Bulk elastic properties of excised lungs and the effect of a transpulmonary pressure gradient

Airway Closure and Expiratory Flow Limitation in Acute Respiratory Distress Syndrome

Acute respiratory distress syndrome (ARDS) is mostly characterized by the loss of aerated lung volume associated with an increase in lung tissue and intense and complex lung inflammation. ARDS has long been associated with the histological pattern of diffuse alveolar damage (DAD). However, DAD is not the unique pathological figure in ARDS and it can also be observed in settings other than ARDS. In the coronavirus disease 2019 (COVID-19) related ARDS, the impairment of lung microvasculature has been pointed out. The airways, and of notice the small peripheral airways, may contribute to the loss of aeration observed in ARDS. High-resolution lung imaging techniques found that in specific experimental conditions small airway closure was a reality. Furthermore, low-volume ventilator-induced lung injury, also called as atelectrauma, should involve the airways. Atelectrauma is one of the basic tenet subtending the use of positive end-expiratory pressure (PEEP) set at the ventilator in ARDS. Recent data revisited the role of airways in humans with ARDS and provided findings consistent with the expiratory flow limitation and airway closure in a substantial number of patients with ARDS. We discussed the pattern of airway opening pressure disclosed in the inspiratory volume-pressure curves in COVID-19 and in non-COVID-19 related ARDS. In addition, we discussed the functional interplay between airway opening pressure and expiratory flow limitation displayed in the flow-volume curves. We discussed the individualization of the PEEP setting based on these findings.

Early onset of airway derecruitment assessed using the forced oscillation technique in subjects with asthma

Derecruitment of air spaces in the lung occurs when airways close during exhalation and is related to ventilation heterogeneity and symptoms in asthma. The forced oscillation technique has been used to identify surrogate measures of airway closure via the reactance (Xrs) versus lung volume relationship. This study used a new algorithm to identify derecruitment from the Xrs versus lung volume relationship from a slow vital capacity maneuver. We aimed to compare two derecruitment markers on the Xrs versus volume curve, the onset reduction of Xrs (DR1_vol) and the onset of more rapid reduction of Xrs (DR2_vol), between control and asthmatic subjects. We hypothesized that the onset of DR1_vol and DR2_vol occurred at higher lung volume in asthmatic subjects. DR1_vol and DR2_vol were measured in 18 subjects with asthma and 18 healthy controls, and their relationships with age and height were examined using linear regression. In the control group, DR1_vol and DR2_vol increased with age (r² = 0.68, P < 0.001 and r² = 0.71, P < 0.001, respectively). DR1_vol and DR2_vol in subjects with asthma [76.58% of total lung capacity (TLC) and 56.79%TLC, respectively] were at higher lung volume compared with control subjects (46.1 and 37.69%TLC, respectively) (P < 0.001). DR2_vol correlated with predicted values of closing capacity (r = 0.94, P < 0.001). This study demonstrates that derecruitment occurs at two points along the Xrs-volume relationship. Both derecruitment points occurred at significantly higher lung volumes in subjects with asthma compared with healthy control subjects. This technique offers a novel way to measure the effects of changes in airways/lung mechanics. NEW & NOTEWORTHY This study demonstrates that the forced oscillation technique can be used to identify two lung volume points where lung derecruitment occurs: 1) where derecruitment is initiated and 2) where onset of rapid derecruitment commences. Measurements of derecruitment increase with age. The onset of rapid derecruitment was highly correlated with predicted closing capacity. Also, the initiation and rate of derecruitment are significantly altered in subjects with asthma.

Airway occlusion assessed by single breath N2 test and lung P-V curve in healthy subjects and COPD patients

PURPOSE

To determine whether the analysis of the slow expiratory transpulmonary pressure-volume (P_L-V) curve provides an alternative to the single-breath nitrogen test (SBN) for the assessment of the closing volume (CV).

METHODS

SBN test and slow deflation P_L-V curve were simultaneously recorded in 40 healthy subjects and 43 COPD patients. Onset of phase IV identified CV in SBN test (CV_SBN), whereas in the P_L-V curve CV was identified by: a) deviation from the exponential fit (CV_exp), and b) inflection point of the interpolating sigmoid function (CV_sig).

RESULTS

In the absence of phase IV, COPD patients exhibited a clearly discernible inflection in the P_L-V curve. In the presence of phase IV, CV_SBN and CV_exp coincided (CV_SBN/CV_exp=1.04±0.04 SD), whereas CV_sig was systematically larger (CV_sig/CV_exp=2.1±0.86).

CONCLUSION

The coincidence between CV_SBN and CV_exp, and the presence of the inflection in the absence of phase IV indicate that the deviation of the P_L-V curve from the exponential fit reliably assesses CV.

A functional mathematical model to simulate the single-breath nitrogen washout

A nonlinear dynamic model is proposed to reproduce and interpret the influence of pulmonary inhomogeneities on the single-breath nitrogen washout (SBNW) curve. The model is characterized by two parallel zones. In each zone, the upper airways are described by a Rohrer resistor. Intermediate airways are represented as a collapsible segment, the volume of which depends on transmural pressure. Smaller airways are described by a resistance which increases when transpulmonary pressure decreases. The respiratory region is modeled as a Voigt element. Three different conditions were simulated: a reference case, characterized by airway-parameter values for normal conditions, and two pathological states corresponding to different levels of disease. In the reference case, a straight line was a good approximation of SBNW phase III and the last point of departure of the nitrogen trace from this line unambiguously identified the onset of phase IV. The slope of phase III rose with disease severity (from a 1.1% increase in nitrogen concentration per 1000 ml of expired volume in the reference case to 3.6% and 7.7% in the pathological cases) and the distinction between phases III and IV became less evident. The results obtained indicate that the slope of phase III depends primarily on nitrogen-concentration differences between lung zones, as determined by different mechanical properties of the respiratory airways. In spite of the simplified representation of the lungs, the similarity of the simulation results to actual data suggests that the proposed model describes important physiological mechanisms underlying changes observed during SBNW in normal and pathological patients.

