The concept of a critical oxygen delivery

Venous hyperoxia after cardiac arrest. Characterization of a defect in systemic oxygen utilization

BACKGROUND

Supranormal mixed venous oxygen saturation (mixed venous hyperoxia), although reported, has never been characterized in humans resuscitated from cardiac arrest (postarrest cardiogenic shock). By contrast, cardiogenic shock without cardiopulmonary arrest (primary cardiogenic shock) is accompanied by mixed venous hypoxia under similar conditions of low oxygen delivery (DO2). The appearance of mixed venous hyperoxia indicates an excessive supply relative to demand in perfused tissue or cellular impairment of oxygen utilization, ie, low systemic oxygen consumption (VO2). Failure to improve VO2 has been associated with a poor outcome in other shock states.

STUDY OBJECTIVE

This study evaluates the clinical significance of mixed venous hyperoxia and its implications for impaired systemic oxygen utilization. The oxygen transport patterns in surviving and nonsurviving cardiac arrest patients are compared for their prognostic and therapeutic implications.

STUDY DESIGN

Consecutive, nonrandomized series.

SETTING

Large urban emergency department (ED).

PARTICIPANTS

Adult normothermic, nontraumatic out-of-hospital cardiac arrest patients presenting to the ED who develop a return of spontaneous circulation (ROSC).

INTERVENTIONS

On arrival to the ED, a fiberoptic catheter was placed in the central venous position for continuous central venous oxygen saturation monitoring (ScvO2). A proximal aortic catheter was placed via the femoral artery for blood pressure monitoring. Upon ROSC, the fiberoptic catheter was advanced to the pulmonary artery. Mean arterial pressure (MAP), cardiac index (CI), VO2, DO2, systemic oxygen extraction ratio (OER), and systemic vascular resistance index (SVRI-dynes.s/cm5.m2) were measured immediately and every 30 min. The duration of cardiac arrest (DCA) in minutes and amount of epinephrine (milligrams) administered during ACLS was recorded.

MEASUREMENTS AND RESULTS

Twenty-three patients were entered into the study. Survivors (living more than 24 h) and nonsurvivors (living less than 24 h) were compared.

CONCLUSIONS

These findings indicate an impairment of systemic oxygen utilization in postarrest cardiogenic shock patients. In spite of a lower DO2 than survivors, the OER in nonsurvivors remained lower than expected. Venous hyperoxia is a clinical manifestation of this derangement. Epinephrine dose may have a causal relationship. The inability to attain a VO2 of greater than 90 ml/min.m2 after the first 6 h of aggressive therapy was associated with a 100 percent mortality in 24 h.

Derived oxygen saturations are not clinically useful for the calculation of oxygen consumption

In critically ill patients, oxygen consumption (VO2) and delivery (DO2) are used to determine optimal haemodynamic management and to grade severity of illness. VO2 may be measured by indirect calorimetry with metabolic gas monitoring systems or derived using the reverse Fick principle. Oxygen saturation (SaO2) may be measured directly by co-oximetry or derived by equations for incorporation into reverse Fick equations. A prospective study comparing VO2 measured by these methods was performed in 20 critically ill patients. The mean VO2 measured by the metabolic gas monitoring system (308 +/- 63.9 ml/min) was significantly greater than that measured by reverse Fick using measured SaO2 (284 +/- 72.0 ml/min) (P < 0.01). This difference may be due to intrapulmonary VO2. When SaO2 was calculated from three logarithmic equations and incorporated into the reverse Fick equations, calculated VO2's were significantly greater (P < 0.001) than those measured by indirect calorimetry. Correlation was poor and wide limits of agreement (-118 to +350 ml/min) were demonstrated. VO2 should ideally be measured by indirect calorimetry in the critically ill, or if reverse Fick is used, SaO2 should be measured by co-oximetry as the use of equations for clinical measurement of SaO2 is clinically suspect.

