Comparison of formulae used to estimate oxygen saturation of mixed venous blood from caval samples

Inferior and Superior Vena Cava Saturation Monitoring After Neonatal Cardiac Surgery

OBJECTIVES

Superior vena cava oxygen saturation (SVC O 2 ) monitoring is well described for early detection of hemodynamic deterioration after neonatal cardiac surgery but inferior vena cava vein oxygen saturation (IVC O 2 ) monitoring data are limited.

DESIGN

Retrospective cohort study of 118 neonates with congenital heart disease (52 single ventricle) from February 2008 to January 2014.

SETTING

Pediatric cardiac ICU.

PATIENTS

Neonates (< 30 d) with concurrent admission IVC O 2 and SVC O 2 measurements after cardiac surgery with cardiopulmonary bypass.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

The primary aim was to correlate admission IVC O 2 and SVC O 2 . Secondary aims included: correlate flank or cerebral near-infrared spectroscopy with IVC O 2 and SVC O 2 , respectively, and exploratory analysis to evaluate associations between oximetry data and a composite adverse outcome defined as any of the following: increasing serum lactate or vasoactive support at 2 hours post-admission, cardiac arrest, or mortality. Admission IVC O 2 and SVC O 2 correlated ( r = 0.54; p < 0.001). However, IVC O 2 measurements were significantly lower than paired SVC O 2 (mean difference, -6%; 95% CI, -8% to -4%; p < 0.001) with wide variability in sample agreement. Logistic regression showed that each 12% decrease in IVC O 2 was associated with a 12-fold greater odds of the composite adverse outcome (odds ratio [OR], 12; 95% CI, 3.9-34; p < 0.001). We failed to find an association between SVC O 2 and increased odds of the composite adverse outcome (OR, 1.8; 95% CI, 0.99-3.3; p = 0.053). In an exploratory analysis, the area under the receiver operating curve for IVC O 2 and SVC O 2 , and the composite adverse outcome, was 0.85 (95% CI, 0.77-0.92) and 0.63 (95% CI, 0.52-0.73), respectively. Admission IVC O 2 had strong correlation with concurrent flank near-infrared spectroscopy value ( r = 0.74; p < 0.001). SVC O 2 had a weak association with cerebral near-infrared spectroscopy ( r = 0.22; p = 0.02).

CONCLUSIONS

In postoperative neonates, admission IVC O 2 and SVC O 2 correlate. Lower admission IVC O 2 may identify a cohort of postsurgical neonates at risk for low cardiac output and associated morbidity.

Validation of intracardiac shunt using thoracic bioimpedance and inert gas rebreathing in adults before and after percutaneous closure of atrial septal defect in a cardiology research unit: study protocol

INTRODUCTION

Intrathoracic shunt quantification is a major factor for appropriate clinical management of heart and pulmonary diseases. Intracardiac shunts quantified by pulmonary to systemic output ratio (Qp/Qs) are generally assessed by Doppler echocardiography, MRI or catheterisation. Recently, some authors have suggested the concomitant use of thoracic bioimpedance (TB) and inert gas rebreathing (IGR) techniques for shunt quantification. The purpose of this study is to validate the use of this approach under conditions where shunt fraction is directly quantified such as in patients with isolated atrial septal defect (ASD).

METHODS AND ANALYSIS

This trial is a prospective, observational single-centre, non-blinded study of adults seen for percutaneous closure of ASD. Qp/Qs ratio will be directly measured by Doppler echocardiography and direct Fick. IGR and TB will be used simultaneously to measure the cardiac output before and after closure: the ratio of outputs measured by IGR and TB reflecting the shunt fraction. The primary outcome will be the comparison of shunt values measured by TB-IGR and Doppler echocardiography.

ETHICS AND DISSEMINATION

The study has been approved by an independent Research Ethics Committee (2017-A03149-44 Fr) and registered as an official clinical trial. The results will be published in a peer-reviewed journal.

TRIAL REGISTRATION NUMBER

NCT03437148; Pre-results.

Efficacy and safety of transcatheter closure in adults with large or small atrial septal defects

Background

In most patients with secundum atrial septal defects (ASD), transcatheter closure is the preferred treatment strategy, but whether device size affects clinical outcomes is unknown. We sought to study the efficacy and safety of large closure devices compared to the use of smaller devices.