Murine pulmonary acinar mechanics during quasi-static inflation using synchrotron refraction-enhanced computed tomography

We visualized pulmonary acini in the core regions of the mouse lung in situ using synchrotron refraction-enhanced computed tomography (CT) and evaluated their kinematics during quasi-static inflation. This CT system (with a cube voxel of 2.8 μm) allows excellent visualization of not just the conducting airways, but also the alveolar ducts and sacs, and tracking of the acinar shape and its deformation during inflation. The kinematics of individual alveoli and alveolar clusters with a group of terminal alveoli is influenced not only by the connecting alveolar duct and alveoli, but also by the neighboring structures. Acinar volume was not a linear function of lung volume. The alveolar duct diameter changed dramatically during inflation at low pressures and remained relatively constant above an airway pressure of ∼8 cmH2O during inflation. The ratio of acinar surface area to acinar volume indicates that acinar distension during low-pressure inflation differed from that during inflation over a higher pressure range; in particular, acinar deformation was accordion-like during low-pressure inflation. These results indicated that the alveoli and duct expand differently as total acinar volume increases and that the alveolar duct may expand predominantly during low-pressure inflation. Our findings suggest that acinar deformation in the core regions of the lung is complex and heterogeneous.

Static inflation and deflation pressure-volume curves from excised lungs of marine mammals

Excised lungs from eight marine mammal species [harp seal (Pagophilus groenlandicus), harbor seal (Phoca vitulina), gray seal (Halichoerus grypush), Atlantic white-sided dolphin (Lagenorhynchus acutus), common dolphin (Delphinus delphis), Risso's dolphin (Grampus griseus), long-finned pilot whale (Globicephala melas) and harbor porpoise (Phocoena phocoena)] were used to determine the minimum air volume of the relaxed lung (MAV, N=15), the elastic properties (pressure-volume curves, N=24) of the respiratory system and the total lung capacity (TLC). Our data indicate that mass-specific TLC (sTLC, l kg(-1)) does not differ between species or groups (odontocete vs phocid) and agree with that estimated (TLC(est)) from body mass (M(b)) by applying the equation: TLC(est)=0.135 M(b)(0.92). Measured MAV was on average 7% of TLC, with a range from 0 to 16%. The pressure-volume curves were similar among species on inflation but diverged during deflation in phocids in comparison with odontocetes. These differences provide a structural basis for observed species differences in the depth at which lungs collapse and gas exchange ceases.

Closing volume: a reappraisal (1967-2007)

Measurement of closing volume (CV) allows detection of presence or absence of tidal airway closure, i.e. cyclic opening and closure of peripheral airways with concurrent (1) inhomogeneity of distribution of ventilation and impaired gas exchange; and (2) risk of peripheral airway injury. Tidal airway closure, which can occur when the CV exceeds the end-expiratory lung volume (EELV), is commonly observed in diseases characterised by increased CV (e.g. chronic obstructive pulmonary disease, asthma) and/or decreased EELV (e.g. obesity, chronic heart failure). Risk of tidal airway closure is enhanced by ageing. In patients with tidal airway closure (CV > EELV) there is not only impairment of pulmonary gas exchange, but also peripheral airway disease due to injury of the peripheral airways. In view of this, the causes and consequences of tidal airway closure are reviewed, and further studies are suggested. In addition, assessment of the "open volume", as opposed to the "closing volume", is proposed because it is easier to perform and it requires less equipment.

The structural basis of airways hyperresponsiveness in asthma

We hypothesized that structural airway remodeling contributes to airways hyperresponsiveness (AHR) in asthma. Small, medium, and large airways were analyzed by computed tomography in 21 asthmatic volunteers under baseline conditions (FEV1= 64% predicted) and after maximum response to albuterol (FEV1= 76% predicted). The difference in pulmonary function between baseline and albuterol was an estimate of AHR to the baseline smooth muscle tone (BSMT). BSMT caused an increase in residual volume (RV) that was threefold greater than the decrease in forced vital capacity (FVC) because of a simultaneous increase in total lung capacity (TLC). The decrease in FVC with BSMT was the major determinant of the baseline FEV1( P < 0.0001). The increase in RV correlated inversely with the relaxed luminal diameter of the medium airways ( P = 0.009) and directly with the wall thickness of the large airways ( P = 0.001). The effect of BSMT on functional residual capacity (FRC) controlled the change in TLC relative to the change in RV. When the FRC increased with RV, TLC increased and FVC was preserved. When the relaxed large airways were critically narrowed, FRC and TLC did not increase and FVC fell. With critical large airways narrowing, the FRC was already elevated from dynamic hyperinflation before BSMT and did not increase further with BSMT. FEV1/FVC in the absence of BSMT correlated directly with large airway luminal diameter and inversely with the fall in FVC with BSMT. These findings suggest that dynamic hyperinflation caused by narrowing of large airways is a major determinant of AHR in asthma.

Assessment of airway closure from deflation lung volume–pressure curve: sigmoidal equation revisited

OBJECTIVE

To assess a sigmoidal equation for describing airway closure.

DESIGN

Experimental study.

SETTING

University laboratory.

PARTICIPANTS

Eight piglets mechanically ventilated on zero end-expiratory pressure (ZEEP).

INTERVENTIONS

Control and lung saline lavage.

MEASUREMENTS AND RESULTS

Lungs were inflated up to transpulmonary pressure of 30 cmH(2)O at constant flow (0.12l s(-1)) then deflated at the same flow rate up to the point at which oesophageal pressure was constant, which was assumed to represent complete airway closure. The deflation volume-transpulmonary pressure curve was fitted to: (1) a sigmoidal equation focusing on inflexion point and pressure at maximal compliance increase and (2) an exponential equation above an inflexion point determined by eyeballing. Data deviate from the exponential equation at the point of airway closure onset. The zero-volume intercept was determined. Complete airway closure was reached at -8.3+/-3.5cmH(2)O in control conditions and at -1.3+/-3.7 cmH(2)O after lavage (p < 0.05). Between control and lavage, onset of airway closure was 3.0+/-1.9 vs. 6.0+/-2.8 cmH(2)O (p <0.05), inflexion point 3.2+/-1.8 vs. 7.7+/-2.6 cmH(2)O (p <0.001), pressure at maximal compliance increase -1.9+/-0.7 vs. -0.03+/-2.1cmH(2)O (p <0.05) and zero-volume intercept -1.5+/-1.4 vs. 0.3+/-2.3cmH(2)O (p <0.05).

CONCLUSIONS

During mechanical ventilation airways stay open and close around ZEEP in control but are closed above ZEEP after lavage. Inflexion point might reflect onset of airways closure in control. Pressure at maximal compliance increase was not a marker of complete airways closure. In control and lavage, pressure at maximal compliance increase and zero-volume intercept were reasonably equivalent.

