Oxygen uptake/oxygen supply dependency: fact or fiction?

The ratios of central venous to arterial carbon dioxide content and tension to arteriovenous oxygen content are not associated with overall anaerobic metabolism in postoperative cardiac surgery patients

Background

The aim of the present study was to evaluate the ability of the ratios of central venous to arterial carbon dioxide content and tension to arteriovenous oxygen content to predict an increase in oxygen consumption (VO₂) upon fluid challenge (FC).

Methods and results

110 patients admitted to cardiothoracic ICU and in whom the physician had decided to perform an FC (with 500 ml of Ringer's lactate solution) were included. The arterial pressure, cardiac index (Ci), and arterial and venous blood gas levels were measured before and after FC. VO₂ and CO₂-O₂ derived variables were calculated. VO₂ responders were defined as patients showing more than a 15% increase in VO₂. Of the 92 FC responders, 43 (46%) were VO₂ responders. At baseline, pCO₂ gap, C(a-v)O₂ were lower in VO₂ responders than in VO₂ non-responders, and central venous oxygen saturation (ScvO₂) was higher in VO₂ responders. FC was associated with an increase in MAP, SV, and CI in both groups. With regard to ScvO_2, FC was associated with an increase in VO₂ non-responders and a decrease in VO₂ responders. FC was associated with a decrease in pvCO₂ and pCO₂ gap in VO₂ non-responders only. The pCO₂ gap/C(a-v)O₂ ratio and C(a-v)CO₂ content /C(a-v)O₂ content ratio did not change with FC. The CO₂ gap content/C(a-v)O₂ content ratio and the C(a-v)CO₂ content /C(a-v)O₂ content ratio did not predict fluid-induced VO₂ changes (area under the curve (AUC) [95% confidence interval (CI)] = 0.52 [0.39‒0.64] and 0.53 [0.4–0.65], respectively; p = 0.757 and 0.71, respectively). ScvO₂ predicted an increase of more than 15% in the VO₂ (AUC [95%CI] = 0.67 [0.55‒0.78]; p<0.0001).

Conclusions

Our results showed that the ratios of central venous to arterial carbon dioxide content and tension to arteriovenous oxygen content were not predictive of VO₂ changes following fluid challenge in postoperative cardiac surgery patients.

My paper 20 years later: effects of dobutamine on the VO₂/DO₂ relationship

INTRODUCTION

Oxygen uptake (VO₂) is independent of oxygen delivery (DO₂) over a wide range of values, because Ov extraction can readily adapt to changes in DO₂. However, VO₂ can become DO₂-dependent in acute circulatory failure.

METHODS

Various methods of evaluating the presence of VO₂/DO₂ dependency were conducted, but criticized because of potential problems with mathematical coupling of data. Some 20 years ago, we proposed using a dobutamine test and showed similar relationships using direct and indirect measurements. Since these studies on the systemic VO₂/DO₂ relationship, investigators have also explored regional alterations.

CONCLUSION

The relationship between VO₂ and DO₂ remains an important concept. Abnormal global VO₂/DO₂ dependency does not exist in stable, critically ill patients, but can exist in circulatory shock of all etiologies. It can also occur regionally in septic patients, and microcirculatory alterations may contribute. Patient management should be titrated individually with careful assessment to identify those who will benefit from increased DO₂.

Uncoupling mitochondrial activity maintains body [Formula: see text] during hemorrhage-induced O2 deficit in the anesthetized rat

During a hemorrhagic shock (HS), O2 uptake ( [Formula: see text] ) decreases as soon as the rate of O2 delivery ( [Formula: see text] ) drops below a "critical level", a response accounted for by the reduction in mitochondrial O2supply. In urethane-anesthetized rats, [Formula: see text] was decreased within 20min from 21.5 to 2.8mlmin(-1) by slowly withdrawing 18mlkg(-1) of blood. This led to a reduction in [Formula: see text] from 6.1 to 2.4mlmin(-1) (n=5, p<0.01). Decoupling mitochondrial oxidative activity by injecting 2,4-DNP (6mgkg(-1), iv) before HS elevated [Formula: see text] to 11.9±1.2mlmin(-1) (n=6, p<0.01), which remained above control HS values throughout most of the hemorrhage. This was associated with higher levels of O2 extraction, cardiac output and ventilation than in control HS. [Formula: see text] relationship was shifted upward and to the left following DNP. In conclusion, cellular and systemic mechanisms, decreasing O2demand, account for a large part of HS induced [Formula: see text] decline resulting in an additional reduction in [Formula: see text] .

