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

Csorba, Zsofia MD*; Petak, Ferenc PhD; Nevery, Kitti MD*; Tolnai, Jozsef PhD; Balogh, Adam L. MD*; Rarosi, Ferenc MSc; Fodor, Gergely H. MD; Babik, Barna MD, PhD*

doi: 10.1213/ANE.0000000000001185
Technology, Computing, and Simulation: Research Report

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.

Published ahead of print February 29, 2016

From the *Department of Anesthesiology and Intensive Therapy, University of Szeged, Szeged, Hungary; and Department of Medical Physics and Informatics, University of Szeged, Hungary.

Accepted for publication December 18, 2015.

Published ahead of print February 29, 2016

Funding: Funded by a Hungarian Basic Research Grant (OTKA K81179). This research was supported by the European Union and the State of Hungary and co-financed by the European Social Fund in the framework of TÁMOP 4.2.4. A/2-11-1-2012-0001 National Excellence Program.

The authors declare no conflicts of interest.

Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal’s website.

Reprints will not be available from the authors.

Address correspondence to Ferenc Petak, PhD, Department of Medical Physics and Informatics, University of Szeged, Korányi Fasor 9, Hungary H-6720. Address e-mail to

The capnogram is a curve reflecting the concentration change of carbon dioxide (CO2) as an endogenous indicator during expiration. 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 geometry1–8 and respiratory tissue stiffness,6–8 and it serves as a valuable tool for the recognition of pulmonary circulatory abnormalities.9–11 International recommendations for standards require the monitoring of ventilation with capnography in all patients undergoing sedation or general anesthesia.12,13

Characterization of the relations between standard lung function parameters and capnographic indices provides a valuable tool facilitating an understanding of the various shapes of the capnogram.1,2,14 However, the previous studies demonstrating associations between the capnographic slope factors with the forced expiratory volume in 1 second1,14 and the peak expiratory flow2,14 were limited to spontaneously breathing subjects. Despite the particular importance of recognizing adverse alterations in the pulmonary system in mechanically ventilated patients, details as to how the resistive and/or elastic properties of the pulmonary system affect the various indices derived from the capnogram are essentially lacking from the literature. In the present study, therefore, we set out 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 lung tissue mechanics, expiratory flow, and gas exchange. To gain an insight into within-subject alterations in the pulmonary condition, a large cohort of ventilated patients was examined during cardiac surgery and a cardiopulmonary bypass (CPB) being applied to generate a temporary complex realignment in the pulmonary mechanics and circulation.

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Detailed description of patient characteristics, methodology, and results can be found in the Supplemental Digital Content (

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After providing written informed consent, 101 patients (women/men: 30/71, 62 ± 9 years) undergoing elective open heart surgeries were examined in a prospective, consecutive cross-sectional manner. The study protocol was approved by the Human Research Ethics Committee of Szeged University, Szeged, Hungary (no. World Health Organization 2788). Patients were excluded in the event of severe cardiopulmonary disorders (pleural effusion >300 mL, ejection fraction <30%, body mass index >35 kg/m2, or intraoperative acute asthma exacerbation).

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Anesthesia and Surgery

IV midazolam (30 μg/kg), sufentanil (0.4–0.5 μg/kg), and propofol (0.3–0.5 mg/kg) were applied to induce anesthesia. The maintenance of anesthesia and muscle relaxation was ensured by an IV infusion of propofol (50 µg/kg/min) and IV boluses of rocuronium (0.2 mg/kg every 30 minutes).

Endotracheal intubation was performed, and the patients were mechanically ventilated in volume-controlled mode with descending flow (Dräger Zeus, Lübeck, Germany). The tidal volume was set to 7 mL/kg with a ventilator frequency of 9 to 14 breaths/min, a positive end-expiratory pressure of 4 cm H2O, and an inspired oxygen fraction (Fio2) of 0.5. Arterial blood gas samples were analyzed to calculate the Horowitz coefficient (HQ = Pao2/Fio2). During cardioplegic cardiac arrest, the lungs were not ventilated while maintaining no positive airway pressure. The lungs were then inflated 3 to 5 times to a peak airway pressure of 30 cm H2O before declamping of the aorta to perform lung recruitment.

