Capnography in airway obstruction

The expiratory capnogram represents the elimination of gas from the alveolar units and the resulting waveform may be influenced by lung abnormalities. In a

homogeneous normal lung, the VlQ ratio, the time constant, and the C02 partial pressure are the same for all respiratory units, and there is a well-defined transition point between phase II (steep concentration change) and phase III (alveolar plateau). However, when the mixing of inspired gas with lung resident gas is incomplete, mainly because of regional inequalities of ventilation and perfusion, CO 2 concentration increases in the airways from proximal to distal. Therefore the increasing expiratory CO2 slope may be due to serial mixing of gas flowing from units with different time constants. As a result, faster units empty more rapidly at first while gas

from slower units, presumably with very low VlQ ratios and high PC02, forms a proportionately greater part of the end-expiratory gas. Consequently, the CO 2

concentration in the exhaled gas progressively increases and contributes to an increase in the slope during phase III of the capnogram. Thus, given a relatively

uniform perfusion distribution, slow hypercapnic units, and sequential emptying, the CO 2 elimination waveform does reflect the VlQ relationship in many patients.

Bronchial obstruction is associated with regional decreases in airflow with consequent reductions in alveolar ventilation responsible for the heterogeneity of the ventilation-perfusion relationship. Therefore the increasing expired CO 2 slope observed in some patients may be due to the serial mixing of gas flowing from units with different time constants. Several studies conducted in asthmatic patients showed the existence of a significant correlation between indices describing the shape of the capnogram and the usual spirometric parameters.

Critically ill patients on mechanical ventilation may often exhibit high respiratory system resistance ( Rrs) for a variety of reasons. The expiratory capnogram is correlated with spirometric indices. In ventilated critically ill patients regression analysis revealed a close correlation between Rrs and expired CO2 slope (r = 0.86; p < 0.001) with 95 per cent confidence intervals for Rrs of ±7.39 cmH2O/l/s from the predicted value obtained by the regression equation, where

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