Physiological effects of flow and pressure triggering during non- invasive mechanical ventilation in patients with chronic obstructive pulmonary disease

BACKGROUND: The effect of the type of trigger system on inspiratory effort has been studied in intubated patients, but no data are available in non-invasive mechanical ventilation where the "trigger variable" may be even more important since assisted modes of ventilation are often employed from the beginning of mechanical ventilation. METHODS: The effect of flow triggering (1 and 5 1/min) and pressure triggering (-1 cm H2O) on inspiratory effort during pressure support ventilation (PSV) and assisted controlled mode (A/C) delivered non-invasively with a full face mask were compared in patients with chronic obstructive pulmonary disease (COPD) recovering from an acute exacerbation. The patients were studied during randomised 15 minute runs at zero positive end expiratory pressure (ZEEP). The oesophageal pressure time product (PTPoes), dynamic intrinsic PEEP (PEEPi,dyn), fall in maximal airway pressure (delta Paw) during inspiration, and ventilatory variables were measured. RESULTS: Minute ventilation, respiratory pattern, dynamic lung compliance and resistances, and changes in end expiratory lung volume (delta EELV) were the same with the two triggering systems. The total PTPoes and its pre-triggering phase (PTP due to PEEPi and PTP due to valve opening) were significantly higher during both PSV and A/C with pressure triggering than with flow triggering at both levels of sensitivity. delta Paw was larger during pressure triggering, and PEEPi,dyn was significantly reduced during flow triggering in the A/C mode only. CONCLUSIONS: In patients with COPD flow triggering reduces the inspiratory effort during both PSV and A/C modes compared with pressure triggering. These findings are likely to be due to a reduction in PEEPi,dyn and in the time of valve opening with a flow trigger.


     

Abdominal muscle recruitment and PEEPi during bronchoconstriction in chronic obstructive pulmonary disease

BACKGROUND: It has been recently shown that, when breathing at rest, many patients with severe chronic obstructive pulmonary disease (COPD) contract abdominal muscles during expiration, and that this contraction is an important determinant of positive end expiratory alveolar pressure (PEEPi). In this study the effects of acute bronchoconstriction on abdominal muscle recruitment in patients with severe COPD were studied, together with the consequence of abdominal muscle action on chest wall mechanics. METHODS: Breathing pattern, pleural (PPL) and gastric (PGA) pressures, and changes in abdomen anteroposterior (AP) diameter were studied in 14 patients with COPD (mean forced expiratory volume in one second (FEV1) 1.06 (0.08) 1) under control conditions and during histamine-induced bronchoconstriction. RESULTS: The analysis of plots of PGA versus the AP diameter of the abdomen revealed that during maximal broncho- constriction (decrease in FEV1 of 34.8% (95% confidence intervals (CI) 29.9 to 39.7)) the expiratory rise in PGA increased significantly whereas end expiratory abdomen AP diameter decreased, indicating marked abdominal muscle recruitment. As a consequence, the rib cage compartment accounted for all of the volume of hyperinflation during bronchoconstriction (mean value 0.66 I, 95% CI 0.49 to 0.83). Positive end expiratory alveolar pressure during progressive bronchoconstriction was related directly to the expiratory rise in PGA and inversely to the expiratory time. CONCLUSIONS: The results indicate that, in patients with severe COPD, the abdominal muscles are recruited during acute bronchoconstriction. This recruitment probably preserves diaphragm length at the beginning of inspiratory muscle contraction despite the hyperinflation, and contributes significantly to positive end expiratory alveolar pressure. The degree of dynamic pulmonary hyperinflation during bronchoconstriction can be overestimated if abdominal muscle contraction is not assessed.


     

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.     

