H12‐(ADP)‐liposomes for hemorrhagic shock in thrombocytopenia: Mesenteric artery injury model in rabbits

Background

Objective

Methods

Results

Conclusion

Cellular and Biochemical Effects of Combined X-Ray Radiation and Storage on Whole Blood

Background

Evaluating the impact of ionizing radiation on stored blood is relevant since blood banks are major assets in emergency conditions such as radiation incident/attack. This study aimed to fill our knowledge gap of combined radiation and storage effects on blood.

Methods

Blood collected from 16 anesthetized rats was anticoagulated, aliquoted into storage bags, and assigned to 8 groups using protocols combining storage (1-day vs 3-day 4^oC) plus irradiation (75 Gy vs 0 Gy - control). Bags were positioned inside an X-ray irradiator (MultiRad-350). Complete blood count, differential white blood cell count, biochemistry, and hemostasis were analyzed (≥7 bags/group).

Results

Na⁺, bicarbonate, glucose, and pH significantly reduced, while K⁺, Cl⁻, and lactate increased by storage. Coagulation measures were not significantly altered after radiation. White blood cell count and most cell types were numerically reduced after radiation, but changes were statistically significant only for monocytes. No significant alterations were noted in aggregation or rotational thromboelastometry parameters between irradiated and control.

Conclusions

Evaluating cellular/biochemical parameters aids in assessing stored blood adequacy after radiation. Data suggest that fresh or cold-stored blood can sustain up to 75 Gy without major critical parameter changes and may remain suitable for use in critically ill patients in military/civilian settings.

Role of albumin on endothelial basement membrane and hemostasis in a rat model of hemorrhagic shock

BACKGROUND

We sought to determine the extent of loss of endothelial basement membrane (BM), leukocyte recruitment, and changes in coagulation after hemorrhagic shock, followed by limited-volume resuscitation (LVR) with 5% albumin (ALB).

METHODS

Anesthetized rats were bled 40% of blood volume and assigned to treatment groups: untreated (n = 6), LVR with normal saline (NS; n = 8), or LVR with ALB (n = 8). Sham rats (n = 6) underwent all procedures except hemorrhage or resuscitation. Blood samples were assayed for active proteases, such as metalloproteinase 9 (MMP-9) and a disintegrin and metalloproteinase 10 (ADAM-10), BM-type heparan sulfate proteoglycan (perlecan), cell count, and coagulation function. Leukocyte transmigration was used to estimate the net efficiency of leukocyte recruitment in cremaster venules.

RESULTS

Hemorrhage significantly lowered red cell count, but white cell and platelet counts did not change (vs. sham). Ionized calcium in plasma was significantly reduced in untreated and remained so after NS. In contrast, ionized calcium was normalized after ALB. Plasma expansion after NS and ALB further reduced leukocyte and platelet counts. Metalloproteinase 9, ADAM-10, and perlecan were significantly higher in untreated rats (vs. sham). Albumin normalized MMP-9, ADAM-10, and perlecan levels, while NS further increased MMP-9, ADAM-10, and perlecan (vs. sham). Transmigrated leukocytes doubled in the untreated group and remained elevated after NS (vs. sham) but normalized after ALB. Albumin reduced every stage of the leukocyte recruitment process to sham levels.

CONCLUSION

Despite similar plasma expansion, NS weakened platelet function contrary to ALB. Plasma expansion with ALB resulted in restoration of BM integrity and attenuation of leukocyte recruitment to tissues, in contrast to NS. Albumin plays a critical role in restoring BM integrity, attenuating leukocyte recruitment to tissues, and optimizing hemostasis by increasing ionized calcium in plasma.

