The relationship between oxygen demand,oxygen uptake,and oxygen supply

Results of the relationships between oxygen supply, demand, and uptake can be used to interpret cardiac output values, identify types of acute circulatory failure, guide attempts to improve cellular function, and thus prevent the development of multiple organ failure and death. Five steps in the interpretation of cardiac output values are recommended: (1) relate cardiac output to the patient's size; (2) determine the presence of anemia or hypoxemia; (3) measure mixed venous O₂ saturation and (4) blood lactate levels; and (5) evaluate O₂ uptake before and after a transient increase in cardiac output.     

Oxygen saturation calculation procedures: A critical analysis of six equations for the determination of oxygen saturation

Photometrically measured values of O₂-saturation (SO₂) were compared with SO₂ predictions on the basis of the equations proposed by Heck, Kelman, Lutz, Marsoner, Severinghaus, and Siggaard-Andersen on 1350 occasions. Capillary, venous and mixed venous blood samples from 23 healthy subjects and 30 patients, suffering from cardio-pulmonary diseases were analyzed at rest, during exhaustion, limited exercise and during the recovery phase of ergometric tests. Overall the best agreements with the measured values were found for the equations of Kelman, Severinghaus and Siggaard-Andersen. The results of Heck's, Lutz', and Marsoner's preditions were in reasonable agreement with the measured SO₂ only under special physiological conditions. No calculation mode can be performed with constant accuracy and reliability when covering a wide range of acid-base values. If SO₂ values are used for further calculations e.g. for determination of cardiac output, measured values are preferred.     

Measurement of tissue perfusion by oxygen transport patterns in experimental shock and in high-risk surgical patients

Survivors of high-risk general (noncardiac) surgery were observed to have cardiac index (CI) values averaging 4.5 l/min·m², oxygen delivery ( O₂) of >600 ml/min·m², and oxygen consumption ( O₂) of 170 ml/min·m². In contrast, these values were relatively normal in patients who subsequently died. A very early predictive index based on these observations was found to predict outcome in 94% of high-risk patients. The hypotheses that increased- O₂ and O₂ in the survivors represent compensatory physiologic responses and that these values were appropriate therapeutic goals were tested in prospective randomized clinical trials and found to reduce mortality and morbidity significantly. The optimal goals were more easily attained with colloids, red cells, dobutamine, and vasodilators, according to their capacity to improve tissue perfusion, as reflected by increased flow and oxygen transport. The extremely complex interactions between- O₂ and O₂ are reviewed.     

Calculation Of O2 Consumption During Low-Flow Anesthesia from Tidal Gas Concentrations, Flowmeter, and Minute Ventilation

We present the principles of a new method to calculate O₂ consumption (VO₂) during low-flow anesthesia with a circle circuit when the source gas flows, end-tidal O₂ concentrations and patient inspired minute ventilation are known. This method was tested in a model with simulated O₂ uptake and CO₂ production. The difference between calculated VO₂ and simulated VO₂ was 0.01 ± 0.02 L/min. A similar approach can be used to calculate uptake of inhaled anesthetics. At present, with this method, the limiting factor in precision of measurement of VO₂ and uptake of anesthetic is the precision of measurement of gas flow and gas concentration (especially O₂ concentration in end-tidal gas, FETO₂) available in clinical anesthetic units. This new and readily available method to calculate O₂ consumption allows us to continuously monitor the patient’s well being, because O₂ consumption is an important physiologic indicator of tissue perfusion and organ function.     

