Unreliability of oxygen tension-based indices in reflecting intrapulmonary shunting in critically ill patients

Measurement of intrapulmonary shunting (Qsp/Qt), a widely used method for monitoring disturbances of pulmonary oxygen transfer in critically ill patients, involves calculation of arterial and mixed venous oxygen contents. In circumstances where mixed venous blood samples are not readily available, oxygen tension-based indices such as the alveolar to arterial oxygen tension differences (P[A-a]O2), arterial oxygen tension to alveolar oxygen tension ratio (PaO2/PAO2), PaO2 to FIO2 ratio (PaO2/FIO2) and respiratory index (RI) are widely utilized to reflect Qsp/Qt. Oxygen content-based indices such as the estimated shunt are not as widely utilized as the oxygen tension indices. In 75 critically ill patients in whom a pulmonary artery catheter was being utilized to augment clinical care, comparisons were made between Qsp/Qt and P(A-a)O2, PaO2/PAO2, PaO2/FIO2, RI, and estimated shunt to determine which index best reflected Qsp/Qt. Correlations between Qsp/Qt and estimated shunt were good (r = .94) and poor for the P(A-a)O2 (r = .62), PaO2/PAO2 (r = .72), PaO2/FIO2 (r = .71), and RI (r = .74). We conclude that there are no real substitutes for venous oxygen contents in critically ill patients. When pulmonary artery blood is not available for analysis, oxygen tension-based indices are unreliable reflectors of Qsp/Qt while the estimated shunt, an oxygen content-based index, provides a more reliable reflection of Qsp/Qt.     

Effect of dopamine on intrapulmonary shunt fraction and oxygen transport in severe sepsis with circulatory and respiratory failure.

The hemodynamic response to a dopamine HCl infusion (10 microgram/kg per min) was measured in 25 adult patients with severe sepsis: there were 6 patients with circulatory hyperdynamic states, 9 patients with myocardial failure, and 10 with hypovolemia. Each patient also had acute respiratory failure. Changes of intrapulmonary shunt fraction (Qs/Qt), arterial and mixed venous oxygen tension (PaO2 and PvO2), oxygen transport, and oxygen consumption (VO2) were evaluated before and after dopamine infusion. Dopamine infusion produced clinical improvement and increased cardiac output. The hemodynamic response seemed to differ slightly according to the pattern of circulatory failure: chronotropic effect appeared to be predominant in hyperdynamic states, whereas inotropic effect appeared to be predominant in myocardial failure or hypovolemia. Moreover, in hypovolemic patients we noted a rise in pulmonary capillary wedge pressure suggesting an additional increase in venous return. During this treatment, we also noted a worsening of the Qs/Qt despite the increase in pulmonary blood flow; this worsening did not prevent significant improvements in VO2, but the improvement in PVO2 was offset by increased Qs/Qt and PaO2 remained unchanged.     

Oxygen tensions and oxyhemoglobin saturations in the assessment of pulmonary gas exchange

We studied the theoretical basis for continuous monitoring of pulmonary gas exchange using arterial and mixed venous oximetry by examining the mathematical relationships between the calculated venous admixture (Qsp/Qt) and the ventilation-perfusion index, which is derived from oxyhemoglobin saturations. We compared this relationship with that between Qsp/Qt and its commonly used estimates: inspired oxygen concentration to arterial blood oxygen tension ratio, arterial to alveolar oxygen tension ratio, and alveolar-arterial oxygen tension difference. The relationship between Qsp/Qt and the oxygen tension-based indices is nonlinear and substantially influenced by changes in inspired oxygen concentration and arteriovenous oxygen content difference. Therefore, it is inaccurate within the clinically acceptable range of arterial blood oxygenation. In contrast, calculation of ventilation-perfusion index from arterial and mixed venous blood oxyhemoglobin saturations provides a linear estimate of Qsp/Qt that is minimally affected by alterations in inspired oxygen concentration or oxygen uptake and, therefore, will allow accurate continuous assessment of pulmonary gas exchange.     

