dialyzers, which also could be applied to patient studies. In a series of lab-bench experiments, the mean transit times (τd) of indocyanine green (ICG) passing the dialyzer were optically measured under different operating conditions and compared with mean transit times calculated from the known volume of the blood compartment (τV) using a mathematical model. τd was always larger than τV. The relative difference in mean transit times (1 - τV/τd) was related to the average cumulative filtration rate (Qfil). The internal filtration fraction Fb = Qfil/Qb was largely independent of blood flow (Qb) and not different from theoretical predictions obtained from a mathematical model. The dispersion of a nondiffusible indicator such as ICG can be used to quantify the magnitude of internal filtration and backfiltration in HF dialyzers using available technology. This information could be useful for testing the HF dialyzers in everyday situations.

Model-based prediction of autoregulatory exhaustion in response to lower-body negative pressure-induced shock

BACKGROUND

We assessed the ability of a normalized autonomic nervous system (ANS) stress measure defined as an increase in the percentage of pulse rate from a baseline homeostasis state to identify corresponding changes in circulating blood volume to quantitatively recognize hypovolemia and predict subsequent autoregulatory exhaustion. Autoregulatory exhaustion is defined as the point where decreased circulatory volume exceeds the compensatory mechanism capacity to maintain flow and pressure. We derived frequency-based measures of pulse rate and pulse strength using a reflective pulse oximeter waveform of a photoplethysmograph placed on the forehead.

METHODS

This study was performed at the Penn State Heart and Vascular Institute, Hershey, Pennsylvania, in June 2010. Ten healthy subjects (5 each male and female) were placed supine in a lower-body negative-pressure chamber to induce central volume loss. Systolic blood pressure was continuously measured, and a value of less than 90 mm Hg defined autoregulatory exhaustion. Derived measures of circulating blood volume were compared with echocardiographic measures to access photoplethysmograph-derived circulatory volume measure relative to traditional cardiac hemodynamics.

RESULTS

All 10 subjects produced consistent patterns of response characterized as a progressively increasing ANS stress in response to increasing lower-body negative pressure. Three subjects exhibited autoregulatory exhaustion, and ANS stress increased markedly on the step before displaying hypotension in these subjects but not the others.

CONCLUSION

Results demonstrate the potential to use model-based measures to serve as a definitive presymptom predictive tool to recognize an impending hypovolemic condition, making this approach suitable for chronic care or for the management of hemodialysis patient where resting baseline measures can be obtained.

Coagulation changes during presyncope and recovery

Orthostatic stress activates the coagulation system. The extent of coagulation activation with full orthostatic load leading to presyncope is unknown. We examined in 7 healthy males whether presyncope, using a combination of head up tilt (HUT) and lower body negative pressure (LBNP), leads to coagulation changes as well as in the return to baseline during recovery. Coagulation responses (whole blood thrombelastometry, whole blood platelet aggregation, endogenous thrombin potential, markers of endothelial activation and thrombin generation), blood cell counts and plasma mass density (for volume changes) were measured before, during, and 20 min after the orthostatic stress. Maximum orthostatic load led to a 25% plasma volume loss. Blood cell counts, prothrombin levels, thrombin peak, endogenous thrombin potential, and tissue factor pathway inhibitor levels increased during the protocol, commensurable with hemoconcentration. The markers of endothelial activation (tissue factor, tissue plasminogen activator), and thrombin generation (F1+2, prothrombin fragments 1 and 2, and TAT, thrombin-antithrombin complex) increased to an extent far beyond the hemoconcentration effect. During recovery, the markers of endothelial activation returned to initial supine values, but F1+2 and TAT remained elevated, suggestive of increased coagulability. Our findings of increased coagulability at 20 min of recovery from presyncope may have greater clinical significance than short-term procoagulant changes observed during standing. While our experiments were conducted in healthy subjects, the observed hypercoagulability during graded orthostatic challenge, at presyncope and in recovery may be an important risk factor particularly for patients already at high risk for thromboembolic events (e.g. those with coronary heart disease, atherosclerosis or hypertensives).

