Total and regional deposition of inhaled aerosols in supine healthy subjects and subjects with mild-to-moderate COPD

Despite substantial development of sophisticated subject-specific computational models of aerosol transport and deposition in human lungs, experimental validation of predictions from these new models is sparse. We collected aerosol retention and exhalation profiles in seven healthy volunteers and six subjects with mild-to-moderate COPD (FEV1 = 50-80%predicted) in the supine posture. Total deposition was measured during continuous breathing of 1 and 2.9 μm-diameter particles (tidal volume of 1 L, flow rate of 0.3 L/s and 0.75 L/s). Bolus inhalations of 1 μm particles were performed to penetration volumes of 200, 500 and 800 mL (flow rate of 0.5 L/s). Aerosol bolus dispersion (H), deposition, and mode shift (MS) were calculated from these data. There was no significant difference in total deposition between healthy subjects and those with COPD. Total deposition increased with increasing particle size and also with increasing flow rate. Similarly, there was no significant difference in aerosol bolus deposition between subject groups. Yet, the rate of increase in dispersion and of decrease in MS with increasing penetration volume was higher in subjects with COPD than in healthy volunteers (H: 0.798 ± 0.205 vs. 0.527 ± 0.122 mL/mL, p=0.01; MS: -0.271±0.129 vs. -0.145 ± 0.076 mL/mL, p=0.05) indicating larger ventilation inhomogeneities (based on H) and increased flow sequencing (based on MS) in the COPD than in the healthy group. In conclusion, in the supine posture, deposition appears to lack sensitivity for assessing the effect of lung morphology and/or ventilation distribution alteration induced by mild-to-moderate lung disease on the fate of inhaled aerosols. However, other parameters such as aerosol bolus dispersion and mode shift may be more sensitive parameters for evaluating models of lungs with moderate disease.

Comparison of CT-derived ventilation maps with deposition patterns of inhaled microspheres in rats

PURPOSE

Computer models for inhalation toxicology and drug-aerosol delivery studies rely on ventilation pattern inputs for predictions of particle deposition and vapor uptake. However, changes in lung mechanics due to disease can impact airflow dynamics and model results. It has been demonstrated that non-invasive, in vivo, 4DCT imaging (3D imaging at multiple time points in the breathing cycle) can be used to map heterogeneities in ventilation patterns under healthy and disease conditions. The purpose of this study was to validate ventilation patterns measured from CT imaging by exposing the same rats to an aerosol of fluorescent microspheres (FMS) and examining particle deposition patterns using cryomicrotome imaging.

MATERIALS AND METHODS

Six male Sprague-Dawley rats were intratracheally instilled with elastase to a single lobe to induce a heterogeneous disease. After four weeks, rats were imaged over the breathing cycle by CT then immediately exposed to an aerosol of ∼ 1 μm FMS for ∼ 5 minutes. After the exposure, the lungs were excised and prepared for cryomicrotome imaging, where a 3D image of FMS deposition was acquired using serial sectioning. Cryomicrotome images were spatially registered to match the live CT images to facilitate direct quantitative comparisons of FMS signal intensity with the CT-based ventilation maps.

RESULTS

Comparisons of fractional ventilation in contiguous, non-overlapping, 3D regions between CT-based ventilation maps and FMS images showed strong correlations in fractional ventilation (r = 0.888, p < 0.0001).

CONCLUSION

We conclude that ventilation maps derived from CT imaging are predictive of the 1 μm aerosol deposition used in ventilation-perfusion heterogeneity inhalation studies.

Airway tree segmentation in serial block-face cryomicrotome images of rat lungs

A highly automated method for the segmentation of airways in the serial block-face cryomicrotome images of rat lungs is presented. First, a point inside of the trachea is manually specified. Then, a set of candidate airway centerline points is automatically identified. By utilizing a novel path extraction method, a centerline path between the root of the airway tree and each point in the set of candidate centerline points is obtained. Local disturbances are robustly handled by a novel path extraction approach, which avoids the shortcut problem of standard minimum cost path algorithms. The union of all centerline paths is utilized to generate an initial airway tree structure, and a pruning algorithm is applied to automatically remove erroneous subtrees or branches. Finally, a surface segmentation method is used to obtain the airway lumen. The method was validated on five image volumes of Sprague-Dawley rats. Based on an expert-generated independent standard, an assessment of airway identification and lumen segmentation performance was conducted. The average of airway detection sensitivity was 87.4% with a 95% confidence interval (CI) of (84.9, 88.6)%. A plot of sensitivity as a function of airway radius is provided. The combined estimate of airway detection specificity was 100% with a 95% CI of (99.4, 100)%. The average number and diameter of terminal airway branches was 1179 and 159 μm, respectively. Segmentation results include airways up to 31 generations. The regression intercept and slope of airway radius measurements derived from final segmentations were estimated to be 7.22 μm and 1.005, respectively. The developed approach enables the quantitative studies of physiology and lung diseases in rats, requiring detailed geometric airway models.

Heterogeneity and matching of ventilation and perfusion within anatomical lung units in rats

Prior studies exploring the spatial distributions of ventilation and perfusion have partitioned the lung into discrete regions not constrained by anatomical boundaries and may blur regional differences in perfusion and ventilation. To characterize the anatomical heterogeneity of regional ventilation and perfusion, we administered fluorescent microspheres to mark regional ventilation and perfusion in five Sprague-Dawley rats and then using highly automated computer algorithms, partitioned the lungs into regions defined by anatomical structures identified in the images. The anatomical regions ranged in size from the near-acinar to the lobar level. Ventilation and perfusion were well correlated at the smallest anatomical level. Perfusion and ventilation heterogeneity were relatively less in rats compared to data previously published in larger animals. The more uniform distributions may be due to a smaller gravitational gradient and/or the fewer number of generations in the distribution trees before reaching the level of gas exchange, making regional matching of ventilation and perfusion less extensive in small animals.

Determination of regional ventilation and perfusion in the lung using xenon and computed tomography

We propose a model to measure both regional ventilation (V) and perfusion (Q) in which the regional radiodensity (RD) in the lung during xenon (Xe) washin is a function of regional V (increasing RD) and Q (decreasing RD). We studied five anesthetized, paralyzed, mechanically ventilated, supine sheep. Four 2.5-mm-thick computed tomography (CT) images were simultaneously acquired immediately cephalad to the diaphragm at end inspiration for each breath during 3 min of Xe breathing. Observed changes in RD during Xe washin were used to determine regional V and Q. For 16 mm(3), Q displayed more variance than V: the coefficient of variance of Q (CV(Q)) = 1.58 +/- 0.23, the CV of V (CV(V)) = 0.46 +/- 0.07, and the ratio of CV(Q) to CV(V) = 3.5 +/- 1.1. CV(Q) (1.21 +/- 0.37) and the ratio of CV(Q) to CV(V) (2.4 +/- 1.2) were smaller at 1,000-mm(3) scale, but CV(V) (0.53 +/- 0.09) was not. V/Q distributions also displayed scale dependence: log SD of V and log SD of Q were 0.79 +/- 0.05 and 0.85 +/- 0.10 for 16-mm(3) and 0.69 +/- 0.20 and 0.67 +/- 0.10 for 1,000-mm(3) regions of lung, respectively. V and Q measurements made with CT and Xe also demonstrate vertically oriented and isogravitational heterogeneity, which are described using other methodologies. Sequential images acquired by CT during Xe breathing can be used to determine both regional V and Q noninvasively with high spatial resolution.