Albumin Saturated With Fatty Acids Prevents Decompensation in a Rat Hemorrhagic Shock Trauma Model With Tourniquet and Hypotensive Resuscitation

ABSTRACT

Decompensation is a major prehospital threat to survival from trauma/hemorrhage shock (T/HS) after controlling bleeding. We recently showed higher than expected mortality from a combat-relevant rat model of T/HS (27 mL/kg hemorrhage) with tourniquet (TQ) and permissive hypotensive resuscitation (PHR) with Plasmalyte. Mortality and fluid requirements were reduced by resuscitation with 25% albumin presaturated with oleic acid (OA-sat) compared with fatty-acid -free albumin or Plasmalyte. The objective of this follow-up analysis was to determine the role of decompensation and individual compensatory mechanisms in those outcomes. We observed two forms of decompensation: slow (accelerating fluid volumes needed to maintain blood pressure) and acute (continuous fluid administration unable to prevent pressure drop). Combined incidence of decompensation was 71%. Nearly all deaths (21 of 22) were caused by acute decompensations that began as slow decompensations. The best hemodynamic measure for predicting acute decompensation was diastolic arterial pressure. Decompensation was due to vascular decompensation rather than loss of cardiac performance. Albumin concentration was lower in decompensating groups, suggesting decreased stressed volume, which may explain the association of low albumin on admission with poor outcomes after trauma. Our findings suggest that acute decompensation may be common after trauma and severe hemorrhage treated with TQ and PHR and OA-sat albumin may benefit early survival and reduce transfusion volume by improving venous constriction and preventing decompensation.

Cremaster muscle perfusion, oxygenation, and heterogeneity revealed by a new automated acquisition system in a rodent model of prolonged hemorrhagic shock

Local blood flow/oxygen partial pressure (Po₂) distributions and flow-Po₂ relationships are physiologically relevant. They affect the pathophysiology and treatment of conditions like hemorrhagic shock (HS), but direct noninvasive measures of flow, Po₂, and their heterogeneity during prolonged HS are infrequently presented. To fill this void, we report the first quantitative evaluation of flow-Po₂ relationships and heterogeneities in normovolemia and during several hours of HS using noninvasive, unbiased, automated acquisition. Anesthetized rats were subjected to tracheostomy, arterial/venous catheterizations, cremaster muscle exteriorization, hemorrhage (40% total blood volume), and laparotomy. Control animals equally instrumented were not subjected to hemorrhage/laparotomy. Every 0.5 h for 4.5 h, noninvasive laser speckle contrast imaging and phosphorescence quenching were employed for nearly 7,000 flow/Po₂ measurements in muscles from eight animals, using an automated system. Precise alignment of 16 muscle areas allowed overlapping between flow and oxygenation measurements to evaluate spatial heterogeneity, and repeated measurements were used to estimate temporal heterogeneity. Systemic physiological parameters and blood chemistry were simultaneously assessed by blood samplings replaced with crystalloids. Hemodilution was associated with local hypoxia, but increased flow prevented major oxygen delivery decline. Adding laparotomy and prolonged HS resulted in hypoxia, ischemia, decreased tissue oxygen delivery, and logarithmic flow/Po₂ relationships in most regions. Flow and Po₂ spatial heterogeneities were higher than their respective temporal heterogeneities, although this did not change significantly over the studied period. This quantitative framework establishes a basis for evaluating therapies aimed at restoring muscle homeostasis, positively impacting outcomes of civilian and military trauma/HS victims.NEW & NOTEWORTHY This is the first study on flow-Po₂ relationships during normovolemia, hemodilution, and prolonged hemorrhagic shock using noninvasive methods in multiple skeletal muscle areas of monitored animals. Automated flow/Po₂ measurements revealed temporal/spatial heterogeneities, hypoxia, ischemia, and decreased tissue oxygen delivery after trauma/severe hemorrhage. Hemodilution was associated with local hypoxia, but hyperemia prevented a major decline in oxygen delivery. This framework provides a quantitative basis for testing therapeutics that positively impacts muscle homeostasis and outcomes of trauma/hemorrhagic shock victims.

Automated noninvasive evaluation of blood flow and oxygenation in rats integrated with systemic physiological monitoring

BACKGROUND

Many studies evaluating blood flow and oxygen partial pressure (PO2) do not directly measure both parameters, are confined to few locations/microvessels, and depend on investigator's selection of measuring sites. Moreover, clinically/physiologically relevant systemic parameters are not simultaneously recorded. We implemented an automated system for prolonged blood flow/PO2 acquisition in large areas while collecting relevant systemic information.

METHODS

In anesthetized animals, cardiorespiratory parameters were continuously recorded. Other data were collected at baseline and hourly after 4 hours of hemorrhagic shock. A cremaster muscle was spread over a pedestal fixed to a motorized stage. One 2-dimensional tissue scan allowed 16 noninvasive PO2 measurements using oxygen-dependent phosphorescence quenching and fiber optics. Blood flow was estimated using laser speckle contrast imaging in the same areas used for PO2 measurements. At each timepoint, blood was sampled for extensive biochemistry/coagulation profile.