Respiratory index/pulmonary shunt relationship: quantification of severity and prognosis in the post-traumatic adult respiratory distress syndrome

The relationship between the respiratory index (RI = alveolar-arterial oxygen gradient [P(A-a)O2] normalized by PaO2) and the pulmonary shunt (Qsp/Qt) has been examined in 929 studies from 240 critically ill post-traumatic patients. Of these, 88 patients (443 studies) were individuals who developed post-traumatic adult respiratory distress syndrome (ARDS) and 152 were patients (486 studies) who did not develop ARDS. This study demonstrates that the RI to Qsp/Qt [RI/(Qsp/Qt)] relationship was significantly (p less than .0001) increased in patients who developed fatal ARDS compared with those who did not develop ARDS, or with those whose ARDS resolved. Because of the increased oxygen consumption (VO2) in ARDS patients in association with their severe limitations in gas exchange (RI) and increased Qsp/Qt, surviving ARDS patients had a significant increase in the cardiac index which resulted in a higher oxygen delivery to VO2 ratio. ARDS patients showed significant (p less than .0001) evidence of increased pulmonary vascular tone, correlated with the increase in the RI/(Qsp/Qt) relationship. In addition, those patients with high RI/(Qsp/Qt) also had increased right ventricular (RVSW) to left ventricular work (LVSW) ratios which were shown to be a direct function of the rise in RI. This increase in both RVSW/LVSW and RI/(Qsp/Qt) ratios was significantly (p less than .0001) correlated with an increased mortality. Thus, the RI/(Qsp/Qt) relationship, which can be obtained from arterial and mixed venous blood gases and saturations only, can be used to predict the severity of the ARDS process as well as important pulmonary vascular and right ventricular overload consequences.     

Arterial blood gas derived variables as estimates of intrapulmonary shunt in critically ill children

Oxygen transport data, prospectively collected from 52 critically ill children, were analyzed to determine whether any derived variable accurately estimated intrapulmonary shunt (Qsp/Qt). Arterial hemoglobin saturation was more closely correlated with Qsp/Qt than was PaO2, alveolar-arterial oxygen gradient, arterial mixed venous oxygen difference (C[a-v]O2), arterial/alveolar oxygen ratio, and the ratio of PaO2 to inspired oxygen (FIO2) (r = 0.8, p less than .0001). When C(a-v)O2 was normal, hemoglobin saturation became a very accurate (r = 0.96) assessment of Qsp/Qt. We conclude that various arterial blood gas derived variables do not accurately reflect Qsp/Qt in critically ill children. In these patients, a pulmonary artery catheter is needed to accurately assess intrapulmonary shunt.     

Extrapulmonary factors influencing the ratio of arterial oxygen tension to inspired oxygen concentration in burn patients.

The ratio of arterial oxygen tension to inspired oxygen concentration (PaO2/FIO2) as an index of respiratory function was evaluated in 22 patients with body surface area burns of 15--80%. These results indicate that this ratio is limited in its applicability because extrapulmonary factors, such as cardiac output, oxygen consumption, and arterial oxygen content, can affect this index by alterations in the amount of venous desaturation. Useful estimates of intrapulmonary right to left shunt (Qs/Qt) from PaO2/FIO2 were obtained only when arteriovenous oxygen content differences (avDO2) were between 3--5 ml/dl. There were avDO2 values above and below 3--5 ml/dl in at least 35% of the observations. Under these circumstances, PaO2/FIO2 did not correctly reflect changes in Qs/Qt. Blood gases from central venous catheters did not mirror changes in true mixed venous blood and, thus, can lead to erroneous estimations of Qs/Qt. Rational therapy of reduced arterial oxygen saturation requires measurement of both extra- and intrapulmonary factors contributing to arterial desaturation. Measurement of PaO2/FIO2 alone will not estimate these factors.     

Revised standards for normal resting dead-space volume and venous admixture in men and women

1. Data have been combined from three previous series to provide revised standards for the prediction of physiological dead-space volume (VD), arterial oxygen tension (Pa,O2), alveolar-to-arterial oxygen-tension difference (PAO2--Pa,O2) and venous admixture fraction (QVa/Qt) in the sitting position. 2. These standards, based on measurements in 96 healthy men and women aged from 20 to 74 years, largely confirm conclusions drawn from the first series of 48 subjects. 3. VD is best predicted on age, height, tidal volume and the reciprocal of respiratory frequency. Pa,O2,(PA,O2--Pa,O2) and Qva/Qt are adequately predicted on age alone.     

