Prone position reduces lung stress and strain in severe acute respiratory distress syndrome.

The present authors hypothesised that in severe acute respiratory distress syndrome (ARDS), pronation may reduce ventilator-induced overall stress (i.e. transpulmonary pressure (P(L))) and strain of lung parenchyma (i.e. tidal volume (V(T))/end-expiratory lung volume (EELV) ratio), which constitute major ventilator-induced lung injury determinants. The authors sought to determine whether potential pronation benefits are maintained in post-prone semirecumbent (SR(PP)) posture under pressure-volume curve-dependent optimisation of positive end-expiratory pressure (PEEP). A total of 10 anesthetised/paralysed, mechanically ventilated (V(T) = 9.0+/-0.9 mL.kg(-1) predicted body weight; flow = 0.91+/-0.04 L.s(-1); PEEP = 9.4+/-1.3 cmH(2)O) patients with early/severe ARDS were studied in pre-prone semirecumbent (SR(BAS)), prone, and SR(PP) positions. Partitioned respiratory mechanics were determined during iso-flow (0.91 L.s(-1)) experiments (V(T) varied within 0.2-1.0 L), along with haemodynamics, gas exchange, and EELV. Compared with SR(BAS), pronation/SR(PP) resulted in reduced peak/plateau P(L) at V(T)s> or =0.6 L; static lung elastance and additional lung resistance decreased and chest wall elastance (in prone position) increased; EELV increased (23-33%); V(T)/EELV decreased (27-33%); arterial oxygen tension/inspiratory oxygen fraction and arterial carbon dioxide tension improved (21-43/10-14%, respectively), and shunt fraction/physiological dead space decreased (21-50/20-47%, respectively). In early/severe acute respiratory distress syndrome, pronation under positive end-expiratory pressure optimisation may reduce ventilator-induced lung injury risk. Pronation benefits may be maintained in post-prone semirecumbent position.     

Influence of progressive and of transient hypoxia on upper airway resistance in normal humans

In order to evaluate the influence of hypoxia on upper airway patency, we measured the response of upper airway resistance (UAR) to progressive (P) normocapnic hypoxia (Rebuck's method) and transient (T) hypoxia (three to five breaths to 100% N2) in 11 normal men. Breath-by-breath inspiratory UAR was calculated at isoflow during exclusive nasal breathing. The UAR response to hypoxia was characterized by the changes in nasal resistance and pharyngeal resistance (PR) as a function of SaO2, mean inspiratory flow (VT/Tl), and changes in the end-expiratory lung volume (EELV) measured with an inductance vest. The ventilatory response to hypoxia was greater during T (-0.31 +/- 0.03 L/min/%SaO2; mean +/- SEM) than during P (-0.27 +/- 0.03 L/min/%SaO2, p = 0.05). UAR decreased as SaO2 decreased; this decrease was steeper during T than during P hypoxia (delta PR/%SaO2: 3.9 +/- 0.5 during P and 2.5 +/- 0.2 during T, p = 0.05). For the whole group, there was no difference in the slope of the decrease in UAR with increasing VT/Tl between the two hypoxic tests (delta PR/delta VT/Tl: -0.85 +/- 0.1 during P and -0.70 +/- 0.1 during T, p greater than 0.05). However, in four subjects, the slope of the relationship PR/VT/Tl during T remained steeper than during P. EELV increased as SaO2 decreased, with a greater increase during progressive than during transient hypoxia.(ABSTRACT TRUNCATED AT 250 WORDS)     

Static pressure-volume curves and body posture in severe chronic bronchitis.

