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(Chest. 2005;127:1122-1131.)
© 2005 American College of Chest Physicians

Does Thoracic Bioimpedance Accurately Determine Cardiac Output in COPD Patients During Maximal or Intermittent Exercise?*

Valerie Bougault, BS; Evelyne Lonsdorfer-Wolf, MD, PhD; Anne Charloux, MD, PhD; Ruddy Richard, MD, PhD; Bernard Geny, MD, PhD and Monique Oswald-Mammosser, MD, PhD

* From the Department of Respiratory, Cardiocirculatory, and Exercise Physiology, University Hospital of Strasbourg, Strasbourg, France.

Correspondence to: Monique Oswald-Mammosser, MD, PhD, Service des Explorations Fonctionnelles Respiratoires et de l’Exercice, Hôpital Civil, BP 426, 67091 Strasbourg Cedex, France; e-mail: Monique.Oswald{at}chru-strasbourg.fr


   Abstract
TOP
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 Conclusion
 References
 
Study objectives: The monitoring of cardiac output (CO) during exercise rehabilitation in patients with COPD, often including strenuous exercise, is advisable. Invasive methods (thermodilution, Fick method) are accurate, but for clinical routine use noninvasive CO estimation is required. We have shown that impedance cardiography (Physio Flow; Manatec Biomédical; Macheren, France) is reliable in COPD patients at rest and during a recumbent, light-intensity exercise. The aim of our study was to evaluate the validity of this noninvasive device in COPD patients during a maximal incremental exercise test (IET) and also during a strenuous intermittent work exercise test (IWET).

Design: Prospective comparative study of the impedance cardiograph vs the direct Fick method applied to oxygen.

Patients: Eight patients with moderate-to-severe COPD (59 ± 6 years old; FEV1, 38 ± 15% predicted; residual volume, 194 ± 64% predicted) [mean ± SD].

Measurements and main results: Forty-nine simultaneous measurements of CO by means of the direct Fick method (COFICK) and CO measured by the impedance cardiograph (COPF) were obtained during the IET, and 108 measurements were made during the IWET. The correlation coefficients between the two measurements were r = 0.85 and r = 0.71 for the IET and the IWET, respectively. COPF was higher than COFICK. The difference between the two methods was 3.2 ± 2.9 L/min during the IET and 2.5 ± 2.1 L/min during the IWET. Expressed as a percentage of the mean of the two measurements, this corresponded to 31 ± 21% and 25 ± 20%, respectively.

Conclusions: The relatively high number of values differing by > 20% precludes the use of impedance cardiography in clinical routine in such a difficult setting (hyperinflated patients and intense exercise).

Key Words: cardiac output • COPD • exercise • Fick principle • impedance cardiography


   Introduction
TOP
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 Conclusion
 References
 
COPD patients have impaired exercise tolerance due to ventilatory limitation and peripheral muscle weakness and thus excessive dyspnea during exercise.1 To counteract these impairments, exercise rehabilitation has been proposed. It has been demonstrated that exercise rehabilitation improves exercise capacity and quality of life, with no change in resting pulmonary function.1 Before beginning physical training rehabilitation, patients are evaluated in most laboratories by a maximal incremental exercise test (IET). While ventilatory parameters are easily measured during an IET, cardiovascular parameters determination, apart from heart rate (HR), are more difficult to obtain. However, it is important to follow hemodynamics evolution and especially cardiac output (CO) during IET and also during high-intensity training, in order to investigate the safety of such work rates. Noninvasive methods for CO measurement are thus required since invasive methods cannot be used routinely. The latter, including dye-dilution, thermodilution, and direct Fick method, considered as reference methods, are expensive, often difficult to use, especially during exercise, and require skilled personal.2 The CO2 rebreathing method is an attractive noninvasive method to measure CO in COPD patients, but is difficult to use, requires steady state, and its accuracy has not been proven during exercise.34 Impedance cardiography, a noninvasive method, easy to use, and which can provide real-time CO data, appears promising to measure hemodynamic parameters in normal subjects56789 and in patients.101112131415 Several devices exist16 using different Kubicek et al17 or Bernstein18 formulas. But until now, they differ in their approach to the position of the electrodes,19 the use of blood-specific resistivity values.101620 and the need to measure baseline impedance (Z). We used a recent impedance cardiograph device (Physio Flow PF-05; Manatec Biomedical; Macheren, France) with a modified stroke volume (SV) calculation formula that requires neither baseline Z measurement nor blood resistivity estimation, and for which the position of electrodes is not critical. We observed in our laboratory accurate measurements of CO (compared to the direct Fick method) at rest, during an upright IET in healthy subjects,5 and also during a constant, recumbent, light-intensity exercise in COPD patients.11 However, whether impedance cardiography may be useful in patients with more severe COPD and during more intense and/or intermittent exercise (largely used for exercise training) remains to be determined.

