Lactate levels as predictors of the relationship between oxygen delivery and consumption in ARDS

The Physiology of Oxygen Transport by the Cardiovascular System: Evolution of Knowledge

The heart, vascular system, and red blood cells play fundamental roles in O₂ transport. The fascinating research history that led to the current understanding of the physiology of O₂ transport began in ancient Egypt in 3000 BC, when it was postulated that the heart was a pump serving a system of distributing vessels. Over 4 millennia elapsed before William Harvey (1578-1657) made the revolutionary discovery of blood circulation, but it was not until the 20th century that a lucid and integrative picture of O₂ transport finally emerged. This review describes major research achievements contributing to this evolution of knowledge. These achievements include the discovery of the systemic and pulmonary circulations, hemoglobin within red blood cells and its ability to bind O₂, and diffusion of O₂ from the capillary as the final step in its delivery to tissue. The authors also describe the classic studies that provided the initial description of the basic regulatory mechanisms governing heart function (Frank-Starling law) and the flow of blood through blood vessels (Poiseuille's law). The importance of technical advances, such as the pulmonary artery catheter, the blood gas analyzer and oximeter, and the radioactive microsphere technique to measure the regional blood flow in facilitating O₂ transport-related research, is recognized. The authors describe how religious and cultural constraints, as well as superstition-based medical traditions, at times impeded experimentation and the acquisition of knowledge related to O₂ transport.

Effects of blood transfusion on oxygen extraction ratio and central venous saturation in children after cardiac surgery

BACKGROUND

Red blood cell transfusion is common in critically ill children after cardiac surgery. Since the threshold for hemoglobin (Hb) transfusion need is not well defined, the threshold Hb level at which dependent critical oxygen uptake-to-delivery (VO2-DO2) status compensation is uncertain.

OBJECTIVES

To assess the effects of blood transfusion on the oxygen extraction ratio (O2ER) and central venous oxygen saturation (ScvO2) to identify a critical O2ER value that could help us determine the critical need for blood transfusion.

DESIGN

Prospective, observational cohort study.

SETTING

Cardiac Surgical Intensive Care Unit at Prince Sultan Cardiac Center in Qassim, Saudi Arabia.

PATIENTS AND METHODS

Between January 2013 and December 2015, we included all children with cardiac disease who underwent surgery and needed a blood transfusion. Demographic and laboratory data with physiological parameters before and 1 and 6 hours after transfusion were recorded and O2ER before and 6 hours after transfusion was computed. Cases were divided into two groups based on O2ER: Patients with increased O2ER (O2ER > 40%) and normal patients without increased O2ER (O2ER < =40%) before transfusion.

MAIN OUTCOME MEASURE(S)

Changes in O2ER and ScvO2 following blood transfusion.

RESULTS

Of 103 patients who had blood transfusion, 75 cases had normal O2ER before transfusion while 28 cases had increased O2ER before transfusion. Following blood transfusion, O2ER and ScvO2 improved in the group that had increased O2ER before transfusion, but not in the group that had normal O2ER before transfusion.

CONCLUSIONS

The clinical and hemodynamic indicators O2ER and ScvO2 may be considered as markers that can indicate a need for blood transfusion.

LIMITATIONS

The limitation of this study is the small number of patients that had increased O2ER before transfusion. There were few available variables to assess oxygen consumption.

A Review of the Role of Blood Lactate Measurements in the ICU

The production of hyperlactatemia in the critically ill patient has previously been thought to be primarily related to anaerobic conditions. Newer data suggests that lactate accumulates under aerobic conditions in clinical settings that were previously thought to solely represent anaerobic processes. This review summarizes the present understanding of lactate metabolism and reviews the data supporting use of lactate determinations in the critical care setting. The use of lactate as a marker of inadequate tissue perfusion as well as its role as a prognostic indicator are discussed. The utility of managing lactic acidosis with various buffers is also reviewed.

