Elaheh Rahbar

Determinants of valve gating in collecting lymphatic vessels from rat mesentery

Secondary lymphatic valves are essential for minimizing backflow of lymph and are presumed to gate passively according to the instantaneous trans-valve pressure gradient. We hypothesized that valve gating is also modulated by vessel distention, which could alter leaflet stiffness and coaptation. To test this hypothesis, we devised protocols to measure the small pressure gradients required to open or close lymphatic valves and determine if the gradients varied as a function of vessel diameter. Lymphatic vessels were isolated from rat mesentery, cannulated, and pressurized using a servo-control system. Detection of valve leaflet position simultaneously with diameter and intraluminal pressure changes in two-valve segments revealed the detailed temporal relationships between these parameters during the lymphatic contraction cycle. The timing of valve movements was similar to that of cardiac valves, but only when lymphatic vessel afterload was elevated. The pressure gradients required to open or close a valve were determined in one-valve segments during slow, ramp-wise pressure elevation, either from the input or output side of the valve. Tests were conducted over a wide range of baseline pressures (and thus diameters) in passive vessels as well as in vessels with two levels of imposed tone. Surprisingly, the pressure gradient required for valve closure varied >20-fold (0.1-2.2 cmH(2)O) as a passive vessel progressively distended. Similarly, the pressure gradient required for valve opening varied sixfold with vessel distention. Finally, our functional evidence supports the concept that lymphatic muscle tone exerts an indirect effect on valve gating.

Early resuscitation intensity as a surrogate for bleeding severity and early mortality in the PROMMTT study.

BACKGROUND The classic definition of massive transfusion, 10 or more units of red blood cells (RBCs) in 24 hours, has never been demonstrated as a valid surrogate for severe hemorrhage and can introduce survival bias. In addition, the definition fails to capture other products that the clinician may have immediately available, and may use, during the initial resuscitation. Assuming that units of resuscitative fluids reflect patient illness, our objective was to identify a rate of resuscitation intensity (RI) that could serve as an early surrogate of sickness for patients with substantial bleeding after injury. METHODS Adult patients surviving at least 30 minutes after admission and receiving one or more RBCs within 6 hours of admission from 10 US Level 1 trauma centers were enrolled in the PRospective Observational Multicenter Major Trauma Transfusion (PROMMTT) study. Total fluid units were calculated as the sum of the number of crystalloid units (1 L = 1 U), colloids (0.5 L = 1 U), and blood products (1 RBC = 1 U, 1 plasma = 1 U, 6 pack platelets = 1 U). Univariable and multivariable logistic regressions were used to evaluate associations between RI and 6-hour mortality, adjusting for age, center, penetrating injury, weighted Revised Trauma Score (RTS), and Injury Severity Score (ISS). RESULTS A total of 1,096 eligible patients received resuscitative fluids within 30 minutes, including 620 transfused with blood products. Despite varying products used, the total fluid RI was similar across all sites (3.2 ± 2.5 U). Patients who received four or more units of any resuscitative fluid had a 6-hour mortality rate of 14.4% versus 4.5% in patients who received less than 4 U. The adjusted odds ratio of 6-hour mortality for patients receiving 4 U or more within 30 minutes was 2.1 (95% confidence interval, 1.2–3.5). CONCLUSION Resuscitation with four or more units of any fluid was significantly associated with 6-hour mortality. This study suggests that early RI regardless of fluid type can be used as a surrogate for sickness and mortality in severely bleeding patients. LEVEL OF EVIDENCE Epidemiologic/prognostic study, level III.

Are we delivering two standards of care for pelvic trauma? Availability of angioembolization after hours and on weekends increases time to therapeutic intervention.

