Can Mixed Venous Saturation Be Monitored Non-invasively?
Can Mixed Venous Saturation Be Monitored Non-invasively?
Abstract & Commentary
By David J. Pierson, MD, Editor, Professor, Pulmonary and Critical Care Medicine, Harborview Medical Center, University of Washington, Seattle, is Editor for Critical Care Alert.
Synopsis: Although it may be able to detect large changes in mixed venous saturation, estimating this variable using the NICO monitor's CO2 rebreathing method for cardiac output determination and several mathematical assumptions correlates poorly with direct measurements of SvO2.
Source: Kotake Y, et al. Can mixed venous hemoglobin oxygen saturation be estimated using a NICO monitor? Anesth Analg 2009;109:119-123.
The NICO monitor (Respironics, Inc.) uses co2 production, estimated from changes in exhaled CO2 during a brief period of rebreathing, to determine cardiac output via the Fick relationship. Mixed venous oxyhemoglobin saturation (SvO2) can theoretically be estimated from this cardiac output value using an assumed value for the relationship between CO2 production and O2 consumption along with measured arterial saturation from pulse oximetry (SpO2). The equation used for this by the current paper's authors was: SvO2 = SpO2 - [VCO2 (mL/min)/0.85] ÷ [O2 content (1.36 × hemoglobin concentration) × cardiac output (L/min) × 0.1].
Kotake et al examined the relationship between spectrophotometrically measured SvO2, using blood drawn from a pulmonary artery catheter, and SvO2 calculated in the above manner using the NICO monitor, in 23 patients undergoing aortic aneurysm repair. They used the Bland-Altman method to evaluate this relationship, and examined the ability of the NICO-estimated values to track the change in SvO2 using correlation analysis in comparison to measured SvO2.
The range of directly measured SvO2 values was 44%-94%, with a mean of 78%. The bias ± limits of agreement of the estimated SvO2 against measured SvO2 was -2.1% ± 11.2%. Change in estimated SvO2 was only modestly correlated with change in measured SvO2. The authors concluded that SvO2 derived from the values measured by the NICO monitor cannot be used interchangeably with the values measured spectrophotometrically using actual mixed venous blood samples.
Commentary
Early goal-directed therapy (EGDT) in the treatment of severe sepsis and septic shock1 has become a widely mandated standard of care, and many hospitals (including mine) have adopted protocols and "code sepsis" systems to implement it. Serial measurements of venous O2 are a core component of EGDT. Central venous saturation has been accepted clinically as a surrogate measurement for true SvO2, even though it is not a perfect substitute and requires several assumptions about its reflection of global tissue oxygenation. Central venous saturation may correlate poorly with SvO2, especially in patients with severe shock and other situations where tissue oxygenation in different body regions may vary. The findings of the present study — that SvO2 estimated from several mathematically derived values, themselves based on assumptions, correlates poorly with actual SvO2 — are not surprising. Whether this non-invasive but questionably reliable estimation of SvO2 is a clinically acceptable substitute for central venous saturation (itself a surrogate for the true variable in question) deserves to be studied before it is widely adopted in clinical practice.
Reference
- Rivers E, et al. Early goal-directed therapy in the treatment of severe sepsis and septic shock. N Engl J Med 2001;345:1368-1377.
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