EFFECT OF GAS DENSITY AND VISCOSITY ON THE MAXIMAL EXPIRATORY FLOW-VOLUME RELATIONSHIP

Expiratory flow limitation

Expiratory flow limitation occurs when flow ceases to increase with increasing expiratory effort. The equal pressure point concept has been largely successful in providing intuitive understanding of the phenomenon, wherein maximal flows are determined by lung recoil and resistance upstream of the site where bronchial transmural pressure is zero (the EPP). Subsequent work on the fluid dynamical foundations led to the wave-speed theory of flow limitation, where flow is limited at a site when the local gas velocity is equal to speed of propagation of pressure waves. Each is a local theory; full predictions require knowledge of both density-dependent Bernoulli pressure drops and viscosity-dependent pressure losses due to dissipation. The former is dominant at mid to high lung volumes, whereas the latter is more important at low lung volumes as the flow-limiting site moves peripherally. The observation of relative effort independence of the maximal flow versus volume curves is important clinically insofar as such maneuvers, when carefully performed, offer a unique window into the mechanics of the lung itself, with little confounding effects. In particular, the important contributions of lung recoil and airways resistance can often be assessed, with implications and applications to diagnosis and management of pulmonary disease.

Mechanical properties of the upper airway

The importance of the upper airway (nose, pharynx, and larynx) in health and in the pathogenesis of sleep apnea, asthma, and other airway diseases, discussed elsewhere in the Comprehensive Physiology series, prompts this review of the biomechanical properties and functional aspects of the upper airway. There is a literature based on anatomic or structural descriptions in static circumstances, albeit studied in limited numbers of individuals in both health and disease. As for dynamic features, the literature is limited to studies of pressure and flow through all or parts of the upper airway and to the effects of muscle activation on such features; however, the links between structure and function through airway size, shape, and compliance remain a topic that is completely open for investigation, particularly through analyses using concepts of fluid and structural mechanics. Throughout are included both historically seminal references, as well as those serving as signposts or updated reviews. This article should be considered a resource for concepts needed for the application of biomechanical models of upper airway physiology, applicable to understanding the pathophysiology of disease and anticipated results of treatment interventions.

Inert gas transport in blood and tissues

This article establishes the basic mathematical models and the principles and assumptions used for inert gas transfer within body tissues-first, for a single compartment model and then for a multicompartment model. From these, and other more complex mathematical models, the transport of inert gases between lungs, blood, and other tissues is derived and compared to known experimental studies in both animals and humans. Some aspects of airway and lung transfer are particularly important to the uptake and elimination of inert gases, and these aspects of gas transport in tissues are briefly described. The most frequently used inert gases are those that are administered in anesthesia, and the specific issues relating to the uptake, transport, and elimination of these gases and vapors are dealt with in some detail showing how their transfer depends on various physical and chemical attributes, particularly their solubilities in blood and different tissues. Absorption characteristics of inert gases from within gas cavities or tissue bubbles are described, and the effects other inhaled gas mixtures have on the composition of these gas cavities are discussed. Very brief consideration is given to the effects of hyper- and hypobaric conditions on inert gas transport.

Techniques for assessing small airways function: Possible applications in asthma and COPD

In recent years special interest has been expressed for the contribution of small airways in the pathophysiology, clinical manifestations and treatment of asthma and COPD. Small airways contribute little to the total respiratory resistance so that extensive damage of small airways may occur before the appearance of any symptoms, and this is the reason why they are characterized as the "silent zone" of airways. Furthermore, the peripheral localization of the small airways and their small diameter constitutes difficult their direct assessment. Thus, they are usually studied indirectly, taking advantage of the effects of their obstruction, such as premature closure, air trapping, heterogeneity of ventilation, and lung volume dependence of airflow limitation. Today, several heterogeneous methods for the assessment of small airways are available. These can be either functional (spirometry, plethysmography, resistance measurements, nitrogen washout, alveolar nitric oxide, frequency dependence of compliance, flow-volume curves breathing mixture of helium-oxygen) or imaging (mainly through high resolution computed tomography). The above-mentioned methods are summarized in Table 1. However, no method is currently considered as the "gold standard" and it seems that combinations of tests are needed. Furthermore, it is not clear whether the small airways are affected in all patients with asthma or COPD and their clinical significance remains under investigation. Well-designed future studies with large numbers of patients are expected to reveal which of the methods for assessing the small airways is the most accurate, reliable and reproducible, for which patients, and which can be used for the evaluation of the effects of treatment.

Asthma in patients climbing to high and extreme altitudes in the Tibetan Everest region

OBJECTIVES

The aim of this study was to investigate the behavior of asthma in patients traveling to high and extreme altitudes.