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Forced Oscillatory Measurements

The low-frequency forced oscillation technique was applied to measure the lung mechanical properties, as detailed previously.15 The volume history was standardized by inflating the lungs to a pressure of 30 cm H2O before the oscillatory measurements. Forced oscillatory signal was introduced into the lungs during short (15 seconds) apneic periods. The input impedance of the lung (ZL) was computed from the power spectra of the airway opening pressure and tracheal airflow. A model16 containing a frequency-independent airway resistance (Raw) and inertance (Iaw) and a constant-phase tissue compartment characterized by the coefficients of damping (G) and elastance (H) was fitted to the mean ZL data. The lung tissue resistance (Rti) and the total lung resistance (RL) at the ventilation frequency (0.2 Hz) were also calculated from the ZL spectra.

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Recording and Analyses of the Expiratory Capnogram

A mainstream capnograph (Novametrix; Capnogard®, Andover, MA) and another central airflow meter (Piston Ltd., Budapest, Hungary) were connected into the ventilatory circuit at the Y-piece, and 15-second CO2 and ventilator flow traces were recorded simultaneously. The CO2 and ventilator flow traces were digitized and imported into custom-made signal analysis software. The slopes of phase III of the capnogram in the time (SIII,T) and in the volumetric (SIII,V) domains were determined by fitting a linear regression line to the last two-thirds of each phase III traces (Fig. 1).17,18 The phase II slopes of the time (SII,T) and volumetric (SII,V) capnograms were determined by calculating the slopes of the best-fitting line around the inflection point (±20%). Each slope was divided by the average corresponding CO2 concentration in the mixed expired gas to obtain normalized time (SnII,T and SnIII,T) and volumetric (SnII,V and SnIII,V) phase II and phase III slopes.11,19,20 This normalization was made only for the slope indices, as performed earlier before and after CPB.11 The angle (αcap) formed by the phase II and phase III limbs of the expiratory time capnogram was also calculated by using a standard monitoring speed of 12.5 mm/s. The transition rates of change from phase II to phase III in the time (D2min) and volumetric (D2Vmin) capnograms reflecting the curvature were calculated as the minima of the second-order time and volumetric derivatives.21

Figure 1.

Figure 1.

Besides these shape factors, dead-space parameters were derived from the volumetric capnograms. The Fowler22,23 dead space (VDF), reflecting the anatomic dead-space volume of the conducting airways, was determined by calculating the expired gas volume until the inflection point of phase II was reached in the volumetric capnogram. The physiologic dead space, including also the alveolar volume not involved in gas exchange, was assessed by the Bohr method (VDB).24 The dead space according to Enghoff modification (VDE) was calculated; this takes also into account the not ventilated, but still perfused alveoli.25

We also calculated the differences between the Enghoff and Bohr dead-space parameters (VDE − VDB) representing the pulmonary shunt circulation. The intrapulmonary shunt blood flow (Qs/Qt) was additionally assessed via the Fick equation.

Under both experimental conditions, 3 to 5 expiratory traces in each recording were analyzed, resulting in the ensemble averaging of 10 to 12 values for further analysis in each patient.

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Analysis of the Expiratory Flow

To characterize the expiratory flow pattern, the expiratory phases of each V′ recordings were analyzed by fitting an exponential function to the elevating limb26:

where Vpl is the plateau flow before the beginning of the next inspiration, PF is the peak expiratory flow, and LF is related to the curvature of the expiratory curve. A larger value of LF indicates a more concave shape in the late flow. Model fitting to the serial data points from the peak flow was performed until 90% of the equilibrium value of V′(t) was reached.

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Measurement Protocol

Two sets of measurements were made under the open-chest condition 5 minutes before the CPB and 5 minutes after the patient was weaned from the CPB. Recruitment maneuvers were performed before the patient was weaned from the CPB. Each data collection period started with recordings of 3 to 5 capnogram traces. During this period, an arterial blood gas sample was taken to measure Pao2 and Paco2 for the calculation of HQ and VDE, respectively. The total lung resistance (Rvent) and compliance (Cvent) displayed by the respiratory monitor of the ventilator were registered at this stage of the protocol. The data collections under both conditions were supplemented by recordings of 3 to 5 ZL data epochs at 1-minute intervals.

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Data Analyses

Sample size estimation was applied to involve sufficient number of patients for the detection of clinically relevant significances. The type 1 error rate was set to 0.05, the statistical power was set to 0.85, and the clinically relevant effect size (alternative hypothesis) was considered to detect correlation coefficients r = 0.3 vs r = 0. The necessary sample size was 96.

