Effect of bronchodilation on expiratory flow limitation and resting lung mechanics in COPD.

Bronchodilator drugs produce variable improvements in forced expiratory volume in 1 s (FEV(1)), but larger changes in end-expiratory lung volume (EELV) in chronic obstructive pulmonary disease (COPD), which were suggested to be related to the presence of expiratory flow limitation (EFL) at rest. We tested this concept in 42 COPD patients (FEV(1) 42.3+/-13.8% predicted) during spontaneous breathing before and after 5 mg nebulised salbutamol. EFL was detected by within-breath changes in respiratory system reactance measured by a multifrequency forced oscillation method, while changes in EELV were assessed by inspiratory capacity (IC). Bronchodilation (BD) increased IC (from 1.8+/-0.5 to 2.1+/-0.6 L, p<0.001) and reduced inspiration resistance ((insp)) at 5 Hz (from 5.1+/-1.6 to 4.2+/-1.5 cmH(2)OxsxL(-1), p<0.001). (insp) identified BD responders with a discriminative power of 80.1%. In total, 20 patients were flow-limited before BD. They showed worse spirometry and higher residual volume, but significant improvements in IC were seen in all patients irrespective of flow limitation. Changes in (insp) were confined to flow-limited patients, as were reactance changes. BD reduced the degree of heterogeneity in the respiratory system, a change best seen with inspiratory values. BD has complex effects on lung mechanics in COPD, and EFL affects both this and the response of some respiratory variables to treatment. However, changes in EELV are consistently seen, irrespective of the presence of flow limitation at rest.

Respiratory impedance during weaning from mechanical ventilation in a mixed population of critically ill patients.

BACKGROUND: Worsening of respiratory mechanics during a spontaneous breathing trial (SBT) has been traditionally associated with weaning failure, although this finding is based on studies with chronic obstructive pulmonary disease patients only. The aim of our study was to assess the course of respiratory impedance non-invasively measured by forced oscillation technique (FOT) during a successful and failed SBT in a mixed population. METHODS: Thirty-four weaning trials were reported in 29 consecutive mechanically ventilated patients with different causes of initiation of ventilation. During the SBT, the patient was breathing through a conventional T-piece connected to the tracheal tube. FOT (5 Hz, +/- 1 cm H(2)O, 30 s) was applied at 5, 10, 15, 20, 25, and 30 min. Respiratory resistance (Rrs) and reactance (Xrs) were computed from pressure and flow measurements. The frequency to tidal volume ratio f/V(t) was obtained from the flow signal. At the end of the trial, patients were divided into two groups: SBT success and failure. RESULTS: Mixed model analysis showed no significant differences in Rrs and Xrs over the course of the SBT, or between the success (n=16) and the failure (n=18) groups. In contrast, f/V(t) was significantly (P<0.001) higher in the failure group. CONCLUSIONS: Worsening of respiratory impedance measured by FOT is not a common finding during a failed SBT in a typically heterogeneous intensive care unit population of mechanically ventilated patients.

Plasma proteomics reveals gestational age-specific responses to mechanical ventilation and identifies the mechanistic pathways that initiate preterm lung injury.

The preterm lung is particularly vulnerable to ventilator-induced lung injury (VILI) as a result of mechanical ventilation. However the developmental and pathological cellular mechanisms influencing the changing patterns of VILI have not been comprehensively delineated, preventing the advancement of targeted lung protective therapies. This study aimed to use SWATH-MS to comprehensively map the plasma proteome alterations associated with the initiation of VILI following 60 minutes of standardized mechanical ventilation from birth in three distinctly different developmental lung states; the extremely preterm, preterm and term lung using the ventilated lamb model. Across these gestations, 34 proteins were differentially altered in matched plasma samples taken at birth and 60 minutes. Multivariate analysis of the plasma proteomes confirmed a gestation-specific response to mechanical ventilation with 79% of differentially-expressed proteins altered in a single gestation group only. Six cellular and molecular functions and two physiological functions were uniquely enriched in either the extremely preterm or preterm group. Correlation analysis supported gestation-specific protein-function associations within each group. In identifying the gestation-specific proteome and functional responses to ventilation we provide the founding evidence required for the potential development of individualized respiratory support approaches tailored to both the developmental and pathological state of the lung.