BACKGROUND We hypothesized that patients with pelvic fractures and hemorrhage admitted during daytime hours were undergoing interventional radiology (IR) earlier than those admitted at night and on weekends, thereby establishing two standards of time to hemorrhage control. METHODS The trauma registry (January 2008 to December 2011) was reviewed for patients admitted with pelvic fractures, hemorrhagic shock, and transfusion of at least 1 U of blood. The control group (DAY) was admitted from 7:30 AM to 5:30 PM Monday to Friday, while the study group (after hours [AHR]) was admitted from 5:30 PM to 7:30 AM, on weekends or holidays. RESULTS A total of 191 patients met the criteria (45 DAY, 146 AHR); 103 died less than 24 hours and without undergoing IR (29% DAY group vs. 62% AHR, p < 0.001). Sixteen patients (all in AHR group) died while awaiting IR (p = 0.032). Eighty-eight patients (32 DAY, 56 AHR) survived to receive IR. Among these, the AHR group were younger (median, 30 years vs. 54 years; p = 0.007), more tachycardic (median pulse, 119 beats/min vs. 90 beats/min; p = 0.001), and had more profound shock (median base, −10 vs. −6; p = 0.006) on arrival. Time from admission to IR (median, 301 minutes vs. 193 minutes; p < 0.001) and computed tomographic scan to IR (176 minutes vs. 87 minutes, p = 0.011) were longer in the AHR group. There was no difference in the 30-day mortality by univariate analysis. However, after controlling for age, arrival physiology, injury severity, and degree of shock, the AHR group had a 94% increased risk of mortality. CONCLUSION The current study demonstrated that patients admitted at night and on weekends have a significant increase in time to angioembolization compared with those arriving during the daytime and during the week. Multivariate regression noted that AHR management was associated with an almost 100% increase in mortality. While this is a single-center study and retrospective in nature, it suggests that we are currently delivering two standards of care for pelvic trauma, depending on the day and time of admission. LEVEL OF EVIDENCE Therapeutic study, level II.

A model of a radially expanding and contracting lymphangion

The lymphatic system is an extensive vascular network featuring valves and contractile walls that pump interstitial fluid and plasma proteins back to the main circulation. Immune function also relies on the lymphatic systems ability to transport white blood cells. Failure to drain and pump this excess fluid results in edema characterized by fluid retention and swelling of limbs. It is, therefore, important to understand the mechanisms of fluid transport and pumping of lymphatic vessels. Unfortunately, there are very few studies in this area, most of which assume Poiseuille flow conditions. In vivo observations reveal that these vessels contract strongly, with diameter changes of the order of magnitude of the diameter itself over a cycle that lasts typically 2-3s. The radial velocity of the contracting vessel is on the order of the axial fluid velocity, suggesting that modeling flow in these vessels with a Poiseuille model is inappropriate. In this paper, we describe a model of a radially expanding and contracting lymphatic vessel and investigate the validity of assuming Poiseuille flow to estimate wall shear stress, which is presumably important for lymphatic endothelial cell mechanotransduction. Three different wall motions, periodic sinusoidal, skewed sinusoidal and physiologic wall motions, were investigated with steady and unsteady parabolic inlet velocities. Despite high radial velocities resulting from the wall motion, wall shear stress values were within 4% of quasi-static Poiseuille values. Therefore, Poiseuille flow is valid for the estimation of wall shear stress for the majority of the lymphangion contractile cycle.

Measuring thrombin generation as a tool for predicting hemostatic potential and transfusion requirements following trauma.