Early and innovative interventions for severe sepsis and septic shock: taking advantage of a window of opportunity

The pathogenic, diagnostic and therapeutic landscape of sepsis is no longer confined to the intensive care unit: many patients from other portals of entry to care, both outside and within the hospital, progress to severe disease. Approaches that have led to improved outcomes with other diseases (e.g., acute myocardial infarction, stroke and trauma) can now be similarly applied to sepsis. Improved understanding of the pathogenesis of severe sepsis and septic shock has led to the development of new therapies that place importance on early identification and aggressive management. This review emphasizes approaches to the early recognition, diagnosis and therapeutic management of sepsis, giving the clinician the most contemporary and practical approaches with which to treat these patients.

Matching total body oxygen consumption and delivery: a crucial objective?

The strength of the rationale for incorporating total body oxygen consumption (VO(2)) and delivery (DO(2)) into our decision making strategies contrasts with the absence of demonstrated benefits of bedside calculations in clinical practice. This situation mandates a careful reappraisal of the theoretical limitations of bedside calculations of DO(2) and VO(2), including a re-evaluation of the clinical situations in which these calculations are valid. Three levels of complexity can be distinguished when analysing a patient's hemodynamic status: 1) simple cases where investigations can be limited to clinical monitoring, including lactate changes over time; 2) intermediate situations requiring invasive investigations in which continuous monitoring of VO(2)-related variables such as cardiac output and mixed venous oxygen saturation often provide enough information to guide clinical decision; and 3) complex situations where assessment of VO(2) and VO(2)/DO(2) analysis might be recommended. Although studies that support such recommendations are limited they are based on a widely accepted physiological model. VO(2) and DO(2) analysis is also limited by theoretical and technical difficulties. In this article, we discuss the validity of these limitations in the bedside assessment of VO(2) and DO(2), and review data supporting the use of VO(2)/DO(2) analysis in the clinical evaluation of complex cases.

A simple but useful method of screening for mesenteric ischemia secondary to acute aortic dissection

BACKGROUND

In spite of recent improvements in treatment for acute aortic dissection, mesenteric ischemia secondary to aortic dissection is still challenging. We propose a simple screening method to detect mesenteric ischemia secondary to acute aortic dissection.

METHODS

From 1991 to 2002, 245 patients with acute aortic dissection were admitted to our hospital. Nine (3.7%) of those were complicated with mesenteric ischemia. The clinical records of those 9 patients were retrospectively analyzed. The ratios of the diameter of the superior mesenteric vein (SMV) to that of the superior mesenteric artery (SMA) were calculated in patients with mesenteric ischemia (group M) and in patients without mesenteric ischemia (group C). Blood test data, including results of arterial blood gas analysis, in the 2 groups were also compared.

RESULTS

The SMV/SMA ratios in groups M and C were 1.16 +/- 0.33 and 1.78 +/- 0.29, respectively (P=.003). A cutoff value of the SMV/SMA ratio was 1.5 (sensitivity, 88.9%; specificity, 88.9%) with an odds ratio of 64.0. Although there were differences between the 2 groups in glutamate oxaloacetate transaminase, lactate dehydrogenase, creatine phosphate kinase, pH, and lactate values, the measurement of lactate was especially useful (P=.002).

CONCLUSIONS

The combination of the SMV/SMA ratio and lactate concentration is a useful screening method to detect mesenteric ischemia secondary to acute aortic dissection.

Life-threatening severe malarial anaemia

Despite our improved understanding of the pathophysiology of severe malaria, major changes in clinical management have not been forthcoming. However, in the case of life-threatening severe malarial anaemia, preliminary evidence suggests that changes in current clinical practice rather than the introduction of novel interventions may improve child survival. This review argues that further research into the clinical physiology of this syndrome is required and could provide compelling evidence for changes in practice particularly with regard to blood transfusion. We focus on the syndrome of severe, symptomatic malarial anaemia associated with a metabolic acidosis which has a high fatality rate. However, it should be remembered that a far greater number of children without signs of life-threatening disease nonetheless experience significant morbidity from severe anaemia. Many of these less-severely ill children may also require blood transfusion. However, the mode and rationale for transfusion in this less-severely ill group is specifically not addressed. Indeed, the arguments presented should not be extrapolated to suggest a uniform approach to transfusion is warranted, the role of blood in the less-critically ill child with severe malaria anaemia being a further area that requires urgent research.