Clinical review: use of venous oxygen saturations as a goal - a yet unfinished puzzle

Shock is defined as global tissue hypoxia secondary to an imbalance between systemic oxygen delivery and oxygen demand. Venous oxygen saturations represent this relationship between oxygen delivery and oxygen demand and can therefore be used as an additional parameter to detect an impaired cardiorespiratory reserve. Before appropriate use of venous oxygen saturations, however, one should be aware of the physiology. Although venous oxygen saturation has been the subject of research for many years, increasing interest arose especially in the past decade for its use as a therapeutic goal in critically ill patients and during the perioperative period. Also, there has been debate on differences between mixed and central venous oxygen saturation and their interchangeability. Both mixed and central venous oxygen saturation are clinically useful but both variables should be used with insightful knowledge and caution. In general, low values warn the clinician about cardiocirculatory or metabolic impairment and should urge further diagnostics and appropriate action, whereas normal or high values do not rule out persistent tissue hypoxia. The use of venous oxygen saturations seems especially useful in the early phase of disease or injury. Whether venous oxygen saturations should be measured continuously remains unclear. Especially, continuous measurement of central venous oxygen saturation as part of the treatment protocol has been shown a valuable strategy in the emergency department and in cardiac surgery. In clinical practice, venous oxygen saturations should always be used in combination with vital signs and other relevant endpoints.

Management of sepsis: early resuscitation

Key links in the chain of survival for the management of severe sepsis and septic shock are early identification and comprehensive resuscitation of high-risk patients. Multiple studies have shown that the first 6 hours of early sepsis management are especially important from a diagnostic, pathogenic, and therapeutic perspective, and that steps taken during this period can have a significant impact on outcome. The recognition of this critical time period and the robust outcome benefit realized in previous studies provides the rationale for adopting early resuscitation as a distinct intervention. Sepsis joins trauma, stroke, and acute myocardial infarction in having "golden hours," representing a critical opportunity early on in the course of disease for actions that offer the most benefit.

Early goal-directed therapy in severe sepsis and septic shock revisited: concepts, controversies, and contemporary findings

Studies of acute myocardial infarction, trauma, and stroke have been translated into improved outcomes by earlier diagnosis and application of therapy at the most proximal stage of hospital presentation. Most therapies for these diseases are instituted prior to admission to an ICU; this approach to the sepsis patient has been lacking. In response, a trial comparing early goal-directed therapy (EGDT) vs standard care was performed using specific criteria for the early identification of high-risk sepsis patients, verified definitions, and a consensus-derived protocol to reverse the hemodynamic perturbations of hypovolemia, vasoregulation, myocardial suppression, and increased metabolic demands. Five years after the EGDT publication, there has been much discussion generated with regard to the concepts of EGDT, as well as debate fueled regarding diagnostic and therapeutic interventions. However, during this time period further investigations by the primary investigators and others have brought additional contemporary findings. EGDT modulates some of the components of inflammation, as reflected by improved organ function. The end points used in the EGDT protocol, the outcome results, and the cost-effectiveness have subsequently been externally validated, revealing similar or even better findings than those from the original trial. Although EGDT is faced with challenges, a coordinated approach to sepsis management is necessary to duplicate the progress in outcomes seen in patients with conditions such as acute myocardial infarction, stroke, and trauma.

