MIXED VENOUS OXYGEN MONITORING

Mixed venous O₂ saturation (Sv̄O₂ ) is related to a number of factors:





In Equation 23, [Hb] is the hemoglobin concentration (g/dL), 13.9 is a constant (i.e., O₂ combining power of Hb [mL/10 g]), and is cardiac output. It can be seen that low SaO₂ , low , low [Hb], or an elevated O₂ level may decrease SO₂ . All these conditions could produce impairment of O₂ delivery to the tissues. This single measurement (Sv̄O₂ ) may be uniquely helpful in detecting any condition that may result in impaired tissue oxygenation. Sv̄O₂ can be monitored by intermittent measurement of blood withdrawn through a pulmonary artery catheter or may be continuously monitored by using a pulmonary artery catheter equipped with fiberoptic bundles. These catheters have two fiberoptic bundles carrying an incident light beam and a reflected light beam. Using the fact that the reflected spectrum of hemoglobin depends on the degree of oxygenation, the appropriate calculations can be performed by the instrument and a continuous display of Sv̄O₂ provided. Low Sv̄O₂ (usually less than 60%) may sensitively reflect an abnormality of one or more of the factors on the right-hand side of Equation 23. However, several factors may artifactually elevate Sv̄O₂ , including wedging of the catheter and mitral regurgitation (which tend to bring the catheter tip into contact with arterialized blood), sepsis, and intracardiac or peripheral left-to-right shunts.

Monitoring of Pv̄O₂ instead of Sv̄O₂ requires additional considerations. Although Sv̄O₂ is a function of four variables (Equation 23), other factors acting on the Hb-O₂ binding curve may alter Pv̄O₂ independently. In Figure 36-12 , a normal curve is shown adjacent to curves depicting increased and decreased Hb-O₂ affinity. For the same SaO₂ and Sv̄O₂ (and the arteriovenous O₂ content difference), substantially different values for Pv̄O₂ occur under the three conditions. Correct interpretation of Pv̄O₂ values must therefore depend on the position of the Hb-O₂ dissociation curve, as dictated by pH, PCO₂ , body temperature, and erythrocyte 2,3-diphosphoglycerate (DPG) concentration.

Sv̄O₂ measurement has been carried one step further by Räsänen and coworkers,^[152] who have used this instrument (to measure Sv̄O₂ ) and a pulse oximeter (to measure SaO₂ ) to calculate shunt fraction continuously according to Equation 11. Räsänen and colleagues^[152] demonstrated that continuous reading of S/T using this principle could be used as a guide to providing an optimal level of continuous positive airway pressure (CPAP) in a group of patients whose tracheas were intubated and in whom lowest S/T was desired. The investigators found that

Figure 36-12 Hemoglobin-oxygen (Hb-O₂ ) saturation curves. Dotted lines indicate common arterial and venous values: SaO₂ = 90% and Sv̄O₂ = 70%. The middle curve (A₀ , V₀ ) represents the relationship with a pH of 7.40 and body temperature (T) of 37°C. The right-hand curve (A₁ , V₁ ) would occur if pH = 7.20 or T = 41°C. The left-hand curve (A₂ , V₂ ) represents the relationship when T = 33°C. Increased levels of any of the four factors shown (i.e., temperature, pH [H⁺ ], erythrocyte 2,3-diphosphoglycerate [2,3 DPG], and partial pressure of carbon dioxide [PCO₂ ]) decreases Hb-O₂ affinity (i.e., rightward shift); decreased levels raise Hb-O₂ affinity (i.e., leftward shift). The same arterial and venous Hb-O₂ saturation under different conditions can therefore result in a wide range of arterial and venous gas tension values.