The aim of our study was thus to compare CO measured by means of electrical impedance (COPF) with CO measured by the Fick (COFICK) principle applied to oxygen, during two types of exercises in COPD patients: maximal IET, which is necessary to estimate the exercise capacity in order to optimize the exercise training programs,121 and during a high-intensity intermittent work exercise test (IWET) we use for exercise training. The latter consists of six stages of 5-min exercise with 4-min cycling at a moderate work intensity, and 1-min cycling at very high intensity work (see below). We have shown2223 that CO (after the first minutes of exercise) is stable during the IWET in healthy and COPD subjects. This is due to increasing HR and decreasing SV during this prolonged 30-min exercise. To our knowledge, no study has investigated impedance cardiography in such conditions in which HR and SV change with a slow component from the beginning to the end of exercise and with a faster component within the 5-min stages, these parameters being lower during the moderate-intensity cycling and higher during high-intensity cycling.


   Materials and Methods
TOP
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 Conclusion
 References
 
Eight COPD patients (seven men and one woman; age range, 51 to 69 years; mean ± SD, 59 ± 6 years) participated in the study. They were all in a stable disease state. Dyspnea graded using a 5-level scale24 ranged from 2 to 4. None of the patients were receiving long-term oxygen therapy. No patient had overt ischemic or other cardiac disease. Systolic left and right ventricular function was normal at rest on echocardiography (performed in seven patients, not done in patient 3). The local ethics committee approved the experimental protocol, and the patients gave their written informed consent.

Lung Function
Lung volumes were obtained by means of a body plethysmograph (Bodyscope; Ganshorn Medizin Electronic; Münnerstadt/Niederlauer, Germany). Reference values are from Quanjer et al.25 Diffusion capacity of the lung for carbon monoxide (DLCO) was measured by the single-breath method in all patients.

Exercise Protocols
In all patients, we simultaneously measured COFICK and COPF. The measurements were performed at rest and during two exercise tests: an IET and an IWET. These tests were performed on the same day, in the same temperature conditions, in the morning after a light breakfast for the IET, and in the afternoon for the IWET, 2 to 3 h after resting during which the patients had a light meal (Fig 1 ). The IET was performed to estimate the exercise capacities and to adapt the intensity of the IWET (see below).



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  		Figure 1.. Chronology of the two exercise tests during the day of investigation: the IET to determine VT and MTP; and the IWET, consisting of six stages of 5 min with 4-min exercise set at VT (base) and 1 min set at 90% MTP (peak).

 
Prior to the IET in the morning, a floated fluid-filled catheter (Flexopulmocath 4F; Plastimed; Saint-leu-la forêt, France) was inserted into the pulmonary artery via a brachial vein under local anesthesia. This permitted mixed venous blood sampling. Arterial blood sampling was performed through a pediatric radial artery catheter (Seldinger Catheter 20G-8cm; Vygon; Ecouen, France). In one patient, only transcutaneous O2 saturation could be obtained. The two exercise tests were performed with the patient sitting on an electronically braked cycle ergometer (Medifit 1000S; Medifit; Maarn, Holland). Each patient performed an IET up to exhaustion to determine his maximal tolerated power (MTP) and his ventilatory threshold (VT) determined graphically using the method of Beaver et al.26 The workload was increased by 10 W every minute after a 3-min warm-up period (14 ± 5 W). The IWET consisted of a 3-min warm-up period followed by six stages of 5 min, with 4 min of cycling at an intensity corresponding to VT called base, alternating with 1 min of cycling at a power corresponding to 90% MTP called peak. For the two exercise tests, patients were asked to maintain pedaling frequency constant at a rate of 60 revolutions per minute. HR and ECG were monitored continuously with a 12-derivation ECG (Cardiovit CS-200; Schiller AG; Baar, Switzerland).