Methods

Using a single-center, prospective registry of adult patients undergoing transcatheter ASD closure, patients receiving a large closure device (waist diameter ≥25 mm, n = 41) were compared to patients receiving smaller devices (waist diameter ≤24 mm, n = 66). We analyzed pre-interventional clinical, hemodynamic and echocardiographic data, interventional success and complication rates, and 6-month clinical and echocardiographic outcomes. The primary efficacy outcome was successful ASD closure achieved by a single procedure and confirmed by lack of a significant residual shunt at 6 months. The primary safety outcome was a composite of device embolization, major bleeding, and new-onset atrial arrhythmia occurring within 6 months.

Results

Transcatheter ASD closure using large devices was successful in 90 % compared to 97 % of patients receiving smaller devices as defined by the primary efficacy outcome (p = 0.20). The primary safety outcome occurred in 4 patients of the large and 6 patients of the small device group, resulting in an event-free rate of 90 and 91 %, respectively (p = 0.89). Similar significant symptomatic improvement was observed in both treatment groups after 6 months, indicated by a 50 % increase in the fraction of patients in NYHA class I (p < 0.0001 vs. baseline).

Conclusions

Transcatheter closure in this cohort of patients with large or small ASD was effective with similar complication rates during short-term follow-up irrespective of the size of the implanted device.

Direct en face imaging of secundum atrial septal defects by velocity-encoded cardiovascular magnetic resonance in patients evaluated for possible transcatheter closure

BACKGROUND

Atrial septal defect (ASD) flow can be measured indirectly by velocity-encoded cardiovascular magnetic resonance (veCMR) of the pulmonary artery and aorta. Imaging the secundum ASD en face could potentially enable direct flow measurement and provide valuable information about ASD size, shape, location, and proximity to other structures.

METHODS AND RESULTS

Forty-four patients referred for possible transcatheter ASD closure underwent a comprehensive standard evaluation, including transesophageal and/or intracardiac echocardiography and invasive oximetry. CMR was performed in parallel and included direct en face veCMR after an optimal double-oblique imaging plane was determined that accounted for ASD flow direction and cardiac-cycle interatrial septal motion. ASD flow measured by direct en face veCMR correlated better with invasive oximetry than indirect (pulmonary artery and aorta) veCMR (r=0.80 versus r=0.66). Additionally, 95% limits of agreement were narrower (+/-3.9 versus +/-5.1 L/min). En face veCMR determined that defects usually were eccentrically shaped (major/minor axis length >1.5) rather than circular, with 16% having extreme eccentricity (major/minor >2.0). Overall, ASD size by both veCMR and intracardiac echocardiography correlated with final device size; however, in small to medium defects (<3 cm(2)) and extremely eccentric defects, veCMR correlated better with final device size than did intracardiac echocardiography. Importantly, CMR identified additional information in 9 patients (20%) that altered clinical management. Specifically, en face veCMR detected additional defects (n=3), large ASD with insufficient rim tissue (n=2), and sinus venosus defect with anomalous pulmonary vein (n=1). Cine and/or morphological imaging detected interrupted inferior vena cava (n=2) and sinus of Valsalva aneurysm (n=1).

CONCLUSIONS

En face veCMR with an optimized imaging plane can determine ASD flow, size, and morphology. CMR provided information incremental to comprehensive standard evaluation that altered clinical management in 20% of patients.

Evaluation of intracardiac shunts with cardiac magnetic resonance

Intracardiac shunts including atrial septal defect, ventricular septal defect, endocardial cushion defects, and surgical baffles may be identified, localized, and quantified using cardiac MRI methods. Both dark-blood and bright-blood techniques are helpful to identify anatomy. Contrast enhancement is especially useful for identifying associated vascular anomalies. Dynamic first-pass contrast agent signal-time studies may demonstrate rapid recirculation and shunting. Volumetric and phase contrast cine methods are useful to quantify flow. Pulmonary to systemic (Qp/Qs) flow ratios may be calculated noninvasively by comparing the pulmonary artery flow to the aortic flow measurement.

Relation of cerebral tissue oxygenation index to central venous oxygen saturation in children

OBJECTIVE

To evaluate the relationship between the cerebral tissue oxygenation index measured by near-infrared spectroscopy and central venous oxygen saturation (SvO2) after corrective surgery of congenital heart defects in children.

DESIGN

Prospective observational clinical study.

SETTING

A tertiary neonatal and paediatric intensive care unit for paediatric cardiology.

