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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Many patients with emphysema are able to meet ventilatory demands during resting conditions, but
they show severe limitations during exercise. To examine the effect of lung volume reduction (LVR)
surgery on exercise performance and the mechanism of possible improvement, we measured ventilatory mechanics (pulmonary resistance [RL], work of breathing [WOB], dynamic intrinsic positive end-expiratory pressure [PEEPi,dyn], peak expiratory flow rate [PEFR]), breathing pattern, oxygen uptake
(
O2), and carbon dioxide removal ( CO2) at rest and during cycle ergometry in eight patients before and 3 mo after LVR surgery. Ventilatory mechanics were evaluated assessing esophageal pressure
and air flow. Three months after LVR surgery, the tolerated workload was doubled when compared with the preoperative value (p < 0.0005), associated with a reduction of RL (p < 0.05), PEEPi,dyn
(p < 0.005), and WOB (p < 0.005) at comparable workloads. Maximal ventilatory capacity and maximal tidal volume (VT) increased significantly (p < 0.01). Maximal O2 increased from 474 ± 23 to
601 ± 16 ml/min (p < 0.005) and maximal CO2 from 401 ± 13 to 558 ± 21 ml/min (p < 0.005), though no significant difference at comparable workloads could be observed. In conclusion, emphysema surgery leads to an improvement of ventilatory mechanics at rest and during exercise. Higher maximal
VT and minute ventilation were observed, resulting in improvement of maximal O2 and CO2 and exercise capacity.
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Many patients suffering from emphysema are able to meet ventilatory demands adequately during resting conditions, but they exhibit severe limitations during exercise (1). A major factor responsible for the exercise limitation in this situation is the development of dynamic pulmonary hyperinflation (DPH), which accompanies the increased ventilatory demands (2).
An improvement in elastic recoil (5, 6) and ventilatory mechanics (7) during resting conditions has been recently demonstrated after lung volume reduction (LVR) surgery, a surgical procedure for the treatment of severe emphysema (8, 9). Since exercise limitation is one of the main causes of impaired quality of life in patients suffering from severe emphysema (2), it is important to investigate the effect of LVR surgery on exercise capacity.
The effect of LVR surgery on muscle recruitment was recently published, indicating improved diaphragmatic function
after surgery (10). Nevertheless, Benditt and colleagues (10)
did not determine parameters closely related to ventilatory
mechanics such as total resistive work of breathing (WOB),
pulmonary resistance (RL), dynamic intrinsic positive end-expiratory pressure (PEEPi,dyn) (11), peak expiratory flow rate
(PEFR), oxygen uptake (O2), and carbon dioxide removal
(CO2) during exercise. Therefore, we have undertaken a prospective study to determine whether patients with severe emphysema demonstrate significant improvement in exercise capacity following LVR surgery and to determine the mechanisms of any possible improvement. We hypothesized that improvement of ventilatory mechanics following surgery would lead to
a reduction in the extent of DPH during exercise, leading to
an increase in maximal ventilatory capacity (MVC) and thus an
increase in O2 and CO2, enabling a significant improvement
in exercise capacity. To investigate this potential improvement
in ventilatory mechanics, we measured WOB, RL, PEEPi,dyn,
PEFR, ventilatory pattern, O2, and CO2 in these patients at
rest and exercise both before and 3 mo after surgery.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Patients
Eight consecutive patients with nonbullous emphysema undergoing bilateral LVR surgery were included in this study (Table 1). The study was approved by the local institutional review board, and all patients gave written informed consent before the operation. Hyperinflation accompanied by functional limitation despite optimal pharmacotherapy was the primary indication for LVR surgery. Baseline patient data, which were determined after a standardized pulmonary rehabilitation and exercise program for at least 4 wk, are shown in Table 1.
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General Patient Assessment
Preoperative assessment included patient history, physical examination, standard pulmonary function testing, whole body plethysmography, arterial blood gas values, quantitative ventilation/perfusion scan, and catheterization of the right side of the heart. Ischemia of the brain, psychological disorders, myocardial infarction, or severe general disease, ventilation/perfusion mismatch, and a mean pulmonary artery pressure exceeding 40 mm Hg at rest were regarded as exclusion criteria for LVR. Morphologic assessment of the lungs was based on high-resolution computed tomography scan (HRCT) with 1-mm slice thickness at intervals of 15 mm. Patients with bullous emphysema, defined as air spaces of greater than one-third of a hemithorax, were excluded from the study.