Ventilation/perfusion indices do not correlate with the difference between oxygen consumption measured by the Fick principle and metabolic monitoring systems in critically ill patients.

OBJECTIVE: To determine whether the difference between oxygen consumption (VO2) measured by metabolic gas monitoring systems and by the Fick principle is related to venous admixture, deadspace/tidal volume ratio, or alveolar-arterial oxygen tension gradient in critically ill patients. DESIGN: A prospective study. SETTING: An 11-bed general ICU in a 900-bed teaching hospital. PATIENTS: Twenty critically ill patients admitted to the ICU who required mechanical ventilation, right heart catheterization, and arterial and mixed venous gas measurements for normal clinical management. RESULTS: Thirty-three recordings were analyzed. The mean VO2 measured by the metabolic gas monitoring system was 308 +/- 63.9 (SD) mL/min and was significantly greater than the mean VO2 measured by the Fick principle of 284 +/- 72.0 mL/min. The difference between the two measurements of 24.3 +/- 47.6 mL/min correlated poorly with venous admixture (r2 = .0009), dead-space/tidal volume ratio (r2 = .0064) and alveolar-arterial oxygen tension gradient (r2 = .017). CONCLUSIONS: If the difference in VO2 measured by metabolic gas monitoring systems and the Fick principle is due to intrapulmonary VO2 then in critically ill patients the ventilation/perfusion indices of venous admixture, deadspace/tidal volume ratio and alveolar-arterial oxygen tension gradient correlate poorly with intrapulmonary VO2.     

Cardiovascular adjustments and gas exchange during extreme hemodilution in humans

OBJECTIVE: To examine the cardiovascular adjustments and pattern of gas exchange that occur during hemodilution. DESIGN: Nonrandomized prospective study. SETTING: Operating room in a university hospital. PATIENTS: Seven patients undergoing elective aortocoronary artery bypass surgery. INTERVENTIONS: Before initiating cardiopulmonary bypass, the patients' hematocrit levels were decreased to approximately 15%. This hemodilution was done by removing a sufficient amount of autologous blood from the aortic cannula and replacing it with a sufficient amount of crystalloid solution. After the discontinuation of cardiopulmonary bypass, measurements were made at a hematocrit of approximately 15%. Then, after autologous blood infusion, measurements were made at a hematocrit of 20%, followed by more blood infusion to approximately 25% with repeat measurements. MEASUREMENTS AND MAIN RESULTS: The following measurements were made before hemodilution and then at all three levels of hemodilution: heart rate, mean arterial pressure (MAP), right atrial pressure, mean pulmonary artery pressure, pulmonary artery occlusion pressure, and cardiac output. From these measurements, the following derived variables were calculated: cardiac index, systemic vascular resistance, and pulmonary vascular resistance. From measurements of arterial oxygen content, mixed venous oxygen content, and cardiac output, intrapulmonary shunt (Qsp/Qt), oxygen uptake (VO2), oxygen extraction ratio, and oxygen delivery (DO2) were derived. The MAP was lowest (57 +/- 3 [SD] vs. 92 +/- 3 mm Hg) at the lowest hematocrit. The cardiac index was highest (4.0 +/- 0.3 vs. 2.3 +/- 0.6 L/min.m2) at the lowest hematocrit. DO2 was lowest at the lowest hematocrit but VO2 remained constant at all levels of hematocrit. The oxygen extraction ratio increased as hematocrit decreased. With progressive increases in hematocrit, DO2 increased and Qsp/Qt decreased. CONCLUSIONS: The data suggest that, during hemodilution, tissue autoregulation of VO2 and utilization are not impaired, but gas exchange function (Qsp/Qt) is impaired.     

Systemic and muscle oxygen uptake/delivery after dopexamine infusion in endotoxic dogs.