Isocapnic hyperventilation increases carbon monoxide elimination and oxygen delivery

Hyperventilation with mixtures of O2 and CO2 has long been known to enhance carbon monoxide (CO) elimination at low HbCO levels in animals and humans. The effect of this therapy on oxygen delivery (DO2) has not been studied. Isocapnic hyperventilation utilizing mechanical ventilation may decrease cardiac output and therefore decrease DO2 while increasing CO elimination. We studied the effects of isocapnic hyperventilation on five adult mechanically ventilated sheep exposed to multiple episodes of severe CO poisoning. Five ventilatory patterns were studied: baseline minute ventilation (RR. VT), twice (2. RR) and four times (4. RR) baseline respiratory rate, and twice (2. VT) and four times (4. VT) baseline tidal volume. The mean carboxyhemoglobin (HbCO) washout half-time (t1/2) was 14.3 +/- 1.6 min for RR. VT, decreasing to 9.5 +/- 0.9 min for 2. RR, 8.0 +/- 0.5 min for 2. VT, 6.2 +/- 0.5 min for 4. RR, and 5.2 +/- 0.5 min for 4. VT. DO2 was increased during hyperventilation compared with baseline ventilation for 2. VT, 4. RR, and 4. VT ventilatory patterns. Isocapnic hyperventilation, in our animal model, did not alter arterial or pulmonary blood pressures, arterial pH, or cardiac output. Isocapnic hyperventilation is a promising therapy for CO poisoning.

Local blockade of allergic airway hyperreactivity and inflammation by the poxvirus-derived pan-CC-chemokine inhibitor vCCI

Allergen-induced asthma is characterized by chronic pulmonary inflammation, reversible bronchoconstriction, and airway hyperreactivity to provocative stimuli. Multiple CC-chemokines, which are produced by pulmonary tissue in response to local allergen challenge of asthmatic patients or experimentally sensitized rodents, chemoattract leukocytes from the circulation into the lung parenchyma and airway, and may also modify nonchemotactic function. To determine the therapeutic potential of local intrapulmonary CC-chemokine blockade to modify asthma, a recombinant poxvirus-derived viral CC-chemokine inhibitor protein (vCCI), which binds with high affinity to rodent and human CC-chemokines in vitro and neutralizes their biological activity, was administered by the intranasal route. Administration of vCCI to the respiratory tract resulted in dramatically improved pulmonary physiological function and decreased inflammation of the airway and the lung parenchyma. In contrast, vCCI had no significant effect on the circulating levels of total or allergen-specific IgE, allergen-specific cytokine production by peripheral lymph node T cells, or peritoneal inflammation after local allergen challenge, indicating that vCCI did not alter systemic Ag-specific immunity or chemoattraction at extrapulmonary sites. Together, these findings emphasize the importance of intrapulmonary CC-chemokines in the pathogenesis of asthma, and the therapeutic potential of generic and local CC-chemokine blockade for this and other chronic diseases in which CC-chemokines are locally produced.

Selected contribution: redistribution of pulmonary perfusion during weightlessness and increased gravity

To compare the relative contributions of gravity and vascular structure to the distribution of pulmonary blood flow, we flew with pigs on the National Aeronautics and Space Administration KC-135 aircraft. A series of parabolas created alternating weightlessness and 1.8-G conditions. Fluorescent microspheres of varying colors were injected into the pulmonary circulation to mark regional blood flow during different postural and gravitational conditions. The lungs were subsequently removed, air dried, and sectioned into approximately 2 cm(3) pieces. Flow to each piece was determined for the different conditions. Perfusion heterogeneity did not change significantly during weightlessness compared with normal and increased gravitational forces. Regional blood flow to each lung piece changed little despite alterations in posture and gravitational forces. With the use of multiple stepwise linear regression, the contributions of gravity and vascular structure to regional perfusion were separated. We conclude that both gravity and the geometry of the pulmonary vascular tree influence regional pulmonary blood flow. However, the structure of the vascular tree is the primary determinant of regional perfusion in these animals.

Hemodynamic effects of 15-microm-diameter microspheres on the rat pulmonary circulation

The microsphere method has been used extensively to measure regional blood flow in large laboratory animals. A fundamental premise of the method is that microspheres do not alter regional flow or vascular tone. Whereas this assumption is accepted in large animals, it may not be valid in the pulmonary circulation of smaller animals. Three studies were performed to determine the hemodynamic effects of microspheres on the rat pulmonary circulation. Increasing numbers of 15-microm-diameter microspheres were injected into a fully dilated, isolated-lung preparation. Vascular resistance increased 0.8% for every 100,000 microspheres injected. Microspheres were also injected into an isolated-lung preparation in which vascular tone was increased with hypoxia. Microspheres did not induce vasodilatation, as reported in other vascular beds. Fluorescent microspheres were injected via tail veins into awake rats, and the spatial locations of the microspheres were determined. Regional distributions remained highly correlated when microspheres of one color were injected after microspheres of another color. This indicates that the initial injection did not alter regional perfusion. We conclude that, when used in appropriate numbers, 15-microm-diameter microspheres do not alter regional flow or vascular tone in the rat pulmonary circulation.

Effect of zonal conditions and posture on pulmonary blood flow distribution to subpleural and interior lung

Observations made on vessels seen directly beneath the pleura may not accurately reflect what occurs in vessels located deeper in the interior of the lung. We quantified flow to subpleural and deeper, interior regions under zone 1 or 2 conditions in excised (n = 5) and in vivo (n = 6) rabbit lungs, in the head-up or inverted position. After infusion of radiolabeled microspheres, lungs were dried at alveolar pressure of 25 cmH(2)O and sliced in 1-cm sections along the gravitational plane and in three planes in the dorsal-ventral axis. Regions located <1 mm from the pleural surface were dissected away from the remaining tissue. In both zonal conditions, 1) weight-normalized flow to the interior exceeded that found in subpleural regions; and 2) flow followed the gravitational gradient, with the correlation varying with the scale of measurement. We conclude that flow through subpleural vessels is less than that which occurs deeper in the interior, but the regional distributions of flow and the effects of zonal conditions are similar in the two regions.

