High-resolution computed tomographic evaluation of airway distensibility and the effects of lung inflation on airway caliber in healthy subjects and individuals with asthma

The effects of a deep inspiration (DI) in individuals with asthma differ from those observed in healthy subjects. It has been postulated that the beneficial effect of lung inflation is mediated by airway stretch. One hypothesis to explain the defects in the function of lung inflation in asthma is that a DI may be unable to stretch the airways. This may result from attenuation of the tethering forces between the airways and the surrounding parenchyma. In the current study, we used high-resolution computed tomography (HRCT) to examine the ability of a DI to distend the airways of subjects with asthma (n = 10) compared with healthy subjects (n = 9) at baseline and after increasing airway tone with methacholine (MCh). We found that both at baseline and after the induction of smooth muscle tone with MCh, a DI distended the airways of healthy and asthmatic subjects to a similar extent, indicating that abnormal interdependence between the lung parenchyma and the airways is unlikely to play a major role in the loss or attenuation of the beneficial effect of lung inflation that characterizes asthma. Furthermore, we observed that after constriction had already been induced by MCh, following a DI, bronchodilation occurred in the healthy subjects but further bronchoconstriction occurred in the subjects with asthma. Our findings suggest that an abnormal excitation contraction mechanism in the airway smooth muscle of subjects with mild asthma counteracts the bronchodilatory effect of a DI. Therefore, the mechanism for reduced bronchodilation after DIs in subjects with mild asthma could be intrinsic to the airway smooth muscle.

The lack of the bronchoprotective and not the bronchodilatory ability of deep inspiration is associated with airway hyperresponsiveness

In healthy subjects, deep inspiration (DI) acts both as a bronchodilator and a bronchoprotector. The latter is impaired in asthmatics. We have now evaluated whether the lack of bronchoprotection is related to bronchial hyperresponsiveness (BHR), and whether the bronchodilatory effect is also lost in asthmatics. Ten healthy subjects (PC20 > 75 mg/ml), 12 asthmatics with moderate to severe BHR (PC20 < 1 mg/ml), 14 asthmatics with mild to borderline BHR (1 < PC20 < 25 mg/ml), and 10 rhinitics with mild to borderline BHR (1 < PC20 < 25 mg/ml) underwent single-dose methacholine provocations inducing at least 20% reduction in FEV1 after 20 min of DI inhibition. To measure the bronchodilatory effect, DIs were taken immediately after the postmethacholine spirometry, and lung function was again tested. To measure the bronchoprotective effect, DIs were taken before the administration of spasmogen. All four groups achieved the same reductions in FEV1 and FVC, in the absence of deep breaths (analysis of variance [ANOVA], p = 0.49). Only healthy subjects showed bronchoprotection (percent bronchoprotection, mean +/- SEM; healthy: 79 +/- 4.0; asthmatics with moderate to severe BHR: 12 +/- 14.5; asthmatics with mild to borderline BHR: -7 +/- 19.7; rhinitics with mild to borderline BHR: 2 +/- 14.0). In contrast, DIs were able to partially reverse bronchial obstruction in all four groups, albeit percent bronchodilation in healthy subjects was somewhat stronger. The dissociation between bronchoprotection and bronchodilation suggests that the two effects involve different mechanisms.

Effect of changing vascular volume on measurement of protein reflection coefficient in ischemic lungs

In ischemic organs, the protein reflection coefficient (sigma) can be estimated by measuring blood hematocrit (Hct) and protein after increasing static vascular pressure (P(v)). Our original equation for sigma (J Appl Physiol 73: 2616-2622, 1992) assumed a constant vascular volume during convective fluid flux (). In this study, we 1) quantified the rate of vascular volume change (dV/dt) still present in ischemic single ferret lungs after 20 min of P(v) = 30 Torr and 2) developed an equation for sigma that allowed a finite dV/dt. In 25 lungs, we estimated the dV/dt after 20 min at P(v) = 30 Torr by subtracting from the rate of lung weight gain (W(L)). The relationship between (0.15 +/- 0.02 ml/min) and W(L) (0.24 +/- 0.02 g/min) was significant (R = 0.66, P < 0.001), but the slope was <1 (0.41 +/- 0.10, P < 0.05). dV/dt (0.10 +/- 0.02 ml/min) was similar in magnitude to at 20 min. The modified equation for sigma revealed that a finite dV/dt caused the original sigma measurement to underestimate true sigma. A low sigma, high, high baseline Hct, and long filtration time enhanced the error. The error was small, however, and could be minimized by adjusting experimental parameters.

Airways hyperresponsiveness and the effects of lung inflation

Lung inflation has a beneficial effect on the airways of healthy subjects. It acts as a bronchoprotector, that is to prevent bronchoconstriction, and as a bronchodilator, in that it reverses bronchial obstruction. The bronchoprotective effect of deep inspiration is more potent than the bronchodilatory one, and the two phenomena appear to advocate different mechanisms. Asthmatics and rhinitics with airways hyperresponsiveness show an impairment in bronchoprotection induced by deep breaths, whereas the bronchodilatory effect, although reduced, is still effective. The lack of the bronchoprotective effect of deep inspiration may contribute to the development of airways hyperresponsiveness. The mechanisms through which lung inflation exerts its beneficial role in healthy subjects, and the factors impairing such an effect in those with airways hyperresponsiveness, are currently under investigation.

Deep inspiration-induced bronchoprotection is stronger than bronchodilation

We have previously shown that in healthy subjects, deep inspiration (DI) has not only a bronchodilatory but also a bronchoprotective effect that is absent in asthmatic subjects. We conducted the study reported here to test the hypothesis that the bronchoprotective effect is stronger than the bronchodilatory effect, and to determine the extent to which these two effects are related. Ten healthy subjects underwent provocations in which single doses of methacholine, previously shown to reduce FEV(1) by 10% to 20% (Dose 1) and by 20% to 40% (Dose 2) were administered after a 20-min period devoid of DI. To measure the bronchodilator effect, DIs were performed immediately after the first spirometry after methacholine, and were followed by another lung function test. To measure their bronchoprotective effect, DIs were performed before administration of a single dose of methacholine, and the FEV(1) after methacholine was compared with that of another single-dose challenge in which DIs were not included. From these outcomes, bronchodilation and bronchoprotection indices were constructed and compared with each other. At Dose 1 (mild obstruction), the ability of DIs to reverse methacholine-induced obstruction was equal to their ability to prevent it (bronchodilation index [BDI] versus bronchoprotection index [BPI]: 1.62 +/- 0.21 versus 2.02 +/- 0.40 [mean +/- SEM], p = 0.26). At Dose 2, the relative potency of both the bronchodilating and bronchoprotective effects of DIs increased, but bronchoprotection was significantly stronger (BDI versus BPI: 3.40 +/- 0.43 versus 6.98 +/- 1.42, p = 0.02). Correlation analysis of the two indices indicated that as the BPI increased, the BDI reached a plateau. We conclude that in healthy humans, the bronchoprotective effect of lung inflation is stronger than the bronchodilatory effect.

Potent bronchoprotective effect of deep inspiration and its absence in asthma

In the absence of deep inspirations, healthy individuals develop bronchoconstriction with methacholine inhalation. One hypothesis is that deep inspiration results in bronchodilation. In this study, we tested an alternative hypothesis, that deep inspiration acts as a bronchoprotector. Single-dose methacholine bronchoprovocations were performed after 20 min of deep breath inhibition, in nine healthy subjects and in eight asthmatics, to establish the dose that reduces forced expiratory volume in 1 s by >15%. The provocation was repeated with two and five deep inspirations preceding methacholine. Additional studies were carried out to assess optimization and reproducibility of the protocol and to rule out the possibility that bronchoprotection may result from changes in airway geometry or from differential spasmogen deposition. In healthy subjects, five deep inspirations conferred 85% bronchoprotection. The bronchoprotective effect was reproducible and was not attributable to increased airway caliber or to differential deposition of methacholine. Deep inspirations did not protect the bronchi of asthmatics. We demonstrated that bronchoprotection is a potent physiologic function of lung inflation and established its absence, even in mild asthma. This observation deepens our understanding of airway dysfunction in asthma.