Relationship of oxygen consumption and oxygen delivery in surgical patients with ARDS

Previous studies have described oxygen delivery (DO2) and oxygen consumption (VO2) relationships in patients with ARDS and other acute diseases that suggest occasions when VO2 may be dependent on the supply of oxygen, ie, DO2. We studied 127 postoperative patients who developed ARDS to evaluate the relationship of DO2 to VO2. We found a weak correlation between DO2 and VO2 in the total series (r = 0.49) as well as in several clinical subgroups of patients with ARDS. Moreover, we observed similar relationships in a concurrent group of 218 postoperative patients who did not develop ARDS. We also examined the DO2/VO2 data of individual patients with ARDS to identify instances where flow-dependent VO2 patterns developed into flow-independent VO2 patterns. We were able to identify an apparent plateau in the DO2/VO2 relationships in 29/50 (58 percent) patients where multiple measurements were obtained over a short period of time. Our data are consistent with the concept that the DO2/VO2 relationship in acutely ill early postoperative patients with and without ARDS is affected by antecedent circulatory problems that may lead to tissue hypoxia and tissue oxygen deficiencies that are manifest by flow dependency.

Effects of cardiovascular drugs on oxygen consumption/oxygen delivery relationship in patients with congestive heart failure

The oxygen consumption (VO2)/oxygen delivery (DO2) relationship was analyzed in ten patients with severe congestive heart failure (CHF) and normal blood lactate levels. First dobutamine and then enoximone, after a washout period, were administered to each patient to increase cardiac output by at least 15 percent. Similar increases in DO2 were obtained with both drugs: from 285 +/- 46 to 393 +/- 87 ml/min/m2 for dobutamine, and from 285 +/- 54 to 392 +/- 99 ml/min/m2 for enoximone. However, while VO2 did not change (132 +/- 24 vs 132 +/- 21 ml/min/m2) (VO2/DO2 independency) with a dobutamine infusion (mean dose of 10 +/- 2 micrograms/kg/min), a significant increase in VO2 from 134 +/- 22 to 157 +/- 21 ml/min/m2 was observed with a bolus infusion of enoximone (mean dose of 1.7 +/- 0.5 mg/kg). These results, observed in patients with CHF without patent oxygen debt, suggest that an artefactual VO2/DO2 dependency might be induced by the cardiovascular drug used to elevate DO2, probably because of a drug-induced oxygen demand increase.

Effects of vasopressor agents and epoprostenol on systemic hemodynamics and oxygen transport in fulminant hepatic failure

Hypotension is a serious complication in patients with fulminant hepatic failure, because it is associated with tissue hypoxia and a further compromise to end-organ function. In this study we investigated the effects of epinephrine and norepinephrine on hemodynamics and oxygen transport variables in 30 patients with fulminant hepatic failure. All had a mean arterial pressure of less than 60 mm Hg, despite adequate intravascular filling pressures. Both epinephrine (n = 15) and norepinephrine (n = 15) improved mean arterial pressure (p less than 0.001 epinephrine and norepinephrine), although this was not associated with a rise in oxygen delivery. Oxygen consumption fell (p less than 0.05 epinephrine, p less than 0.001 norepinephrine) because of a lower oxygen extraction ratio (p less than 0.01 epinephrine and norepinephrine). The addition of epoprostenol, a microcirculatory vasodilator, in 10 patients from each group led to an increase in oxygen consumption (p less than 0.001 epinephrine and norepinephrine) because of a rise in oxygen delivery (p less than 0.05 epinephrine, p less than 0.01 norepinephrine) and oxygen extraction ratio (p less than 0.01 epinephrine, p less than 0.001 norepinephrine), without a fall in mean arterial pressure. The fall in oxygen consumption after the institution of vasopressor therapy could exacerbate tissue hypoxia and thus contribute to further organ damage in an already susceptible patient. In patients with fulminant hepatic failure who are given vasopressor support, the addition of epoprostenol may prevent the development of tissue hypoxia.

Shock index: a re-evaluation in acute circulatory failure

STUDY OBJECTIVE

To evaluate the relationship between the shock index SI (ratio of heart rate to systolic arterial pressure) and cardiac function and oxygen transport in an experimental model of hemorrhage and clinical septic shock.