Decision Making Concepts for the Remote, Personalized Evaluation of COPD Patients' Health Status.

INTRODUCTION: This article is part of the Focus Theme of Methods of Information in Medicine on "Biosignal Interpretation: Advanced Methods for Neural Signals and Images". OBJECTIVES: This paper presents the main concepts of a decision making approach for the remote management of COPD patients based on the early detection of disease exacerbation episodes. METHODS: An e-diary card is defined to evaluate a number of physiological variables and clinical parameters acquired remotely by means of wearable and environmental sensors deployed in patients' long-stay settings. The automatic evaluation of the card results in a so-called Chronic Status Index (CSI) whose computation is tailored to patients' specific manifestation of the disease (i.e., patient's phenotype). The decision support method relies on a parameterized analysis of CSI variations so as to early detect worsening changes, identify exacerbation severity and track the patterns of recovery. RESULTS: A preliminary study, carried out in real settings with 30 COPD patients monitored at home, has shown the validity and sensitivity of the method proposed, which was effectively able to timely and correctly identify patients' critical situation. CONCLUSION: The preliminary results showed that the proposed e-diary card, which presents several novel features with respect to other solutions presented in the literature, can be practically used to remotely monitor COPD patients.

Positional effects on lung mechanics of ventilated preterm infants with acute and chronic lung disease.

BACKGROUND: The role of prone position in preterm infants has not been completely clarified. We investigated prone versus supine posture-related changes in respiratory system resistance (Rrs) and reactance (Xrs) measured by the Forced Oscillation Technique (FOT) in mechanically ventilated preterm newborns. METHODS: Patients were studied in the supine versus prone positions in random order. Oxygen saturation, transcutaneous partial pressure of oxygen (ptcO2 ), carbon dioxide (ptcCO2 ), Rrs and Xrs were measured in each position. RESULTS: Nine patients with respiratory distress syndrome (RDS) and nine with evolving broncho-pulmonary dysplasia (BPD) were studied. Rrs was, on average, 9.8 (1.3, 18.3 as 95%CI) cmH2 O*s/l lower in the prone compared to the supine position (P = 0.02), while no differences in Xrs, ptcO2 , ptcCO2 , and breathing pattern were observed between postures. Only patients with evolving BPD showed a significant reduction of Rrs from 69.0 ± 27.4 to 53.0 ± 16.7 cmH2 O*s/l, P = 0.01. No significant correlations were found between changes in lung mechanics and ptcO2 , ptcCO2 , or breathing pattern. CONCLUSIONS: On short-term basis, prone positioning does not offer significant advantages in lung mechanics in mechanically ventilated infants with RDS, while it is associated with lower Rrs values in patients with evolving BPD.

Effect of changing the gravity vector on respiratory output and control.

We studied the respiratory output in five subjects exposed to parabolic flights [gravity vector 1, 1.8 and 0 gravity vector in the craniocaudal direction (Gz)] and when switching from sitting to supine (legs bent at the knees). Despite differences in total respiratory compliance (highest at 0 Gz and in supine and minimum at 1.8 Gz), no significant changes in elastic inspiratory work were observed in the various conditions, except when comparing 1.8 Gz with 1 Gz (subjects were in the seated position in all circumstances), although the elastic work had an inverse relationship with total respiratory compliance that was highest at 0 Gz and in supine posture and minimum at 1.8 Gz. Relative to 1 Gz, lung resistance (airways plus lung tissue) increased significantly by 52% in the supine but slightly decreased at 0 Gz. We calculated, for each condition, the tidal volume changes based on the energy available in the preceding phase and concluded that an increase in inspiratory muscle output occurs when respiratory load increases (e.g., going from 0 to 1.8 Gz), whereas a decrease occurs in the opposite case (e.g., from 1.8 to 0 Gz). Despite these immediate changes, ventilation increased, going to 1.8 and 0 Gz (up to approximately 23%), reflecting an increase in mean inspiratory flow rate, tidal volume, and respiratory frequency, while ventilation decreased (approximately -14%), shifting to supine posture (transition time approximately 15 s). These data suggest a remarkable feature in the mechanical arrangement of the respiratory system such that it can maintain the ventilatory output with small changes in inspiratory muscle work in face of considerable changes in configuration and mechanical properties.