Airway nitric oxide diffusion in asthma: Role in pulmonary function and bronchial responsiveness

If the nitric oxide (NO) diffusing capacity of the airways (DNO) is the quantity of NO diffusing per unit time into exhaled gas (q) divided by the difference between the concentration of NO in the airway wall (Cw) and lumen, then DNO and C(w) can be estimated from the relationship between exhaled NO concentration and expiratory flow. In 10 normal subjects and 25 asthmatic patients before and after treatment with inhaled beclomethasone, DNO averaged 6.8 +/- 1.2, 25.5 +/- 3.8, and 22.3 +/- 2.7 nl/s/ppb x 10(-3), respectively; C(w) averaged 149 +/- 31.9, 255.3 +/- 46.4, and 108.3 +/- 14.3 ppb, respectively; and DNOC(w) (the maximal from diffusion) averaged 1,020 +/- 157.5, 6,512 +/- 866, and 2,416 +/- 208.5 nl/s x 10(-3), respectively. DNO and DNOC(w) in the asthmatic subjects before and after steroids were greater than in normal subjects (p < 0.0001), but C(w) was not different. Within asthmatic subjects, steroids caused C(w) and DNOC(w) to fall (p < 0.0001), but DNO was unchanged. DNOC(w) after steroids, presumably reflecting maximal diffusion of constitutive NO, was positively correlated with methacholine PC(20) and FEV(1)/FVC before or after steroids. The increased DNO measured in asthmatic patients may reflect upregulation of nonadrenergic, noncholinergic, NO-producing nerves in airways in compensation for decreased sensitivity of airway smooth muscle to the relaxant effects of endogenous NO.

Airway narrowing in healthy humans inhaling methacholine without deep inspirations demonstrated by HRCT

Normal subjects prevented from taking a deep breath show changes in airflow similar to those of asthmatics when challenged with methacholine (MCh). To confirm airway narrowing by MCh in this setting and to determine its location, we concurrently measured changes in airway lumenal area using high resolution computed tomography (HRCT) and airflow using partial spirometry in five normal subjects challenged with increasing doses of MCh under prohibition of deep breaths. In an attempt to improve imaging accuracy, we corrected for the changes in lung volume during bronchoprovocation. At every step of the provocation, scanning was performed at approximately the same lung volume. On the HRCT images, airway area decreased in response to the increasing doses of MCh to 91 +/- 2%, 88 +/- 2%, and 80 +/- 2% of baseline at the doses of MCh 0.25, 0.75, and 2.5 mg/ml, respectively (p < 0.001). Airway narrowing showed no predilection for particular airway sizes and occurred in a heterogeneous pattern. The changes in the mean airway lumenal area as measured by HRCT and the mean partial spirometric outcomes were highly correlated: FEV(1)p (r(2) = 0.46, p = 0.001), FVCp (r(2) = 0.20, p = 0.05), FEV(1)/FVCp (r(2) = 0.55, p = 0.002), MMEFp (r(2) = 0.31, p = 0.01), and taup (r(2) = 0.51, p = 0.0004). We conclude that in normal subjects who are prevented from taking a deep breath, the spirometric changes occurring with aerosol MCh challenge are associated with conducting airway narrowing.

Effect of ventilation on vascular permeability and cyclic nucleotide concentrations in ischemic sheep lungs

Ventilation during ischemia attenuates ischemia-reperfusion lung injury, but the mechanism is unknown. Increasing tissue cyclic nucleotide levels has been shown to attenuate lung ischemia-reperfusion injury. We hypothesized that ventilation prevented increased pulmonary vascular permeability during ischemia by increasing lung cyclic nucleotide concentrations. To test this hypothesis, we measured vascular permeability and cGMP and cAMP concentrations in ischemic (75 min) sheep lungs that were ventilated (12 ml/kg tidal volume) or statically inflated with the same positive end-expiratory pressure (5 Torr). The reflection coefficient for albumin (sigmaalb) was 0.54 +/- 0.07 and 0.74 +/- 0. 02 (SE) in nonventilated and ventilated lungs, respectively (n = 5, P < 0.05). Filtration coefficients and capillary blood gas tensions were not different. The effect of ventilation was not mediated by cyclic compression of alveolar capillaries, because negative-pressure ventilation (n = 4) also was protective (sigmaalb = 0.78 +/- 0.09). The final cGMP concentration was less in nonventilated than in ventilated lungs (0.02 +/- 0.02 and 0.49 +/- 0. 18 nmol/g blood-free dry wt, respectively, n = 5, P < 0.05). cAMP concentrations were not different between groups or over time. Sodium nitroprusside increased cGMP (1.97 +/- 0.35 nmol/g blood-free dry wt) and sigmaalb (0.81 +/- 0.09) in nonventilated lungs (n = 5, P < 0.05). Isoproterenol increased cAMP in nonventilated lungs (n = 4, P < 0.05) but had no effect on sigmaalb. The nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester had no effect on lung cGMP (n = 9) or sigmaalb (n = 16) in ventilated lungs but did increase pulmonary vascular resistance threefold (P < 0.05) in perfused sheep lungs (n = 3). These results suggest that ventilation during ischemia prevented an increase in pulmonary vascular protein permeability, possibly through maintenance of lung cGMP by a nitric oxide-independent mechanism.

Rapid reversibility of the allergen-induced pulmonary late-phase reaction by an intravenous beta2-agonist

This study was performed to determine the degree to which beta2-adrenergic receptor agonists can reverse the allergen-induced late reduction in lung function. On two occasions, seven asthmatic subjects were administered terbutaline or its vehicle by intravenous infusion 7 h after inhaled allergen, at which point the forced expiratory volume in 1 s was 57% of baseline. On another occasion, terbutaline was infused at baseline to determine maximal attainable bronchodilation. After allergen challenge, terbutaline rapidly improved lung function. At the end of terbutaline infusion, the forced expiratory volume in 1 s reached 100 +/- 1.3% of baseline and 84.2 +/- 4.3% of maximal attainable value, but the bronchodilating effect of the beta-agonist did not plateau. The values for forced vital capacity were 102 +/- 1.3% of baseline and 95.1 +/- 3% of maximal attainable value. The kinetics of the terbutaline effect, when it was infused at baseline, were similar to those in the late phase. Because the late-phase reduction in lung function is rapidly reversible by beta2-adrenergic agonists, we conclude that it is caused mainly by bronchial smooth muscle spasm.

Lung volume reduction surgery and airflow limitation

Interest has recently been renewed in lung volume reduction surgery (LVRS) for end-stage emphysema. However, numerous questions about its role in the treatment of emphysema remain, including the clinical characteristics of optimal candidates and its mechanism of improvement in pulmonary function. In this report, we develop a mathematical analysis and graphic depiction of the mechanism of improvement in expiratory airflow and vital capacity. This analysis is based on consideration of the interaction between lung function and respiratory muscle function. We also reexamine previously published pulmonary mechanics in patients with alpha1-antitrypsin deficiency, chronic obstructive pulmonary disease, and asthma. We find a major determinant of airflow limitation common to these diseases is the ratio of residual volume to total lung capacity (RV/TLC). Moreover, RV/TLC is found to be the single most important determinant of the improvement in pulmonary function after LVRS. Regardless of the specific underlying lung disease, the impairment of airflow is due primarily to mismatch between the sizes of the lung and the chest wall, and the effects of LVRS are almost exclusively due to improvement of that match. This analysis can be used to develop testable hypotheses to guide patient selection for this procedure.

Direct assessment of small airways reactivity in human subjects

Our knowledge of airways reactivity to inflammatory agonists is derived predominantly from tests dominated by large airway responsiveness. To determine directly, the histamine responsiveness of the smallest airways, eight normal and 11 asymptomatic asthmatic subjects were studied utilizing a wedged bronchoscope technique. A fiberoptic bronchoscope was wedged in the anterior segment of the right upper lobe and a double-lumen catheter was advanced through the working channel to its tip. With a constant flow of gas (5% CO2 in air) through one lumen of the catheter, pressure at the tip of the bronchoscope was measured with the subject breath-holding at FRC. Peripheral airways resistance (Rp) was measured at baseline and after saline, histamine (10, 50, 100 mg/ml) and isoproterenol (2 mg/ml) challenge through the bronchoscope. Baseline Rp of asthmatics (0.041 +/- 0.015 cm H2O/ml/min; mean +/- SE) was significantly greater than normal subjects (0.011 +/- 0.003 cm H2O/ml/min; p = 0.019). The log of the concentration of histamine that caused a 100% increase in peripheral airways response was greater in the normal subjects than in the asthmatic subjects (p = 0.0114) and correlated with whole lung responsiveness to histamine in asthmatics (r = 0.847, p < 0.05). Isoproterenol reversed completely the increase in Rp in normal subjects but not asthmatic subjects. The results of this study demonstrate that the resistance of the smallest peripheral airways, when measured directly, increased when challenged locally with histamine in both normal subjects and asthmatic subjects. However, the peripheral airways responsiveness was significantly enhanced in asthmatic subjects relative to normal controls.

