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

Abstract

Introduction: The slope of phase III of the capnogram (SIII) relates to progressive emptying of the alveoli, a ventilation/perfusion mismatch, and ventilation inhomogeneity. S(III) 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 S(III) were measured in time domain (S(T-III)) and in a smaller cohort (n = 68) by volumetry (S(V-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: S(T-III) and S(V-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-S(T-III) relationship, which was primarily affected by Crs. Decreases in Raw with increasing PEEP were reflected in sharp falls in S(III) in Group HC, and in moderate decreases in S(III) in Group MC, whereas S(T-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 S(T-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.

Figure 1

Timeline of the experimental protocol. Rrs and Crs, readings from the respirator display; CO₂, recording of capnogram curves; Zrs, forced oscillatory measurement of respiratory system impedance. CO₂, carbon dioxide; Crs, dynamic respiratory compliance; Rrs, total respiratory resistance; Zrs, input impedance of the respiratory system.

Figure 2

Forced oscillatory airway (Raw, airway resistance) and respiratory tissue (G, damping and H, elastance) mechanical parameters, the slope of the third phase of the capnogram as expressed in the time domain before (S_T-III) and after normalization for the mean expired CO2 (Sn_T-III), or as a function of expired volume before (S_V-III) and after (Sn_V-III) normalization, partial pressure of oxygen in the arterial blood (PaO₂) and dynamic compliance (Crs) displayed by the respirator in patients with healthy lungs (Group HL), and in patients with respiratory symptoms with Crs in the lowest tenth percentile (Group LC), with Crs between the tenth and the ninetieth percentile (Group MC) and with Crs above the ninetieth percentile in (Group HC). *, P < 0.05 vs. the variable at the previous PEEP level; #, P < 0.05 vs. a variable at a PEEP of 3 cmH₂O. α, P < 0.05 vs. Group HL within a PEEP; β, P < 0.05 vs. Group HC within a PEEP; γ, P < 0.05 vs. Group MC within a PEEP.

Figure 3

Relationship between forced oscillatory airway resistance (Raw) and phase III slope of time capnogram (S_T-III) at PEEP levels of 3 (X), 6 (▼) and 9 cm H₂O (▲) in patients with healthy lungs (Group HL), and in patients with respiratory symptoms with dynamic respiratory compliance (Crs) in the lowest tenth percentile (Group LC), with Crs between the tenth and ninetieth percentile (Group MC) and with Crs above the ninetieth percentile (Group HC). Thin grey lines denote individual patients; thick black lines with symbols show group mean and SE values.

Figure 4

Effects of body mass index (BMI) and ejection fraction (EF) on dynamic respiratory compliance (Crs). The best fit plane is demonstrated with a mesh surface.