PATIENTS

Neonates and children consecutively admitted to the paediatric cardiology intensive care unit after corrective surgery of non-cyanotic congenital heart defects.

MEASUREMENTS AND RESULTS

Forty-three children were studied. Cerebral tissue oxygenation index, measured non-invasively by near-infrared spectroscopy, was compared to SvO2, measured by a catheter placed in the right atrium, and to haemodynamic and respiratory parameters. Pearson's correlation coefficients and p values were calculated. Simultaneously measured values for SvO2 (62.2+/-9.8%, 39.8-80.4%) and cerebral tissue oxygenation index (56.7+/-8.8%, 35.8-71.2%) showed a significant correlation ( r=0.52, p<0.001).

CONCLUSION

Cerebral tissue oxygenation index and SvO2 are not interchangeable parameters, but cerebral tissue oxygenation index reflects the haemodynamic influence on cerebral oxygenation after cardiovascular surgery. Further work is necessary to confirm the clinical role of continuous non-invasive measurement of cerebral tissue oxygenation index with regard to the variations of global systemic oxygen consumption after cardiac surgery in children.

Noninvasive Quantification of Left-to-Right Shunt in Pediatric Patients

Background —Blood flow can be quantified noninvasively by phase-contrast cine MRI (PC-MRI) in adults. Little is known about the feasibility of the method in children with congenital heart disease.

Methods and Results —In 50 children (mean age 6.2 years, range 1.1 to 17.7 years) with an atrial- or ventricular-level shunt, blood flow rate in the great vessels was determined by PC-MRI, and the ratio of pulmonary to aortic flow (Q̇p/Q̇s) was compared with Q̇p/Q̇s by oximetry. We found a difference of 2% and a range of −20% to +26% (limits of agreement, mean±2 SD). In another 7 children with congenital heart disease but no cardiac shunting (mean age 7.9 years, range 1.3 to 13.5 years), Q̇p/Q̇s by PC-MRI was 1.02 (SD ±0.06). No difference between systemic venous and aortic flow volumes was found (range −17% to +20%, n=37). Blood flow through a secundum atrial septal defect as assessed by PC-MRI (n=24) overestimated the shunt compared with the difference between pulmonary and aortic flows. The mean difference between 3 repeated PC-MRI measurements in each location was 5.3% (SD ±4.0%, n=522), demonstrating good precision. The interobserver variability was low. The accuracy of PC-MRI was confirmed by in vitro experiments.

Conclusions —Determination of Q̇p/Q̇s by PC-MRI in children is quick, safe, and reliable compared with oximetry. Systemic venous flow can be quantified by PC-MRI, whereas through-plane shunt measurement within an atrial septal defect is inaccurate.

A critical reappraisal of the oximetric assessment of intracardiac left-to-right shunting in adults

Although the oximetric analysis of blood from the right heart chambers is the most commonly used method for assessing the presence of intracardiac left-to-right shunting, the data the analysis is based on are limited. In addition, uncertainty exists concerning the best way of estimating the mixed venous oxygen content in subjects with intraatrial left-to-right shunting. In 102 adults without left-to-right shunting, blood was obtained from the venae cavae and right heart chambers to measure oxygen content. The limits of normality of oxygen content differences were 0.5 ml/dl from venae cavae to right atrium, 0.6 ml/dl from right atrium to right ventricle, and 0.9 ml/dl from right ventricle to pulmonary artery. The pulmonary arterial oxygen content was best estimated by combining the superior and inferior vena caval oxygen contents according to the formula (2[SVC] + 3[IVC]) divided by 5, where SVC is the superior vena cava and IVC is the inferior venae cava. These data provide new oximetric criteria for establishing the presence of intracardiac left-to-right shunting in adults.

The modified Fontan procedure: physiology and anesthetic implications

The modified Fontan operation has gained wide acceptance as a functional corrective procedure for patients with CHD with single ventricle physiology. Long-term survival and palliation of symptoms are excellent with most patients able to lead normal lives. The absence of a pulmonary contractile ventricle means that the single ventricle is responsible for perfusion of both the pulmonary and systemic circulations. Elevated systemic venous pressure is required to overcome PVR and this state of systemic venous hypertension has a significant impact on the anesthetic and postoperative care of these patients.