Assessment of protein permeability in normal human systemic circulation

Vascular permeability to oncotic agents is an important determinant of transvascular fluid flux (J) and systemic fluid balance. In this study, a technique was developed to measure protein reflection coefficients (omega) for albumin (Alb), immunoglobulin (Ig) G, and IgM in the intact human systemic circulation to evaluate the role of vascular protein permeability in health and disease. A mathematical model was developed to calculate omega in the forearm circulation from changes in venous hematocrit and protein concentration that occur during edema formation. Assumptions required for the model were validated in an initial set of experiments in normal subjects when edema was induced by inflating a pneumatic cuff on the upper arm. A second series of experiments assessed omega for Alb, IgG, and IgM in men (n = 7) and in women in the follicular (n = 5) and luteal (n = 4) phases of the menstrual cycle. There was an increasing trend in omega with molecular size in aggregated subjects [omega Alb = 0.81 +/- 0.12 (SE), omega IgG = 0.88 +/- 0.12, omega IgM = 0.92 +/- 0.18; P = 0.088]. These values were consistent with those obtained with in vitro preparations. omega values were lower in women in the luteal than in the follicular phase (P = 0.047). We conclude that the assumptions required for this model can be achieved in the intact forearm circulation and that there are menstrual phase-related differences in vascular protein permeability in normal women.

Structural basis for alterations in upper airway collapsibility

To determine the structural basis for alterations in upper airway (UA) collapsibility, the pharyngeal critical pressure (Pcrit) was measured in an isolated feline upper airway preparation. The effect of airway elongation and dilation was explored by displacing the trachea caudally and the tongue anteriorly, respectively. With caudal-tracheal displacement, Pcrit fell progressively, a result that can be attributed to increased tension within the pharyngeal mucosa. In contrast, anterior-tongue displacement decreased Pcrit when the trachea had been caudally displaced but not with the trachea in the neutral position. These findings suggest that longitudinal tension within the airway mucosa modulates both Pcrit and the response in Pcrit to dilating forces. A mechanical model to account for these findings is discussed.

Effect of tracheal and tongue displacement on upper airway airflow dynamics

We have previously shown that caudal tracheal displacement alters the airflow dynamics of the upper airway. In the present study, we specifically examined the effects of tongue and tracheal displacement on upper airway airflow dynamics. To determine how tongue and tracheal displacement modulate maximal inspiratory airflow (VImax), we analyzed the pressure-flow relationships obtained in the isolated upper airway of paralyzed cats. VImax and its determinants, the pharyngeal critical pressure (Pcrit) and the nasal resistance (Rn) upstream to the flow-limiting site, were measured as tongue displacement and tracheal displacement were systematically varied. Four results were obtained: 1) there was no independent effect of tongue displacement on VImax, Pcrit, or Rn; 2) there was an increase in VImax with 2 cm of tracheal displacement, which was associated with a decrease in Pcrit and an increase in Rn; 3) there was an interactive effect of tongue and tracheal displacement on VImax and Pcrit but not on Rn; and 4) there was a large increase in VImax with tongue displacement > 2.5 cm with the trachea nondisplaced, which was associated with a large decrease in Pcrit and a large increase in Rn. We conclude that tongue and tracheal displacement exert differing influences on airflow dynamics and present a mechanical model of the upper airway that explains these results.

Airway hyperresponsiveness in asthma: a problem of limited smooth muscle relaxation with inspiration

We hypothesized that hyperresponsiveness in asthma is caused by an impairment in the ability of inspiration to stretch airway smooth muscle. If the hypothesis was correct, we reasoned that the sensitivity to inhaled methacholine in normal and asthmatic subjects should be the same if the challenge was carried out under conditions where deep inspirations were prohibited. 10 asthmatic and 10 normal subjects received increasing concentrations of inhaled methacholine under conditions where forced expirations from a normal end-tidal inspiration were performed. When no deep inspirations were allowed, the response to methacholine was similar in the normal and asthmatic subjects, compatible with the hypothesis we propose. Completely contrary to our expectations, however, was the marked responsivity to methacholine that remained in the normal subjects after deep breaths were initiated. 6 of the 10 normal subjects had > 20% reduction in forced expiratory volume in one second (FEV 1) at doses of methacholine < 8 mg/ml, whereas there was < 15% reduction with 75 mg/ml during routine challenge. The ability of normal subjects to develop asthmatic responses when the modulating effects of increases in lung volume was voluntarily suppressed suggests that an intrinsic impairment of the ability of inspiration to stretch airway smooth muscle is a major feature of asthma.

CPAP reduces inspiratory work more than dyspnea during hyperinflation with intrinsic PEEP

Hyperinflation with intrinsic positive end-expiratory pressure (PEEPi) loads the respiratory muscles and causes dyspnea in obstructive lung disease. Continuous positive airway pressure (CPAP) has shown some efficacy in reducing inspiratory work and dyspnea. However, in obstructive lung disease, inspiratory work and dyspnea may be increased by additional factors that may not be affected by CPAP. Therefore, to study the effects of hyperinflation with intrinsic PEEP and CPAP in isolation, we used a mechanical analog of airway closure to increase end-expiratory lung volume in normal subjects. In five subjects in whom inspiratory work was measured, increasing end-expiratory lung volume by 1 and 2 L increased inspiratory work per breath from 0.42 +/- 0.04 J to 1.17 +/- 0.15 J (p < 0.05 compared with baseline) and 1.58 +/- 0.22 J (p < 0.05 compared with baseline and to the lesser level of hyperinflation). Although CPAP reduced work per breath and per minute to levels not significantly different from baseline, it had little effect on dyspnea. In ten subjects hyperinflated to 2.4 +/- 0.12 L above FRC, breathing could be sustained 19.5 +/- 4.5 min before quitting the load. This was increased to 26.7 +/- 5.2 min by 10 cm H2O CPAP (p = 0.052). Inspiratory dyspnea was modestly reduced by CPAP during these endurance trials. We conclude that CPAP can substantially ameliorate the respiratory work load induced by hyperinflation with intrinsic PEEP. However, the effects of CPAP on dyspnea and endurance are more limited. This suggests that the limits to breathing at high lung volumes are related to factors in addition to respiratory muscle work, and that CPAP may be of more value in reducing the work than in relieving the distress of obstructive lung disease.

CPAP reduces inspiratory work more than dyspnea during hyperinflation with intrinsic PEEP

Hyperinflation with intrinsic positive end-expiratory pressure (PEEPi) loads the respiratory muscles and causes dyspnea in obstructive lung disease. Continuous positive airway pressure (CPAP) has shown some efficacy in reducing inspiratory work and dyspnea. However, in obstructive lung disease, inspiratory work and dyspnea may be increased by additional factors that may not be affected by CPAP. Therefore, to study the effects of hyperinflation with intrinsic PEEP and CPAP in isolation, we used a mechanical analog of airway closure to increase end-expiratory lung volume in normal subjects. In five subjects in whom inspiratory work was measured, increasing end-expiratory lung volume by 1 and 2 L increased inspiratory work per breath from 0.42 +/- 0.04 J to 1.17 +/- 0.15 J (p < 0.05 compared with baseline) and 1.58 +/- 0.22 J (p < 0.05 compared with baseline and to the lesser level of hyperinflation). Although CPAP reduced work per breath and per minute to levels not significantly different from baseline, it had little effect on dyspnea. In ten subjects hyperinflated to 2.4 +/- 0.12 L above FRC, breathing could be sustained 19.5 +/- 4.5 min before quitting the load. This was increased to 26.7 +/- 5.2 min by 10 cm H2O CPAP (p = 0.052). Inspiratory dyspnea was modestly reduced by CPAP during these endurance trials. We conclude that CPAP can substantially ameliorate the respiratory work load induced by hyperinflation with intrinsic PEEP. However, the effects of CPAP on dyspnea and endurance are more limited. This suggests that the limits to breathing at high lung volumes are related to factors in addition to respiratory muscle work, and that CPAP may be of more value in reducing the work than in relieving the distress of obstructive lung disease.