Scatters in measured variables are expressed as SEM values. In the event of passing the normality test (marked in footnotes), paired t tests were used to examine the statistical significance of the changes induced in the parameters by the CPB. Wilcoxon signed rank tests were applied otherwise to verify the significance of the changes in the mechanical, capnographic, or gas exchange parameters. The Pearson test was applied to analyze the correlations between the different variables. The comparison of Pearson correlation coefficients was made by Steiger Z test; these tests were performed between the particular and the nearest r values. Subgroups of patients were formed based on the initial HQ level (high and low 25 percentile) and based on the extremity of changes after surgery (top 25 percentile increase and bottom 25 percentile decrease in HQ, respectively). Time domain capnogram slope indices and Raw and Cvent and their changes after the surgery were also correlated in these subgroups and were compared with the results obtained from the pooled population. Values of P <0.05 were considered to be statistically significant.

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Parameters reflecting lung mechanics and the expiratory flow are demonstrated in Figure 2. All the resistive parameters, including those reflecting the flow resistance of the airways (Raw) or of the lung tissues (Rti) or the combination of these compartments (Rvent and RL), exhibited marked and statistically significant increases after CPB (P < 0.0001 for each). Conversely, more moderate but still highly significant decreases were observed after CPB in the compliance parameters determined at end-expiratory lung volume by the oscillometry (CL) or at end-inspiratory lung volume by the ventilator (Cvent) (P < 0.0001 for both). CPB induced no statistically detectable changes in PF (P = 0.5), whereas the parameter LF, reflecting the curvature of the late flow, increased significantly (P < 0.0001). The CPB-induced adverse lung mechanical changes were also reflected in the significant decrease in HQ (from 371 ± 11 to 350 ± 14 mm Hg; P = 0.038).

Figure 2.

Figure 2.

Figure 3 depicts the indices derived from the time and volumetric capnographic measurements before and after the CPB. Marked and statistically significant increases were observed in the time and volumetric parameters reflecting the phase III slope of the expired CO2 (P < 0.0001 for SIII,T, SnIII,T, SIII,V, and SnIII,V) after the CPB. The slopes of phase II revealed significant decreases after CPB (P < 0.0001 for both SII,T and SII,V), whereas these drops were no longer detectable after normalization to the CO2 concentration in the mixed expired gas (P = 0.4 and 0.9 for SnII,T and SnII,V, respectively1). CPB increased the curvature representing the transition from phase II to phase III (P < 0.0001 for both D2min and D2Vmin). Uniform decreases were detected in VDF and VDB (P < 0.0001) after the CPB, whereas VDE increased significantly (P < 0.0001). These changes in the dead-space parameters resulted in significant elevations in the shunt parameters reflecting the alterations in lung ventilation (P = 0.02 and P < 0.0001 for VDB − VDF and VDE − VDB, respectively) and perfusion (Qs/Qt, P < 0.0001).

Figure 3.

Figure 3.

Figure 4 illustrates the strengths of the correlations between the lung mechanical parameters (x-axis) and the time and volumetric capnographic parameters reflecting the slopes, transitions, dead space, and shunt fractions (y-axis).

Figure 4.

Figure 4.

The lung resistive parameters exhibited the closest associations with the phase III slope capnographic parameters (P < 0.0001), particularly after the CPB, when all the indices reflecting the resistive properties of the pulmonary system were markedly elevated (P < 0.0001; Fig. 4, top panels). Significant, but somewhat weaker, correlations were observed between the lung resistive parameters and the ventilation dead-space parameters VDF (P < 0.0001) and VDB (P < 0.0001). More specifically, the mechanical parameter representing the flow resistance of the airways (Raw) correlated best (P < 0.0001) with the SIII,T (r = 0.63 and r = 0.68 for SIII,T before and after the CPB, respectively; P < 0.0001). Moreover, Raw correlated significantly with SIII,V (r = 0.43 and r = 0.55 for SIII,V before and after CPB, respectively, P < 0.0001). Normalization of the phase III slopes to the CO2 concentration in the mixed expired gas did not affect these relations noticeably (P = 0.71). Conversely, the mechanical parameters characterizing lung tissue elasticity (H and Cvent) showed the closest (P = 0.006) relations with the time capnographic parameters describing the phase II (r = 0.65 and r = 0.41 between H and SII,T before and after the CPB, respectively; P < 0.0001). The pulmonary elastance and compliance parameters also revealed close associations with the capnographic indices reflecting the curvatures of the transitions between the phases, particularly before the CPB (r = −0.57 between H and D2min; P < 0.0001). The early- and late-phase expiratory flow parameters revealed strong associations between PF and the dead-space indices both before and after the CPB. LF exhibited the strongest correlation with SnIII,V (r = 0.53; P < 0.0001).

