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Central venous oxygenation: when physiology explains apparent discrepancies.

Squara P - Crit Care (2014)

Bottom Line: 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 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.

View Article: PubMed Central - PubMed

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.

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Relationships between mixed venous oxygen hemoglobin saturation and its components. To create these curves, we modeled a standard ICU population of 1,000 patients (Excel, Microsoft) with normal distribution of arterial oxygen hemoglobin saturation (SaO2; 0.95 ± 0.05 limited to 1) and normal distributions of total body oxygen consumption (VO2) needs (140 ± 30 ml/minute.m2), cardiac output (CO; 3.0 ± 0.5 L/minute.m2) and hemoglobin concentration (Hb; 100 ± 15 g/L). Only one of the four components was sequentially changed (Y-variables) to look at its specific relationship with mixed venous oxygen saturation (SvO2) (X-variable). When unchanged, variables were set to their mean value. The horizontal arrows indicate the fluctuations of the Y-variable around its mean value (±2 standard deviations). The vertical arrows show the corresponding fluctuations of SvO2. We can see that reference ranges (mean ±2SD) of SaO2, VO2 needs, CO, and Hb are compensated for by a 26%, 50%, 47%, and 40% change in SvO2, respectively. Thus, CO is not necessarily the predominant component of SvO2 except when low (left, flat part of the relationship).
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Fig1: Relationships between mixed venous oxygen hemoglobin saturation and its components. To create these curves, we modeled a standard ICU population of 1,000 patients (Excel, Microsoft) with normal distribution of arterial oxygen hemoglobin saturation (SaO2; 0.95 ± 0.05 limited to 1) and normal distributions of total body oxygen consumption (VO2) needs (140 ± 30 ml/minute.m2), cardiac output (CO; 3.0 ± 0.5 L/minute.m2) and hemoglobin concentration (Hb; 100 ± 15 g/L). Only one of the four components was sequentially changed (Y-variables) to look at its specific relationship with mixed venous oxygen saturation (SvO2) (X-variable). When unchanged, variables were set to their mean value. The horizontal arrows indicate the fluctuations of the Y-variable around its mean value (±2 standard deviations). The vertical arrows show the corresponding fluctuations of SvO2. We can see that reference ranges (mean ±2SD) of SaO2, VO2 needs, CO, and Hb are compensated for by a 26%, 50%, 47%, and 40% change in SvO2, respectively. Thus, CO is not necessarily the predominant component of SvO2 except when low (left, flat part of the relationship).

Mentions: For any cell, tissue, and organ, VO2 is the difference between arterial and venous O2 flows. For the whole body, if we ignore the O2 dissolved in the plasma water, which represents only a few percent of the total O2 blood content, if we consider that arterial and venous blood flows are represented by the cardiac output (CO), and if we assume that arterial and venous blood flow have a similar hemoglobin concentration (Hb), then we can write:2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \mathrm{V}{\mathrm{O}}_2=\mathrm{C}\mathrm{O}\times \mathrm{H}\mathrm{b}\times 1.34\times \left(\mathrm{S}\mathrm{a}{\mathrm{O}}_2-\mathrm{Sv}{\mathrm{O}}_2\right) $$\end{document}VO2=CO×Hb×1.34×SaO2−SvO2where VO2 is in ml/minute.m2, CO is in L/minute.m2, 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, therefore, dimensionless percentages. The constant 1.34 is the carrying capacity of the oxygenated Hb in milliliters of O2 per gram. This equation can be reformulated as a function of each variable, but its reformulation as a function of SvO2 is one of the most popular because SvO2 measurements are precise, accurate, time responsive and quite easy to monitor [10,11]. Figure 1 shows that the relationships between SvO2 and its components are not equivalent and not necessarily linear. As a consequence, a large change in one variable may be compensated by a small change in another - for example, small changes in low CO that are compensated for by large changes in SvO2 and large changes in high CO that are compensated for by small changes in SvO2:3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \mathrm{S}\mathrm{v}{\mathrm{O}}_2=\mathrm{S}\mathrm{a}{\mathrm{O}}_2-\mathrm{V}{\mathrm{O}}_2/\left(\mathrm{C}\mathrm{O}\times \mathrm{H}\mathrm{b}\times 1.34\right) $$\end{document}SvO2=SaO2−VO2/CO×Hb×1.34Figure 1


Central venous oxygenation: when physiology explains apparent discrepancies.

