The use of central venous oxygen sturation measurements in a coronary care unit

Monitoring of Tissue Oxygenation: an Everyday Clinical Challenge

Purpose of review

The aim of this article is to study the overview of pathophysiology and clinical application of central venous oxygen saturation monitoring in critically ill patients and during the perioperative period.

Recent findings

There are several clinical studies and animal experiments evaluating the effects of goal-directed hemodynamic stabilization on critically ill patients. Recent systematic reviews and meta-analyses found that advanced hemodynamic endpoints-targeted management has a positive effect on outcome in high-risk surgical patients. As all interventions aim to improve tissue oxygenation, it is of utmost importance to monitor the balance between oxygen delivery and consumption. For this purpose, central venous blood gas analysis provides an easily available tool in the everyday clinical practice. The adequate interpretation of central venous oxygen saturation renders the need of careful evaluation of several physiological and pathophysiological circumstances. When appropriately evaluated, central venous oxygen saturation can be a valuable component of a multimodal individualized approach, in which components of oxygen delivery are put in the context of the patients' individual oxygen consumption. In addition to guide therapy, central venous oxygen saturation may also serve as an early warning sign of inadequate oxygen delivery, which would otherwise remain hidden from the attending physician.

Summary

With the incorporation of central venous oxygen saturation in the everyday clinical routine, treatment could be better tailored for the patients' actual needs; hence, it may also improve outcome.

Single transpulmonary thermodilution and continuous monitoring of central venous oxygen saturation during off-pump coronary surgery

BACKGROUND

Off-pump coronary artery bypass grafting (OPCAB) requires thorough monitoring of hemodynamics and oxygen transport. Our aim was to find out whether therapeutic guidance during and after OPCAB, using an algorithm based on advanced monitoring, influences perioperative hemodynamic and fluid management as well as the length of post-operative ICU and hospital stay.

METHODS

Patients were randomized into two groups of hemodynamic monitoring: the conventional monitoring (CM) group (n=20) and the advanced monitoring (AM) group (n=20). In the CM group, therapy was guided by central venous pressure, mean arterial pressure (MAP) and heart rate (HR), and in the AM group by the intrathoracic blood volume index, MAP, HR, central venous oxygen saturation (ScvO(2)) and cardiac index (CI). The measurements were performed before and during surgery, and at 2, 4 and 6 h post-operatively.

RESULTS

In the AM group, colloids and dobutamine were given more frequently and were accompanied by increments in ScvO(2), CI and oxygen delivery compared with baseline. The percentage of ephedrine administration was higher in the CM group. The algorithm guided by AM decreased time until achieving the status of 'fit for ICU discharge' and post-operative hospital stay by 15% and 25%, respectively.

CONCLUSIONS

A goal-directed algorithm based on advanced hemodynamic monitoring and continuous measurement of ScvO(2) facilitates early detection and correction of hemodynamic changes and influences the strategy for fluid therapy that can improve the course of post-operative period after coronary artery bypass grafting on the beating heart.

Correlation between central venous oxygen saturation and oxygen delivery changes following fluid therapy

BACKGROUND

The rationale for using central venous oxygen saturation (ScvO(2)) in various clinical scenarios is that it reflects the balance between oxygen delivery (DO(2)) and demands. In this study, we evaluated the correlation between ScvO(2) and DO(2) changes (Delta Do(2), DeltaScvO(2)) in patients receiving fluid therapy following coronary surgery. We also correlated the changes of mean arterial pressure (Delta MAP) and central venous pressure (Delta CVP), with Delta DO(2).

METHODS

Sixty consecutive sedated and mechanically ventilated adult patients, with cardiac index <or=2.3 L/min/m(2) and a pulmonary artery occlusion pressure <or=12 mmHg following coronary surgery, were included. Concomitant hemodynamic parameters, arterial and venous blood gases were measured before (T0) and after (T1) administration of a 500 ml bolus of an isotonic crystalloid solution over 30 min. The correlations between Delta DO(2) and DeltaScvO(2), Delta MAP or Delta CVP were evaluated by linear regression analysis and Pearson test.

RESULTS

Cardiac index (1.9+/-0.2 vs 2.3+/-0.5 ml/min/m(2)), MAP (83+/-11 vs 94+/-13 mm Hg) and CVP (5.7+/-3 vs 7.1+/-3 mmHg) were significantly higher at T1 compared with T0. The correlation of Delta DO(2) with DeltaScvO(2) was positive, significant (r=0.41; P=0.004) and superior to its correlation with Delta MAP (r=0.30; P=0.01) or Delta CVP (r=0.03; P=0.78).