Based on prior data, the current authors hypothesised that beneficial pronation effects on gas exchange and respiratory mechanics might be maximised in severely hyperinflated chronic bronchitis patients. The current authors also sought to elucidate underlying mechanisms and to determine whether pronation effects are reflected by postural changes in inspiratory pressure-volume (P-V) curve characteristics. A total of 16 mechanically ventilated patients (for 16-36 h) with chronic bronchitis exacerbation were studied in pre-prone semirecumbent (SREC), prone and post-prone SREC postures. Static respiratory system intrinsic positive end-expiratory pressure (PEEPi,rs) was >12 cmH2O. Haemodynamics, partitioned respiratory mechanics, gas exchange, and lung volumes were determined at zero external positive end-expiratory pressure. P-V curves were constructed from functional residual capacity. End-expiratory lung volume exceeded opening volume. Prone position versus pre-prone SREC resulted in 20% reduced pressure at the lower inflection point (LIP) and 17% increased volume at the upper inflection point of the lung P-V curve, improved lung mechanics and volumes, oxygenation, and carbon dioxide arterial tension (Pa,CO2). In multiple linear regression, postural decreases in PEEPi,rs and additional lung resistance independently predicted postural decreases in lung LIP pressure and Pa,CO2), respectively. In conclusion, in severely hyperinflated patients, pronation reduces lung lower inflection point pressure and increases lung upper inflection point volume. Pronation effects on ventilation homogeneity and carbon dioxide arterial tension are maximised, implying that pronation can be useful during early controlled ventilation.     

Exercise-induced changes in functional residual capacity

We used a helium-rebreathe technique in nine healthy subjects to determine the effects of exercise intensity and duration on end-expiratory lung volume (EELV). The rebreathe functional residual capacity (FRC) technique was shown: (a) to be similar to that measured in the body plethysmograph, at rest; (b) to agree closely with volitionally induced changes in EELV as determined by inductance plethysmography, at rest; (c) to be reproducible within subjects between trials conducted at rest or exercise on different days (r = 0.96, coefficient of variation +/- 3%); (d) to correlate significantly with coincident changes in end-expiratory esophageal pressure from rest to exercise, with increasing exercise intensity and over time at a constant exercise load. Exercise-induced reductions in EELV occurred in all subjects, averaging 0.3 L (-0.1 to -0.7 L) in light exercise and 0.79 L (-0.5 to -1.2 L) in heavy or maximum exercise. This reduction in EELV accounted for slightly more than one-half of the increase in VT during light exercise and slightly less than one-half of the increased VT in heavy exercise. In heavy prolonged exercise lasting 8-15 min, EELV fell in the initial 2 min and was either sustained at this reduced level or fell further with exercise duration to exhaustion. We found that FRC was reduced even in very light exercise when changes in TE and VE from rest were minimal; further reductions in EELV occurred as end-inspiratory lung volume increased and expiratory time shortened with increasing exercise intensity and duration. Based on these types of changes we speculate that active expiration during exercise in humans may be controlled by a combination of locomotor-related feed-forward and lung volume related feed-back mechanisms.     

Perceived inspiratory difficulty during inspiratory threshold and hyperinflationary loadings

Dynamic hyperinflation loads the inspiratory muscles by increasing end-expiratory lung volume (EELV) and imposing intrinsic positive end-expiratory pressure (PEEPi), the latter behaving as an inspiratory threshold load (ITL). The major purpose of this study was to describe the independent effects of the imposed ITL and changes in operating lung volume on the perception of inspiratory difficulty. In eight healthy subjects, independent increases in EELV and ITL were induced by continuous positive airway pressure (CPAP) and external ITL applications, respectively; increase in both EELV and PEEPi (thus the imposed ITL) was induced by application of positive end-expiratory pressure (PEEP). The perceived inspiratory difficulty increased significantly when either EELV or ITL was increased, and was always greater during combined increase in EELV and the imposed ITL (during PEEP) than when either factor was increased independently, suggesting that the imposed ITL and EELV each contribute independently to inspiratory difficulty. Inspiratory difficulty of each subject under all conditions was then fitted into a step-forward multiple regression model. The imposed ITL was a significant contributor to inspiratory difficulty in all subjects and was the first parameter to be selected in six of the eight subjects. When the results of all the subjects were pooled, the imposed ITL alone explained 40% of variations in inspiratory difficulty. Adding the change in end-inspiratory lung volume (DeltaEILV) to the model explained an additional 24% of variations in inspiratory difficulty. The coefficients (slopes) of the imposed ITL and DeltaEILV were 0.21 +/- 0.02 cm H2O-1 and 0.051 +/- 0.006 %IC-1, respectively. It is concluded that under our experimental conditions, the imposed ITL is a better predictor for explaining the variability of the perceived inspiratory difficulty than the operating lung volume.     