The role of the inhibitory nonadrenergic noncholinergic system in antigen-induced pulmonary hypersensitivity

To study the role of the inhibitory nonadrenergic noncholinergic (i-NANC) system in regulating bronchial reactivity during antigen challenge, we first tested a blocker of the i-NANC system (oxyhemoglobin, HbO2, 2.5 microm) on the relaxation response of guinea pig tracheal strips (n=6) in vitro to electrical field stimulation (ES) in the presence of atropine (1 microg/ml) and propranolol (2 microg/ml). Fresh HbO2 significantly inhibited 35.3+/-4.5% (P<0.001) of the NANC relaxation response. Secondary, 26 anesthetized, ovalbumin-sensitized animals were divided into three groups: antigen challenged (n=10), pretreated with HbO2 (13 mg/kg) and challenged (n=9), and treated with HbO2 only (n=7). Pulmonary resistance (RL) and dynamic compliance (Cdyn) were measured 15-20 min prior to (baseline) and up to 30 min after antigen or HbO2 injection. Antigen challenge alone induced early maximal respiratory changes: RL increased 1646+/-115% above baseline (2 min) whereas Cdyn decreased 42+/-10% below baseline (4 min). These changes returned to baseline within 15 min. Pretreatment with HbO2 increased peak respiratory responses induced by antigen [RL, 3728+/-1680% above baseline; Cdyn, 69+/-7% below baseline (P<0.05)]. HbO2 delayed significantly (P<0.05) the time for recovery of RL and Cdyn. HbO2 alone had little effect on respiratory parameters. We conclude that HbO2 may antagonize the i-NANC system in the airway and this antagonism may accentuate pulmonary hypersensitivity during acute antigen challenge.

Blockade of CD49d (alpha4 integrin) on intrapulmonary but not circulating leukocytes inhibits airway inflammation and hyperresponsiveness in a mouse model of asthma

Immunized mice after inhalation of specific antigen have the following characteristic features of human asthma: airway eosinophilia, mucus and Th2 cytokine release, and hyperresponsiveness to methacholine. A model of late-phase allergic pulmonary inflammation in ovalbumin-sensitized mice was used to address the role of the alpha4 integrin (CD49d) in mediating the airway inflammation and hyperresponsiveness. Local, intrapulmonary blockade of CD49d by intranasal administration of CD49d mAb inhibited all signs of lung inflammation, IL-4 and IL-5 release, and hyperresponsiveness to methacholine. In contrast, CD49d blockade on circulating leukocytes by intraperitoneal CD49d mAb treatment only prevented the airway eosinophilia. In this asthma model, a CD49d-positive intrapulmonary leukocyte distinct from the eosinophil is the key effector cell of allergen-induced pulmonary inflammation and hyperresponsiveness.

Influence of the route of allergen administration and genetic background on the murine allergic pulmonary response

We used various ovalbumin sensitization and challenge protocols to determine the importance of the route of allergen administration and the genetic background in modulating the physiologic, inflammatory, and immunologic features characteristic of allergen-induced asthma. In BALB/c mice, induction of maximal airway hyperresponsiveness and airspace eosinophilia required administration of ovalbumin by both the intraperitoneal and the intranasal routes (combination protocol), whereas intraperitoneal immunization alone resulted in maximal ovalbumin-specific IgE plasma levels. Thus, a systemic immune response to allergen, in addition to, or independent of IgE production, as well as local allergen challenge were necessary for maximal induction of pulmonary disease. BALB/c mice treated with ovalbumin by the combination protocol had increased Th2-type cytokine mRNA levels in bronchial lymph node tissue compared with control mice. In contrast, C57BL/6 mice treated with ovalbumin by the combination protocol had significantly decreased responses compared with BALB/c mice for all parameters of allergic pulmonary disease examined, with the exception of airspace eosinophilia. Genetic background has a striking and selective effect on the phenotype of murine allergic pulmonary disease. Further analysis of this murine model should be useful in helping define the critical pathogenetic events in allergen-induced asthma.

Comparison of five measures derived from in vivo pulmonary vascular pressure-flow curves

In order to facilitate evaluation of acute changes in lung vascular characteristics in vivo, pressure-flow (delta P-Q) curves of the pulmonary circulation were obtained by step-wise flow reduction. A balloon-tipped Swan-Ganz catheter was inserted via the jugular vein into the inferior vena cava of anesthetized, open chest, ventilated rabbits. Pulmonary arterial (Ppa) and left atrial (Pla) pressures were measured via catheters, and cardiac output Q by an electromagnetic flow probe on the aorta. Inflation of the balloon for 10 s reduced Q by 30-70%. delta P-Q curves were constructed by plotting a series of values of Q against corresponding delta P (= Ppa-Pla). To evaluate the feasibility and sensitivity of the method, these curves were compared under three conditions each paired with control (normoxia): hypoxia (8% O2), isoproterenol infusion, and serotonin infusion. Within our measured flow ranges, most delta P-Q plots were fairly linear, and extrapolated to a positive delta P intercept. Comparing slope, intercept, resistance, delta P at fixed Q, and Q at fixed delta P, we found that the latter two provided the more sensitive index to differentiate vasomotor changes. Since delta P-Q curves generally miss the origin, calculated pulmonary vascular resistance must depend on Q. Therefore, using the shifts in entire delta P-Q curves to select common range of Q and/or delta P is in general more quantitatively reliable for defining altered vascular characteristics.

The importance of leukotrienes in airway inflammation in a mouse model of asthma

Inhalation of antigen in immunized mice induces an infiltration of eosinophils into the airways and increased bronchial hyperreactivity as are observed in human asthma. We employed a model of late-phase allergic pulmonary inflammation in mice to address the role of leukotrienes (LT) in mediating airway eosinophilia and hyperreactivity to methacholine. Allergen intranasal challenge in OVA-sensitized mice induced LTB4 and LTC4 release into the airspace, widespread mucus occlusion of the airways, leukocytic infiltration of the airway tissue and broncho-alveolar lavage fluid that was predominantly eosinophils, and bronchial hyperreactivity to methacholine. Specific inhibitors of 5-lipoxygenase and 5-lipoxygenase-activating protein (FLAP) blocked airway mucus release and infiltration by eosinophils indicating a key role for leukotrienes in these features of allergic pulmonary inflammation. The role of leukotrienes or eosinophils in mediating airway hyperresponsiveness to aeroallergen could not be established, however, in this murine model.

Effects of exogenous surfactant on lung pressure-volume characteristics during liquid ventilation

Total liquid ventilation (LV) lowers airway pressures and potentially reduces barotrauma in models of hyaline membrane disease. LV eliminates surface tension by eliminating the air-perfluorochemicals (PFC) interface but does not eliminate interfacial tension (IT) at the lung/PFC interface. We hypothesized that pretreatment with exogenous surfactant before LV would shift the overall pressure-volume (PV) curve to the left, compared with LV without surfactant. Sequential quasi-static PV curves were obtained in 10 excised lungs (saline, air, PFC), with one-half randomized to exogenous surfactant replacement before LV. Analysis revealed that maximal inflation pressures were reduced during LV compared with baseline air curves. Addition of exogenous surfactant to LV further reduced maximal inflation pressures. A novel approach was used to transform these PV curves to estimates of in situ IT-volume curves. Estimated maximal IT at 20 ml/kg in preterm lamb lungs on air inflation after surfactant was 51 mN/m, compared with 40 mN/m for LV alone and with 27 mN/m for the combination of surfactant and LV. We conclude that the IT-reducing properties of the PFC studied (perflubron) can be augmented through the use of exogenous surfactant.