Operative Technique
All patients were operated on bilaterally using a videoendoscopic approach. Surgery was first performed on the side that showed more severe abnormality in lung morphology. The goal was to remove 20% to 30% of the volume of each lung, concentrating on the regions showing greatest destruction. When the emphysematous process was homogeneously distributed, portions of all lobes were excised.
Assessment of Ventilatory Mechanics
Ventilatory mechanics were assessed 5-10 d before surgery and 3 mo
after the procedure. Ventilatory mechanics were assessed during rest
and cycle ergometer testing. Air flow () was measured by a flow sensor (Varflex, Bicore CP-100 monitor; Bicore Monitoring Systems,
Inc., Irvine, CA), connected to a tightly adjusted face mask (12). The
fit of the face mask was evaluated by comparing inspiratory and expiratory volumes. A difference of less than 5% was regarded as sufficiently tight. Tidal volume was obtained by integration of the flow signal. Airway pressure (Paw) was measured through a catheter attached
to the flow sensor and the esophageal pressure (Pes) by a nasogastric
tube incorporating an esophageal balloon. The correct position of the
balloon catheter was verified using the occlusion test (13). The esophageal balloon and the flow sensor were connected to a portable monitor (CP-100 cardiopulmonary monitor) that provided a real-time display of , volume (V), Paw, and Pes tracings. Loops of Pes versus
and Paw versus relationships were derived from this device. The accuracy of the measurements provided by this monitoring system has
recently been found satisfactory (14). In addition, we performed an
evaluation of the Bicore CP-100 measurements and the calculations
performed by the software of the device. The results of this evaluation are given in the Appendix .
Minute ventilation (E) and breathing pattern, i.e., tidal volume
(VT), respiratory frequency (f), the duration of inspiration (TI) and
expiration (TE), and the duty cycle (TI/Ttot) were analyzed from the
flow signal. Total resistive work of breathing (WOB) values were provided directly by the monitor, which calculated the area under the Pes
versus lung volume curve (15). The monitoring system provides WOB
for each breath. WOB is obtained by integrating the area subtended
by Pes and lung volume during a complete respiratory cycle. RL was
determined by the device during inspiration and expiration, and the
presented values represent mean pulmonary resistance. The following
formulas were used for calculation of RL and WOB:
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(1) |
where RL = pulmonary resistance in cm H2O/L/s, Paw = airway pressure in cm H2O, Pes = esophageal pressure in cm H2O, PTP = transpulmonary pressure in cm H2O, I = inspiratory airflow at the mouth
in L/s, and E = expiratory airflow at the mouth in L/s.
Chest wall compliance was not included in our WOB calculations
because the subjects were breathing spontaneously during the entire
measuring period. Therefore, the formula used for calculation of
WOB was WOB = (Pee 
Pes) · dt, where Pes = esophageal pressure at the end of inspiration; Pee = end-expiratory esophageal pressure immediately before the onset of inspiratory flow; dt = volume, calculated from the time-integrated flow signal, determined by the
flow transducer.
PEEPi,dyn was measured, whereby PEEPi,dyn is equal to the absolute change in Pes from the onset of inspiratory effort to the onset
of inspiratory (16). All resting measurements were performed in
patients breathing room air quietly in an upright sitting position on
the bicycle ergometer. During resting conditions WOB, PEEPi,dyn,
and RL were calculated from 39 breaths, after excluding values that
differed from the mean by more than two standard deviations. These
extreme values are most likely artifacts due to coughing or swallowing. During exercise, values of the above-mentioned variables were
calculated from the last 10 breaths at a given workload. Maximal peak
expiratory flow rates (PEFRmax) were calculated by determining the
mean value of PEFR from the last 10 breaths at maximal workload
(Wmax).
Exercise Test Procedure
Resting measurements were made with the patient seated on a bicycle
ergometer with electrical brakes (Jaeger, Wuerzburg, Germany), breathing room air through a one-way non-rebreathing circuit. The
flow transducer (Varflex) from the Bicore CP-100 monitor was connected tightly to the face mask and the circuit. The patient then began
pedaling at 60 revolutions per minute at an initial work load of 10 watts (W), which was incremented by 10 W every 2 min until the test
was terminated due to patient exhaustion or decision of the supervising physician. The following data were collected continuously at rest
and throughout exercise: heart rate, blood pressure, and carbon dioxide (CO2) and oxygen (O2) gas concentrations (EOS Sprint; Jaeger,
Wuerzburg, Germany) for calculation of O2 and CO2. Arterial
blood samples were drawn at the end of each work rate for subsequent analysis.