METHODS AND RESULTS

This study was conducted in a hypovolemic circulatory failure model; 40% hemorrhage in the anesthetized pig and normovolemic hyperdynamic septic patients in the intensive care unit (ICU). Hemodynamic and oxygen transport variables were measured and their relationships to SI was examined. SI was inversely related to blood loss, cardiac index (CI), stroke volume (SV), mean arterial pressure (MAP) and left ventricular stroke work (LVSW) (r = -0.73, -0.75, -0.89 and -0.75, respectively P less than 0.01) following hemorrhage in the anesthetized pig. Oxygen transport variables, i.e. oxygen delivery (DO2) and mixed venous oxygen saturation (SvO2P) (r = -0.68 and -0.74, respectively, P less than 0.01) were also inversely related to the SI. Oxygen consumption (VO2) increased initially with increasing SI and fell when SI was greater than 3.0. In clinical septic shock and following blood volume expansion, the SI was not correlated to CI, SVI, MAP or systemic vascular resistance (SVR) (r = -0.01, -0.47, -0.34 and -0.14, respectively, P-value NS) but was inversely related to LVSWI (r = -0.68, P less than 0.01). There were no relationships between the SI and oxygen transport variables (DO2, SvO2) (r = -0.02 and -0.17, P-value NS) in septic shock.

CONCLUSION

SI provides a non-invasive means to monitor deterioration or recovery of LVSW during acute hypovolemic and normovolemic circulatory failure and its therapy. SI may be of limited value in the assessment of systemic oxygen transport and response to therapy in clinical shock.

[Hemodynamic parameters in the severely burnt patient during the 1st 72 hours]

The haemodynamic time course of 16 patients with severe burn injury was investigated using a flow-directed balloon-tipped pulmonary artery catheter. The patients, aged 33.8 +/- 5.5 years, were burnt over 60 +/- 10% of body surface area, with a UBS score of 228 +/- 43. The measurements were obtained every six hours after insertion of the catheter. Fluid load was determined with Evans' formula, and modified according to the haemodynamic data. Catecholamines were introduced when this and a trial of fluid loading with 5 ml.kg-1 of macromolecules during a 20 min period had failed, starting with dobutamine or dopamine, followed by adrenaline as required. During the first hours after the injury, circulatory shock was partly linked to hypovolaemia: mean arterial pressure was 60.1 +/- 7.8 mmHg, right auricular pressure 4.5 +/- 2 mmHg, pulmonary wedge pressure 4.7 +/- 2 mmHg, cardiac index 3.5 +/- 0.8 l.min-1 x m-2. However, during the second and third days, cardiac output increased, with a cardiac index at 4.7 +/- 0.6 l.min-1 x m-2 and 5.2 +/- 0.2 l.min-1 x m-2 respectively, and arterial vascular resistances were decreased (536 +/- 125 dyn.s.cm-5). These data suggest a specific haemodynamic profile in severe burn patients which justifies invasive monitoring, and the use of catecholamines, in those patients that do not respond to fluid loading. The link between these data and the concomitant metabolic disturbances due to the burn injury has not yet been established. The increase in cardiac index could be related to the inflammatory response.(ABSTRACT TRUNCATED AT 250 WORDS)

Necrotizing myopathy in critically-ill patients

Skeletal muscle wasting is commonly observed in critically-ill patients and has been attributed to catabolic fibre atrophy and to neuropathy. This study describes the occurrence of a necrotizing myopathy in 15 out of 31 critically-ill patients who had percutaneous biopsies taken from the tibialis anterior muscles. While most cases showed necrosis of isolated fibres, 5 of the 12 patients who had serial biopsies showed progressive necrosis of up to 95 per cent of the fibres. One other case showed infarction and one case had staphylococcal vasculitis. Atrophy of type 1 and/or type 2 fibres was documented by morphometry in 12 cases. Myoglobin-containing casts were demonstrated immunohistochemically in renal tubules on either biopsy or necropsy material in 5 out of 7 cases. The presence of muscle necrosis was a clinically unexpected finding which may contribute to weakness, complicate the interpretation of tissue biochemistry and energy balance studies, and potentiate renal failure. The necrosis is probably multifactorial in origin, with ischaemia and sepsis contributing factors.

