PERICAPILLARY GAS AND WATER DISTRIBUTION VOLUMES OF THE LUNG CALCULATED FROM MULTIPLE INDICATOR DILUTION CURVES

Preoperative estimation of pulmonary extravascular thermal volume in patients undergoing pneumonectomy

Pulmonary extravascular thermal volume (PETV) was measured during pulmonary artery occlusion in 18 patients preoperatively and 7 patients postoperatively who were undergoing pneumonectomy. We found that the PETV decreased from 6.6±2.3 ml·kg^-1 before occlusion to 4.1±1.6 ml·kg^-1 during occlusion. There was a significant correlation between the PETVs before and during occlusion multiplied by the fraction of pulmonary perfusion (r=0.77,P<0.001). Although the PETV increased in two patients and decreased in four within 48 h after pneumonectomy, it returned to the value during occlusion at 3 weeks after pneumonectomy in seven patients. There was a significant correlation between the PETV during occlusion and that at 3 weeks after pneumonectomy (r=0.66,P<0.05). In conclusion, PETV during pulmonary artery occlusion is a reliable baseline value in the assessment of postoperative pneumonectomy values.

Correlation of oxygenation with vascular permeability-surface area but not with lung water in humans with acute respiratory failure and pulmonary edema

We used a single-pass multiple tracer technique to measure cardiac output, extravascular lung water (EVLW) and lung vascular [14C]urea permeability-surface area (PSu) in 14 patients with acute respiratory failure and pulmonary edema. All patients had increased EVLW, but EVLW in the 10 surviving patients (0.26 +/- 0.06 SE ml/ml total lung capacity [TLC]) was not significantly different from that in the five patients who died (0.22 +/- 0.05). EVLW did not correlate with intravascular pressures or with alveolar-arterial oxygen pressure difference (A-aDO2). PSu was lower in surviving patients (0.50 +/- 0.16 SE ml/s X liter TLC) than in patients who died (3.44 +/- 0.36; P less than 0.05) and also lower than in previously reported data in patients with normal PSu. PSu correlated significantly with A-aDO2. Serial studies showed that PSu returned from a low value toward normal in a patient who survived but remained high in a patient who died. We conclude that the amount of edema in the lungs measured by indicator methods was not the principal determinant of either the magnitude of oxygenation defect or survival in the patients studied. We interpret the low PSu in surviving patients as decreased surface area and infer that the ability of the lung circulation to reduce perfusion of damaged and edematous areas was important in preserving oxygenation. A high PSu, presumably reflecting perfusion of areas with increased permeability, was a sign of especially poor prognosis. Multiple tracer techniques for measuring lung vascular PSu may help to define the pathogenesis and to evaluate therapies of acute lung injury in humans. Such measurements may be a more useful clinical tool than measurements of lung water in patients with acute respiratory failure and pulmonary edema.

Regional extravascular lung density and fractional pulmonary blood volume in patients with chronic pulmonary venous hypertension

Using a technique based on positron transmission and emission tomography, we measured regional extravascular lung density (lung tissue and interstitial water per unit thoracic volume) and fractional pulmonary blood volume (volume of blood per unit thoracic volume) in normal subjects and in patients with cardiomyopathy and chronic pulmonary venous hypertension. We found an increase in extravascular lung density in the patients. Extravascular density was increased in all parts of the lung studied, but higher values were seen in the dorsocaudal portion than in the ventrocaudal portion of the lung (average 151% and 130% of normal, respectively). Fractional blood volume was markedly reduced in the dorsocaudal part of the lung (mean 74% of normal). The increase in extravascular density probably reflects structural changes in the lung as well as accumulation of interstitial fluid. The distribution of extravascular density is consistent with previous radiological and morphological studies, and may reflect a higher tendency to oedema formation in the dependent parts of the lung. The measurements of fractional blood volume suggest that intrapulmonary blood volume is reduced in chronic pulmonary venous hypertension. This may reflect a decrease in the distensibility of the pulmonary vessels due to structural abnormalities, as well as functional changes in the pulmonary vasculature.