Teaching fluid shifts during orthostasis using a classic paper by Foux et al

Hypovolemic and orthostatic challenge can be simulated in humans by the application of lower body negative pressure (LBNP), because this perturbation leads to peripheral blood pooling and, consequently, central hypovolemia. The classic paper by Foux and colleagues clearly shows the effects of orthostasis simulated by LBNP on fluid shifts and homeostatic mechanisms. The carefully carried out experiments reported in this paper show the interplay between different physiological control systems to ensure blood pressure regulation, failure of which could lead to critical decreases in cerebral blood flow and syncope. Here, a teaching seminar for graduate students is described that is designed in the context of this paper and aimed at allowing students to learn how Foux and colleagues have advanced this field by addressing important aspects of blood regulation. This seminar is also designed to put their research into perspective by including important components of LBNP testing and protocols developed in subsequent research in the field. Learning about comprehensive protocols and carefully controlled studies can reduce confounding variables and allow for an optimal analysis and elucidation of the physiological responses that are being investigated. Finally, in collaboration with researchers in mathematical modeling, in the future, we will incorporate the concepts of applicable mathematical models into our curriculum.

Volume regulating hormone responses to repeated head-up tilt and lower body negative pressure

BACKGROUND

We hypothesized the existence of different hormonal response patterns to repeated lower body negative pressure (LBNP) and head-up tilt (HUT) in healthy males. We compared hormonal, cardiovascular and plasma volume changes from rest to stress within- and between-LBNP and HUT applications. Hormones investigated included adrenocorticotropic hormone (ACTH), aldosterone, plasma renin activity (PRA), atrial natriuretic peptide (ANP) and arginine vasopressin (AVP).

MATERIALS AND METHODS

Three sequential 30-min bouts of LBNP at -55mmHg (n=14) or 70° HUT (n=9) were preceded by 30-min supine rest, and a 60-min supine rest followed the 3rd stimulus.

RESULTS

Plasma renin activity increases above baseline, in relation to aldosterone, were larger with LBNP than with HUT. The 3rd HUT application resulted in a greater increase in aldosterone compared to LBNP. Mean arterial blood pressure was elevated significantly during 1st and 3rd HUT application. ACTH responses were highly correlated with those of aldosterone in both LBNP and HUT (r(2) =0·96). AVP responses, in contrast to ANP, to the three consecutive stress situations were not significantly different, both with LBNP and HUT.

CONCLUSIONS

We speculate that the observed differences in blood pressure and hormonal responses to LBNP and HUT are caused by divergent effects of blood pooling in the splanchnic region, despite similar reductions in splanchnic perfusion. Apparently with repeated central hypovolaemia, especially by the 3rd application of stress, plasma aldosterone levels rise (along with ACTH), conceivably increasing its volume-guarding effect.

Pulse Decomposition Analysis of the digital arterial pulse during hemorrhage simulation

BACKGROUND

Markers of temporal changes in central blood volume are required to non-invasively detect hemorrhage and the onset of hemorrhagic shock. Recent work suggests that pulse pressure may be such a marker. A new approach to tracking blood pressure, and pulse pressure specifically is presented that is based on a new form of pulse pressure wave analysis called Pulse Decomposition Analysis (PDA). The premise of the PDA model is that the peripheral arterial pressure pulse is a superposition of five individual component pressure pulses, the first of which is due to the left ventricular ejection from the heart while the remaining component pressure pulses are reflections and re-reflections that originate from only two reflection sites within the central arteries. The hypothesis examined here is that the PDA parameter T13, the timing delay between the first and third component pulses, correlates with pulse pressure.T13 was monitored along with blood pressure, as determined by an automatic cuff and another continuous blood pressure monitor, during the course of lower body negative pressure (LBNP) sessions involving four stages, -15 mmHg, -30 mmHg, -45 mmHg, and -60 mmHg, in fifteen subjects (average age: 24.4 years, SD: 3.0 years; average height: 168.6 cm, SD: 8.0 cm; average weight: 64.0 kg, SD: 9.1 kg).

RESULTS

Statistically significant correlations between T13 and pulse pressure as well as the ability of T13 to resolve the effects of different LBNP stages were established. Experimental T13 values were compared with predictions of the PDA model. These interventions resulted in pulse pressure changes of up to 7.8 mmHg (SE = 3.49 mmHg) as determined by the automatic cuff. Corresponding changes in T13 were a shortening by -72 milliseconds (SE = 4.17 milliseconds). In contrast to the other two methodologies, T13 was able to resolve the effects of the two least negative pressure stages with significance set at p < 0.01.

