Flow redistribution during progressive hemorrhage is a determinant of critical O2 delivery

ISCCM Guidelines for Hemodynamic Monitoring in the Critically Ill

Hemodynamic assessment along with continuous monitoring and appropriate therapy forms an integral part of management of critically ill patients with acute circulatory failure. In India, the infrastructure in ICUs varies from very basic facilities in smaller towns and semi-urban areas, to world-class, cutting-edge technology in corporate hospitals, in metropolitan cities. Surveys and studies from India suggest a wide variation in clinical practices due to possible lack of awareness, expertise, high costs, and lack of availability of advanced hemodynamic monitoring devices. We, therefore, on behalf of the Indian Society of Critical Care Medicine (ISCCM), formulated these evidence-based guidelines for optimal use of various hemodynamic monitoring modalities keeping in mind the resource-limited settings and the specific needs of our patients. When enough evidence was not forthcoming, we have made recommendations after achieving consensus amongst members. Careful integration of clinical assessment and critical information obtained from laboratory data and monitoring devices should help in improving outcomes of our patients.

How to cite this article

Kulkarni AP, Govil D, Samavedam S, Srinivasan S, Ramasubban S, Venkataraman R, et al. ISCCM Guidelines for Hemodynamic Monitoring in the Critically Ill. Indian J Crit Care Med 2022;26(S2):S66-S76.

The pathophysiology of uncontrolled hemorrhage in horses

BACKGROUND

Hemorrhagic shock in horses may be classified in several ways. Hemorrhage may be considered internal versus external, controlled or uncontrolled, or described based on the severity of hypovolemic shock the patient is experiencing. Regardless of the cause, as the severity of hemorrhage worsens, homeostatic responses are stimulated to ameliorate the systemic and local effects of an oxygen debt. In mild to moderate cases of hemorrhage (<15% blood volume loss), physiological adaptations in the patient may not be clinically apparent. As hemorrhage worsens, often in the uncontrolled situation such as a vascular breach internally, the pathophysiological consequences are numerous. The patient mobilizes fluid and reserve blood volume, notably splenic stored and peripherally circulating erythrocytes, to preferentially supply oxygen to sensitive organs such as the brain and heart. When the global and local delivery of oxygen is insufficient to meet the metabolic needs of the tissues, a cascade of cellular, tissue, and organ dysfunction occurs. If left untreated, the patient dies of hemorrhagic anemic shock.

CLINICAL IMPORTANCE

An understanding of the pathophysiological consequences of hemorrhagic shock in horses and their clinical manifestations may help the practitioner understand the severity of blood volume loss, the need for referral, the need for transfusion, and potential outcome. In cases of severe acute uncontrolled hemorrhage, it is essential to recognize the clinical manifestations quickly to best treat the patient, which may include humane euthanasia.

KEY POINTS

Uncontrolled hemorrhage may be defined as the development of a vascular breach and hemorrhage that cannot be controlled by interventional hemostasis methods such as external pressure, tourniquet, or ligation. Causes of uncontrolled hemorrhage in horses may be due to non-surgical trauma, surgical trauma, invasive diagnostic procedures including percutaneous organ biopsy, coagulopathy, hypertension, cardiovascular anomaly, vascular damage, neoplasia such as hemangiosarcoma, toxicity, or idiopathic in nature. When a critical volume of blood is lost, the respondent changes in heart rate, splenic blood mobilization, and microcirculatory control can no longer compensate for decreasing oxygen delivery to the tissues In spite of organ-specific microvascular responses (eg, myogenic responses, local mediator modulation of microvasculature, etc), all organs experience decreases in blood flow during severe hypovolemia Acute, fatal hemorrhagic shock is characterized by progressive metabolic acidosis, coagulopathy, and hypothermia, often termed the "triad of death," followed by circulatory collapse.

The fetal inflammatory response syndrome: the origins of a concept, pathophysiology, diagnosis, and obstetrical implications

The fetus can deploy a local or systemic inflammatory response when exposed to microorganisms or, alternatively, to non-infection-related stimuli (e.g., danger signals or alarmins). The term "Fetal Inflammatory Response Syndrome" (FIRS) was coined to describe a condition characterized by evidence of a systemic inflammatory response, frequently a result of the activation of the innate limb of the immune response. FIRS can be diagnosed by an increased concentration of umbilical cord plasma or serum acute phase reactants such as C-reactive protein or cytokines (e.g., interleukin-6). Pathologic evidence of a systemic fetal inflammatory response indicates the presence of funisitis or chorionic vasculitis. FIRS was first described in patients at risk for intraamniotic infection who presented preterm labor with intact membranes or preterm prelabor rupture of the membranes. However, FIRS can also be observed in patients with sterile intra-amniotic inflammation, alloimmunization (e.g., Rh disease), and active autoimmune disorders. Neonates born with FIRS have a higher rate of complications, such as early-onset neonatal sepsis, intraventricular hemorrhage, periventricular leukomalacia, and death, than those born without FIRS. Survivors are at risk for long-term sequelae that may include bronchopulmonary dysplasia, neurodevelopmental disorders, such as cerebral palsy, retinopathy of prematurity, and sensorineuronal hearing loss. Experimental FIRS can be induced by intra-amniotic administration of bacteria, microbial products (such as endotoxin), or inflammatory cytokines (such as interleukin-1), and animal models have provided important insights about the mechanisms responsible for multiple organ involvement and dysfunction. A systemic fetal inflammatory response is thought to be adaptive, but, on occasion, may become dysregulated whereby a fetal cytokine storm ensues and can lead to multiple organ dysfunction and even fetal death if delivery does not occur ("rescued by birth"). Thus, the onset of preterm labor in this context can be considered to have survival value. The evidence so far suggests that FIRS may compound the effects of immaturity and neonatal inflammation, thus increasing the risk of neonatal complications and long-term morbidity. Modulation of a dysregulated fetal inflammatory response by the administration of antimicrobial agents, anti-inflammatory agents, or cell-based therapy holds promise to reduce infant morbidity and mortality.