Spirometry and Plethysmography and Maximum Voluntary Ventilation
Spirometry and whole body plethysmography were performed in all patients before and at 3 mo following surgery. Spirometry was performed using an open system with integration of the flow system; whole-body plethysmography was carried out by the constant volume method (Jaeger). The patients' lung function parameters were compared with the reference value given by the European Community for Steel and Coal (17). Maximum voluntary ventilation (MVV) was determined by means of the generally used method (18).
Statistical Analysis
Statistical analysis was performed with the Wilcoxon matched-pairs
signed-ranks t test for determining differences in WOB, PEEPi,dyn,
RL, PEFRmax, f, E, VT, TI/Ttot, O2, CO2, PaO2, and PaCO2, as well
as spirometric parameters caused by surgery (19). Normalized PEFRmax after surgery was correlated with the increase in exercise capacity
after surgery by means of linear regression. Values are expressed as
mean ± standard error of the mean (SEM). Value p < 0.05 was considered as the level of significance.
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RESULTS |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Exercise Test Results
Preoperatively, all but one patient (7/8) could perform 4 min of exercise testing (20-W workload). Exercise testing had to be interrupted at 10 W in one patient before surgery, and only one out of eight patients was able to finish 6 min exercise testing (30 W) preoperatively. The cause for stopping exercise was shortness of breath in all patients. In contrast, all patients were able to perform 8 min (40 W) of ergometer testing 3 mo after surgery (p < 0.0005), and one patient could reach up to 70 W postoperatively. In all of the pre- and postoperative tests exercise was terminated by the patient.
Ventilatory Mechanics and Breathing Pattern
Dynamic intrinsic positive end-expiratory pressure (PEEPi,dyn) (Figure 1) and RL (Figure 2) were significantly lower during cycle ergometer testing at comparable workloads after LVR surgery (Table 2). Total resistive work of breathing (Figure 3) decreased at rest and comparable workloads 3 mo after surgery (Table 2). Maximal peak expiratory flow rate increased in all patients after surgery as shown in Figure 4. Mean PEFRmax increased from 0.89 ± 0.04 L/s before surgery to 1.27 ± 0.09 L/s (p < 0.005) after surgery. Normalized PEFRmax was correlated with the increase in Wmax: r = 0.84; p < 0.01 (Figure 5).
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1 · s
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No significant difference in TI/Ttot was observed when preoperative values were compared with postoperative values. Duty cycle was 0.41 ± 0.02 versus 0.44 ± 0.02 at rest (preoperative value versus postoperative value), 0.41 ± 0.02 versus 0.44 ± 0.02 at 10-W workload, and 0.42 ± 0.03 versus 0.43 ± 0.01 at 20-W workload. Preoperatively, respiratory frequency during rest was 18.5 ± 0.7 breaths/min versus 18.5 ± 1.1 breaths/min postoperatively, and increased at 10 W to 23.9 ± 1.4 preoperatively versus 20.3 ± 0.4 breaths/min (p < 0.025) postoperatively. At 20-W workload respiratory frequency was 25.7 ± 1.7 before versus 21.9 ± 0.4 breaths/min (p < 0.01) after LVR surgery. Maximal VT observed during exercise increased significantly (p < 0.01) 3 mo after LVR surgery (0.89 ± 0.04 L preoperatively versus 1.11 ± 0.04 L postoperatively), and was significantly (p < 0.05) higher at 20 W (Figure 6).
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Oxygen Uptake, Carbon Dioxide Removal, and Arterial Blood Gases
Maximal oxygen uptake and maximal carbon dioxide removal
were significantly (p < 0.005) higher after surgery, as shown in Table 3. Nevertheless, at comparable workloads there was no
significant difference in O2 (Figure 7) or CO2 (Figure 8). Minimal PaO2 during exercise showed great interindividual differences pre- and postoperatively and was not significantly improved after surgery, whereas maximal PaCO2 during exercise
was significantly (p < 0.005) reduced after surgery (Table 3).