Effect of CPAP on intrinsic PEEP,inspiratory effort,and lung volume in severe stable COPD

BACKGROUND: Intrinsic positive end expiratory pressure (PEEPi) constitutes an inspiratory threshold load on the respiratory muscles, increasing work of breathing. The role of continuous positive airway pressure (CPAP) in alleviating PEEPi in patients with severe stable chronic obstructive pulmonary disease is uncertain. This study examined the effect of CPAP on the inspiratory threshold load, muscle effort, and lung volume in this patient group. METHODS: Nine patients were studied at baseline and with CPAP increasing in increments of 1 cm H(2)O to a maximum of 10 cm H(2)O. Breathing pattern and minute ventilation (I), dynamic PEEPi, expiratory muscle activity, diaphragmatic (PTPdi/min) and oesophageal (PTPoes/min) pressure-time product per minute, integrated diaphragmatic (EMGdi) and intercostal EMG (EMGic) and end expiratory lung volume (EELV) were measured. RESULTS: Expiratory muscle activity was present at baseline in one subject. In the remaining eight, PEEPi was reduced from a mean (SE) of 2.9 (0.6) cm H(2)O to 0.9 (0.1) cm H(2)O (p<0.05). In two subjects expiratory muscle activity contributed to PEEPi at higher pressures. There were no changes in respiratory pattern but I increased from 9.2 (0.6) l/min to 10.7 (1.1) l/min (p<0.05). EMGdi remained stable while EMGic increased significantly. PTPoes/min decreased, although this did not reach statistical significance. PTPdi/min decreased significantly from 242.1 (32.1) cm H(2)O.s/min to 112.9 (21.7) cm H(2)O.s/min). EELV increased by 1.1 (0.3) l (p<0.01). CONCLUSION: High levels of CPAP reduce PEEPi and indices of muscle effort in patients with severe stable COPD, but only at the expense of substantial increases in lung volume.     

Severity Related Differences in Lung Attenuation in Men With COPD

Background and objectivesWe compare the inspiratory and expiratory regional lung densities between different levels of COPD severity (as assessed by the GOLD scale and by the BODE index), and to assess the relationship between regional lung densities and functional lung parameters.Patients and methodsFifty-five stable moderate-severe COPD men were selected. Functional evaluation included dyspnoea scale, blood gases, spirometry, plethysmography, diffusing capacity and six-minute walk test. Severity was classified according the GOLD scale and the BODE index. High resolution computed tomography (HRCT) scans of the entire lung at full inspiration and two sections at full expiration were obtained. Densitometry software was used to calculate the densities of the lung areas.ResultsInspiratory and expiratory mean lung densities (MLD) of the lower lobes were significantly lower in very severe and severe COPD patients than in moderate patients. In contrast, we only found differences between the upper lobe MLD values of moderate and severe COPD patients. Inspiratory and expiratory HRCT densities were similar among all BODE quartiles, for both the upper and lower lobes. In a multiple regression analysis, airway obstruction parameters were mainly related to the expiratory MLD of the lower lobes, whereas lung hyperinflation parameters were predicted by the inspiratory MLD of the lower lobes. Lastly, diffusion capacity was independently related to the expiratory/inspiratory MLD of the lower lobes and to the inspiratory MLD of the upper lobes.ConclusionsThere are differences in lung attenuation measurements by HRCT between the varying levels of COPD severity as assessed by the GOLD scale.     

Breathing pattern and carbon dioxide retention in severe chronic obstructive pulmonary disease.

BACKGROUND: The factors leading to chronic hypercapnia and rapid shallow breathing in patients with severe chronic obstructive pulmonary disease (COPD) are not completely understood. In this study the interrelations between chronic carbon dioxide retention, breathing pattern, dyspnoea, and the pressure required for breathing relative to inspiratory muscle strength in stable COPD patients with severe airflow obstruction were studied. METHODS: Thirty patients with COPD in a clinically stable condition with forced expiratory volume in one second (FEV1) of < 1 litre were studied. In each patient the following parameters were assessed: (1) dyspnoea scale rating, (2) inspiratory muscle strength by measuring minimal pleural pressure (PPLmin), and (3) tidal volume (VT), flow, pleural pressure swing (PPLsw), total lung resistance (RL), dynamic lung elastance (ELdyn), and positive end expiratory alveolar pressure (PEEPi) during resting breathing. RESULTS: Arterial carbon dioxide tension (PaCO2) related directly to RL/PPLmin, and ELdyn/PPLmin, and inversely to VT and PPLmin. There was no relationship between PaCO2 and functional residual capacity (FRC), total lung capacity (TLC), or minute ventilation. PEEPi was similar in eucapnic and hypercapnic patients. Expressing PaCO2 as a combined function of VT and PPLmin (stepwise multiple regression analysis) explained 71% of the variance in PaCO2. Tidal volume was directly related to inspiratory time (TI), and TI was inversely related to the pressure required for breathing relative to inspiratory muscle strength (PPLsw, %PPLmin). There was an association between the severity of dyspnoea and both the increase in PPLsw (%PPLmin) and the shortening in TI. CONCLUSIONS: The results indicate that, in stable patients with COPD with severe airflow obstruction, hypercapnia is associated with shallow breathing and inspiratory muscle weakness, and rapid and shallow breathing appears to be linked to both a marked increase in the pressure required for breathing relative to inspiratory muscle strength and to the severity of the breathlessness.     