BACKGROUND Thrombin is the central coagulation protease that activates clotting proteins, triggers platelet aggregation, and converts fibrinogen to fibrin. Relationships between thrombin generation (TG) and clinical outcomes have not been defined following trauma. We hypothesize that TG is predictive of transfusion requirements and patient outcomes. METHODS Plasma was collected from 406 highest-level activation trauma patients upon admission and 29 healthy donors. Standard coagulation tests were performed, and TG was measured by calibrated automated thrombogram. Mann-Whitney U-tests were used to compare healthy versus trauma patients, and subgroup analyses were used to compare hypocoagulable versus nonhypocoagulable patients. Hypocoagulability was determined by area under the receiver operating characteristic curve analysis and was defined as peak TG of less than 250 nM. Multiple logistic regressions were used to assess the ability of TG to predict massive transfusion and mortality. RESULTS The median (interquartile range) age was 39 years (28–52 years), with an Injury Severity Score (ISS) of 17 (9–26). The trauma patients had greater TG (peak, 316.2 nM [270.1–355.5 nM]) compared with the healthy controls (124.6 nM [91.1–156.2 nM]), p < 0.001. The overall rate of hypocoagulability was 17%. The patients with peak TG of less than 250 nM were more severely injured (ISS, 25 [13–30] vs. 16 [9–25], p = 0.003); required more transfusions of red blood cells (p = 0.02), plasma (p = 0.003), and platelets (p = 0.006); had fewer hospital-free days (p = 0.001); and had increased mortality (10% vs. 3% at 24 hours, p = 0.006, and 29% vs. 11% at 30 days, p = 0.0004). Peak TG of less than 250 nM was predictive of massive transfusion (odds ratio, 4.18; p = 0.01) and 30-day mortality (odds ratio, 2.78; p = 0.02). Finally, peak TG was inversely correlated with standard coagulation tests. CONCLUSION While the physiologic response to injury is to upregulate plasma procoagulant activity, the patients with reduced TG required more transfusions and had poorer outcomes. Measuring TG may provide an exquisitely sensitive tool for detecting disturbances in the enzymatic phases of coagulation in critically injured patients. LEVEL OF EVIDENCE Prognostic/epidemiologic study, level III.

Passive pressure-diameter relationship and structural composition of rat mesenteric lymphangions.

BACKGROUND Lymph flow depends on both the rate of lymph production by tissues and the extent of passive and active pumping. Here we aim to characterize the passive mechanical properties of a lymphangion in both mid-lymphangion and valve segments to assess regional differences along a lymphangion, as well as evaluating its structural composition. METHODS AND RESULTS Mesenteric lymphatic vessels were isolated and cannulated in a microchamber for pressure-diameter (P-D) testing. Vessels were inflated from 0 to 20 cmH(2)O at a rate of 4 cmH(2)O/min, and vessel diameter was continuously tracked, using an inverted microscope, video camera, and custom LabVIEW program, at both mid-lymphangion and valve segments. Isolated lymphatic vessels were also pressure-fixed at 2 and 7 cmH(2)O and imaged using a nonlinear optical microscope (NLOM) to obtain collagen and elastin structural information. We observed a highly nonlinear P-D response at low pressures (3-5 cmH(2)O), which was modeled using a three-parameter constitutive equation. No significant difference in the passive P-D response was observed between mid-lymphangion and valve regions. NLOM imaging revealed an inner elastin layer and outer collagen layer at all locations. Lymphatic valve leaflets were predominantly elastin with thick axially oriented collagen bands at the insertion points. CONCLUSIONS We observed a highly nonlinear P-D response at low pressures (3-5 cmH(2)O) and developed the first constitutive equation to describe the passive P-D response for a lymphangion. The passive P-D response did not vary among regions, in agreement with the composition of elastin and collagen in the lymphatic wall.

Lymph transport in rat mesenteric lymphatics experiencing edemagenic stress

To assess lymphatic flow adaptations to edema, we evaluated lymph transport function in rat mesenteric lymphatics under normal and increased fluid volume (edemagenic) conditions in situ.

Trauma, Time, and Transfusions: A Longitudinal Analysis of Coagulation Markers in Severely Injured Trauma Patients Receiving Modified Whole Blood or Component Blood Products.