RESULTS

The system was used successfully by different operators. A set of flow/PO2 measurements was completed in less than 90 seconds. Muscle flow and PO2 correlated with some but not several systemic parameters, emphasizing the importance of performing both local and systemic evaluations.

CONCLUSION

System advantages include integration between local and over 40 systemic parameters, unbiased data collection/analysis, improved performance/sampled area, easy expansion, implementation and maintenance, no customized programming, and simplified training. Combining this system with trauma/prolonged HS models will enhance our ability to investigate tissue stability and select better resuscitation strategies to improve outcomes and survival.

LEVEL OF EVIDENCE

Diagnostic test, level V.

Low-volume resuscitation with normal saline is associated with microvascular endothelial dysfunction after hemorrhage in rats, compared to colloids and balanced crystalloids

Background

Restoration of endothelial glycocalyx (EG) barrier may be an essential therapeutic target for successful resuscitation. The aim of this study was to compare in vivo the effects of resuscitation with normal saline (NS) to lactated Ringer’s solution (LR), 5% albumin and fresh frozen plasma (FFP) on their ability to maintain EG and barrier function integrity, mitigate endothelial injury and inflammation, and restore vascular homeostasis after hemorrhagic shock.

Methods

Anesthetized rats (N = 36) were subjected to hemorrhagic shock (bled 40% of total blood volume), followed by resuscitation with 45 ml/kg NS or LR, or 15 ml/kg 5% albumin or FFP. Microhemodynamics, EG thickness, permeability, leukocyte rolling and adhesion were assessed in >180 vessels from cremaster muscle, as well as systemic measures.

Results

After hypotensive resuscitation, arterial pressure was 25% lower than baseline in all cohorts. Unlike FFP, resuscitation with crystalloids failed to restore EG thickness to baseline post shock and shedding of glycocalyx proteoglycan was significantly higher after NS. NS decreased blood flow and shear, and markedly increased permeability and leukocyte rolling/adhesion. In contrast, LR had lesser effects on increased permeability and leukocyte rolling. Albumin stabilized permeability and white blood cell (WBC) rolling/adhesion post shock, comparable to FFP.

Conclusions

Resuscitation with NS failed to inhibit syndecan-1 shedding and to repair the EG, which led to loss of endothelial barrier function (edema), decline in tissue perfusion and pronounced leukocyte rolling and adhesion. Detrimental effects of NS on endothelial and microvascular stabilization post shock may provide a pathophysiological basis to understand and prevent morbidity associated with iatrogenic resuscitation after hemorrhagic shock.

Plasma syndecan-1 and heparan sulfate correlate with microvascular glycocalyx degradation in hemorrhaged rats after different resuscitation fluids

The endothelial glycocalyx plays an essential role in many physiological functions and is damaged after hemorrhage. Fluid resuscitation may further change the glycocalyx after an initial hemorrhage-induced degradation. Plasma levels of syndecan-1 and heparan sulfate have been used as indirect markers for glycocalyx degradation, but the extent to which these measures are representative of the events in the microcirculation is unknown. Using hemorrhage and a wide range of resuscitation fluids, we studied quantitatively the relationship between plasma biomarkers and changes in microvascular parameters, including glycocalyx thickness. Rats were bled 40% of total blood volume and resuscitated with seven different fluids (fresh whole blood, blood products, and crystalloids). Intravital microscopy was used to estimate glycocalyx thickness in >270 postcapillary venules from 58 cremaster preparations in 9 animal groups; other microvascular parameters were measured using noninvasive techniques. Systemic physiological parameters and blood chemistry were simultaneously collected. Changes in glycocalyx thickness were negatively correlated with changes in plasma levels of syndecan-1 ( r = −0.937) and heparan sulfate ( r = −0.864). Changes in microvascular permeability were positively correlated with changes in both plasma biomarkers ( r = 0.8, P < 0.05). Syndecan-1 and heparan sulfate were also positively correlated ( r = 0.7, P < 0.05). Except for diameter and permeability, changes in local microcirculatory parameters (red blood cell velocity, blood flow, and wall shear rate) did not correlate with plasma biomarkers or glycocalyx thickness changes. This work provides a quantitative framework supporting plasma syndecan-1 and heparan sulfate as valuable clinical biomarkers of glycocalyx shedding that may be useful in guiding resuscitation strategies following hemorrhage.