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.     

Effect of expiratory resistance on gas-exchange and breathing pattern in chronic obstructive pulmonary disease (COPD) patients being weaned from the ventilator

BACKGROUND: The majority of patients with severe chronic obstructive pulmonary disease (COPD) have flow limitation, which has deleterious side effects. If these patients are mechanically ventilated, this often results in difficult weaning. Spontaneously breathing COPD patients experience a beneficial effect of pursed lip breathing. We investigated whether in intubated COPD patients application of an external resistance could produce the same beneficial effects on breathing pattern and gas-exchange as pursed lip breathing. METHODS: Ten COPD patients with flow limitation were studied during pressure support mechanical ventilation. Two types of expiratory resistances were applied: one fixed level of resistance and one with a resistive pressure decay. Each resistance was applied in 5 patients and the highest level was chosen that did not cause hyperinflation. Blood gas values and breathing pattern with and without resistance were compared. RESULTS: With resistance 1, gas-exchange and breathing pattern did not change significantly; average PCO2 changed from 8.0 to 8.1 kPa, PO2 from 10.2 to 10.3 kPa, tidal volume from 0.380 to 0.420 l, respiratory rate from 25 to 23 bpm and inspiratory:expiratory ratio from 1:1.9 to 1:2.0. With resistance 2, gas-exchange and breathing pattern did not change significantly; average PCO2 changed from 5.8 to 6.0 kPa, PO2 from 11.1 to 12.1 kPa, tidal volume from 0.733 to 0.695 l, respiratory rate from 16 to 18 bpm and inspiratory:expiratory ratio from 1:2.3 to 1:2.9. CONCLUSION: In intubated COPD patients being weaned from the ventilator, application of an external resistance did not have the same beneficial effects as pursed lip breathing.     

Cardiovascular collapse associated with extreme iatrogenic PEEPi in patients with obstructive airways disease

Chronic obstructive pulmonary disease (COPD) is commonly associated with positive alveolar pressure at end-expiration (intrinsic PEEP or PEEPi) caused by a prolonged expiratory time constant. Positive pressure ventilation (PPV) with large tidal volumes and high ventilatory frequencies may cause pulmonary hyperinflation, with increases in intrathoracic pressure and cardiopulmonary effects. We report two cases, one of fatal pulseless electrical activity, the other of life-threatening hypotension, both during vigorous manual PPV, in patients with severe COPD. This phenomenon has been well-recognized by intensivists but is reported poorly more widely.      

EFFECTS OF PEEP ON DYNAMIC HYPERINFLATION IN PATIENTS WITH AIRFLOW LIMITATION

We have studied the effects of extrinsically applied PEEP (PEEPe)and intrinsic PEEP (PEEPi) on lung volume and peak airway pressure(Paw,peak) in 10 patients with airflow limitation during mechanicalventilation. PEEPe was applied in 2-4 cm H2O increments untilvalues greater than PEEPi were reached. Total lung hyperinflationwas quantified by measuring the expired volume resulting fromdeflation (starting at end inspiration) until cessation of expiratoryflow. The previous expired tidal volume was subtracted fromthis volume to obtain the change in functional residual capacity(8FRC), the hyperinflation resulting from PEEPi and PEEPe. PEEPi(0.49–1.66 kPa) was demonstrated in all patients beforethe application of PEEPe and correlated with 5FRC (r = 0.71),with 5FRC increasing by 582 ml/kPa PEEPi (P < 0.05). PEEPeat pressures less than PEEPi increased SFRC by (mean) 186 (SEM)34m//kPa PEEPe (P < 0.05) and increased Paw,peak by 0.6 (0.12)kPa/kPa PEEPe (P < 0.05). In contrast, PEEPe at pressuresgreater than PEEPi, increased SFRC by 695 (128) ml/kPa PEEPe(P < 0.05) and Paw.peak by 1.8 (0.26) kPa/kPa PEEPe. We concludethat PEEPe may be applied cautiously at values less than PEEPiwhen clinically indicated, but the application of PEEPe at valuesgreater than PEEPi may substantially aggravate lung hyperinflation.(Br. J. Anaesth. 1993; 70: 267–272)     