Independent oxygen uptake and oxygen delivery in septic and postoperative patients

Previous studies reporting pathologic oxygen supply dependency calculated VO2 as CO x C(a-v)O2. We investigated whether pathologic oxygen supply dependency exists in septic and postoperative patients if VO2 and DO2 are assessed independently. In septic patients, VO2 was 164 +/- 31 and DO2 was 633 +/- 209 ml/min/m2. The slope (b) of the VO2-DO2 regression line VO2 = b x DO2 + a ranged from -0.10 to 0.08 (mean, 0.02 +/- 0.01, p less than 0.05) and was statistically significant in two patients (b = 0.05 and b = 0.08, p less than 0.05). In postoperative patients VO2 was 136 +/- 19 and DO2 was 481 +/- 160 ml/min/m2; b ranged from -0.07 to 0.09 (mean, 0.04 +/- 0.01, p less than 0.001) and was statistically significant in one patient (b = 0.09, p less than 0.01). The lack of a close relationship between independently measured VO2 and DO2 may indicate that septic and postoperative patients in stable hemodynamic condition have no pathologic oxygen supply dependency. Analysis of the VO2-DO2 relationship may not be useful to guide therapy or predict outcome.

Assessment of oxygenation in the critically ill

This paper reviews the physiology and pathology of oxygen transport from the atmosphere to the cells in critically ill patients. A thorough understanding of this allows a rational approach to monitoring and managing an hypoxic patient in the intensive care setting.

Oxygen delivery and oxygen uptake in postoperative and septic patients

Pathologic dependency of VO2 on DO2 has been reported in postoperative and septic patients. We studied the influence of an artifact due to calculation of VO2 from CO and AV content difference. In 13 postoperative and seven septic patients, the relationships between DO2 and cVO2 and between DO2 and mVO2 were analyzed by linear regression. In ten patients, cVO2 and DO2 were significantly correlated, but in only two of these patients mVO2 also correlated with DO2. In two patients, there was a significant correlation between mVO2 and DO2. The main contribution to the correlation and to the slope of the regression line between cVO2 and DO2 may be due to mathematic coupling of cVO2 and DO2. With cVO2, erroneous conclusions concerning the VO2-DO2 relationship may be drawn. In four patients, DO2 and mVO2 were significantly related, reflecting either physiologic coupling or pathologic supply dependency of VO2.

Improved survival in ARDS patients associated with a reduction in pulmonary capillary wedge pressure

The survival and ICU length of stay of 40 ARDS patients admitted to the ICU were analyzed to determine if a management strategy of lowering the pulmonary capillary wedge pressure (Ppw) was associated with an increased survival or a decreased ICU length of stay. ARDS was defined as three or four quadrant alveolar filling roentgenographically, a PaO2 less than 80 mm Hg with an FIO2 greater than .5 and a Ppw less than 18 mm Hg. Patients were divided into two groups: group 1 included all patients in whom there was a reduction of Ppw by at least 25 percent, and group 2 included patients in whom there was no, or less than a 25 percent reduction in Ppw. Survival was statistically different between the groups with 12 of 16 group 1 patients and seven of 24 group 2 patients surviving to hospital discharge. This difference remained statistically significant after stratifying patients by age and the APACHE II severity of illness index. We conclude that this retrospective analysis supports the notion that treatment of low pressure pulmonary edema with reduction of Ppw is associated with an increased survival.