Chest wall kinematic determinants of diaphragm length by optoelectronic plethysmography and ultrasonography.

To estimate diaphragm fiber length from thoracoabdominal configuration, we measured axial motion of the right-sided area of apposition by ultrasonography and volumes displaced by chest wall compartments [pulmonary, abdominal rib cage, and abdomen (Vab)] by optoelectronic plethysmography in four normal men during quiet breathing and incremental exercise without and with expiratory flow limitation. Points at the cephalic area of apposition border were digitized from echo images and mapped into three-dimensional space, and the axial distance from the xyphoidal transverse plane (D(ap)) was measured simultaneously with the volumes. Linear regression analysis between changes (Delta) in D(ap) and the measured volume changes under all conditions showed that 1) DeltaD(ap) was linearly related more to DeltaVab than to changes in pulmonary and abdominal rib cage volumes; and 2) this was highly repeatable between measures. Multiple stepwise regression analysis showed that DeltaVab accounted for 89-96% of the variability of DeltaD(ap), whereas the rib cage compartments added <1%. We conclude that, under conditions of quiet breathing and exercise, with and without expiratory flow limitation, instantaneous DeltaD(ap) can be estimated from DeltaVab.

Respiratory and leg muscles perceived exertion during exercise at altitude.

We compared the rate of perceived exertion for respiratory (RPE,resp) and leg (RPE,legs) muscles, using a 10-point Borg scale, to their specific power outputs in 10 healthy male subjects during incremental cycle exercise at sea level (SL) and high altitude (HA, 4559 m). Respiratory power output was calculated from breath-by-breath esophageal pressure and chest wall volume changes. At HA ventilation was increased at any leg power output by ∼ 54%. However, for any given ventilation, breathing pattern was unchanged in terms of tidal volume, respiratory rate and operational volumes of the different chest wall compartments. RPE,resp scaled uniquely with total respiratory power output, irrespectively of SL or HA, while RPE,legs for any leg power output was exacerbated at HA. With increasing respective power outputs, the rate of change of RPE,resp exponentially decreased, while that of RPE,legs increased. We conclude that RPE,resp uniquely relates to respiratory power output, while RPE,legs varies depending on muscle metabolic conditions.

Pulmonary V(O)(2) kinetics at the onset of exercise is faster when actual changes in alveolar O2 stores are considered.

Breath-by-breath (BbB) oxygen uptake rate (V(O)(2)) was measured at the mouth (MO) and at the alveolar level, at the onset of square wave cycling exercise of moderate intensity in six healthy male subjects. Alveolar BbB V(O)(2) values were calculated correcting MO V(O)(2) values by (i) estimating (GR); and (ii) measuring (opto-electronic plethysmography, OEP) BbB lung O(2) store changes.V(O)(2) kinetics was then described by a bi-exponential model. GR yielded larger values of the time constants (tau2) of the primary phase of V(O)(2) kinetics. The mean response times (MRTs) calculated by analysing GR BbB V(O)(2) values were larger than (i) those obtained by using MO and OEP at 90W; and (ii) that by using MO at 120W. OEP corrected V(O)(2) yielded the highest normalised amplitude of the cardiodynamic phase of the V(O)(2) on-response. Correction of BbB V(O)(2) for actual BbB changes of lung O(2) stores by OEP thus seems more appropriate for the study of the early cardiodynamic phase of V(O)(2) kinetics than GR.

Effects of volume shift on the pressure-volume curve of the respiratory system in ALI/ARDS patients.