METHODS

Twenty-four Dutch patients with mild asthma did a trekking at high and extreme altitudes (up to 6410 m = 21030 ft) in the Tibetan Everest region. Asthma symptoms, use of asthma medication, symptoms of acute mountain sickness, spirometry, peripheral oxygen saturation, and heart rate were measured at 1300 m (baseline), and at 3875, 4310, 5175, and 6410 m. Asthma symptoms were assessed by means of a modified version of the Asthma Control Test. Symptoms of acute mountain sickness were scored by the Lake Louise self-report questionnaire. The expedition staff, consisting of seven healthy persons, acted as a control group.

RESULTS

In both asthmatics and controls, forced vital capacity (FVC) and forced expiratory volume in one second (FEV1) decreased with increasing altitude, whereas FEV1 as percent of FVC (FEV1%FVC) did not change. In both groups, peak expiratory flow (PEF) increased with increasing altitude. In general, differences in spirometric values between asthmatics and controls were not significant. Asthma symptoms did not change with increasing altitude. During ascent, less than half of the asthma patients increased their medication use. According to the Lake Louise score, no acute mountain sickness occurred, except for in the asthma group at 6410 m, which showed mild acute mountain sickness at that altitude. As expected, peripheral oxygen saturation decreased with increasing altitude in asthmatics and controls, differences between the two groups not being significant. In general, heart rate (at rest) did not change with altitude, except for an increase in asthmatics at 6410 m.

CONCLUSIONS

These results suggest that traveling to high and extreme altitudes is safe for patients with mild asthma.

The Use of a Helium-Oxygen Mixture during Maximum Expiratory Flow to Demonstrate Obstruction in Small Airways in Smokers

We measured the response to breathing a mixture of 80% helium and 20% oxygen (He) during a maximum expiratory flow-volume (MEFV) maneuver in 66 nonsmokers and 48 smokers, aged 17-67. All of the subjects studied had (forced expiratory volume in 1 s/forced vital capacity [FEV(1.0)/FVC]) x 100 of greater than 70%. While the flow rates of the smokers were within +/-2 SD of those of the nonsmokers at 50% VC (Vmax(50)), both groups showed a reduction in flow with age (nonsmokers: r=-0.34, P<0.01; smokers r=-0.52, P<0.001). Nonsmokers showed no significant reduction with age in response to breathing He, while smokers showed a marked reduction with age (r=-0.63, P<0.001 at Vmax(50)). We also measured the lung volume at which maximum expiratory flow (Vmax) while the subject was breathing He became equal to Vmax while he was breathing air, and expressed it as a percent of the VC. This was the most sensitive method of separating smokers from nonsmokers. These results indicate that the use of He during an MEFV maneuver affords sufficient sensitivity to enable detection of functional abnormalities in smokers at a stage when Vmax while they are breathing air is normal.

Impaired exercise ventilatory mechanics with the self-contained breathing apparatus are improved with heliox

The effect of the self-contained breathing apparatus (SCBA) with compressed air (BA-A) on ventilatory mechanics, work of breathing (WOB), pulmonary function, and respiratory muscle fatigue, was compared with that of a low resistance breathing valve (LRV). Further, the effect of unloading the respiratory muscles with heliox with the SCBA (BA-H) was compared with BA-A and LRV. Twelve men completed three randomized exercise trials on separate days, each consisting of three 10 min bouts of stepping exercise (Bouts 1, 2, and 3) separated by a 5 min recovery. Subjects wore firefighter protective equipment including the SCBA. At rest, FEV(1) and peak expiratory flow rates were lower with BA-A than with LRV, but were higher with BA-H than either with BA-A or LRV. After Bout 3, expiratory reserve volume, expiratory resistive WOB, and inspiratory elastic WOB were increased in BA-A compared to LRV but these were lower with BA-H compared to BA-A. After Bout 3, maximal inspiratory and expiratory pressures were reduced with BA-A, but not with LRV or BA-H. In summary, we found that the SCBA reduced resting pulmonary function, and increased expiratory reserve volume, work of breathing, and respiratory muscle fatigue during stepping exercise, and these changes can be reduced with the use of heliox.

Location of flow limitation in liquid-filled rabbit lungs

The effects of end-inspiratory lung volume (EILV) and expiratory flow rate (Q) on the location of flow limitation in liquid-filled lungs were investigated by measuring pressure along the airways and by radiographic imaging. The lungs of New Zealand white rabbits were filled with perfluorocarbon to the randomly selected EILV of 20, 30, or 40 ml/kg, and the volume was actively drained at one of three Q: 2.5, 5.0, or 7.5 ml/s. The minimum pressures recorded by a movable catheter at locations along the airways show that flow limitation occurred in the main bronchi and trachea, and was independent of EILV and Q. The minimum pressure at the trachea was -80 mm Hg compared with values that were more positive than -10 mm Hg at a location 3 cm distal to the carina for all EILV and Q combinations. This location was confirmed by the lung images. The airway diameters gradually decreased with time, until flow limitation occurred. In airways distal to the collapse, there was not a significant decrease in diameter. Based on these data, we conclude that flow limitation in liquid-filled lungs occurs in the trachea and main bronchi and its location is independent of EILV or Q.

