Abstract
Central venous oxygen saturation (ScvO2) >70% or mixed venous oxygen saturation (SvO2) >65% is recommended for both septic and non-septic patients. Although it is the task of experts to suggest clear and simple guidelines, there is a risk of reducing critical care to these simple recommendations. This article reviews the basic physiological and pathological features as well as the metrological issues that provide clear evidence that SvO2 and ScvO2 are adaptative variables with large inter-patient variability. This variability is exemplified in a modeled population of 1,000 standard ICU patients and in a real population of 100 patients including 15,860 measurements. In these populations, it can be seen how optimizing one to three of the four S(c)vO2 components homogenized the patients and yields a clear dependency with the fourth one. This explains the discordant results observed in large studies where cardiac output was increased up to predetermined S(c)vO2 thresholds following arterial oxygen hemoglobin saturation, total body oxygen consumption needs and hemoglobin optimization. Although a systematic S(c)vO2 goal-oriented protocol can be statistically profitable before ICU admission, appropriate intensive care mandates determination of the best compromise between S(c)vO2 and its four components, taking into account the specific constraints of each individual patient.
Highlights
Central venous oxygen saturation (ScvO2) >70% or mixed venous oxygen saturation (SvO2) >65% is recommended for both septic and non-septic patients
Small change in another - for example, small changes in low cardiac output (CO) that are compensated for by large changes in Mixed venous oxygen saturation (SvO2) and large changes in high CO that are compensated for by small changes in SvO2: where Total body oxygen consumption (VO2) is in ml/minute.m2, CO is in L/minute.m2, hemoglobin concentration (Hb) is in g/L, and arterial oxygen hemoglobin saturation (SaO2) and SvO2 are the ratios of arterial and venous oxygenated Hb over the total Hb per blood unit and, dimensionless percentages
Basic physiology tells us that SvO2 is not a regulated variable but an adaptive variable depending on four elementary regulated components: VO2 needs, Arterial oxygen hemoglobin saturation (SaO2), Hb and CO
Summary
Basic physiology tells us that SvO2 is not a regulated variable but an adaptive variable depending on four elementary regulated components: VO2 needs, SaO2, Hb and CO. There is no physiological argument for targeting particular values of SvO2 (or its surrogate ScvO2) by specific interventions except in homogenized populations, where optimizing one to three of the four SvO2 components may yield a clear dependency with the fourth one. This explains the apparently contradictory results observed in large studies where CO was increased up to specific SvO2 thresholds and confirms the basic physiology predicting large inter-patient variability. Competing interests The author declares that he has no competing interests
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