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Spirometric Findings and MVV
As displayed in Table 4, FEV1 was significantly (p < 0.01) improved in our patient population 3 mo after surgery. Residual volume (RV) decreased significantly (p < 0.01), similar to total lung capacity (p < 0.01). Maximum voluntary ventilation improved significantly (p < 0.01) after LVR. The ratio of MVC to MVV was significantly lower after surgery (p < 0.01).
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Higher workloads were tolerated by patients after emphysema surgery. Changes in ventilatory mechanics involved significantly reduced PEEPi,dyn, RL, and WOB at comparable workloads as well as higher maximal PEFR during exercise.
Ventilatory Mechanics
Preoperatively, our patients showed significant airflow obstruction at rest and during exercise, which improved after surgery (Figure 4, Table 4). Chronic airflow obstruction is known to lead to reduced MVC, which in turn leads to reduced exercise capacity (20). Maximal ventilatory capacity improved significantly after surgery, probably leading to improved exercise capacity. These findings are comparable with the findings of O'Donnell and coworkers (21) in patients undergoing bullectomy and ipsilateral lung volume reduction surgery.
PEEPi,dyn, an indirect sign for the amount of air trapping
and closely related to the amount of DPH (16), was present
even during resting conditions in our patients before surgery
(Figure 1). The presence of PEEPi,dyn during rest is an indirect sign of DPH caused by expiratory flow limitation at rest,
which is frequently present in patients suffering from severe
chronic obstructive pulmonary disease (COPD) (22). If expiratory flow limitation is present during resting breathing, any
further increase in ventilation should result in enhanced DPH,
and thus increased PEEPi,dyn (23). We observed steadily increasing PEEPi,dyn with increasing workloads before and after surgery, but the increase in PEEPi,dyn was lower after the
surgical procedure (Figure 1). This finding indicates that DPH
is decreased by means of the surgical procedure. Dynamic pulmonary hyperinflation, and thus PEEPi,dyn, during rest and exercise is mainly caused by expiratory flow limitation in patients suffering from severe COPD (23). Since we could demonstrate improved PEFRmax (Figure 4) and a good correlation
between the increase in PEFRmax and the increase in Wmax
(Figure 5), improved expiratory air flow is very likely a major
factor causing reduced PEEPi,dyn at comparable workloads
after surgery. Hoppin predicted on a theoretical basis that
LVR surgery can lower the lung volume at which a given expiratory flow can be achieved (24). This theoretical prediction
seems to be confirmed by our findings of postoperatively increased PEFRmax accompanied by a reduction in PEEPi,dyn,
at isowatt, indicating a reduced DPH at comparable E.
Our patients showed reduced WOB at comparable workloads after surgery (Figure 3), probably due to decreased PEEPi,dyn (Figure 1) and decreased RL (Figure 2), since PEEPi,dyn and RL influence WOB (25). Severely increased WOB has been observed in COPD patients with acute respiratory insufficiency (26). In these patients acute respiratory insufficiency has been attributed to respiratory muscle fatigue (26), because DPH leads to elevated inspiratory WOB and impaired inspiratory muscle function, since the inspiratory muscles are forced to operate at an unfavorable length for pressure generation (27). Exercise termination occurred in our patients at comparable maximal values of WOB (Figure 3) before and after surgery, but these maximal values were reached at higher workloads postoperatively. Although we have no direct evidence for respiratory muscle fatigue due to high WOB in our patients, the above-mentioned findings indicate that decreased WOB at isowatt exercise contributed to increased exercise tolerance. In addition, increased maximal transdiaphragmatic pressure has been observed in patients with COPD after LVR surgery (28). This is not surprising since maximal transdiaphragmatic pressure is dependent on lung volume (29), which is reduced after LVR surgery. Therefore, improved diaphragmatic function due to a reduction in lung volume could probably also contribute to improved exercise capacity.
Respiratory Pattern
The decrease in PEEPi,dyn we observed in our patients (Figure 1) could also be caused by changes in respiratory pattern,
such as a reduction of E, VT, and/or increase in TE (30).