As concerns the relations between the CPB-induced changes in the lung mechanical and capnographic indices (Fig. 4, bottom panel), the marked elevations in Raw correlated best (P = 0.001) with the decreases in the phase II slope parameters of the time capnogram (r = −0.72 and r = −0.70 for SII,T and SnII,T, respectively; P < 0.0001). The CPB-induced airway narrowing was also reflected in the elevated phase III slope parameters of the time and volumetric capnograms (r = 0.49 for both SIII,T and SIII,V; P < 0.0001) and the curvature of the transition between the phases in the time domain (r = 0.6 for D2min; P < 0.0001). The changes in the other mechanical parameters reflecting the tissue (G) or total lung resistance (RL or Rvent) displayed similar relations with the alterations in the various capnographic indices after the CPB. Assessment of the mild CPB-induced stiffening of the lung tissue also revealed statistically significant correlations between the changes in Cvent and those in the phase III slope parameters in both the time and volumetric capnograms (r = −0.48 for both SnIII,T and SnIII,V; P < 0.0001). Neither the absolute values of HQ nor the changes after CPB exhibited close relations with any other mechanical or capnographic indices; an association was observed with VDE before the CPB (r = 0.31; P < 0.0002).

The relations between the initial fundamental lung mechanical and capnographic indices for the subgroups of patients based on starting HQ are depicted on Figure 5A. Strong positive significant correlations were observed between Raw and phase III slope parameters (P = 0.002) and between Cvent and phase II slope parameters independently of the subgroup allocation (P = 0.001). The initial Cvent-SIII,T relation was not significantly correlated (P = 0.20), whereas the Raw-SII,T correlation appeared significant only for the pooled patient population (P = 0.0045). The changes in Raw correlated to those in both slope variables (P < 0.0001), whereas the alterations in Cvent were significantly related with those in phase III slopes (P = 0.023; Fig. 5B).

Figure 5.

Figure 5.

Findings reflecting interindividual variability and demonstrating individual changes are included in the Supplemental Digital Content (Table 1S and Figure 1S, A large range was obtained for the coefficient of variation in the initial lung mechanical and capnogram parameters (ranging from 25% to 169% for VDB and SnIII,T, respectively). Further interdependences of the main capnogram shape factors and lung mechanical parameters are illustrated in Figure 2S in the Supplemental Digital Content (

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Capnography is an essential part of the monitoring in patients requiring mechanical ventilation. The present study was motivated to elucidate how changes in the mechanical properties of the different lung compartments are reflected in the alterations in the shape, dead space, and pulmonary shunt circulation parameters obtained from the time or volumetric capnograms. A detailed characterization of the airway and lung tissue mechanics was combined with a comprehensive evaluation of the capnographic indices before and after a lung function deterioration induced by a CPB. Our study revealed the specific influence of the lung resistive and elastic parameters on the capnogram shape indices in ventilated patients.

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Phase III Slope

The results demonstrated significant increases in both SIII,T and SIII,V immediately after the CPB. The elevations also appeared after normalization to the CO2 concentration in the mixed expired gas11 (Fig. 3). This finding differs from that observed previously in a smaller cohort of ventilated cardiac surgery patients, where no major changes were observed in the phase III slope after CPB.11 The discrepancy may be attributed to the more aggressive maneuvers applied to recruit the lungs after the CPB in this previous study, to the application of a higher positive end-expiratory pressure (7 vs 4 cm H2O), and to the somewhat delayed measurement time after the CPB (15 vs 5 minutes).

The phase III slope of the capnogram is thought to reflect the summation of the ventilation inhomogeneities relating to the working alveolar compartments with different time constants and the ventilation-perfusion mismatch as concerns the dead-space and/or intrapulmonary shunt. 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 or high compliance.6 Raw, therefore, exerted the primary influence on the SIII parameters before the CPB (Fig. 4; and Figure 1S, Supplemental Digital Content,, independent from the initial lung function (i.e., HQ; Fig. 5A). This finding is in accordance with the postulate of the close link between the airway cross-sectional area and SIII, based on spirometric data obtained previously in spontaneously breathing patients.1,3,27 Representing (G) or incorporating lung parenchymal resistive component (RL and Rvent) weakened the correlation substantially (P < 0.0001; Fig. 4). This suggests that under baseline conditions, the internal friction in the lung tissue does not exert a major effect on the capnographic SIII indices, along with the lesser role of compliance. After the CPB, significant associations appeared between the overall resistive and capnographic phase III slope parameters because of the greatly elevated tissue resistance (Fig. 4). Thus, an elevated SIII may indicate the presence of lung disorders affecting not only the airways but also the tissue resistive properties, such as observed during interstitial edema in sepsis or cardiac failure.28 These phenomena are comprehensively confirmed by the significantly elevated concavity of the late-expiratory flow (Fig. 2), the increase in VDE, and the diminished HQ (Fig. 3).

