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October 17-21, 2009 New Orleans, LA Home Abstract Archive Search Abstracts 2009 Session Grid 2009 Meeting Website Policy Statements ASA Website Feedback 
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A1471
October 21, 2009 9:00 AM - 11:00 AM Room Area F |
| Evaluation of SvO 2 Estimate Based on Inverse Fick Calculation |
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* Lara M. Brewer, M.S., Joseph A. Orr, Ph.D., Matthias Görges, M.S. Anesthesiology, University of Utah, Salt Lake City, Utah |
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Introduction:
Mixed venous oxygen saturation (SvO
2
) measurement can be used to assess the global oxygen supply-demand relationship. SvO
2
measurement requires blood gas analysis of mixed venous blood and placement of either a fiber optic central venous catheter or a pulmonary artery catheter. SvO
2
can be calculated using the inverse Fick equation if oxygen uptake (VO
2
), cardiac output (Q) hemoglobin (Hb), and arterial oxygen saturation (SaO
2
) are known. We used the data provided by a noninvasive cardiac output computer, which utilizes the partial Fick method, as the basis for the SvO
2
calculation. We compared the accuracy of this method against directly measured blood gas samples in five anesthetized pigs.
Methods: Using the inverse Fick equation, SvO 2 is calculated as the arterial oxygen saturation less the VO 2 divided by the product of Q and arterial oxygen content (CaO2). The NM3 partial rebreathing cardiac output monitor (Philips, Wallingford, CT) measures CO 2 excretion (VCO 2 ), arterial oxygen saturation via pulse oximetry (SpO2) and cardiac output via partial CO 2 rebreathing (Qp). Assuming a value for the respiratory quotient of 0.85, the oxygen uptake (VO 2 ) was calculated from directly measured CO 2 production (VCO 2 ). CaO 2 was calculated from Hemoglobin content (Hb). Five pigs (30-40 Kg) were intubated and anesthetized with 1 MAC of isoflurane. A pulmonary artery catheter (Edwards Lifesciences, Irvine, CA) was placed so that mixed venous blood could be sampled from the catheter tip. Arterial blood samples were also drawn to measure Hb. The partial rebreathing cardiac output monitor sensor was placed in the breathing circuit between the endotracheal tube and the wye-piece. Qp, VCO 2 , SpO 2 and other parameters measured by the NM3 monitor were saved to a computer so that inverse Fick SvO 2 could be calculated off line. Continuous infusions of dobutamine or norepinephrine were used to raise cardiac output. Blood gas samples were drawn at baseline levels and during periods of high or low cardiac output as indicated by the noninvasive monitor. We compared corresponding estimated and direct SvO 2 measurements. Results: The average estimated SvO 2 was 67.8% and the average directly measured SvO2 was 68% (range of 30.8 to 89.6%). The bias was -0.16% and the standard deviation of the difference was 6.4%. Figure 1 shows a scatter plot of the estimated measurement versus the direct measurement (R 2 = 0.92).[figure1] Discussion: Clinical SvO 2 measurement is invasive and expensive. These data show that an estimate of SvO 2 may be calculated with the inverse Fick equation from the parameters provided by a partial rebreathing cardiac output monitor and Hb. This method makes assumptions about the respiratory quotient that are only valid during periods of respiratory stability. This assumption is often valid during the maintenance phase of the typical general anesthetic. Another possible source of error is the use of SpO 2 in place of the SaO 2 from the blood gas. If invalid SpO 2 measurement is suspected, the SaO 2 could be substituted in the calculation. From Proceedings of the 2009 Annual Meeting of the American Society Anesthesiologists. |
Figure 1
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