BACKGROUND AND METHODS: This study was designed to test whether dopexamine, a dopaminergic and beta 2-adrenergic agonist, would a) increase systemic oxygen delivery (DO2) in endotoxic dogs, and b) interfere with the ability of resting skeletal muscle to extract oxygen. There were three treatment groups (n = 6 in each group): control, endotoxin alone (E) 4 mg/kg iv, and endotoxin + dopexamine (E + D) 12 micrograms/kg.min. Data were analyzed between and within groups by split-plot analysis of variance with significance of identified differences tested post hoc by Duncan's multiple range test. Donor RBC and dextran were used after endotoxin to maintain adequate perfusion pressures, with Hct kept near 40%. Blood flow to left hindlimb muscles was decreased in controlled steps of 15 min each after stabilization. RESULTS: In E group, cardiac output (Qt), mean arterial pressure (MAP), systemic DO2, and oxygen uptake (VO2) decreased despite blood volume expansion. In E + D group with similar volume expansion, dopexamine maintained Qt, systemic DO2, and VO2 near the control levels, although MAP and systemic vascular resistance were reduced. In comparison with control subjects, endotoxin increased critical DO2 in the isolated limb muscles from 4.6 to 7. mL/kg.min and decreased critical oxygen extraction from 81% to 68%. The pressure/flow relationship in the limb became flattened, indicating loss of vascular reactivity. In the E + D group, there was no further change in the pressure/flow curve nor in the critical oxygen extraction level. CONCLUSIONS: Dopexamine provided hemodynamic support for endotoxic dogs, thereby increasing total DO2 and VO2, while not altering oxygen extraction in the muscle.     

Sequential cardiorespiratory patterns in septic shock

Sequential hemodynamic and oxygen transport monitoring was performed in 33 patients with septic shock to define the temporal pattern of physiologic events. Measurements taken over a 24-h period before the hypotensive crisis, defined as the lowest initial mean arterial pressure (MAP), were compared to those taken during the 48 h thereafter. In the 24-h period before the hypotensive crisis, there were increases in cardiac index (CI), central venous pressure (CVP), pulmonary capillary wedge pressure (WP), pulmonary vascular resistance index (PVRI), and pulmonary shunt (Qsp/Qt), but decreases in MAP, systemic vascular resistance index (SVRI) and oxygen delivery (Do2). When sequential cardiorespiratory patterns were examined, oxygen consumption (VO2) fell transiently to significantly low values 12 h before as well as at the time of the hypotensive crisis. SVRI fell and CI rose to values significantly different from normal in the 4 h before the low MAP. During the subsequent 48 h after the hypotensive crisis, CI, CVP, WP, PVRI and Qsp/Qt remained elevated. Values for MAP, SVRI, DO2, and VO2 were significantly reduced. These results demonstrate the existence of antecedent cardiorespiratory alterations that precede the hypotensive episode in septic shock and suggest that flow maldistribution in the systemic circulation is an early event with possible pathogenic significance.     

Comparison of oxygen consumption measurements: indirect calorimetry versus the reversed Fick method

OBJECTIVE: To compare measurement of oxygen consumption (VO2) by spirometry and the reversed Fick method. DESIGN: Within-patient comparison using simultaneous measurements by the two methods, one previously calibrated on a metabolic simulator. PATIENTS: Twenty sets of observations on eight patients (57 to 83 yrs) requiring mechanical ventilation in a critical care unit. INTERVENTIONS: None during or immediately before the measurements. MEASUREMENTS AND MAIN RESULTS: Duplicate pairs of measurements of VO2 were made with a previously validated spirometric technique and the reversed Fick method (Qt[CaO2 - CVO2]), where Qt is cardiac output, CaO2 is arterial oxygen content, and CVO2 is mixed venous oxygen content. The coefficient of variation of the difference between duplicate measurements by the former technique was only 2.53% compared with 10.4% for the latter. The mean VO2 measurement by the spirometric method was 285.7 +/- 40.7 (SD) mL/min standard temperature and pressure, dry (STPD) and for the reversed Fick method, the mean VO2 measurement was 249.3 +/- 38.5 mL/min STPD. The mean difference was 36.4 +/- 28.5 mL/min STPD (p less than .001). CONCLUSIONS: The repeatability of the spirometric method was four times better than the reversed Fick method. The latter gave a significantly lower value that probably, in part, reflects the VO2 of the lung, which is included in the spirometric method but not in the reversed Fick measurement.     