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Phase II Slope

The phase II slope was decreased after the CPB in both the time and the volumetric capnograms. This agrees with the results of the only previous study, where the changes in SII were measured 2 minutes after establishment of the full pulmonary blood flow.11 However, normalization of SII to a possible lower CO2 content of the expired gas (i.e., SnII) after weaning from the CPB eliminated these changes (Fig. 3) because the intensity of axial gas mixing depends on the CO2 concentration.11

Phase II of the capnogram represents the overall width of the moving airway-alveolar gas front and its slope may be explained by opposing effects. The heterogeneous start of lung emptying, the reduced airway lumen, and increased tissue damping may all contribute to the decreases, whereas an elevated elastic recoil and a low alveolar CO2 content may counteract these changes in SII.1,3,11,29 Before the CPB, the correlation analyses of the SII parameters indicated their close relation with the elastic properties of the lungs (Fig. 4), independent of the starting gas exchange ability of the lungs (Fig. 5A). This finding is in accordance with a wider phase II observed previously in emphysematous patients30,31 and increases in SII after compliance elevation through recruitment maneuvers.29 Because PF is determined more by the lung tissue stiffness (r = 0.34; P < 0.0001 for H) than by the airway caliber (r = 0.09; P = 0.34 for Raw), the significant correlation of PF with the phase II capnographic indices may also be attributed to the influence of the lung elastance during early expiration.

Independent of the direction and magnitude of change in HQ, the CPB-induced changes in SII exhibited close correlations with the markedly elevated Raw (Fig. 5B) and lung resistance parameters (RL and Rvent; Fig. 4). This finding indicates that inhomogeneous airway constriction leads to a more sequential emptying of lung compartments with different CO2 content, even at the beginning of expiration, and thereby widens the airway-alveolar gas front with subsequent decreases in the phase II slope. The loss of correlations between the changes in SII,T and Cvent (Fig. 5B) may be due to the complex and opposing phenomena affecting SII,T, as described earlier.

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Capnographic Parameters Reflecting Phase Transitions

The transition indices (αcap, D2min, and D2Vmin) reside in the same part of the capnogram, but their meanings are different. αcap is the angle between SII and SIII, that is, the relation between the overall gas front and the alveolar gas volume, whereas D2min and D2Vmin are related to the internal surface of the moving CO2 diffusion front in the airways during expiration. The elevation observed in αcap after the CPB reflects the combined alterations in SII and SIII, whereas both second derivative parameters approached 0 after the CPB (Fig. 3), demonstrating blunted (less cornered) transitions between capnographic phases II and III. This finding may be attributed to the highly heterogeneous severe airway constriction that develops after the CPB, which blurs the resulting diffusion front measured in the central airway. The close associations between forced expiratory volume in 1 second and the capnographic indices reflecting the phase II to phase III transition in spontaneously breathing patients is in accordance with this result.1 Our data further demonstrate that, in ventilated patients, the low compliance associated with the normal airway patency compresses the flow profile, resulting in a sharper phase II to phase III transition (Supplemental Digital Content, Supplemental Figure 1S, C, This finding reveals the sensitivity of D2min and D2Vmin parameters to changes in lung compliance as opposed to αcap, which demonstrates rather resistive properties (Fig. 4).