Doppler color-flow imaging assessment of shunt size in atrial septal defect

Two-dimensional echocardiography and pulsed-Doppler studies have not proved to be reliable methods of assessing left-to-right shunt size in atrial septal defect. Doppler color-flow imaging displays the transatrial jet, providing a new dimension with the potential capability of quantifying left-to-right shunt size. Twenty-three patients with atrial septal defect were studied by color-flow imaging and cardiac catheterization. The defect size measured by two-dimensional echocardiography, the maximal color-flow jet width in the atrial septum, and the maximal color-flow jet area in the right atrium were correlated with cardiac catheterization-derived left-to-right shunt size. Correlation coefficients were 0.57 (p less than 0.01), 0.67 (p less than 0.001), and 0.65 (p less than 0.01), respectively. Atrial septal color-flow jet width distinguished patients with less than a 2:1 left-to-right shunt size ratio (eight patients, jet width less than 15 mm in in all) from patients with greater than a 2:1 left-to-right shunt size ratio (15 patients, jet width greater than 15 mm in all). These results indicate that Doppler color-flow imaging can distinguish left-to-right shunt size in atrial septal defect accurately enough to influence decisions with regard to subsequent patient management.

Bedside nuclear probe for detection and quantification of left to right intracardiac shunts

A cadmium telluride nuclear probe with an Elscint gamma camera was used to detect and measure left to right intracardiac shunts at the bedside in 34 patients. Fifteen also had right heart catheterisation and oximetric measurement of the shunt. For the nuclear technique 740 MBq (20 mCi) of technetium-99m pertechnetate was injected into the right antecubital vein and the pulmonary to systemic flow ratio (QP:QS) was measured by the gamma variate technique. Data were not obtained in four patients because the nuclear probe failed in three patients and one storage disc was corrupted. Data from the gamma camera were lost in another patient. When the size of the shunt measured by the nuclear probe was compared with that measured by the oximetric technique the mean difference (SD of mean difference) was 0.36 (SD 0.78) and when it was compared with the gamma camera it was 0.08 (SD 0.67). Analysis of scatter plots showed that the larger the shunt, the larger the discrepancy. Twenty four of 29 data sets showed complete agreement between the nuclear probe and gamma camera on the size of the shunt. Any differences were small. These data indicate that left to right intracardiac shunts may be measured accurately by a nuclear probe at the bedside in either the coronary care unit or outpatient department.

Accuracy of Doppler echocardiography in quantification of left to right shunts in adult patients with atrial septal defect

In previous experimental and pediatric studies, the ratio of pulmonary to systemic flow (Qp/Qs) was accurately estimated by Doppler echocardiography in various cardiac shunt lesions. The purpose of this study was to assess the accuracy of pulsed Doppler echocardiography in determining the magnitude of shunt flow in adult patients with an ostium secundum type atrial septal defect. In 32 patients with high quality echocardiograms and excellent Doppler signals, blood flow was measured in the right and left ventricular outflow tract by Doppler echocardiography. In 16 patients without heart disease, the correlation (r) between systemic (Qs) and pulmonary (Qp) blood flow was 0.96 (SEE = 0.417 liter/min, y = 1.05x - 0.21) and the mean Qp/Qs ratio was 1.01 +/- 0.09. In 16 patients with an atrial septal defect, the Qp/Qs ration measured by oximetry ranged from 1.34 to 4.61 and by pulsed Doppler echocardiography from 1.31 to 4.46 (p = NS). In these 16 patients, the correlation between the Qp/Qs ratio determined by oximetry and pulsed Doppler echocardiography was significant (r = 0.82, SEE = 0.54). In the total group of 32 patients, the correlation was stronger (r = 0.93, SEE = 0.37). Systematic differences between the invasive and noninvasive shunt calculations did not occur. Thus, in adult patients with an atrial septal defect of the secundum type and high quality echocardiograms, the magnitude of left to right shunt can be accurately assessed by pulsed Doppler echocardiography. In the absence of pulmonary hypertension, pulsed Doppler echocardiography provides precise information for the decision to undertake conservative or operative treatment.