Oxygen transport in adult respiratory distress syndrome and other acute circulatory problems: relationship of oxygen delivery and oxygen consumption

OBJECTIVE: To evaluate the evidence that oxygen consumption (VO2) is pathologically dependent on oxygen delivery (DO2). DATA SOURCES: Studies published since 1972 with their relevant bibliographies and computerized search of MEDLINE. STUDY SELECTION: All clinical papers reporting the relationship of: VO2 to DO2 in the adult respiratory distress syndrome (ARDS), sepsis, other critically ill patients, and normal individuals; cardiac output determined by measured VO2 to calculated VO2 from the arterial-mixed venous oxygen difference; blood lactate to DO2; and selected basic science studies. DATA EXTRACTION: Study quality was assessed and all pertinent data were summarized. RESULTS OF DATA EXTRACTION: Normal individuals display physiologic dependence of VO2 at very low levels of DO2 (330 mL/min.m2). Pathologic dependence of VO2 on DO2 entails two concepts: a) VO2 varies directly with DO2 over a wide range of DO2 and b) of particular import, tissue oxygen extraction is compromised. This pathologic supply dependence was initially identified in patients with ARDS; subsequently, it has been demonstrated in patients with sepsis and in a variety of other critically ill individuals. There are substantial, but not uniform, data documenting this dependence of VO2 on DO2 in ARDS. In some studies, this relationship correlates best with increased lactate concentrations. However, increased blood lactate concentrations do not accurately track other evidence of tissue hypoxia. Some researchers have attributed the finding of this supply dependency to artifact, when VO2 is determined by the arterial-mixed venous oxygen difference. However, when these methods are compared, the correlation is excellent. Others have raised the concern that appreciable changes in VO2, even over short periods of time, may result in physiologic increases in DO2. However, when "control" groups have been contemporaneously compared with patients with ARDS using the same methodology, they have not shown supply dependency. Interwoven throughout the studies reviewed is overwhelming and uniform evidence that both mixed venous oxygen tension (PVO2) and mixed venous oxygen content (CVO2) correlate poorly with cardiac output, DO2, or VO2. The inconsistencies in identifying pathologic DO2 dependency may well reflect the unknown variables that exist in patients with ARDS, perhaps better labeled, multiple organ system failure. CONCLUSIONS: Pathologic dependence of VO2 on DO2, especially the inability to increase tissue oxygen extraction, is present in most patients with ARDS and many other critically ill individuals. PVO2 and CVO2 are both unreliable indicators of cardiac output, DO2, or VO2.     

The AaDO2 and venous admixture at varying inspired oxygen concentrations in chronic obstructive pulmonary disease

The alveolar to arterial oxygen pressure difference (AaDO2) and pulmonary venous admixture (Qs/Qt) were measured in 32 patients with chronic obstructive pulmonary disease during right heart catheterization at inspired oxygen concentrations (FIO2) of 21, 24, 28, 35, and 40%. Patients without chronic hypercapnia (PaCO2 is less than 45 torr, group A) had Qs/Qt less than 25% while breathing room air; their AaDO2 rose at a rate of 3 torr for each percent increase in FIO2. In those with chronic hypercapnia (PaCO2 greater than 44 torr., (group B), THE Qs/Qt was always greater than 24% during air breathing and the AaDO2 rose at a rate of 5 torr for each percentage increase in FIO2. These changes should be considered in the interpretation of the AaDO2 in patients with COPD in whom the FIO2 is changed during the course of therapy. The Qs/Qt fell curvilinearly with increasing FIO2 but the rates of fall were quantitatively different in groups A and B. A physiological explanation for the changes in Qs/Qt and ADO2 which result from changes in FIO2 is presented.     

Effect of hydralazine on intrapulmonary shunt

We compared the acute effects of bilateral arteriovenous may be related to levels of PvO2. The hydralazine-associated (p less than .05) decrease in resistance. Mixed venous oxygen fistulas to those of hydralazine infusion on hemodynamics and pulmonary gas exchange in dogs with pulmonary edema induced by administration of oleic acid. Oleic acid significantly (p less than .01) increased intrapulmonary shunt (Qsp/Qt) and pulmonary and systemic vascular resistance, and reduced cardiac output. Once the lesion stabilized, both opening the fistula and infusing hydralazine produced a similar and significant (p less than .01) increase in cardiac output, and a significant (p less than .05) decrease in resistance. Mixed venous oxygen tension (PvO2) closely followed the changes in cardiac output; however, PaO2 did not change. Qsp/Qt significantly (p less than .01) increased with the fistulas open and with hydralazine infusion. Closure of the fistulas or bleeding the animal at the end of the experiment reversed the changes in cardiac output and Qsp/Qt. The comparable increases in cardiac output and Qsp/Qt produced by opening the fistulas or infusing hydralazine may be related to levels of PvO2. The hydralazine-associated PvO2 increase indicates that this drug increased oxygen transport to the tissues even as Qsp/Qt became larger.     