AIM: The pressure-volume (PV) curve in acute lung injury and acute respiratory distress syndrome (ALI/ARDS) patients has been proposed for estimating the underlying pathology, lung recruitment and setting mechanical ventilation. The supersyringe method may lead to artifacts due to thermodynamics and gas exchange. Another possible confounding factor is the volume shift, primarily blood, out of the chest wall when the intrathoracic pressures rise. We set out to quantify the volume shift and investigate its mechanisms. METHODS: Ten ALI/ARDS patients (5 males/5 females, PaO(2)/FiO(2) 222+/-67) were studied in the Intensive Care Unit, University Hospital. PV curve was performed by a supersyringe (0.100 L, 14 steps Delta-Vgas) while recording the chest wall volume difference (Delta-Vcw) by the optoelectronic plethysmography. Differences in airway (Delta-Paw) and esophageal (Delta-Pes) pressures were measured during the maneuver. Volume shift was defined as Delta-Vcw-Delta-Vgas, corrected for thermodynamic and gas exchange. RESULTS: Starting compliance (P<0.05), inflation/deflation compliance (P<0.01), hysteresis (P<0.01) and unrecovered volume (P<0.01) were significantly affected by volume shift. The volume shift was directly correlated to the product Delta-Paw*inflation time (R2=0.87, P<0.001), to the ratio of Delta-Pes to Delta-Paw (R2=0.80, P<0.01) and to central venous pressure (R2=0.42, P<0.05) and inversely correlated with the deflation time (R2=0.58, P<0.05). At 20 cmH2O of airway pressure the volume shift between the inflation and deflation limbs of the PV curve amounted to 0.099+/-0.058 L. CONCLUSIONS: The volume shift, constituted mainly of blood, significantly affects both inspiratory and expiratory PV curve. Caution is needed when interpreting the PV parameters (Minerva Anestesiol 2007;73:1-10).

Expiratory flow limitation detected by forced oscillation and negative expiratory pressure.

The within-breath change in reactance (Delta(rs)) measured by forced oscillation technique (FOT) at 5 Hz reliably detects expiratory flow limitation in chronic obstructive pulmonary disease (COPD). The present study compared this approach to the standard negative expiratory pressure (NEP) method. In total, 21 COPD patients were studied by applying both techniques to the same breath and in 15 patients the measurements were repeated after bronchodilator. For each patient and condition five NEP tests were performed and independently scored by three operators unaware of the FOT results. In 180 tests, FOT classified 53.3% as flow limited. On average, the operators scored 27.6% of tests flow limited and 47.6% non-flow limited, but could not score 24.8%. The methods disagreed in 7.9% of cases; in 78% of these the NEP scores differed between operators. Bronchodilation reduced NEP and DeltaX(rs) scores, with only the latter achieving significance. Averaging the operators' NEP scores, a threshold between 24.6-30.8% of tidal volume being flow limited by NEP produced 94% agreement between methods. In conclusion, when negative expiratory pressure and forced oscillation technique were both available they showed good agreement. As forced oscillation technique is automatic and can measure multiple breaths over long periods, it is suitable for monitoring expiratory flow limitation continuously and identifying patients' breathing close to the onset of expiratory flow limitation, where intermittent sampling may be unrepresentative.

Noninvasive detection of expiratory flow limitation in COPD patients during nasal CPAP.