Renal autoregulation in experimental septic shock and its response to vasopressin and norepinephrine administration

Evidence suggests that septic shock patients with chronic arterial hypertension may benefit from resuscitation targeted to achieve higher blood pressure values than other patients, possibly as a result of altered renal autoregulation. The effects of different vasopressor agents on renal autoregulation may be important in this context. We investigated the effects of arginine vasopressin (AVP) and norepinephrine (NE) on renal autoregulation in ovine septic shock. Sepsis was induced by fecal peritonitis. When shock developed (decrease in mean arterial pressure to <65 mmHg and no fluid-responsiveness), animals were randomized to receive NE or AVP in a crossover design. Before the switch to the second vasopressor, the first vasopressor was discontinued for 30 minutes to ensure complete washout of the first vasopressor. Renal autoregulation was evaluated by recording the change in renal blood flow (RBF) in response to manual, stepwise reductions in renal inflow pressure. In this model, the lower limit of renal autoregulation was not significantly altered 6 hours after sepsis induction (59±9 vs. 64±7 mmHg at baseline, p=0.096). After development of shock, the autoregulatory threshold was lower with AVP than with NE (59±5 vs. 65±7 mmHg, p=0.010). However, RBF was higher with NE both at the start of autoregulatory measurements (206±58 vs. 170±52 mL/min; p=0.050) and at the autoregulatory threshold (191±53 vs. 150±47 mL/min; p=0.008). As vasopressors may have different effects on renal autoregulation, blood pressure management in patients with septic shock should be individualized and take into account drug-specific effects.

Venous oxygen saturation in critical illness

OBJECTIVE

To review clinically relevant features of systemic oxygen delivery and consumption and the technique and use of venous oxygenation monitoring in human and veterinary medicine.

DATA SOURCES

Veterinary and human peer-reviewed medical literature including scientific reviews, clinical and laboratory research articles, and authors' clinical research experience.

SUMMARY

Measurement of venous hemoglobin oxygen saturation (venous oxygenation) provides insight into the balance between oxygen supply and tissue demand. In people, measurement of venous oxygen saturation can reveal decompensation that is missed by physical examination and other routinely monitored parameters. Therefore, measurement of mixed or central venous oxygenation measurement may help guide therapy and predict outcome of critically ill patients. In dogs, low central venous oxygen saturation has been associated with impaired cardiopulmonary function and poor outcome in several small studies of experimental shock or severe clinical illness, suggesting that monitoring this variable may assist the treatment of severe illness in this species as well.

CONCLUSION

Venous oxygenation reflects systemic oxygenation status and can be used to guide treatment and estimate prognosis in critically ill patients. Measurement of venous oxygenation in veterinary patients is feasible and is a potentially valuable tool in the management of patients with severe disease. This review is intended to increase the understanding and awareness of the potential role of venous oxygen measurement in veterinary patients.

Renal autoregulation and blood pressure management in circulatory shock

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

Diffuse optical monitoring of peripheral tissues during uncontrolled internal hemorrhage in a porcine model

Reliable, continuous and noninvasive blood flow and hemoglobin monitoring in trauma patients remains a critical, but generally unachieved goal. Two optical sensing methods - diffuse correlation spectroscopy (DCS) and diffuse reflectance spectroscopy (DRS) - are used to monitor and detect internal hemorrhage. Specifically, we investigate if cutaneous perfusion measurements acquired using DCS and DRS in peripheral (thighs and ear-lobe) tissues could detect severe hemorrhagic shock in a porcine model. Four animals underwent high-grade hepato-portal injury in a closed abdomen, to induce uncontrolled hemorrhage and were subsequently allowed to bleed for 10 minutes before fluid resuscitation. DRS and DCS measurements of cutaneous blood flow were acquired using fiber optical probes placed on the thigh and earlobe of the animals and were obtained repeatedly starting from 1 to 5 minutes pre-injury, up to several minutes post shock. Clear changes were observed in measured optical spectra across all animals at both sites. DCS-derived cutaneous blood flow decreased sharply during hemorrhage, while DRS-derived vascular saturation and hemoglobin paralleled cardiac output. All derived optical parameters had the steepest changes during the rapid initial hemorrhage unambiguously. This suggests that a combined DCS and DRS based device might provide an easy-to-use, non-invasive, internal-hemorrhage detection system that can be used across a wide array of clinical settings.

Effects of controlled hypoxemia or hypovolemia on global and intestinal oxygenation and perfusion in isoflurane anesthetized horses receiving an alpha-2-agonist infusion

BACKGROUND

Aim of this prospective experimental study was to assess effects of systemic hypoxemia and hypovolemia on global and gastrointestinal oxygenation and perfusion in anesthetized horses. Therefore, we anesthetized twelve systemically healthy warmblood horses using either xylazine or dexmedetomidine for premedication and midazolam and ketamine for induction. Anesthesia was maintained using isoflurane in oxygen with either xylazine or dexmedetomidine and horses were ventilated to normocapnia. During part A arterial oxygen saturation (SaO2) was reduced by reducing inspiratory oxygen fraction in steps of 5%. In part B hypovolemia was induced by controlled arterial exsanguination via roller pump (rate: 38 ml/kg/h). Mean arterial blood pressure (MAP), heart rate, pulmonary artery pressure, arterial and central venous blood gases and cardiac output were measured, cardiac index (CI) was calculated. Intestinal microperfusion and oxygenation were measured using laser Doppler flowmetry and white-light spectrophotometry. Surface probes were placed via median laparotomy on the stomach, jejunum and colon.

RESULTS

Part A: Reduction in arterial oxygenation resulted in a sigmoid decrease in central venous oxygen partial pressure. At SaO2 < 80% no further decrease in central venous oxygen partial pressure occurred. Intestinal oxygenation remained unchanged until SaO2 of 80% and then decreased. Heart rate and pulmonary artery pressure increased significantly during hypoxemia. Part B: Progressive reduction in circulating blood volume resulted in a linear decrease in MAP and CI. Intestinal perfusion was preserved until blood loss resulted in MAP and CI lower 51 ± 5 mmHg and 40 ± 3 mL/kg/min, respectively, and then decreased rapidly.

CONCLUSIONS

Under isoflurane, intestinal tissue oxygenation remained at baseline when arterial oxygenation exceeded 80% and intestinal perfusion remained at baseline when MAP exceeded 51 mmHg and CI exceeded 40 mL/kg/min in this group of horses.

TRIAL REGISTRY NUMBER

33.14-42,502-04-14/1547.

Specific Etiologies Associated With the Multiple Organ Dysfunction Syndrome in Children: Part 2

OBJECTIVE

To describe a number of conditions and therapies associated with multiple organ dysfunction syndrome presented as part of the Eunice Kennedy Shriver National Institute of Child Health and Human Development Multiple Organ Dysfunction Workshop (March 26-27, 2015). In addition, the relationship between burn injuries and multiple organ dysfunction syndrome is also included although it was not discussed at the workshop.

DATA SOURCES

Literature review, research data, and expert opinion.

STUDY SELECTION

Not applicable.

DATA EXTRACTION

Moderated by an expert from the field, issues relevant to the association of multiple organ dysfunction syndrome with a variety of conditions and therapies were presented, discussed, and debated with a focus on identifying knowledge gaps and the research priorities.

DATA SYNTHESIS

Summary of presentations and discussion supported and supplemented by relevant literature.