Tissue oxygenation and microvascular hemodynamics in experimental arterial gas embolism

Microvascular hemodynamic responses to arterial gas embolism (AGE) and local oxygen tensions (PO2) have never been evaluated in vivo using intravital microscopy. A system was implemented to study AGE in real time using brightfield and phosphorescence microscopy as well as laser-induced microvessel occlusion. Bubble dynamics, microhemodynamics and oxygenation were studied following AGE in 61 microvessels and 41 interstitial spaces from 19 anesthetized rats. AGE was induced by direct air injection into the femoral artery ipsilateral to the studied cremaster muscle. Bubble-induced vaso-occlusion was investigated, and microvascular blood flow redistribution were associated with changes in intravascular and interstitial PO2. Microvascular blood flow as well as intravascular and tissue PO2 decreased after microvascular occlusion following microembolism. However, certain areas did not become fully hypoxic since redistribution of blood allowed oxygen to be supplied by nearby microvessels with blood (or plasma) flow or tissue gas diffusion. A linear correlation between interstitial and intravascular PO2 was found during baseline and after AGE. Because some microvessels remain flowing even after AGE, our observations suggest that intravascular therapeutic agents administered during severe AGE may reach microvascular networks and provide additional oxygenation to tissue areas where blood flow is compromised due to occlusion of some microvessels.

Early physiologic responses to hemorrhagic hypotension

The identification of early indicators of hemorrhagic hypotension (HH) severity may support early therapeutic approaches and bring insights into possible mechanistic implications. However, few systematic investigations of physiologic variables during early stages of hemorrhage are available. We hypothesized that, in certain subjects, early physiologic responses to blood loss are associated with the ability to survive hemorrhage levels that are lethal to subjects that do not present the same responses. Therefore, we examine the relevance of specific systemic changes during and after the bleeding phase of HH. Stepwise hemorrhage, representing prehospital situations, was performed in 44 rats, and measurements were made after each step. Heart and respiratory rates, arterial and venous blood pressures, gases, acid-base status, glucose, lactate, electrolytes, hemoglobin, O(2) saturation, tidal volume, and minute volume were measured before, during, and after bleeding 40% of the total blood volume. Fifty percent of rats survived 100 min (survivors, S) or longer; others were considered nonsurvivors (NS). Our findings were as follows: (1) S and NS subjected to a similar hemorrhage challenge showed significantly different responses during nonlethal levels of bleeding; (2) survivors showed higher blood pressure and ventilation than NS; (3) although pH was lower in NS at later stages, changes in bicarbonate and base excess occurred already during the hemorrhage phase and were higher in NS; and (4) plasma K(+) levels and glucose extraction were higher in NS. We conclude that cardiorespiratory and metabolic responses, essential for the survival at HH, can differentiate between S and NS even before a lethal bleeding was reached.

Measurement of hemoglobin oxygen saturation using Raman microspectroscopy and 532-nm excitation

The resonant Raman enhancement of hemoglobin (Hb) in the Q band region allows simultaneous identification of oxy- and deoxy-Hb. The heme vibrational bands are well known at 532 nm, but the technique has never been used to determine microvascular Hb oxygen saturation (So2) in vivo. We implemented a system for in vivo noninvasive measurements of So2. A laser light was focused onto areas of 15–30 μm in diameter. Using a microscope coupled to a spectrometer and a cooled detector, Raman spectra were obtained in backscattering geometry. Calibration was performed in vitro using blood at several Hb concentrations, equilibrated at various oxygen tensions. So2 was estimated by measuring the intensity of Raman signals (peaks) in the 1,355- to 1,380-cm−1 range (oxidation state marker band ν4), as well as from the ν19 and ν10 bands (1,500- to 1,650-cm−1 range). In vivo observations were made in microvessels of anesthetized rats. Glass capillary pathlength and Hb concentration did not affect So2 estimations from Raman spectra. The Hb Raman peaks observed in blood were consistent with earlier Raman studies using Hb solutions and isolated cells. The correlation between Raman-based So2 estimations and So2 measured by CO-oximetry was highly significant for ν4, ν10, and ν19 bands. The method allowed So2 determinations in all microvessel types, while diameter and erythrocyte velocity could be measured in the same vessels. Raman microspectroscopy has advantages over other techniques by providing noninvasive and reliable in vivo So2 determinations in thin tissues, as well as in solid organs and tissues in which transillumination is not possible.

