Effects of respiratory mechanics on the capnogram phases: importance of dynamic compliance of the respiratory system

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Introduction: The slope of phase III of the capnogram (SIII) relates to progressive emptying of the alveoli, a ventilation/perfusion mismatch, and ventilation inhomogeneity. SIIIdepends 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 SIIIfor 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 SIIIwere measured in time domain (ST-III) and in a smaller cohort (n = 68) by volumetry (SV-III) with and without normalization to the average CO2phase 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-IIIand SV-IIIexhibited 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-IIIrelationship, which was primarily affected by Crs. Decreases in Raw with increasing PEEP were reflected in sharp falls in SIIIin Group HC, and in moderate decreases in SIIIin Group MC, whereas ST-IIIwas insensitive to changes in airway caliber in Groups LC and HL.Conclusions: SIIIassessed in the time domain and by volumetry provide meaningful information about alterations in airway caliber, but only within an individual patient. Although ST-IIImay 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. © 2012 Babik et al.; licensee BioMed Central Ltd.




Babik, B., Csorba, Z., Czövek, D., Mayr, P. N., Bogáts, G., & Peták, F. (2012). Effects of respiratory mechanics on the capnogram phases: importance of dynamic compliance of the respiratory system. Critical Care, 16(5). https://doi.org/10.1186/cc11659

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