Multiple pressure-volume loops recorded with sinusoidal low flow in a porcine acute respiratory distress syndrome model

OBJECTIVES

To develop a method for automatic recording of multiple dynamic elastic pressure-volume (P(el)/V) loops. To analyse the relationship between multiple dynamic P(el)/V loops and static P(el)/V loops in a porcine model of acute lung injury/acute respiratory distress syndrome (ALI/ARDS). To test the hypothesis that increasing lung collapse and re-expansion with decreasing positive end expiratory pressure (PEEP) can be characterized by hysteresis of the P(el)/V loops. MATERIAL AND INTERVENTIONS: In eight anaesthetized and paralysed pigs, ALI/ARDS was induced by inhalation of dioctyl sodium sulfosuccinate and large tidal volume ventilation.

MEASUREMENTS AND RESULTS

The physiological and histopathological findings indicated a status mimicking an early stage of ALI/ARDS. Automatically, a series of dynamic P(el)/V loops from different PEEP levels were recorded with the sinusoidal flow modulation method using a computer-controlled ventilator. During expiration, resistance increased more than twofold. For each step of lower starting pressure, the inspiratory limb was displaced towards lower volume indicating derecruitment. Recruitment occurred between 20 and 40 cm H(2)O. The expiratory curves, all starting from 50 cm H(2)O, overlapped. Hysteresis increased significantly in loops recorded from lower PEEP levels. Viscoelasticity explained differences between static and dynamic P(el)/V loops.

CONCLUSIONS

Automated multiple P(el)/V loop determination is feasible and provides comprehensive information on lung derecruitment and recruitment. It requires determination of volume dependence of expiratory resistance. An expiratory curve serves as a reference to inspiratory curves and provides information about hysteresis.

Low positive end-expiratory pressure does not exacerbate nebulized-acid lung injury in dogs

It is not clear if low end-expiratory pressures contribute to ventilator-induced lung injury in large animals. We sought to determine whether ventilation with a low level of positive end-expiratory pressure (PEEP) worsens preexisting permeability lung injury in dogs. Lung injury was initiated in 20 mongrel dogs by ventilating with nebulized 3N hydrochloric acid until a lower inflection point (LIP) appeared on the respiratory system pressure-volume loop. One group of 10 dogs was then ventilated for 4 hours with PEEP set below the LIP (low PEEP), whereas the remaining group of dogs was ventilated for the same time period with similar tidal volumes but with PEEP set above the LIP (high PEEP). We found histologic evidence of reduced alveolar volumes in the low-PEEP animals. However, there were no differences in neutrophil infiltration, lung lobe weights, pulmonary capillary hemorrhage or congestion, or arterial endothelin-1 concentration between the 2 protocol groups. In conclusion, we were unable to demonstrate that ventilation with PEEP set below the LIP exacerbates hydrochloric acid-induced lung injury in dogs.

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.

Elastic pressure-volume curves in acute lung injury and acute respiratory distress syndrome

BACKGROUND

The principal features of elastic pressure-volume curves of lungs or the respiratory system (P(el)/V curves) recorded during reexpansion of collapsed lungs and subsequent deflation have been known since the 1950s. In acute respiratory failure and acute respiratory distress syndrome such curves have recently attracted increasing interest because new knowledge can be acquired from them, and because such curves may be useful as guidelines in setting the ventilator so as to avoid ventilator-induced lung injury.

DISCUSSION

This article reviews recording methods, underlying physiology and utility of P(el)/V curves in research and clinical work.

Resting and exercise physiology in interstitial lung diseases

Interstitial lung diseases (ILDs) are a heterogeneous group of parenchymal pulmonary disorders with varying histologic appearances. Pulmonary function tests have gained a prominent role in the diagnosis and management of patients with these disorders. Although their role in the differential diagnosis of the various ILDs is limited, physiologic measurements are pivotal studies providing clues regarding disease severity, prognosis,and response to therapy.

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.

On defining total lung capacity in the mouse

Maximal lung volume or total lung capacity in experimental animals is dependent on the pressure to which the lungs are inflated. Although 25-30 cmH2O are nominally used for such inflations, mouse pressure-volume (P-V) curves show little flattening on inflation to those pressures. In the present study, we examined P-V relations and mean alveolar chord length in three strains (C3H/HeJ, A/J, and C57BL/6J) at multiple inflation pressures. Mice were anesthetized, and their lungs were degassed in vivo by absorption of 100% O2. P-V curves were then recorded in situ with increasing peak inflation pressure in 10-cmH2O increments up to 90 cmH2O. Lungs were quickly frozen at specific pressures for morphometric analysis. The inflation limbs never showed the appearance of a plateau, with lung volume increasing 40-60% as inflation pressure was increased from 30 to 60 cmH2O. In contrast, parallel flat deflation limbs were always observed, regardless of the inflation pressure, indicating that the presence of a flat deflation curve cannot be used to justify measurement of total lung capacity in mice. Alveolar size increased monotonically with increasing pressure in all strains, and there was no evidence of irreversible lung damage from these inflations to high pressures. These results suggest that the mouse lung never reaches a maximal volume, even up to nonphysiological pressures >80 cmH2O.

Contribution of airway closure to chronic postbronchiolitis airway dysfunction in rats

Genetically susceptible Brown Norway rats develop a chronic asthmalike syndrome after recovering from viral bronchiolitis at an early age. We hypothesized that airway closure is an important mechanism of airflow obstruction in postbronchiolitis rats. Rats were studied 8–12 wk after inoculation with Sendai virus or sterile vehicle at 3–4 wk of age. Under light pentobarbital anesthesia, rats were instrumented with an orotracheal catheter and an esophageal pressure monitor and placed in a total body plethysmograph. Lung volumes and forced-expiratory maneuvers were measured using the Boyle's law method and software-controlled valving of positive and negative pressures to elicit lung inflations and rapid deflations; pulmonary resistance was measured during spontaneous tidal breathing; and quasi-static pressure-volume curves were obtained with passive inflations and deflations in fully anesthetized, paralyzed rats. Compared with controls, the postbronchiolitis rats had elevated pulmonary resistance and reduced forced-expiratory volume in 0.2 s. Most of the reduced forced-expiratory volume in 0.2 s was associated with reduced forced vital capacity, indicating premature airway closure as a prominent mechanism. The reduced airflow in postbronchiolitis rats was highly dependent on lung volume, being nearly normal at 70% lung capacity, but sevenfold less than normal at 30% lung capacity. Increased respiratory system hysteresis between functional reserve capacity and total lung capacity was evidence for increased airway closure at normal end-expiratory lung volumes in postbronchiolitis rats. We conclude that airway instability and closure is a prominent mechanism of the chronic airway dysfunction in rats that have recovered from viral bronchiolitis at an early age.