Microvascular blood flow and oxygenation during hemorrhagic hypotension

Understanding microvascular oxygen transport requires the knowledge of microvessel topology and geometry, blood flow and oxygen levels. Microvascular hemodynamic responses to hemorrhagic hypotension (HH) such as size-dependent vasoconstriction and blood flow reduction could lead to increased longitudinal oxygen partial pressure (PO(2)) gradients. However, the mesenteric microvascular PO(2) has never been evaluated during HH. Therefore, we studied hemodynamic variables and PO(2) distribution in 165 mesenteric microvessels from 39 anesthetized rats to investigate whether HH-induced vasoconstriction and blood flow reduction were associated with changes in longitudinal PO(2) gradients. Vessels were analyzed according to their position in the network, as well as a few interstitial PO(2) areas. We found that during baseline a small PO(2) gradient exists, but HH is accompanied by more pronounced microvascular longitudinal PO(2) gradients. Decreased blood flow did not seem to completely explain these findings, since blood flow was uniformly diminished in arterioles and venules, independent of diameter and position in the network. During HH, some microvessels presented higher PO(2) than during baseline despite blood flow reduction, possibly due to a combination of systemic hyperoxia and low oxygen consumption of mesentery. The data suggest that blood flow measurements may be a poor indicator of the oxygenation status in some regions of the mesentery. The enhanced mesenteric longitudinal PO(2) gradient may lead to regions with different levels of other physiologically active compounds.

[Oxygen concentrators: evolution of inspired concentration of oxygen and repercussions in an anesthetized patient with CO2 absorber system. Pilot study.]

BACKGROUND AND OBJECTIVES

Resolution 1355/92 of the Conselho Federal de Medicina approved minimal standards for the installation and operation of oxygen concentrators (PSA) and recommended University Hospitals to undertake a prospective analysis in order to improve the system. It motivated this pilot study whose objective was to determine the clinical viability of using PSA oxygen by analyzing the variation in oxygen concentration in the fresh gas flow (FGF) outlet and in the inspired concentration of oxygen.

METHODS

An observational study with 30 patients, ASA I, undergoing upper abdominal surgeries using a CO2 absorber system and fresh gas flow (FGF) O(2)93 at 500 mL.min-1. Weight, age, type and duration of the surgery, inspired and expired fraction of CO2 (FiCO2, P ET CO2); inspired fraction of O2 (FiO2); and O2 concentration (O2ent) in the FGF. The following parameters were measured after intubation and every 10 minutes until the end of the procedure: P ET CO2, FiO2, and O2. Results underwent statistical analysis and p < 0.05 was considered significant.

RESULTS

The inspired fraction of carbon dioxide was equal to zero in all patients and moments of the study, but there was a significant reduction in P ET CO2 with time. The variables O2ent and FiO2 had similar tendencies with time (p = 0.1283), but the variable O2ent presented higher means (p < 0.001); evolution of mean O2ent and FiO2 was observed (p < 0.05).

CONCLUSIONS

This study demonstrated that the use of PSA oxygen, within the conditions proposed for the experiment, is safe and induced a progressive increase in O(2)93 in the FGF and of FiO2.

Oxygenation monitoring of tissue vasculature by resonance Raman spectroscopy

Resonance Raman spectroscopy offers a mechanism for the noninvasive measurement of in vivo and in situ hemoglobin oxygen saturation (HbO(2)Sat) in living tissue. Clinically informative signals can be provided by resonance enhancement with deep violet excitation. It is notable that fluorescence does not significantly degrade the quality of the signals. During the controlled hemorrhage and resuscitation of rats, signal intensity ratios of oxy- vs. deoxyhemoglobin from sublingual mucosa correlated with co-oximetry values of blood withdrawn from a central venous catheter. The spectroscopic application described here has potential as a noninvasive method for the diagnosis of clinical shock and guidance of its therapy.