The difference between mean inspiratory and expiratory respiratory reactance (delta(rs)) measured with forced oscillation technique (FOT) at 5 Hz allows the detection of expiratory flow limitation (EFL) in chronic obstructive pulmonary disease (COPD) patients breathing spontaneously. This aim of this study was to evaluate whether this approach can be applied to COPD patients during noninvasive pressure support. Delta(rs) was measured in seven COPD patients subjected to nasal continuous positive airway pressure (CPAP) at 0, 4, 8 and 12 cmH2O in sitting and supine positions. Simultaneous recording of oesophageal pressure and the Mead and Whittenberger (M-W) method provided a reference for scoring each breath as flow-limited (FL), non-flow-limited (NFL) or indeterminate (I). For each patient, six consecutive breaths were analysed for each posture and CPAP level. According to M-W scoring, 47 breaths were FL, 166 NFL and 51 I. EFL scoring using FOT coincided with M-W in 94.8% of the breaths. In the four patients who were FL in at least one condition, delta(rs) was reduced with increasing CPAP. These data suggest that the forced oscillation technique may be useful in chronic obstructive pulmonary disease patients on nasal pressure support by identifying continuous positive airway pressure levels that support breathing without increasing lung volume, which in turn increase the work of breathing and reduce muscle effectiveness and efficiency.

Effect of salbutamol on lung function and chest wall volumes at rest and during exercise in COPD.

BACKGROUND: Inhaled bronchodilators can increase exercise capacity in chronic obstructive pulmonary disease (COPD) by reducing dynamic hyperinflation, but treatment is not always effective. This may reflect the degree to which the abdomen allows dynamic hyperinflation to occur. METHOD: A double blind, randomised, crossover trial of the effect of 5 mg nebulised salbutamol or saline on endurance exercise time was conducted in 18 patients with COPD of mean (SD) age 67.1 (6.3) years and mean (SD) forced expiratory volume in 1 second (FEV1) of 40.6 (15.0)% predicted. Breathing pattern, metabolic variables, dyspnoea intensity, and total and regional chest wall volumes were measured non-invasively by optoelectronic plethysmography (OEP) at rest and during exercise. RESULTS: Salbutamol increased FEV1, forced vital capacity (FVC) and inspiratory capacity and reduced functional residual capacity (FRC) and residual volume significantly. OEP showed the change in resting FRC to be mainly in the abdominal compartment. Although the mean (SE) end expiratory chest wall volume was 541 (118) ml lower (p<0.001) at the end of exercise, the endurance time was unchanged by the bronchodilator. Changes in resting lung volumes were smaller when exercise duration did not improve, but FEV1 still rose significantly after active drug. After the bronchodilator these patients tried to reduce the end expiratory lung volume when exercising, while those exercising longer continued to allow end expiratory abdominal wall volume to rise. The change to a more euvolumic breathing pattern was associated with a lower oxygen pulse and a significant fall in endurance time with higher isotime levels of dyspnoea. CONCLUSIONS: Nebulised salbutamol improved forced expiratory flow in most patients with COPD, but less hyper-nflated patients tried to reduce the abdominal compartmental volume after active treatment and this reduced their exercise capacity. Identifying these patients has important therapeutic implications, as does an understanding of the mechanisms that control chest wall muscle recruitment.

Influence of expiratory flow-limitation during exercise on systemic oxygen delivery in humans.

To determine the effects of exercise with expiratory flow-limitation (EFL) on systemic O(2) delivery, seven normal subjects performed incremental exercise with and without EFL at approximately 0.8 l s(-1) (imposed by a Starling resistor in the expiratory line) to determine maximal power output under control (W'(max,c)) and EFL (W'(max,e)) conditions. W'(max,e) was 62.5% of W'(max,c), and EFL exercise caused a significant fall in the ventilatory threshold. In a third test, after exercising at W'(max,e) without EFL for 4 min, EFL was imposed; exercise continued for 4 more minutes or until exhaustion. O(2) consumption (V'(O)(2)) was measured breath-by-breath for the last 90 s of control, and for the first 90 s of EFL exercise. Assuming that the arterio-mixed venous O(2) content remained constant immediately after EFL imposition, we used V'(O)(2) as a measure of cardiac output (Q'(c)). Q'(c) was also calculated by the pulse contour method with blood pressure measured continuously by a photo-plethysmographic device. Both sets of data showed a decrease of Q'(c) due to a decrease in stroke volume by 10% (p < 0.001 for V'(O)(2)) with EFL and remained decreased for the full 90 s. Concurrently, arterial O(2) saturation decreased by 5%, abdominal, pleural and alveolar pressures increased, and duty cycle decreased by 43%. We conclude that this combination of events led to a decrease in venous return secondary to high expiratory pressures, and a decreased duty cycle which decreased O(2) delivery to working muscles by approximately 15%.