Objective: The current study leveraged data from the Early Whole Blood (EWB) trial to explore the effects of modified whole blood (mWB) versus component (COMP) transfusions on coagulation parameters over time using longitudinal statistical methods. Study Design and Methods: The EWB study was a single-center randomized controlled trial, approved by the local IRB. Adult patients at highest-level trauma activations were randomized into mWB or COMP groups. Coagulation status was evaluated (at times 0, 3, 6, 12, and 24 h postadmission) using thrombelastography, platelet aggregometry, and calibrated automated thrombograms. Longitudinal statistical analyses with generalized estimating equations (GEE) were used to evaluate the effects of group, time, transfusion types, and their respective interactions on changes in measured coagulation markers. Results: A total of 59 patients were enrolled and adhered to protocol in the EWB trial, 25 in the mWB group, and 34 in the COMP group. Patients in both the mWB and COMP groups demonstrated a significant decline in their thrombelastography parameters during the first 3–6 h, specifically K-time, &agr;-angle, maximum amplitude, G, and LY30. Patients receiving mWB exhibited improved thrombin potential than those receiving COMP. Platelet count and function declined over time in both mWB and COMP groups; however, platelet aggregation in response to ristocetin in the mWB group was significantly improved at 12 h compared with the COMP group. The longitudinal GEE model revealed significant group-time interactive effects on the changes in coagulation markers and significant effect of platelet transfusions on improvements in coagulation profile. Conclusions: We observed significant interactive group-time effects, indicating that the types of transfusion as well as the time of transfusion significantly affect the patients coagulation status. Our pilot data suggest that there is an improvement in platelet function with mWB, but further studies are needed. Regardless, platelet transfusions were associated with improvements in coagulation over time in both the groups.

Plasma colloid osmotic pressure is an early indicator of injury and hemorrhagic shock.

ABSTRACT Objective: Hemorrhagic shock is the leading cause of traumatic deaths; many could be potentially prevented with appropriate resuscitation. However, to initiate resuscitation, one must identify patients with hemorrhagic shock early. In this article, we determined the associations between plasma colloid osmotic pressure (COP) and clinical outcomes in severely injured trauma patients. Methods: Plasma samples were collected from 104 trauma patients upon admission to the emergency department and 10 healthy volunteers to serve as control subjects. Plasma osmolality, COP, and serum protein were measured and correlated to clinical data. Thrombelastography and impedance aggregometry were performed to assess coagulopathy. Commercial enzyme-linked immunosorbent assays were used to quantify syndecan 1. Results: Plasma COP was significantly reduced in trauma patients compared to control subjects 17.7 ± 2.6 vs. 20.7 ± 2.1 mmHg (P < 0.05) and strongly correlated to serum protein values (R2 = 0.7). We divided our cohort into low (COP ⩽16.5 mmHg) and normal (COP >16.5 mmHg) subgroups, illustrating significantly higher Injury Severity Score scores in patients with low COP (21 vs. 10, P = 0.007), despite no differences in vital signs. Patients with low COP received more red blood cells, plasma, and platelets (4 vs. 0 total units, P = 0.0005) within 24 h of admission. Syndecan 1 levels were significantly higher (184 vs. 52 ng/mL, P = 0.027) in patients with low COP. Conclusions: Reduced plasma COP and serum protein in trauma patients are indicative of injury severity. In the absence of significant alterations in vital signs, plasma COP levels were associated with increased requirements for blood products and increased syndecan 1 shedding. We believe that plasma COP provides new insight in guiding resuscitation.

Three-dimensional Analysis of Flow Disturbances Caused by Clots in Inferior Vena Cava Filters

PURPOSE To assess the hemodynamics associated with clot captured within two different types of inferior vena cava (IVC) filters. MATERIALS AND METHODS Computational flow models were constructed for different clot sizes and shapes captured within the Greenfield (GF) (Medi-tech/Boston Scientific, Watertown, Massachusetts) and TrapEase (Cordis, Miami Lakes, Florida) IVC filters. Two models were employed; one was a straight tube (ST), and the other was a realistic model (RM) that included iliac and renal veins and lumbar curvature, with filter deployment between these inflows. Calculations were based on the Lattice Boltzmann method (LBM), allowing for accurate modeling of flows that are in transition from laminar to turbulent. RESULTS Flow disturbances were noted downstream of captured clots, with turbulence intensities reaching 41%. Disturbances were strongest with large clot volumes and in ST models. The RM vessel geometry greatly reduced the level of flow disturbance (majority of <2%; maximum turbulence intensity of 11%). Implementing flow rate representative of the infrarenal vena cava (rather than suprarenal) was also shown to reduce the amount of flow disturbance in ST models. CONCLUSIONS Although there is a mild amount of flow disturbance caused by captured clots, these flow patterns are not of the variety that have been shown to trigger platelet activation in other studies. Turbulence intensities were lower in the RMs, indicating the need to perform such flow studies under physiologic conditions.