Analysis of ventilation and performance abnormalities by washout in alveolar air and arterial blood continuous measurement of inert gas

Estimation of ventilation-perfusion (VA/Q) inequality by the multiple inert gas elimination technique requires knowledge of arterial, mixed venous, and mixed expired concentrations of six gases. Until now, arterial concentrations have been directly measured and mixed venous levels either measured or calculated by mass balance if cardiac output was known. Because potential applications of the method involve measurements over several days, we wished to determine whether inert gas levels in peripheral venous blood ever reached those in arterial blood, thus providing an essentially noninvasive approach to measuring VA/Q mismatch that could be frequently repeated. In 10 outpatients with chronic obstructive pulmonary disease, we compared radial artery (Pa) and peripheral vein (Pven) levels of the six gases over a 90-min period of infusion of the gases into a contralateral forearm vein. We found Pven reached 90% of Pa by approximately 50 min and 95% of Pa by 90 min. More importantly, the coefficient of variation at 50 min was approximately 10% and at 90 min 5%, demonstrating acceptable intersubject agreement by 90 min. Since cardiac output is not available without arterial access, we also examined the consequences of assuming values for this variable in calculating mixed venous levels. We conclude that VA/Q features of considerable clinical interest can be reliably identified by this essentially noninvasive approach under resting conditions stable over a period of 1.5 h.

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Validation of measurements of ventilation-to-perfusion ratio inequality in the lung from expired gas

Journal of Applied Physiology ◽

10.1152/japplphysiol.00662.2002 ◽

2003 ◽

Vol 94 (3) ◽

pp. 1186-1192 ◽

Cited By ~ 10

Author(s):

G. Kim Prisk ◽

Harold J. B. Guy ◽

John B. West ◽

James W. Reed

Keyword(s):

Gas Exchange ◽

Respiratory Exchange Ratio ◽

Blood Gases ◽

Inert Gas ◽

Phase Iii ◽

Strongly Correlated ◽

Arterial Blood Gases ◽

Arterial Blood ◽

Expired Gas ◽

The Relationship

The analysis of the gas in a single expirate has long been used to estimate the degree of ventilation-perfusion (V˙a/Q˙) inequality in the lung. To further validate this estimate, we examined three measures ofV˙a/Q˙ inhomogeneity calculated from a single full exhalation in nine anesthetized mongrel dogs under control conditions and after exposure to aerosolized methacholine. These measurements were then compared with arterial blood gases and with measurements of V˙a/Q˙ inhomogeneity obtained using the multiple inert gas elimination technique. The slope of the instantaneous respiratory exchange ratio (R slope) vs. expired volume was poorly correlated with independent measures, probably because of the curvilinear nature of the relationship due to continuing gas exchange. When R was converted to the intrabreathV˙a/Q˙ (iV˙/Q˙), the best index was the slope of iV˙/Q˙ vs. volume over phase III (iV˙/Q˙slope). This was strongly correlated with independent measures, especially those relating to inhomogeneity of perfusion. The correlations for iV˙/Q˙ slope and R slope considerably improved when only the first half of phase III was considered. We conclude that a useful noninvasive measurement ofV˙a/Q˙ inhomogeneity can be derived from the intrabreath respiratory exchange ratio.

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