Seven healthy males and 5 patients with COPD were studied with increasing dead space; 250, 500, and 750ml, respectively. The slope of the P
CO2-ventilation relationship and the slope of the metabolic hyperbola are defined as S and S
L, respectively. The product of S and S
L is defined as “gain”, which represents the ability of homeostatic control mechanism for blood gas levels. In fact, the magnitude of P
ACO2 change is assumed to be as great as (gain+1) times the actual change with the lack of chemical control of respiration.
In both normal subjects and patients, the values of gain were found to be between 10 to 20 which were well in accord with the value calculated from actual P
ACO2 change and S
L. The agreement was best in the experiments with VD increment of 500ml. The reason why no differences in the gain were seen between the normals and patients was assumed to be due to a slight depression in pulmonary functions in the latter.
The degree of scatter in S was far greater than that in SL, thus the values of gain were mostly determined by individual differences in S values. Therefore, when S
L was calculated by using allometric coefficients, the value of gain can be estimated only by measuring P
ACO2 respiratory frequency and body weight. This calculation was conducted by the following equation:
Estimated gain=-ΔV
D×(P
ACO2)
2/0.863×V
CO2×ΔP
ACO2-1
Reasonable agreements were seen between actual gain and this simplified estimations. This approach is intended to be used in routine examination for evaluating the ability of homeostatic control mechanism for blood gas levels.
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