Sigmoidal equation for lung and chest wall volume-pressure curves in acute respiratory failure

To assess incidence and magnitude of the "lower inflection point" of the chest wall, the sigmoidal equation was used in 36 consecutive patients intubated and mechanically ventilated with acute lung injury (ALI). They were 21 primary and 5 secondary ALI, 6 unilateral pneumonia, and 4 cardiogenic pulmonary edema. The lower inflection point was estimated as the point of maximal compliance increase. The low constant flow inflation method and esophageal pressure were used to partition the volume-pressure curves into their chest wall and lung components on zero end-expiratory pressure. The sigmoidal equation had an excellent fit with coefficients of determination >0.90 in all instances. The point of maximal compliance increase of the chest wall ranged from 0 to 8.3 cmH2O (median 1 cmH2O) with no difference between ALI groups. The chest wall significantly contributed to the lower inflection point of the respiratory system in eight patients only. The occurrence of a significant contribution of the chest wall to the lower inflection point of the respiratory system is lower than anticipated. The sigmoidal equation is able to determine precisely the point of the maximal compliance increase of lung and chest wall.

Differences in the deflation limb of the pressure-volume curves in acute respiratory distress syndrome from pulmonary and extrapulmonary origin

OBJECTIVE

To assess the differences in the deflation pressure-volume (PV) curves between acute respiratory distress syndrome from pulmonary (ARDSp) and extrapulmonary (ARDSe) origin.

DESIGN

. Prospective study.

SETTING

Twenty-bed intensive care unit in an university hospital.

PATIENTS

Ten patients within the first 24 h from meeting ARDS criteria, classified as ARDSp or ARDSe in a clinical basis.

INTERVENTIONS

A deflation PV curve was recorded by means of decreasing steps of continuous positive airway pressure (CPAP) from 35 to 0 cmH(2)O.

RESULTS

The simultaneous recording of pressure at the airway opening (Pao), esophageal pressure (Pes) and volumes (V) allows us to trace the Pao-V, Pes-V and transpulmonary pressure (Ptp)-V curves. These data were fitted to a sigmoid model and ARDSp and ARDSe groups were compared. ARDSp has lower lung compliance and higher chest wall compliance than ARDSe (35.9+/-11.3 vs. 77.2+/-50.6 and 199.6+/-44.4 vs. 125.5+/-16.5 ml/cmH(2)O, respectively, P<0.05). The Pao-V curve in ARDSp is shifted down and right with respect to ARDSe. The Ptp-V curve shows a similar displacement. The Pes-V curve in the ARDSp group is, however, shifted to the left. When relative values (percentage to the maximum volume achieved at 35 cmH(2)O) are considered, these differences persist, but, in the Ptp-V curves, are only significant in the low-pressure range.

CONCLUSIONS

Differences between ARDSp and ARDSe PV curves are present all along the pressure axis and are related to differences not only in the Pes-V curve, but also in the Ptp-V curve.

Effects of positive end-expiratory pressure on gas exchange and expiratory flow limitation in adult respiratory distress syndrome

OBJECTIVE

To assess the effects of different positive end-expiratory pressure (PEEP) levels (0, 5, 10, and 15 cm H2O) on tidal expiratory flow limitation (FL), regional intrinsic positive end-expiratory pressure (PEEPi) inhomogeneity, alveolar recruited volume (Vrec), respiratory mechanics, and arterial blood gases in mechanically ventilated patients with acute respiratory distress syndrome (ARDS).

DESIGN

Prospective clinical study.

SETTING

Multidisciplinary intensive care unit of a university hospital.

PATIENTS

Thirteen sedated, mechanically ventilated patients during the first 2 days of ARDS.

INTERVENTIONS

Detection of tidal FL and evaluation of total dynamic PEEP (PEEPt,dyn), total static PEEP (PEEPt,st), respiratory mechanics, and Vrec from pressure, flow, and volume traces provided by the ventilator. The average (+/-sd) tidal volume was 7.1 +/- 1.5 mL/kg, the total cycle duration was 2.9 +/- 0.45 secs, and the duty cycle was 0.35 +/- 0.05.

MEASUREMENTS

Tidal FL was assessed using the negative expiratory pressure technique. Regional PEEPi inhomogeneity was assessed as the ratio of PEEPt,dyn to PEEPt,st (PEEPi inequality index), and Vrec was quantified as the difference in lung volume at the same airway pressure between quasi-static inflation volume-pressure curves on zero end-expiratory pressure (ZEEP) and PEEP.

RESULTS

On ZEEP, seven patients exhibited FL amounting to 31 +/- 8% of tidal volume. They had higher PEEPt,st and PEEPi,st ( p<.001) and lower PEEPi inequality index ( p<.001) than the six nonflow-limited (NFL) patients. Two FL patients became NFL with PEEP of 5 cm H2O and five with PEEP of 10 cm H2O. In both groups, PaO2 increased progressively with PEEP. In the FL group, there was a significant correlation of PaO2 to PEEPi inequality index ( p=.002). For a given PEEP, Vrec was greater in NFL than FL patients, and a significant correlation of Pao to Vrec ( p<.001) was found only in the NFL group.

CONCLUSIONS

We conclude that on ZEEP, tidal FL is common in ARDS patients and is associated with greater regional PEEPi inhomogeneity than in NFL patients. With PEEP of 10 cm H2O, flow limitation with concurrent cyclic dynamic airway compression and re-expansion and the risk of "low lung volume injury" were absent in all patients. In FL patients, PEEP induced a significant increase in PaO2, mainly because of the reduction of regional PEEPi inequality, whereas in the NFL group, arterial oxygenation was improved satisfactorily because of alveolar recruitment.

Time dependence of recruitment and derecruitment in the lung: a theoretical model

Recruitment and derecruitment (R/D) of air spaces within the lung is greatly enhanced in lung injury and is thought to be responsible for exacerbating injury during mechanical ventilation. There is evidence to suggest that R/D is a time-dependent phenomenon. We have developed a computer model of the lung consisting of a parallel arrangement of airways and alveolar units. Each airway has a critical pressure (Pcrit) above which it tends to open and below which it tends to close but at a rate determined by how far pressure is from Pcrit. With an appropriate distribution of Pcrit and R/D velocity characteristics, the model able to produce realistic first and second pressure-volume curves of a lung inflated from an initially degassed state. The model also predicts that lung elastance will increase transiently after a deep inflation to a degree that increases as lung volume decreases and as the lung becomes injured. We conclude that our model captures the time-dependent mechanical behavior of the lung due to gradual R/D of lung units.

