To evaluate the distribution of inhaled gases in an obstructive patient, which phase of the nitrogen elimination test should be observed?

Inhaled gases' distribution in obstructive patients can effectively be assessed during Phase III of the nitrogen elimination test. This phase represents the alveolar plateau on the exhaled nitrogen concentration curve, where the nitrogen concentration becomes stable after the more rapid exhalation of nitrogen from the conducting airways and the initial portions of the alveolar gas.

During Phase III, the nitrogen concentration reflects gas exchange occurring in the alveoli and provides insights into ventilation and distribution in the lungs. For patients with obstructive lung disease, Phase III can demonstrate an increase in the slope of the curve, indicating uneven distribution of ventilation due to airway resistance issues. Analyzing this phase helps clinicians identify potential ventilation-perfusion mismatching and assess the severity of airflow obstruction.

Observing the closing volume could provide information about airway closure but doesn't specifically focus on the distribution of inhaled gases throughout the respiratory cycle in the same detailed manner as Phase III does. Additionally, Phases I and II are related to different aspects of nitrogen elimination, such as the initial washout and the transitional phase of the test, which are less directly related to evaluating gas distribution in obstructive lung disease.