Differences of cardiac output measurements by open-circuit acetylene uptake in pulmonary arterial hypertension and chronic thromboembolic pulmonary hypertension: a cohort study

Background As differences in gas exchange between pulmonary arterial hypertension (PAH) and chronic thromboembolic pulmonary hypertension (CTEPH) have been demonstrated, we asked if cardiac output measurements determined by acetylene (C2H2) uptake significantly differed in these diseases when compared to the thermodilution technique. Method Single-breath open-circuit C2H2 uptake, thermodilution, and cardiopulmonary exercise testing were performed in 72 PAH and 32 CTEPH patients. Results In PAH patients the results for cardiac output obtained by the two methods showed an acceptable agreement with a mean difference of -0.16 L/min (95% CI -2.64 to 2.32 L/min). In contrast, the agreement was poorer in the CTEPH group with the difference being -0.56 L/min (95% CI -4.96 to 3.84 L/min). Functional dead space ventilation (44.5 ± 1.6 vs. 32.2 ± 1.4%, p < 0.001) and the mean arterial to end-tidal CO2 gradient (9.9 ± 0.8 vs. 4.1 ± 0.5 mmHg, p < 0.001) were significantly elevated among CTEPH patients. Conclusion Cardiac output evaluation by the C2H2 technique should be interpreted with caution in CTEPH, as ventilation to perfusion mismatching might be more relevant than in PAH.


Introduction
The assessment of cardiac output is a crucial factor in the risk stratification and management of patients with pulmonary hypertension (PH) as it is directly related to the clinical severity of the disease as well as being one of the most important prognostic factors [1].
Several methods have been introduced to measure cardiac output in humans. The thermodilution technique has been validated against the direct Fick method, which represents the "gold standard" when evaluating cardiac output [2]. It is used routinely to assess cardiac output in PH patients. However, the thermodilution method is an invasive technique requiring right heart catheterization of the patient. A reliable non-invasive method to determine the cardiac output would allow serial measurements and, thus, would facilitate the follow-up management of PH patients. Among the noninvasive techniques the acetylene (C 2 H 2 ) rebreathing method has been validated against different other techniques and has gained wide acceptance [2][3][4]. A drawback to the method, however, is the build-up of carbon dioxide (CO 2 ) as a result of rebreathing. Therefore, open-circuit methods have been developed [5,6]. C 2 H 2 is a non-toxic, inert gas that has a low solubility in lung tissue but a high solubility in blood. When inhaled, C 2 H 2 is rapidly taken up in the pulmonary blood stream at a rate proportional to the effective pulmonary blood flow. The rate of disappearance allows calculating the cardiac output in the absence of a relevant pulmonary shunt blood flow. Yet, this technique is expected to fail whenever there are conditions that may affect the distribution of the gas in the lungs. The alveolar distribution of C 2 H 2 should not be affected in PH patients. However, significant differences in gas exchange between pulmonary arterial hypertension (PAH) and chronic thromboembolic pulmonary hypertension (CTEPH) have been demonstrated recently, most likely resulting from different lung perfusion patterns and additional thromboembolic vascular occlusion in CTEPH [7].
We therefore aimed to clarify if cardiac output measurements done by a single-breath open-circuit C 2 H 2 uptake method significantly differ in PAH and CTEPH patients when compared to the cardiac output assessment performed by thermodilution.

Study design and study population
This retrospective cohort study included patients who were referred to our PH centre between January 2007 and December 2010. All study participants fulfilled the diagnostic criteria for PAH or CTEPH according to current guidelines [1]. All patients underwent right heart catheterization to establish the diagnosis. CTEPH was ruled out or confirmed by ventilation/perfusion scan or pulmonary angiography in every patient. No subject in the study received any specific pulmonary artery medication upon inclusion in the trial. Most of our study participants were referred to our hospital for the evaluation of PH or unexplained dyspnea on exertion. Therefore, all patients were extensively diagnosed for the presence of lung disease. Patients with pulmonary comorbidity (such as asthma, chronic obstructive pulmonary disease, lung fibrosis or other symptomatic interstitial lung diseases) or concomitant extra-cardiac diseases limiting exercise performance were excluded. The catheter examination was part of the routine diagnostic work-up in all patients. All procedures adhered to commonly accepted ethical guidelines and written informed consent was obtained from every patient.