Gas exchange parameters (minute ventilation, tidal volume, breathing frequency, oxygen uptake [O2]; CO2 output) were measured by means of an open-circuit metabolic chart on a breath-by-breath basis (SensorMedics; Yorba Linda, CA). Data were averaged over 20-s periods. Blood gas analyses (pH, PaCO2, PaO2, and oxyhemoglobin measurements) were also performed (Ciba-Corning 270–278 Co-oxymeter; Bayer; Puteaux, France).

COFICK
COFICK was measured every 2 min from rest to peak effort during the IET, and at rest at the first and third minute of each IWET stage. The formula used was as follows: COFICK = O2/(CaO2 – vO2), where CaO2 = systemic arterial blood content in oxygen, and vO2 = mixed venous blood content in oxygen. The oxygen content was calculated as follows:

where Hb is the blood hemoglobin concentration in grams per 100 mL, SO2 is hemoglobin saturation in percentage, and 1.34 is the oxygen-carrying capacity expressed in milliliters of oxygen per gram hemoglobin. SV by the Fick principle (SVFICK) was calculated as the ratio of COFICK to HR by the Fick principle.

COPF
SV measured by the bioimpedance method (SVPF) is based on changes in transthoracic impedance (Z) during cardiac ejection. The device we used (Physio Flow PF-05; Manatec Biomédical; Macheren, France) emits an alternating electrical current of 1.8 mA and 75 kHz via electrodes (Ag/AgCl, Blue Sensor VL; Medicotest; Oelstykke, Denmark). Two sets of two electrodes, one transmitting and one sensing, are applied above the supraclavicular fossa at the left base of the neck and along the xiphoid, respectively. Another set of two electrodes is used to monitor a single ECG signal (Fig 2 ). With the Physio Flow device, there is no need to measure baseline Z; thus, positioning of the electrodes is not critical.



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  		Figure 2.. Physio Flow device electrode positioning. Z1 and Z3 = transmitting electrodes; Z2 and Z4 = sensing electrodes; ECG1 and ECG2 = ECG monitoring electrodes.

 
SVPF measurement is based on the following formula:

, where HRPF is the HR (beats per minute) based on the R-R interval determined on the ECG first derivative; SVi is SV index (milliliters per meters squared of BSA; SVi = SV/BSA); and BSA is the body surface area (meters squared), calculated according to the Haycock formula:

, where BM is the body mass (kilograms), and H is height (centimeters).

A first SVi evaluation is done during the calibration (SViCAL), and is based on 30 consecutive heart beats, the patient being immobile and relaxed sitting on the ergocycle. This evaluation records the largest Z variation during ventricular systole (maximum Z – minimum Z) and the largest variation rate of the Z signal, called contractility index (dZ/dtMAX). SVi depends on the left ventricular ejection time. The Physio Flow device uses another slightly different parameter, the thoracic flow inversion time (TFIT) [in milliseconds; Fig 3 ]. The TFIT is measured from the first mathematical derivative of the impedance signal (dZ/dt) and is the time interval between the first zero value following the beginning of the cardiac cycle (beginning of QRS on the ECG) and the first nadir after the peak of the ejection velocity (dZ/dtMAX). The obtained TFIT is weighted using a specific algorithm that in addition of the electrical signal includes two parameters: HR measured by the impedance cardiograph (HRPF) and arterial tension difference (systolic arterial tension – diastolic arterial tension measured with a sphygmomanometer). Indeed, impedance wave form is depending on two parameters affecting aorta: aortic compliance and HR. Thus, SViCAL is determined according to the following formula:

where k is a constant, and SViCAL represents the baseline reference. During the data acquisition phase, the parameters described above are measured and compared to the measurement made during the calibration phase. Finally, the equation used to measure SV is as follows:

For this experiment, cardiac parameters (HRPF, SVPF, and COPF) were measured continuously and averaged every 20 s.



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  		Figure 3.. Waveforms obtained with the Physio Flow device. {Delta}Z = bioimpedance signal; Zmax = maximum bioimpedance signal.

 
Statistics
A paired t test was used to compare values measured with the Fick method and the Physio Flow device. Linear regression analysis was performed to assess the relationship between the two methods. In addition, to estimate the agreement between the two measurements, the Bland and Altman approach was used.27


   Results
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 Conclusion
 References
 
Resting Data
Anthropometric and pulmonary function data are presented in Table 1 . As a mean for the whole group, obstruction was moderate to severe and DLCO was decreased.


