Relationships between capnogram parameters and respiratory mechanics in ventilated patients

Capnography is one of the most frequently used monitoring methods in anaesthesia and intensive therapy. However, details as to how the resistive and/or elastic properties of the respiratory system affect the various indices derived from the capnogram curve are lacking from the literature. The aims of the present thesis were therefore to establish the connections between the various phase, shape, dead space or pulmonary shunt circulation parameters of the time or volumetric capnogram and those reflecting the airway and respiratory tissue mechanics, expiratory flow and gas exchange. A large cohort of patients scheduled for elective cardiac surgery was enrolled in this thesis. After induction of total intravenous anaesthesia, the patients were intubated and ventilated. Forced oscillation technique was applied to measure airway resistance (Raw), tissue damping (G) and elastance (H). Time and volumetric capnography were performed to assess parameters reflecting the phase II (SII) and III slopes (SIII), their transition (D2min), and the deadspace indices according to Fowler, Bohr and Enghoff approach. The respiratory resistance (Rrs) and the dynamic compliance (Crs) displayed by the ventilator were registered, and arterial and central venous blood gas analysis were performed. In the first study (Study 1) the measurement was performed in open-chest condition before and 5 min after cardiopulmonary bypass (CPB), whereas in the second study (Study 2) of this thesis, the measurements were accomplished at positive end-expiratory pressure (PEEP) levels of 3, 6 and 9 cm H2O in patients with healthy lungs, and in patients with respiratory symptoms involving low (Group LC), medium (Group MC) or high Crs (Group HC). In Study 1, SII and D2min exhibited the closest associations with H (0.65 and 0.57; p<0.0001, respectively), whereas SIII correlated most strongly with Raw (r=0.63; p<0.0001) before CPB, whereas significant elevations in Raw and G, with smaller but still significant increases in H were induced by CPB. These adverse mechanical changes were reflected consistently in SII, SIII and D2min, with weaker correlations with the dead-space indices. The intrapulmonary shunt expressed as the difference between the Enghoff and Bohr dead-space parameters was increased after CPB (95±5% vs. 143±6%; p<0.001). The results confirm that the capnographic parameters from the early phase of expiration (SII and D2min) are linked to the pulmonary elastic recoil, while the effect of airway patency on SIII dominates over the lung tissue stiffness in mechanically ventilated patients. However, severe deteriorations in lung resistance or elastance affect both capnogram slopes. In Study 2, SIII,T and SIII,V exhibited similar PEEP dependencies and distribution between the protocol groups formed on the bases of Crs. A wide inter-individual scatter was observed in the overall RawSIII,V relationship, which was primarily affected by Crs. Decreases in Raw with increasing PEEP were reflected in sharp falls in SIII in Group HC, whereas SIII,T was insensitive to changes Raw in Groups LC and HL. In Group HC, SIII was the highest and the oxygenation was similar than in the healthy group, in Group LC, the SIII was similar than that in the healthy patients, but the oxygenation was the worst. According to our data, SIII provide meaningful information about alterations in airway caliber, but only within an individual patient. The sensitivity of SIII,T depends on Crs. Thus, assessment of the capnogram shape should always be coupled with Crs when the airway resistance or oxygenation are evaluated. In conclusion, evaluation of the relationship between capnography and respiratory mechanics help the anaesthesiologist and intensive therapist to have a deeper understanding of the shape indices of the capnogram, and in a broad sense, to bridge the gap between the physiological and the clinical knowledge in adequate bedside monitoring.