Airway hyperreactivity is associated with specific leukocyte subset infiltration in a mouse model of allergic airway inflammation

Airway hyperreactivity is defined as an increased bronchoconstrictor response to physical, pharmacological, or other stimuli. Patients with asthma develop airway hyperreactivity as well as peribronchial inflammation. We employed an established schistosome soluble egg antigen (SEA)-induced murine model of allergic inflammation to examine the temporal relationship between airway hyperreactivity and leukocyte subset infiltration. Dose response curves of intravenous methacholine were used in mice to characterize airway reactivity at various time points after intranasal SEA rechallenge. Cellular infiltration into the airspace was assessed by bronchoalveolar lavage. Airway hyperreactivity increased as early as 1 h postchallenge. Peak hyperreactivity occurred at 8 h postchallenge. Subsequently, reactivity decreased at 24 h and fell to the level observed in controls by 48 h. Neutrophil influx correlated directly with the increase in airway reactivity, as neutrophils were observed as early as 1 h, peaked at 8 h, diminished by 24 h and were not detected at 48 h post-SEA challenge. In contrast, eosinophil infiltration was not observed until 24 h and peaked at 48 h post-SEA rechallenge when increases in airway reactivity were not detected. Airway resistance induced by methacholine correlated with neutrophil (r2 = 0.90) but not eosinophil (r2 = 0.1) infiltration. These results suggest that the airway hyperreactivity observed during allergic airway inflammation correlates with airways neutrophilia and weakly eosinophil accumulation.

Perfusion through vessels open in zone 1 contributes to gas exchange in rabbit lungs in situ

We previously found that up to 15% of the normal cardiac output can flow through lungs that are entirely in zone 1 and that the zone 1 pathway utilizes alveolar corner vessels. Because of the proximity of these vessels to alveoli, we hypothesized that lungs perfused under zone 1 conditions would exchange gas. We used the multiple inert gas elimination technique to assess the ventilation-perfusion (VA/Q) distribution under zones 1 and 2 in six rabbit lungs perfused with tris(hydroxymethyl)aminomethane-buffered Tyrode solution containing 1% albumin, 4% dextran, and papaverine (25 mg/l). High-frequency oscillation (tidal volume = 2.8 ml at 20 Hz, bias flow = 1 l/min) kept alveolar pressure (PA) nearly constant at 10 or 20 cmH2O. Pulmonary arterial pressure was set 2.5 cmH2O below or 5 cmH2O above PA (zones 1 and 2, respectively). Pulmonary venous pressure was kept at 0 cmH2O, with zero reference being the bottom of the lung. At PA of 10 cmH2O, flow was 64 +/- 40 and 5 +/- 3 ml/min (P < 0.05) and the mean VA/Q for perfusion was 1.1 +/- 0.4 and > 5 (P < 0.05) in zones 2 and 1, respectively. At PA of 20 cmH2O, flow was 89 +/- 36 and 22 +/- 13 ml/min (P < 0.05) and the mean VA/Q for perfusion was 0.8 +/- 0.3 and 3.7 +/- 2.4 (P < 0.05) in zones 2 and 1, respectively. Shunt averaged < 5% of total flow in all conditions. Blood flowing through vessels remaining open under zone 1 conditions 1) exchanges gas, 2) does not occur through anatomic or physiological shunts, and 3) may explain the high VA/Q seen with positive end-expiratory pressure.

Flow pulsatility does not increase mean microvascular pressure or filtration in zone 3 rabbit lungs

We previously reported that mean pulmonary arterial pressure (Ppa) during pulsatile flow exceeded that for steady flow when flow was greater than the normal resting value and speculated that this was due to irregularities of the flow profiles in precapillary vessels, mainly the larger arteries. From this we hypothesized that neither mean microvascular pressure nor the rate of fluid filtration would be affected by flow pulsatility. We therefore compared the effects of steady vs. pulsatile flow on the double-occlusion pressure (Pdo) and on edema formation (rate of weight gain) in zone 3 rabbit lungs. Excised left lungs (n = 19) were perfused with Tyrode solution and ventilated with an end-expiratory pressure of 2.5 cmH2O. A diaphragm pump generated pulsatile flow with a stroke volume of 1.0 ml (approximately 0.8 the normal resting value for rabbit left lung). Nonpulsatile flow was generated by raising an arterial reservoir. Flow rate was set at 100 or 400 ml/min (approximately 0.4 or 1.6 x the normal resting cardiac output, respectively). Vascular pressures (referenced to the bottom of the lung) were measured after ventilation, at end expiration, was interrupted. Pdo values were obtained in random order at 15 time points that were evenly distributed within the pulse cycle, averaged across pulses to obtain the mean capillary pressure profile, and then averaged over time. At the lower flow of 100 ml/min, mean Ppa and Pdo were slightly lower (3-4%) during pulsatile compared with nonpulsatile conditions. At the higher flow of 400 ml/min, mean Ppa was higher under pulsatile conditions (13%), whereas downstream the mean Pdo values were equal.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism by which the prone position improves oxygenation in acute lung injury

The mechanism by which oxygenation improves when patients with ARDS are turned from supine to prone position is not known. From results of our previous studies we reasoned that (1) when supine, in the setting of lung injury, transpulmonary pressure will be less than airway opening pressure and (2) atelectasis will develop preferentially in dorsal lung areas, and (3) both ventilation and ventilation/perfusion ratios would improve in these regions on turning prone. To study this directly, we measured regional ventilation and perfusion using 81mKr and 99mTc-MAA, respectively, and single photon emission computed tomography, both prone and supine, in four control animals and four given oleic acid. After oleic acid, the prone position improved (1) oxygenation (mean +/- SD PaO2 = 140 +/- 112 versus 453 +/- 54 mm Hg), (2) median ventilation/perfusion ratios (0.77 versus 0.95), (3) ventilation/perfusion heterogeneity (coefficient of variation 86 +/- 15 versus 61 +/- 6), and (4) the gravitational ventilation/perfusion gradient (dependent to non-dependent slopes of 0.22 versus -0.02, all p < 0.05). The prone position generates a transpulmonary pressure sufficient to exceed airway opening pressure in dorsal lung regions, i.e., in regions where atelectasis, shunt, and ventilation/perfusion heterogeneity are most severe, without adversely affecting ventral lung regions.