CONCLUSIONS

The agreement of observations and measurements provides a preliminary validation of the PDA model regarding the origin of the arterial pressure pulse reflections. The proposed physical picture of the PDA model is attractive because it identifies the contributions of distinct reflecting arterial tree components to the peripheral pressure pulse envelope. Since the importance of arterial pressure reflections to cardiovascular health is well known, the PDA pulse analysis could provide, beyond the tracking of blood pressure, an assessment tool of those reflections as well as the health of the sites that give rise to them.

Gravity, the hydrostatic indifference concept and the cardiovascular system

Gravity, like any acceleration, causes a hydrostatic pressure gradient in fluid-filled bodily compartments. At a force of 1G, this pressure gradient amounts to 10 kPa/m. Postural changes alter the distribution of hydrostatic pressure patterns according to the body's alignment to the acceleration field. At a certain location--referred to as hydrostatically indifferent--within any given fluid compartment, pressure remains constant during a given change of position relative to the acceleration force acting upon the body. At this specific location, there is probably little change in vessel volume, wall tension, and the balance of Starling forces after a positional manoeuvre. In terms of cardiac function, this is important because arterial and venous hydrostatic indifference locations determine postural cardiac preload and afterload changes. Baroreceptors pick up pressure signals that depend on their respective distance to hydrostatic indifference locations with any change of body position. Vascular shape, filling volume, and compliance, as well as temperature, nervous and endocrine factors, drugs, and time all influence hydrostatic indifference locations. This paper reviews the physiology of pressure gradients in the cardiovascular system that are operational in a gravitational/acceleration field, offers a broadened hydrostatic indifference concept, and discusses implications that are relevant in physiological and clinical terms.

Changes in hepatic blood flow during whole body hyperthermia

PURPOSE

Changes in blood flow distribution are important for heat dispersion and for supportive therapeutic strategies such as simultaneous whole body hyperthermia (WBH) and administration of chemotherapy. The aim of this clinical study was to determine changes in hepatic blood flow during WBH for the treatment of metastatic cancer.

MATERIALS AND METHODS

This observational clinical study was part of a phase I/II feasibility study of WBH. WBH was induced using a radiant heat device. Hepatic blood flow was estimated using indocyanine green clearance measurements. The plasma disappearance rate of indocyanine green (PDR-ICG) was recorded in percent/min. We used an invasive thermo-dye-dilution technique to estimate hepatic blood flow, cardiac output, and volume status. Mean arterial blood pressure was also measured invasively. To determine the effects of hyperthermia the measurements were performed at defined temperature points.

RESULTS

In 10 of 22 treatments the PDR-ICG fell below normal values during hyperthermia, which represented a significant fall in hepatic blood flow. Cardiac output, volume status, and mean arterial blood pressure did not differ between patients whose liver blood flow was reduced and those whose liver blood flow remained unchanged.

CONCLUSIONS

We observed distinct reductions in hepatic blood flow during WBH, which suggested a significant redistribution of blood flow away from the core during WBH. This was not mirrored by global circulatory parameters.

Patterns of cardiovascular control during repeated tests of orthostatic loading

To investigate patterns of cardiovascular control, a protocol of head up tilt (HUT) followed by lower body negative pressure (LBNP), which represents a significant cardiovascular control challenge, was employed. Linear regression of beat-to-beat heart rate (HR) and mean blood pressure (MBP) data collected over repeated tests was used to analyze control response during the LBNP phase of the combined HUT + LBNP protocol. Four runs for each of 10 healthy young males reaching presyncope were analyzed. Subjects were classified into 2 groups based on the consistency of MBP regulation in response to central hypovolemia induced by LBNP. The consistent group tended to exhibit consistent HR slope (rate of change of HR over time as calculated by linear regression) whereas subjects in the inconsistent group could not be easily classified. Subjects with consistent MBP maintenance exhibited patterns suggesting a consistency of response in cardiovascular control whereas subjects less successful in maintaining MBP exhibited less clearly defined patterns over four runs.