Expiratory flow limitation and intrinsic positive end-expiratory pressure at zero positive end-expiratory pressure in patients with adult respiratory distress syndrome

It has been suggested that in patients with adult respiratory distress syndrome (ARDS), intrinsic positive end-expiratory pressure (PEEPi) is generated by a disproportionate increase in expiratory flow resistance. Using the negative expiratory pressure (NEP) technique, we assessed whether expiratory flow limitation (EFL) and PEEPi were present at zero PEEP in 10 semirecumbent, mechanically ventilated ARDS patients. Because bronchodilators may decrease airway resistance, we also investigated the effect of nebulized salbutamol on EFL, PEEPi, and respiratory mechanics in these patients, and in seven patients we measured the latter variables in the supine position as well. In the semirecumbent position, eight of the 10 ARDS patients exhibited tidal EFL, ranging from 5 to 37% of the control tidal volume (VT), whereas PEEPi was present in all 10 subjects, ranging from 0.4 cm H(2)O to 7.7 cm H(2)O. The onset of EFL was heralded by a distinct inflection point on the expiratory flow-volume curve, which probably reflected small-airway closure. Administration of salbutamol had no statistically significant effect on PEEPi, EFL (as %VT), or respiratory mechanics. EFL (%VT) and PEEPi were significantly higher in the supine position than in the semirecumbent position, whereas the other respiratory variables did not change. Our results suggest that in the absence of externally applied PEEP, most ARDS patients exhibit EFL associated with small-airway closure and a concomitant PEEPi.     

Effects of end-expiratory lung volume on lung mechanics in normal and edematous lungs

This study determined the effects of end-expiratory pressures (EEP) and alterations in end-expiratory lung volume (EELV) on lung compliance (CL) and pulmonary resistance to gas flow (RP) in 20 cats with normal and edematous lungs. EELV was varied using EEP ranging from -8 to +10 cm H2O. Negative EEP was used to decrease EELV of the healthy lung causing CL to decrease and RP to increase. Positive EEP in the healthy lung also caused CL to decrease but did not significantly affect RP. After inducing pulmonary edema using alloxan, functional residual capacity (FRC) decreased 38%, CL decreased 66% and RP increased 106% (p less than 0.001). An EEP of 4 cm H2O returned EELV to normal FRC levels and produced maximum values for CL. Increases in EEP to 4 cm H2O also caused decreases in RP in the edematous lungs but further increase did not cause significant changes in RP. These results show that (1) relatively low levels of EEP returned EELV to normal FRC levels in alloxan-induced pulmonary edema, and (2) optimal lung mechanics were obtained when EELV was equal to or slightly above normal FRC values in both healthy and edematous lungs.     

Recruitment maneuvers in three experimental models of acute lung injury. Effect on lung volume and gas exchange

Recruitment maneuvers (RM), consisting of sustained inflations at high airway pressures, have been advocated as an adjunct to mechanical ventilation in acute respiratory distress syndrome (ARDS). We studied the effect of baseline ventilatory strategy and RM on end-expiratory lung volume (EELV) and oxygenation in 18 dogs, using three models of acute lung injury (ALI; n = 6 in each group): saline lavage (LAV), oleic acid injury (OAI), and intratracheal instillation of Escherichia coli (pneumonia; PNM). All three models exhibited similar degrees of lung injury. The PNM model was less responsive to positive end-expiratory pressure (PEEP) than was the LAV or OAI model. Only the LAV model showed an oxygenation response to increasing tidal volume (VT). After RM, there were transient increases in Pa(O(2)) and EELV when ventilating with PEEP = 10 cm H(2)O. At PEEP = 20 cm H(2)O the lungs were probably fully recruited, since the plateau airway pressures were relatively high ( approximately 45 cm H(2)O) and the oxygenation was similar to preinjury values, thus making the system unresponsive to RM. Sustained improvement in oxygenation after RM was seen in the LAV model when ventilating with PEEP = 10 cm H(2)O and VT = 15 ml/kg. Changes in EELV correlated with changes in Pa(O(2)) only in the OAI model with PEEP = 10 cm H(2)O. We conclude that responses to PEEP, VT, and RM differ among these models of ALI. RM may have a role in some patients with ARDS who are ventilated with low PEEP and low VT.     