原位肝移植非静-静脉转流围术期肺氧合及肺内分流的变化

目的观察非体外静静脉转流术下原位肝移植(OLT)围术期肺氧合功能及肺内分流的改变。方法对19例晚期肝病行OLT手术的患者采用静吸复合麻醉,无肝期均未使用体外静静脉转流术。常规经右颈内静脉放置SwanGanz导管持续监测心排血量(CO),测定肺氧合功能及肺内分流率。分别在麻醉诱导后、无肝前30min、无肝30min、新肝30min和术毕抽取桡动脉血和肺动脉血进行血气分析,记录不同时期的动脉血氧分压(PaO2)、动脉血二氧化碳分压(PaCO2)和心排血指数(CI),计算肺泡动脉氧分压差(PAaO2)。根据肺内分流标准模型公式计算肺内分流率(Qs/Qt)。结果麻醉诱导后吸入氧浓度(FiO2)1.00情况下,PaO2为(385.0±56.4)mmHg(1mmHg=0.133kPa),PAaO2和Qs/Qt均明显高于正常参考值。麻醉诱导后各项指标与无肝前30min相比差异均无显著性;无肝30minCO和CI明显下降(P均<0.01),Qs/Qt明显下降(P<0.05);新肝30minPaO2、PaCO2均明显升高(P均<0.05),PAaO2明显下降(P<0.05),CO和CI均明显升高(P均<0.01);术毕CO和CI也均明显升高(P均<0.05),并维持在较高的水平,Qs/Qt明显下降(P<0.05),但PaO2、PaCO2、PAaO2则均无明显变化。结论OLT患者非静静脉转流术前和术中均存在明显的肺氧合功能障碍。     

Maximal oxygen consumption in patients with lung disease.

A theoretical model for oxygen transport assuming a series linkage of ventilation, diffusion, oxygen uptake by erythrocytes, cardiac output, and oxygen release was used to calculate expected values for maximal oxygen intake (VO2max) of patients with various pulmonary disorders 22 patients with either restrictive or obstructive ventilatory impairment were studied at rest and maximal exercise. When exercise measurements of maximal pulmonary blood flow (QCmax), oxygen capacity, membrane diffusing capacity for CO, pulmonary capillary blood volume, alveolar ventilation, and mixed venous oxygen saturation were employed as input values, predictions of VO2max from the model correlated closely with measured values (r = 0.978). Measured VO2max was 976+/-389 ml/min (45.3+/-13% of predicted normal), and VO2max predicted from the model was 1,111+/-427 ml/min. The discrepancy may in part reflect uneven matching of alveolar ventilation, pulmonary capillary blood flow, and membrane diffusing capacity for CO within the lung; uniform matching is assumed in the model so that mismatching will impair gas exchange beyond our predictions. Although QCmax was less than predicted in most patients (63.6+/-19.6% of predicted) the model suggests that raising QCmax to normal could have raised VO2max only 11.6+/-8.8% in the face of existent impairment of intrapulmonary gas exchange. Since pulmonary functions measured at rest correlated well with exercise parameters needed in the model to predict VO2max we developed a nomogram for predicting VO2max from resting CO diffusing capacity, the forced one second expired volume, and the resting ratio of dead space to tidal volume. The correlation coefficient between measured and predicted VO2max, by using this nomogram, was 0.942.     