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  		Table 1.. Anthropometric and Pulmonary Function Data of the Subjects Included in the Study*

 
The results of CO, SV, and HR measured at rest are represented on Table 2 . There was no difference in CO between the morning and the afternoon when measured with the same method, but COPF was consistently and significantly higher than COFICK. This was mainly explained by a significantly higher SVPF than SVFICK, HRPF being no or only slightly higher than heart rate by the Fick principle. With both methods, SV tended to be lower in the afternoon compared to the morning, but this was not significant. HR was significantly higher in the afternoon for both methods.


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  		Table 2.. CO, SV, and HR Measured by the Two Methods at Rest, Before Exercise*

 
Exercising Data
In the 8 patients, 49 CO measurements were obtained during the IET in the morning and 108 measurements were obtained during the IWET in the afternoon. The correlation between COFICK and COPF during the IET and the IWET were rather good with r = 0.85 (p < 0.001) and r = 0.71 (p < 0.001), respectively (Fig 4 , top left, A, and top right, B). The differences between the two measurements plotted against their mean value according to Bland and Altman27 are represented in Figure 4, bottom left, C, and bottom right, D. During IET, the mean difference (COPF – COFICK) was 3.2 L/min with the limits of agreement being – 2.6 L/min and 9 L/min. Expressed as a percentage of the mean of COPF and COFICK, this corresponded to a mean difference of 31%, with limits of agreement of – 11% and 73%. During the IWET, the mean difference (COPF – COFICK) was 2.5 L/min, with limits of agreement of – 1.7 L/min and 6.7 L/min; in percentage, mean + 25%, and limits of agreement, – 15% and + 65%. The differences between the two measurements were ≥ 20% in 16 occasions (33%) during the IET and in 37 occasions (34%) during the IWET.



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  		Figure 4.. Top panels: Correlation plots between COPF and COFICK in the eight COPD patients. Top left, A: During the IET (49 measures). Top right, B: During the IWET (108 measures). Bottom panels: Differences and limits of agreement between the two measures of CO (COPF – COFICK) vs their mean ([COPF + COFICK]/2) [Bland and Altman27], in the eight COPD patients. Bottom left, C: IET; bottom right, D: IWET.

 
The correlation between SVPF and SVFICK during the IET and the IWET are represented in Figure 5 , top left, A, and top right, B, with r = 0.61 (p < 0.001) and r = 0.66 (p < 0.001), respectively. During the IET, the mean difference (SVPF – SVFICK) was 30 mL, with limits of agreement being – 16.8 mL and 76.8 mL (Fig 5, bottom left, C). Expressed as a percentage, this corresponded to a mean difference of 31%, with limits of agreement of – 13% and 75%. During the IWET, the mean difference (SVPF – SVFICK) was 19.9 mL, with limits of agreement – 16.3 mL and 56.1 mL (Fig 5, bottom right, D). Expressed as a percentage, this corresponded to a mean difference of 23%, with limits of agreement of – 17% and 63%. HR measured with the two methods correlated well, with r = 0.96 (p < 0.001) for the IET and r = 0.99 (p < 0.001) for the IWET, the difference being on average approximately 1 to 2 beats/min.



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  		Figure 5.. Top panels: Correlation between SVPF and SVFICK in the eight COPD patients. Top left, A: During the IET (49 measures). Top right, B: During the IWET (108 measures). Bottom panels: Differences and limits of agreement between the two measures of stroke volume (SVPF – SVFICK) vs their mean ([SVPF + SVFICK]/2) [Bland and Altman27], in the eight COPD patients. Bottom left, C: IET. Bottom right, D: IWET.

 
Individual values of CO measurements during the IET and the IWET are represented in Figure 6 . As can be seen, adequacy between the two methods was not obtained with accuracy in most cases. In only one patient (patient 3) did the two measurements not differ by > 20%, either during the IET or during the IWET, except for the IWET resting measurement (difference of 37%). In another patient (patient 8), adequacy between the two measurements was good during the IWET, but not during the IET. These two patients presented with lesser pulmonary distension (residual volume [RV]/total lung capacity [TLC], 45% and 39%, respectively). Nevertheless, the differences between COPF and COFICK were neither correlated with RV/TLC nor with FEV1/vital capacity (VC) for the whole group.



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  		Figure 6.. Individual values of CO during the IET and the IWET. Solid lines indicate COFICK; dashed lines indicate COPF.

 


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  		Figure 6.. Continued.