Changes in renal vein,renal surface,and urine oxygen tension during hypoxia in pigs

To determine whether ureteral urine oxygen tension could serve as a monitor of renal hypoxia and its relationship to other renal O₂ tension parameters, we simultaneously measured femoral artery (PaO₂), renal vein (Pr O₂), renal surface (PrsO₂), and ureteral urine (PuO₂) oxygen tensions in 8 anesthetized pigs while incrementally decreasing the inspired oxygen concentration (FiO₂) from 21% to 12%. Renal artery blood flow, measured by transit time ultrasound, renal oxygen consumption, and thermodilution cardiac output, was constant. Changes in PaO₂, Pr O₂, PrsO₂, and PuO₂ caused by decreasing FiO₂ were evaluated by one-way analysis of variance. The relationships between PuO₂ and the other O₂ tension parameters were evaluated by correlation coefficient and linear regression statistics. Of six possible O₂ decrements (combinations of 3, 6, and 9%), only Pr O₂ significantly decreased with all six decrements. PuO₂ decreased when FiO₂ decreased 6% or more. PuO₂ is not a sensitive indicator of systemic hypoxia. Under constant renal perfusion and oxygen consumption, PuO₂ had a correlation coefficient of 0.80 and a regression equation of PuO₂=0.84 (Pr O₂)+11.6, with Pr O₂. PuO₂ is related to Pr O₂ when renal perfusion is constant.     

Commonly used reference values underestimate oxygen uptake in healthy, 50‐year‐old Swedish women

Cardiopulmonary exercise testing (CPET) is the gold standard among clinical exercise tests. It combines a conventional stress test with measurement of oxygen uptake (V_O2) and CO₂ production. No validated Swedish reference values exist, and reference values in women are generally understudied. Moreover, the importance of achieved respiratory exchange ratio (RER) and the significance of breathing reserve (BR) at peak exercise in healthy individuals are poorly understood. We compared V_O2 at maximal load (peakV_O2) and anaerobic threshold (V_O2@AT) in healthy Swedish individuals with commonly used reference values, taking gender into account. Further, we analysed maximal workload and peakV_O2 with regard to peak RER and BR. In all, 181 healthy, 50‐year‐old individuals (91 women) performed CPET. PeakV_O2 was best predicted using Jones et al. (100·5%). Furthermore, underestimation of peakV_O2 in women was found for all studied reference values (P<0·001) and was largest for Hansen‐Wasserman: women had 115% of predicted peakV_O2, while men had 103%. PeakV_O2 was similar in subjects with peak RER of 1–1·1 and RER > 1·1 (2 328·7 versus 2 176·7 ml min^?1, P = 0·11). Lower BR (≤30%) related to significantly higher peakV_O2 (P<0·001). In conclusion, peakV_O2 was best predicted by Jones. All studied reference values underestimated oxygen uptake in women. No evidence for demanding RER > 1·1 in healthy individuals was found. A lowered BR is probably a normal response to higher workloads in healthy individuals.     

Toe temperature versus transcutaneous oxygen tension monitoring during acute circulatory failure

Measurements of toe temperature and transcutaneous PO₂ (P_tcO₂) have been both suggested for non-invasive assessment of peripheral blood flow in acute circulatory failure. The underlying principle of the two methods is that cutaneous vasoconstriction occurs early when tissue perfusion is altered. In 15 patients, we compared the two measurements during cardiogenic shock (27 measurements) or septic shock (29 measurements). Toe-ambiant temperature gradient and P_tcO₂ correlated well together (r=0.66, p(0.001) especially in hyperkinetic septic shock (r=0.79, p(0.001). In cardiogenic shock, toe-ambiant temperature correlated well with cardiac index (r=0.63), stroke index (r=0.64) and oxygen transport (r=0.65), and these correlations were stronger than for P_tcO₂. In septic shock, both techniques were poor indicators of blood flow indexes but P_tcO₂ rather correlated with arterial pressure (r=0.66) and left ventricular work (r=0.66). Trend evaluation of data revealed in cardiogenic shock that the increase in toe temperature usually preceded the increase in P_tcO₂. Since measurement of P_tcO₂ is technically more complicated, correlates less well with standard hemodynamic parameters and later reflects cardiovascular improvement, it has no advantage over measurement of toe temperature in circulatory shock. In cardiogenic shock, measurements of toe temperature can reliably track cardiac output changes. In septic states, however, non-invasive assessment of skin perfusion is of limited interest.     