Physiologic effects and side effects of prostaglandin E1 in the adult respiratory distress syndrome

Because PGE1 previously has been reported to increase survival of patients with ARDS, we evaluated physiologic effects and side effects of PGE1 in a prospective open-label study of patients with ARDS. Seventeen patients with ARDS who did not have significant renal or hepatic dysfunction received PGE1 by continuous central venous infusion (30 ng/kg/min). Seventeen control patients with ARDS without renal or hepatic dysfunction who had similar APACHE II and ARDS scores and causes of ARDS did not receive PGE1. Prostaglandin E1 significantly decreased the SVRI and oxygen extraction ratio. Concentrations of total and polymorphonuclear leukocytes, but not platelets, increased significantly during PGE1 infusion, but did not change in control patients. There was no change in the Do2I and Vo2I during the course of the PGE1 infusion. There were no differences in Do2I and Vo2I during PGE1 infusion between survivors and nonsurvivors. Prostaglandin E1 was infused for a mean of 5.9 +/- 1.8 days (+/- SD) and was discontinued on ten occasions in seven patients because of supraventricular dysrhythmias (n = 4), hypotension (n = 3), thrombocytopenia (n = 3), and cardiac arrest (n = 2). Nonsurvivors had PGE1 discontinued prematurely more frequently than survivors (56 percent [5/9] vs 25 percent [2/8], respectively). The prevalence of multiple-system organ failure and the in-hospital mortality of both PGE1-treated and control patients were not different. Although PGE1 causes significant systemic vasodilation and possibly decreased intrapulmonary polymorphonuclear leukocyte sequestration, PGE1 does not influence multiple-system organ failure or mortality of patients with ARDS without renal or hepatic dysfunction.

Optimal oxygen delivery in critically ill patients

Standard hemodynamic support in septic shock is to increase pulmonary capillary wedge pressure to above 15 mmHg by volume replacement and to give inotropic support if the mean arterial pressure (MAP) is not adequate. In an attempt to decrease mortality in critically ill patients, oxygen delivery (DO2) was increased by switching inotropic support from dobutamine alone or in combination with norepinephrine to dopamine alone, or by adding dopexamine, prostacyclin, or hypertonic saline to the treatment. DO2 increased significantly in all patients, but the increase in DO2 was accompanied by only a 10% increase in oxygen consumption (VO2). The increase in VO2 was similar in survivors and nonsurvivors and in patients with and without septic shock. The results indicate that if adequate volume and inotropic support is provided for critically ill patients, the detectable oxygen debt is small and has little effect on patient outcome. When DO2 is adequate, factors other than a tissue oxygen deficit seem to determine patient outcome.

The oxygen status of the arterial blood in the critically ill

In Critical Care medicine the concepts of Oxygen Delivery, Oxygen Consumption and Tissue Oxygenation have become fundamental in clinical practice but measurements of Oxygen Content and O2 Transport variables require invasive procedures that could be dangerous for critically ill patients and trigger a septic process. Derived indices obtained combining data from a Blood Gas Analyzer with the data from a multi-wavelength spectrophotometer and using the Ole Siggaard-Andersen pH/Blood Gas computerised algorithm might be the non-invasive answer. On 115 arterial blood samples from critically ill patients, we measured pH, pCO2, pO2, oxygen saturation, total hemoglobin concentration and fractions of carboxy- and methemoglobin. The new algorithm was used to calculate: active hemoglobin concentration, total oxygen concentration, actual half-saturation tension, 2,3-diphosphoglycerate concentration, estimated functional shunt, uncompensated mixed venous pO2 (assuming an arterio-venous oxygen difference of 2.3 mmol/L based on a standard oxygen consumption of 11.2 mmol/min and a standard cardiac output of 4.9 L/min) and the cardiac oxygen compensation factor. In Intensive Care all the oxygen parameters may be determined with sufficient accuracy and precision provided the oxygen saturation level is less than 0.97 and provided the definition of oxygen saturation is properly settled and measurements are performed according to the highest state of the art. However, in critically ill patients in evolution our aim is to maintain an 'optimal' paO2 on the plateau of the Oxygen Dissociation Curve (ODC) and the use of mechanical ventilation, high FIO2, fluid challenges and the rapid improvement of the patient's conditions can cause a value for sO2 greater than or less than 0.97 and an improvement or worsening of the paO2. The p50 calculation both in simultaneously drawn arterial and venous blood permits utilisation of derived indices (pO2uv-, CQ) for sO2 greater than 0.97. The Ole Siggaard-Andersen algorithm seems to give correct p50 values, at high saturation values, particularly when discarding unrealistic values for calculated cDPG. The correlation between p50 calculated by the Ole Sigaard-Andersen algorithm with that derived from classical formula shows the superiority of the findings obtained by means of the new algorithm. In critically ill patients the ODC is usually shifted to the right. The new parameters, pO2uv- and CQ, contain useful informations for clinical practise; but rapid changes in Cardiac Index (CI) and VO2/m2 can be ignored by the new algorithm, if these changes are not associated with a rise in ctO2 or pH and pCO2 changes.(ABSTRACT TRUNCATED AT 400 WORDS)