Pulsatile and nonpulsatile pressure-flow relationships in zone 3 excised rabbit lungs

We compared the effects of pulsatile vs. nonpulsatile flow (Q) on pulmonary arterial pressure (Ppa)-Q relationships in zone 3 over wide ranges of pulse rate, stroke volume (SV), and Q. Excised left lungs of rabbits (n = 15) were perfused with tris(hydroxymethyl)aminomethane-buffered Tyrode solution containing 4% dextran, 1% albumin, and 10 mg/l of indomethacin and were ventilated with room air. Pulsatile Q was generated by a diaphragm pump delivering SV of 0.5, 1, or 2 ml (representing approximately 0.3, 0.6, and 1.2 times, respectively, the normal resting SV for rabbit left lung) and adjusting the pump frequency. Nonpulsatile Q was generated by raising an arterial reservoir to the required height. Mean pulmonary arterial (Ppa) and left atrial pressures were measured at end exhalation (positive end-expiratory pressure = 2.5 cmH2O) near the tips of the perfusion cannulas and were referenced to the lung base. Left atrial pressure was held constant at 7 cmH2O.Q was alternated between pulsatile and nonpulsatile, increasing Q stepwise from 100 to 600 ml/min (Q from approximately 0.3 to 2 times the normal resting Q for rabbit left lung), after which Q was reduced stepwise back to initial values. For the smallest SV there were no differences between Ppa-Q curves under pulsatile and nonpulsatile conditions. At the largest SV, Ppa was greater during pulsatile than nonpulsatile Q at Q > 100 ml/min. The slopes of the Ppa-Q curves were greater during pulsatile Q at the two larger SV values. These results can be explained by increasing Q turbulence and less ideal velocity profiles at higher peak Q resulting from the effects of rapidly changing inertial forces.

Lung inflation distends small arteries (< 1 mm) in excised dog lungs

We utilized microfocal fluoroscopic angiography to study the influence of lung inflation on small (0.2- to 1.3-mm-diam) pulmonary arteries in isolated left lower lobes from dog lungs during both flow and no-flow conditions. Alveolar pressure, which in this preparation was equal to transpulmonary pressure, was set at 2, 8, or 14 mmHg while vascular pressure was varied from 0 to 24 mmHg. The diameters of these small arterial vessels increased with lung inflation. No differences were observed between the results obtained during flow and no-flow conditions. Thus, arteries in this diameter range can be considered as extra-alveolar, and the effect of lung inflation on these small extra-alveolar arteries was qualitatively similar to that previously described for larger extra-alveolar vessels. Quantitatively, the degree of vessel distension was about the same per unit increase in transpulmonary pressure at constant vascular pressure as for a change in vascular pressure at constant transpulmonary pressure. Accordingly, inflation produced a decrease in perivascular pressure surrounding these small arteries that was approximately equal to the increase in transpulmonary pressure.

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

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

Volume infusion produces abdominal distension, lung compression, and chest wall stiffening in pigs

The effect of severe generalized edema on respiratory system mechanics is not well described. We measured airway pressure, gastric pressure, and four vertical pleural pressures in 13 anesthetized paralyzed pigs ventilated in the upright position. Pressure-volume relationships of the respiratory system, chest wall, and lung were measured on deflation from total lung capacity to residual volume and during tidal breathing both before (control) and 50 min after one of two interventions. In one series of experiments, a volume equal to 15-20% of the pig's body weight was infused intravenously. In a second series, a balloon was placed in the peritoneal space to distend the abdomen to the same gastric pressures as achieved in the first series. Measurements were compared before and after either abdominal balloon inflation or volume infusion. Volume infusion increased the pleural pressure in dependent lung regions, decreased both total lung capacity (34%) and functional residual capacity (62%) (both P less than 0.05), and markedly shifted the respiratory system and chest wall pressure-volume curves to the right, but it only moderately affected the lung deflation curve. Tidal compliances of the respiratory system, chest wall, and lung decreased 36, 31, and 49%, respectively (all P less than 0.05). The effect of abdominal balloon inflation on respiratory system mechanics was similar to that of volume infusion. We conclude that infusing large volumes of fluid markedly alters chest wall mechanics, mainly by causing abdominal distension that prohibits descent of the diaphragm.

Gravity is a minor determinant of pulmonary blood flow distribution

Regional pulmonary blood flow in dogs under zone 3 conditions was measured in supine and prone postures to evaluate the linear gravitational model of perfusion distribution. Flow to regions of lung that were 1.9 cm3 in volume was determined by injection of radiolabeled microspheres in both postures. There was marked perfusion heterogeneity within isogravitational planes (coefficient of variation = 42.5%) as well as within gravitational planes (coefficient of variation = 44.2 and 39.2% in supine and prone postures, respectively; P = 0.02). On average, vertical height explained only 5.8 and 2.4% of the flow variability in the supine and prone postures, respectively. Whereas the gravitational model predicts that regional flows should be negatively correlated when measured in supine and prone postures, flows in the two postures were positively correlated, with an r2 of 0.708 +/- 0.050. Regional perfusion as a function of distance from the center of a lung explained 13.4 and 10.8% of the flow variability in the supine and prone postures, respectively. A linear combination of vertical height and radial distance from the centers of each lung provided a better-fitting model but still explained only 20.0 and 12.0% of the flow variability in the supine and prone postures, respectively. The entire lung was searched for a region of contiguous lung pieces (22.8 cm3) with high flow. Such a region was found in the dorsal area of the lower lobes in six of seven animals, and flow to this region was independent of posture. Under zone 3 conditions, neither gravity nor radial location is the principal determinant of regional perfusion distribution in supine and prone dogs.

Abdominal distension alters regional pleural pressures and chest wall mechanics in pigs in vivo

Abdominal distension (AD) occurs in pregnancy and is also commonly seen in patients with ascites from various causes. Because the abdomen forms part of the "chest wall," the purpose of this study was to clarify the effects of AD on ventilatory mechanics. Airway pressure, four (vertical) regional pleural pressures, and abdominal pressure were measured in five anesthetized, paralyzed, and ventilated upright pigs. The effects of AD on the lung and chest wall were studied by inflating a liquid-filled balloon placed in the abdominal cavity. Respiratory system, chest wall, and lung pressure-volume (PV) relationships were measured on deflation from total lung capacity to residual volume, as well as in the tidal breathing range, before and 15 min after abdominal pressure was raised. Increasing abdominal pressure from 3 to 15 cmH2O decreased total lung capacity and functional residual capacity by approximately 40% and shifted the respiratory system and chest wall PV curves downward and to the right. Much smaller downward shifts in lung deflation curves were seen, with no change in the transdiaphragmatic PV relationship. All regional pleural pressures increased (became less negative) and, in the dependent region, approached 0 cmH2O at functional residual capacity. Tidal compliances of the respiratory system, chest wall, and lung were decreased 43, 42, and 48%, respectively. AD markedly alters respiratory system mechanics primarily by "stiffening" the diaphragm/abdomen part of the chest wall and secondarily by restricting lung expansion, thus shifting the lung PV curve as seen after chest strapping. The less negative pleural pressures in the dependent lung regions suggest that nonuniformities of ventilation could also be accentuated and gas exchange impaired by AD.