Flow Limitation in Liquid-Filled Lungs: Effects of Liquid Properties

Flow limitation in liquid-filled lungs is examined in intact rabbit experiments and a theoretical model. Flow limitation (“choked” flow) occurs when the expiratory flow reaches a maximum value and further increases in driving pressure do not increase the flow. In total liquid ventilation this is characterized by the sudden development of excessively negative airway pressures and airway collapse at the choke point. The occurrence of flow limitation limits the efficacy of total liquid ventilation by reducing the minute ventilation. In this paper we investigate the effects of liquid properties on flow limitation in liquid-filled lungs. It is found that the behavior of liquids with similar densities and viscosities can be quite different. The results of the theoretical model, which incorporates alveolar compliance and airway resistance, agrees qualitatively well with the experimental results. Lung compliance and airway resistance are shown to vary with the perfluorocarbon liquid used to fill the lungs. Surfactant is found to modify the interfacial tension between saline and perfluorocarbon, and surfactant activity at the interface of perfluorocarbon and the native aqueous lining of the lungs appears to induce hysteresis in pressure–volume curves for liquid-filled lungs. Ventilation with a liquid that results in low viscous resistance and high elastic recoil can reduce the amount of liquid remaining in the lungs when choke occurs, and, therefore, may be desirable for liquid ventilation.

Small airways function in patients with inflammatory bowel disease

It has been reported that in patients with inflammatory bowel disease (IBD), the airways are involved, and a number of clinical manifestations have been described. The aim of this study was to investigate the function of the small airways in IBD. Thirty patients with IBD (mean age, 47 yr), 12 with Crohn's disease and 18 with ulcerative colitis, were studied and compared with a control group of 16 normal subjects. Maximal expiratory flow-volume curves were performed breathing room air and a mixture of 80% helium, 20% oxygen. The differences of flows at 50% of FVC (delta Vmax50) and the volume of equal flows (Visov) were calculated as indices of small airways function. In addition, spirometry, lung volumes, and diffusing capacity were measured. Visov was statistically significantly greater in patients with either CD or UC than in control subjects (x +/- SD) (24.99 +/- 1.35 and 25.95 +/- 1.5 versus 20.1 +/- 1.39), (p < 0.01 and p < 0.001, respectively). A reduction in TL(CO) was noticed in the active stage of the disease in both groups of patients (p < 0.05). This may indicate that lung parenchyma is also involved in active IBD. Our results suggest that the function of the small airways and diffusion capacity of the lungs are affected in patients with IBD.

[The utility of the flow-volume curve in detecting fixed extrathoracic stenoses in patients with chronic obstructive pulmonary disease]

The flow-volume curve is the usual noninvasive diagnostic test for upper airways obstruction. In order to assess its usefulness for the detection of fixed upper airways obstruction in chronic obstructive pulmonary disease (COPD), we plotted flow-volume curves using maximum inspiratory and expiratory maneuvers in 60 COPD patients and in 15 healthy controls. Baseline readings were taken, followed by readings after random application of fixed external resistances with diameters of 4, 6, 8 and 10 mm in all cases. Although PEF and FIF50 decreased significantly with resistance of 10 mm, even in the group with the most severe ventilatory limitation it was necessary to reduce the internal diameter to 6 mm to detect changes in FEV1. In patients with baseline values under 50% of theoretical values, 4 mm stenosis was required to provoke changes in FEV1. The fall in FEV1 and PEF was less evident as the severity of COPD increased, with both parameters correlating with percent baseline FEV1 only at diameters of 6 mm (p < 0.01) and 4 mm (p < 0.001). The sensitivities of the usual indices for detecting upper airways obstruction, such as FEV1/PEF and FEV1/FEV0-5, were low (below 50%) in all groups at diameters over 6 mm, and in the most severe cases of COPD, even with stenosis of 4 mm. Nor did other indices, such as FEF50/FIF50 and FEV1/FIV1,, give better diagnostic yield. We conclude that the flow-volume curve may not detect the existence of upper airways obstruction in COPD, and that in the most severely affected patients alternative diagnostic methods should therefore be considered.

Effect of altitude on hand-held peak flowmeters

OBJECTIVE

To quantify the effect of altitude on the operational characteristics of hand-held peak flowmeters.

DESIGN

Altitude simulation within a hypobaric chamber combined with five constant simulated peak flows delivered from a computerized pump were used to test commercially available peak flowmeters.

SETTING

F.G. Hall Hyperbaric/Hypobaric facilities located at Duke University School of Medicine.

MEASUREMENTS

Two each of nine models of commercially available hand-held peak flowmeters and a volume spirometer were tested at six simulated altitudes (100, 500, 1,000, 1,500, 2,000, and 3,000 m) using five target peak flows. Each peak flow was injected into each meter twice. Forward stepwise regression was used to check for nonlinear relationships between altitude and peak expiratory flowmeter readings. Linear regression equations were fit to the data at each target flow across altitude. Effect of absolute peak flow was tested by analysis of covariance.