CONCLUSIONS

Sepsis and trauma are the two conditions most commonly associated with multiple organ dysfunction syndrome both in children and adults. However, many other pathophysiologic processes may result in multiple organ dysfunction syndrome. In this article, we discuss conditions such as liver failure and pancreatitis, pathophysiologic processes such as ischemia and hypoxia, and injuries such as trauma and burns. Additionally, therapeutic interventions such as medications, blood transfusions, transplantation may also precipitate and contribute to multiple organ dysfunction syndrome. The purpose of this article is to describe the association of multiple organ dysfunction syndrome with a variety of conditions and therapies in an attempt to identify similarities, differences, and opportunities for therapeutic intervention.

Renal perfusion in sepsis: from macro- to microcirculation

The pathogenesis of sepsis-associated acute kidney injury is complex and likely involves perfusion alterations, a dysregulated inflammatory response, and bioenergetic derangements. Although global renal hypoperfusion has been the main target of therapeutic interventions, its role in the development of renal dysfunction in sepsis is controversial. The implications of renal hypoperfusion during sepsis probably extend beyond a simple decrease in glomerular filtration pressure, and targeting microvascular perfusion deficits to maintain tubular epithelial integrity and function may be equally important. In this review, we provide an overview of macro- and microcirculatory dysfunction in experimental and clinical sepsis and discuss relationships with kidney oxygenation, metabolism, inflammation, and function.

Platelet closure time in anesthetized Greyhounds with hemorrhagic shock treated with hydroxyethyl starch 130/0.4 or 0.9% sodium chloride infusions

OBJECTIVE

To measure platelet closure time (PCT) in dogs during controlled hemorrhagic shock and after fluid resuscitation with hydroxyethyl starch (HES) 130/0.4 or 0.9% sodium chloride.

DESIGN

Experimental interventional study.

SETTING

University veterinary teaching hospital.

ANIMALS

Eleven healthy Greyhounds.

INTERVENTIONS

Dogs were anesthetized and had 48 mL/kg of blood removed to induce hemorrhagic shock. Dogs received 20 mL/kg of HES 130/0.4 (n = 6) or 80 mL/kg of 0.9% sodium chloride (NaCl; n = 5) intravenously over 20 minutes. PCT was measured using the Platelet Function Analyzer-100 with collagen and adenosine-diphosphate cartridges at: T0 = 60 minutes after induction of anesthesia prior to hemorrhage, T1 = during hemorrhagic shock, and T2 = 40 minutes after completion of fluid bolus. Packed cell volume and platelet count were concurrently measured.

MEASUREMENT AND MAIN RESULTS

Hemorrhagic shock did not significantly change PCT, with no difference between T0 and T1. Both the HES 130/0.4 and 0.9% NaCl group had a significantly increased mean PCT at T2 of 91.4 seconds (95% CI 69.3-113.4) and 95.5 seconds (95% CI 78.2-112.8), respectively, compared to T1. The magnitude of change was significantly greater for the 0.9% NaCl group than the HES 130/0.4 group. There was no difference in the magnitude of change in PCV and platelet count between the 2 groups. The PCV and platelet count were >25% and >100,000/μL, respectively, in all dogs, except for dogs in the HES 130/0.4 group at T2 where platelet counts were <100,000/μL.

CONCLUSION

Controlled hemorrhagic shock in Greyhounds under anesthesia did not cause a significant change in PCT. Both HES 130/0.4 and 0.9% NaCl administration after induction of shock increased PCT. These results do not support that HES 130/0.4 causes relevant platelet dysfunction beyond hemodilution.

Personalizing blood pressure management in septic shock

This review examines the available evidence for targeting a specific mean arterial pressure (MAP) in sepsis resuscitation. The clinical data suggest that targeting an MAP of 65-70 mmHg in patients with septic shock who do not have chronic hypertension is a reasonable first approximation. Whereas in patients with chronic hypertension, targeting a higher MAP of 80-85 mmHg minimizes renal injury, but it comes with increased risk of arrhythmias. Importantly, MAP alone should not be used as a surrogate of organ perfusion pressure, especially under conditions in which intracranial, intra-abdominal or tissue pressures may be elevated. Organ-specific perfusion pressure targets include 50-70 mmHg for the brain based on trauma brain injury as a surrogate for sepsis, 65 mmHg for renal perfusion and >50 mmHg for hepato-splanchnic flow. Even at the same MAP, organs and regions within organs may have different perfusion pressure and pressure-flow relationships. Thus, once this initial MAP target is achieved, MAP should be titrated up or down based on the measures of organ function and tissue perfusion.

Snake constriction rapidly induces circulatory arrest in rats

As legless predators, snakes are unique in their ability to immobilize and kill their prey through the process of constriction, and yet how this pressure incapacitates and ultimately kills the prey remains unknown. In this study, we examined the cardiovascular function of anesthetized rats before, during and after being constricted by boas (Boa constrictor) to examine the effect of constriction on the prey's circulatory function. The results demonstrate that within 6 s of being constricted, peripheral arterial blood pressure (PBP) at the femoral artery dropped to 1/2 of baseline values while central venous pressure (CVP) increased 6-fold from baseline during the same time. Electrocardiographic recordings from the anesthetized rat's heart revealed profound bradycardia as heart rate (fH) dropped to nearly half of baseline within 60 s of being constricted, and QRS duration nearly doubled over the same time period. By the end of constriction (mean 6.5±1 min), rat PBP dropped 2.9-fold, fH dropped 3.9-fold, systemic perfusion pressure (SPP=PBP−CVP) dropped 5.7-fold, and 91% of rats (10 of 11) had evidence of cardiac electrical dysfunction. Blood drawn immediately after constriction revealed that, relative to baseline, rats were hyperkalemic (serum potassium levels nearly doubled) and acidotic (blood pH dropped from 7.4 to 7.0). These results are the first to document the physiological response of prey to constriction and support the hypothesis that snake constriction induces rapid prey death due to circulatory arrest.

Long-term and short-term effects of hemodialysis on liver function evaluated using the galactose single-point test

Aim. The galactose single-point (GSP) test assesses functioning liver mass by measuring the galactose concentration in the blood 1 hour after its administration. The purpose of this study was to investigate the impact of hemodialysis (HD) on short-term and long-term liver function by use of GSP test. Methods. Seventy-four patients on maintenance HD (46 males and 28 females, 60.38 ± 11.86 years) with a mean time on HD of 60.77 ± 48.31 months were studied. The GSP values were compared in two groups: (1) before and after single session HD, and (2) after one year of maintenance HD. Results. Among the 74 HD patient, only the post-HD Cr levels and years on dialysis were significantly correlated with GSP values (r = 0.280, P < 0.05 and r = −0.240, P < 0.05, resp.). 14 of 74 patients were selected for GSP evaluation before and after a single HD session, and the hepatic clearance of galactose was similar (pre-HD 410 ± 254 g/mL, post-HD 439 ± 298 g/mL, P = 0.49). GSP values decreased from 420.20 ± 175.26 g/mL to 383.40 ± 153.97 g/mL after 1 year maintenance HD in other 15 patients (mean difference: 19.00 ± 37.66 g/mL, P < 0.05). Conclusions. Patients on maintenance HD for several years may experience improvement of their liver function. However, a single HD session does not affect liver function significantly as assessed by the GSP test. Since the metabolism of galactose is dependent on liver blood flow and hepatic functional mass, further studies are needed.