Effects of PEEP on respiratory mechanics after open heart surgery.

Respiratory dysfunction, particularly atelectasis, is common after open heart surgery. Routine use of PEEP (5 to 10 cm H2O) in these patients has been advocated. We studied the effects of different levels of PEEP on respiratory mechanics in ten mechanically ventilated open heart surgery patients in the immediate postoperative period. PEEP was studied in increasing increments and decreasing decrements. This procedure was repeated three times. Flow, tidal volume, and airway pressure were measured. We used the rapid airway occlusion technique to determine static compliance of the respiratory system (Cst,rs) and intrinsic PEEP (PEEPi). The changes in end-expiratory lung volume (delta EELV) were measured with respiratory inductive plethysmography. Recruitment of lung units (Vrec) was estimated as the difference in lung volume between PEEP and zero end-expiratory (ZEEP) for the same static inflation pressure (15 cm H2O). We found that (1) Cst,rs at ZEEP was significantly reduced (60 +/- 2 ml/cm H2O); (2) while PEEP of 5 cm H2O did not cause significant recruitment, higher levels of PEEP (10 to 15 cm H2O) were effective; (3) Cst,rs, Vrec, and delta EELV were higher during stepwise PEEP decrease; (4) after the first and second stepwise PEEP increase-decrease run, there was a small persistent increase in EELV and Cst,rs at ZEEP. No further changes were found after the third run. We conclude that after open heart surgery, PEEP less than 10 cm H2O is not effective to reopen atelectatic lung units.     

Volumetric capnography in patients with acute lung injury: effects of positive end-expiratory pressure.

The aim of the study was to analyse the effects of positive end-expiratory pressure (PEEP) on volumetric capnography and respiratory system mechanics in mechanically ventilated patients. Eight normal subjects (control group), nine patients with moderate acute lung injury (ALI group) and eight patients with acute respiratory distress syndrome (ARDS group) were studied. Respiratory system mechanics, alveolar ejection volume as a fraction of tidal volume (VAE/VT), phase III slopes of expired CO2 beyond VAE and Bohr's dead space (VD/VT(Bohr)) at different levels of PEEP were measured. No differences in respiratory system resistances were found between the ALI and ARDS groups. VD/VT(Bohr) and expired CO2 slope beyond VAE were higher in ALI patients (0.52+/-0.01 and 13.9+/-0.7 mmHg x L(-1), respectively) compared with control patients (0.46+/-0.01 and 7.7+/-0.4 mmHg x L(-1), p<0.01, respectively) and in ARDS patients (0.61+/-0.02 and 24.9+/-1.6 mmHg x L(-1), p<0.01, respectively) compared with ALI patients. VAE/VT differed similarly (0.6+/-0.01 in control group, 0.43+/-0.01 in ALI group and 0.31+/-0.01 in ARDS group, p<0.01). PEEP had no effect on VAE/VT, expired CO2 slope beyond VAE and VD/VT(Bohr) in any group. A significant correlation (p<0.01) was found between VAE/VT and expired CO2 slope beyond VAE and lung injury score at zero PEEP. Indices of volumetric capnography are affected by the severity of the lung injury, but are unmodified by the application of positive end-expiratory pressure.     

Respiratory and haemodynamic effects of the prone position at two different levels of PEEP in a canine acute lung injury model.