Regional control of venous return: liver blood flow

The aim of the study was to determine whether closing pressures or vascular distensibility can be used to describe liver venous hemodynamics when right atrial pressure is raised. The study was performed using a vascularly isolated pig liver preparation that allowed the independent control of portal vein and hepatic artery inflows and of outflow pressure (Pout). Pressure-flow (P-Q) relationships of both liver vessels were generated at multiple levels of Pout. At Pout of 0 mm Hg, the portal vein P-Q relationship was linear, with a convexity toward the pressure axis at low flows (5 to 10 ml/min/kg). The zero flow pressure was 1.5 +/- 0.2 mm Hg, greater than Pout (p < 0.05). On raising Pout from 0 to 15 mm Hg, the shape of the portal vein P-Q relationships became progressively more linear, with a decrease in slope; no difference between zero flow pressure and Pout was observed. At Pout of 0 mm Hg, the hepatic artery presented a zero flow pressure > Pout. Raising Pout from 0 to 15 and 30 mm Hg resulted in a zero flow pressure always > Pout (p < 0.05). The behavior of the liver vein system is characterized by a zero flow pressure mimicking a classic vascular waterfall and by distensibility, once the waterfall is exceeded. Both factors act to minimize the reduction in venous return with an increased central venous pressure. Flow through the hepatic artery is affected by an increase in backpressure occurring upstream from the sinusoids, reducing arterial inflow for a constant perfusion pressure.

Observations of ventilation during resuscitation in a canine model

BACKGROUND

Fear of infection limits the willingness of laymen to do cardiopulmonary resuscitation (CPR). This study assessed the time course of change in arterial blood gases during resuscitation with only chest compression (no ventilation) in an effort to identify the time for which ventilation could be deferred.

METHODS AND RESULTS

Aortic pressures and arterial blood gases were monitored in seven 20- to 30-kg dogs in ventricular fibrillation (VF) at 2-minute intervals during chest compression alone (no ventilation) at 80 to 100 compressions per minute. Before the induction of ventricular fibrillation, all animals were intubated and ventilated with room air, 10 mL/kg. The endotracheal tube was removed when VF was induced. Pre-VF arterial pH, PCO2, and O2 saturation were (mean +/- SEM) 7.39 +/- 0.02, 27.0 +/- 1.5 mm Hg, and 97.5 +/- 0.5%, respectively, with aortic pressures being 143.2 +/- 5.7/116.2 +/- 4.6 mm Hg. At 4 minutes of chest compression alone, the corresponding values were 7.39 +/- 0.03, 24.3 +/- 3.1 mm Hg, and 93.9 +/- 3.0%, with an arterial pressure of 48.1 +/- 7.7/22.6 +/- 3.9 mm Hg. Mean minute ventilation during the fourth minute of CPR, measured with a face mask-pneumotachometer, was 5.2 +/- 1.1 L/min.

CONCLUSIONS

These data suggest that in the dog model of witnessed arrest, chest compression alone during CPR can maintain adequate gas exchange to sustain O2 saturation > 90% for > 4 minutes. The need for immediate ventilation during witnessed arrest should be reexamined.

Hyperinflation with intrinsic PEEP and respiratory muscle blood flow

Increased end-expiratory lung volume and intrinsic positive end-expiratory pressure (PEEP) are common in obstructive lung disease, especially during exacerbations or exercise. This loads the respiratory muscles and may also stress the circulatory system, causing a reduction or redistribution of cardiac output. We measured the blood flow to respiratory muscles and systemic organs using colored microspheres in 10 spontaneously breathing anesthetized tracheotomized dogs. Flows during baseline breathing (BL) were compared with those during hyperinflation (HI) induced by a mechanical analogue of airway closure and with those during an inspiratory resistive load (IR) that produced an equivalent increase in inspiratory work and time-integrated transdiaphragmatic pressure. Cardiac output was unchanged during IR (3.19 +/- 0.27 l/min at BL, 3.09 +/- 0.34 l/min during IR) but was reduced during HI (2.14 +/- 0.29 l/min; P < 0.01). Among the organs studied, flow was unaltered by IR but decreased to the liver and pancreas and increased to the brain during HI. For the respiratory muscles, flow to the diaphragm increased during IR. However, despite a 1.9-fold increase in inspiratory work per minute and a 2.5-fold increase in integrated transdiaphragmatic pressure during HI, blood flow to the diaphragm was unchanged and flow to the scalenes and sternomastoid fell. The only respiratory muscle to which flow increased during HI was the transversus abdominis, an expiratory muscle. We conclude that the circulatory effects of hyperinflation in this model impair inspiratory muscle perfusion and speculate that this may contribute to respiratory muscle dysfunction in hyperinflated states.

Effects of hyperinflation and CPAP on work of breathing and respiratory failure in dogs

Increased end-expiratory lung volume (EELV) and airway resistance are both characteristic features of obstructive lung disease. Increased EELV alone loads the respiratory muscles and may cause respiratory failure, changes that could be reversed by continuous positive airway pressure (CPAP). To study the effects of elevated EELV on respiration without increased airway resistance, we used a mechanical analogue of airway closure to increase EELV in six spontaneously breathing anesthetized dogs. Hyperinflation of 0.84 +/- 0.11 liter for 30 min decreased minute ventilation from 4.8 +/- 0.37 to 3.5 +/- 0.21 l/min and increased arterial PCO2 from 40.3 +/- 1.5 to 73.2 +/- 8.1 Torr (both P < 0.01). Inspiratory work per breath increased 3-fold, work per liter increased 3.7-fold, and work per minute increased 2.8-fold (all P < 0.01). CPAP at 15 cmH2O restored minute ventilation to 4.3 +/- 0.3 l/min and reduced arterial PCO2 to 54 +/- 6.6 Torr (NS vs. baseline). All measurements of inspiratory work were also restored to baseline, but cardiac output was reduced (baseline 3.09 +/- 0.36, hyperinflation 2.71 +/- 0.36, hyperinflation + CPAP 1.94 +/- 0.29 l/min; P < 0.05, baseline vs. hyperinflation + CPAP). We conclude that increases in EELV mimic important features of airway obstruction, increase inspiratory work, and can cause respiratory failure independent of increased airway resistance. This respiratory failure is reversed by CPAP at the potential expense of hemodynamic compromise.

Effect of hypoxia on lung fluid balance in ferrets

To determine how hypoxia may alter determinants of pulmonary transvascular fluid flux, adult male ferrets were exposed to either room air (C) or hypoxia (H; FIO2 = 0.12) for 24 h. After anesthesia and ventilation with C or H, the mean pulmonary artery pressures were 18.4 +/- 2.2 (SEM) and 27.3 +/- 2.9 mm Hg, respectively (p < 0.025). The right lung was then removed for gravimetric analysis of lung water and the left lung was blood-perfused (approximately 142 ml/kg/min) and continuously weighed for 15 min at left atrial pressures of 20, 25, and 30 mm Hg. Filtration coefficient (Kf) was estimated from the slopes of the relationships of rate of weight gain versus change in vascular pressure over the last 5 min of each interval. Extravascular lung water/blood-free dry lung weight for C and H were 2.95 +/- 0.06 (SEM) and 3.53 +/- 0.09 ml/g, respectively (p < 0.01). Kf for C and H were 0.0645 +/- 0.0190 (SEM) and 0.0662 +/- 0.0085 ml/min/mm Hg/100 g, respectively (NS). In a second group of experiments, in which lungs were removed from ferrets after 24 h exposures to C or H, protein reflection coefficients (sigma) were estimated by comparing the increases in perfusate hematocrit and protein concentrations during edema formation. Reflection coefficients for albumin were 0.64 +/- 0.03 (SEM) and 0.39 +/- 0.07 with C and H, respectively (p < 0.01). The sigma values for IgG and IgM were not affected.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of positive end-expiratory pressure and body position on pressure in the thoracic great veins