Regional microvascular function and vascular reactivity in patients with different degrees of multiple organ dysfunction syndrome

The pathophysiology of multiple organ dysfunction syndrome (MODS) is believed to be related to that of microcirculatory dysfunction. We hypothesized that the severity of MODS is determined by measuring regional variables of microvascular function and vascular reactivity in critically ill patients. Therefore, we compared (a) reactive hyperemia response in the forearm using transcutaneous Po2/Pco2 electrodes and laser Doppler velocimetry, (b) microvascular permeability assessed by strain-gauge plethysmography in legs, and (c) variables derived from gastric tonometry in hemodynamically stable patients with moderate (n = 15) and severe (n = 15) MODS. There were no differences in systemic oxygen delivery, consumption, and oxygen extraction ratio between the groups. Mortality was 20% in patients with moderate MODS and 60% in patients with severe MODS (P = 0.025). Patients with a high MODS score had significantly larger arterial lactate concentrations (3.81 +/- 2.7 mmol/L) than patients with moderate MODS (1.66 +/- 0.82 mmol/L; P = 0.006). No significant differences in gastric pHi, gastric regional-to-arterial Pco2 difference, capillary filtration coefficient, isovolumetric venous pressure, and skin reactive hyperemia response were observed between patients with moderate and severe MODS. Once MODS is established, regional variables of microvascular function and vascular reactivity measured in this study do not reflect severity of organ dysfunction.

Sepsis-induced failure of hepatic energy metabolism

HYPOTHESIS

Recent evidence suggests that sepsis may induce an uncoupling of oxidative phosphorylation. The purpose of this study was to quantify temporal changes in hepatic oxygen consumption and cellular energy state with increasing severity of sepsis and thus assess the interrelationship of these parameters as either primary defect or compensatory response.

MAIN OUTCOME MEASURES

Pseudomonas aeruginosa was infused intravenously in eight instrumented anesthetized swine inducing a progressive severity of sepsis to shock. Eight other animals served as instrumented controls. Hepatic blood flow, oxygen use, and concentrations of ATP, ADP, AMP, NAD(+), and NADH were measured at baseline and then sequentially during the study.

RESULTS

Except for an increase in heart rate, there were no temporal changes in measured values for the control animals. For swine receiving P. aeruginosa, hepatic oxygen delivery and consumption increased with early sepsis whereas there were no alterations in the concentrations of adenine nucleotides or NAD(+)/NADH within liver. Septic shock was notable for a decrease in oxygen delivery yet oxygen consumption remained elevated because of an increase in percent oxygen extraction. The hepatic concentrations of ATP and NADH decreased during septic shock.

CONCLUSIONS

These findings suggest that any sepsis-induced limitation in phosphorylation may be initially compensated by an increase in oxygen use. This study also suggests that decreases in NADH availability may be a principal factor in the decompensation of sepsis to shock.

Increased tissue oxygen extraction and acidosis with progressive severity of sepsis

BACKGROUND

Lactic acidosis and increased production of CO(2) are common in septic shock. Presumably, both acidosis and CO(2) enhance the release of oxygen from hemoglobin. The purpose of this study was to assess the relationship of oxygen utilization, CO(2) production, acidosis, and hemoglobin oxygen (Hgb-O(2)) dissociation with progressive severity of sepsis to shock.

MATERIALS AND METHODS

Femoral arterial and vein, hepatic vein, portal vein, and pulmonary artery catheters were placed in 16 anesthetized swine. Organ blood flow was determined by timed injections of colored microspheres. After baseline measurements, Pseudomonas aeruginosa was infused in eight animals. This bacterial slurry was continued inciting a progression of sepsis to shock. Eight animals served as instrumented controls.

RESULTS

With sepsis and shock, there was a progressive decrease in pH and an increase in pCO(2) in plasma with all sampling sites (P < 0.01 septic shock versus baseline versus control). Blood flow to the liver and intestines increased with sepsis (P < 0.01) but then returned to near baseline control values during shock. VO(2) and/or percent O(2) extraction increased with sepsis and septic shock for the whole body and for the liver, intestine and leg (P < 0.01). There was a strong correlation between venous O(2) saturation, acidosis, and pCO(2) to percent O(2) extraction (r > 60; P < 0.0001). However, calculated P(50) values for Hgb-O(2) dissociation remained unchanged.