Low-volume ventilation causes peripheral airway injury and increased airway resistance in normal rabbits

Lung mechanics and morphometry of 10 normal open-chest rabbits (group A), mechanically ventilated (MV) with physiological tidal volumes (8-12 ml/kg), at zero end-expiratory pressure (ZEEP), for 3-4 h, were compared with those of five rabbits (group B) after 3-4 h of MV with a positive end-expiratory pressure (PEEP) of 2.3 cmH(2)O. Relative to initial MV on PEEP, MV on ZEEP caused a progressive increase in quasi-static elastance (+36%) and airway (Rint; +71%) and viscoelastic resistance (+29%), with no change in the viscoelastic time constant. After restoration of PEEP, quasi-static elastance and viscoelastic resistance returned to control levels, whereas Rint remained elevated (+22%). On PEEP, MV had no effect on lung mechanics. Gas exchange on PEEP was equally preserved in groups A and B, and the lung wet-to-dry ratios were normal. Both groups had normal alveolar morphology, whereas only group A had injured respiratory and membranous bronchioles. In conclusion, prolonged MV on ZEEP induces histological evidence of peripheral airway injury with a concurrent increase in Rint, which persists after restoration of normal end-expiratory volumes. This is probably due to cyclic opening and closing of peripheral airways on ZEEP.

Optimal peep in ARDS. Changing concepts and current controversies

From many recently performed studies, it is clear that a criterion standard for determining the optimal positive end-expiratory pressure (PEEP) level in patients with acquired respiratory distress syndrome (ARDS) does not exist. What is evident and consistent, however, are several points such the optimal PEEP level ultimately represents a balance between regional areas of overstretching and regional derecruitment; higher levels of PEEP may be required early in ARDS, independent of oxygenation requirements; and the exact method for titrating PEEP in patients with ARDS remains to be determined. These points and others are delineated and discussed in this article.

Pulmonary function tests in interstitial lung disease: what role do they have?

Pulmonary function tests have been widely accepted and utilized in the management of interstitial lung diseases. Although the tests performed have changed little over the past several decades, extensive literature has been published highlighting their clinical role in the diagnosis, staging, prognostication, and follow-up of patients with a wide variety of interstitial lung diseases.

Pentoxifylline rescue preserves lung function in isolated canine lungs injured with phorbol myristate acetate

OBJECTIVE

We hypothesized that pentoxifylline, administered after phorbol myristate acetate (PMA), would diminish the severity of lung injury.

SETTING

Animal research laboratory.

DESIGN

Comparative study.

SUBJECTS

Mongrel dogs (n = 33).

INTERVENTIONS

Baseline measurements were obtained from the isolated blood-perfused dog lung lobes after 1 h of stable perfusion and ventilation. Four different measures of lung compliance were obtained along with WBC and neutrophil counts. Pulmonary vascular resistance (PVR) and capillary filtration coefficient (Kf) were calculated, and the ratio of a normalized maximal enzymatic conversion rate to the Michaelis-Menten constant (Amax/Km) was used to assess perfused capillary surface area. The control lobes (n = 8) were ventilated and perfused for an additional 40 min while the injured lobes (n = 17) received PMA (0.1 microg/mL of perfusate). The pentoxifylline-protected lobes (n = 8) were treated with pentoxifylline (1 mg/mL of perfusate) 10 min after injury with PMA. All measurements were then repeated.

MEASUREMENT AND MAIN RESULTS

The three groups did not differ significantly at baseline. The control lobes remained relatively stable over time. The injured lobes demonstrated marked deterioration in compliance: 8.79 +/- 0.7 to 5.97 +/- 0.59 mL/cm H(2)O (p < 0.05) vs 10.1 +/- 1.0 to 8.07 +/- 0.72 mL/cm H(2)O and 9.6 +/- 1.1 to 9.9 +/- 0.85 mL/cm H(2)O in the control and protected lobes, respectively. Both groups receiving PMA had similar drops in WBC and neutrophil counts, but the pentoxifylline-protected lobes had preservation of all four compliance measures. PVR increased from 37.8 +/- 1.8 to 118.6 +/- 12.7 cm H(2)O/L/min (p < 0.05) in the injured lobes vs 35.4 +/- 0.5 to 36.3 +/- 2.8 cm H(2)O/L/min and 40.4 +/- 0.04 to 46.7 +/- 2.8 cm H(2)O/L/min (p < 0.05) in the control and protected lobes, respectively. Kf increased < 25% in the protected group but more than tripled in the injured group. Amax/Km dropped from 559 +/- 36 to 441 +/- 33 mL/min (p < 0.05) in the injured lobes vs 507 +/- 14 to 490 +/- 17 mL/min and 609 +/- 34 to 616 +/- 37 mL/min in the control and pentoxifylline-protected lobes, respectively.

CONCLUSIONS

The use of pentoxifylline as a rescue agent prevented the PMA-induced deterioration of lung compliance, vascular integrity, and endothelial metabolic function in this acute lung injury model, despite significant pulmonary neutrophil sequestration.

Ventilation above closing volume reduces pulmonary vascular resistance hysteresis

The aim of this study was to determine the relationship of pulmonary vascular resistance (PVR) hysteresis and lung volume, with special attention to the effects of ventilation around closing volume (CV). Isolated, blood-perfused canine left lower lung lobes (LLL) were incrementally inflated and deflated. Airway and pulmonary artery pressures (PAP) were recorded after each stepwise volume change. Constant blood flow was provided (600 ml/min) and the pulmonary vein pressure (PVP) was held constant at 5 cm H2O. PAP changes, therefore, were a direct index of PVR changes. Group 1 lobes underwent a full inflation from complete collapse to total lobe capacity (TLC) followed by a full deflation. Group 2 lobes underwent two deflation/inflation cycles, after an initial full inflation. These cycles, both beginning at TLC, had deflation end above and below CV, respectively. Significant PVR hysteresis was noted when the first inflation and deflation were compared. The maximum difference in PAP on deflation was 3.3 cm H2O or 11%. The mean decrease was 2.7 cm H2O for 18 lobes (p < 0.0001). The PAPs on all subsequent inflations or deflations that began above CV remained 9% lower than the initial inflation (n = 9, p < 0.0001), but were not different from each other. However, the final inflation which began from below CV resulted in a 30% return of PVR hysteresis (mean increase in PAP of 0.8 cm H2O, n = 7, p < 0.004). We conclude that there is hysteresis in the PVR response during ventilation, with decreased PVR during deflation relative to the initial inflation, that this hysteresis is absent when lung volume is maintained greater than CV, and that hysteresis returns when inflation occurs after deflation below CV.