Fluid management in the critically ill child

Fluid management has a major impact on the duration, severity and outcome of critical illness. The overall strategy for the acutely ill child should be biphasic. Aggressive volume expansion to support tissue oxygen delivery as part of early goal-directed resuscitation algorithms for shock--especially septic shock--has been associated with dramatic improvements in outcome. Recent data suggest that the cost-benefit of aggressive fluid resuscitation may be more complex than previously thought, and may depend on case-mix and the availability of intensive care. After the resuscitation phase, critically ill children tend to retain free water while having reduced insensible losses. Fluid regimens that limit or avoid positive fluid balance are associated with a reduced length of hospital stay and fewer complications. Identifying the point at which patients change from the 'early shock' pattern to the later 'chronic critical illness' pattern remains a major challenge. Very little data are available on the choice of fluids, and most of the information that is available arises from studies of critically ill adults. There is therefore an urgent need for high-quality trials of both resuscitation and maintenance fluid regimens in critically ill children.

Physiologic responses to severe hemorrhagic shock and the genesis of cardiovascular collapse: can irreversibility be anticipated?

BACKGROUND

The causes of cardiovascular collapse (CC) during hemorrhagic shock (HS) are unknown. We hypothesized that vascular tone loss characterizes CC, and that arterial pulse pressure/stroke volume index ratio or vascular tone index (VTI) would identify CC.

METHODS

Fourteen Yorkshire-Durock pigs were bled to 30 mmHg mean arterial pressure and held there by repetitive bleeding until rendered unable to compensate (CC) or for 90 min (NoCC). They were then resuscitated in equal parts to shed volume and observed for 2 h. CC was defined as a MAP < 30 mmHg for 10 min or <20 mmHg for 10 s. Study variables were recorded at baseline (B0), 30, 60, 90 min after bleeding and at resuscitation (R0), 30, and 60 min afterward.

RESULTS

Swine were bled to 32% ± 9% of total blood volume. Epinephrine (Epi) and VTI were low and did not change in NoCC after bleeding compared with CC swine, in which both increased (0.97 ± 0.22 to 2.57 ± 1.42 mcg/dL, and 173 ± 181 to 939 ± 474 mmHg/mL, respectively), despite no differences in bled volume. Lactate increase rate (LIR) increased with hemorrhage and was higher at R0 for CC, but did not vary in NoCC. VTI identified CC from NoCC and survivors from non-survivors before CC. A large increase in LIR was coincident with VTI decrement before CC occurred.

CONCLUSIONS

Vasodilatation immediately prior to CC in severe HS occurs at the same time as an increase in LIR, suggesting loss of tone as the mechanism causing CC, and energy failure as its probable cause.

Mitochondrial function and tissue vitality: bench-to-bedside real-time optical monitoring system

BACKGROUND

The involvement of mitochondria in pathological states, such as neurodegenerative diseases, sepsis, stroke, and cancer, are well documented. Monitoring of nicotinamide adenine dinucleotide (NADH) fluorescence in vivo as an intracellular oxygen indicator was established in 1950 to 1970 by Britton Chance and collaborators. We use a multiparametric monitoring system enabling assessment of tissue vitality. In order to use this technology in clinical practice, the commercial developed device, the CritiView (CRV), is tested in animal models as well as in patients.

METHODS AND RESULTS

The new CRV enables the optical monitoring of four different parameters, representing the energy balance of various tissues in vivo. Mitochondrial NADH is measured by surface fluorometry/reflectometry. In addition, tissue microcirculatory blood flow, tissue reflectance and oxygenation are measured as well. The device is tested both in vitro and in vivo in a small animal model and in preliminary clinical trials in patients undergoing vascular or open heart surgery. In patients, the monitoring is started immediately after the insertion of a three-way Foley catheter (urine collection) to the patient and is stopped when the patient is discharged from the operating room. The results show that monitoring the urethral wall vitality provides information in correlation to the surgical procedure performed.

Delaying blood transfusion in experimental acute anemia with a perfluorocarbon emulsion

BACKGROUND

To avoid unnecessary blood transfusions, physiologic transfusion triggers, rather than exclusively hemoglobin-based transfusion triggers, have been suggested. The objective of this study was to determine systemic and microvascular effects of using a perfluorocarbon-based oxygen carrier (PFCOC) to maintain perfusion and oxygenation during extreme anemia.

METHODS

The hamster (weight, 55-65 g) window chamber model was used. Two isovolemic hemodilution steps were performed using hydroxyethyl starch, 10%, at normoxic conditions to a hematocrit of 19% (hemoglobin, 5.5 g/dl), the point at which the transfusion trigger was reached. Two additional hemodilution exchanges using the PFCOC (Oxycyte) and increasing the fraction of inspired oxygen to 1.0 were performed to reduce the hematocrit to 11% (hemoglobin, 3.8 g/dl) and 6% (hemoglobin, 2.0 g/dl), respectively. No control group was used in the study because this concentration of hemodilution is lethal with conventional plasma expanders. Systemic parameters, microvascular perfusion, functional capillary density, and oxygen tensions across the microvascular network were measured.

RESULTS

At 6% hematocrit, the PFCOC maintained mean arterial pressure, cardiac output, systemic oxygen delivery, and oxygen consumption. As hematocrit was decreased from 11% to 6%, functional capillary density, calculated microvascular oxygen delivery, and oxygen consumption decreased; and the oxygen extraction ratio was close to 100%. Peripheral tissue oxygenation was not predicted by systemic oxygenation.

CONCLUSIONS

The PFCOC, in conjunction with hyperoxia, was able to sustain organ function and partially provide systemic oxygenation during extreme anemia during the observation period. The PFCOC can work as a bridge until erythrocytes are available for transfusion or when additional oxygen is required, despite the possible limitations in peripheral tissue oxygenation.

Oxygen transport characterization of a human model of progressive hemorrhage

BACKGROUND

Hemorrhage continues to be a leading cause of death from trauma sustained both in combat and in the civilian setting. New models of hemorrhage may add value in both improving our understanding of the physiologic responses to severe bleeding and as platforms to develop and test new monitoring and therapeutic techniques. We examined changes in oxygen transport produced by central volume redistribution in humans using lower body negative pressure (LBNP) as a potential mimetic of hemorrhage.