However, VE and TI/Ttot were not significantly different at
comparable workloads after surgery, VT was increased at 20-W
workload (Figure 6), even though PEEPi,dyn was decreased
(Figure 1), and TI/Ttot was not changed at comparable workloads after surgery. Therefore, changes in breathing pattern
were not responsible for reduced PEEPi,dyn during rest and
exercise in our patients.
After surgery patients were able to reach significantly
higher E by generating higher VT (Figure 6), whereas TI/Ttot
was unchanged and respiratory frequency decreased at comparable workloads. In healthy subjects a rise in E is mainly
achieved by increasing VT (31) by means of a reduction in
FRC and an increase in inspiratory capacity (23). Patients suffering from severe COPD, like our patients undergoing LVR
surgery, exhibit low expiratory flow rates and breathe at elevated FRC to improve maximal expiratory flow during exercise (22). This reduces the ability of these patients to increase
VT and thus limits maximal exercise ventilation (23). An increase in E is achieved by an increase in respiratory frequency, leading to the unfavorable rapid shallow breathing
pattern (32), which we observed in our patients preoperatively. After LVR surgery our patients could generate significantly higher maximal VT (Figure 6) and E, probably due to
improved expiratory flow rates (Figure 4).
Changes in O2 and CO2
At comparable workloads and E, no significant changes in
O2 (Figure 7) and CO2 (Figure 8) were observed. In addition, we observed significantly higher maximal O2 after surgery (Figure 7) and CO2 (Figure 8), which can be explained
by improved E after surgery, and is the precondition for sufficient oxygen supply at higher workloads. Recently, Koulouris and colleagues (22) reported good correlation between
the degree of flow limitation and O2max as well as the reduction in VTmax. These findings correspond with our findings of
improved PEFRmax accompanied by increased O2max (Figure 7) and increased VTmax (Figure 6), enabling improved exercise performance after LVR surgery.
In summary, our results show that maximal exercise capacity is doubled after LVR surgery, probably owing to improved
expiratory flow, which leads to a reduction of PEEPi,dyn and
WOB, while enabling an increase in VT at comparable E. As
exercise limitation is known to be a major cause of decreased
quality of life (1), LVR surgery seems, at least by short-term
results, a proper therapeutic approach for patients suffering from
emphysema, especially because conventional therapy cannot
provide improvement of similar magnitude (33).
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Footnotes |
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Correspondence and requests for reprints should be addressed to Edda M. Tschernko, M.D., Department of Clinical Pharmacology, Vienna General Hospital, Waehringer Guertel 18-20 A-1090, Vienna, Austria. E-mail: Edda.Tschernko{at}univie.ac.at
(Received in original form February 20, 1997 and in revised form June 5, 1998).
Edda M. Tschernko holds a scholarship from the Erwin Schroedinger Research Foundation, Austria.Acknowledgments: The authors thank Martin J. Tobin, M.D., for carefully reading the manuscript.
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References |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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26. Fleury, B., D. Murciano, C. Talamo, M. Aubier, R. Pariente, and J. Milic-Emili. 1985. Work of breathing in patients with chronic obstructive pulmonary disease in acute respiratory failure. Am. Rev. Respir. Dis. 131: 822-827 [Medline].
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28. Tschernko, E. M., W. Wisser, T. Wanke, M. A. Rajek, M. Kritzinger, H. Lahrmann, M. Kontrus, H. Benditte, and W. Klepetko. 1997. Changes in ventilatory mechanics and diaphragmatic function after lung volume reduction in patients with COPD. Thorax 52: 545-550 [Abstract].
29. Wanke, T., G. Schenz, H. Zwick, W. Popp, L. Ritschka, and M. Flicker. 1990. Dependence of maximal sniff generated mouth and transdiaphragmatic pressure on lung volume. Thorax 45: 352-355 [Abstract].
30. Tuxen, D. V., and S. Lane. 1987. The effects of ventilatory pattern on hyperinflation, airway pressures, and circulation in mechanical ventilation of patients with severe air-flow obstruction. Am. Rev. Respir. Dis. 136: 872-879 [Medline].
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33. Belman, M. J., W. C. Botnick, and J. W. Shin. 1996. Inhaled bronchodilators reduce dynamic hyperinflation during exercise in patients with chronic obstructive pulmonary disease. Am J. Respir. Crit. Care Med. 153: 967-975 [Abstract].
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APPENDIX |
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To compare conventional measuring techniques with the measurements of the Bicore CP-100 monitor and the breath-by-breath calculations of the device, the following experiments were performed.