The oxygen status of the arterial blood revised: relevant oxygen parameters for monitoring the arterial oxygen availability

The new generation of very accurate multi-wavelength oximeters, e.g. OSM3, for in vitro measurement of the hemoglobin oxygen saturation, total hemoglobin concentration, and carboxy- and methemoglobin fractions opens new aspects of oxygen monitoring. Combined with the data from the blood gas analyzer (e.g. ABL300) these very accurate measurements allow the calculation of several derived oxygen parameters on the basis of a set of newly developed calculation algorithms. The traditional parameters obtained from an arterial sample are the oxygen tension (pO2) and the hemoglobin oxygen saturation (sO2). Clinical examples illustrate that the pO2 and the sO2 even in combination may give misleading information. The new algorithm calculates three extra oxygen parameters. 1) The oxygen extraction tension, px, defined as the tension required to extract 2.3 mmol of oxygen per liter blood. It signals the mixed venous pO2 level on the assumption that the arterio-venous oxygen difference is normal (2.3 mmol/L). 2) The concentration of extractable oxygen, cx, defined as the concentration of oxygen extracted at a tension of 5.0 kPa. 3) The oxygen compensation factor, Qx, derived as (2.3 mmol/L)/cx. It may be interpreted as the increase in cardiac output necessary to maintain a normal mixed venous pO2 of 5 kPa. These three parameters indicate the oxygen availability of the blood and summarize important properties of the arterial blood in relation to oxygen supply of the tissues, including the arterial pO2, the 'active' hemoglobin concentration (equivalent to the oxygen capacity), and the hemoglobin oxygen affinity (p50). The set of data measured with the blood gas analyzer, e.g. the ABL300 combined with the data measured with the OSM3 contains much more information than is routinely utilized. This information is extracted and summarized by our calculation algorithm. Omitting the calculation of the extra oxygen parameters involves a risk of losing valuable information.

Relationship between mixed venous oxygen saturation and cardiac index in patients with chronic congestive heart failure

The use of mixed venous oxygen saturation (SvO2) in patients with chronic congestive heart failure (CHF) has been advocated to analyze the action of therapy on cardiac index (CI). To evaluate the relationship between CI and SvO2, ten CHF patients (mean age 65 years) were studied before and one, two, three, four (T4), six, eight and 24 hours after oral administration of an angiotensin converting enzyme (ACE) inhibitor (perindopril, 4 mg). At T4, a 12 percent increase in CI (p less than 0.01) was associated with a 16 percent decrease in arteriovenous oxygen difference (p less than 0.01), a 13 percent increase in mixed venous oxygen pressure (PvO2) (p less than 0.01), and a 9 percent increase in SvO2 (p less than 0.05) with no significant change in arterial oxygen pressure. There was no correlation between CI and SvO2 (r = 0.22) and between CI and PvO2 (r = 0.23). Individual analyses were performed and patients were divided into two groups based on CI versus SvO2 r value; group 1, n = 6, r greater than 0.65 (0.65-0.90), group 2, n = 4, r less than 0.65 (0.14-0.20). The lack of correlation in group 2 was due to a drug-dependent increase in oxygen consumption (VO2) +18 percent vs -3 percent in group 1 (p less than 0.05) associated with a lack of increase in PvO2 +3 percent vs +14 percent in group 1 (p less than 0.05) despite a similar increase in oxygen availability +19 percent versus +16 percent. It was concluded that (1) a correlation between CI and SvO2 is not found in every patient with CHF; (2) the lack of correlation in four out of our ten patients was due to an associated and significant increase in CI and VO2 in group 2; (3) group 2 patients probably had an important oxygen debt before treatment; (4) SvO2 cannot be used instead of CI to determine the hemodynamic consequences of the use of cardiovascular drugs.