Peripheral oxygenation in hypotensive preterm babies

Monitoring oxygenation in peripheral tissues of preterm babies may be useful in understanding the redistribution of blood flow during hypotension. Hemoglobin flow and venous saturation were measured in the forearm using near infrared spectroscopy with venous occlusion and were used to calculate fractional oxygen extraction, oxygen delivery, and oxygen consumption. Thirty ventilated preterm babies (median birth weight 976 g) were studied; 15 were hypotensive and 15 normotensive. Treatment for hypotension was dopamine alone (median dose 5 microg/kg/min) in eight cases, 4.5% human albumin solution (20 mL/kg) with dopamine in five cases, and only a blood transfusion (20 mL packed cells/kg) in two cases. There was a weak correlation between hemoglobin flow and mean arterial blood pressure (r = 0.40, p = 0.03). In hypotensive compared with normotensive babies, there was a significantly lower median hemoglobin flow (10.2 versus 20.2 micromol/100 mL/min, p = 0.0006), forearm oxygen delivery (37.8 versus 75.2 micromol/100 mL/min, p = 0.0008), and oxygen consumption (11.0 versus 23.9 micromol/100 mL/min, p = 0.006), but the fractional oxygen extraction (0.327 versus 0.306, p = 0.48) and the blood lactate concentration (1.22 versus 1.20 mmol/L, p = 0.44) were similar. Following treatment of hypotension, oxygen delivery (p = 0.02) and oxygen consumption (p = 0.04) increased to 64.2 and 21.7 micromol/100 mL/min, respectively, but fractional oxygen extraction (p = 0.81) and blood lactate concentration (p = 0.94) after treatment were unchanged. VO2 was variable in the forearm of human infants. It reduced when DO2 was low, and there was no evidence of tissue injury or switch to anaerobic metabolism. Measurements of peripheral tissue oxygenation seem to be of some value in understanding the pathophysiologic changes that occur with hypotension.

Assessment of three methods to reduce the influence of mathematical coupling on oxygen consumption and delivery relationships

STUDY OBJECTIVES

To determine if oxygen consumption/oxygen delivery (VO2/DO2) relationships derived using calorimetry (which are not influenced by shared measurement error) agreed with those obtained using the pulmonary artery (PA) catheter alone. To evaluate three strategies to reduce the influence of shared measurement error to determine if agreement between the two methods could be improved.

METHODS

Twenty-seven patients were studied following coronary artery bypass surgery. Calorimetric VO2, six thermodilution cardiac outputs (COs), and arterial and mixed venous oxygen content measurements were made at baseline and were repeated 30 min following dobutamine administrations of 3 microg/kg/min and 5 microg/kg/min.

RESULTS

Dobutamine produced a dose-dependent increase in DO2, from 378+/-65 mL/min/m2 to 446+/-78 mL/min/m2 (p<0.01) and in both PA catheter and calorimetric-derived VO2, from 104+/-18 mL/min/Mi2 to 114+/-22 mL/min/m2 (p<0.05) and from 117+/-15 mL/min/m2 to 126+/-19 mL/min/m2 (p<0.01), respectively. Agreement was poor (bias=12%, SD=21%) between the calorimetric and PA catheter methods of determining VO2/DO2 slope. When three CO measurements were used to calculate VO2, and three separate CO measurements were used to calculate DO2, the level of agreement between the two methods improved (bias=2%, SD=15%). Increasing the number of COs resulted in a similar improvement in the level of agreement between the two methods. Weighting the slope to the observed change in DO2 was the best method to improve the level of agreement (bias=2%, SD=6% for three COs).

CONCLUSIONS

To reduce the influence of shared measurement error, the best strategy to improve the measurement of VO2/DO2 slope is to maximize the change in DO2 (optimally over 100 mL/min/m2).

Oxygen supply dependency can characterize septic shock

OBJECTIVE

To demonstrate that oxygen consumption (VO2) can be dependent on oxygen delivery (DO2) during hemodynamic instability and independent of DO2 following stabilization.

DESIGN

We retrospectively reviewed hemodynamic and blood gas data collected from ten patients in whom DO2 was acutely altered during an episode of septic shock (phase A) and after recovery from this episode (phase B).

SETTING

General intensive care unit of a university hospital.

PATIENTS

10 critically ill adult patients (aged 55 +/- 19 years).