CONCLUSION

A significant correlation between ScvO(2) and DO(2) changes was found in patients receiving fluid therapy following coronary surgery. ScvO(2) could be used as an indicator to track DO(2) and to guide volume loading.

The value of venous oximetry

PURPOSE OF REVIEW

Maintenance of adequate tissue oxygenation is an important task in intensive care units. In this context, venous oximetry by obtaining mixed venous oxygen saturation or central venous oxygen saturation has been discussed as useful monitoring parameters. This review discusses the physiology and clinical application of these parameters.

RECENT FINDINGS

No study has so far demonstrated that venous oxygen saturation monitoring can reduce mortality in critically ill patients although length of stay has been decreased in cardiac surgery patients. Furthermore, pulmonary artery catheter usage does not affect outcome in critically ill patients. In contrast, early goal directed therapy for patients with severe sepsis or septic shock, which includes treatment goals for mean arterial pressure, central venous pressure, and central venous oxygen saturation, was able to increase survival in these patients. There is also evidence that central venous oxygen saturation measurement is beneficial in other types of shock.

SUMMARY

Early goal directed therapy should be implemented in the initial resuscitation of septic patients. Measurement of central venous oxygen saturation can easily be applied in intensive care unit patients and offers a useful indirect indicator for the adequacy of tissue oxygenation.

Transcutaneous oxymetry measurements during acute blood loss

Transcutaneous PO2 (TcPO2) was measured in healthy adult blood donors to test the sensitivity of this method as a noninvasive means of diagnosing occult blood loss. TcPO2, the energy required to heat the electrode (MW), and postural changes in blood pressure and pulse, were measured before and after a 450-ml blood donation. There was a significant increase (P less than .005) in postural pulse of 4.8/min, but no significant change (P greater than .05) in postural blood pressure. There was no significant change (P greater than .05) in TcPO2, but there was a significant decrease (P less than .001) in MW. These MW changes probably reflect physiologic changes associated with blood loss.

Continuous monitoring of right atrial oxygen tension in patients with myocardial infarction

Right atrial oxygen tension (RAPvO2) was measured continuously in 26 patients admitted to a coronary care unit with acute myocardial infarction. A catheter incorporating a Clark type oxygen sensor at its tip was inserted percutaneously into the right atrium. Insertion was simple, safe and comparable to the introduction of a standard central venous pressure line. RAPvO2 correlated well with the patients' clinical condition and reflected both cardiac and pulmonary function. When breathing air 11 of the patients had sustained RAPvO2 levels of less than 34 mmHg (4.53 kPa). In this group there were eight deaths. Fifteen patients had an RAPvO2 greater than 34 mmHg (4.53 kPa) except for transient falls related to movement and in this group there were no deaths (p less than 0.002). The correction of arterial hypoxaemia by oxygen therapy raised RAPvO2 and lowered the heart rate. In some patients Dopamine and transvenous pacing raised RAPvO2 and could be adjusted with reference to the continuous recording. Movement often caused marked falls in RAPvO2 especially in the seriously ill. Evidence relating RAPvO2 to mixed venous oxygen and tissue oxygen is reviewed.

Detection of pulmonary edema in acute myocardial infarction

To evaluate methods for detecting pulmonary edema, pulmonary extravascular water volume was measured at 24 hour intervals (total 72 hours) in 25 patients with acute myocardial infarction. Measured lung water was compared with results of clinical, blood gas, X-ray and hemodynamic methods for detecting pulmonary edema. Increased pulmonary extravascular water volume on one or more measurements was observed in 18 of the 25 patients and was associated with an abnormal chest radiograph and increased pulmonary arterial wedge, pulmonary arterial diastolic and right atrial pressures. It was associated less well with clinical, blood gas and other hemodynamic measurements. Pulmonary arterial diastolic or pulmonary wedge pressure was a significant predictor of lung water 24 hours later. Both "preclinical pulmonary edema" and the "therapeutic phase lag" could be predicted from the pulmonary wedge pressure. Clinical, blood gas, radiographic and other hemodynamic measurements were not predictive.

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.