Flow through zone 1 lungs utilizes alveolar corner vessels

We have previously observed flows equivalent to 15% of the resting cardiac output of rabbits occurring through isolated lungs that were completely in zone 1. To distinguish between alveolar corner vessels and alveolar septal vessels as a possible zone 1 pathway, we made in vivo microscopic observations of the subpleural alveolar capillaries in five anesthetized dogs. Videomicroscopic recordings were made via a transparent thoracic window with the animal in the right lateral position. From recordings of the uppermost surface of the left lung, alveolar septal and corner vessels were classified depending on whether they were located within or between alveoli, respectively. Observations were made with various levels of positive end-expiratory pressure (PEEP) applied only to the left lung via a double-lumen endotracheal tube. Consistent with convention, flow through septal vessels stopped when PEEP was raised to the mean pulmonary arterial pressure (the zone 1-zone 2 border). However, flow through alveolar corner vessels continued until PEEP was 8-16 cmH2O greater than mean pulmonary arterial pressure (8-16 cm into zone 1). These direct observations support the idea that alveolar corner vessels rather than patent septal vessels provide the pathway for blood flow under zone 1 conditions.

Surfactant replacement improves lung recoil in rabbit lungs after acid aspiration

We tested the hypothesis that surfactant replacement would be beneficial in the acid-aspiration model of acute lung injury. HCl (0.1 N, 2 ml/kg) was injected into the trachea of excised rabbit lungs (n = 8). Control lungs (n = 4) had no intervention. All were perfused with Tyrode's solution mixed 1:1 with autologous whole blood at 40 ml/min/kg for 30 min, and then degassed. A modified natural surfactant (Survanta, Ross Laboratories) was then injected into the trachea of four lungs injured with HCl (100 mg/kg at 25 mg/ml). Two quasi-static pressure-volume curves were determined. The mean alveolar pressures at 50, 60, 70, 80, and 90% of TLC were greater in the HCl group than in the control group (p less than 0.05). However, no difference was observed between the control lungs and those that received HCl + Survanta. In 13 anesthetized, paralyzed, and ventilated rabbits, deflation pressure-volume curves were determined from TLC to FRC (measured by helium dilution). Then, 0.1 N HCl (3 ml/kg) was injected into the trachea and, in seven, Survanta was instilled 5 min later. The mean alveolar pressures at 60, 70, 80, and 90% TLC were higher at 15 and 60 min in the HCl group compared with their pre-HCl time point (p less than 0.05). In the HCl + Survanta group, no differences were seen at 15 min, and only slight increased were seen at 60 min. No effect of surfactant replacement on arterial blood gases was observed. HCl aspiration increased recoil in both excised and in vivo lungs, and surfactant replacement with Survanta returned recoil to normal.

Flow characteristics of open vessels in zone 1 rabbit lungs

To describe the flow characteristics of vessels open in zone 1, we perfused isolated rabbit lungs with Tyrode's solution containing 1% albumin, 4% dextran, and papaverine (0.05 mg/ml). Lungs were expanded by negative pleural pressure (Ppl) of -10, -15, -20, and -25 cmH2O. Pulmonary arterial (Ppa) and venous (Ppv) pressures were varied relative to alveolar pressure (PA = 0) and measured 5-10 mm inside the pleura (i) and outside (o) of the lungs. With Ppa(o) at -2.5 cmH2O, we constructed pressure-flow (P-Q) curves at each Ppl by lowering Ppv(o) until Q reached a maximum, indicating fully developed zone 1 choke flow. Maximum flows were negligible until Ppl fell below -10 cmH2O, then increased rapidly at Ppl of -15 and -20 cmH2O, and at Ppl of -25 cmH2O reached about 15 ml.min-1.kg body wt-1. The Ppv(o) at which flow became nearly constant depended on degree of lung inflation and was 5-8 cmH2O more positive than Ppl. As Ppv(o) was lowered below Ppa(o), Ppv(i) remained equal to Ppv(o) until Ppv(i) became fixed at a pressure 2-3 cmH2O more positive than Ppl. At this point the choke flow was therefore located in veins near the pleural boundary. No evidence of choke flow (only ohmic resistance) was seen in the intrapulmonary segment of the vessels remaining open in zone 1. With Ppv(o) held roughly at Ppl, Q could be stopped by lowering Ppa(o), at which time Ppa(i) was several cmH2O above Ppv(i), showing that intrapulmonary vessel closure had occurred.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of leukotrienes B4, C4, and D4 on segmental pulmonary vascular pressures

Leukotrienes (LTs) C4 and D4 are vasoconstrictors and are thought to increase both systemic and pulmonary vascular permeability. However, we and others have observed that LTC4 and LTD4 cause pulmonary vasoconstriction but do not increase the fluid filtration coefficient of excised guinea pig lungs perfused with a cell-depleted perfusate. To determine what vascular segments were exposed to an LT-induced increase in intravascular hydrostatic pressure we measured pulmonary arterial (Ppa), pulmonary arterial occlusion (Po,a), venous (Po,v) and double occlusion (Pdo) pressures in isolated guinea pig lungs perfused with a cell-depleted buffered salt solution before and after injecting 4 micrograms of LTB4, LTC4, or LTD4 into the pulmonary artery. All three LTs increased airway pressures and also increased Ppa, Po,a, and Pdo. Histamine (15 micrograms) as well as serotonin (20 or 200 micrograms) had the same effect. In excised rabbit lungs, histamine and serotonin increased only Ppa, and Po,a. LTC4 had no vasoactivity. There are marked species variations with regard to the activity and site of action of histamine, serotonin, and LTC4 on the pulmonary circulation.

Sites of leakage in three models of acute lung injury

Fluid leaking from arterial and venous extra-alveolar vessels (EAV's) may account for up to 60% of the total transvascular fluid flux when edema occurs in the setting of normal vascular permeability. We determined if the permeability and relative contribution of EAV's was altered after inducing acute lung injury in rabbits by administering oleic acid (0.1 ml/kg) into the pulmonary artery, HCl (5 ml/kg of 0.1 N) into the trachea, or air emboli (0.03 ml.kg-1.min-1) into the right atrium for 90 min. Subsequently, the lungs were excised and continuously weighed while they were maintained in a warmed, humidified chamber with alveolar and pulmonary vascular pressures controlled and the lungs either ventilated or distended with 5% CO2 in air. The vascular system was filled with autologous blood and saline (1:1) to which papaverine (0.1 mg/ml) was added to inhibit vasospasm. Vascular pressures were referenced to the lung base. After a transient hydrostatic stress to maximize recruitment, vascular pressures were set at 5 cmH2O, and lungs were allowed to become isogravimetric (30-60 min). A fluid filtration coefficient (Kf) was determined by the use of a modification of the method of Drake and colleagues [Am. J. Physiol. 234 (Heart Circ. Physiol. 3): H266-H274, 1978]. EAV's were isolated by zoning techniques. In control preparations arterial and venous EAV's accounted for 26% (n = 9) and 38% (n = 11) of the total leakage, respectively. In all three models Kf increased two- to fourfold when the lungs were in zone 3 (alveolar vessels and arterial and venous EAV's contributing to the leakage).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of lung inflation on fluid flux in zone 1 lungs

