Rebreathing K(CO) and single breath K(CO) measured at the same mean alveolar volume reveal a physiological relationship between unequal diffusion and unequal ventilation in normal subjects.
BACKGROUND
The single breath (SB) method for determining the transfer factor for carbon monoxide is influenced by an unequal distribution of diffusion and ventilation. The rebreathing (RB) method is thought not to be influenced by these inequalities. Therefore, a comparison of the results of the two methods enables us to investigate unequal ventilation and diffusion. We have previously shown that even in normal subjects unequal ventilation influences the relation between the transfer factor as measured by the RB method and by the SB method.


OBJECTIVE
To investigate to what extent unequal diffusion influences the relation between the RB and SB transfer factor in normal subjects.


METHODS
Measurements were performed in 8 normal subjects. Differences in alveolar volume as measured by the RB and SB method were prevented from influencing the results by comparing the RB and SB transfer factor per liter of alveolar volume, i.e. the RB and SB Krogh factor, instead of the RB and SB transfer factor. The effect of the known dependence of the Krogh factor on inspired volume was prevented by measuring the SB Krogh factor at an inspired volume equal to the mean inspired volume of the RB measurement. This SB Krogh factor was compared with the RB Krogh factor calculated as the quotient of the RB transfer factor and the mean alveolar volume during the RB measurement.


RESULTS
This RB Krogh factor was always larger than the corresponding SB Krogh factor. The difference was significant (p = 0.006). A high percentage (80%) of the variance in the quotient of the two Krogh factors, which can be considered as a parameter for unequal diffusion, was explained by the variance in the quotient of the mean RB volume and the corresponding SB volume. The latter quotient is a measure of unequal ventilation.


CONCLUSIONS
The finding suggests a physiological relation between unequal diffusion and unequal ventilation. A plausible explanation was found in the decrease of the Krogh factor with an increasing ratio of inspired volume to residual volume for each part of the lung separately. The consequence is that in the absence of unequal ventilation the RB transfer factor is equal to the SB transfer factor measured at an inspired volume equal to the mean RB inspired volume.
Lung Function Department of the Heart Lung Division and Centre of Biostatistics, Utrecht University, Utrecht, The Netherlands.
H
H
Jansons
J K
JK
Fokkens
I
I
van der Tweel
J
J
Kreukniet
D000328:Adult; D002248:Carbon Monoxide; D004058:Diffusion; D005260:Female; D006801:Humans; D008176:Lung Volume Measurements; D008297:Male; D008875:Middle Aged; D011312:Pressure; D011650:Pulmonary Alveoli / Q000502:physiology*; D012016:Reference Values; D012119:Respiration; D013997:Time Factors
D002248:Carbon Monoxide