Positive end-expiratory pressure (PEEP) commonly decreases cardiac output. The major cause of this is believed to be decreased venous return due to increased right atrial pressure. We hypothesized that when the lungs were hyperinflated they could also restrict venous return by directly compressing the thoracic vena cavae. We measured the longitudinal distribution of pressure in the thoracic vena cavae of 10 dogs on and off 10 mm Hg PEEP, in the supine (S), prone (P), right lateral (RL), and left lateral decubitus (LL) positions. In the superior vena cava (SVC) both on and off PEEP, and in the inferior vena cava (IVC) off PEEP, pressure fell uniformly from the thoracic inlet to the right atrium. However, in the IVC on PEEP, intravascular pressure fell abruptly by up to 5 mm Hg. This pressure drop occurred in a discrete (1 to 2-cm) segment of the IVC, suggesting a localized increased in extravascular surface pressure. When this pressure inflection was present, changes in right atrial pressure had no effect on pressure in the IVC upstream of the inflection, consistent with a "vascular waterfall." These observations were most prominent in the LL, least common in the RL, and variably present in the P and S positions. Occlusion of the right bronchus intermedius prior to PEEP (preventing right lower, middle, and accessory lobe inflation) prevented the appearance of the pressure inflection during PEEP in the LL but not in the S or P positions. We conclude that PEEP impedes venous return partly by direct compression of the IVC, predominantly in positions in which the IVC is non-dependent.(ABSTRACT TRUNCATED AT 250 WORDS)

Tracheal and neck position influence upper airway airflow dynamics by altering airway length

Upper airway obstruction during sleep is characterized by inspiratory airflow limitation and reductions in maximal inspiratory airflow (VImax). To determine how mechanical factors modulate VImax, we analyzed pressure-flow relationships obtained in the isolated upper airway of paralyzed cats. VImax and its determinants, the pharyngeal critical pressure (Pcrit) and the nasal resistance (Rn) upstream to the flow-limiting site (FLS), were measured as caudal tracheal displacement, neck position, and airway length were systematically varied. As the proximal tracheal stump was displaced caudally, graded increases in VImax from 145.3 +/- 90.8 (SD) to 285.9 +/- 117.5 ml/s (P < 0.02) and decreases in Pcrit from -3.0 +/- 3.0 to -9.5 +/- 3.4 cmH2O (P < 0.002) were seen without any significant change in Rn. During neck flexion, significant decreases in VImax from 192.1 +/- 68.5 to 87.2 +/- 48.4 ml/s (P = 0.001), increases in Pcrit from -5.3 +/- 2.03 cmH2O to -1.6 +/- 1.4 cmH2O (P < 0.001), and decreases in Rn from 29.7 +/- 12.2 cmH2O.l-1.s to 16.2 +/- 8.9 cmH2O.l-1.s (P < 0.001) were noted compared with the neutral or extended neck position. Relative to the neutral airway length, upper airway length was found to decrease by 1.15 +/- 0.14 cm during neck flexion and to lengthen by 0.45 +/- 0.12 cm during neck extension. When tracheal displacement and neck position were altered, VImax and Rn correlated directly and Pcrit correlated inversely with airway length (P < 0.001). We conclude that alterations in airflow mechanics with caudal tracheal displacement and changes in neck positions are primarily due to alterations in airway length.

Influence of diffusion on estimations of protein reflection coefficient by double-indicator method

In isolated perfused organs, vascular protein reflection coefficients (sigma) can be calculated from the changes in hematocrit and perfusate protein concentration (CP) that occur during edema formation. This technique requires the assumption that transvascular protein flux by diffusion is negligible. To assess diffusion-induced errors in calculations of sigma, we derived an expression for CP that includes determinants of diffusive protein flux: protein permeability-surface area product (PS), transvascular fluid flux (J), true sigma, and transvascular protein concentration. We used this expression to obtain values of CP under various experimental conditions and then calculated values of sigma (measured sigma) for those conditions. Diffusion causes measured sigma to be lower than true sigma. The diffusion-induced error is larger and potentially substantial when J/PS is low and when true sigma is high. Diffusion-induced error is also larger when the amount of edema formation is greater. In recent isolated canine lung experiments where J/PS was approximately 2.7, diffusion-induced errors in measured sigma for albumin would have been approximately 0.06 (at true sigma = 0.5) and approximately 0.18 (at true sigma = 0.9). When J/PS was higher, the potential for diffusion-induced errors was much smaller. We conclude that diffusion causes underestimation of true sigma and that the error in measured sigma may be substantial when J/PS is < 5 and when true sigma is > 0.5.

Developmental differences in catalase activity and hypoxic-hyperoxic effects on fluid balance in isolated lamb lungs

The effects of hypoxia (95% N2/5% CO2) followed by hyperoxia (95% O2/5% CO2) were determined in isolated lungs of premature (gestational age 128 to 135 d) and full-term (postnatal age 0 to 5 d) lambs perfused with autologous blood (100 mL.min-1.kg body weight-1). In full-term lungs, hypoxia-hyperoxia compared with hypoxia alone decreased pulmonary artery pressure and increased weight gain and extravascular lung water. In premature lungs, the increase in weight gain was greater and was associated with hemorrhage and increased pulmonary arterial and peak airway pressures. Papaverine eliminated reoxygenation-induced differences in pulmonary artery pressure, peak airway pressure, and weight gain in both age groups. Osmotic reflection coefficients for total protein and albumin, measured by a modification of the filtered volume technique, averaged 0.591 +/- 0.054 (SEM) and 0.465 +/- 0.054 (SEM), respectively, and were not altered by reoxygenation or age. Catalase activity in lung tissue and erythrocytes was lower in premature lambs, but there were no age-related differences in superoxide dismutase or glutathione peroxidase activities. These results demonstrate that hypoxia-hyperoxia in isolated lamb lungs increased lung weight due to edema formation in full-term lamb lungs and hemorrhage in premature lamb lungs and that this increase was greater in premature lamb lungs. We speculate that the weight gain caused by reoxygenation was due to a vasodilation-induced increase in surface area in full-term lamb lungs and a vasoconstriction-induced increase in vascular pressure in premature lamb lungs.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of electrical stimulation of the hypoglossal nerve on airflow mechanics in the isolated upper airway

To determine the influence of electrical hypoglossal (HG) nerve stimulation on upper airway airflow mechanics, we analyzed pressure-flow relationships obtained during bilateral supramaximal HG nerve stimulation over a range of frequencies from 0 to 100 Hz in the isolated feline upper airway. Inspiratory airflow (VI), hypopharyngeal pressure (Php), and pharyngeal pressure (Pph) immediately upstream from the flow-limiting site (FLS) were recorded while Php was rapidly lowered to achieve inspiratory flow limitation in the isolated upper airway. Pressure-flow relationships were analyzed to determine the maximum in VI (VImax) and the mechanical determinants of VImax, the upper airway critical pressure (Pcrit) and the nasal resistance (RN) upstream to the FLS. In groups of decerebrate spinally anesthetized (n = 6) and unanesthetized (n = 6) cats, graded increases in VImax (p < 0.05) and decreases in Pcrit (p < 0.001) were observed as the stimulation frequency of the intact HG nerves was increased. In the cats with and without spinal anesthesia, VImax increased by 139 and 201%, and Pcrit decreased by 159 and 280%, respectively. RN was also correlated with stimulation frequency in the cats without spinal anesthesia (p = 0.01) and increased in four of six cats with spinal anesthesia. In an additional six decerebrate cats, significant increases in VImax (p < 0.001) and decreases in Pcrit (p = 0.01) were elicited by stimulating the distal cut HG nerve ends (50 Hz), whereas no changes were noted in these parameters when the proximal ends were stimulated. The findings suggest that HG stimulation increases VImax by decreasing Pcrit, which indicates a decrease in upper airway collapsibility at the FLS.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of maximal inspiratory airflow by neuromuscular activity: effect of CO2