The effect of neuromuscular blockade on oxygen consumption in sedated and mechanically ventilated pediatric patients after cardiac surgery

Objective  To measure the effect of intense neuromuscular blockade (NMB) on oxygen consumption (VO₂) in deeply sedated and mechanically ventilated children on the first day after complex congenital cardiac surgery. Design  Prospective clinical interventional study. Setting  Pediatric intensive care unit of an university medical centre. Measurements and results  Nine mechanically ventilated and sedated children (weight 2.8–8.7 kg) were included. All children were treated with vasoactive drugs. The level of sedation was quantified using the comfort score, Ramsay score and bispectral index (BIS). The intensity of NMB was quantified using acceleromyography and VO₂ was measured using indirect calorimetry. Analgo-sedation using various intravenous agents was targeted at a deep level (comfort score < 18, BIS < 60 and Ramsay score > 4). NMB was achieved by intravenous administration of rocuronium. All measurements were conducted before, during and after recovery from a period of intense NMB. Baseline values were VO₂ 6.1 ml/(kg min) (SD 1.3), comfort score 13 (SD 0.7), BIS 42.5 (SD 14.2), mean blood pressure 54.0 mmHg (SD 10.5), mean heart rate 129.9 bpm (SD 28.9) and mean core temperature 36.7°C (SD 0.5). There were no significant differences in VO₂ or other parameters between baseline, during NMB and the recovery phase. Conclusion  Neuromuscular blocking agents do not reduce oxygen consumption in deeply sedated and mechanically ventilated children after congenital cardiac surgery.     

Oxygen delivery and consumption in surfactant-depleted newborn piglets

Objective: To study the relationship between oxygen (O₂) delivery (DO₂) and O₂ consumption (VO₂) in surfactant-depleted newborn piglets. Design: Prospective animal study. Setting: Hospital surgical research laboratory. Subjects: Twenty-six anesthetized and ventilated newborn piglets. Interventions: Twenty of the animals were subjected to repeated saline lung lavages, and then assigned to either the saline group or the L-NAME group. The other six animals without lavage were studied as the control group. Piglets in the L-NAME group and the control group received 3 mg/kg of N^w-nitro-L-arginine methyl ester (L-NAME, an inhibitor of NO synthase) i. v.; and those in the saline group received the same volume of saline i. v. Measurements and results: Cardiac output (CO) was measured and arterial and mixed venous blood gases were analyzed. DO₂, O₂ extraction ratio (O₂ER) and VO₂ were calculated. Plasma hypoxanthine was analyzed. In the lung lavaged groups, cardiac index, DO₂ and VO₂ decreased significantly after L-NAME i. v. but not after saline i. v. Further, the decrease in VO₂ in the L-NAME group correlated with the decrease in DO₂ (r = 0.83, p < 0.001). In the control group, cardiac index and DO₂, but not VO₂, decreased significantly after L-NAME i. v. Simultaneously, O₂ER increased significantly. Plasma hypoxanthine was not modified by lung lavage but increased after L-NAME i. v. in both the L-NAME and control groups. Conclusion: These data suggest that O₂ supply dependency is present in surfactant-depleted newborn piglets. Received: 30 June 1997 Accepted: 26 January 1998     

Comparison between cardiac output measured by thermodilution technique and calculated by O2 and modified CO2 Fick methods using a new metabolic monitor