METHODS AND RESULTS

In 20 healthy volunteers, systemic oxygen delivery and oxygen consumption, skeletal muscle oxygenation and oral mucosa perfusion were measured over increasing levels of LBNP to the point of hemodynamic decompensation. With sequential reductions in central blood volume, progressive reductions in oxygen delivery and tissue oxygenation and perfusion parameters were noted, while no changes were observed in systemic oxygen uptake or markers of anaerobic metabolism in the blood (e.g., lactate, base excess). While blood pressure decreased and heart rate increased during LBNP, these changes occurred later than the reductions in tissue oxygenation and perfusion.

CONCLUSIONS

These findings indicate that LBNP induces changes in oxygen transport consistent with the compensatory phase of hemorrhage, but that a frank state of shock (delivery-dependent oxygen consumption) does not occur. LBNP may therefore serve as a model to better understand a variety of compensatory physiological changes that occur during the pre-shock phase of hemorrhage in conscious humans. As such, LBNP may be a useful platform from which to develop and test new monitoring capabilities for identifying the need for intervention during the early phases of hemorrhage to prevent a patient's progression to overt shock.

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.

Assessment of splanchnic blood flow using magnetic resonance imaging

BACKGROUND AND AIMS

The splanchnic circulation has an important function in the body under both physiological and pathophysiological conditions. Despite its importance, no reliable noninvasive procedures for estimating splanchnic circulation have been established. The aim of this study was to evaluate MRI as a tool for assessing intra-abdominal blood flows of the aorta, portal vein (VPO) and the major intestinal and hepatic vessels.

METHODS

In nine healthy volunteers, the proximal aorta (AOP) and distal abdominal aorta (AOD), superior mesenteric artery (SAM), celiac trunk (CTR), hepatic arteries (common and proper hepatic arteries, AHC and AHP, respectively), and VPO were localized on contrast-enhanced magnetic resonance angiography images. Volumetric flow was measured using a two-dimensional cine echocardiogram-gated phase contrast technique. Measurements were taken before and 30 min after continuous intravenous infusion of somatostatin (250 microg/h) and were independently evaluated by two investigators.

RESULTS

Blood flow measured by MRI in the VPO, SAM, AOP, AHP, and CTR significantly decreased after drug infusion. Flows in the AOD and AHC showed a tendency to decrease (P>0.05). Interrater agreement on flows in MRI was very good for large vessels (VPO, AOP, and AOD), with a concordance correlation coefficient of 0.94, as well as for smaller vessels such as the CTR, AHC, AHP, and SAM (concordance correlation coefficient =0.78).

CONCLUSION

Somatostatin-induced blood flow changes in the splanchnic region were reliably detected by MRI. MRI may be useful for the noninvasive assessment of blood flow changes in the splanchnic region.

[Effects of sedative agents on metabolic demand]

Literature about the effects of sedative drugs on the metabolic demand of critically ill patients is relatively old and of relatively poor quality. Most are experimental or observational studies. Level of evidence is therefore relatively low corresponding to "expert opinion". The effects of analgesics and hypnotics on tissue metabolic demand associated remain difficult to be adequately quantified. They are essentially related to a decreased neuro-humoral response to stress. This response involves principally the sympathetic system, which could be effectively blocked by most of the anesthetic agents. Other factors could participate to the observed reduction in tissue metabolic demand, as a decrease in spontaneous muscular activity, a reduction in work of breathing and/or a decrease in body temperature. The relative contribution of these different factors will depend on the clinical situation of the patient. Proper effects of anesthetic agents on cellular metabolism are limited as they can only decrease the functional component of this metabolism especially at the level of the heart and to some extent, at the level of the brain. Although the control of the sympathetic activity may be beneficial in critically ill patient, complete sympathetic blockade could be detrimental. Indeed, when oxygen transport to the tissues is acutely reduced, the sympathetic system plays an important role in the redistribution of blood flow according of local metabolic demand. The complete blunting of the neuro-humoral response to stress and therefore of the sympathetic system alters this physiological mechanism and results in a decrease in tissue oxygen extraction capabilities. An imbalance between tissue oxygen demand and delivery could appear with the development of cellular hypoxia. The institution of sedation in a critically ill patient requires careful evaluation of the sedation level using an appropriate scale. In patients in whom a reduction in metabolic demand is specifically requested, but also in patients with limited oxygen transport, the effects of sedative agents on the oxygen consumption-oxygen delivery relationship must also be monitored. The choice of the different agents to be administered will depend on the predefined objectives. As far as intravenous agents are concerned, there is no evidence than one association is more efficient in reducing patient's metabolic demand.

Effects of low abdominal blood flow and dobutamine on blood flow distribution and on the hepatic arterial buffer response in anaesthetized pigs

Low cardiac output impairs the hepatic arterial buffer response (HABR). Whether this is due to low abdominal blood flow per se is not known. Dobutamine is commonly used to increase cardiac output, and it may further modify hepatosplanchnic and renal vasoregulation. We assessed the effects of isolated abdominal aortic blood flow changes and dobutamine on hepatosplanchnic and renal blood flow. Twenty-five anesthetized pigs with an abdominal aorto-aortic shunt were randomized to 2 control groups [zero (n = 6) and minimal (n = 6) shunt flow], and 2 groups with 50% reduction of abdominal blood flow and either subsequent increased abdominal blood flow by shunt reduction (n = 6) or dobutamine infusion at 5 and 10 microg kg(-1) min(-1) with constant shunt flow (n = 7). Regional (ultrasound) and local (laser Doppler) intra-abdominal blood flows were measured. The HABR was assessed during acute portal vein occlusion. Sustained low abdominal blood flow, by means of shunt activation, decreased liver, gut, and kidney blood flow similarly and reduced local microcirculatory blood flow in the jejunum. Shunt flow reduction partially restored regional blood flows but not jejunal microcirculatory blood flow. Low-but not high-dose dobutamine increased gut and celiac trunk flow whereas hepatic artery and renal blood flows remained unchanged. Neither intervention altered local blood flows. The HABR was not abolished during sustained low abdominal blood flow despite substantially reduced hepatic arterial blood flow and was not modified by dobutamine. Low-but not high-dose dobutamine redistributes blood flow toward the gut and celiac trunk. The jejunal microcirculatory flow, once impaired, is difficult to restore.