Transfer impedance of the respiratory system by forced oscillation technique and optoelectronic plethysmography.

To estimate the transfer impedance of the respiratory system (Ztr), we applied pressure forcing at the mouth from 1 to 24 Hz in eight healthy subjects and used optoelectronic plethysmography (OEP) to measure volume changes of the chest wall and its different compartments: pulmonary rib cage (RCp), abdominal rib cage (RCa) and abdomen (AB). Spectral analysis allowed assessment of input impedance (Zin) and total (Ztr) and compartmental (ZRCP, ZRCa, and ZAB) transfer impedances. As expected, averaged values of Zin and Ztr were similar at low frequencies (< 8 Hz) while they progressively differed at high frequencies. The percentage contributions of ZRCp, ZRCa , and ZAB to Ztr were, respectively, 35.3 +/- 1.4SD, 13.8 +/- 1.4, and 50.8 +/- 2.8 at low frequencies (< 8 Hz) and 63.1 +/- 5.5, 20.7 +/- 5.2, and 16.2 +/- 2.3 at higher frequencies (> 10 Hz). The validation of our approach was based on the comparison with a physical model comprised of a rubber membrane stretched over and attached to the lip of a bowl. We conclude that the combination of forced oscillations with OEP provides the simultaneous assessment of Zin and Ztr, it does not require the use of a plethysmographic chamber and it allows the separation between the different rib cage-abdominal pathways.

Spatial distribution of human respiratory system transfer impedance.

Transfer impedance (Ztr) of the respiratory system provides specific information on airways and tissues, but little is known about its spatial distribution in the different thoracoabdominal regions. To study Ztr distribution on the chest wall surface we analyzed five healthy subjects in the supine position by applying a sinusoidal forcing pressure (4, 8, and 12 Hz) at the mouth and measuring airway opening pressure and flow. Three-dimensional positions of 68 reflective markers placed on the chest wall over selected reference points were simultaneously measured by an optoelectronic motion analyzer. A subset of ten points placed on the midline were used to measure chest wall movements in the craniocaudal direction. While the motion of rib cage markers was synchronous, the abdominal markers demonstrated surface waves propagating caudally. The amplitude and phase of these waves were strongly dependent on position and frequency. We used a new method to measure total and local chest wall volume variations to compute the distribution of Ztr over the chest wall. Above 4 Hz we found that Ztr was inhomogeneously distributed and strongly dependent on position and frequency, mainly in the abdomen where the phase was often more more than 180 degrees with high values of modulus. For this reason, we conclude that above 8 Hz Ztr represents rib cage mechanics almost exclusively.

Regional chest wall volumes during exercise in chronic obstructive pulmonary disease.

BACKGROUND: Dynamic hyperinflation of the lungs impairs exercise performance in chronic obstructive pulmonary disease (COPD). However, it is unclear which patients are affected by dynamic hyperinflation and how the respiratory muscles respond to the change in lung volume. METHODS: Using optoelectronic plethysmography, total and regional chest wall volumes were measured non-invasively in 20 stable patients with COPD (mean (SD) forced expiratory volume in 1 second 43.6 (11.6)% predicted) and dynamic hyperinflation was tracked breath by breath to test if this was the mechanism of exercise limitation. Resting ventilation, breathing pattern, symptoms, rib cage and abdominal volumes were recorded at rest and during symptom limited cycle ergometry. Pleural, abdominal, and transdiaphragmatic pressures were measured in eight patients. RESULTS: End expiratory chest wall volume increased by a mean (SE) of 592 (80) ml in 12 patients (hyperinflators) but decreased by 462 (103) ml in eight (euvolumics). During exercise, tidal volume increased in euvolumic patients by reducing end expiratory abdominal volume while in hyperinflators tidal volume increased by increasing end inspiratory abdominal and rib cage volumes. The maximal abdominal pressure was 22.1 (9.0) cm H(2)O in euvolumic patients and 7.6 (2.6) cm H(2)O in hyperinflators. Euvolumic patients were as breathless as hyperinflators but exercised for less time and reached lower maximum workloads (p<0.05) despite having better spirometric parameters and a greater expiratory flow reserve. CONCLUSIONS: Dynamic hyperinflation is not the only mechanism limiting exercise performance in patients with stable COPD. Accurate measurement of chest wall volume can identify the different patterns of respiratory muscle activation during exercise.