Because of conflicting data in the literature, we studied the effect of positive-pressure inflation on transvascular fluid filtration in zone 1 lungs. Lungs from New Zealand White rabbits (n = 10) were excised, perfused with saline and autologous whole blood (1:1), ventilated, and continuously weighed. Pulmonary arterial and venous pressures (Pvas) were referenced to the most dependent part of the lung. A change in vascular volume (delta Vvas) and a fluid filtration rate (FFR) were calculated from the change in lung weight that occurred from 0 to 30 s and from 3 to 5 and 5 to 10 min, respectively, after changing alveolar pressure (PA). FFR's and delta Vvas's were measured with Pvas equal to 2 or 10 cmH2O and PA changing from 15 to 30 cmH2O when the lungs were normal and after they were made edematous. When Pvas = 2 cmH2O, increasing PA increased the Vvas and the FFR in both normal and edematous lungs. However, when Pvas = 10 cmH2O, increasing PA only slightly changed the Vvas and reduced the FFR in the normal lungs, and decreased Vvas and markedly decreased the FFR in the presence of edema. Inflating zone 1 lungs by positive pressure has an effect on transvascular fluid flux that depends on the Pvas. The results suggest that the sites of leakage in zone 1 also vary depending on Pvas and PA.

Continuity of arterial and venous extra-alveolar interstitium in excised rabbit lungs

We studied the interdependence of arterial and venous extra-alveolar vessel (EAV) leakage on the rate of pulmonary vascular fluid filtration (measured as the change in lung weight over time). Edema was produced in continually weighed, excised rabbit lungs kept in zone 1 (alveolar pressure = 25 cmH2O) by increasing pulmonary arterial (Ppa) and/or venous (Ppv) pressure from 5 to 20 cmH2O (relative to the lung base) and continuing this hydrostatic stress for 3-5 h. Raising Ppa and Ppv simultaneously produced a lower filtration rate than the sum of the filtration rates obtained when Ppa and Ppv were raised separately, while the lung gained from 20 to 95% of its initial weight. When vascular pressure was elevated in either EAV segment, fluid filtration always decreased rapidly as the lung gained up to 30-45% of its initial weight. Filtration then decreased more slowly. The lungs became isogravimetric at 60 and 85% weight gain when the Ppa or Ppv was elevated, respectively; when Ppa and Ppv were raised simultaneously substantial fluid filtration continued even after 140% weight gain. We conclude that the arterial and venous EAV's share a common interstitium in the zone 1 condition, this interstitium cannot be represented as a single compartment with a fixed resistance and compliance, and arterial and venous EAV leakage influences leakage from the other segment.

Factors affecting massive postmortem bronchoconstriction in guinea pig lungs

To examine endogenous factors affecting the development of the massive bronchoconstriction in the postmortem guinea pig lung, 58 anesthetized open-chest animals were divided into three groups: 1) exsanguination only (n = 13), 2) pulmonary perfusion with 5% dextran and 1% bovine serum albumin (BSA) in Tyrode's solution (Ca2+ perfusate) (n = 21), and 3) pulmonary perfusion with 5% dextran and 1% BSA in saline (Ca2+-free perfusate) (n = 24). These groups were further divided into several subgroups according to treatments: 1) substance P depletion by chronic administration of capsaicin, 2) acute capsaicin treatment to release substance P, 3) dazoxiben treatment to block endogenous synthesis of thromboxane A2, 4) diethylcarbamazine treatment to eliminate leukotriene (LT) synthesis, and 5) FPL 55712 treatment to antagonize actions of LT. Vital capacity from the deflation pressure-volume (PV) curve of the lung was used as the indicator of bronchoconstriction. Most PV curves were performed for 30 min following exsanguination or artificial perfusion. Ca2+-free perfusate enhanced the airway spasm at 5-10 min, but the spasm disappeared gradually after 10 min. Substance P depletion significantly decreased (P less than 0.01) the bronchial constriction at 20-30 min, whereas substance P release induced severe airway spasm (P less than 0.01) during the entire study. In addition, FPL 55712 reduced the bronchospasm (P less than 0.05) in Ca2+ perfusate at 30 min. Thus Ca2+ and several endogenous mediators may be involved with the airway spasm of the postmortem guinea pig lung.

Histamine and leukotrienes mediate pulmonary hypersensitivity to antigen in guinea pigs

To study roles of histamine and slow-reacting substance of anaphylasis (SRS-A) in mediating airway responses following antigen challenge, mediator antagonists were administered to guinea pigs sensitized with ovalbumin 10 days before the study. Twenty-three animals were divided into the following five treatment groups: 1) saline only (control 1, n = 5); 2) antigen challenged (n = 5); 3) antigen + methapyrilene (antihistamine, n = 5); 4) FPL 55712 only (SRS-A antagonist, control 2; n = 4), and 5) antigen + FPL 55712 (n = 4). Control groups were not sensitized. Experimental values were compared with those of control 1 at equal times after injections. Pulmonary resistance (RL), dynamic compliance (Cdyn), breathing frequency (f), tidal volume, minute ventilation (VE) and systemic arterial pressure were measured for 15-20 min just before (base line) and for up to 30 min after saline or antigen administration. Antigen challenge alone induced maximal respiratory changes at 5 min. RL increased 131 +/- 28% above base line (P less than 0.05), whereas Cdyn decreased slightly (28 +/- 10%, P less than 0.05). Antihistamine almost eliminated all changes in RL but did not affect decreased Cdyn. On the other hand, FPL 55712 eliminated changes in both RL and Cdyn. Both antagonists blocked the transient increase in VE, but neither blocked the rise in f at 5 min. We conclude that antigen-induced bronchoconstriction (RL) may be primarily mediated by histamine, whereas simultaneous alterations in Cdyn may depend mainly on leukotrienes and those in f depend on neither.

Gas trapping in excised rabbit lungs depends on volume history and method of degassing

Trapped gas volume (Vtg) was obtained after 5 and 10 repeated inflation-deflation cycles between transpulmonary pressure (Ptp) = 0 and 30 cmH2O in 12 experimental groups of freshly excised rabbit lungs. Gas flow rate was 1.0 ml/s except in one group (0.4 ml/s). In lungs degassed by O2 absorption (Dabs), Vtg increased from an initial 12-15% total lung capacity (TLC) (1st cycle) to 40% TLC (10th cycle), whereas in vacuum-degassed lungs (Dvac) the final Vtg was almost unchanged, remaining at less than 20% TLC. However, with the slower flow rate, Vtg in Dvac became 60% TLC. Increased lung water was not found in Dabs and therefore could not account for the above difference. In lungs not degassed after excision, Vtg increased roughly in proportion to the duration of passive collapse at Ptp = 0. However, a single brief exposure to a negative airway pressure (Pao = -10 cmH2O) resulted in a greater rate of increase of Vtg than 15-min collapse. When any of the foregoing groups were vacuum degassed after 5 cycles, they then resembled the Dvac group and showed almost no increase of Vtg in successive cycles. In Dvac, negative Pao and 15-min collapse had only minor effects on increasing Vtg. Thus, at a flow rate of 1 ml/s vacuum degassing almost eliminated all tendencies to trap gas in rabbit lungs, but the tendency was more than restored at slower flows. Brief airway closure by negative tracheal pressure can markedly enhance subsequent trapping of collapsed lungs. Differences arising from degassing methods might be due to effects on bronchomotor tone or on the physical characteristics of airway lining.