This study was designed to examine whether the oxygenation response in the prone position differs in magnitude depending on the level of positive end-expiratory pressure (PEEP) applied in the supine position, and whether cardiac output (CO) increases in the prone position. In seven supine dogs, acute lung injury was established by saline lavage (arterial oxygen tension (Pa,O2)/inspiratory oxygen fraction (FI,O2) 17.8+/-9.6 kPa (134+/-72 mmHg)), and inflection point (Pflex) of the respiratory system was measured (6.6+/-1.4 cmH2O). Pa,O2/FI,O2 and CO of the supine and prone positions were obtained under the application of low PEEP and then under optimal PEEP (2 cmH2O below and above Pflex, respectively). The net increase in Pa,O2/FI,O2 by prone positioning was greater at low PEEP (27.3+/-12.0 kPa (205+/-90 mmHg)) than at optimal PEEP (4.4+/-13.0 kPa (33+/-98 mmHg)) (p=0.006). CO decreased significantly with optimal PEEP in the supine position (2.4+/-0.5 versus 3.1+/-0.4 L x min(-1) at baseline, p<0.001), and increased to 3.4+/-0.6 and 3.6+/-0.7 L x min(-1) in the prone position at 5 min and 30 min, respectively (both p=0.018). When the dogs were turned supine at optimal PEEP, CO again decreased (2.4+/-0.5 L x min(-1), p<0.001). In conclusion, the prone position augmented the effect of relatively low positive end-expiratory pressure on oxygenation, and attenuated the haemodynamic impairment of relatively high positive end-expiratory pressure in a canine acute lung injury model.     

Comparison of lung area by chest radiograph, with estimation of lung volume by helium dilution during prone and supine positioning in mechanically ventilated preterm infants: a pilot study

Measurement of lung volume may be useful in determining the degree of lung disease and for optimizing an infant's mechanical ventilator settings. A chest radiograph (CXR) is often used to estimate lung volume, because direct measurement, e.g., functional residual capacity (FRC), is neither practical nor possible in the neonatal intensive care unit. In supinely positioned infants, good correlation was found between lung area determined by CXR and lung volume, e.g., functional residual capacity (FRC). Whether this is true for the prone position is unknown. Since positioning may affect oxygenation and pulmonary function, we studied the relationship between lung area measured from CXR and FRC during both supine and prone positioning in 14 mechanically ventilated preterm infants. Lung area was determined from CXRs using computed radiography and FRCs obtained by helium dilution at end-expiration in both supine and prone positions. Reproducibility of lung area measurements was demonstrated by high correlations between two observers (R2 = 0.92 and 0.99 for supine and prone, respectively). When supine, lung area was 15.4 +/- 3.1 cm2, and FRC was 19.5 +/- 7.3 ml. In prone position, lung area was 16.7 +/- 4.2 cm2, and FRC 23.0 +/- 9.4 ml. There was a moderate to strong positive correlation between lung area and FRC for both positions (supine: r = 0.57, P < 0.03; prone: r = 0.63, P < 0.02). Lung area measured by computed radiography is a reproducible and practical method for estimating lung volume from routine chest X-rays in both supine and prone positions in mechanically ventilated preterm infants.     

The effects of body weight on airway calibre.

Increased wheeze and asthma diagnosis in obesity may be due to reduced lung volume with subsequent airway narrowing. Asthma (wheeze and airway hyperresponsiveness), functional residual capacity (FRC) and airway conductance (Gaw) were measured in 276 randomly selected subjects aged 28-30 yrs. Data were initially adjusted for smoking and asthma before examining relationships between weight and FRC (after adjustment for height), and between body mass index (BMI = weight.height(-2)) and Gaw (after adjustment for FRC) by multiple linear regression, separately for females and males. For males and females, BMI (+/-95% confidence interval) was 27.0+/-4.6 kg.m(-2) and 25.6+/-6.0 kg.m(-2) respectively, Gaw was 0.64+/-0.04 L.s(-1).cmH2O(-1) and 0.57+/-0.03 L.s(-1).cmH2O(-1), and FRC was 85.3+/-3.4 and 84.0+/-2.9% of predicted. Weight correlated independently with FRC in males and females. BMI correlated independently and inversely with Gaw in males, but only weakly in females. In conclusion, obesity is associated with reduced lung volume, which is linked with airway narrowing. However, in males, airway narrowing is greater than that due to reduced lung volume alone. The mechanisms causing airway narrowing and sex differences in obesity are unknown.     