Objective: To calculate cardiac output from dual oximetry with carbon dioxide production (VCO₂) and oxygen consumption (VO₂) measured by a new metabolic monitor, and to compare these values with measurements made simultaneously using the thermodilution method during the steady state condition. Design: Prospective, comparative clinical study. Setting: The adult postsurgical intensive care unit (ICU) of a University Hospital. Patients: Twenty mechanically ventilated postsurgical patients (70.7 ± 7.8 years of age; range 50–84). Measurements and results: A new metabolic monitor (Puritan-Bennett 7250, Carlsbard, USA) connected to a ventilator (Puritan-Bennett 7200) was used to measure VCO₂ and VO₂. Measurements of arterial (SaO₂) and mixed venous (SvO₂) oxygen saturations were made using pulse and venous fiberoptic oximeters. Cardiac output starting from VCO₂ (CO_VCO2) was obtained according to Mahutte's formula: CO_VCO2 = VCO₂/[k (SaO₂− SvO₂)], where k represents a constant. The value for each patient was determined from the initial measurements of thermodilution cardiac output (COtd), VCO₂, SaO₂ and SvO₂. CO_VCO2 calculated from the previous equation was compared to the COtd. Cardiac output calculated from the traditional O₂ Fick equation (CO_VO2) was compared to the COtd. All patients were studied over a period of 120 min at 15-min intervals in reasonably stable conditions. CO_VCO2 was closely related to COtd (r = 0.94; SEE = 0.79; p = 0.0001; n = 180) with a bias of − 0.10 and a precision of 0.45 l/min. The mean percent difference between the two methods was − 2.2 ± 8.3 %. CO_VO2 was related to COtd (r = 0.77; SEE = 0.79; p = 0.0001; n = 180) with a bias of − 0.57 and precision of 0.86 l/min. The mean percent difference between the two methods was − 10.8 ± 16.0 %. Conclusions: In stable patients, cardiac output measurements obtained from dual oximetry with VO₂ and VCO₂ measured by this new metabolic monitor, show good correlation with measurements made using the thermodilution method. The values of cardiac output calculated from VCO₂ are more accurate and precise than values from VO₂. The validity of these measurements in hemodynamically unstable patients and during various modes of mechanical ventilation seems warranted. Received: 5 February 1997 Accepted: 16 June 1997     

Delivery dependent oxygen consumption in patients with septic shock: Daily variations,relationship with outcome and the sick-euthyroid syndrome

Delivery dependent oxygen consumption (DDOC) is observed in patients with sepsis and vital organ dysfunction, and has been related to outcome. Similarly the sick-euthyroid syndrome is associated with a high mortality. We examined the daily variations of DDOC and its relation to hormonal changes, particularly those of the thyroid. In 22 patients, 14 with septic shock and 8 post-operative controls, oxygen delivery was increased by increasing cardiac output with vasodilation by phentolamine, during a total of 207 days. DDOC varied markedly between consecutive days in individual patients with sepsis, in both survivors and non-survivors. DDOC was related to severity of illness, assessed by APACHE II score (r=0.50,p=0.017), and plasma levels of triiodothyronine (T₃),r=–0.49,p=0.011, and thyroxine (T₄),r=–0.53,p=0.012. No correlation was observed between DDOC and outcome, nor blood levels of lactate, epinephrine, norepinephrine, dopamine or cortisol. In conclusion, we observed a marked disturbance of systemic oxygen uptake autoregulation in patients with septic shock which varied during the clinical course and was related to the sick-euthyroid syndrome.With the technical assistance of J. Lopez     

Skeletal muscle oxygen saturation does not estimate mixed venous oxygen saturation in patients with severe left heart failure and additional severe sepsis or septic shock

Introduction  

Low cardiac output states such as left heart failure are characterized by preserved oxygen extraction ratio, which is in contrast to severe sepsis. Near infrared spectroscopy (NIRS) allows noninvasive estimation of skeletal muscle tissue oxygenation (StO₂). The aim of the study was to determine the relationship between StO₂ and mixed venous oxygen saturation (SvO₂) in patients with severe left heart failure with or without additional severe sepsis or septic shock.     

Variations in oxygen consumption and carbon dioxide production during parenteral nutrition

To verify that variations caused by total parenteral nutrition (TPN) in O₂ intake (VO₂) and CO₂ output (VCO₂) can affect respiratory function of non-hypercatabolic patients, we studied 18 patients in two groups; group I (control): eight patients receiving 75–100g glucose/24 h, and group II: ten patients fed intravenously on 13.6 g N₂ and approximately 2,800 kcal/24 h given as a) 62% glucose+38% fats (TPN-G+F) and b) 100% glucose (TPN-G). VO₂, VCO₂, respiratory quotient (RQ) and minute ventilation (VE) were measured in all patients. We found that VCO₂ was 27% higher in intravenously fed patients (p<0.01 andp<0.02). similarly, VE was 26% higher in intravenously fed patients (p<0.001 andp<0.02). Comparison of TPN-G+F and TPN-G results showed no differences in VCO₂; by contrast, VO₂ was 21% less during TPN-G(p<0.01).     