INTERVENTIONS

DO2 was altered by fluid challenge, administration of vasoactive agents, or application of positive end-expiratory pressure.

RESULTS

In phase A, changes in VO2 (121 +/- 32 vs 165 +/- 36 ml/min.m2; p < 0.001) paralleled changes in DO2 (415 +/- 153 vs 607 +/- 217 ml/min.m2; p < 0.001), but oxygen extraction (O2ER) remained stable (31.9 +/- 11.2 vs. 30.2 +/- 8.9%; NS). In phase B, changes in DO2 (412 +/- 118 vs 526 +/- 152 ml/min.m2; p < 0.001) were associated with opposite changes in O2ER (36.1 +/- 4.2 vs 28.9 +/- 4.9%; p < 0.001), and VO2 was unchanged (147 +/- 35 vs 149 +/- 33 ml/min.m2; NS). The mean VO2/DO2 slope was greater in phase A than in phase B (0.26 +/- 0.09 vs. 0.08 +/- 0.08; p < 0.004). Blood lactate levels were higher in phase A than in phase B (3.3 +/- 1.8 vs 1.6 +/- 0.6 mEq/l; p < 0.05).

CONCLUSIONS

Oxygen supply independency and dependency can be found at different times in the same critically ill patient. Our findings are consistent with the concept that VO2/DO2 dependency is a marker of septic shock. Interventions to increase DO2 are probably justified when this phenomenon is present.

Effect of intravenous lactated Ringer's solution infusion on the circulating lactate concentration: Part 3. Results of a prospective, randomized, double-blind, placebo-controlled trial

OBJECTIVES

We previously discovered that small amounts of lactated Ringer's solution, which are inadequately cleared from an intravenous catheter, falsely increase the circulating lactate concentration in blood samples collected from that catheter. That finding prompted us to test the hypothesis that intravenous lactated Ringer's solution, infused at a rate used in resuscitation, would increase the circulating lactate concentration.

DESIGN

A prospective, randomized, double-blinded, placebo-controlled study.

SETTING

A critical care research laboratory.

SUBJECTS

Twenty-four normal, healthy, adult volunteer subjects.

INTERVENTIONS

Two intravenous catheters were placed. One was used for the infusion of the test solution and the other catheter was used for blood sampling. Blood samples were serially collected for the determination of blood lactate concentrations.

MEASUREMENTS AND MAIN RESULTS

Twenty-four healthy adult volunteers were randomized to receive a 1-hr infusion of either lactated Ringer's solution (n = 6), 0.9% saline (n = 6), 5% dextrose in lactated Ringer's solution (D5RL) (n = 6), or 5% dextrose in water (D5W) (n = 6). Each subject received nothing by mouth after midnight. At 0800 hrs, catheters were inserted and each subject received 1 L of the assigned solution over 1 hr. Throughout the study, the subjects were at rest. Three-milliliter samples of venous blood were collected before, during (at 15, 30, 45, and 60 mins), and after (at 90, 120, and 240 mins) the infusion. Blood samples were placed on ice immediately after collection and analyzed within 5 mins of collection. Lactate concentrations were determined using an ion-selective, amperometric electrode, which we have previously validated. Lactate concentrations were compared between subjects receiving lactated Ringer's solution vs. subjects receiving normal saline. A similar comparison was made between subjects receiving D5RL vs. D5W at similar time points during the study. There were no clinically or statistically significant differences in lactate values at the time points studied in those subjects receiving lactated Ringer's solution vs. those persons receiving normal saline (p > .05; n = 12; Student-Newman-Keuls' multiple comparison test) or those subjects receiving D5W vs. those subjects infused with D5RL (p > .05; n = 12; Student-Newman-Keuls' multiple comparison test). In no case did the circulating lactate values exceed 2 mmol/L (the upper limit of normal).

CONCLUSIONS

The short-term infusion of lactated Ringer's solution in normal adults (hemodynamically stable) does not falsely increase circulating lactate concentrations when 1 L is given over 1 hr. Therefore, clinicians should not disregard increased lactate concentrations in patients receiving a rapid infusion of lactated Ringer's solution.