Evaluation of impulse oscillation system: comparison with forced oscillation technique and body plethysmography.

The impulse oscillation system (IOS) has been developed recently to measure respiratory system resistance (Rrs) and reactance (Xrs) at different frequencies up to > or = 25 Hz. IOS has, however, not been validated against established techniques. This study compared IOS with the classical pseudorandom noise forced oscillation technique (FOT) and body plethysmographic airway resistance (Raw) in 49 subjects with a variety of lung disorders and a wide range of Raw (0.10-1.28 kPa x L(-1) x s). Rrs,IOS was slightly greater than Rrs,FOT, especially at lower frequencies, with a mean +/- SD difference at 5-6 Hz of 0.14 +/- 0.09 kPa x L(-1) x s. Comparisons with the wave-tube technique applied on two analogues indicated an overestimation by IOS. Xrs,IOS and Xrs,FOT were very similar, with a slightly higher resonant frequency with IOS than with FOT (mean difference +/- SD 1.35 +/- 3.40 Hz). Raw was only moderately correlated with Rrn,FOT and Rrs-IOS; although the mean differences were small (0.04 +/- 0.14 kPa x L(-1)s for Rrs6,FOT and -0.10 +/- 0.14 kPa x L(-1) x s for Rrs5,IOS), IOS and FOT markedly underestimated high resistance values. In conclusion, the impulse oscillation system yields respiratory system resistance and reactance values similar, but not identical to those provided by the forced oscillation technique.     

The effects of ventilatory pattern on hyperinflation, airway pressures, and circulation in mechanical ventilation of patients with severe air-flow obstruction

Patients with severe air-flow obstruction receiving mechanical ventilation are at risk of inadvertent pulmonary hyperinflation with morbidity and mortality caused by pneumothorax and circulatory depression. Nine patients with severe air-flow obstruction (5 asthma, 4 chronic air-flow obstruction) requiring mechanical ventilation were studied while sedated and therapeutically paralyzed. Pulmonary hyperinflation during steady-state ventilation was quantified by measuring total exhaled volume during 20- to 40-s apnea (end-inspiratory lung volume, VEI). Patients were studied at 3 levels of minute ventilation (VE) (10, 16, and 26 L/min) and at each VE, 3 levels of tidal volume (VT) (0.6, 1.0, and 1.6 L) and 3 levels of inspiratory flow (VI) (40, 70, and 100 L/min for VT = 1.0 L). There were progressive increases in VEI when VT was increased or when expiratory time (TE) was decreased either by an increase in rate (and hence VE) or by a decrease in VI (at a constant VE) reaching lung volumes as high as 3.6 +/- 0.4 L above FRC. Alveolar, central venous, and esophageal pressure rose in parallel with lung volumes, and hypotension was seen in most patients at highest lung volumes. Peak airway pressure (Ppk) was predominantly related to inspiratory flow and did not reflect changes in lung volume. Levels of ventilation required for normocapnia prior to paralysis (15.7 +/- 2.3 L/min) were associated with hypotension in 7 patients and probable hyperinflation in excess of 1.96 +/- 0.17 L above FRC. VEI is a simple, reproducible measurement of pulmonary hyperinflation and may be more important than Ppk in the causation of barotrauma.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of PEEP on inspiratory resistance in mechanically ventilated COPD patients.