Near infrared spectroscopy for evaluation of the trauma patient: a technology review

Clinicians now realize the limitations of the physical examination in detecting compensated shock states, the severity of uncompensated states, and in determining the adequacy of resuscitation in order to prevent subsequent post-traumatic multisystem organ failure and death. A renewed interest has developed in interrogating the state of oxygen transport at the end-organ level in the trauma patient. Although used as a research tool and now clinically to monitor cerebral oxygenation during complex cardiovascular and neurosurgery, near infrared absorption spectroscopy (NIRS) is being more aggressively investigated and now marketed clinically as a noninvasive means to assess tissue oxygenation in the trauma patient at the end organ level. This paper will describe the principles of NIRS and the basis for its proposed use in the trauma patient to assess tissue oxygenation. This includes its known limitations, current controversies, and what will be needed in the future to make this technology a part of the initial and ongoing assessment of the trauma patient. The ultimate goal of such techniques is to prevent misassessment of patients and inadequate resuscitation, which are believed to be major initiators in the development of multisystem organ failure and death.

Systemic Responses to Hemodilution After Transfusion with Stored Blood and with a Hemoglobin-Based Oxygen Carrier

We assessed the systemic effects of exchanges with blood or hemoglobin (Hb) raffimer under conditions of critical oxygen delivery (Do(2)crit). We compared Do(2)crit in animals receiving Hb-based oxygen carrier (HBOC; Hemolink), fresh blood (collected <24 h), or stored blood (10 days) before hemodilution. Rats were randomized to control, blood, or HBOC isovolemic exchange. Oxygen consumption was measured by using expired gas (o(2)a) and blood (o(2)b) samples, whereas whole-body oxygen delivery (Do(2)) was calculated from cardiac output and arterial oxygen content. After exchange, rats were subjected to stepwise isovolemic hemodilution. Blood pressure, gases, acid-base status, glucose, Hb oxygen saturation, heart rate, and total peripheral resistance were also measured. We found that 1) HBOC-treated rats showed an increased mean arterial blood pressure and total peripheral resistance throughout the hemodilution, 2) Do(2)crit calculated with o(2)a or o(2)b gave identical results, 3) Do(2)crit was not different between animals receiving blood and those receiving HBOC, 4) the terminal Hb concentration (1.8 +/- 0.1 g/dL) and Do(2) (5 +/- 1 mL . min(-1) . kg(-1)) were similar for all animals, and 5) most oxygen transport and biochemical variables changed similarly during hemodilution. The data suggest that tolerance to Do(2)crit is not altered by 50% replacement of native Hb by stored blood or Hb raffimer.

Hemoglobin oxygen saturation measurements using resonance Raman intravital microscopy

A system is described for in vivo noninvasive measurements of hemoglobin oxygen saturation (HbO2Sat) at the microscopic level. The spectroscopic basis for the application is resonant Raman enhancement of Hb in the violet/ultraviolet region, allowing simultaneous identification of oxy- and deoxyhemoglobin with the same excitation wavelength. The heme vibrational bands are well known, but the technique has never been used to determine microvascular HbO2Sat in vivo. A diode laser light (power: 0.3 mW) was focused onto sample areas 15-30 microm in diameter. Raman spectra were obtained in backscattering geometry by using a microscope coupled to a spectrometer and a cooled detector. Calibration was performed in vitro by using glass capillaries containing blood at several Hb concentrations, equilibrated at various oxygen tensions. HbO2Sat was estimated using the Raman band intensities at 1,360 and 1,375 cm(-1). Glass capillary path length and Hb concentration had no effect on HbO2Sat estimated from Raman spectra. In vivo observations were made in blood flowing in microvessels of the rat mesentery. The Hb Raman peaks observed in oxygenated and deoxygenated blood were consistent with earlier Raman studies that used Hb solutions and isolated cells. The method allowed HbO2Sat determinations in the whole range of arterioles, venules, and capillaries. Tissue transillumination allowed diameter and erythrocyte velocity measurements in the same vessels. Raman microspectroscopy offers distinct advantages over other currently used techniques by providing noninvasive and reliable in vivo determinations of HbO2Sat in thin tissues as well as in solid organs and tissues, which are unsuitable for techniques requiring transillumination.