Lung Function Tests
Pulmonary function tests included spirometry, body plethysmography and measurement of diffusing capacity employing the single-breath method (Master Screen Body and MS-PFT, Jaeger, Cardinal Health, USA). Each parameter was calculated as percentage of the predicted one. The following parameters were measured: forced vital capacity (FVC), total lung capacity (TLC), forced expiratory volume in one second (FEV1) and diffusing capacity for carbon monoxide (TLco). Blood gas analysis (ABL 725, Radiometer, Copenhagen, Denmark) was performed in arterialized capillary blood from the ear lobe without supplemental oxygen (O 2 ).

Cardiopulmonary exercise testing
Cardiopulmonary exercise testing (CPET) was performed using a standardized protocol [6]. Work rate was continuously increased by 5 -15 watts/min to a maximum tolerated level on an electromagnetically braked cycle ergometer (ViaSprint 150 p, Ergoline, Germany). Patients were encouraged to exercise until symptoms were intolerable. Blood gas analysis was done at rest and during peak exercise. Heart rate was monitored continuously and non invasive blood pressure was taken every 2 minutes. The maximum work rate was recorded. O 2 uptake (VO2), minute ventilation (Ve) and CO 2 output (VCO2) were measured breath by breath using an adult facemask (Vmax spectra 229 D, Sensor Medics, USA). O 2 pulse, alveolar-arterial O 2 difference (AaDO2) and functional dead space ventilation (Vd/Vt) were calculated as described by Wasserman et al. [8]. The anaerobic threshold (AT) was chosen at the peak VO2 at which the ventilatory equivalent for O 2 (Ve/VO2) increased while the ventilatory equivalent for CO 2 (Ve/ VCO2) decreased or remained constant. Peak VO2 was defined as the value of averaged data during the final 15 seconds of exercise. The Ve/VCO2 slope was determined as the linear regression slope of Ve and VCO2 from the start of exercise until the respiratory compensation point (the time point at which ventilation is stimulated by acidaemia and the end-tidal CO 2 (etCO2) begins to decrease).

Right heart catheterization and thermodilution
Patients received no medication on the morning of the procedure, resulting in a discontinuation of treatment of at least 12 hours. A thermodilution catheter (7.5 F quadruple-lumen, balloon-tipped, flow-directed, "S" Tip Swan-Ganz Catheter, Edwards Lifesciences, Irvine, USA) was inserted via the right or left femoral vein. Hemodynamic measurements were performed in supine position and included heart rate, pressure in wedge-position (PCWP), pulmonary arterial pressure (PAP) and right atrium pressure (RAP). O 2 saturation (SO2) was measured in mixed venous blood samples (ABL 725, Radiometer, Copenhagen, Denmark). The cardiac output was measured by thermodilution with 10 ml of sterile, icecold isotonic (0.9%) saline, which was injected through the right atrial lumen of the catheter; the drop in temperature at the distal thermistor was then recorded. The injectate temperature was determined by a thermistor which was placed directly behind the right atrial inlet of the catheter. Cardiac output was calculated using a computer system (Com-2, Cardiac Output Computer, Edwards Lifesciences, Irvine, USA). In each patient, a minimum of 3 measurements were performed; the mean value was calculated if the variability of values was less than 10%. The pulmonary vascular resistance (PVR) was calculated using a standard formula [PVR = (mean PAP -PCWP)/cardiac output].

C 2 H 2 Technique
C 2 H 2 uptake was measured by an open-circuit singlebreath, slow-expiration method using a commercially available system (Vmax spectra 229 D, Sensor Medics, USA). The precision and the reliability of this system have been validated by others [5] and the instructions of the manufacturer were followed in detail. Briefly, the patients were instructed to breathe through the mouthpiece of the apparatus. The nostrils were occluded with a nose clip. The inhaled gas was a mixture of 0.298% methane, 0.300% carbon monoxide, 0.300% C 2 H 2 , 21.100% O 2 and 78.002% nitrogen. The manoeuvre started at end-expiration with a maximal inspiration. The breath hold time was 1-3 seconds. Then the patients slowly exhaled the total volume. Thereafter, cardiac output was calculated by an integrated computer from the disappearance curve of C 2 H 2 . In each patient, a minimum of 3 measurements were performed; the mean value was calculated if the variability of values was less than 10%. The procedure was tolerated without any problems by all patients.