Pulmonary gas distribution during ventilation with different inspiratory flow patterns in experimental lung injury -- a computed tomography study

BACKGROUND: There is still controversy about the optimal inspiratory flow pattern for ventilation of patients with acute lung injury. The aim of this study was to compare the effects of pressure-controlled ventilation (PCV) with a decelerating inspiratory flow with volume-controlled ventilation (VCV) with constant inspiratory flow on pulmonary gas distribution (PGD) in experimentally induced ARDS. METHODS: Sixteen adult sheep were randomized to be ventilated with PCV or VCV after surfactant depletion by repeated bronchoalveolar lavage. Positive end-expiratory pressure (PEEP) was increased in a stepwise manner from zero end-expiratory pressure (ZEEP) to 7, 14 and 21 cm H(2)O in hourly intervals. Respiratory rate, inspiration-to-expiration ratio and tidal volume were kept constant. Central hemodynamics, gas exchange and airway pressures were measured. Electron beam computed tomographic (EBCT) scans of the entire lungs were performed at baseline (preinjury) and each level of end-expiratory pressure during an inspiratory and expiratory hold maneuver. The lungs were three-dimensionally reconstructed and volumetric assessments were made separating the lungs into four subvolumes classified as overinflated, normally aerated, poorly aerated and nonaerated. RESULTS: Pressure-controlled ventilation led to a decrease in peak airway pressure and an increase in mean airway pressure. No differences between groups were found regarding plateau pressures, hemodynamics and gas exchange. Recruitment, defined as a decrease in expiratory lung volume classified as nonaerated, was similar in both groups and predominantly associated with PEEP. Overinflated lung volumes were increased with PCV. CONCLUSIONS: In this model of acute lung injury, ventilation with decelerating inspiratory flow had no beneficial effects on PGD when compared with ventilation with constant inspiratory flow, while the increase in overinflated lung volumes may raise concerns regarding potential ventilator-associated lung injury.     

Effect of salmeterol on respiratory muscle activity during exercise in poorly reversible COPD

BACKGROUND: Some patients with irreversible chronic obstructive pulmonary disease (COPD) experience subjective benefit from long acting bronchodilators without change in forced expiratory volume in 1 second (FEV(1)). Dynamic hyperinflation is an important determinant of exercise induced dyspnoea in COPD. We hypothesised that long acting bronchodilators improve symptoms by reducing dynamic hyperinflation and work of breathing, as measured by respiratory muscle pressure-time products. METHODS: Sixteen patients with "irreversible" COPD (<10% improvement in FEV(1) following a bronchodilator challenge; mean FEV(1) 31.1% predicted) were recruited into a randomised, double blind, placebo controlled, crossover study of salmeterol (50 micro g twice a day). Treatment periods were of 2 weeks duration with a 2 week washout period. Primary outcome measures were end exercise isotime transdiaphragmatic pressure-time product and dynamic hyperinflation as measured by inspiratory capacity. RESULTS: Salmeterol significantly reduced the transdiaphragmatic pressure-time product (294.5 v 348.6 cm H(2)O/s/min; p = 0.03), dynamic hyperinflation (0.22 v 0.33 litres; p = 0.002), and Borg scores during endurance treadmill walk (3.78 v 4.62; p = 0.02). There was no significant change in exercise endurance time. Improvements in isotime Borg score were significantly correlated to changes in tidal volume/oesophageal pressure swings, end expiratory lung volume, and inspiratory capacity, but not pressure-time products. CONCLUSIONS: Despite apparent "non-reversibility" in spirometric parameters, long acting bronchodilators can cause both symptomatic and physiological improvement during exercise in severe COPD.     

Inspiratory muscle strength and endurance during hyperinflation and histamine induced bronchoconstriction.