Squara P - Crit Care (2014)

Relationships between mixed venous oxygen hemoglobin saturation and its components. To create these curves, we modeled a standard ICU population of 1,000 patients (Excel, Microsoft) with normal distribution of arterial oxygen hemoglobin saturation (SaO2; 0.95 ± 0.05 limited to 1) and normal distributions of total body oxygen consumption (VO2) needs (140 ± 30 ml/minute.m2), cardiac output (CO; 3.0 ± 0.5 L/minute.m2) and hemoglobin concentration (Hb; 100 ± 15 g/L). Only one of the four components was sequentially changed (Y-variables) to look at its specific relationship with mixed venous oxygen saturation (SvO2) (X-variable). When unchanged, variables were set to their mean value. The horizontal arrows indicate the fluctuations of the Y-variable around its mean value (±2 standard deviations). The vertical arrows show the corresponding fluctuations of SvO2. We can see that reference ranges (mean ±2SD) of SaO2, VO2 needs, CO, and Hb are compensated for by a 26%, 50%, 47%, and 40% change in SvO2, respectively. Thus, CO is not necessarily the predominant component of SvO2 except when low (left, flat part of the relationship).
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4282012&req=5

Fig1: Relationships between mixed venous oxygen hemoglobin saturation and its components. To create these curves, we modeled a standard ICU population of 1,000 patients (Excel, Microsoft) with normal distribution of arterial oxygen hemoglobin saturation (SaO2; 0.95 ± 0.05 limited to 1) and normal distributions of total body oxygen consumption (VO2) needs (140 ± 30 ml/minute.m2), cardiac output (CO; 3.0 ± 0.5 L/minute.m2) and hemoglobin concentration (Hb; 100 ± 15 g/L). Only one of the four components was sequentially changed (Y-variables) to look at its specific relationship with mixed venous oxygen saturation (SvO2) (X-variable). When unchanged, variables were set to their mean value. The horizontal arrows indicate the fluctuations of the Y-variable around its mean value (±2 standard deviations). The vertical arrows show the corresponding fluctuations of SvO2. We can see that reference ranges (mean ±2SD) of SaO2, VO2 needs, CO, and Hb are compensated for by a 26%, 50%, 47%, and 40% change in SvO2, respectively. Thus, CO is not necessarily the predominant component of SvO2 except when low (left, flat part of the relationship).
Mentions: For any cell, tissue, and organ, VO2 is the difference between arterial and venous O2 flows. For the whole body, if we ignore the O2 dissolved in the plasma water, which represents only a few percent of the total O2 blood content, if we consider that arterial and venous blood flows are represented by the cardiac output (CO), and if we assume that arterial and venous blood flow have a similar hemoglobin concentration (Hb), then we can write:2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \mathrm{V}{\mathrm{O}}_2=\mathrm{C}\mathrm{O}\times \mathrm{H}\mathrm{b}\times 1.34\times \left(\mathrm{S}\mathrm{a}{\mathrm{O}}_2-\mathrm{Sv}{\mathrm{O}}_2\right) $$\end{document}VO2=CO×Hb×1.34×SaO2−SvO2where VO2 is in ml/minute.m2, CO is in L/minute.m2, 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, therefore, dimensionless percentages. The constant 1.34 is the carrying capacity of the oxygenated Hb in milliliters of O2 per gram. This equation can be reformulated as a function of each variable, but its reformulation as a function of SvO2 is one of the most popular because SvO2 measurements are precise, accurate, time responsive and quite easy to monitor [10,11]. Figure 1 shows that the relationships between SvO2 and its components are not equivalent and not necessarily linear. As a consequence, a large change in one variable may be compensated by a small change in another - for example, small changes in low CO that are compensated for by large changes in SvO2 and large changes in high CO that are compensated for by small changes in SvO2:3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \mathrm{S}\mathrm{v}{\mathrm{O}}_2=\mathrm{S}\mathrm{a}{\mathrm{O}}_2-\mathrm{V}{\mathrm{O}}_2/\left(\mathrm{C}\mathrm{O}\times \mathrm{H}\mathrm{b}\times 1.34\right) $$\end{document}SvO2=SaO2−VO2/CO×Hb×1.34Figure 1

Bottom Line: 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 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.

View Article: PubMed Central - PubMed

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.

Show MeSH
Related in: MedlinePlus