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Dead-Space and Shunt Parameters

The anatomical dead space (VDF) was decreased slightly but consistently after the CPB (Fig. 3). Because this change was associated with marked increases in Raw and LF (Fig. 4, bottom), the compromised lumen of the conducting airways and/or their exclusion from the ventilation may explain this finding. The dead-space parameter incorporating the additional volume of the unperfused alveoli (VDB) followed very similar changing and correlation patterns, indicating the negligible unperfused but ventilated alveolar compartment after the CPB. The bronchoconstriction resulting from the additive effects of systemic inflammatory response syndrome (SIRS) and local hypocapnia may contribute to the low alveolar dead space. Conversely, supine position, surgery, and CPB led to elevations in VDE (Fig. 3), suggesting a substantial enlargement of the volume of the not ventilated but perfused alveoli because of the persistent atelectasis after the CPB.32,33

The difference between VDE and VDB, which approximates the extent of the pulmonary shunt, was increased markedly after the CPB (Fig. 3). It is noteworthy that VDE − VDB exhibited parallel changes and a significant correlation (r = 0.47; P < 0.0001) with the shunt fraction obtained from the classical shunt equation (Qs/Qt), highlighting the additional usefulness of volumetric capnography in the assessment of the intrapulmonary shunt (Fig. 3).

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Methodological Aspects

Although patients with severe cardiopulmonary disorders were excluded from the present study, the pulmonary status of the participating subjects varied widely from relatively healthy lungs to obstructive and restrictive disorders. Such interindividual variety of pulmonary symptoms with additional demographic and anthropometric differences is expected to occur in all health care units providing ventilatory support. Therefore, this feature of the study is particularly favorable and also facilitates the performance of the correlation analyses.

It is also noteworthy that the results represent an open-chest condition. Significant alteration in lung-thorax dynamics is expected to influence both the capnography indices and forced oscillatory data reflecting airway and tissue mechanics.34 The capnogram parameters are determined by the heterogeneity of the lungs, geometry of the airway tree, and the forces exerted by the tissue resistive and elastic properties of the lungs and the chest wall.6 Although our study allows an insight into the mechanisms coupling the capnogram and mechanical parameters, a further study in intact chest patients may be needed to generalize our findings.

A further important methodological aspect of the present study is related to the use of correlation analyses to assess the associations between parameters obtained by 2 different techniques. As a general rule, the existence of significant correlations between variables is necessary but not sufficient to imply a causal relation. In the present study, the lung mechanical and capnographic parameters are linked to each other through local common mechanisms governing lung emptying. Furthermore, the individual correlation results from consistent physiologic and clinical findings. These considerations verify that causation can be inferred with great certainty.

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In conclusion, we characterized the relations between the time or volumetric capnographic parameters and the lung mechanical indices reflecting the airway and the lung tissue viscoelastic properties in cardiac surgery patient underwent open heart surgery. The lung tissue stiffness predominantly determines the capnographic parameters in the early phase of expiration because the elastic forces are maximal at high lung volumes. Thus, in most of the cases, the phase II slope of the capnogram is predominantly determined by pulmonary elastic recoil. Conversely, the resistive properties of the lungs become increasingly important during the later phase of expiration and thus the phase III slope is shaped overwhelmingly by the airway resistance. However, markedly elevated lung resistance additionally worsens the capnogram phase II slope. Similarly, severely compromised lung elastance also distorts the capnogram phase III slope. Because computational methods could be incorporated into the modern anesthesia machines to quantify capnographic shape factors, these parameters, together with the traditional bedside mechanical indices, promise to improve differential diagnoses and advance guiding respiratory therapy.

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Name: Zsofia Csorba, MD.

Contribution: This author helped conduct the study and analyze the data.

Attestation: Zsofia Csorba has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Ferenc Petak, PhD.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Attestation: Ferenc Petak has seen the original study data, reviewed the analysis of the data, and approved the final manuscript and is the author responsible for archiving the study files.

Name: Kitti Nevery, MD.

Contribution: This author helped conduct the study and analyze the data.

Attestation: Kitti Nevery has seen the original study data and approved the final manuscript.

Name: Jozsef Tolnai, PhD.

Contribution: This author helped conduct the study and analyze the data.

Attestation: Jozsef Tolnai has seen the original study data and approved the final manuscript.

Name: Adam L. Balogh, MD.

Contribution: This author helped conduct the study and analyze the data.

Attestation: Adam L. Balogh has seen the original study data and approved the final manuscript.

Name: Ferenc Rarosi, MSc.

Contribution: This author helped analyze the data.

Attestation: Ferenc Rarosi has seen the original study data and approved the final manuscript.

Name: Gergely H. Fodor, MD.

Contribution: This author helped analyze the data.

Attestation: Gergely H. Fodor has seen the original study data and approved the final manuscript.

Name: Barna Babik, MD, PhD.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Attestation: Barna Babik has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

This manuscript was handled by: Maxime Cannesson, MD, PhD.

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