Statistical Analysis
Statistical analysis was performed with SPSS software for Windows version 12.0 (SPSS, IBM Inc., Chicago, USA). All data are presented as mean ± standard error of mean (SEM). The results obtained from the PAH and CTEPH group were compared using the Student's t-test for unpaired probes. The agreement between the thermodilution and C 2 H 2 method was analyzed as described by Bland and Altman [9]. Agreement bias was expressed as the mean of the differences obtained by the different techniques. The limits of agreement were expressed as the mean ± 2 SD, and the 95% confidence interval (95% CI) of the bias was calculated. Simple linear regression analysis was performed in order to compare the quality of agreement between the cardiac output measurements obtained by the different techniques in both cohorts. A probability value of less than 0.05 was considered statistically significant; all reported p values are two-tailed.

Clinical Characteristics of the study population
A total of 104 patients were included in the study (Table 1). 72 patients were diagnosed having PAH and 32 study participants were suffering from CTEPH. CTEPH patients were significantly older than subjects in the PAH subgroup (70.4 ± 1.4 vs. 52.5 ± 2.3 years, p < 0.001). Gender and body mass indices (BMI) were equally distributed in both groups.

Comparison of cardiac output measurements by the C 2 H 2 method and thermodilution
The average cardiac output as determined by thermodilution was 3.96 ± 0.15 L/min and 3.68 ± 0.22 L/min by the C 2 H 2 technique for the whole study population. Among PAH patients, the mean cardiac output was 4.09 ± 0.14 L/min measured by thermodilution and 3.94 ± 0.18 L/min evaluated by C 2 H 2 uptake. In the CTEPH group, mean cardiac output was determined to be 3.68 ± 0.18 L/min by thermodilution and 3.10 ± 0.32 L/min by the C 2 H 2 technique. Figure 1 plots the results obtained by thermodilution with those obtained by C 2 H 2 uptake. Figure 2 shows the Bland & Altman plot of the differences between thermodilution and C 2 H 2 technique against the mean of both values. In PAH patients the results obtained by the two methods showed an acceptable agreement with the mean difference being -0.16 L/min (95% CI -2.64 to 2.32 L/ min). In contrast, the agreement was poorer in the CTEPH group with the difference being -0.56 L/min (95% CI -4.96 to 3.84 L/min). Simple linear regression analysis revealed that values for cardiac output measured by thermodilution and C 2 H 2 uptake were significantly associated in the PAH subgroup (R 2 = 0.122, p = 0.004). In contrast, cardiac output measurements obtained by the different techniques in the CTEPH cohort were not significantly associated (R 2 = 0.032, p = 0.35).