Pulmonary gas exchange improves in the prone position with abdominal distension

Arterial blood oxygenation in patients with adult respiratory distress syndrome is often improved in the prone position. Critically ill patients often have abdominal distension and whether similar improvements in gas exchange occur with the prone position is not known. We therefore studied the effect of posture on gas exchange in eight ketamine-anesthetized pigs with abdominal distension. A rubber balloon, placed in the abdominal cavity, was filled with water to increase intra-abdominal pressure. The animals were mechanically ventilated with FIO2 = 0.4, and PaCO2 was kept constant. Gas exchange was measured in the supine and prone positions, with and without abdominal distension, in random order, using the multiple inert gas elimination technique (MIGET). When the abdomen was normal, the prone position increased PaO2 by 16 +/- 21 mm Hg (p < 0.05), accompanied by a small, but statistically insignificant, decrease in AaPO2 (p = 0.08) and no change in ventilation/perfusion (V A/Q) heterogeneity measured by MIGET. In the presence of abdominal distension, the prone position increased Pa O2 by 26 +/- 18 mm Hg (p < 0.01) and decreased AaPO2 (p < 0.05) and V A/Q heterogeneity as measured by the log standard deviation of the perfusion distribution (p < 0.01) and the arterial-alveolar difference area (p < 0.05). In addition, intragastric pressure was lower in the prone position (p < 0.01). We conclude that in anesthetized, mechanically ventilated pigs, the prone position improves pulmonary gas exchange to a greater degree in the presence of abdominal distension than when the abdomen is normal.

Computational model of oscillatory airflow in a bronchial bifurcation

Airflow distribution in the bronchial tree is an important factor that controls gas mixing in the lungs, especially, in diseased lungs or during high frequency ventilation. A nonlinear analog model has been developed to investigate the dependency of airflow distribution in asymmetric bronchial bifurcations on structural and physiological parameters. The system parameters (electrical analogs) are time-dependent and were extracted from laboratory studies of airway models and physiological measurements. The model was used to study flow distribution in peripheral pathways of normal and pathological airways during different modes of quiet breathing as well as high frequency ventilation. Model simulations revealed that (i) increasing of ventilation frequency or stroke volume increases the time and percentage of pendelluft in each cycle, (ii) diameter asymmetry between parallel pathways is more dominant than length asymmetry and enhances the degree of asynchronous ventilation to peripheral pathways, and (iii) asymmetry in the compliance of peripheral airways and lung parenchyma greatly increases the degree of asynchronous ventilation.

High-frequency oscillatory ventilation versus conventional ventilation in a piglet model of early meconium aspiration

OBJECTIVE

To compare the cardiopulmonary effects of high-frequency oscillatory ventilation (HFO) and conventional ventilation (CV) in a piglet model of meconium aspiration syndrome.

DESIGN

Prospective, randomized control study.

SUBJECTS

Piglets 1 to 2 wks of age.

INTERVENTIONS

Meconium aspiration was induced in 30 piglets. They were then randomized to CV, HFO at 10 Hz, or HFO at 15 Hz.

MEASUREMENTS AND MAIN RESULTS

Arterial blood gas, and systemic and pulmonary hemodynamics were measured serially. Airway opening pressure (P-Flex), static lung compliance (Crs), and trapped gas volume (TGV) were derived. Meconium instillation produced similar stable decreases in Crs (6.7 +/- 0.7 [SEM] to 4.7 +/- 0.4 mL/cm) and increases in pulmonary vascular resistance (68 +/- 6.4 vs. 91.9 +/- 8.5 mm Hg/mL/kg/min). A greater proportion of animals (40%, p< .007) remained hypercarbic during HFO at 15 Hz. Oxygenation indices were similar for all groups. In regards to high-frequency support, both power and deltaP were higher in the HFO at 15 Hz group (p< .001). When compared with both CV and HFO at 10 Hz, the TGV in the HFO at 15 Hz group was significantly higher following randomization to ventilator type. P-Flex was also greatest in the 15 Hz group, followed by the 10 Hz group and the CV group. Higher airway opening pressures, given identical compliance, suggest that HFO at 15 Hz resulted in greater large airway obstruction. With HFO's inherent low tidal volumes, progression of meconium to the distal airways may be delayed.

CONCLUSIONS

Early institution of HFO at 15 Hz in meconium aspiration may exacerbate air trapping. HFO at lower rates may be the optimal method of respiratory support in meconium aspiration syndrome. HFO may extend the window of time available for removal of meconium.

Relationships between alveolar size and fibre distribution in a mammalian lung alveolar duct model

A finite element model, comprising an assemblage of tetrakaidecahedra or truncated octahedra, is used to represent an alveolar duct unit. The dimensions of the elastin and collagen fibre bundles, and the surface tension properties of the air-liquid interfaces, are based on available published data. Changes to the computed static pressure-volume behavior with variation in alveolar dimensions and fibre volume densities are characterized using distensibility indices (K). The air-filled lung distensibility (Ka) decreased with a reduction in the alveolar airspace length dimensions and increased with a reduction of total fibre volume density. The saline-filled lung distensibility (Ks) remained constant with alveolar dimensions and increased with decreasing total fibre volume density. The degree of geometric anisotropy between the duct lumen and alveoli was computed over pressure-volume cycles. To preserve broadly isotropic behavior, parenchyma with smaller alveolar airspace length dimensions required higher concentrations of fibres located in the duct and less in the septa in comparison with parenchyma of larger airspace dimensions.

Respiratory system mechanics in sedated, paralyzed, morbidly obese patients

The effects of inspiratory flow and inflation volume on the mechanical properties of the respiratory system in eight sedated and paralyzed postoperative morbidly obese patients (aged 37.6 +/- 11.8 yr who had never smoked and had normal preoperative seated spirometry) were investigated by using the technique of rapid airway occlusion during constant-flow inflation. With the patients in the supine position, we measured the interrupter resistance (Rint,rs), which in humans probably reflects airway resistance, the "additional" resistance (delta Rrs) due to viscoelastic pressure dissipation and time-constant inequalities, and static respiratory elastance (Est,rs). Intra-abdominal pressure (IAP) was measured by using a bladder catheter, and functional residual capacity was measured by the heliumdilution technique. The results were compared with a previous study on 16 normal anesthetized paralyzed humans. Compared with normal persons, we found that in obese subjects: 1) functional residual capacity was markedly lower (0.645 +/- 0.208 liter) and IAP was higher (24 +/- 2.2 cmH2O); 2) alveolar-arterial oxygenation gradient was increased (178 +/- 59 mmHg); 3) the volume-pressure curve of the respiratory system was curvilinear with an "inflection" point; 4) Est,rs, Rint,rs, and delta Rrs were higher than normal (29.3 +/- 5.04 cmH2O/l, 5.9 +/- 2.4 cmH2O.l-1.s, and 6.4 +/- 1.6 cmH2O.l-1.s, respectively); 5) Rint,rs increased with increasing inspiratory flow, Est,rs did not change, and delta Rrs decreased progressively; and 6) with increasing inflation volume, Rint,rs and Est,rs decreased, whereas delta Rrs rose progressively. Overall, our data suggest that obese subjects during sedation and paralysis are characterized by hypoxemia and marked alterations of the mechanical properties of the respiratory system, largely explained by a reduction in lung volume due to the excessive unopposed IAP.