To determine how maximal inspiratory airflow (VImax) is modulated by changes in airway neuromuscular activity, we analyzed pressure-flow relationships obtained during inspiration and expiration in isolated upper airways of anesthetized hyperoxic dogs at different levels of CO2. Inspiratory airflow (VI), hypopharyngeal pressure (Php), pharyngeal pressure at the flow-limiting site (FLS), and alae nasi (AN) and genioglossus (GG) electromyographic (EMG) activity were recorded while VI limitation was produced by rapidly lowering Php until VI plateaued at VImax. VImax and its mechanical determinants, pharyngeal critical pressure (Pcrit) and nasal resistance (Rn) upstream to the FLS, were measured. During hypercapnia (high CO2), VImax increased significantly during inspiration (217.3) and expiration (184.1%). These increases were associated with significant increases in phasic but not tonic AN and GG activity. They were also associated with decreases in Pcrit from -6.2 +/- 1.6 (SE) at hypocapnia to -9.3 +/- 3.0 and -11.8 +/- 3.4 cmH2O at high CO2 during expiration and inspiration, respectively. No significant changes in Rn occurred. When phasic neuromuscular activity was abolished by complete neuromuscular blockade in three dogs, these increases in VImax and decreases in Pcrit at high CO2 were eliminated. When phasic EMG activity was accentuated in four vagotomized dogs, significant increases in VImax and decreases in Pcrit were demonstrated during inspiration vs. expiration at high CO2. These findings indicate that upper airway neuromuscular activity increases VImax in the isolated upper airway by decreasing collapsibility (Pcrit) at the FLS site when neuromuscular activity is stimulated by hypercapnia.

Factors influencing measurement of protein reflection coefficient by filtered volume technique

In isolated perfused organs, the protein reflection coefficient (sigma) can be estimated by comparing increases in hematocrit (Hct) and protein concentration (CP) during transvascular fluid filtration. In this study, we developed an equation for sigma to examine the potential influences of perfusate leak, evaporation, and hemolysis-induced changes in red blood cell volume and perfusate water. We also performed experiments in isolated ferret lungs to quantitate the magnitude of these potential sources of error and the effects of free hemoglobin on measurements of CP. These studies demonstrated that 1) perfusate leak does not cause an error because its effects on changes in Hct and CP counteract each other; 2) evaporation causes an overestimation of sigma, but in our experiments this effect was small; 3) hemolysis-induced changes in red blood cell and perfusate water volumes may cause an over- or underestimation of sigma, but these effects are small; 4) overestimations of CP due to increasing free perfusate hemoglobin concentration can cause substantial overestimations of sigma; and 5) values of sigma calculated from previous equations and from our equation were virtually identical, suggesting that the assumptions necessary for the previous equations were not significant sources of error. In agreement with previous workers, we conclude that the most important potential source of error is hemolysis-induced increases in free perfusate hemoglobin.

Production of granulocyte/macrophage colony-stimulating factor in human airways during allergen-induced late-phase reactions in atopic subjects

Granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-3 (IL-3) are hematopoietic growth factors that have been shown to induce proliferation and activation of inflammatory cells, and may play a role in allergic reactions. Since little is known about the involvement of cytokines in allergic inflammation in the lung, the levels of GM-CSF and IL-3 were measured in bronchoalveolar lavage (BAL) fluids obtained in the late phase after segmental lung antigen (Ag) challenge in 14 allergic rhinitis subjects with or without bronchial asthma. BAL fluids either after Ag (ragweed, dust mite, or timothy) or saline control challenge were recovered 19 h later. In 6 of the 14 patients, BAL fluids were concentration-dialyzed (20x) and assayed for cytokine activity. Cytokine assays were performed using the human megakaryocytic leukemic cell line M-07e, which is responsive to either GM-CSF or IL-3. The level of GM-CSF-equivalents was approximately 25 times higher in Ag-challenged sites (49.9 +/- 12.7 pg/ml; mean +/- SEM), compared to saline challenge sites (2.2 +/- 1.0, p < 0.01, n = 9). Neutralization experiments using a polyclonal specific antibody (Ab) against GM-CSF and IL-3 revealed that the bulk of the activity was GM-CSF. BAL fluids from Ag- and saline-challenged sites in one nonatopic subject contained no significant GM-CSF activity. Furthermore, the level of GM-CSF in Ag-challenged BAL fluid and the percentage of eosinophils in BAL from each subject correlated significantly (r = 0.73, p < 0.005, n = 14).(ABSTRACT TRUNCATED AT 250 WORDS)

Separate effects of ischemia and reperfusion on vascular permeability in ventilated ferret lungs

In systemic organs, ischemia-reperfusion injury is thought to occur during reperfusion, when oxygen is reintroduced to hypoxic ischemic tissue. In contrast, the ventilated lung may be more susceptible to injury during ischemia, before reperfusion, because oxygen tension will be high during ischemia and decrease with reperfusion. To evaluate this possibility, we compared the effects of hyperoxic ischemia alone and hyperoxic ischemia with normoxic reperfusion on vascular permeability in isolated ferret lungs. Permeability was estimated by measurement of filtration coefficient (Kf) and osmotic reflection coefficient for albumin (sigma alb), using methods that did not require reperfusion to make these measurements. Kf and sigma alb in control lungs (n = 5), which were ventilated with 14% O2-5% CO2 after minimal (15 +/- 1 min) ischemia, averaged 0.033 +/- 0.004 g.min-1.mmHg-1.100 g-1 and 0.69 +/- 0.07, respectively. These values did not differ from those reported in normal in vivo lungs of other species. The effects of short (54 +/- 9 min, n = 10) and long (180 min, n = 7) ischemia were evaluated in lungs ventilated with 95% O2-5% CO2. Kf and sigma alb did not change after short ischemia (Kf = 0.051 +/- 0.006 g.min-1.mmHg-1.100 g-1, sigma alb = 0.69 +/- 0.07) but increased significantly after long ischemia (Kf = 0.233 +/- 0.049 g.min-1 x mmHg-1 x 100 g-1, sigma alb = 0.36 +/- 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of systolic and diastolic positive pleural pressure pulses with altered cardiac contractility

Positive pleural pressure (Ppl) decreases left ventricular afterload and preload. The resulting change in cardiac output (CO) in response to these altered loading conditions varies with the baseline level of cardiac contractility. In an isolated canine heart-lung preparation, we studied the effects of positive Ppl applied phasically during systole or diastole on CO and on the cardiac function curve (the relationship between CO and left atrial transmural pressure). When baseline cardiac contractility was enhanced by epinephrine infusion, systolic and diastolic positive Ppl decreased CO equally (1,931 +/- 131 to 1,419 +/- 124 and 1,970 +/- 139 to 1,468 +/- 139 ml/min, P less than 0.01) and decreased the pressure gradient driving venous return. However, neither shifted the position of the cardiac function curve, suggesting that the predominant effect of positive Ppl was decreased preload. When baseline cardiac contractility was depressed by severe respiratory acidosis, diastolic positive Ppl pulses caused no significant change in CO (418 +/- 66 to 386 +/- 52 ml/min), the cardiac function curve, or the pressure gradient for venous return. However, systolic positive Ppl pulses increased CO from 415 +/- 70 to 483 +/- 65 ml/min (P less than 0.01) and significantly shifted the cardiac function curve to the left. Thus the effect of Ppl pulsations on CO works through different mechanisms, depending on the state of cardiac contractility.

Peripheral airways resistance in smokers

To determine peripheral airways resistance (Rp) in asymptomatic smokers, we used a wedged bronchoscope technique to study 19 volunteers (18 to 44 yr of age) who actively smoked for 2 to 28 pack-years. A fiberoptic bronchoscope was wedged in a subsegmental bronchus of the right upper lobe. Using a double lumen catheter inserted through the working channel of the bronchoscope, we infused 5% CO2 in air through one lumen and measured pressure through the second lumen. Rp was determined as the average of the peripheral resistance measured at three or more flow rates. This resistance ranged from 0.003 to 0.075 cm H2O/ml/min in the 19 subjects. We have previously shown normal subjects to have an average Rp of 0.009 +/- 0.002 cm H2O/ml/min (mean +/- SE) and asthmatic subjects an average of 0.069 +/- 0.017 cm H2O/ml/min. Thus, despite normal pulmonary function as assessed by spirometry, these asymptomatic smokers demonstrated a wide range of Rp values from normal to that observed in asthmatic subjects. These findings are consistent with a mechanism that considers the high resistance to result from inflammatory changes in the small airways.