RESULTS

For these altitudes, linear relationships were found between altitude and measured peak flow. For all meters tested, the average decrease in peak flow ranged from -8.7% at the lowest target flow (123 L/min) to -6.5% at the highest target flow (702 L/min) for each 100 mm Hg decrease in barometric pressure (PB). Individual meters ranged from -12.3% at the lowest target flow to -4.4% at the highest target flow for 100 mm Hg decrease in PB. The spirometer had no significant changes associated with changes in PB. In all cases, the magnitude of the altitude effect, measured by percent change, decreased with increasing peak flow.

CONCLUSIONS

Peak expiratory flowmeters underread PEF as a function of both increasing altitude and increasing target peak flow.

Ventilatory effects of pneumoperitoneum monitored with continuous spirometry

Inspiratory and expiratory tidal volume, peak and plateau airway pressure, compliance of the respiratory system, pressure-volume and flow-volume loops were monitored continuously and recorded in seven women undergoing laparoscopy with carbon dioxide insufflation to an intra-abdominal pressure of 1.6 kPa. All patients were anaesthetised using a total intravenous technique and a constant minute ventilation was maintained. Peak airway and plateau airway pressures increased by 50% and 81% respectively, whilst the compliance of the respiratory system decreased by 47% during the period of increased intra-abdominal pressure. Following release of the pneumoperitoneum, peak and plateau pressures remained elevated by 37% and 27% respectively, and the compliance was 86% of the pre-insufflation value. On-line monitoring of respiratory volumes, pressures and compliance may be helpful during general anaesthesia for laparoscopic procedures to avoid the potential harmful effects of increased airway pressures occurring with increased intra-abdominal pressure.

The effects of diuresis and transfusion on pulmonary function in children with thalassemia major

Previous pulmonary function studies in subjects with thalassemia major (TM) who were on regular transfusion programs have demonstrated results ranging from small airway obstruction to a restrictive pattern. Ten subjects with TM were studied pre- and postdiuresis, and again 24 hr after transfusion, in order to evaluate the role of possible fluid overload in altering pulmonary function. Subjects underwent spirometry and had lung volume and flow volume curves (MEFVC) measured in a volume displacement plethysmograph while breathing air and a mixture of 80% helium and 20% oxygen (HeO2). Six patients had pulmonary mechanics measured with esophageal balloons in place. Baseline function was normal and no change occurred following diuresis. Following transfusion, the volume of isoflow (VisoV) decreased, but other parameters did not change. Subsequent analysis revealed 5 subjects with an initial VisoV greater than 20 (% FVC) but, paradoxically, less evidence of flow limitation in the small airways than those with a VisoV less than 20 (% FVC). Posttransfusion, in those subjects with an initially high VisoV, the Vmax25(air) tended to fall without a change of MEFVC in HeO2, resulting in a decreased VisoV. This was interpreted as evidence of subtle abnormalities in the small airways caused by volume expansion, raising doubts about the value of the VisoV as a measure of small airway disease. As a group, our subjects did not demonstrate any abnormalities in baseline function. Some subjects had mild flow limitation in small airway while other developed comparable levels of flow limitation following the volume expansion associated with transfusion.

Peak expiratory flow at altitude

The mini Wright peak flow meter is a useful, portable instrument for field studies but being sensitive to air density will under-read at altitude. True peak expiratory flow will increase at altitude, however, because of the decreased air density, given that dynamic resistance is unchanged. The effect of simulated altitude on peak expiratory flow (PEF) was determined in six subjects with both the mini Wright meter and a volumetric spirometer (which is unaffected by air density). With increasing altitude PEF as measured by the spirometer increased linearly with decreasing pressure, so that at a barometric pressure of 380 mm Hg* (half an atmosphere, corresponding to an altitude of 5455 m) there was a 20% increase over sea level values. The mini Wright flow meter gave readings 6% below sea level values for this altitude--that is, under-reading by 26%. Measurements of PEF made at altitude with the mini Wright meter should be corrected by adding 6.6% per 100 mm Hg drop in barometric pressure.

Effects of an external resistance on maximum flow in chronic obstructive lung disease: implications for recognition of coincident upper airway obstruction

To determine how the presence of generalised airflow limitation due to chronic obstructive lung disease affects the recognition of simulated upper airway obstruction, a study was carried out in 12 patients (mean (SD) age 57 (7) years) with chronic obstructive lung disease (FEV1% predicted 53 (22), range 21-70) and 12 matched control subjects. Patients and control subjects performed maximal inspiratory and expiratory flow-volume curves in a variable volume plethysmograph with and without upper airway obstruction simulated at the mouth with a series of polythene washers of internal diameter 4, 6, 8, 10, and 12 mm. In patients, as in normal subjects, peak expiratory flow (PEF) and maximum inspiratory flow at 50% of vital capacity (Vmax50) were more sensitive to upper airway obstruction than were FEV1 or maximum expiratory flow at 50% VC (VEmax50); but the reductions in all indices caused by simulated upper airway obstruction were smaller in the patients than in the controls. The fall in PEF (whether expressed in absolute units or as a percentages) consequent on severe (4 mm) upper airway obstruction became smaller with increasing severity of chronic obstructive lung disease. The subjects also produced flow-volume curves with and without 6 mm upper airway obstruction while breathing helium and oxygen (heliox). In both groups the effects of heliox on PEF and Vmax50 were increased when upper airway obstruction was simulated. It was confirmed that the functional recognition of upper airway obstruction is more difficult in patients with chronic obstructive lung disease than in normal subjects and this difficulty increases with severity of disease; an unusually large increase in PEF or Vmax50 while the patient is breathing heliox should raise the suspicion of coexisting upper airway obstruction, but such a pattern is not specific.

