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

Abstract

IntroductionThe 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.MethodsSidestream 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).ResultsST-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.ConclusionsSIII 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.

Capnographic Parameters in Ventilated Patients: Correspondence with Airway and Lung Tissue Mechanics

“…The overall and the regional lung emptying are determined by the opposite effects of Raw, G and the lung recoil tendencies (6). The role of the lung tissue stiffness decreases dynamically toward the end of expiration, and the elastic recoil affects SIII in patients with low 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 or high compliance (6). Raw, therefore, exerted the primary influence on the SIII parameters before the CPB (Figs 4 and 1S), independent from the initial lung function (i.e.…”

“…In addition to verifying the correctness of the airway management, capnography provides information about the uniformity of lung emptying and adverse changes in the overall airway geometry (1)(2)(3)(4)(5)(6)(7)(8) and respiratory tissue stiffness (6)(7)(8), and it serves as a valuable tool for the recognition of pulmonary circulatory abnormalities (9)(10)(11).…”

Methacholine-Induced Variations in Airway Volume and the Slope of the Alveolar Capnogram Are Distinctly Associated with Airflow Limitation and Airway Closure

“…In another series of 19 asthmatics, airway responsiveness was correlated with Sacin, an index of ventilation heterogeneity in the diffusing regions of the lung [ 39 ]. A possible explanation for the lack of an association between baseline capnographic indices and airway responsiveness may be the high interindividual variability in the former [ 29 ].…”

“…Because of the lack of effects of methacholine on the vasculature of the respiratory system [ 25 ][ 26 ], we assumed that the probability that alterations in lung perfusion determine variations in the slope of the alveolar capnogram in subjects with mild symptoms and without previous respiratory disease would be very low. Because slope3 is associated with the extent of small airway disease in resected lungs [ 23 ][ 24 ], as well as with disease severity in conditions where alterations of small airways play key roles, such as COPD [ 27 ] or cystic fibrosis [ 28 ], and because Slope3 is associated with airway resistance in humans [ 29 ], this index has been proposed as an indicator of small airway dysfunction.…”

Respiratory consequences of perioperative complications related to anaesthesia

“…The extent of emptying of lung compartments containing various CO 2 concentrations during mechanical ventilation and the shape of the resulting capnogram are determined primarily by the airway geometry (Blanch et al, 1999;Bohm et al, 2009;Nik Hisamuddin et al, 2009;Stromberg and Gustafsson, 2000;Yaron et al, 1996), and the driving pressure exerted by the elastic recoil of the respiratory tissues (Babik et al, 2012). The marked elevations in these parameters in the aspirated lungs suggest the presence of uneven lung ventilation, particularly at the lower PEEP level (Fig.…”