Effects of positive end-expiratory pressure on the canine venous return curve

To study the mechanism whereby positive end-expiratory pressure (PEEP) decreases venous return, we used a closed-chest canine venous bypass preparation to study the effects of 10 mm Hg PEEP on the systemic venous pressure-flow curves from the superior and inferior vena cava (SVC and IVC). These curves were characterized by three variables: the critical downstream pressure below which venous return was maximal (PCRIT), the conductance to venous return (GVR), and the effective upstream pressure driving venous return. PEEP reduced venous return by decreasing the maximal venous return even when the pressures at the outflow of the IVC and SVC were maintained below zero. PEEP increased PCRIT in the SVC and IVC (SVC: -0.31 +/- 0.53 to 3.21 +/- 0.84; IVC: -0.41 +/- 0.64 to 5.23 +/- 1.02 (SE) mm Hg; p less than 0.005). GVR in the SVC was reduced (52.5 +/- 26 to 37.8 +/- 5.3 (SE) ml/min/mm Hg; p less than 0.005), but changes in the IVC did not reach statistical significance. These changes were partially offset by increases in the upstream pressure driving venous return (SVC: 9.44 +/- 0.54 to 12.25 +/- 0.71; IVC: 9.42 +/- 0.69 to 12.51 +/- 1.02 (SE) mm Hg; p less than 0.01). Analysis of these findings suggests that PEEP may alter venous return through effects on the peripheral circulation, independent of its effects on the heart.

Effect of weight loss on upper airway collapsibility in obstructive sleep apnea

Previous investigators have demonstrated in patients with obstructive sleep apnea that weight reduction results in a decrease in apnea severity. Although the mechanism for this decrease is not clear, we hypothesize that decreases in upper airway collapsibility account for decreases in apnea severity with weight loss. To determine whether weight loss causes decreases in collapsibility, we measured the upper airway critical pressure (Pcrit) before and after a 17.4 +/- 3.4% (mean +/- SD) reduction in body mass index in 13 patients with obstructive sleep apnea. Thirteen weight-stable control subjects matched for age, body mass index, gender (all men), and non-REM disordered breathing rate (DBR) also were studied before and after usual care intervention. During non-REM sleep, maximal inspiratory airflow was measured by varying the level of nasal pressure and Pcrit was determined by the level of nasal pressure below which maximal inspiratory airflow ceased. In the weight loss group, a significant decrease in DBR from 83.3 +/- 31.0 to 32.5 +/- 35.9 episodes/h and in Pcrit from 3.1 +/- 4.2 to -2.4 +/- 4.4 cm H2O (p less than 0.00001) was demonstrated. Moreover, decreases in Pcrit were associated with nearly complete elimination of apnea in each patient whose Pcrit fell below -4 cm H2O. In contrast, no significant change in DBR and a minimal reduction in Pcrit from 5.2 +/- 2.3 to 4.2 +/- 1.8 cm H2O (p = 0.031) was observed in the "usual care" group. We conclude that (1) weight loss is associated with decreases in upper airway collapsibility in obstructive sleep apnea, and that (2) the resolution of sleep apnea depends on the absolute level to which Pcrit falls.

Upper airway collapsibility in snorers and in patients with obstructive hypopnea and apnea

During sleep, mild reduction in inspiratory airflow is associated with snoring, whereas obstructive hypopneas and apneas are associated with more marked reductions in airflow. We determined whether the degree of inspiratory airflow reduction was associated with differences in the collapsibility of the upper airway during sleep. Upper airway collapsibility was defined by the critical pressure (Pcrit) derived from the relationship between maximal inspiratory airflow and nasal pressure. In 10 asymptomatic snorers, six patients with obstructive hypopneas, and 10 patients with obstructive apneas, during nonrapid eye movement sleep, Pcrit ranged from -6.5 +/- 2.7 cm H2O to -1.6 +/- 1.4 and 2.5 +/- 1.5 cm H2O, respectively (mean +/- SD, p less than 0.001). Moreover, higher levels of Pcrit were associated with lower levels of maximal inspiratory airflow during tidal breathing during sleep (p less than 0.005). We conclude that differences in upper airway collapsibility distinguish among groups of normal subjects who snore and patients with periodic hypopneas and apneas. Moreover, the findings suggest that small differences in collapsibility (Pcrit) along a continuum are associated with reduced airflow and altered changes in pattern of breathing.

Effects of positive end-expiratory pressure on the gradient for venous return

The major mechanism whereby positive end-expiratory pressure (PEEP) decreases cardiac output is believed to be a decrease in the pressure gradient for venous return. However, although PEEP increases right atrial pressure (PRA), It may also elevate mean systemic pressure (PMS), the static circulatory filling pressure that is the upstream pressure for venous return. In an intact canine preparation, we studied the effects of 15 cm H2O PEEP on cardiac output, PRA, and PMS (the equilibrium PRA during ventricular fibrillation). To examine the role of neurovascular reflexes, PEEP was applied before and after either carotid sinus and vagal denervation (CSV) or total spinal anesthesia with arterial pressure restored by epinephrine infusion (SAE). To examine the effects of PEEP-induced elevations of abdominal pressure, the abdomen was bound or widely opened and the abdominal contents exteriorized. With reflexes intact, neither binding nor opening the abdomen altered the rise in PMS during PEEP. CSV attenuated the rise in Pms by 17% (Control, 4.89 +/- 0.3 SE; CSV, 4.04 +/- 0.22 mmHg; p less than 0.01), and SAE attenuated it by 49% (Control, 4.21 +/- 0.27; SAE, 2.14 +/- 0.31 mmHg; p less than 0.00005). After either CSV or SAE, the rise in Pms was not affected by binding. PEEP decreased (Pms-PRA) only when the abdomen was bound because of a greater rise in PRA, or during SAE because of a lesser rise in Pms. Under control conditions, PEEP increased Pms and PRA equally [(PRA-Pms) = 3.89 +/- 0.26 without PEEP versus 4.13 +/- 0.29 mm Hg with PEEP]. We conclude that PEEP increases Pms by both reflex and mechanical means independent of increased abdominal pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Flow-volume characteristics in the pulmonary circulation

Isolated ferret and canine lungs were used to validate a method for assessing determinants of vascular volume in the pulmonary circulation. With left atrial pressure (Pla) constant at 5 mmHg, flow (Q) was raised in steps over a physiological range. Changes in vascular volume (delta V) with each increment in Q were determined as the opposite of changes in perfusion system reservoir weight or from the increase in lung weight. At each level of Q, the pulmonary arterial and left atrial cannulas were simultaneously occluded, allowing all vascular pressures to equilibrate at the same static pressure (Ps), which was equal to the compliance-weighted average pressure in the circulation before occlusion. Hypoxia (inspired PO2 25 Torr) in ferret lungs, which causes intense constriction in arterial extra-alveolar vessels, had no effect on the slope of the Ps-Q relationship, interpreted to represent the resistance downstream from compliance (control 0.025 +/- 0.006 mmHg.ml-1.min, hypoxia 0.030 +/- 0.013). The Ps-axis intercept increased from 8.94 +/- 0.50 to 13.43 +/- 1.52 mmHg, indicating a modest increase in the effective back-pressure to flow downstream from compliant regions. The compliance of the circulation, obtained from the slope of the relationship between delta V and Ps, was unaffected by hypoxia (control 0.52 +/- 0.08 ml/mmHg, hypoxia 0.56 +/- 0.08). In contrast, histamine in canine lungs, which causes constriction in veins, caused the slope of the Ps-Q relationship to increase from 0.013 +/- 0.007 to 0.032 +/- 0.006 mmHg.ml-1.min (P less than 0.05) and the compliance to decrease from 3.51 +/- 0.56 to 1.68 +/- 0.37 ml/mmHg (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for elevated levels of histamine, prostaglandin D2, and other bronchoconstricting prostaglandins in the airways of subjects with mild asthma