Cytopathic hypoxia in sepsis

Diminished availability of oxygen at the cellular level might account for organ dysfunction in sepsis. Although the classical forms of tissue hypoxia due to hypoxemia, anemia, or inadequate perfusion all might be important under some conditions, it seems increasingly likely that a fourth mechanism, namely cytopathic hypoxia, might play a role as well. The term cytopathic hypoxia is used to denote diminished production of adenosine triphosphate (ATP) despite normal (or even supranormal) PO2 values in the vicinity of mitochondria within cells. At least in theory, cytopathic hypoxia could be a consequence of several different (but mutually compatible) pathogenic mechanisms, including diminished delivery of a key substrate (e.g., pyruvate) into the mitochondrial tricarboxylic acid (TCA) cycle, inhibition of key mitochondrial enzymes involved in either the TCA cycle or the electron transport chain, activation of the enzyme, poly-(ADP)-ribosylpolymerase (PARP), or collapse of the protonic gradient across the inner mitochondrial membrane leading to uncoupling of oxidation (of NADH and FADH) from phosphorylation of ADP to form ATP. Tantalizing, but limited, data support the view that cytopathic hypoxia occurs in both animals and patients with sepsis or endotoxemia.

Oxygen uptake/oxygen supply dependency: fact or fiction?

More than a decade ago, observations of co-variance between VO2 and DO2 led to the identification of a condition known as pathological O2 supply dependency. This condition was subsequently observed in critically ill patients with sepsis and acute circulatory failure. More recently, other authors have challenged the existence of this condition, often citing methodologic problems or mathematical coupling to account for spurious observations in the earlier studies. Here, we review the evidence for and against pathological O2 supply dependency. We find that many of the arguments have some validity but only in specific circumstances. We conclude, therefore, that pathological O2 supply dependency is a hallmark of acute circulatory failure and that an effective therapeutic approach should be based on an evaluation of organ system function in each individual case. Parameters such as blood lactate, pHi and veno-arterial PCO2 may be useful in this respect.

Splanchnic metabolism associated with liver metastasis

Metastatic liver disease can modify the metabolic response to critical illness. Systemic lactic acidosis may arise from an increased production due to inadequate peripheral tissue oxygen transport, altered metabolic function such as depressed pyruvate oxidation or insufficient hepatic clearing capacity due to tumor replacement of functional liver mass. Hepatic venous catheterization in a patient with extensive metastatic melanoma to the liver and adult respiratory distress syndrome indicated a marked disparity between whole body and liver oxygenation which may arise due to a markedly stepped up splanchnic oxygen utilization unmatched by a proportionate rise in regional oxygen delivery. Since some neoplasms may exhibit increased metabolic activity, it is suspected that these metastatic lesions may have contributed to the observed regional hypermetabolism thereby worsening hepatic hypoxia and exacerbating lactic acidosis. This case also illustrates the difficulties in interpreting global indicators of metabolic function and oxygenation in critically ill patients.

Prostacyclin in septic shock

OBJECTIVE

Investigation of the hypothesis that the infusion of 10 ng/kg/min prostacyclin (epoprostenol) (PGI2) improves O2 uptake in patients with hyperdynamic septic shock.

DESIGN

Prospective, single cohort design.

SETTING

ICU, university hospital.

PATIENTS

Fifteen postoperative patients with septic shock.

INTERVENTIONS

Infusion of 10 ng/kg/min of PGI2 for 60 min.

MEASUREMENTS

Complete hemodynamic profile with O2 transport-related variables (simultaneous measurements of VO2 from the respiratory gases and by cardiovascular Fick) and blood lactate levels before start of the PGI2-infusion and 60 min thereafter.

MAIN RESULTS

Oxygen delivery increased significantly (14 percent) from its already high value, 750 +/- 238 to 852 +/- 214 ml/min/m2. The O2 extraction ratio remained unchanged. When VO2 was measured from the respiratory gases, it was unchanged. When VO2 was measured by cardiovascular Fick, it increased slightly (p < 0.05).

CONCLUSIONS

We conclude that in this O2 challenge test with PGI2 in patients with septic shock, an increase in O2 delivery was not matched by an increase in VO2. We believe that the adequate conventional support of these patients may have prevented the PGI2 from revealing a "covert" O2 debt. The PGI2 test did not predict mortality by O2 supply dependency. The small increase in VO2 as calculated indirectly suggests a degree of mathematical coupling of O2 delivery and uptake.