Hemorrhagic shock in obstetrics

Acute postpartum hemorrhage is the leading worldwide cause of maternal mortality, such deaths being usually related to the development of hemorrhagic shock and its consequences, especially the multiple organ dysfunction syndrome. Obstetricians should be aware of the clinical manifestations and principles of management of hemorrhagic shock. Initial assessment of the bleeding patient requires monitoring blood pressure, pulse, capillary refill, mental status and urinary output. This allows estimation of the amount and the rate of blood loss and helps direct treatment. Hemorrhagic shock is a condition in which inadequate perfusion of organs results in insufficient availability of oxygen to satisfy the metabolic needs of the tissues. A catabolic state develops. The consequences of these changes are inflammation, endothelial dysfunction, and disruption of normal metabolic processes in vital organs. Once these events become established, the process of shock is often irreversible, even if volume and red cell deficits are corrected. The principal goals of management are controlling the source of the blood loss; restoring adequate oxygen carrying capacity; and maintaining adequate tissue perfusion. Patients with severe postpartum hemorrhage are at risk of developing hypothermia, an insidious complication that contributes substantially to morbidity and mortality. It must be prevented or treated promptly. Successful treatment of exsanguinating postpartum hemorrhage depends on efficient collaboration among all members of the patient care team, and a management plan based on an understanding of the pathophysiology of shock and tailored to the individual patient's situation.

Effects of isovolemic resuscitation with hemoglobin-based oxygen carrier Hemoglobin glutamer-200 (bovine) on systemic and mesenteric perfusion and oxygenation in a canine model of hemorrhagic shock: a comparison with 6% hetastarch solution and shed blood

OBJECTIVE

To study Hemoglobin glutamer-200 bovine (Hb-200), 6% hetastarch (HES) and shed whole blood (WB) resuscitation in canine hemorrhagic shock.

STUDY DESIGN

Prospective laboratory investigation. Animals Twelve adult dogs [29 +/- 1 kg (mean +/- SD)].

METHODS

Anesthetized dogs were instrumented for recording systemic and mesenteric hemodynamic parameters and withdrawal of arterial, mixed and mesenteric venous blood, in which hematological, oxygenation, blood gas and acid-bases variables were determined. Recordings were made before [baseline (BL)], after 1 hour of hypovolemia and immediately and 3 hours post-resuscitation with 30 mL kg(-1) of either Hb-200, HES, or WB.

RESULTS

Blood withdrawal (average 34 +/- 2 mL kg(-1)) caused significant hemodynamic changes, metabolic acidosis and hyperlactatemia characteristic for hemorrhagic shock. Only WB transfusion restored all variables. Hemoglobin glutamer-200 bovine infusion returned most hemodynamic parameters including cardiac output and mesenteric arterial blood flow to BL but increased mean arterial pressure above BL (p < 0.05). However, Hb-200 failed to restore total Hb and arterial oxygen content (CaO2), leaving systemic (DO2I) and mesenteric O2 delivery (DO2Im) below BL (p < 0.05). Nevertheless, acid-base variables recovered completely after Hb-200 resuscitation, and met-hemoglobin (Met-Hb) levels increased (p < 0.05). Hetastarch resuscitation returned hemodynamic variables to or above BL but further decreased total Hb and CaO2, preventing recovery of sDO2I and mDO2I (p < 0.05). Thus, systemic and mesenteric O2 extraction stayed above BL (p < 0.05) while acid-base variables recovered to BL, although slower than in Hb-200 and WB groups (p < 0.05).

CONCLUSIONS AND CLINICAL RELEVANCE

Resuscitation with Hb-200 seemed to resolve metabolic acidosis and lactatemia more rapidly than HES, but not WB; yet it is not superior to HES in improving DO2I and DO2Im. The hyperoncotic property of solutions like Hb-200 that results in rapid volume expansion with more homogenous microvascular perfusion and the ability to facilitate diffusive O2 transfer accelerating metabolic recovery may be the key mechanisms underlying their beneficial effects as resuscitants.

The systemic delivery and consumption of oxygen in the infant after cardiac surgery

“All the vital mechanisms, however varied they may be, have only one object, that of preserving constant the conditions of life in the internal environment.”1

An essential function of the cardiopulmonary system is to generate sufficient flow of oxygenated blood around the circulation in order to maintain normal cellular metabolism. The systemic delivery of oxygen is a function of the cardiac output and the content of oxygen in the systemic arterial blood, while the extent to which metabolising tissues require this oxygen for the maintenance of their integrity and function defines the systemic consumption of oxygen. As metabolising tissues have no mechanism for storing oxygen, they depend on its continuous supply, which must at least match their changing demands. As a result, it is a fundamental requirement of survival that the systemic consumption of oxygen, at all times, is matched by appropriate levels of its delivery.

Redistribution of cardiac output during hemorrhagic shock in sheep

OBJECTIVE

Our goal was to evaluate the robustness of one of the assumptions used by esophageal Doppler monitors to compute systemic stroke volume and cardiac output; i.e., a constant flow proportion between supra-aortic vessels and descending aorta. For this purpose, we measured ascending and descending aortic blood flows during acute hemorrhage in anesthetized ewes.

DESIGN

Prospective, experimental study.

SETTING

Animal research facility.

SUBJECTS

Adult ewes.

INTERVENTIONS

Anesthetized animals were implemented with an aortic pressure transducer and two ultrasound transit time flowmeters placed around ascending and descending aorta, respectively. After baseline measurements, three incremental blood withdrawals were followed by progressive blood restitution in three similar steps.

MEASUREMENTS AND MAIN RESULTS

Ascending and descending aortic blood flows were reduced in a proportional manner after hemorrhage (-48% and -46%, respectively; p < .05 vs. baseline). Following blood restitution, flows were not fully restored, but ascending aortic flow was reduced by 27% with respect to initial control values while descending aortic flow was only 15% below. The agreement between ascending aortic flow and cardiac output calculated as descending aortic flow divided by 0.7 was characterized by a bias of 0.07 L/min and limits of agreement of +1.24 L/min and -1.10 L/min.

CONCLUSIONS

Minor blood flow redistribution between supra-aortic and descending aortic territories was seen only following blood restitution but not during hemorrhage in these anesthetized ewes. This observation supports the robustness of the assumption of constant flow proportion used by the esophageal Doppler monitor to calculate systemic stroke volume from descending aortic flow measurements.

Critical oxygen delivery during cardiopulmonary bypass in dogs

BACKGROUND AND OBJECTIVE

To determine the minimal oxygen delivery and pump flow that can maintain systemic oxygen uptake during normothermic (37 degrees C) pulsatile and non-pulsatile cardiopulmonary bypass in dogs.

METHODS

Eighteen anaesthetized dogs were randomly assigned to receive either non-pulsatile (Group C; n = 9) or pulsatile bypass flow (Group P; n = 9). Oxygen delivery was reduced by a progressive decrease in pump flow, while arterial oxygen content was maintained constant. In each animal, critical oxygen delivery was determined from plots of oxygen uptake vs. oxygen delivery and from plots of blood lactate vs. oxygen delivery using a least sum of squares technique. Critical pump flow was determined from plots of lactate vs. pump flow.