Oxygen delivery-dependent oxygen consumption in acute respiratory failure

OBJECTIVE: To investigate whether oxygen consumption (VO2) is dependent on oxygen delivery (DO2) in adult respiratory distress syndrome (ARDS) and non-ARDS acute respiratory failure. DESIGN: Intervention study of a consecutive sample of patients admitted to the ICU with the diagnosis of acute respiratory failure. SETTING: Tertiary care center. PATIENTS: Thirteen consecutive patients with a diagnosis of ARDS and 11 with a diagnosis of respiratory failure not due to ARDS. Patients were monitored with an oximetric pulmonary artery catheter and mechanically ventilated. INTERVENTIONS: DO2 was decreased by the application of positive end-expiratory pressure (PEEP) (20 cm H2O), and subsequently increased by an iv infusion of dobutamine (10 micrograms/kg.min). RESULTS: After the application of PEEP, DO2 decreased significantly in both groups. However, VO2 decreased significantly (p less than .01) only in the ARDS group. When dobutamine was infused, DO2 increased significantly (p less than .01) in both groups, but VO2 increased only in ARDS patients. DO2 correlated significantly with VO2 both in ARDS (r2 = .81, p less than .01) and in non-ARDS (r2 = .38, p less than .05) patients. The correlation coefficient was significantly higher for ARDS than for non-ARDS patients. Comparing the slopes of the regression lines, a stronger dependency of VO2 on DO2 was found in ARDS than in non-ARDS respiratory failure (p less than .001). The oxygen extraction ratio correlated with DO2 in non-ARDS patients (r2 = .49, p less than .05), but not in ARDS patients. CONCLUSIONS: VO2 is dependent on DO2 over a wide range of DO2 values in acute respiratory failure. This dependency phenomenon is much stronger in ARDS than in respiratory failure due to other causes. Due to the abnormal dependency of VO2 on DO2, changes in the oxygenation status may not be reflected by changes in mixed venous oxygen saturation in ARDS.     

The effects of inversed ratio ventilation (IRV) on arterial oxygenation during mechanical ventilation in patients with acute respiratory failure.

We investigated the effects of inversed ratio ventilation by altering the inspiratory:expiratory (I:E) ratio and assessing the time course changes in the intrapulmonary shunting (Qs/Qt) in 14 patients with acute respiratory failure. Stepwise prolongation of the I:E ratio from 1:1.9 to 2:1 and then to 2.6 or 4:1 was applied when PEEP failed to raise the PaO2 above 80 mmHg while breathing oxygen. A significant decrease in Qs/Qt was observed following prolongation of the I:E ratio from 1:1.9 (Qs/Qt = 45 +/- 9%) to 2:1 (Qs/Qt = 29 +/- 9%) but not with further prolongation of the I:E ratio (Qs/Qt = 27 +/- 7%). Improvement of the pulmonary ventilation/perfusion imbalance became more marked with continued IRV and a significant increase in PaO2 was observed at 6 h after initiating prolongation of the inspiratory time (P less than 0.05). There were no significant changes in hemodynamics, PaCO2, or peak inspiratory pressure during IRV. This ventilatory pattern may be indicated when PEEP fails to improve PaO2, but prolongation of the inspiratory time above an I:E ratio of 2:1 did not produce a greater improvement in Qs/Qt and further increases in PaO2 did not occur after more than 10 h of IRV in our 14 patients.     

Agreement of an Inert Gas Rebreathing Device with Thermodilution and the Direct Oxygen Fick Method in Measurement of Pulmonary Blood Flow

Objective. Inert gas rebreathing is a well established method for the non-invasive measurement of pulmonary blood flow. We tested the agreement in measurement of pulmonary blood flow (Qt rb) by a new inert soluble gas rebreathing device, the Innocor (Innovision, Copenhagen), with bolus thermodilution (Qt td) and the direct oxygen Fick (Qt Fick) method. Methods. 9 patients pre- and post-cardiac surgery were recruited resulting in 20 sets of measurements overall. Arterial and mixed venous blood samples were collected simultaneously with a thermodilution measurement and rebreathing manoeuvre to measure both V _O2 and effective pulmonary capillary blood flow. Results. Mean bias (95% confidence limits) was: Qt rb − Qt td 0.01 (± 0.42) L/min; Qt rb − Qt Fick + 0.34 (± 0.59) L/min. The standard deviation of the difference between paired measurements was: Qt td − Qt rb ± 0.89 L/min; Qt Fick − Qt rb ± 1.26 L/min. Conclusions. Acceptable overall agreement between the Innocor and these reference standards was demonstrated.