Functional and anatomical correlates in atrial septal defect. An echocardiographic analysis

The results of cross sectional echocardiography, intracardiac contrast echocardiography, and balloon sizing techniques and conventional haemodynamic assessment were correlated in 40 consecutive patients evaluated for an isolated left to right shunt at atrial level. Echo free areas along the septum were identified in 23 of 25 patients with a secundum defect, but not in two with a fenestrated defect, and in the upper atrial septum in three of four patients with a sinus venosus defect. No false positive results occurred in 11 patients with a probe patent foramen ovale. Saline contrast injection into the left atrium showed significant left to right shunting in all patients with atrial septal defect; inferior vena caval injection produced right to left shunting in 15 of 29 patients and a negative contrast effect in eight of 29 patients with an atrial septal defect, although neither correlated quantitatively with defect diameter or magnitude of the left to right shunt. Echocardiographic assessment of defect size as small, moderate, or large showed a highly significant correlation with balloon measurement of defect diameter, although some overlap between the groups was evident. In contrast, the correlation between defect diameter and pulmonary to systemic blood flow ratio was poor, mainly because of highly variable shunting in patients with an anatomically large defect. Cross sectional echocardiography has high sensitivity and specificity in the diagnosis of the non-fenestrated atrial septal defect and provides quantitative information about defect diameter. Contrast studies do not add to the diagnostic value of imaging from the subcostal position. The poor correlation between defect size and the measured shunt suggests that the latter may not be the best criterion for surgical management and that size could be an important factor likely to influence both the long term prognosis and the decision for closure.

Superiority of radionuclide over oximetric measurement of left to right shunts

In 100 children with suspected left to right shunts the ratio of pulmonary to systemic flow was measured both by oximetry and first pass radionuclide angiography. The pulmonary time activity curve from the radionuclide study was analysed by the method of gamma variate fits. There was strong correlation between the two techniques; weaker correlation was found when the shunt was at atrial rather than ventricular level. This difference can be explained only by problems with the oximetric rather than the radionuclide technique. Although there are important limitations to the radionuclide method, it is the more precise and less invasive of the two and is to be preferred when the accurate measurement of left to right shunts is required.

Persistence of ductus arteriosus with left to right shunt in the older patient

Eight hundred and four patients with persistence of the ductus arteriosus were seen in Edinburgh between 1940 and 1979. Thirty-seven of them reached the age of 50 years, and in 32 the shunt was exclusively from left to right. Fifteen of the 32 were subsequently treated surgically. None of the 32 was lost to follow-up. Duration of clinical observation averaged 17 years and extended to over 30 years in eight patients. Their features have been correlated with those from reports of 48 comparable patients in an attempt to clarify the management of the persistent ductus in the older patient. Impairment of left ventricular function is shown as the major risk, even when the ductus is small. Bacterial endarteritis is infrequent. Surgical treatment carries greater risk than in childhood and early adult life but usually reduces heart size and restores exercise tolerance. Left ventricular dysfunction, however, occasionally vitiates the benefits; symptoms are then incompletely relieved and death from heart failure may occur months or years after operation. Experience in older patients thus emphasises the value of elective operation in childhood, however well the child, however trivial the shunt. It is concluded that in older patients, the presence or the development of symptoms or cardiac enlargement are almost always indications for surgical treatment. As age increases, especially by the eighth decade, medical treatment may be preferable. Continued follow-up of symptomless patients without cardiomegaly is important because increase in heart size usually precedes further deterioration which can then be prevented by timely surgical treatment.

Physiologic effects of increasing hemoglobin concentration in left-to-right shunting in infants with ventricular septal defects

We studied the acute effects of increasing hemoglobin concentration and hematocrit on the pulmonary and systemic circulations of nine infants with large left-to-right shunts. After isovolemic exchange transfusion, which was designed to raise hemoglobin but keep blood volume constant, a consistent rise in systemic and pulmonary vascular resistances occurred. This rise was comparable to those previously found in isolated circulations showing a linear relation between hematocrit and loge of the vascular resistance. These changes in resistance were accompanied by decreases in systemic and pulmonary blood flow and a marked decline in left-to-right shunt. Despite the decrease in systemic blood flow, there was no decline in systemic oxygen transport, and there may have been a marginal decrease in left ventricular stroke work. These observations help explain why the newborn with a large ventricular septal defect and a high hemoglobin concentration does not have clinical signs of a large left-to-right shunt, and also suggest that the postnatal decline in hematocrit has a substantial role in the normal fall in pulmonary vascular resistance after birth.