This study aimed to investigate the effect of increased lung volume with positive end-expiratory pressure (PEEP) on respiratory resistance in patients with chronic obstructive pulmonary disease (COPD). Ten patients with COPD were mechanically ventilated for acute respiratory failure. PEEP was set at 0, 5, 10 and 15 cm H2O. Using the rapid airway occlusion technique, the total inspiratory resistance of the respiratory system was partitioned into interrupter (Rint,rs) and additional effective (deltaRrs) resistances. At each level of PEEP, at constant inflation flow, the inflation volume (deltaV) was varied from 0.2-1 L, and, at constant deltaV, the inflation flow was varied from 0.2-1.2 L x s(-1). The changes in end-expiratory lung volume (deltaEELV) induced by PEEP were also measured. The difference between the EELV and the relaxation volume of the respiratory system (deltaFRC) increased significantly with PEEP of 10 and 15 cm H2O as compared to a PEEP of 0, the increase being associated with a significant reduction of Rint,rs. By contrast, deltaRrs was independent of deltaFRC. At constant deltaV, Rint,rs fitted Rohrer's equation (Rint,rs = K1 + K2 x flow). While K2 significantly declined with AFRC, K1 did not change. At all levels of PEEP, deltaRrs was not influenced by deltaFRC. With increasing lung volume induced by positive end-expiratory pressure, the inspiratory airway resistance decreased, whereas the viscoelastic behaviour of the respiratory system, as reflected by additional effective resistance, did not change.     

Chest wall kinematics and breathlessness during pursed-lip breathing in patients with COPD

BACKGROUND: Pursed-lip breathing (PLB) is a strategy often spontaneously employed by patients with COPD during distress situations. Whether and to what extent PLB affects operational lung volume is not known. Also, conflicting reports deal with PLB capability of decreasing breathlessness. Participants and measurements: Twenty-two patients with mild-to-severe COPD were studied. Volumes of chest wall (CW) compartments (rib cage [RC] and abdomen) were assessed using an optoelectronic plethysmograph. Dyspnea was assessed by a modified Borg scale. RESULTS: Compared to spontaneous breathing, patients with PLB exhibited a significant reduction (mean +/- SD) in end-expiratory volume of the CW (VCW) [VCWee; - 0.33 +/- 0.24 L, p < 0.000004], and a significant increase in end-inspiratory VCW (VCWei; + 0.32 +/- 0.43 L, p < 0.003). The decrease in VCWee, mostly due to the decrease in end-expiratory volume of the abdomen (VAbee) [- 0.25 +/- 0.21 L, p < 0.00002], related to baseline FEV(1) (p < 0.02) and to the increase in expiratory time (TE) [r(2) = 0.49, p < 0.0003] and total time of the respiratory cycle (TTOT) [r(2) = 0.35, p < 0.004], but not to baseline functional residual capacity (FRC). Increase in tidal volume (VT) of the chest wall (+ 0.65 +/- 0.48 L, p < 0.000004) was shared between VT of the abdomen (0.31 +/- 0.23 L, p < 0.000004) and VT of the rib cage (+ 0.33 +/- 0.29 L, p < 0.00003). Borg score decreased with PLB (p < 0.04). In a stepwise multiple regression analysis, decrease in VCWee accounted for 27% of the variability in Borg score at 99% confidence level (p < 0.008). CONCLUSIONS: Changes in VCWee related to baseline airway obstruction but not to hyperinflation (FRC). By lengthening of TE and TTOT, PLB decreases VCWee and reduces breathlessness.     

Differences in pharyngeal properties between snorers with predominantly central sleep apnea and those without sleep apnea

The underlying cause of idiopathic central sleep apnea syndrome is not well understood. We therefore examined the possibility that patients with idiopathic central sleep apnea may have abnormalities of upper airway mechanics that might contribute to the pathogenesis of central apneas. The acoustic reflection technique was used to assess pharyngeal size, lung volume dependence, and pharyngeal "compliance" in 8 patients with idiopathic central sleep apnea, all of whom were snorers, and in 8 weight-matched, snoring control subjects with normal sleep studies. Patients with central sleep apnea when compared with control subjects exhibited markedly increased specific pharyngeal "compliance" (0.12 +/- 0.05 versus 0.03 +/- 0.01 cm H2O-1; p less than 0.001), increased change in pharyngeal area from FRC to RV (0.8 +/- 0.5 versus 0.03 +/- 0.3 cm2; p less than 0.05), and a larger pharyngeal area at FRC (4.7 +/- 0.9 versus 3.8 +/- 0.8 cm2; p less than 0.03). We conclude that increased pharyngeal "compliance" and lung volume dependence may play a role in the etiology of central apneas in this syndrome.     