Blood-gas and hemodynamic responses to oxygen in acute myocardial infarction

Blood-gas (Pa o o2 ) and hemodynamic responses to the inhalation of oxygen were studied in 60 patients with acute myocardial infarction. Patients who were not in heart failure on admission and did not develop signs of heart failure within the next 5 days achieved the same Pa o o2 level while breathing 100% oxygen as did patients without acute myocardial infarction. Patients with pulmonary edema or cardiogenic shock had a very poor Pa o o2 response to oxygen inhalation. Patients in mild heart failure at the time of study and patients who developed heart failure subsequent to the study had a Pa o o2 response intermediate between the other two groups. This rise of Pa o o2 with oxygen correlated with the cardiac index and right atrial oxygen prior to inhalation of oxygen. Uncomplicated patients responded to inhalation of oxygen with a decrease of heart rate, cardiac index, stroke index, and cardiac work, and an increase of peripheral resistance. Patients in pulmonary edema or cardiogenic shock or with a low cardiac index or low Pa o o2 responded with only a slight increase in peripheral resistance. The Pa o o2 achieved while breathing oxygen appeared to determine the type of hemodynamic response to oxygen. Administration of oxygen to patients with acute myocardial infarction is useful in identifying latent heart failure and in predicting the subsequent clinical course of these patients.

Mixed venous oxygen saturation in relation to cardiac output in myocardial infarction

Mixed venous oxygen saturations derived from measurement of mixed venous oxygen tension were compared with dye dilution cardiac output determinations in 26 patients with acute myocardial infarction. Mixed venous oxygen saturation was greatly reduced in patients with shock or failure complicating myocardial infarction. The level of oxygen saturation correlated with cardiac output determinations. The measurement of mixed venous oxygen saturation, which is relatively simple and does not require elaborate equipment, should be an important aid to the rational treatment of patients with low output states complicating acute myocardial infarction.

Ultrasonic cardiac echography for determining stroke volume and valvular regurgitation

The ventricular dimensions of 51 patients with heart disease were determined by ultrasonic echography during cardiac catheterization. These data were used to calculate end-diastolic and end-systolic volumes and stroke volume, using a prolate ellipse as a geometric model of the left ventricle. In 30 patients without valvular regurgitation the stroke volumes determined by the echographic method were compared with those determined simultaneously by the standard Fick method with a correlation coefficient of r = 0.966. In 21 patients with valvular regurgitation, the severity of regurgitation was estimated by comparing the forward stroke volume determined by the Fick method with the total left ventricular stroke volume determined by the echographic method. These calculations of regurgitation correlated reasonably well with the degree of valvular regurgitation estimated from angiocardiographic study. It is suggested that these echographic determinations of stroke volume are an atraumatic, safe, and acceptable method in patients without valvular regurgitation. Moreover, these preliminary studies suggest that the severity of valvular regurgitation can be estimated by utilizing ultrasound echocardiography.

Critical assessment of use of central venous oxygen saturation as a mirror of mixed venous oxygen in severely ill cardiac patients

Bedside catheterization of the right heart with a percutaneously introduced flowdirected catheter was carried out in 24 critically ill patients; 71 determinations of mixed venous oxygen saturation (MVo 2 ) were obtained. A second catheter was inserted either into a central intrathoracic vein or the right atrium for simultaneous sampling of central venous (CVo 2 ) or right atrial (RAo 2 ) oxygen saturation.

For the group as a whole, mean CVo 2 (57.9% ± 15.25) was significantly greater than mean MVo 2 (53.3% ± 14.84) ( P <0.001), but there was no significant difference and correlation was good between changes in central venous compared to changes in mixed venous oxygen saturation. On the other hand, there was no significant mean difference as well as an excellent correlation between individual values of RAo 2 and MVo 2 (r=+0.95).

Patients with heart failure or shock showed poor correlation between CVo 2 and simultaneously determined MVo 2 . Furthermore, subjects with shock showed a mean CVo 2 (58.0%±13.05) that was significantly greater than mean MVo 2 (47.5% ± 15.11). In contrast, there was no significant difference between mean RAo 2 and mean MVo 2 and excellent correlation of individual values in patients with either heart failure or shock. Although CVo 2 is a poor reflection of MVo 2 in subjects with severe heart failure or shock, there was a better correlation between changes in CVo 2 with corresponding changes in MVo 2 . In addition, RAo 2 correlated closely with corresponding values of MVo 2 . The reversal of the normal relationship between CVo 2 and MVo 2 under these circumstances is compatible with the thesis that low output states are attended by redistribution of blood flow away from femoral, splanchnic, and renal circulation with preferential preservation of cerebral blood flow.