Massive postmortem bronchoconstriction in guinea pig lungs

A special phenomenon (difficult to inflate and deflate) occurring in the postmortem guinea pig lungs was studied in 40 animals. Thirty minutes after excision of the lungs or exsanguination, less than 50% of the lungs could be inflated even at high inflation pressure (34 cmH2O), and most gas was trapped during deflation. The amount of trapped gas volume at 30 min was related to the degree of lung inflation maintained during the 5- to 30-min period after exsanguination. Since stiffness of the lung tissue was unlikely to explain the phenomenon, we speculated airway obstruction as the major factor. No foam or bubbles were found in larger airways and we thus hypothesized that the obstruction was due to bronchoconstriction. This was confirmed histologically in that the lumina of both bronchi and bronchioles were constricted. The latent period to the onset of this constriction was short (approximately 5 min). It was not associated with O2 availability but was delayed an additional 15 min by a thromboxane inhibitor (dazoxiben). Neither maintaining lung temperature at 37 degrees C nor vagotomy and/or cervical transection prevented the constriction. Without exsanguination, onset of bronchoconstriction was delayed by about 1 h. We conclude that postmortem bronchoconstriction may be caused by release of an endogenous constrictor agent.

Sampling and analysis of cerebrospinal fluid for chronic studies in awake rats

A method is described whereby cerebrospinal fluid (CSF) may be sampled repeatedly from awake rats over a period of 1 to 10 wk. Stainless steel or Teflon catheters (22-gauge) were implanted in the cisterna magna of anesthetized rats (n = 19) via a midline hole immediately rostral to the interparietal-occipital suture. Several days later almost simultaneous CSF and blood samples from the same air-breathing animals were slowly drawn into glass capillary tubes. pH was determined immediately by electrode and total CO2 by microgasometer. Because sampling via Tygon microbore tubing attached to the implanted catheters allowed part of CSF CO2 content to be lost through the tube wall, stainless steel tube is preferred to make this connection. For mock CSF, total CO2 calculated from pH and PCO2 values was closely comparable to that directly measured by microgasometer. CSF pH, PCO2, and [HCO-3] in five awake rats were found to be similar to those reported in the literature for the briefly anesthetized rat. The sampling procedure did not of itself significantly alter breathing patterns (n = 14). Thus, chronic CSF sampling is feasible in awake rats for purposes of studies of ventilatory control.

In vitro buffer value of brain tissue during prolonged hypercapnia

Ventilatory adaptation to CO2 has been related to a return toward normal pH via an increase in CSF and arterial [HCO3(-)]. To examine whether the overall brain tissue can contribute HCO3(-) to surrounding fluids, we measured the in vitro CO2 buffer value (beta CO2) of control and hypercapnic rat brain homogenates and compared values with reported in vivo data. Hypercapnic rats were exposed to 7% CO2 for 3 days or 1 week. Brain homogenate was continuously tonometered for 3 h at 37 degrees C with 2%, 5% and 15% CO2 in O2. In addition, we used KOH to determine the brain buffering in the pH range 6.8-10.25. During CO2 titration, [HCO3(-)] increased gradually with time up to 90 min by about 10-15%, but the increase was blocked by a metabolic inhibitor, NaF, beta CO2, estimated per kg brain tissue from the dilute homogenate, ranged between 23.4 +/- 2.7 (SD) in controls and 26.0 +/- 1.3 meq/pH in the 7 day group, which were not significantly different. Over the same 7 days, CO2 dissociation curves were shifted upwards with similar slopes by about 6 ml/100 g tissue in association with a rise in pH of about 0.06, consistent with an accumulation of HCO3(-) without any change in buffers. No significant differences between groups were found from KOH titration curves, either in slope or position, consistent with lack of alteration in buffers as well. In vitro brain tissue beta CO2 (about 25) was less than reported in vivo values in the literature (around 40), possibly because H+ adjustments by whole body occur so rapidly in vivo. In addition, other investigators demonstrated that a major part of the increased brain cell [HCO3(-)] in prolonged hypercapnia could not be accounted for by the fixed acid production (Acta Physiol. Scand. 83: 344, 1971). By assuming the in vitro beta CO2 measures the available non-carbonic buffers, the data may be interpreted as showing that the overall brain tissue accumulates HCO3(-) from surrounding fluid during hypercapnia.

End-expiratory volume in the rat: effects of consciousness and body position

Functional residual capacity (FRC), tidal volume (VT), and frequency (f) were compared in 23 rats while either awake and unrestrained or anesthetized. FRC was determined from gas compression with closed airway inside a cone-shaped body plethysmograph. In the awake state (mean +/- SD), FRC was 1.02 +/- 0.22 ml/100 g, VT was 0.38 +/- 0.06 ml/100 g, and f was 142 +/- 22 breaths/min. During anesthesia, FRC decreased (P less than 0.01) to 52.9% of awake values, VT increased (P less than 0.01) to 147.4%, and f decreased (P less than 0.01) to 71.8%, leaving minute ventilation almost unchanged. An additional seven rats were used to examine postural effects on FRC during anesthesia, and in another seven animals pleural pressure changes were monitored. Dynamic lung compliance (0.80 ml . kg-1 X cmH2O-1) was not altered by anesthesia, but the pressure-volume curve was shifted 6 cmH2O higher. Thoracic compression, followed by a time-dependent effect of volume history, may account for the major change in FRC. The remainder of the decrease in FRC may be due to lower breathing frequency, loss of inspiratory muscle activity, and/or less airway resistance after anesthesia. Peak diaphragmatic electromyogram per unit VT was shown to increase almost linearly with FRC, indicating that diaphragmatic efficiency was decreased as lung volume was elevated.

Mechanical properties of rat lung during prolonged hypercapnia

Young adult rats were exposed to short-term (2 or 4 h) and prolonged (1 day to 3 wk) 5% CO2 to determine whether changes in lung mechanical properties might contribute to ventilatory adaptation. Anaesthetized rats were tracheotomized for measurement of functional residual capacity (FRC) and respiratory rates (chest intact), and for pressure-volume (PV) curves (chest open). Total lung capacity, minimal volume, lung compliance, and lung wet weight-to-dry weight ratio did not change significantly (P greater than 0.05) from air control values with CO2 exposure. From 4 h to 3 wk of hypercapnia, FRC was increased significantly (P less than 0.01) by about 25%; some of this rise could be associated with dynamic factors such as increased breathing frequency. Lung PV curves showed a small (0.5 cmH2O) left shift early in the short exposure period; this reversed into a slight (0.5 cmH2O) right shift after prolonged CO2. It is unlikely that these relatively small changes in FRC and lung recoil could contribute significantly to the previously reported long-term ventilatory adaptation to CO2.