Systolic time intervals in isolated ventricular septal defect in the adult

Little information is available on the length of the systolic time intervals in adult patients with isolated ventricular septal defects (VSD). In the present study the external carotid pulse and the phono- and electrocardiogram were recorded in 17 patients, mean age 29 years, with angiographically proved VSD. They had unidirectional left-to-right shunts with ratios of pulmonary-to-systemic blood flow (Qp/Qs) of 1-5.22. Their right ventricular pressures were normal or only moderately elevated. Left ventricular ejection time was consistently abbreviated, the degree of abbreviation relating significantly with Qp/Qs (r = -0.70, p less than 0.01). The preejection period was prolonged but the relationship between its degree of prolongation and Qp/Qs did not reach statistical significance (r = 0.4), p greater than 0.05). The relationship between the preejection period/left ventricular ejection time ratio (PEP/LVET) and Qp/Qs was statistically significant (r = 0.51, p less than 0.05). We conlcude that in adult VSD patients with normal right ventricular pressures, a hemodynamically important shunting, i.e., Qp/Qs above 1.4 or left-to-right shunt exceeding 30% of pulmonary blood flow, may be excluded in the presence of a normal left ventricular ejection time or a normal PEP/LVET ratio.

Relation between caval and pulmonary artery oxygen saturation in children

The flow relation between that in the superior vena cava and inferior vena cava was studied in order to estimate the oxygen saturation of mixed venous blood in connection with heart catheterisation of children between the ages of 5 and 8 years without cardiac shunts. The investigation includes a group of 19 children examined during halothane anaesthesia and a group of 40 examined under local anaesthesia. The groups were comparable in respect of age and body surface. The results show that the oxygen saturation of mixed venous blood (MVB) is higher under general anaesthesia than during local anaesthesia, and that the oxygen saturation in the former group is highest in the superior vena cava (SVC) and lowest in the inferior vena cava (IVC), while the reverse is the case during examination under local anaesthesia. Based on multiple regression analysis, it is advisable, for children between the ages of 5 and 8 years, to use the formula MVBcalc = -0.85 + 0.81 SVC + 0.18 IVC for the estimation of the oxygen saturation in the mixed venous blood in children under general anaesthesia and the formula MVBcalc = 10.21 + 0.52 SVC + 0.34 IVC for children during local anaesthesia. When estimating mixed venous blood in younger children and during general anaesthesia or both, more emphasis should be laid on the flow through the superior vena cava.

Calculation of oxygen saturation of mixed venous blood in infants

A retrospective survey of the heart catheterizations of twenty-two children below the age of three years was carried out in order to evaluate different formulae for calculation of the oxygen saturation in mixed venous blood (MVB) from the oxygen saturation in blood from the superior caval vein (SVC) and inferior caval vein (IVC). The formula MVB = (3SVC + IVC)/4 was found satisfactory, also during halothane anesthesia.

Afterload reduction in the management of postinfarction ventricular septal defect

The primary goal in the medical management of ventricular septal defect complicating myocardial infarction is to support cardiac function and control symptoms, if possible, for a period of 4 to 6 weeks. If the patient survives this period, surgical correction of the defect is technically easier and safer. In many cases, However, cardiac function is severly compromised, intractable biventricular failure develops,early operation is necessary and the likelihood of successful repair is diminished.

A comparison of formulas used to estimate mixed venous saturations

True mixing of venous blood in the absence of shunt occurs in the pulmonary artery. In the presence of left to right shunt at a level proximal to the pulmonary artery, mixed venous blood for oxygen saturation (MVO2) is estimated by using an average of blood samples taken from the chamber proximal to the shunt. In atrial septal defect, the determination of MVO2 is calculated by using blood samples from the superior vena cava (SVC) and the inferior vena cava (IVC). Several formulas have been proposed, utilizing varying combinations of blood samples taken from the SVC and IVC. In the present investigation, 100 patients without evidence of shunt were studied during routine cardiac catheterization. Duplicate blood samples were taken from the pulmonary artery (PA), the SVC, and the IVC, and were analyzed for oxygen-saturation. If one assumes that the PA blood sample represents true venous blood mixing (TMVO2), the following formulas were used for comparison: 1)PA = SVC; 2) PA = IVC; 3) PA = (SVC + IVC)/2; 4) PA = (2SVC + IVC)/3; 5) PA = (3SVC + IVC)/4; and 6) PA = (2IVC + SVC)/3. When one uses the standard two variable regression equations, this study shows that the 90% confidence limits are wide. The correlation, however, is somewhat better if one uses the formulas 3)-6). Therefore, the error that may be introduced in calculating the TMVO2 may be substantial and can critically alter the estimation of the shunted blood volume.