Haemodynamic changes and skeletal muscle oxygen tension during complete blood exchange with ultrapurified polymerized bovine haemoglobin

Objective: The study investigates the effect of continuous blood exchange with ultrapurified, polymerized bovine haemoglobin (UPBH) in comparison to hetastarch on haemodynamics, oxygen transport and skeletal muscle oxygen tension in a canine model. Design: Sixteen anaesthetized beagle dogs underwent haemodilution with lactated Ringer's to a starting haematocrit of 20 % followed by progressive blood exchange with 6 % hetastarch 200,000/0.5 (HES, group 1) or UPBH (haemoglobin 13 ± 1 g · dl⁻¹, molecular weight (MW) 32–500,000, group 2) to haematocrit target levels of 15 %, 10 % and 5 % or less. Measurements and results: Besides haemodynamics, skeletal muscle tissue oxygen tension (tPO₂) was measured using a polarographic needle probe. In HES-treated animals, heart rate, cardiac output and blood flow were higher while systemic vascular resistance, systemic and regional arterio-venous oxygen difference (avDO₂) and oxygen extraction ratios were lower when compared to the UPBH group. In spite of a higher final haematocrit of 5 % in group 1, in comparison to group 2 with 2 %, final muscular oxygen uptake (4.7 ± 4 vs 10.1 ± 2 ml · min⁻¹) and mean tPO₂ (11.8 ± 2.3 vs 51.1 ± 2.9 mm Hg) were lower in group 1 than in group 2. While tPO₂ histograms were continuously shifted to lower oxygen tensions during progressive haemodilution with HES, UPBH-exchanged animals showed tPO₂ histograms shifted to higher values than baseline. Conclusion: In spite of vasoconstriction, UPBH provided more haemodynamic stability and enhanced skeletal muscle tPO₂ during progressive blood exchange when compared to HES. Received: 30 December 1996 Accepted: 16 June 1997     

Effects of various concentrations of inhaled oxygen on tissue dysoxia,oxidative stress,and survival in a rat hemorrhagic shock model

Objective

To test the hypothesis that a fractional inspired oxygen (F_IO₂) of 1.0 compared to 0.4 during hemorrhagic shock (HS) and fluid resuscitation (FR): mitigates tissue dysoxia; however, enhances the oxidative stress; therefore, offsets the benefit on survival.

Methods

Thirty rats underwent: HS for 75 min, during which 3.0 mL/100 g of blood was withdrawn, followed by FR for 75 min, during which 1.0 mL/100 g of shed blood and 3.0 mL/100 g of crystalloid solution were infused. Ten rats were randomized into one of three F_IO₂ (0.21 vs. 0.4 vs. 1.0) groups, and observed for survival until 72 h in each group. Hemodynamics, liver tissue PO₂ (P_TO₂), and, plasma antioxidants levels were also monitored.

Results

Oxygen inhalation increased mean arterial pressure (MAP) and decreased heart rate (HR) during HS and FR. Liver P_TO₂ was less than 10 Torr in all groups throughout HS; while it increased to average 26–35 Torr in oxygen groups during FR, it remained at 10 Torr with F_IO₂ 0.21 (P < 0.01). MAP, HR, and P_TO₂ did not differ significantly between oxygen groups. Plasma antioxidants levels did not differ among the three groups. All rats treated with oxygen, but eight of 10 rats with F_IO₂ 0.21 survived up to 72 h (NS).

Conclusions

Supplemental oxygen does not mitigate tissue dysoxia during HS, but does reduce tissue dysoxia without enhancing oxidative stress during subsequent FR. Increased F_IO₂ appears to prolong survival. These beneficial effects of supplemental oxygen do not differ between an F_IO₂ of 0.4 and 1.0.     