Systemic oxygen extraction can be improved during repeated episodes of cardiac tamponade

We used a tamponade model to study the relationship between oxygen uptake (VO2) and oxygen delivery (DO2) during successive, reversible decreases in blood flow. In 7 pentobarbital-anesthetized and mechanically ventilated dogs, a catheter was introduced via a left thoracotomy into the pericardium to inject and to withdraw saline. Each experiment consisted of three steps. First, cardiac output was reduced by successive pericardial fluid injections until 4 to 6 data points were obtained in the dependent region of the VO2/DO2 plot (step 1). Second, cardiac output was restored by progressive withdrawal of pericardial fluid (step 2). Third, cardiac output was lowered again by reinjection of fluid into the pericardium until death (step 3). Expired gases were collected for determination of VO2. In each animal, critical DO2 (DO2crit), below which VO2 became DO2 dependent, was determined from a plot of VO2 versus DO2. When releasing tamponade, VO2 was restored to baseline. For the 3 steps, DO2crit were 10.5 +/- 2.2 mL/kg/min in step 1, 9.8 +/- 1.8 mL/kg/min in step 2, and 8.3 +/- 1.9 mL/kg/min in step 3 (P < .01 v step 1; P < .05 v step 2, respectively). There was no significant difference in VO2 at DO2crit for the three steps. Hence, critical oxygen extraction ratio (ERO2crit) increased from 60% +/- 12% in step 1 to 64% +/- 11% in step 2 (not significant) and to 73% +/- 12% in step 3 (P < .01). The VO2/DO2 dependency slope was also steeper in step 3 than in step 1 (0.77 +/- 0.31 v 0.54 +/- 0.20, P < .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Response of critically ill patients to treatment aimed at achieving supranormal oxygen delivery and consumption. Relationship to outcome

STUDY OBJECTIVE

To evaluate the response to therapy aimed at achieving supranormal cardiac and oxygen transport variables (cardiac index [CI] > 4.5 L/min/m2, oxygen delivery [DO2] > 600 ml/min/m2, and oxygen consumption [VO2] > 170 ml/min/m2) in a heterogenous group of critically ill patients and to assess its relationship to outcome.

DESIGN

Patients were divided retrospectively into two groups. Group 1 (n = 15) achieved supranormal values for CI, DO2 and VO2 simultaneously during the first 24 h. Group 2 (n = 17) failed to achieve these goals simultaneously at any time point.

SETTING

General intensive care units in a teaching and a district general hospital.

PATIENTS

Thirty-two patients at risk of developing multiple organ failure were studied prospectively.

INTERVENTIONS

Patients received volume expansion and then, if necessary, dobutamine (5 to 200 micrograms/kg/min) to increase CI and DO2 until all three goals were achieved simultaneously.

RESULTS

In group 2, target VO2 could never be reached despite the fact that 11 (65 percent) patients achieved target CI and DO2 simultaneously. In this group, lactate levels did not fall and 16 patients died. In contrast, in group 1, attainment of all goals was associated with a significant reduction (p < 0.05) in blood lactate levels, and all but one of these patients survived. The persistently raised lactate levels in group 2 were associated with significantly higher venous oxygen saturation (SvO2) and lower oxygen extraction ratio (OER); in these patients, SvO2 rose and OER fell in response to increases in DO2.

CONCLUSION

These results suggest that failure to increase VO2 was related predominantly to an inability of the tissues to extract or utilize oxygen rather than a failure to increase DO2. These findings support the hypothesis that in order to survive a critical illness, patients must achieve a high level of VO2. An inability to do so is reflected in persistently elevated blood lactate levels and an extremely poor prognosis.