 
The correlation coefficient between O2 and COFICK was r = 0.95 (p < 0.001), and the regression equation was COFICK = 5.9O2 + 3.1. The correlation coefficient between O2 and COPF was r = 0.88 (p < 0.001), and the regression equation was as follows: COPF = 9.7O2 + 3.2.


   Discussion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
Conclusion
 References
 
The main results of our study are as follows: (1) the Physio Flow PF-05 overestimates CO—compared to the Fick method—both at rest and during IET and IWET exercise in COPD patients, and (2) although a fair correlation is globally observed between both methods, the number of values differing by > 20% is relatively high. In our study, COFICK was considered the "gold standard" of CO measurements during the IET and IWET. With 1-min IET, when obviously a steady state is not obtained, the accuracy of the measurement may be questioned. Nevertheless, Yamabe et al28 compared COFICK obtained during a 1-min IET to COFICK measured during steady-state exercises, performed at 30% and 65% of MTP, in patients with chronic heart failure. Nakanishi et al29 compared a 3-min IET with a 1-min IET in patients with an old myocardial infarction. Both authors observed that COFICK is a valid measurement of CO even with 1-min incrementals in the work rate. In our study, during IET, COPF and COFICK increased linearly with O2. The slope of the O2 vs COFICK regression equation is 5.9 L/L and similar to those described by Proctor et al21 during a 2-min IET in healthy younger and older subjects (range, 5.4 to 5.9 L/L). In contrast, the slope of the correlation of O2 vs COPF in our study is 9.7 L/L, thus much higher; this is more evidence that COPF overestimates CO in our COPD patients.

The higher COPF compared to COFICK is in great part due to higher SVPF than SVFICK, the difference in HR between the two methods being of no clinically importance. At rest, in our study mean SVFICK for the whole group is 63 mL in the morning, and 56 mL in the afternoon. Quite similar low SV values (in comparison to healthy subjects) have been reported in COPD patients by Stewart and Lewis30 with the direct Fick method, and by Bogaard et al10 and Smit et al31 using an impedance cardiograph. Thus, we consider that with our thoracic impedance device, SV is overestimated in our series of moderate-to-severe COPD.

Our results could not be explained by the severity of lung disease since there was no correlation between COPF – COFICK and hyperinflation estimated by RV/TLC or obstruction appreciated by FEV1/VC. Nevertheless, it must be underlined that the two patients for whom there was an adequacy between the two measurements, especially during IWET, had the least pulmonary hyperinflation. A larger series of patients with various degree of hyperinflation should be investigated to estimate this factor as a cause of failure for COPF to estimate CO in COPD patients.

In our study, the device we used for thoracic impedance measurement is the Physio Flow PF-05, the algorithm of which is slightly different from that of the PF-03 investigated in the two previous studies511 performed in our laboratory. In those studies, adequacy between COPF and COFICK was correct. Richard et al5 observed during an IET in healthy people, a mean difference COPF – COFICK of – 0.58 L/min (– 11%), with limits of agreement between –27% and + 21%. Charloux et al11 observed a mean difference in COFICK – COPF of 0.26 L/min (2.4%), with limits of agreement between – 2.2 L/min (– 21%) and + 2.7 L/min (+ 26%), in COPD patients and patients with obstructive sleep apnea syndrome during recumbent exercise on an ergocycle, at a light intensity. LaMantia et al32 stated that if the limits of agreement do not exceed 22%, the two methods of CO measurement are in agreement, in respect with accuracy. Whereas Charloux et al11 and Richard et al5 reported valuable measurements of CO with the Physio Flow device, with few data outside the 22% limit, we observed more discrepancies between COFICK and COPF that may be explained by the changes in the PF-05 algorithm. However, Tordi et al6 reported a good correlation between the COPF measured with the PF-05 and CO measured by the CO2 rebreathing method during steady-state exercises (performed at a work rate corresponding to HRs of 120, 140, and 160 beats/min) in healthy young men, with a mean difference between the two measurements of 0.06 L/min. In this study, mean SVPF was quite similar to SV measured by means of CO2 rebreathing. From this recent work6 and the two previous studies511 performed in our laboratory, it can be concluded that the Physio Flow is a valid device to investigate CO in healthy subjects or in COPD patients in the recumbent position during light-exercise intensity. The present study demonstrated that this is far from being the case in patients with severe COPD in the upright position at rest and during heavy exercise, especially in patients with hyperinflation, as discussed above.

