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Causes of a high physiological dead space in critically ill patients.

Wagner PD - Crit Care (2008)

Bottom Line: In the previous issue of Critical Care, Niklason and colleagues remind us that physiological dead space, while easily measured, consolidates potentially very complex physiological derangements into a single number.The authors show how shunts raise arterial pCO2, thereby increasing dead space, and how changes in other variables such as cardiac output and acid/base state further modify it.A solid understanding of respiratory physiology is required to properly interpret physiological dead space in the critically ill.

View Article: PubMed Central - HTML - PubMed

Affiliation: Division of Physiology, Department of Medicine, University of California at San Diego, 9500 Gilman Drive, 0623A, La Jolla, CA 92093-0623, USA. pdwagner@ucsd.edu

ABSTRACT
Since around 1950, physiological dead space - the difference between arterial and mixed expired pCO2 (partial pressure of carbon dioxide) divided by the arterial pCO2 - has been a useful clinical parameter of pulmonary gas exchange. In the previous issue of Critical Care, Niklason and colleagues remind us that physiological dead space, while easily measured, consolidates potentially very complex physiological derangements into a single number. The authors show how shunts raise arterial pCO2, thereby increasing dead space, and how changes in other variables such as cardiac output and acid/base state further modify it. A solid understanding of respiratory physiology is required to properly interpret physiological dead space in the critically ill.

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Comparison of effects of shunt (top) and ventilation/perfusion ratio () inequality (bottom) on calculated physiological dead space. In general,  inequality leads to greater dead space than shunt does. Log SDQ, second moment (dispersion) of the ventilation/perfusion distribution on a log scale.
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Figure 1: Comparison of effects of shunt (top) and ventilation/perfusion ratio () inequality (bottom) on calculated physiological dead space. In general, inequality leads to greater dead space than shunt does. Log SDQ, second moment (dispersion) of the ventilation/perfusion distribution on a log scale.

Mentions: Fourth, inequality is generally a cause of greater physiological dead space than shunt is (Figure 1) (calculations using algorithms from [2]). For example, it takes a very large, 60% shunt to increase dead space by 20% but a log-normal pattern of only moderate inequality (log SDQ = 1.3) does the same. Normal log SDQ is less than 0.6 [5], and the highest log SDQ values seen are about 2 to 2.5. Log SDQ is a parameter defined for quantifying inequality in the multiple inert gas elimination technique [6,7] and is the second moment (dispersion) of the distribution on a log scale.


Causes of a high physiological dead space in critically ill patients.

Wagner PD - Crit Care (2008)

Comparison of effects of shunt (top) and ventilation/perfusion ratio () inequality (bottom) on calculated physiological dead space. In general,  inequality leads to greater dead space than shunt does. Log SDQ, second moment (dispersion) of the ventilation/perfusion distribution on a log scale.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2481441&req=5

Figure 1: Comparison of effects of shunt (top) and ventilation/perfusion ratio () inequality (bottom) on calculated physiological dead space. In general, inequality leads to greater dead space than shunt does. Log SDQ, second moment (dispersion) of the ventilation/perfusion distribution on a log scale.
Mentions: Fourth, inequality is generally a cause of greater physiological dead space than shunt is (Figure 1) (calculations using algorithms from [2]). For example, it takes a very large, 60% shunt to increase dead space by 20% but a log-normal pattern of only moderate inequality (log SDQ = 1.3) does the same. Normal log SDQ is less than 0.6 [5], and the highest log SDQ values seen are about 2 to 2.5. Log SDQ is a parameter defined for quantifying inequality in the multiple inert gas elimination technique [6,7] and is the second moment (dispersion) of the distribution on a log scale.

Bottom Line: In the previous issue of Critical Care, Niklason and colleagues remind us that physiological dead space, while easily measured, consolidates potentially very complex physiological derangements into a single number.The authors show how shunts raise arterial pCO2, thereby increasing dead space, and how changes in other variables such as cardiac output and acid/base state further modify it.A solid understanding of respiratory physiology is required to properly interpret physiological dead space in the critically ill.

View Article: PubMed Central - HTML - PubMed

Affiliation: Division of Physiology, Department of Medicine, University of California at San Diego, 9500 Gilman Drive, 0623A, La Jolla, CA 92093-0623, USA. pdwagner@ucsd.edu

ABSTRACT
Since around 1950, physiological dead space - the difference between arterial and mixed expired pCO2 (partial pressure of carbon dioxide) divided by the arterial pCO2 - has been a useful clinical parameter of pulmonary gas exchange. In the previous issue of Critical Care, Niklason and colleagues remind us that physiological dead space, while easily measured, consolidates potentially very complex physiological derangements into a single number. The authors show how shunts raise arterial pCO2, thereby increasing dead space, and how changes in other variables such as cardiac output and acid/base state further modify it. A solid understanding of respiratory physiology is required to properly interpret physiological dead space in the critically ill.

Show MeSH
Related in: MedlinePlus