Maximal expiratory flow in the guinea pig

Using the whole body plethysmograph, the maximal expiratory flow-volume (MEFV) curve was performed in anesthetized-paralyzed guinea pigs with intact chest (n = 7) and in anesthetized, chest-open animals following exsanguination (n = 13). The pressure-volume (PV) curve was also measured. Before and after the MEFV and PV maneuvers, lung volume was determined with a neon dilution method. Peak maximal expiratory flow (Vmax) of 178 +/- 7 ml/sec occurring at 83% TLC. After the peak flow, Vmax decreased gradually with reducing lung volume. The Vmax-static recoil pressure curve was relatively linear up to PL = 5 cmH2O. Density-dependence of Vmax (helium-Vmax was significantly higher than air-Vmax) was found at or above 60% TLC but not at lung volume below 60% TLC. For the chest-open postmortem guinea pig, Vmax and TLC decreased while trapped gas volume increased gradually with time after exsanguination, indicating that bronchoconstriction gradually became more severe. The magnitude of this postmortem airway spasm was related to age and anesthetic used.

Removal of observer variability from the determination of the volume of isoflow

The initial attractiveness of the volume of isoflow (VisoV) as an index of early small airways dysfunction has faded, perhaps because of VisoV's high variability, a significant part of which is contributed by the observer. We suggest here that the observer variability can be removed by filtering the flow data in the reciprocal volume domain, by using a modified foreign gas mixture in which some of the helium is replaced by neon, and by reading VisoV from a plot of density dependence vs. lung inflation. Support for these suggestions is drawn from both model simulations and experiments. Model simulations suggested that VisoV would be increased by the substitution of neon for some of the helium in the usual helium-oxygen (Helox) mixture. This was confirmed in tests on matched groups of normal and asymptomatic asthmatic children. The inclusion of 20% neon (Heneox20) was found to reduce the coefficient of variation in the mean value of VisoV in both groups of subjects. An interesting result of this test was that VisoV using Heneox20 was significantly higher in the asthmatic group than in the normal group, whereas VisoV using Helox was not. The results for density dependence (DD) between the 2 groups were not significantly different using Heneox20 but were using Helox. Plots of DD (derived from heavily filtered flow curves) vs. lung inflation showed an unambiguous value for VisoV.

Respiratory impedance in normal humans: effects of bronchodilatation and bronchoconstriction

Total respiratory resistance and reactance from 3 to 30 Hz were determined by the method of forced random noise oscillation in 12 normal male subjects before and after bronchodilatation and bronchoconstriction induced by deep breaths of aerosols of isoproterenol and atropine and of methacholine and histamine. Isoproterenol and atropine induced small decreases in total respiratory resistance at most frequencies, and isoproterenol decreased resonant frequency slightly (P less than 0.01). After administration of both methacholine and histamine, resonance frequency increased (P less than 0.01) and total respiratory resistance became more frequency-dependent, increasing mainly in the lower frequency range. In six of the subjects, we attempted to produce central deposition of methacholine by rapid, shallow breathing and peripheral deposition of the drug by slow, deep breathing. Only two subjects had suggestive evidence of central bronchoconstriction. No difference was noted, however, in the impedance behavior with either type of breathing. In awake humans, impedance analysis does not seem to distinguish between central and peripheral airway constriction.

The effect of low density gas breathing on vesicular lung sounds

Turbulent airflow (largely gas density dependent) in larger airways is believed by many lung sound researchers to be the mechanism responsible for the generation of vesicular lung sounds. To test the validity of this concept, we measured the amplitude of lung and tracheal sounds of 6 subjects alternately breathing air and a low density gas mixture (80% helium, 20% oxygen: He-O2). Lung sounds were recorded from 3 chest wall sites: Anterior right upper lobe (RUL), posterior and posterolateral right lower lobe (RLL), and a site over the proximal trachea below the larynx. The subjects rebreathed into an electronic spirometer filled with the test gas, and achieved a peak inspiratory and expiratory airflow of 2-2.5 L/sec. Lung sound amplitude was determined by an automated, flow-corrected measurement procedure. The mean decrease in sound amplitude when breathing He-O2 compared to air was: trachea, inspiration 44%; trachea, expiration 45%; RUL, inspiration 13%; RUL, expiration 25%; RLL, inspiration 15% (expiration at the RLL was too quiet to record). Cross-correlation and frequency analyses of the sounds recorded at the two RLL sites on both test gases revealed no consistent change in frequency or time relationships, indicating absence of effect of gas density on sound transmission between the sound generating airways and chest wall. These data suggest that the mechanism of production of the inspiratory vesicular lung sound is not simply turbulent airflow but some other relatively gas density independent mechanism. The tracheal and expiratory lung sounds do appear to be produced by a more density dependent turbulent mechanism.