CONCLUSIONS

This study demonstrates that increased oxygen extraction in severe sepsis is related to a fall in tissue oxygen availability and not related to any allosteric change in Hgb-O(2) dissociation. Therefore, acidosis and hypercapnia do not have a demonstrable effect on altering oxygen availability during sepsis.

Dopamine does not correct oxygen consumption/oxygen delivery relation abnormality during vasomotor shock induced by interleukin-1beta

We previously showed that interleukin 1beta (IL-1beta) induces vasomotor shock and impairs the oxygen consumption (VO2)/oxygen delivery (DO2) relation by increasing the slope of the supply-independent line in rabbits. In the present study, we investigated the inotropic effect of dopamine on the VO2/DO2 abnormality induced by IL-1beta. Twelve rabbits were divided into two groups (n = 6, each) and were given 10 microg/kg of IL-1beta or saline (control) intravenously. After baseline measurements were obtained, dopamine was infused continuously at a rate of 20 microg/kg/min throughout the study in both groups. All rabbits were subjected to stepwise cardiac tamponade to reduce the DO2 to <5 mL/min/kg by inflation of a handmade balloon placed into the pericardial sac. The VO2/DO2 relation was then analyzed by the dual-line method. Dopamine failed to correct the IL-1beta-induced decrease in mean arterial pressure to the baseline level. Dopamine significantly increased cardiac index in both groups, resulting in significant increases in DO2 (IL-1beta, 28.5 +/- 6.0 mL/min/kg from baseline 24.1 +/- 3.5 mL/min/kg; control, 27.7 +/- 2.9 mL/min/kg from baseline 22.9 +/- 2.9 mL/min/kg), but did not affect VO2 (IL-1beta, 10.0 +/- 0.5 mL/min/kg from baseline 9.9 +/- 0.7 mL/min/kg; control, 10.2 +/- 0.4 mL/min/kg from baseline 10.2 +/- 0.2 mL/min/kg). The IL-1beta group showed a significantly greater supply-independent line slope than that of controls (IL-1beta, y = 0.14x + 6.3; control, y = 0.06x + 8.6) during stepwise decreases in DO2. These results indicate that continuous infusion of dopamine at 20 microg/kg/min increases DO2 but does not correct the vasomotor disturbance or VO2/DO2 abnormality caused by IL-1beta.

Energy metabolism in critically ill patients: lactate is a major oxidizable substrate

The potential role of an energy defect in acute diseases is still in the centre of the pathophysiological understanding of such states and therefore of our attempts to limit or to reverse the possible deleterious consequences of such defect. In fact several recent experimental works have shown that instead of being a negative consequence, the lactate production and the related metabolic acidosis due to the stimulation of anaerobic ATP-production pathway is rather a protective adapted response.

Treatment of massive hemorrhage with liposome encapsulated human hemoglobin (NRC) and hydroxyethyl starch (HES) in beagles

The efficacy of NRC as a substitute for blood transfusion for treatment of acute, massive hemorrhage was evaluated in this study. Fourteen beagles, anesthetized with a mixture of 50% nitrous oxide, 0.97% sevoflurane and oxygen and ventilated by a respirator, were hemodiluted by withdrawal of 12 ml/kg of blood and infusion of isovolemic HES (hemodilution: HD) four times every 10 min. Then the animals were divided into two groups; an HES group, in which the same HDs were continued and an NRC group, in which the HDs were done with NRC. The hematocrit value decreased to 11% in the HES group after eight HDs and to 13% in the NRC group, which had a 3.9% NRCcrit value during the same period. In the NRC group, Cao2 decreased to 7.9 ml/min, which was significantly higher than the 5.9 ml/dl of the HES group. Oxygen consumption decreased to 56 ml/min in the HES group, but in the NRC group, it dropped to 74 ml/min, which was significantly higher than that of the HES group. Cardiac output increased to 1.2 times that of the control after eight HDs and arterial mean pressure decreased to approximately 60%. The above data indicated that NRC delivered sufficient oxygen to tissues, substituting for circulating red cells and maintained aerobic metabolism. Therefore, it should be possible to use NRC successfully for the treatment of cute, massive hemorrhage.