Effect of temperature on pressure-volume hysteresis of excised lungs

The objective of this study was to determine if the effect of temperature on excised lung pressure-volume (P-V) hysteresis during various P-V maneuvers would be consistent with predicted effects based on the recruitment-derecruitment (R-D) model of lung P-V hysteresis. Three sets of P-V curves were recorded for excised rat lungs at (1) 24 degrees C, (2) either 42 degrees or 45 degrees C, and (3) 24 degrees C. After full inflation of the lung, deflation-inflation (D-I) cycles were performed between total lung capacity (30 cmH2O) and successively decreasing end-expiratory pressures (EEPs). Normalized hysteresis (K) was plotted vs EEP. K remained relatively constant at EEPs > or = +5 cmH2O at 24 degrees C and 42 degrees C and > +5 cmH2O at 45 degrees C. Large increases in K occurred as the EEP was further reduced, with the relationship of K vs EEP being shifted to the right at 42 degrees C and 45 degrees C relative to 24 degrees C, with the greater shift occurring at 45 degrees C. Previous work has shown that the R-D of lung units contributes to P-V hysteresis and is EEP-dependent, increasing at EEPs < or = +4 +/- 1 cmH2O at room temperature (Cheng et al., 1995). This study suggests that at increased temperatures, R-D of lung units is initiated at higher EEPs and is more extensive than at room temperature.

Prone positioning improves pulmonary function in obese patients during general anesthesia

We investigated the effects of prone position on functional residual capacity (FRC), the mechanical properties (compliance and resistance) of the total respiratory system, lung and chest wall, and the gas exchange in 10 anesthetized and paralyzed obese (body mass index more than 30 kg/m2) patients, undergoing elective surgery. We used the esophageal balloon technique together with rapid airway occlusions during constant inspiratory flow to partition the mechanics of the respiratory system into its pulmonary and chest wall components. FRC was measured by the helium dilution technique. Measurements were taken in the supine position and after 15-30 min of prone position maintaining the same respiratory pattern (tidal volume 12 mL/kg ideal body weight, respiratory rate 14 breaths/ min, fraction of inspired oxygen [FIO2]0.4). We found that FRC and lung compliance significantly (P < 0.01) increased from the supine to prone position (0.894 +/- 0.327 L vs 1.980 +/- 0.856 L and 91.4 +/- 55.2 mL/cm H2O vs 109.6 +/- 52.4 mL/cm H2O, respectively). On the contrary, the prone position reduced chest wall compliance (199.5 +/- 58.7 mL/cm H2O vs 160.5 +/- 45.4 mL/cm H2O, P < 0.01), thus total respiratory system compliance did not change. Resistance of the total respiratory system, lung, and chest wall were not modified on turning the patients prone. The increase in FRC and lung compliance was paralleled by a significant (P < 0.01) improvement of PaO2 from supine to prone position (130 +/- 31 vs 181 +/- 28 mm Hg, P < 0.01), while PaCO2 was unchanged. We conclude that, in anesthetized and paralyzed obese subjects, the prone position improves pulmonary function, increasing FRC, lung compliance, and oxygenation.

Contribution of opening and closing of lung units to lung hysteresis

The recruitment and derecruitment of lung units is one explanation of the hysteresis observed in an excised lung during inflation and deflation. A simplified model has been proposed in which the recruitment-derecruitment process is a function of end-expiratory pressure (Frazer, D.G., K.C. Weber and G.N. Franz, Respir. Physiol. 61: 277-288, 1985). The object of this study was to test this model with three experimental procedures. During the first set of experiments, progressively larger pressure-volume (PL-VL) loops were recorded with end-expiratory pressure held at either -5 cmH2O, where all lung units are assumed to be closed, or +5 cmH2O, where all recruited lung units are assumed to be open. In the first case hysteresis is maximal, in the second, minimal. The difference in hysteresis is presumed to arise from the recruitment-derecruitment process. In the second set of experiments, excised lungs are slowly inflated and then deflated at a constant rate while constant-amplitude sinusoidal volume oscillations are superimposed. The end-expiratory pressure of the superimposed loops gradually rose as the lung was inflated and fell as the lung was deflated. Hysteresis was minimal when end-expiratory pressure was above 4 +/- 1 cmH2O even as peak-to-peak loop pressure greatly varied. This supports the notion of an end-expiratory pressure dependent mechanism of recruitment/derecruitment. During the third set of experiments lungs were inflated to either 50%, 75%, or 100% TLC. Volumes of air were then withdrawn and replaced so that the initial volume was restored in sinusoidal fashion as the amplitude of the volume excursions increased. For PL-VL loops with end-expiratory pressures between +4 and -2 cmH2O, pressure amplitudes rose and the hysteresis index (loop area/tidal volume) increased, regardless of the initial lung volume. These results are consistent with the previously described model of Frazer et al. (1985) which assumed that PL-VL curves can be divided into an 'opening' region, an 'open' region and a 'closing' region and that the demarcation of these regions depends on transpulmonary pressure, specifically end-expiratory pressure, and to a much lesser degree on lung volume.

The mechanical behavior of a mammalian lung alveolar duct model

A model for the mechanical properties of an alveolar duct is analyzed using the finite element method. Its geometry comprises an assemblage of truncated octahedral alveoli surrounding a longitudinal air duct. The amounts and distributions of elastin and collagen fiber bundles, modeled by separate stress-strain laws, are based upon published data for dogs. The surface tension of the air-liquid interface is modeled using an area-dependent relationship. Pressure-volume curves are computed that compare well with experimental data for both saline-filled and air-filled lungs. Pressure-volume curves of the separate elastin and collagen fiber contributions are similar in form to the behavior of saline-filled lungs treated with either elastase or collagenase. A comparison with our earlier model, based upon a single alveolus, shows the duct to have a behavior closer to reported experimental data.