Discussion
Our study revealed an acceptable agreement of the cardiac output assessment determined by thermodilution and C 2 H 2 uptake in PAH patients, although we observed a large splay of single measurements. These differences, however, were not larger than those reported from patients with other cardiopulmonary diseases [10]. Our findings are in accordance with an earlier trial by Hoeper et al. who reported that thermodilution and C 2 H 2 rebreathing are equally accurate in patients with idiopathic PAH when being compared with the direct Fick method [2]. In contrast, our study demonstrated that the agreement between thermodilution and the C 2 H 2 technique was poor in patients suffering from CTEPH. Several issues need to be discussed in this context. In our study CTEPH patients had lower CI values compared to the PAH subgroup. As C 2 H 2 uptake tends to underestimate cardiac output in our CTEPH cohort and thermodilution might overestimate cardiac output in the presence of a low CI [11], the poorer agreement of both methods could be explained by a wider divergence of measurements. However, another trial has challenged this hypothesis indicating that thermodilution is equally accurate in a broad spectrum of cardiac output values [2].
Furthermore, mean FEV1 and the averaged FEV1/FVC ratios were significantly reduced in the CETPH cohort, possibly explaining the poorer agreement of both methods. As patients with concurrent lung diseases were excluded from our trial and both groups had similar mean BMI values, these findings might be explained by the older mean age in the CETPH subgroup. As the alveolar distribution of C 2 H 2 might be affected by the reduction of FEV1/FVC, cardiac output measurements obtained by the C 2 H 2 uptake method are probably less accurate in the CTEPH cohort.
In addition to that, an impaired gas exchange might contribute to the poorer agreement of both techniques in the CTEPH cohort, as the C 2 H 2 technique is expected to fail in the presence of a relevant mismatching of ventilation to perfusion. Indeed, we found evidence of an uneven distribution of ventilation to perfusion in CTEPH patients primarily due to two different mechanisms, increased dead space ventilation and heterogeneous pulmonary perfusion.
Although there are some reports indicating that ventilation/perfusion matching is relatively well preserved in both conditions [12][13][14], there is evidence that increased ventilation/perfusion ratios in CTEPH are primarily caused by augmented dead space fractions [15,16]. In our study Vd/Vt at peak exercise was significantly increased in CTEPH compared to PAH. These findings are in accordance with the report of Zhai and colleagues, who were able to show that significant differences in gas exchange exist between CTEPH and PAH due to differences in Vd/Vt [7]. Dead space ventilation in pulmonary thromboembolism increases the gradient between arterial and end-tidal CO 2 [17]. Correspondingly, we found the mean a-etCO2 gradient to be significantly elevated among CTEPH patients compared to the PAH subgroup in our study. Similar findings have recently been made by Scheidl and co-workers [18], who found increased capillary to end-tidal CO 2 differences in patients with CTEPH compared to those suffering from idiopathic PAH. Increased dead space ventilation might be the result of a more uneven perfusion pattern in CTEPH patients, possibly due to additional thrombus formation and a more proximal vascular occlusion, leading to heterogeneous perfusion defects [19]. Areas with diminished blood flow and areas with increased blood flow coexist, while ventilation is more or less homogeneously distributed. As a result, there are areas with an increased ventilation/perfusion ratio or even dead space ventilation and others with a low ventilation/perfusion ratio. In contrast, PAH is characterized by an obstructive vasculopathy that bilaterally involves distal, medium to small size muscular arteries [20], possibly resulting in a more balanced distribution of perfusion and ventilation. Therefore, we speculate that compared to PAH patients, increased heterogeneity in pulmonary blood flow in comparison to ventilation might contribute to an inaccurate determination of cardiac output by the C 2 H 2 method in CTEPH.
Our study is limited by the fact that cardiac output measurements determined by thermodilution and the C 2 H 2 method were not performed simultaneously. Thus, the splay of single measurements might partially be explained by the physiological variations of cardiac output during the day. However, both techniques were performed shortly after each other without any change of medication in all study participants in order to minimize substantial fluctuations. Moreover, the results might have been influenced by different body positions, as right heart catheterizations were performed in supine position while patients were sitting when the C 2 H 2 technique was performed. Thus, measurements by the C 2 H 2 uptake method might tend to underestimate cardiac output, as preload decreases when subjects sit up. This circumstance possibly explains some of the very low cardiac output values measured by C 2 H 2 uptake. Finally, we didn't evaluate the degree of tricuspid regurgitation, which is commonly present in patients with PH and might have influenced the accuracy of thermodilution in our study participants [21]. As a consequence, thermodilution might have underestimated the cardiac output in the presence of severe tricuspid regurgitation.

Conclusions
Despite these limitations we conclude the following: in contrast to PAH, the agreement of cardiac output measurements determined by the thermodilution and the C 2 H 2 uptake method is poorer in CTEPH. The evaluation of cardiac output by the C 2 H 2 technique should be interpreted with caution in CTEPH patients, as ventilation to perfusion mismatching might be more relevant in CTEPH than in PAH.

Disclosure
The study was supported by a grant of Pfizer. The supporting source had no involvement in the study design, data collection, analysis or interpretation of data, in the writing of the report, and in the decision to submit the manuscript for publication.
Authors' contributions Drs. Schwaiblmair, von Scheidt and Berghaus conceived and designed the study. Drs. Schwaiblmair, Faul and Berghaus acquired the study data. Dr. Schwaiblmair performed the statistical analysis. Drs. Schwaiblmair and Berghaus drafted the article. All authors participated in interpreting the data and revised the manuscript for important intellectual content. All authors approved the final version of the manuscript.