Experimental analysis of critical oxygen delivery

Systemic variables were evaluated with respect to O2 delivery to test the hypothesis that critical O2 delivery and critical Hb can be estimated by multiple variables collected simultaneously. Rats were subjected to transfusion with either fresh or stored blood and then subjected to stepwise isovolemic hemodilution. Critical levels were measured by the dual-regression method from plots of systemic variables against O2 delivery and Hb. Delivery was calculated from cardiac index and arterial O2 content. We found that 1) after hemodilution, O2 delivery changed in a nonlinear relationship with Hb; 2) critical delivery calculated using 30 different systemic variables was not statistically different from each other; 3) critical delivery and critical Hb were correlated but were not different between animals receiving fresh or stored blood; and 4) similar critical levels were found using a single variable from several animals and using several variables from the same subject. The best variables to estimate critical delivery were lactate, bicarbonate, base excess, O2 extraction ratio, expired CO2, pulse pressure, cardiac index, and systolic pressure. The data suggest that a multivariable analysis of critical delivery may help determine the physiological oxygenation boundary at the whole body level. This may assist in finding therapeutic triggers on an individual basis using systemic markers of the transition from aerobic to anaerobic metabolism.

Continuous peripheral resistance measurement during hemorrhagic hypotension

We tested the hypotheses that continuous total peripheral resistance (TPR) measurements are superior to intermittent data collection and that variables related to TPR can be used to distinguish between survivors and nonsurvivors (NS), respectively, of prolonged hemorrhagic hypotension (HH). One week after a transit-time ultrasound probe was implanted on their ascending aortas, 21 rats were subjected to 4 h of HH at 40 mmHg. Measurements were made before and up to 4 h after initiation of HH. Additional bleeding or Ringer l-lactate (RL) infusion was used to maintain HH. TPR was continuously measured online using recordings of blood flow and arterial pressure. Approximately 67% of the rats survived ≥3 h; others were considered NS. Data collected at 30-min intervals failed to detect the maximum value of TPR (TPRmax). The times to reach TPRmax were similar for survivors and NS and were strongly correlated with the bleeding end points and with the RL infusion-onset times. However, survivors showed higher TPRmax values than NS ( P < 0.005) and had a significantly longer period than NS during which TPR was above baseline level (116 ± 20 vs. 51 ± 10 min). In conclusion, 1) the transit-time ultrasound technique at high sampling rate allowed continuous and accurate real-time monitoring of TPR, 2) the bleeding end point and RL infusion-onset times may be used as surrogates of the time to TPRmax, 3) TPRmax of survivors and NS could be detected only using a continuous TPR measurement, and 4) differences between survivors and NS could be revealed by the continuous TPR curve.

Systemic responses to prolonged hemorrhagic hypotension

Studies are needed to provide a rigorous examination of the relevance of monitored variables during prolonged hemorrhagic hypotension (HH). This study was designed to investigate the parameters that describe biochemical and O2 transport patterns in animals subjected to HH. Systemic parameters that could differentiate survivors from nonsurvivors were identified. An aortic flow probe was implanted in rats ( n = 21) for continuous measurement of cardiac output. Experiments were performed 6–9 days after surgery. Rats were bled to a mean arterial pressure of 40 mmHg and kept at that level using Ringer-lactate solution. Arterial and venous blood pressures, gases, acid-base status, glucose, lactate, electrolytes, hemoglobin, O2 saturation, heart and respiratory rates, total peripheral resistance, and O2 delivery and consumption were measured before hemorrhage, soon after 40 mmHg was reached, and 0.5, 1, 2, 3, and 4 h later. Fifty-three percent of rats survived ≥3 h (survivors); others were considered nonsurvivors. Nonsurvivors showed a significantly greater degree of metabolic acidosis than survivors. Arterial Po2, respiratory rate, O2 saturation, O2 content, glucose, and pH were significantly higher in survivors. The rate of Ringer-lactate infusion, arterial K+, and Pco2 were lower in survivors. Arterial K+ and respiratory rate were the only parameters significantly different between survivors and nonsurvivors at all time points during HH. Arterial levels of K+ showed the clearest distinction between survivors and nonsurvivors and may explain the sudden death experienced by animals during HH. The data suggest that early respiratory and metabolic compensations are essential for survival of prolonged HH.