Postnatal growth of lung parenchyma in the piglet: morphometry correlated with mechanics

BACKGROUND

A previous study of piglet lung growth (Mansell et. al. 1989. J. Appl. Physiol., 67:1422-1427) showed transient stiffness to changes in shape and volume immediately after birth. Later, elastic recoil was found to increase as the lung grew in weight and volume. The present study uses morphometry to test possible structural correlates of these two mechanical changes.

METHODS

Piglet lungs were fixed near full inflation via the airways during the immediate newborn period (6-12 hours, n = 3), at 3-5 days (n = 6), 25-30 days (n = 5), and 80-85 days (n = 3). Morphometry comprised arithmetic and harmonic mean thicknesses of alveolar septae and average mean surface curvature. Measurements of curvature and airspace volume were combined to differentiate alveolar expansion from septal proliferation as mechanisms for volumetric growth.

RESULTS

The unique mechanical behavior of the newborn lungs was associated with relatively thick alveolar septae. Marked thinning of the septae and resolution of the stiffness to shape and volume change had occurred by 3-5 days. An increase in elastic recoil during the first postnatal month was found to be associated with simple airspace expansion. The second and third months were characterized by septal proliferation and increase in arithmetic mean septal thickness but elastic recoil did not increase further. Harmonic mean septal thickness and airspace volume per gram of lung tissue did not change over the course of the study.

CONCLUSIONS

1) A relative stiffness to shape and volume change in freshly newborn piglet lungs is associated with relatively thick alveolar septal walls; 2) postnatal development of piglet lung parenchyma involves septal lengthening and thinning followed by septal proliferation; 3) the initial phase of septal lengthening, rather than the later phase of septal proliferation, is associated with increase in parenchymal recoil.

Exponential analysis of the pressure-volume characteristics of ovine lungs

Static pressure-volume curves were generated from data obtained from 18 normal anaesthetized adult sheep. Lung volumes were determined by helium dilution. An exponential curve of the form V = Vmax - Ae-KP was fitted to the pressure-volume data from each sheep where P is the static recoil pressure, Vmax represents the volume asymptote, A is the difference between Vmax and the intercept on the volume axis and K defines the slope and hence the shape of the P-V curve. Quality of fit of the data was assessed visually, by means of a sign test and a runs test and by the coefficient of determination (r2). Exponential equations were found to adequately describe the shape of the pressure-volume curve in sheep. The exponent K was not correlated with effective alveolar volume (VAeff) (rs = 0.183; P > 0.05). Static lung compliance was determined over a volume range from the end-expiratory level (VEEL) to VEEL plus 400 ml. Measurements of static lung compliance were significantly correlated with measurements of effective alveolar volume (VAeff) (rs = 0.505; P < 0.025). In the ovine, the exponent K, an index of distensibility, is independent of lung volume and offers a means of assessing lung distensibility in this species.

Mechanical behaviour of the canine respiratory system at very low lung volumes

We studied the changes in dynamic elastance and resistance of the respiratory system in 6 supine, anaesthetized, paralysed, tracheostomised and open chested dogs. Tracheal pressure (Ptr), tracheal flow (V) and 3 alveolar pressures (Palv by alveolar capsule) were measured continuously for 20 min at 5 levels of positive end expiratory pressure (PEEP) between 0.1 and 0.5 kPa. The lungs were inflated to total lung capacity (TLC) at the start of each recording period. Lung elastance (EL) and resistance (RL) were estimated by fitting the equation Ptr = RLV + ELV + K to the measured data for each breath by multiple linear regression (V = volume, K = constant). Airway resistance (Raw) was obtained from the difference between Ptr and Palv. EL increased progressively in the 20 min following lung inflations. The increase in EL over this time was about 45% of its baseline value at a PEEP of 0.1 kPa compared to an increase of only about 10% at a PEEP of 0.5 kPa. In contrast, RL changed very little over the recording period at all levels of PEEP. At low levels of PEEP Palv often bore no resemblance to Ptr indicating that significant airway obstruction or closure had occurred. These results suggest that the increase in EL at low PEEP was primarily due to the accretion of airspace closure, and that nonlinear tissue mechanical properties were responsible for the lack of change in RL.

Prone position alters the effect of volume overload on regional pleural pressures and improves hypoxemia in pigs in vivo

Oxygenation improves in patients with adult respiratory distress syndrome and in animals with oleic acid-induced lung injury when they are turned from the supine to the prone position. Dependent and nondependent pleural pressures (Ppl) were measured in six pigs ventilated in the supine and prone positions before and after volume infusion (VI). Before VI the mean +/- SEM AaPO2 difference was 26 +/- 8 mm Hg when the animals were supine and 10 +/- 2 mm Hg when they were prone (p > 0.05). After VI the AaPO2 was 64 +/- 6 mm Hg when the animals were supine (p < 0.05) and 43 +/- 7 mm Hg when they were prone (p < 0.05). VI increased the Ppl gradient from 0.53 +/- 0.1 to 0.71 +/- 0.1 cm H2O/cm when the animals were supine (p < 0.05) and from 0.17 +/- 0.1 to 0.27 +/- 0.1 cm H2O/cm when they were prone (p < 0.05). Dependent Ppl at FRC was much less positive when the animals were prone versus supine (0.9 +/- 0.3 versus 3.0 +/- 0.5 cm H2O, p < 0.05), suggesting that the airways in these dependent regions would narrow and/or close and that ventilation to these regions would diminish as a result of VI.

Estimating alveolar surface area during life

Alveolar surface area (Sa) may be derived from measurements of total lung capacity (TLC) and mean linear intercept (Lm), an estimate of average airspace size. Because the index of pulmonary distensibility, K, is a function of Lm, we were able to derive Lm from ln K using their respective age regressions and estimated Sa in 147 healthy subjects. K was obtained from exponential analysis of static pressure-volume data. TLC was measured in a body plethysmograph. As an estimate of airspace size, the value used for Lm was shown to be appropriate for the air-inflated lung at TLC. In 95 men (mean age 40 +/- 16 SD years), Sa was 118 (SD 22) m2 and in 52 women (mean age 38 +/- 17 SD years) it was 91 (SD 18) m2 and Sa decreased with age (P less than 0.001). In a morphometric study, Thurlbeck (Am. Rev. Respir. Dis. 95: 765-773, 1967) obtained smaller values for Sa (owing to the use of a smaller lung volume) and a similar decrease in Sa with age as that found here. Providing a standardised methodology is used for measurement of K, the present method allows a reasonable estimate of Sa to be obtained during life.