BACKGROUND: This study investigated whether the inspiratory muscles are susceptible to fatigue during acute airway narrowing because of increased airway resistance and hyperinflation. METHODS: Asthmatic subjects performed up to four series (on separate days) of 18 maximal static inspiratory efforts of 10 seconds' duration with 10 second rest intervals (50% duty cycle; total duration six minutes): at functional residual capacity (FRC) (control); after histamine induced bronchoconstriction, which decreased forced expiratory volume in one second (FEV1) to a mean of 55% (SD 11%) of the initial value; at a voluntarily increased lung volume (initial volume held at 140% control); and after inhalation of histamine at a voluntarily increased lung volume. RESULTS: For the group of subjects the mean (SD) maximal inspiratory pressure (MIP) in the control experiments was 114 (22) cmH2O and the initial volume was 3.5 (1.2) 1. After histamine inhalation the initial lung volume for contractions increased to 118% (5%) of the control volume. In the high lung volume experiments initial volumes were 140% (12%) of the control (volume without histamine) and 140% (15%) (with histamine). The relation between MIP and initial absolute lung volume was determined for each subject before fatigue developed. When the inspiratory pressures for each contraction in the endurance test were normalised to the pressure expected for that lung volume, no significant differences were found between the four experimental conditions for MIP, or between pressures sustained over the 18 contractions. CONCLUSIONS: Histamine induced bronchoconstriction and hyperinflation had no detectable effect on inspiratory muscle strength or endurance in these asthmatic subjects.     

Factors determining maximum inspiratory flow and maximum expiratory flow of the lung

The factors determining maximum expiratory flow and maximum inspiratory flow of the lung are reviewed with particular reference to a model which compares the lung on forced expiration to a Starling resistor. The theoretical significance of the slope of the expiratory maximum flow-volume curve is discussed. A method of comparing maximum expiratory flow with maximum inspiratory flow at similar lung volumes is suggested; this may be applied either to a maximum flow-volume curve or to a forced expiratory and inspiratory spirogram.     

Patterns of dynamic hyperinflation during exercise and recovery in patients with severe chronic obstructive pulmonary disease

BACKGROUND: Not all patients with severe chronic obstructive pulmonary disease (COPD) progressively hyperinflate during symptom limited exercise. The pattern of change in chest wall volumes (Vcw) was investigated in patients with severe COPD who progressively hyperinflate during exercise and those who do not. METHODS: Twenty patients with forced expiratory volume in 1 second (FEV(1)) 35 (2)% predicted were studied during a ramp incremental cycling test to the limit of tolerance (Wpeak). Changes in Vcw at the end of expiration (EEVcw), end of inspiration (EIVcw), and at total lung capacity (TLCVcw) were computed by optoelectronic plethysmography (OEP) during exercise and recovery. RESULTS: Two significantly different patterns of change in EEVcw were observed during exercise. Twelve patients had a progressive significant increase in EEVcw during exercise (early hyperinflators, EH) amounting to 750 (90) ml at Wpeak. In contrast, in all eight remaining patients EEVcw remained unchanged up to 66% Wpeak but increased significantly by 210 (80) ml at Wpeak (late hyperinflators, LH). Although at the limit of tolerance the increase in EEVcw was significantly greater in EH, both groups reached similar Wpeak and breathed with a tidal EIVcw that closely approached TLCVcw (EIVcw/TLCVcw 93 (1)% and 93 (3)%, respectively). EEVcw was increased by 254 (130) ml above baseline 3 minutes after exercise only in EH. CONCLUSIONS: Patients with severe COPD exhibit two patterns during exercise: early and late hyperinflation. In those who hyperinflate early, it may take several minutes before the hyperinflation is fully reversed after termination of exercise.     