Influence of airway geometry on expiratory flow limitation and density dependence

In a study of individuals of the same sex and similar age and size, between subject variability was noted in lung recoil, lung size, density dependence of maximum expiratory flow, tracheal cross-sectional area, and anatomic dead space. No significance was observed in the relationships to one another of lung size or either estimate of airway size, nor did the magnitude of the change in flow with changes in gas density show a consistent relationship to any of the size variables. The results indicate that there is a dissociation between tracheal dimensions and anatomic dead space as well as between airway size and lung size, suggesting that dysanapsis is a general phenomenon. This introduces additional sources of interindividual variability of maximum expiratory flow and measurements of density dependence of flow.

Density dependence of maximal expiratory flows in normal subjects: influence of sex, age, body build and spirographic variables

We studied the correlation between density-dependence parameters (delta He, volume of isoflow) and age, body build, spirography and forced expiratory fows in 69 healthy non-smokers (16 females) aged 18 to 51 years. Density dependence variables showed no significant differences between young (less than 35 years) males and females. Volume of isoflow per cent of forced vital capacity (Viso V % FVC) was poorly significantly related to age (r = +0.25 P less than 0.05) and weight (r = +0.27 P less than 0.05). Significant correlations were found between density dependence at low lung volume and the FEV1/VC ratio (r = 0.31, P = 0.01 for delta He 25%; r = -0.40, P less than 0.001 for Viso V% FVC) or the forced expiratory flows with air (Viso V% FVC vs. FEF 25% r = -0.42, P less than 0.001). These correlations were closer in females as compared to males. Volume of isoflow was well related to delta He 25% (r = -0.64, P less than 0.001), but the relationship with delta He 50% was weaker (r = -0.38; P less than 0.01).

Severe upper airway obstruction caused by bullous pemphigoid: diagnostic usefulness of the flow-volume curve

The site and severity of upper airway obstruction accurately determined by analysis of the flow-volume curve obtained from a dyspneic patient with bullous pemphigoid. The limitation of maximum inspiratory flow to 0.5 L/s and of maximum expiratory flow to 0.7 L/s over most of the vital capacity suggested that the lumen of the extrathoracic trachea was narrowed to a diameter of 3 mm. The marked improvement in flow with the patient breathing a helium-oxygen mixture further confirmed that flow was limited in a large central airway. The predictions made from analysis of the flow-volume curve were confirmed by fiberoptic bronchoscopic examination and by computerized axial tomography, which revealed severe supraglottic obstruction. After a tracheostomy was performed, maximal inspiratory and expiratory flows were normal.

Predominant site of airway resistance in chronic obstructive pulmonary disease

The purpose of the present clinical research was to establish the major site of resistance and obstruction in patients with severe, symptomatic chronic obstructive pulmonary disease. We used a noninvasive technique by measuring airflow in a plethysmograph after patients breathed air and after breathing a mixture of 80 percent helium and 20 percent oxygen. In group 1 were 14 patients (mean age, 63 +/- 7 years [+/- 1 SD]) with bronchitis. The ratio of their forced expiratory volume in one second over the vital capacity (FEV1/VC) was 43 +/- 10 percent (mean +/- 1 SD), and their single-breath carbon monoxide diffusing capacity (Dsb) was normal (125 +/- 41 percent of the predicted value). In group 2 were 13 patients with emphysema (mean age, 60 +/- 10 years), whose FEV1/VC was 33 +/- 11 percent and Dsb was 55 +/- 14 percent of predicted. All patients had abnormally low increases in the maximum expiratory flow at midvital capacity after breathing the helium-oxygen mixture. The range in group 1 was -11 percent to +14 percent and in group 2 was -10 percent to +15 percent. Normal subjects in our laboratory show increases of at least +24 percent. This indicates that the limitation of airflow occurs primarily in the small (less than 2 mm in internal diameter) and not the large airways.

Work performance breathing normoxic nitrogen or helium gas mixtures

Twelve subjects completed a progressive treadmill test to maximal aerobic capacity while breathing air or a 79% helium--21% oxygen gas mixture (HeO2). Metabolic and thermoregulatory responses to work while breathing the two mixtures were compared at rest, 30-40%, 60-70%, and 85-90% of maximal performance, and at maximal effort. Ventilation, ventilatory equivalent, and respiratory rates were increased and oxygen uptakes decreased by breathing HeO2 when the level of work exceeded 85-90% of maximum. Heat loss through the respiratory tract was greater breathing HeO2. The reduction in maximal oxygen uptake is probably due to a reduction in the oxygen cost of breathing a less dense gas. It was not related to a lower body temperature and probably not to O2 transport or circulatory limitation. HeO2 breathing had no effect on maximal mechanical work capacity.

