Monitoring Of Mixed Venous Oxygen Saturation Research Articles

Relation between Mixed Venous Oxygen Saturation and Cardiac Index: Nonlinearity and Normalization for Oxygen Uptake and Hemoglobin

The ability of mixed venous oxygen saturation (SvO₂) monitoring to reflect changes in cardiac index (CI) with therapy in critically ill patients is unclear. To this end, SvO₂and CI were measured before and during an infusion of enoximone and/or dobutamine in 30 patients with advanced heart failure. A nonlinear relationship was observed between SvO₂ and CI with the nonlinear correlation coefficient being 0.52. On normalizing for individual differences in hemoglobin and oxygen consumption, this correlation coefficient became 0.90. Further analysis of individual data was performed using linear regression, and the slopes and correlation coefficients were found to span a wide range (slope: −10.0 to 30.9 min-mVL, r: −0.27 to 0.99). However, the mean slope and correlation coefficient for patients with baseline CI and SvO₂ <21/min/m* and <55 percent were 18.3 min-m²/L and 0.87, respectively, while those for the remainder of patients were only 3.1 min-m²/L and 0.42, respectively. Thus, the nonlinear correlation coefficient of the SvO₂-CI relationship in a group of patients is dependent on the homogeneity of their oxygen consumption and hemoglobin concentration. Furthermore, the ability of SvO₂to serve as a therapeutic indicator in any given patient is dependent on baseline SvO₂ and CI.

Continuous mixed venous oxygen saturation monitoring as a patient assessment tool

The measurement of continuous mixed venous oxygen saturation (S v0 2) monitoring demonstrates the interaction between oxygen supply and demand. This paper will firstly, examine the methods used to monitor continuous S v0 2. Secondly, by utilizing a modified Fick equation, the pathophysiology of oxygen supply/demand dynamics in relation to S v0 2 monitoring will be discussed. Finally, the author will attempt to elucidate the clinical significance of S v0 2 monitoring in the critical care environment.

Con: Perioperative continuous monitoring of mixed venous oxygen saturation should not be routine in high-risk cardiac surgery

M OST CLINICIANS accept that invasive hemodynamic monitoring facilitates diagnosis and treatment of complex cardiovascular disorders, and would use a pulmonary artery catheter (PAC) in the management of patients during high-risk cardiac surgery. Perioperative mixed venous oxygen saturation (SSO,) monitoring via a fiberoptic PAC oximeter seems a small additional step to take in these patients, but this step cannot yet be justified. The increasing cost of health care demands the critical evaluation of all new monitoring technologies and places us more and more in the unfamiliar position of an epidemiologist assessing screening techniques. The problem in question should be common and important, with an accepted backup test, and it should have a well-defined transition from a latent to an overt phase, with evidence that early detection will reduce morbidity or mortality. The test should have an acceptable sensitivity and specificity, and the costs of testing must be balanced against competing health care priorities. Just as laboratory screening has in its time been intuitively felt to be a good thing, without need of rigorous proof, there is a similar danger of uncritical acceptance of new patient monitoring technology. The incorporation of continuous SVO, measurement as an option with PAC use has produced interesting physiological insights, and thus has made the transition to “a major advance,” without fulfilling any of the previous criteria. The authors believe that its usefulness is still unproven in any area of patient care, and that anecdotal reports of mishap detection with its use are insufficient reason for its adoption. The problem is that cardiac output (CO), a measurement of total flow, does not reflect adequacy of tissue perfusion. Just as arterial CO, tension is the parameter that determines the adequacy of minute ventilation in relation to metabolic rate, SSO, was considered to be the parameter by which to assess the adequacy of tissue oxygen supply, and thus indirectly the adequacy of CO, in relation to demand. The validity of SSO, monitoring as an indirect correlate of CO is based on certain assumptions, in particular the body’s ability to maintain a constant oxygen consumption (q02) by compensating for decreases in delivery (DO,) through increased 0, extraction. Oxygen consumption equals the product of CO and arteriovenous oxygen difference:

Continuous mixed venous oxygen saturation monitoring during general anesthesia.

Clinical evaluation of continuous SvO2 monitoring during general anesthesia was made in 21 surgical patients utilizing a fiberoptic reflectometry system combined with a pulmonary artery flow-directed balloon catheter. On-line in vivo values for SvO2 by the system were closely related to those obtained in vitro from a Radiometer ABL-300. There was a good correlation between changes of in vivo SvO2 and corresponding changes in cardiac index. We also observed that there were significant correlations between SvO2 and oxygen extraction ratio, and SvO2 and oxygen delivery. These data indicate that continuous monitoring of SvO2 during general anesthesia can provide on-line information not only about hemodynamic state but also on oxygen transport, which will be especially helpful in managing hemodynamically unstable patients during anesthesia.

Central mixed and splanchnic venous oxygen saturation monitoring.

Central mixed venous oxygen saturation (SvO2) monitoring in critically ill patients to estimate adequacy of peripheral perfusion is gaining increasing popularity. However, a number of unexpected responses, one of which is marked depression of regional (splanchnic) venous oxygen saturation which may coexist with normal or high SvO2, makes interpretation of this parameter difficult. The SvO2 and hepatic venous oxygen saturation levels in seven injured (postoperative) and 15 septic patients were measured. No substantial differences between central and hepatic venous oxygen saturation was noted in nonseptic patients, however, septic subjects exhibited a normal SvO2 of 70.5% +/- 8.7% at a time when the hepatic venous saturation was 55.6% +/- 14.4% which is a significant (p less than 0.05) reduction. This reduced oxygen saturation was noted to arise from an increased regional metabolic rate rather than reduced perfusion. Nevertheless, we conclude that a flow limited regional oxygen consumption may potentially exist despite the presence of a normal SvO2 in certain patient subgroups such as septic subjects. Therefore, a normal SvO2 should not be considered as sole criteria to insure optimal oxygen delivery in critically ill patients.