Breathing patterns during breathing exercises in persons with tetraplegia

STUDY DESIGN: Cross-sectional, observational, controlled study. OBJECTIVES: To survey breathing patterns during breathing at rest, ordinary deep breathing (DB), positive expiratory pressure (PEP) and inspiratory resistance-positive expiratory pressure (IR-PEP) among individuals with a cervical spinal cord lesion (SCL) compared with able-bodied controls. SETTING: Sahlgrenska University Hospital, G?teborg, Sweden. METHOD: Participants consisted of 20 persons with a complete SCL at the C5-C8 level (at least 1 year postinjury) and 20 matched, able-bodied controls. Breathing patterns and static lung volumes were measured using a body plethysmograph. RESULTS: Compared to the controls, breathing patterns at rest among the people with tetraplegia were characterised by a decreased tidal volume, stable respiratory rate and total cycle duration resulting in decreased mean inspiratory and expiratory flow, and alveolar ventilation. All volume and flow parameters increased except respiratory rate, which decreased during DB and PEP. During IR-PEP, tidal volume increased less compared to PEP, and combined with a decreased respiratory rate the alveolar ventilation was lower than during breathing at rest. The functional residual capacity increased during PEP and IR-PEP in people with tetraplegia. CONCLUSION: DB exercises with or without resistance during expiration or the whole breathing cycle affect the breathing pattern in persons with tetraplegia. DB was superior in increasing volumes and flow. PEP and IR-PEP increased FRC but IR-PEP decreased volumes and flows. However, large interindividual differences in the SCL group indicate the need for caution in generalising the results. SPONSORSHIP: This work was supported in part by grants from the Memorial Foundation of the Swedish Association of registered Physiotherapists and the Association of Cancer and Road Accident Victims.     

Experimental studies on lung mechanics, gas exchange and oxygen delivery under open lung conditions Mechanical ventilation with decelerating versus constant inspiratory flow

The treatment of patients with acute respiratory failure poses an ever-present clinical challenge since conventional ventilation sometimes causes further impairment of lung function. Thus, it is important to identify characteristics of different ventilatory patterns and ventilatory strategies that affect airway pressures, lung volumes, pulmonary gas exchange and hemodynamics. In order to mimic acute respiratory failure in the patient, we have used an experimental animal injury model, characterized by highly unstable alveoli. The basic prerequisite for this thesis was a positive end-expiratory pressure (PEEP) sufficient to keep the lung open throughout the whole ventilatory cycle. Different inspiratory flow patterns were studied by using a successful recruitment procedure together with ventilation performed using PEEP levels below, at, and above the inflection-point pressure. To elucidate the effect of prolonged inspiration time, sufficient PEEP was supplied to prevent end-expiratory collapse already during the reference mode with an inspiration/expiration ratio (I:E ratio) of 1:1. When an I:E ratio of 2:1 or above was used, both studies (one with mean airway pressure constant and the other with total PEEP constant) showed impaired hemodynamics, but displayed unaffected arterial oxygen tension. Decelerating inspiratory flow delivers the major part of the tidal volume during early inspiration under sustained constant inspiratory pressure. This reduces serial dead space and improves alveolar ventilation. In all studies, decelerating inspiratory flow showed increased carbon dioxide elimination, i.e. improved alveolar ventilation, but decelerating inspiratory flow was found to be no better for alveolar recruitment or in oxygenation. In an attempt to explain radiological observations when studying the relationship of pulmonary radiological appearance to end-expiratory pressure, Staubs' concept of quantal alveolar behavior was used. This states that the individual alveolus fills essentially independently of its neighbors and has a propensity to be either air-filled and expanded, or completely fluid-filled and collapsed. Stepwise sequential deflation of the lung from full expansion, followed by stepwise inflation of the lung, produced a pressure/attenuation loop. This hysteresis phenomenon implies that the attenuation of the lung relates to the set PEEP, but also to the immediately preceding structural status. For identical end-expiratory pressures the attenuation on the hysteresis down-slope invariably had a higher value than that of the up-slope. Thus, the pressure at which alveolar collapse takes place differs from that of expansion. In conclusion, ventilation with decelerating inspiratory flow revealed lower peak inspiratory and higher mean airway pressure than did constant inspiratory flow. No differences in end-inspiratory pressure were seen. Decelerating inspiratory flow delivers the major part of the tidal volume during early inspiration, which reduces serial dead space and improves alveolar ventilation. The decelerating inspiratory flow was found to be no better than constant inspiratory flow for alveolar recruitment or oxygenation. When the lung was ventilated with PEEP sufficient to prevent end-expiratory collapse, an inspiration-to-expiration ratio at and above 2:1 impaired hemodynamics. Our results suggest that, in order to obtain and maintain open lung conditions, the lung must be recruited and PEEP reduced to the level just above the alveolar collapse pressure.     