Density dependence of expiratory flow and bronchodilator response in asthma

Change in the magnitude of density dependence of the maximal expiratory flow (D/MEF) following inhalation of isoproterenol was used as a test for predicting the long term response to isoproterenol vs atropine in 24 adult patients with longstanding asthma. Eleven subjects showed a decrease in D/MEF manifested by increase in volume of isoflow (VisoV) and/or decrease in Vmax50 Helox/air following isoproterenol inhalation (group 1). Thirteen subjects manifested an opposite response (group 2). Atropine sulphate (0.08 mg/kg) and isoproterenol hydrochloride (2.5 mg) were then administered by inhalation, each four times a day for seven days in a randomized double-blind cross over fashion to all subjects. One of group 1 but ten of group 2 subjects had a greater subjective and objective improvement with atropine than with isoproterenol (P less than .005). An increase in D/MEF following isoproterenol can be used as a test to predict a better response to atropine than to isoproterenol over a one week period. Such a response occurs in almost half of the adult chronic asthmatic patients. The results are consistent with a preferential dilatation of the large airways by atropine.

Sites of airway dilatation in asthma following inhaled versus subcutaneous terbutaline. Comparison of physiologic tests with radionuclide lung images

In 12 asthmatic subjects with mild to severe airways obstruction, we compared the relative magnitude and sites of airway dilatation of a beta-adrenergic stimulant administered by different routes. Specific airway conductance (SGaw), peak expiratory flow, the sum of the absolute volume of isoflow and residual volume as a percentage of total lung capacity (capacity of isoflow) and the ratio of maximal expiratory flow at 50 per cent of vital capacity breathing 80 per cent helium-20 per cent oxygen to that breathing air (ratio Vmax50) were determined before and after the administration of aerosolized terbutaline (0.5 mg), subcutaneous terbutaline (0.5 mg) or placebo. Increases in SGaw and peak expiratory flow of greater than or equal to 25 per cent were considered to indicate significant dilatation of central airways; increases in ratio Vmax50 of greater than or equal to 0.10 and decreases in capacity of isoflow of greater than or equal to 10 per cent were assumed to reflect dilatation of peripheral airways. In addition, radioaerosol and radioxenon lung imaging was performed to determine the relationship between changes in lung imaging patterns and changes in physiologic indices in response to bronchodilator therapy. Placebo caused little change in lung function or lung imaging. After inhaled terbutaline, SGaw and peak expiratory flow increased greater than or equal to 25 per cent in seven subjects, ratio Vmax50 increased greater than or equal to 0.10 in only three subjects, capacity of isoflow decreased greater than or equal to 10 per cent in only one subject, radioaerosol images showed less central deposition in nine subjects and radioxenon images showed improved distribution and/or washout of xenon in five subjects. After the administration of subcutaneous terbutaline, SGaw and peak expiratory flow increased greater than or equal to 25 per cent in 10 subjects, ratio Vmax50 increased greater than or equal to 0.10 in 10 subjects, capacity of isoflow decreased greater than or equal to 10 per cent in 11 subjects, and radioaerosol and xenon images showed improvement in 11 and eight subjects, respectively. These findings are consistent with the action of inhaled terbutaline mainly on large airways and of subcutaneous terbutaline on both large and small airways. Although reduced central radioaerosol deposition correlated well with physiologic evidence of large airway dilatation, improvement in xenon distribution and washout could be attributed to dilatation of either large and/or small airways.

Constancy of effort and variability of maximal expiratory flow rates

In 14 normal subjects and in 13 patients with obstructive pulmonary diseases, we studied the variability within an individual of values for the maximal expiratory flow rate (Vmax) recorded simultaneously vs expired pulmonary volume (at the mouth) and vs thoracic volume (measured with a body plethysmograph). We found that the variance of Vmax within an individual at 25, 50, and 75 percent of the expired vital capacity did not differ statistically whether pulmonary volume was the expired or the thoracic gas volume. In ten healthy subjects on two occasions (at an interval of 12 days, on the average), we measured the peak expiratory flow rate and Vmax at different levels of inflation, with respect to either expired or thoracic volume. There was no statistical differences in Vmax between the first and the last day. A larger variability of Vmax measured vs expired volume implies a change in the expiratory effort from one forced expiration to another and a different degree of compression of intrathoracic air. Since this was not the case, we conclude that muscular effort during repeated forced expirations is similar. The good reproducibility of effort explains in great measure the good reproducibility of Vmax.