RESULTS

At the critical point, oxygen delivery obtained from oxygen uptake was 7.7 +/- 1.1 mL min(-1) kg(-1) in Group C and 6.8 +/- 1.8 mL min(-1) kg(-1) in Group P (n.s.). These values were similar to those obtained from lactate measurements (Group C: 7.8 +/- 1.6 mL min(-1) kg(-1); Group P: 7.6 +/- 2.0 mL min(-1) kg(-1)). Critical pump flows determined from lactate measurements were 55.6 +/- 13.8 mL min(-1) kg(-1) in Group C and 60.8 +/- 13.9 mL min(-1) kg(-1) in Group P (n.s.).

CONCLUSIONS

Oxygen delivery values greater than 7-8 mL min(-1) kg(-1) were required to maintain oxygen uptake during normothermic cardiopulmonary bypass with either pulsatile or non-pulsatile blood flow. Elevation of blood lactate levels during bypass helps to identify inadequate tissue oxygen delivery related to insufficient pump flow.

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.

Assessment and treatment of perfusion abnormalities in the emergency patient

Many patients presented to the emergency veterinarian are suffering from global or local tissue hypoperfusion. Global or systemic hypoperfusion can occur secondary to a reduction in the effective circulating intravascular volume (hypovolemic shock) or reduced ability of the heart to pump blood around the body secondary to reduced cardiac function (cardiogenic shock),obstruction to blood flow (obstructive shock), or maldistribution of the circulating intravascular volume (distributive shock). Initial assessment involving physical examination supplemented by measurement of hemodynamic and metabolic parameters allows the clinician to recognize and treat patients with severe global hypoperfusion. Use of techniques like sublingual capnometry and measurement of central venous oxygen saturation may aid recognition and evaluation of early hypoperfusion. Treatment decisions are made based on an assessment of the severity of the hypoperfusion and its probable underlying cause. Early effective treatment of hypoperfusion is likely to lead to a better outcome for the patient.

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.

Lingual, splanchnic, and systemic hemodynamic and carbon dioxide tension changes during endotoxic shock and resuscitation

Sublingual and intestinal mucosal blood flow and Pco2 were studied in a canine model of endotoxin-induced circulatory shock and resuscitation. Sublingual Pco2 (PsCO2) was measured by using a novel fluorescent optrode-based technique and compared with lingual measurements obtained by using a Stowe-Severinghaus electrode [lingual Pco2 (PlCO2)]. Endotoxin caused parallel changes in cardiac output, and in portal, intestinal mucosal, and sublingual blood flow (Q̇s). Different blood flow patterns were observed during resuscitation: intestinal mucosal blood flow returned to near baseline levels postfluid resuscitation and decreased by 21% after vasopressor resuscitation, whereas Q̇s rose to twice that of the preshock level and was maintained throughout the resuscitation period. Electrochemical and fluorescent Pco2 measurements showed similar changes throughout the experiments. The shock-induced increases in PsCO2 and PlCO2 were nearly reversed after fluid resuscitation, despite persistent systemic arterial hypotension. Vasopressor administration induced a rebound of PsCO2 and PlCO2 to shock levels, despite higher cardiac output and Q̇s, possibly due to blood flow redistribution and shunting. Changes in PlCO2 and PsCO2 paralleled gastric and intestinal Pco2 changes during shock but not during resuscitation. We found that the lingual, splanchnic, and systemic circulations follow a similar pattern of blood flow variations in response to endotoxin shock, although discrepancies were observed during resuscitation. Restoration of systemic, splanchnic, and lingual perfusion can be accompanied by persistent tissue hypercarbia, mainly lingual and intestinal, more so when a vasopressor agent is used to normalize systemic hemodynamic variables.

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.

Clinical review: hemorrhagic shock

This review addresses the pathophysiology and treatment of hemorrhagic shock – a condition produced by rapid and significant loss of intravascular volume, which may lead sequentially to hemodynamic instability, decreases in oxygen delivery, decreased tissue perfusion, cellular hypoxia, organ damage, and death. Hemorrhagic shock can be rapidly fatal. The primary goals are to stop the bleeding and to restore circulating blood volume. Resuscitation may well depend on the estimated severity of hemorrhage. It now appears that patients with moderate hypotension from bleeding may benefit by delaying massive fluid resuscitation until they reach a definitive care facility. On the other hand, the use of intravenous fluids, crystalloids or colloids, and blood products can be life saving in those patients who are in severe hemorrhagic shock. The optimal method of resuscitation has not been clearly established. A hemoglobin level of 7–8 g/dl appears to be an appropriate threshold for transfusion in critically ill patients with no evidence of tissue hypoxia. However, maintaining a higher hemoglobin level of 10 g/dl is a reasonable goal in actively bleeding patients, the elderly, or individuals who are at risk for myocardial infarction. Moreover, hemoglobin concentration should not be the only therapeutic guide in actively bleeding patients. Instead, therapy should be aimed at restoring intravascular volume and adequate hemodynamic parameters.

Cardiovascular monitoring tools: use and misuse

PURPOSE OF REVIEW

To review important areas of current and novel hemodynamic monitoring practice in the intensive care unit and to highlight potential areas of physiologic and clinical use or misuse, as well as areas of uncertainty and ongoing controversy.

RECENT FINDINGS

To truly determine when hemodynamic monitoring tools are misused would require randomized controlled evidence of a measurable improvement in relevant clinical (as opposed to physiologic) outcomes. Unfortunately, little evidence of this kind exists, and that which does exist is highly controversial in nature. Because of the limited evidence of an effect of hemodynamic monitoring on clinical outcomes, the use and misuse of hemodynamic monitoring tools is typically judged on physiologic grounds (Does it improve physiology? Does it predict physiology? Is it physiologically rational?). The relation between physiologic gain and final clinical outcome, however, is tenuous. Recent investigations confirm this lack of a clear link. They also suggest that new technology that is now emerging to less invasively measure cardiac output and intrathoracic fluid compartments is ready for formal evaluations of efficacy and effectiveness.

SUMMARY

The effectiveness of hemodynamic monitoring in the intensive care unit remains inadequately tested and unproven. New tools are now rapidly emerging to challenge established technologies. Formal assessment of their efficacy and effectiveness is needed to avoid a repeat of the pulmonary artery catheter experience.