Bogaard et al10 compared CO measured by means of an impedance cardiography device (IPG-104 impedance; Mini-Lab; Detroit, MI) and by means of the noninvasive CO2 rebreathing method. SV and CO were measured at rest and during steady-state exercises, ranging from light intensity to exhaustion, in patients with moderate COPD.10 The authors found a good correlation between the two methods during exercise, with few data falling outside the limits of agreement of ± 22%. The mean CO difference (impedance – rebreathing) was 0.01 ± 1.28 L/min, which are acceptable values. It must be underlined that these results were obtained in COPD patients with lesser obstruction than our subjects. Bogaard et al10 observed higher impedance CO values compared to rebreathing at low-intensity exercise and lower impedance CO values at high-intensity exercise. It is interesting to note that the slope of the regression analysis of CO vs O2 with their impedance cardiography device was 6.2 L/L; this is much less than what we observed in our study and close to the slope we reported for COFICK. Bogaard et al,33 in a review, hypothesized that different parameters could contribute to the inaccuracy of an electrical impedance cardiography device in COPD patients: increased motion artifacts induced by accessory respiratory muscle movement, increased respiratory artifacts (higher ventilation per minute for the same quantity of work), and change in lung volumes.33 All these factors could be responsible for the discrepancies we observed between COFICK and COPF, but it must be underlined that we observed no correlation between COPF – COFICK and minute ventilation, tidal volume, or breathing frequency. We found no more correlation between COPF – COFICK and hyperinflation or obstruction estimated at rest in our study. This is in agreement with the results reported for healthy subjects breathing with a high resistance.34 Nevertheless, we cannot exclude hyperinflation as a factor explaining the inaccuracy of the COPF measurements in our study. Indeed, Bogaard et al33 explained that baseline Z, which accounts for the basic thoracic Z, compensates for lung volumes. With our device, baseline Z is not measured, thus preventing perhaps such compensation. However, during the calibration, maximum Z – minimum Z, which is probably influenced by lung volumes, is taken into account; this minimizes probably the fact that baseline Z is not measured. However, CO measurement is based on the aortic component of the dZ/dt estimation. During exercise in COPD patients, nonnegligible changes in pulmonary vascular flow and intrathoracic pressure, in addition to hyperinflation, may change the passage of the current emitted by the impedance cardiography device through the chest and thus modify the aortic component of dZ/dt. This may explain that adequacy was not improved during exercise, in contrast to what was observed by Bogaard et al10 in their patients with less severe COPD. Moreover, in their study, Critchley and Critchley35 observed that CO measured by impedance cardiography underestimated the real CO in patients with increased amount of fluid in the chest. They suggested that the aortic component of dZ/dt was minored by the pulmonary component. It is not excluded that in patients with increased amount of air in their chest (in COPD hyperinflated patients), the opposite situation occurs, which could lead to an overestimation of CO.


   Conclusion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Conclusion
References
 
Although a fair correlation is globally observed between COPF and COFICK, and in contrast to what we observed in our laboratory in two previous studies511 in healthy subjects and in recumbent COPD patients during a light exercise, the relatively high number of values differing by > 20% precludes the use of the impedance cardiography in clinical routine in such a difficult setting (hyperinflated COPD patients and intense or intermittent exercise). We hypothesize that the changes in the relationship between the aortic and the pulmonary components of Z due to changes in intrathoracic pressure, pulmonary vascular flow in addition to dynamic hyperinflation during exercise may modify dZ/dt.


   Footnotes
 
Abbreviations: BSA = body surface area; CO = cardiac output; COFICK = CO measured by the direct Fick method; COPF = CO measured by the impedance cardiography; DLCO = diffusion capacity of the lung for carbon monoxide; dZ/dt = mathematical derivative of the impedance signal; dZ/dtMAX = contractility index; HR = heart rate; IET = incremental exercise test; IWET = intermittent work exercise test; MTP = maximal tolerated power; RV = residual volume; SV = stroke volume; SVFICK = stroke volume by the Fick principle; SVi = stroke volume index; SViCAL = stroke volume index measured during calibration; SVPF = stroke volume measured by the bioimpedance method; TFIT = thoracic flow inversion time; TLC = total lung capacity; VC = vital capacity; O2 = oxygen uptake; VT = ventilatory threshold; Z = impedance

Received for publication September 8, 2004. Accepted for publication September 30, 2004.


   References
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Conclusion
 References
 

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