Lactic acidosis

An understanding of the pathophysiology of lactic acidosis is crucial in facilitating the optimal care of critically ill patients. The relevant biochemistry of lactic acidosis is reviewed, and the more controversial aspects relating to the genesis of the acidosis are highlighted. The current system of classification of lactic acidosis divides etiologies on the basis of the presence or absence of clinical signs of tissue hypoperfusion. Several types of lactic acidosis in which clinical evidence of tissue hypoperfusion is lacking demonstrate hemodynamic evidence of occult hypoperfusion. The diagnostic and therapeutic implications of this observation are discussed. Current diagnostic criteria for lactic acidosis include a pH less than 7.35 and blood lactate concentration greater than 5 to 6 mM/L. An important issue relates to the implications of lactate values that are greater than normal but below this diagnostic range. The use of the oxygen flux test may be valuable in the diagnosis of occult tissue hypoperfusion in patients with low-grade elevations in lactate levels. The current therapy for lactic acidosis involves addressing the primary cause and supportive management. The use of bicarbonate in the therapy for lactic acidosis is controversial due to potential adverse effects on cardiac function. The specifics of this controversy are outlined, and newer therapeutic alternatives are reviewed. The use of blood lactate concentration as a prognostic index may be more useful in patients with shock than without shock.

[What is the role of monitoring of mixed venous blood saturation in cardiac surgery?]

The equipment available for mixed venous blood saturation (Svo2) monitoring is now accurate. SvO2 is not a direct measure of cardiac output, because it depends on the balance between oxygen delivery (TaO2) and consumption (VO2). As haemoglobin affinity for oxygen increases during cardio-pulmonary bypass (CPB), the optimal level of SvO2 after CPB should be above 65-70%. There is a critical level of TaO2 below which VO2 is dependent on TaO2. Below this level, SvO2 has no clinical value as it no longer depends on TaO2. Similarly, SvO2 has no clinical value during lactic acidosis. When these limitations are taken into account, SvO2 monitoring is useful for the interpretation of intra- and post-operative haemodynamic alterations occurring during cardiac surgery. It is particularly indicated in patients with preoperative NYHA class III or IV congestive heart failure.

[Relation of oxygen transport and consumption. Role of impaired tissue uptake]

In healthy subjects, when oxygen transport is gradually decreased, oxygen consumption is maintained as tissue oxygen extraction is increased. When delivery is decreased further, there is a critical level below which tissue extraction cannot increase in proportion to the reduced delivery, and oxygen consumption falls. Blood lactate levels then rise, a sign of tissue hypoxia, despite further increases in oxygen extraction as delivery drops below this critical level. There are two major mechanisms which tend to prevent tissue hypoxia in case of reduced oxygen delivery: regional redistribution of blood flow and an increase in the number of perfused capillaries. This possibility of regulating blood flow distribution may be lost during disseminated intravascular coagulation, alpha-adrenergic receptor blockade, hypothermia, arteriovenous shunting. All these alterations have been reported as occurring in sepsis. An abnormal dependency on oxygen supply is observed during bacteriaemia or endotoxinaemia. This is secondary to a reduced ability, at the whole body level, to extract oxygen from a limited supply. The inability to increase oxygen extraction is related to a maldistribution of the cardiac output, with "stealing" of blood, i.e. overperfusion of some organs (skeletal muscle) rather than those where perfusion is rapidly compromised (small intestine). Endotoxin also reduces the efficacy of oxygen extraction within the isolated intestinal segment, whereas that within other organs is preserved. These observations are similar to findings in patients with sepsis who seem to have both an increased demand in oxygen, and a reduced ability to extract it at the tissue level.