The compliance curve for the flow limiting segments of the airway. II. Experiments with human subjects

Maximum effort flow-static recoil curves were obtained in 5 healthy subjects breathing air, He/O2, and SF6/O2 mixtures. In 4 of them maximum effort flows corresponded to really maximal flows and their curves were transformed into compliance curves for the flow limiting segments of the airway and analyzed from the point of view of a previously presented lung model (Pedersen and Nielsen 1976). The results showed, that viscosity dependent pressure losses from the alveoli to the flow limiting segments were minimal for air and SF6/O2, but not for He/O2. When viscosity dependent pressure lossess could be negleted, then expiration of gases of different densities gave almost identical compliance curves for the flow limiting segments. This supported the applicability of the model. The calculated compliance curves for the flow limiting segments were compared with data from the literature, and the findings indicated that flow limitation during expirations with just maximal flows throughout began in the extrapulmonary airways and moved upstream during the expiration.

Relative contributions of large and small airways to flow limitation in normal subjects before and after atropine and isoproterenol

Bronchodilatation was produced in normal subjects by the inhalation of atropine, a parasympatholytic agent, and isoproterenol, a beta adrenergic stimulator. Density dependence of maximal expiratory flow (Vmax), expressed as a ratio of Vmax with an 80% helium-20% oxygen gas mixture to Vmax with air at isolung volumes, indicated that the predominant flow regimes across upstream airways changed differently after each agent was given separately. After atropine Vmax increased, elastic recoil pressure did not change, and density dependence decreased. Utilizing the equal pressure points analysis which defines upstream and downstream segments of the intrathoracic airways at flow limitation, these results suggest a greater relative dilatation of the larger upstream airways such that more of the driving pressure is dissipated across the smaller airways in which flow is less dependent upon gas density. After isoproterenol Vmax increased, elastic recoil pressure did not change, and density dependence increased. This suggests a preferential dilatation of the smaller and more peripheral airways with less density-dependent flow regimes such that more of the driving pressure would be dissipated in the larger airways in which flow is more dependent upon gas density. Systematic decreases after isoproterenol lead independently to the same conclusion. After both agents together, Vmax increased and density dependence and critical alveolar pressures did not change from control, suggesting a relatively uniform dilatation of all the airways comprising the upstream segment.

Small airways dysfunction in influenza A virus infection: therapeutic role and potential mode of action of amantadine

Previous studies have demonstrated alterations in gas exchange and pulmonary mechanics in presumed and documented viral upper respiratory infections. In our laboratory, using sensitive and noninvasive testing, we have demonstrated significant and prolonged alterations of pulmonary mechanics in uncomplicated natural influenza A infection, which implicate increased resistance in small airways as the cause of dysfunction. Amantadine administration in established influenza A infection has been associated with an accelerated improvement of clinical illness and pulmonary physiologic dysfunction. Preliminary study suggests that this is due to neither cholinergic blockade nor bronchodilation.

A method to correct for the influence of gas density on maximal expiratory flow rate

Maximal expiratory flow rate (Vmax) was measured at 20, 35, 50, 65, and 80% vital capacity in 4 young healthy subjects breathing air, SF6/O2, and He/O2 mixtures. The flows of SF6/O2 and He/O2 were corrected to normal alveolar gasflow by means of only the density of the gases. The values for normal alveolar gasflow and corrected SF6/O2 flow were identical at 35% VC and larger volumes while the values for normal alveolar gasflow and corrected He/O2 flow were not. The results indicate that in young healthy subjects it is possible to correct Vmax at lung volumes above 35% VC for the changes induced by an increase in density of the gas breathed, provided viscosity is not much changed. Without correction, Vmax after O2-breathing will be underestimated by about 6%, compared with Vmax for normal alveolar gas, whereas a change in alveolar CO2 concentrations between 3 and 9% only causes a 1% decrease of Vmax.

Severe airway disease due to inhalation of fumes from cleansing agents

A subject had severe airway obstruction and evidence of small-airway disease resulting from inhalation of "fumes" liberated during the course of mixing several drain-cleansing agents. The evidence in favor of the airways as the predominant site of damage was severe reduction of maximal expiratory flow in the presence of normal lung-recoil pressure in a nonsmoker who had no other known industrial exposures or predisposing respiratory conditions.

Reversibility of pulmonary function abnormalities in smokers. A prospective study of early diagnostic tests of small airways disease

A prospective study of the reversibility of pulmonary function abnormalities in cigarette smokers was performed. Base line studies of 50 otherwise healthy cigarett smokers included lung volumes, static volume-pressure curves, closing volume, slope of phase III and maximum expiratory flow-volume curves while the subjects breathed air and while they breathed an 80 per cent helium-20 per cent oxygen mixture (He). Ten subjects, seven women and three men, ages 29 to 61, were restudied 6 to 14 weeks after they had stopped smoking. Two of these people had an abnormal closing volume, four had an abnormal slope of phase III and five had an abnormal response to helium. Static volume-pressure curves, slope of phase III and airflow rates were not improved on the repeat studies. There was increases in maximum expiratory flow rates with He at 50 and 25 per cent of vital capacity, reductions in the lung volume at which the maximum expiratory flow rate of air and He became identical, and decreases in closing volume. These reversible functional changes in smokers are thought to indicate improvement of peripheral airways obstruction.