Estimation of end-expiratory lung volume variations by optoelectronic plethysmography.

OBJECTIVE: To test the capability of optoelectronic plethysmography (OEP) to monitor positive end-expiratory pressure (PEEP)-induced changes of end-expiratory lung volume (EELV) changes in mechanically ventilated paralyzed patients. DESIGN: Laboratory and clinical investigation. SETTING: Intensive care unit of the Ospedale Maggiore Policlinico di Milano. PATIENTS: A total of eight patients with respiratory failure of various degrees, sedated and paralyzed. INTERVENTIONS: PEEP variations (+/-5 cm H2O) relative to the baseline PEEP of 10 cm H2O. MEASUREMENTS AND MAIN RESULTS: In the model protocol, we tested the reproducibility of the OEP by repeating volume measurements of a plastic torso model over a 21-hr period, every 30 mins. The variations of OEP measurements of the torso model (9337 mL value) were encountered in a range of 16 mL (sd = 4 mL). In the patient protocol, we measured the end-expiratory volume of the chest wall (EEVCW) breath-by-breath by OEP before, during, and after the PEEP increase/decrease and we compared its variations with the corresponding variations of EELV measured by helium dilution technique. The regression line between EELV changes measured by helium and EEVCW changes measured by OEP resulted very close to the identity line (slope 1.06, intercept -0.02 L, r(2) = 0.89) and their difference was not related to their absolute magnitude. After PEEP increase, the new steady state of EEVCW was reached approximately in 15 breaths; and, after PEEP decrease, in 3-4 breaths. The slow increase in EEVCW was mainly because of the abdominal compartment. CONCLUSION: OEP measurements of EEVCW accurately reflect the changes of EELV. Furthermore, OEP allows a continuous compartmental analysis, even during unsteady conditions.

Detection of expiratory flow limitation in COPD using the forced oscillation technique.

Expiratory flow limitation (EFL) during tidal breathing is a major determinant of dynamic hyperinflation and exercise limitation in chronic obstructive pulmonary disease (COPD). Current methods of detecting this are either invasive or unsuited to following changes breath-by-breath. It was hypothesised that tidal flow limitation would substantially reduce the total respiratory system reactance (Xrs) during expiration, and that this reduction could be used to reliably detect if EFL was present. To test this, 5-Hz forced oscillations were applied at the mouth in seven healthy subjects and 15 COPD patients (mean +/- sD forced expiratory volume in one second was 36.8 +/- 11.5% predicted) during quiet breathing. COPD breaths were analysed (n=206) and classified as flow-limited if flow decreased as alveolar pressure increased, indeterminate if flow decreased at constant alveolar pressure, or nonflow-limited. Of these, 85 breaths were flow-limited, 80 were not and 41 were indeterminate. Among other indices, mean inspiratory minus mean expiratory Xrs (deltaXrs) and minimum expiratory Xrs (Xexp,min) identified flow-limited breaths with 100% specificity and sensitivity using a threshold between 2.53-3.12 cmH2O x s x L(-1) (deltaXrs) and -7.38- -6.76 cmH2O x s x L(-1) (Xexp,min) representing 6.0% and 3.9% of the total range of values respectively. No flow-limited breaths were seen in the normal subjects by either method. Within-breath respiratory system reactance provides an accurate, reliable and noninvasive technique to detect expiratory flow limitation in patients with chronic obstructive pulmonary disease.