The relationship of blood lactate concentrations, oxygen delivery and oxygen consumption in septic shock and the adult respiratory distress syndrome

Tissue hypoxia is thought to be pivotal to the development of multiple organ failure, but cannot be measured directly in clinical practice. We assessed the relationship between initial arterial blood lactate concentrations and the presence of the phenomenon of delivery-dependent oxygen consumption, both of which may indicate tissue hypoxia. Twenty-three critically ill patients with septic shock and adult respiratory distress syndrome were studied prospectively and allocated to one of two groups according to blood lactate concentrations. In group 1, blood lactate concentration was less than the level widely accepted as significant (2 mmol.l-1); in group 2, the concentration exceeded 2 mmol.l-1. In both groups, resuscitation with colloid, blood and vasoactive drugs resulted in significant increases in oxygen delivery; in group 1 (n = 13), mean (SEM) oxygen delivery increased from 484 (36) to 730 (44) ml.min-1.m-2 (p < 0.005) and in group 2 (n = 10) from 550 (54) to 780 (54) ml.min-1.m-2 (p < 0.05). In neither group was there a significant change in oxygen consumption. However, there were individuals in both groups who exhibited pathological delivery dependence. This suggests that the absence of hyperlactataemia does not preclude delivery dependence of oxygen consumption with the attendant potential for tissue hypoxia.

Modification of oxygen extraction ratio by change in oxygen transport in septic shock

To help clarify the oxygen uptake/transport (VO2/TO2) relationship and because the oxygen extraction ratio (OER) and TO2 share no common variable such as cardiac index, we examined the changes in OER when TO2 was decreased in 12 patients with sepsis in whom a PEEP trial was performed. From zero end-expiratory pressure (ZEEP) to PEEP (12 +/- 3 cm H2O), a significant increase in OER from 30 +/- 10 percent to 38 +/- 12 percent (p less than 0.005) was observed, and individual percentage changes in OER were well correlated with individual percentage changes in TO2. The VO2 measured (VO2m) by respiratory gas analysis was unchanged, while VO2 calculated by the Fick equation (VO2f) decreased, suggesting a mathematical coupling between VO2f and TO2. Patients with hyperlactacidemia (n = 5) exhibited the same relationships between OER, VO2m, and TO2 as those without hyperlactacidemia. Our results suggest an adaptive response in the OER when TO2 is decreased in patients with established septic shock.

Oxygen supply dependency in the critically ill--a continuing conundrum

There was little dispute that endotoxin treatment of experimental animals could recreate the O2 extraction defect that had been observed in critically ill patients. The remaining question was whether or not this necessarily signified pervasive tissue hypoxia. Some limitation to O2 diffusion in the tissues had been postulated because of known effects of endotoxin that ultimately result in damage to endothelium. We were unable to alter the critical DO2 or 0(2)ER in endotoxic dogs by manipulating the arterial PO2. This tended to rule against there being a diffusion limitation created by the endotoxin as a result of endothelial disruption or microvascular dysfunction. The results of the DCA and dopexamine experiments served to remind us that arterial lactate measurements may or may not indicate widespread tissue hypoxia. Sepsis, as emulated by endotoxin infusions, is also a metabolic disease that can cause inactivation of PDH and thus cause lactacidosis without tissue hypoxia. Regional measurements of lactate flux indicated that gut was hypoxic in spite of DO2 above critical because of maldistribution of blood flow between muscularis and mucosa. The questions persist of how much tissue hypoxia is caused by sepsis or endotoxin when DO2 is supported at supposedly adequate levels and whether there are marked regional differences. Such questions still await answers. Newer technological advances that permit assessment of tissue oxygenation by noninvasive methods, such as near infrared spectrophotometry or nuclear magnetic resonance measurement of tissue energy potential, may soon be feasible in critically ill patients. This kind of information will be of vast importance in designing the most effective therapeutic regimen.

Multiple Organ Failure Syndrome—Part I: Epidemiology, Prognosis, and Pathophysiology

The multiple organ failure syndrome (MOFS) is the leading cause of death in intensive care units. Although sepsis is an important cause of MOFS, it is clear that MOFS can occur in the absence of infection. The pathophysiology of MOFS is complex and multifactorial and includes derangements in oxygen delivery and consumption, the release of inflammatory and vasoactive mediators capable of inflicting tissue damage, and alterations in the barrier function of the intestinal mucosa. Although advances have been made in our understanding of MOFS, treatment remains nonspecific and largely supportive. Early and aggressive restoration of tissue perfusion, adequate treatment of infection, timely nutritional support, and support of individual failed organs remain the mainstay of therapy. Therapeutic agents directed against the various mediators associated with the pathophysiology of MOFS may prove useful in the future.