Dopamine-1 receptor stimulation impairs intestinal oxygen utilization during critical hypoperfusion

Effects of a dopamine-1 (DA-1) receptor agonist on systemic and intestinal oxygen delivery (Do(2))-uptake relationships were studied in anesthetized dogs during sequential hemorrhage. Control (group 1) and experimental animals (group 2) were treated similarly except for the addition of fenoldopam (1.0 microg x kg(-1) x min(-1)) in group 2. Both groups had comparable systemic critical Do(2) (Do(2crit)), but animals in group 2 had a higher gut Do(2crit) (1.12 +/- 1.13 vs. 0.80 +/- 0.09 ml. kg(-1) x min(-1), P < 0.05). At the mucosal level, a clear biphasic delivery-uptake relationship was not observed in group 1; thus oxygen consumption by the mucosa may be supply dependent under physiological conditions. Group 2 demonstrated higher peak mucosal blood flow and lack of supply dependency at higher mucosal Do(2) levels. Fenoldopam resulted in a more conspicuous biphasic relationship at the mucosa and a rightward shift of overall splanchnic Do(2crit) despite increased splanchnic blood flow. These findings suggest that DA-1 receptor stimulation results in increased gut perfusion heterogeneity and maldistribution of perfusion, resulting in increased susceptibility to ischemia.

Whole body oxygen consumption and critical oxygen delivery in response to prolonged and severe carbon monoxide poisoning

BACKGROUND

Carbon monoxide (CO) poisoning remains the leading cause of death by poisoning in the world. One of the major proposed mechanisms for CO toxicity is the binding of CO to cytochrome oxidase and interference with cellular oxygen utilization but evidence for this is inconclusive.

AIM OF STUDY

This study examined the effects of prolonged CO exposure on the dynamics of whole body oxygen consumption (VO(2)) and oxygen delivery (DO(2)) in an attempt to observe if CO exposure results in a defect of oxygen utilization defect as determined by a reduction in VO(2) during the course of poisoning prior to reaching the point where VO(2) is directly dependent on DO(2). This critical level of DO(2) (DO(2)crit) produced by CO poisoning was compared to historical values produced by other insults, which decrease global body DO(2).

METHODS

Five small dogs were ventilated for 2 h with 0.25% CO and room air followed by 0.5% CO until death. Cardiac index (Q), DO(2), VO(2), oxygen extraction ratio (OER), and systemic lactate were measured every 15 min until death.

RESULTS

Carboxyhemoglobin (COHb) levels increased linearly over 2.5 h to values above 80% until death. VO(2) remained constant and not significantly different from baseline below a COHb of 80%. At COHb levels above 80%, VO(2) precipitously dropped. Similarly lactate levels were not significantly elevated from baseline until VO(2) dropped. DO(2) decreased by 78% (from 23+/-6 ml/kg/min to 5+/-4 ml/kg/min) over time despite an increase in Q by 58% until levels of COHb were above 80%. OER increased from 19+/-5% to 50+/-11% until death. The calculated DO(2)crit was 10.7+/-4 ml/min/kg, which is not significantly different from values ranging from 7 to 13 ml/min/kg reported in the literature due to other insults, which reduce DO(2).

CONCLUSION

In this canine model of prolonged CO exposure, no gradual reduction in VO(2) or increase in systemic lactate prior to reaching DO(2)crit was noted. In addition, CO exposure does not appear to change the DO(2)crit. The combination of these findings does not support the theory that CO produces a whole body intracellular defect in oxygen utilization.

Systemic and microvascular responses to hemorrhagic shock and resuscitation with Hb vesicles

A phospholipid vesicle encapsulating hemoglobin (Hb vesicle, HbV) has been developed to provide O(2)-carrying capacity to plasma expanders. Its ability to restore systemic and microcirculatory conditions after hemorrhagic shock was evaluated in the dorsal skinfold window preparation of conscious hamsters. The HbV was suspended in 8% human serum albumin (HSA) at Hb concentrations of 3.8 g/dl [HbV(3.8)/HSA] and 7.6 g/dl [HbV(7.6)/HSA]. Shock was induced by 50% blood withdrawal, and mean arterial pressure (MAP) at 40 mmHg was maintained for 1 h by the additional blood withdrawal. The hamsters receiving either HbV(3.8)/HSA or HbV(7.6)/HSA suspensions restored MAP to 93 +/- 14 and 93 +/- 10 mmHg, respectively, similar with those receiving the shed blood (98 +/- 13 mmHg), which were significantly higher by comparison with resuscitation with HSA alone (62 +/- 12 mmHg). Only the HSA group tended to maintain hyperventilation and negative base excess after the resuscitation. Subcutaneous microvascular blood flow reduced to approximately 10-20% of baseline during shock, and reinfusion of shed blood restored blood flow to approximately 60-80% of baseline, an effect primarily due to the sustained constriction of small arteries A(0) (diameter 143 +/- 29 microm). The HbV(3.8)/HSA group had significantly better microvascular blood flow recovery and nonsignificantly better tissue oxygenation than of the HSA group. The recovery of base excess and improved tissue oxygenation appears to be primarily due to the increased oxygen-carrying capacity of HbV fluid resuscitation.

Splanchnic vasoregulation during mesenteric ischemia and reperfusion in pigs

We evaluated the hepatic arterial buffer response (HABR) to portal vein (PV) occlusion during 2 h of reduced superior mesenteric arterial blood flow (median 2 mL min(-1) kg(-1), range of 1-3 mL min(-1) kg(-1)) and 1 h of reperfusion in seven pigs and in seven controls. In animals with reduced mesenteric blood flow, celiac trunk blood flow (Qtr) increased during mesenteric hypoperfusion from 4 +/- 1 mL min(-1) kg(-1) (mean +/- SD) to 16 +/- 3 mL min(-1) kg(-1) (P = 0.028), and hepatic arterial blood flow (Qha) increased from 2 +/- 1 to 10 +/- 4 mL min(-1) kg(-1) (P= 0.018). The extra-hepatic fraction of Qtr (Qtr-Qha) also increased (P = 0.028). In controls, Qtr and Qha also increased, but to lower levels. At baseline, acute PV occlusion increased Qha by 5.0 +/- 2.8 mL min(-1) kg(-1) (P < 0.001), whereas Qtr-Qha decreased by 1.6 +/- 1.6 mL min(-1) kg(-1) (P = 0.007). After 120 min of reduced mesenteric blood flow, the HABR was exhausted (change in Qha to PV occlusion of 0.7 +/- 1.6 mL min(-1) kg(-1) [P= 0.27]). The efficacy of the HABR was also reduced in controls animals. Despite increased cardiac output, all flows from the celiac trunk decreased during reperfusion (P = 0.028) and the HABR partially recovered. We conclude that reduced mesenteric perfusion impairs the HABR, which recovers only partially after reperfusion. The distribution of the increased celiac trunk flow secondary to PV occlusion ranges from increased HABR and decreased non-hepatic blood flow (a steal) to decreased hepatic arterial blood flow and increased non-hepatic blood flow (an inverse steal).