Introduction: The slope of phase III of the capnogram (SIII) relates to progressive emptying of the alveoli, a ventilation/perfusion mismatch, and ventilation inhomogeneity. SIII depends not only on the airway geometry, but also on the dynamic respiratory compliance (Crs); this latter effect has not been evaluated. Accordingly, we established the value of SIII for monitoring airway resistance during mechanical ventilation.Methods: Sidestream capnography was performed during mechanical ventilation in patients undergoing elective cardiac surgery (n = 144). The airway resistance (Raw), total respiratory resistance and Crs displayed by the ventilator, the partial pressure of arterial oxygen (PaO2) and SIII were measured in time domain (ST-III) and in a smaller cohort (n = 68) by volumetry (SV-III) with and without normalization to the average CO2 phase III concentration. Measurements were performed at positive end-expiratory pressure (PEEP) levels of 3, 6 and 9 cmH2O in patients with healthy lungs (Group HL), and in patients with respiratory symptoms involving low (Group LC), medium (Group MC) or high Crs (Group HC).Results: ST-III and SV-III exhibited similar PEEP dependencies and distribution between the protocol groups formed on the basis of Crs. A wide interindividual scatter was observed in the overall Raw-ST-III relationship, which was primarily affected by Crs. Decreases in Raw with increasing PEEP were reflected in sharp falls in SIII in Group HC, and in moderate decreases in SIII in Group MC, whereas ST-III was insensitive to changes in airway caliber in Groups LC and HL.Conclusions: SIII assessed in the time domain and by volumetry provide meaningful information about alterations in airway caliber, but only within an individual patient. Although ST-III may be of value for bedside monitoring of the airway properties, its sensitivity depends on Crs. Thus, assessment of the capnogram shape should always be coupled with Crs when the airway resistance or oxygenation are evaluated.
ASJC Scopus subject areas
- Critical Care and Intensive Care Medicine