Histamine and certain cyclooxygenase products of arachidonic acid have been implicated as mediators of inflammation and are potent constrictors of human airways. Because asthma may represent manifestations of chronic inflammation of the airways, the levels of histamine and six prostanoid mediators were measured in airway fluids obtained by bronchoalveolar lavage (BAL) of 12 normal, 11 allergic rhinitic, and 15 asymptomatic, allergic asthmatic subjects. Simultaneous profiling of prostanoid mediators in individual samples was performed using gas chromatography-mass spectrometry. Levels of PGD2, 9 alpha,11 beta-PGF2 and PGF2 alpha were 12 to 22 times higher in asthmatic than in normal subjects (p less than 0.01), with concentrations in airway fluids of asthmatic subjects after correction for dilution of 3.8, 0.5, and 1.4 nanomolar, respectively. Levels of PGD2 and 9 alpha,11 beta-PGF2 were increased nearly tenfold in asthmatic subjects compared with those in rhinitic subjects (p less than 0.01), distinguishing the subjects with lower airway disease from those with another atopic condition. Histamine levels were increased fourfold in asthmatic subjects compared with those in normal subjects (p less than 0.001); however, similar increases were found in rhinitic subjects. We conclude that elevated levels of multiple mediators with potent bronchoconstricting activity are present in the airways of subjects with mild asthma, indicating that even mild disease is associated with evidence of airway inflammation. The interactions of bronchoconstricting mediators and airway inflammation may play important roles in the pathogenesis of asthma.

Peripheral lung resistance in normal and asthmatic subjects

In obstructive lung disease, peripheral airways are a major site of pathologic abnormalities. However, resistance to airflow in small airways in the periphery of the lung accounts for only a small fraction of total airway resistance. Consequently, abnormalities of small airway function may not be readily detected using routine pulmonary function testing. In the present study, resistance of the peripheral lung was examined directly in six normal subjects and nine mildly asthmatic subjects. There were no significant differences between the normal and asthmatic groups in pulmonary function assessed by spirometry (FEV1, FVC) and body plethysmography (specific airway conductance). Direct measurements of peripheral lung function were made using a fiberoptic bronchoscope wedged into a subsegmental, right upper lobe bronchus. Using a double-lumen catheter inserted into the instrument channel of the bronchoscope, pressures (PB) produced by three or more different levels of gas flow (V) (5% CO2 in air) between 50 and 500 ml/min were measured. All pressure measurements were made at a constant lung volume (i.e., functional residual capacity) confirmed by monitoring transpulmonary pressure with an esophageal balloon. The pressure-flow relationship in both normal and asthmatic subjects could be approximated by a straight line through the origin, demonstrating these airways to be relatively nondistensible. Peripheral lung resistance (Rp) was defined by PB/V and averaged for three or more levels of flow.(ABSTRACT TRUNCATED AT 250 WORDS)

Positive pleural pressure decreases coronary perfusion

Pressure surrounding the heart (PSH) rises with maneuvers that increase pleural pressure. This may decrease left ventricular (LV) oxygen demand by reducing LV afterload. However, positive PSH may also directly impede coronary flow. To study the effects of positive PSH on coronary perfusion, PSH was increased in 10-mmHg increments from 0 to 60 mmHg in an isolated canine heart-lung preparation with constant venous return, arterial pressure, and lung volume. Increased PSH caused a rapid significant (P less than 0.001) fall in left atrial transmural pressure (PLATM) of up to 1.28 +/- 0.31 mmHg. With constant venous return and lung volume, this was interpreted to reflect decreased LV afterload. However, at levels of PSH greater than 30 mmHg, initial decreases in PLATM were followed by sustained increases, suggesting that there was a deterioration in cardiac function despite the lower level of afterload. Increased PSH was also associated with decreases in circumflex coronary artery flow [flow (ml/min) = 52.4 - 0.4PSH, P less than 0.01]. Moreover, when the circumflex coronary artery was maximally dilated with adenosine, the effects of PSH were amplified [flow (ml/min) = 137.9 - 1.78PSH, P less than 0.001], indicating that positive PSH mechanically impeded coronary flow. When PSH was raised to 60 mmHg for 90 s, the aortic-coronary sinus lactate concentration difference fell from 0.71 +/- 0.09 to 0.10 +/- 0.21 mM (mean +/- SE, P less than 0.001, n = 8), suggesting myocardial ischemia. We conclude that positive PSH directly decreases myocardial perfusion. This may lead to ischemic cardiac dysfunction, especially in patients with low arterial pressure or coronary artery disease.

Effect of positive nasal pressure on upper airway pressure-flow relationships

To determine the influence of changes in nasal pressure (Pn) on airflow mechanics in the upper airway, we examined the effect of elevations in Pn on upper airway resistance and critical pressure (Pcrit) during stage I/II sleep in six patients with obstructive sleep apnea. When Pn was elevated above a Pcrit, periodic occlusions of the upper airway were eliminated and inspiratory airflow limitation was demonstrated by the finding that inspiratory airflow (VI) became maximal (VImax) and independent of fluctuations in hypopharyngeal pressure (Php) when Php fell below a specific Php (Php'). As Pn was elevated, VI vs. Php demonstrated 1) marked decreases in early and late inspiratory resistances from 75.9 +/- 34.7 and 54.6 +/- 19.0 to 8.0 +/- 1.7 and 7.6 +/- 1.6 cmH2O.l-1.s (P less than 0.05), respectively, and 2) increases in early and late inspiratory Php' to levels that exceeded Pcrit by 3.0 +/- 0.6 and 3.1 +/- 0.7 cmH2O, respectively, at the highest level of Pn applied (P less than 0.01). This latter finding suggests that elevations in Pn result in increases in Pcrit. We suggest that elevations in Pn produce distinct alterations in upper airway resistance and collapsibility, which may influence oppositely the level of airflow through the upper airway during sleep.

Mechanism of reduced LV afterload by systolic and diastolic positive pleural pressure

To investigate the mechanism by which increased pleural pressure (Ppl) assists left ventricular (LV) ejection, we compared the effects of phasic systolic or diastolic increases in Ppl (40-60 mmHg) with use of an isolated canine heart-lung preparation with constant venous return. Positive Ppl during systole (S) caused left atrial transmural pressure (Platm = Pla - Ppl) to decrease by 1.25 +/- 0.46 (SE) mmHg (P less than 0.025). Central blood volume (CBV), the volume of blood in the heart, lungs, and thoracic great vessels, decreased by 29 +/- 4.0 (SE) ml (P less than 0.001). When Ppl was raised for an equal duration during diastole (D), the decrease in Platm was not significant, but there was a significant decrease in CBV (10.5 +/- 4.1 ml, P less than 0.05). With constant venous return, these changes suggested that phasic elevations in Ppl in either S or D assisted LV ejection by decreasing LV afterload. To test the hypothesis that positive Ppl during D reduced afterload by emptying the thoracic aorta, we compared the effects of diastolic positive Ppl with a rigid aorta vs. a compliant aorta. Although there was no statistical difference in the effects of diastolic positive Ppl on Platm, the decrease in CBV was significantly greater when the aorta was compliant than when it was rigid (23 +/- 2.2 vs. 17 +/- 2.7 ml, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of exercise hemoconcentration and hyperosmolality on exercise responses

We investigated the effects of a decrease in plasma volume (PV) and an increase in plasma osmolality during exercise on circulatory and thermoregulatory responses. Six subjects cycled at approximately 65% of their maximum O2 uptake in a warm environment (30 degrees C, 40% relative humidity). After 30 min of control (C) exercise (no infusion), PV decreased 13.0%, or 419 +/- 106 (SD) ml, heart rate (HR) increased to 167 +/- 3 beats/min, and esophageal temperature (Tes) rose to 38.19 +/- 0.09 degrees C (SE). During infusion studies (INF), infusates were started after 10 min of exercise. The infusates contained 5% albumin suspended in 0.45, 0.9, or 3.0% saline. The volume of each infusate was adjusted so that during the last 10 min of exercise PV was maintained at the preexercise level and osmolality was allowed to differ. HR was significantly lower (10-16 beats/min) during INF than during C. Tes was reduced significantly during INF, with trends for increased skin blood flow and decreased sweating rates. No significant differences in HR, Tes, or sweating rate occurred between the three infusion conditions. We conclude that the decrease in PV, which normally accompanies moderate cycle exercise, compromises circulatory and thermal regulations. Increases in osmolality appear to have small if any effects during such short-term exercise.