Difference in Physiological Components of VO2 Max During Incremental and Constant Exercise Protocols for the Cardiopulmonary Exercise Test

[Purpose] VO₂ is expressed as the product of cardiac output and O₂ extraction by the Fick equation. During the incremental exercise test and constant high-intensity exercise test, VO₂ results in the attainment of maximal O₂ uptake at exhaustion. However, the differences in the physiological components, cardiac output and muscle O₂ extraction, have not been fully elucidated. We tested the hypothesis that constant exercise would result in higher O₂ extraction than incremental exercise at exhaustion. [Subjects] Twenty-five subjects performed incremental exercise and constant exercise at 80% of their peak work rate. [Methods] Ventilatory, cardiovascular, and muscle oxygenation responses were measured using a gas analyzer, Finapres, and near-infrared spectroscopy, respectively. [Results] VO₂ was not significantly different between the incremental exercise and constant exercise. However, cardiac output and muscle O₂ saturation were significantly lower for the constant exercise than the incremental exercise at the end of exercise. [Conclusion] These findings indicate that if both tests produce a similar VO₂ value, the VO₂ in incremental exercise would have a higher ratio of cardiac output than constant exercise, and VO₂ in constant exercise would have a higher ratio of O₂ extraction than incremental exercise at the end of exercise.Key words: Cardiopulmonary exercise test, Fick equation, Near-infrared spectroscopy     

Effects of rapid permissive hypercapnia on hemodynamics,gas exchange,and oxygen transport and consumption during mechanical ventilation for the acute respiratory distress syndrome

Objective To measure the effects of rapid permissive hypercapnia on hemodynamics and gas exchange in patients with acute respiratory distress syndrome (ARDS).Design Prospective study.Setting: 18-bed, medical intensive care unit, university hospital.Patients 11 mechanically ventilated ARDS patients.Intervention Patients were sedated and ventilated in the controlled mode. Hypercapnia was induced over a 30–60 min period by decreasing tidal volume until pH decreased to 7.2 and/or P₅₀ increased by 7.5 mmHg. Settings were then maintained for 2 h.Results Minute ventilation was reduced from 13.5±6.1 to 8.2±4.1l/min (mean±SD), PaCO₂ increased (40.3±6.6 to 59.3±7.2 mmHg), pH decreased (7.40±0.05 to 7.26±0.05), and P₅₀ increased (26.3±2.02 to 31.1±2.2 mmHg) (p<0.05). Systemic vascular resistance decreased (865±454 to 648±265 dyne·s·cm^–5, and cardiac index (CI) increased (4±2.4 to 4.7±2.4 l/min/m²) (p<0.05). Mean systemic arterial pressure was unchanged. Pulmonary vascular resistance was unmodified, and mean pulmonary artery pressure (MPAP) increased (29±5 to 32±6 mmHg,p<0.05). PaO₂ remained unchanged, while saturation decreased (93±3 to 90±3%,p<0.05), requiring an increase in FIO₂ from 0.56 to 0.64 in order to maintain an SaO₂>90%. PvO₂ increased (36.5±5.7 to 43.2±6.1 mmHg,p<0.05), while saturation was unmodified. The arteriovenous O₂ content difference was unaltered. Oxygen transport (DO₂) increased (545±240 to 621±274 ml/min/m²,p<0.05), while the O₂ consumption and extraction ratio did not change significantly. Venous admixture (Q_va/Q_t) increased (26.3±12.3 to 32.8±13.2,p<0.05).Conclusions These data indicate that acute hypercapnia increases DO₂ and O₂ off-loading capacity in ARDS patients with normal plasma lactate, without increasing O₂ extraction. Whether this would be beneficial in patients with elevated lactate levels, indicating tissue hypoxia, remains to be determined. Furthermore, even though hypercapnia was well tolerated, the increase in Q_va/Q_t, CI, and MPAP should prompt caution in patients with severe hypoxemia, as well as in those with depressed cardiac function and/or severe pulmonary hypertension.     