Evaluation of the Accuracy of the Bicore Pneumotachograph
Six patients with COPD were studied during rest and maximal
exercise. Measurements of and V were performed simultaneously by the Bicore CP-100 pneumotachograph and the
pneumotachograph of the bicycle ergometer (EOS Sprint; Jaeger). Measurements were performed at rest and during exercise. In three patients the conventional pneumotachograph
was connected with the face mask and the Bicore pneumotachograph connected to the conventional pneumotachograph. In the remaining three patients the position of the pneumotachographs was exchanged. Values for f and VT were compared during 1 min of rest and during the last minute before
exercise was terminated. Single patient data are given in Table
5. The mean deviation in 12 measurements was 2.0% for f and
5.0% for VT (Table 5).
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Evaluation of PEEPi,dyn Measurements
PEEPi,dyn measurements were performed in 17 patients with COPD by means of the Bicore CP-100 monitor and the conventional technique during rest. The sequence of the measurements was chosen in a random fashion. In all patients conventional and Bicore measurements of PEEPi,dyn were performed on the same day. Data analysis was done by two physicians. The investigator performing Bicore calculations was not aware of the conventional results, and the investigator performing conventional calculations was unaware of the Bicore results.
Bicore measurements were performed as described in
METHODS. The mean value of PEEPi,dyn (given by the Bicore
software) of eight consecutive breaths was calculated after excluding values that differed from the mean more than two
standard deviations (likely artifacts due to coughing or swallowing) and referred to as PEEPi,dyn Bicore. Conventional
measurements were undertaken while the patient was breathing through a mouthpiece. Air flow was measured with a conventional pneumotachograph (Jaeger). Both conventional and
Bicore pneumotachographs were linear over the experimental
range of flows. Pes was measured with a balloon-tipped catheter system: a balloon 10 cm in length connected to a polyethylene catheter with an internal diameter of 1.5 mm and a length
of 120 cm. This catheter was connected to a differential pressure transducer (Jaeger). Pes, Paw, , and its integrated volume signal (V) were continuously recorded at 100 Hz using
an analog-to-digital converter (DATAQ Instruments, Akron,
OH). In eight consecutive breaths PEEPi,dyn was evaluated
as the negative deflection in Pes from the onset of inspiratory
effort to the point of zero flow during unoccluded breathing as
described by Ninane and coworkers (16). This approach assumes that the change in pleural pressure required to initiate inspiratory air flow approximates the opposing level of elastic recoil pressure present at end expiration, i.e., PEEPi,dyn. The mean value of PEEPi,dyn of eight consecutive breaths was
then referred to as the PEEPi,dyn Conventional of the patient. Thereafter, PEEPi,dyn Bicore and PEEPi,dyn Conventional were compared (Table 6). Conventional PEEPi,dyn was
3.5 ± 2.7 cm H2O, and PEEPi,dyn Bicore was 3.2 ± 2.9 cm
H2O. Mean PEEPi,dyn of the evaluated COPD patients at
rest was considerably lower compared with the patients undergoing exercise testing (3.2 ± 2.9 cm H2O versus 5.0 ± 0.9 cm H2O) in the presented study. This is due to the fact that patients displaying a high PEEPi,dyn at rest are preferably selected for LVR surgery at our institution.
|
Evaluation of WOB Measurements
Total resistive WOB was measured in our patients by integrating the area subtended by Pes and lung volume during a complete respiratory cycle. WOB done on the lungs was calculated
in eight consecutive breaths and mean WOB of those eight
breaths was thereafter compared with the mean WOB of eight
consecutive breaths determined by the software of the device.
WOB per breath was obtained by integration of the area subtended by the dynamic change in Pes and lung volume during the breathing cycle for conventional measurements (26).
WOB per liter of ventilation was calculated by dividing WOB
per minute by E. These calculations were performed in six patients with COPD (FEV1% predicted 28.3 ± 8.2%) during
rest. The maximum time that elapsed between breaths used
for conventional calculations and breaths used for Bicore calculations was 2 min. Mean WOB computed by the software of
the device was 1.52 ± 0.34 joules (J)/L versus 1.60 ± 0.29 J/L
when conventional calculations were performed. Thus, the
values derived from the automated computations underestimated WOB by 5%.
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