外源性呼气末正压对慢性阻塞性肺疾病患者呼吸功的影响

目的探讨外源性呼气末正压（PEEPe）对慢性阻塞性肺疾病（COPD）患者呼吸功的影响及其机制。方法选择接受气管插管和机械通气的COPD患者10例,使用BicoreCP-100呼吸力学监测仪进行呼吸力学监测。首先测定PEEPe为0时的动态内源性呼气末正压（PEEPi）,再以此值的0、40%、60%、80%和100%随机设置PEEPe;在每次改变PEEPe水平30min后,测定呼吸力学及心率、血压、动脉血气。结果患者的呼吸功、压力时间乘积、食管压差和动态PEEPi在加用PEEPe后即明显降低,且随着PEEPe的增加继续下降（P值均<0.01）。当PEEPe加至PEEPe=0时所测的动态PEEPi的80%和100%时,呼吸机做功显著增加（P值均<0.01）。呼吸功的变化和动态PEEPi的变化呈直线正相关关系（r=0.609,P<0.01）。结论PEEPe可降低呼吸功,并且是通过降低患者呼气末肺泡与中心气道之间的压力差来降低呼吸功的。     

Ventilatory constraints during exercise in patients with chronic heart failure

We examined the degree of ventilatory constraint in patients with a history of chronic heart failure (CHF; n = 11; mean +/- SE age, 62 +/- 4 years; cardiac index [CI], 2.0 +/- 0.1; and ejection fraction [EF], 24 +/- 2%) and in control subjects (CTLS; n = 8; age, 61 +/- 5 years; CI, 2.6 +/- 0.3) by plotting the tidal flow-volume responses to graded exercise in relationship to the maximal flow-volume envelope (MFVL). Inspiratory capacity (IC) maneuvers were performed to follow changes in end-expiratory lung volume (EELV) during exercise, and the degree of expiratory flow limitation was assessed as the percent of the tidal volume (VT) that met or exceeded the expiratory boundary of the MFVL. CHF patients had significantly (p < 0.05) reduced baseline pulmonary function (FVC, 76 +/- 4%; FEV(1), 78 +/- 4% predicted) relative to CTLS (FVC, 99 +/- 4%; FEV(1), 102 +/- 4% predicted). At peak exercise, oxygen consumption (VO(2)) and minute ventilation (V(E)) were lower in CHF patients than in CTLS (VO(2), 17 +/- 2 vs 32 +/- 2 mL/kg/min; VE, 56 +/- 4 vs 82 +/- 6 L/min, respectively), whereas VE/carbon dioxide output was higher (42 +/- 4 vs 29 +/- 5). In CTLS, EELV initially decreased with light exercise, but increased as VE and expiratory flow limitation increased. In contrast, the EELV in patients with CHF remained near residual volume (RV) throughout exercise, despite increasing flow limitation. At peak exercise, IC averaged 91 +/- 3% and 79 +/- 4% (p < 0.05) of the FVC in CHF patients and CTLS, respectively, and flow limitation was present over > 45% of the VT in CHF patients vs < 25% in CTLS (despite the higher VE in CTLS). The least fit and most symptomatic CHF patients demonstrated the lowest EELV, the greatest degree of flow limitation, and a limited response to increased inspired carbon dioxide during exercise, all consistent with VE constraint. We conclude that patients with CHF commonly breathe near RV during exertion and experience expiratory flow limitation. This results in VE constraint and may contribute to exertional intolerance.