Hemodynamic effects of partial liquid ventilation with perfluorocarbon in acute lung injury

Objective To assess the effect of partial liquid ventilation with perfluorocarbons on hemodynamics and gas exchange in large pigs with induced acute lung injury (ALI).Design Randomized, prospective, double-control, experimental study.Setting Experimental intensive care unit of a university.Materials Eighteen large pigs (50±5 kg body weight) with an average anterior posterior thoracic diameter of 24 cm and induced acute lung injury.Interventions All animals were surfactant depleted by lung lavage to aP _aO₂ below 100 mmHg and randomized to receive either perflubron (n=6) or saline (n=6) in five intratracheal doses of 5 ml/kg at 20-min intervals, or no instillation (n=6).Measurements and results In all animals heart rate, arterial pressures, pulmonary pressures, cardiac output and blood gases were recorded at 20-min intervals. There was no deleterious effect on any hemodynamic parameter in the perflubron group, whereas systolic and mean pulmonary arterial pressure values showed a persistent decrease after the first 5 ml/kg of perflubron, from 48.7±14.1 to 40.8±11.7 mmHg and from 39.7±13.2 to 35.2±12.0 mmHg, respectively. Perflubron resulted in a significant (ANOVAP<0.01), dose-dependent increase inP _aO₂ values from 86.3±22.4 to a maximum of 342.4±59.4 mmHg at a dose of 25 ml/kg; the other groups showed no significant increase inP _aO₂.Conclusions Tracheal instillation of perflubron in induced ALI results in a dose-dependent increase inP _aO₂ and has no deleterious effect on hemodynamic parameters.     

The concept of a critical oxygen delivery

In healthy tissues, decreases in oxygen delivery (QO_n) do not lower oxygen consumption (VO₂) because tissue O₂ extraction increases proportionately. When delivery is reduced below a critical threshold, VO₂ falls because tissue extraction exceeds a critical threshold, and cannot compensate for the reduction in delivery. In the adult respiratory distress syndrome and perhaps in septicemia, tissue extraction capacity is impaired, leading to O₂ supply dependency despite normal or increased overall delivery. This pathologic supply dependency could be caused by a loss in autoregulatory capacity, by disrupted blood flow distribution secondary to peripheral microembolization, or to other factors interfering with efficient tissue distribution of QO₂ with respect to VO₂. Alternatively, the increased VO₂ may be consumed in biochemical pathways not associated with ATP production, or in the production of oxygen radicals or hydrogen peroxide. To the extent this abnormal dependence of VO₂ on QO₂ reflects tissue hypoxia, clinical interventions which decrease systemic delivery should be evaluated with regard to possible deleterious effects on organ system function.     

Red blood cell transfusion compared with gelatin solution and no infusion after cardiac surgery: effect on microvascular perfusion,vascular density,hemoglobin, and oxygen saturation

BACKGROUND: After cardiac surgery, red blood cell (RBC) transfusion may improve systemic hemodynamics and thereby microvascular blood flow and O₂ delivery (DO₂). STUDY DESIGN AND METHODS: In a nonrandomized prospective observational study on post–cardiac surgery patients, systemic hemodynamics and microvascular blood flow, vascular density (sidestream dark‐field imaging), hemoglobin (Hb) content, and saturation (reflectance spectrophotometry) were measured before and 1 hour after start of transfusion of 1 to 2 units of leukoreduced RBCs (270 ± 203 mL), 500 mL of gelatin solution, or control (no infusion), when patients were considered clinically hypovolemic with (RBC group, n = 12) or without (gelatin group, n = 14) anemia (Hb < 10 g/dL) or not (n = 13), respectively. RESULTS: Systemic Hb was lower and increased in the RBC transfusion but not in gelatin and control groups. There were no differences in changes in systemic DO₂, O₂ uptake, and extraction between groups. RBC transfusion, compared with gelatin or control, increased medium‐sized vascular density, Hb content, and saturation in the microcirculation, while blood flow remained unchanged. Changes of microvascular Hb and saturation paralleled changes in systemic Hb. CONCLUSION: The data argue in favor of efficacy of RBC transfusion after cardiac surgery. RBC transfusion increases systemic Hb and this in turn increases medium‐sized vascular density and DO₂ in the sublingual microcirculation, independently of systemic hemodynamics and volume status.