Expiratory flow limitation in morbidly obese postoperative mechanically ventilated patients

Although obesity promotes tidal expiratory flow limitation (EFL), with concurrent dynamic hyperinflation (DH), intrinsic PEEP (PEEPi) and risk of low lung volume injury, the prevalence and magnitude of EFL, DH and PEEPi have not yet been studied in mechanically ventilated morbidly obese subjects. In 15 postoperative mechanically ventilated morbidly obese subjects, we assessed the prevalence of EFL [using the negative expiratory pressure (NEP) technique], PEEPi, DH, respiratory mechanics, arterial oxygenation and PEEPi inequality index as well as the levels of PEEP required to abolish EFL. In supine position at zero PEEP, 10 patients exhibited EFL with a significantly higher PEEPi and DH and a significantly lower PEEPi inequality index than found in the five non-EFL (NEFL) subjects. Impaired gas exchange was found in all cases without significant differences between the EFL and NEFL subjects. Application of 7.5 +/- 2.5 cm H2O of PEEP (range: 4-16) abolished EFL with a reduction of PEEPi and DH and an increase in FRC and the PEEPi inequality index but no significant effect on gas exchange. The present study indicates that: (a) on zero PEEP, EFL is present in most postoperative mechanically ventilated morbidly obese subjects; (b) EFL (and concurrent risk of low lung volume injury) is abolished with appropriate levels of PEEP; and (c) impaired gas exchange is common in these patients, probably mainly due to atelectasis.     

Pressure characteristics of the Ambu CPAP system and the Servo Ventilator 900C in CPAP mode

Spontaneous breathing was stimulated in the Ambu continuous positive airway pressure (CPAP) system and the Servo Ventilator 900C by means of a lung model programmed to mimic the respiratory flow patterns of a healthy volunteer and a patient in severe respiratory distress. Changes in airway pressure, flow and volume were recorded during "breathing" with CPAP at 0.5, 1.0 and 1.5 kPa. In the Ambu system, the airway pressure decreased during inspiration and increased during expiration, while the mean airway pressure was close to the pre-set CPAP value. The pressure changes were minimal when the fresh gas flow was increased from 15 to 25 1 X min-1. The higher fresh gas flow is recommendable during deep or rapid breathing. In the Servo ventilator 900C, there was a short initial inspiratory pressure drop, succeeded by a pressure rise above the CPAP value. The expiratory airway pressure was somewhat higher than CPAP. Both systems were found to be recommendable for clinical use.     

Alternative methods for assessing bronchodilator reversibility in chronic obstructive pulmonary disease

BACKGROUND: Bronchodilator reversibility testing is recommended in all patients with chronic obstructive pulmonary disease (COPD) but does not predict improvements in breathlessness or exercise performance. Two alternative ways of assessing lung mechanics-measurement of end expiratory lung volume (EELV) using the inspiratory capacity manoeuvre and application of negative expiratory pressure (NEP) during tidal breathing to detect tidal airflow limitation-do relate to the degree of breathlessness in COPD. Their usefulness as end points in bronchodilator reversibility testing has not been examined. METHODS: We studied 20 patients with clinically stable COPD (mean age 69.9 (1.5) years, 15 men, forced expiratory volume in one second (FEV(1)) 29.5 (1.6)% predicted) with tidal flow limitation as assessed by their maximum flow-volume loop. Spirometric parameters, slow vital capacity (SVC), inspiratory capacity (IC), and NEP were measured seated, before and after nebulised saline, and at intervals after 5 mg nebulised salbutamol and 500 microg nebulised ipratropium bromide. The patients attended twice and the treatment order was randomised. RESULTS: Mean FEV(1), FVC, SVC, and IC were unchanged after saline but the degree of tidal flow limitation varied. FEV(1) improved significantly after salbutamol and ipratropium (0.11 (0.02) l and 0.09 (0.02) l, respectively) as did the other lung volumes with further significant increases after the combination. Tidal volume and mean expiratory flow increased significantly after all bronchodilators but breathlessness fell significantly only after the combination treatment. The initial NEP score was unrelated to subsequent changes in lung volume. CONCLUSIONS: NEP is not an appropriate measurement of acute bronchodilator responsiveness. Changes in IC were significantly larger than those in FEV(1) and may be more easily detected. However, our data showed no evidence for separation of "reversible" and "irreversible" groups whatever outcome measure was adopted.