title: Capnographic Parameters in Ventilated Patients: Correspondence with Airway and Lung Tissue Mechanics.
abstract: BACKGROUND
  Although the mechanical status of the lungs affects the shape of the capnogram, the relations between the capnographic parameters and those reflecting the airway and lung tissue mechanics have not been established in mechanically ventilated patients. We, therefore, set out to characterize how the mechanical properties of the airways and lung tissues modify the indices obtained from the different phases of the time and volumetric capnograms and how the lung mechanical changes are reflected in the altered capnographic parameters after a cardiopulmonary bypass (CPB).
  
  
  METHODS
  Anesthetized, mechanically ventilated patients (n = 101) undergoing heart surgery were studied in a prospective consecutive cross-sectional study under the open-chest condition before and 5 minutes after CPB. 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 phase III slopes (SIII), their transition (D2min), the dead-space indices according to Fowler, Bohr, and Enghoff and the intrapulmonary shunt.
  
  
  RESULTS
  Before CPB, SII and D2min exhibited the closest (P = 0.006) associations with H (0.65 and -0.57; P < 0.0001, respectively), whereas SIII correlated most strongly (P < 0.0001) with Raw (r = 0.63; P < 0.0001). CPB induced significant elevations in Raw and G and H (P < 0.0001). These adverse mechanical changes were reflected consistently in SII, SIII, and D2min, with weaker correlations with the dead-space indices (P < 0.0001). The intrapulmonary shunt expressed as the difference between the Enghoff and Bohr dead-space parameters was increased after CPB (95% ± 5% [SEM] vs 143% ± 6%; P < 0.001).
  
  
  CONCLUSIONS
  In mechanically ventilated patients, the capnographic parameters from the early phase of expiration (SII and D2min) are linked to the pulmonary elastic recoil, whereas the effect of airway patency on SIII dominates over the lung tissue stiffness. However, severe deterioration in lung resistance or elastance affects both capnogram slopes.
authors.affiliation: From the *Department of Anesthesiology and Intensive Therapy, University of Szeged, Szeged, Hungary; and †Department of Medical Physics and Informatics, University of Szeged, Hungary.
authors.forename: Zsofia
authors.initials: Z
authors.lastname: Csorba
authors.forename: Ferenc
authors.initials: F
authors.lastname: Petak
authors.forename: Kitti
authors.initials: K
authors.lastname: Nevery
authors.forename: Jozsef
authors.initials: J
authors.lastname: Tolnai
authors.forename: Adam L
authors.initials: AL
authors.lastname: Balogh
authors.forename: Ferenc
authors.initials: F
authors.lastname: Rarosi
authors.forename: Gergely H
authors.initials: GH
authors.lastname: Fodor
authors.forename: Barna
authors.initials: B
authors.lastname: Babik
mesh_terms: D000368:Aged; D000403:Airway Resistance; D000768:Anesthesia, General; D015415:Biomarkers / Q000378:metabolism; D019296:Capnography; D002245:Carbon Dioxide / Q000378:metabolism*; D006348:Cardiac Surgical Procedures; D002315:Cardiopulmonary Bypass; D003430:Cross-Sectional Studies; D004548:Elasticity; D005260:Female; D006801:Humans; D008168:Lung / Q000378:metabolism / Q000502:physiology*; D008176:Lung Volume Measurements; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D011446:Prospective Studies; D012121:Respiration, Artificial; D015656:Respiratory Mechanics; D013997:Time Factors
chemical_list: D015415:Biomarkers; D002245:Carbon Dioxide