MIXED VENOUS OXYGEN SATURATION
Fick's equation: SvO2 = SaO2 -VO2 / 13.9 x Q x [Hb]
Continuous SVO2 monitoring allows the minute-to-minute assessment of total tissue oxygen balance (i.e., the relationship between oxygen delivery and oxygen consumption). SVO2 varies directly with cardiac output, Hb, and SaO2, and inversely with VO2 (oxygen consumption.). The normal SVO2 is 75%, which indicates that under normal conditions, tissues extract 25% of the oxygen delivered. An increase in VO2 or a decrease in arterial oxygen content (SaO2 x Hb) is compensated by increasing CO or tissue oxygen extraction. When the SVO2 is less than 30%, tissue oxygen balance is compromised, and anaerobic metabolism ensues. A normal SVO2 does not ensure a normal metabolic state but suggests that oxygen kinetics are either normal or compensated.
Complex peripheral vascular surgical procedures often require invasive hemodynamic monitoring. Direct measurement of the arterial pressure is regarded as a necessity. However, it is not always possible to attain arterial access or even noninvasive blood pressure monitoring. When a complex case must be undertaken without the availability of arterial blood pressure monitoring, an alternative continuous monitor is advisable. Although it is seldom considered useful intraoperatively, mixed venous saturation (SvO2) is a continuous monitor of a patient's overall oxygen delivery status.
Current anesthesia literature would indicate that oximetric PACs are not fully utilized during anesthesia. Intraoperative values of SvO2 are routinely well above normal. This is presumably due to the increased FiO2 and the anesthetic-induced decreased metabolism or the inability of tissues to extract oxygen. 1,2 Under general anesthesia, patients may have mixed venous saturations near or above 90%. At that level, small changes in the partial pressure of oxygen in the pulmonary artery will not be indicated by discernable changes in the saturation. At a normal mixed venous partial pressure oxygen of 40 torr and saturation of 75%, small changes in the oxygen tension will be easily noted with the corresponding changes in the saturation. As our cases showed, patients in poor cardiovascular condition may not have abnormally high venous saturations while undergoing anesthesia and surgery. Thus, changes in the cardiovascular status of our patients were easily reflected by changes in the mixed venous saturation. We were able to take advantage of this situation and safely monitor our patients through difficult surgical procedures.
The SvO2 is dependent on several variables. Changes in hemoglobin, cardiac output, arterial saturation, or tissue oxygen requirements can result in changes in the mixed venous saturation. As such, it is not a very specific indicator of a patients condition. As noted above, in many circumstances during anesthesia, the mixed venous saturation is not a very sensitive indicator of a patients condition. In reality, very few of our monitors are truly specific. The pulse oximeter may indicate a drop in arterial saturation, however, this could be due to a low inspired oxygen, a decrease in cardiac output, a fall in the temperature of the extremity, an injection of dye, or nail polish. The EKG can note tachycardia, but this may be due to pain, inadequate anesthesia, hypovolemia, or a medication we have just injected. Likewise, a fall in the heart rate can have as many causes. The blood pressure might be reduced with hypovolemia, tachycardia/bradycardia, decrease in venous return, or a relative overdose of an anesthetic. The pulmonary capillary wedge pressure (PCWP) might decrease with a fall in volume or because the cardiac output has increased. An increased PCWP could indicate a change in volume status or impending cardiac failure. Urine output may depend on volume status, previous diuretic therapy, cardiac output, or blood pressure.
None of these are really specific. Clinicians use their experience and assessment of all the variables before deciding on the cause of an alteration in their patients condition. Only then can the appropriate response be determined and carried out. The use of mixed venous saturation monitoring can provide the clinician with another source of information to facilitate proper management. Evaluation of a change in saturation is similar to the assessment given to changes noted on other monitors.
Other possible monitors such as a pulmonary artery catheter with continuous cardiac output capability or transesophageal echocardiography were not available at the times of these patients surgeries.
Anesthetic management of patients with severe peripheral vascular disease is always a challenge. Extensive surgery and the potential blood losses, in addition to the patients underlying condition makes continuous monitoring of hemodynamics mandatory. However, our usual sites for vascular access may not be available. Measurement of urine output as a marker of adequacy of perfusion and volume status may also not be possible. These two cases showed the efficacy of an oximetric pulmonary artery catheter as a continuous intraoperative monitor. The use of an oximetric PAC should be considered in those intraoperative situations when the continuous monitoring of a patients oxygen delivery status is required.
Baele PL, McMichan JC, Marsh HM, Sil JC, Southorn PA: Continuous mixed venous oxygen saturation in critically ill patients. Anesth Analg 61:513-7, 1982.
Birman H, Haq A, Hew E, Aberman A: Continuous monitoring of mixed venous oxygen saturation in hemodynamically unstable patients. Chest 86:753-756, 1984.
Cariou A, Monchi M, Dhainaut JF. Continuous cardiac output and mixed venous saturation monitoring. J Crit Care 13:198-213, 1997.
Lugo G, Arizpe D, Dominguez G, Ramirez M, Tamariz O: Relationship between oxygen consumption and oxygen delivery during anesthesia in high-risk surgical patients. Crit Care Med 21:64-69, 1993.
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