The exchange of small molecules as a measure of normal and abnormal lung microvascular function

Evidence exists for the utility of measures of small-molecule exchange in assessing the normal and abnormal transport status of the lung vasculature. The evidence for the usefulness of PS for 14C-urea is: 1. PSu compares to other small molecules as would be expected from free-diffusion coefficients. 2. The extraction of 14C-urea decreases with increase flow as would be expected of a diffusion-limited indicator. 3. PSu is unaffected by moderate pressure increase, it increases when lymph protein flow indicated permeability increase, and it decreases when surface area is reduced. 4. Lung injuries such as E. coli endotoxemia in sheep and ARDS in patients can both reduce PSu, presumably through surface area reduction, and increase PSu, through increased permeability. Two events that accompany clinically important vascular injury complicate the interpretation of lung MT curves. These events are surface area loss and heterogeneity of flow and transport properties in the lung. Additional work is needed to precisely measure these variables and assess their importance.

Lung water and capillary permeability in pulmonary sarcoid-like granulomatosis

The subject of our research was the use of multiple indicators to study the lungs of rats during a pathological process induced by complete Freund's adjuvant (CFA). The parameters studied were extravascular water (idQwl), the capillary permeability of [14C]sucrose, and the unrecovered fraction of [3H5glucose. We ascertained that idQwl gives a valuable estimate of total pulmonary water (r=0.91, n=30) and allows a satisfactory quantification of granulomatosis, as is shown by the comparison between the values of idQwl measured at different stages of the lesional process with average number of nodules per slice of right middle pulmonary lobe. The capillary permeability of sucrose is normal both at early and late stages of the CFA granulomatosis process. The fraction of [3H]glucose not recovered increases significantly (p less than 0.001) during the initial stage of the CFA process. This apparent loss of indicator can be explained by the existence of a slow cellular compartment.

In vivo assessment of pulmonary vascular integrity in experimental pulmonary edema

During single pass indicator studies across the lungs [(14)C]urea remains in the vascular compartment, but its molecular size and solubility suggest it might escape abnormally permeable vessels. To test the hypothesis that [(14)C]urea might be used to distinguish pulmonary edema due to acutely increased intravascular pressure from that due to vascular damage by alloxan, we studied [(51)Cr]erythrocytes (r), [(125)I]albumin (a), [(14)C]urea (u), and tritiated water as dilution indicators in the pulmonary circulation of anesthetized dogs. In addition, the adequacy of albumin as an intravascular indicator was evaluated. Indicator curves, blood gases, hematocrit, and vascular pressures were determined during a base-line period and repeated 30 and 60 min after treatment in five groups of dogs: (a) saline control. (b) alloxan edema. (c) epinephrine infusion, (d) volume overload, and (e) left atrial (LA) balloon obstruction.Groups b, d, and e developed a similar degree of edema judging by wet/dry lung weights and histology. Groups a and c did not develop edema. In alloxan edema, differences between the mean transit time volume of u and r (V(v-r)) increased over base line at 30 (P < 0.001) and 60 min (P < 0.02); the differences between the mean transit time volume of a and r (V(e-r)) increased slightly at 30 (P < 0.03) and 60 min (P < 0.02); and V(u-r) significantly exceeded V(a-r) at 30 (mean difference = 9 ml, P < 0.02) and 60 min (mean difference = 11, P < 0.04). In none of the other groups did V(u-r) significantly exceed V(a-r). Thus, comparison of V(u-r) with V(a-r) may permit distinction between "high pressure" and "increased permeability" pulmonary edema. Albumin was not a consistently reliable indicator of intravascular volume as compared with composite red cell and albumin curve.

Indicator dilution measurements of lung volumes and alveolar air exchange during breathing

A new triple tracer indicator dilution technique has been used to measure alveolar ventilation as well as air and tissue volumes in the lungs of experimental animals and man. The tracers indocyanine green, [(121)I]antipyrine and xenon-133 were rapidly injected into the right atrium, while sampling was carried out from a peripheral artery. Blood flow and tissue volumes were obtained by classical analysis of the indocyanine green and antipyrine concentration-time curves. A double exit-port, constant air flow model was used to analyze the xenon curves, because ventilatory loss led to incomplete recovery of the gas tracer in effluent blood. Uniform ventilation and perfusion were assumed. This analysis permitted calculation of alveolar ventilation (VA(Xe)) and functional residual capacity (FRC(Xe)) during normal breathing. In control studies, VA(Xe) was similar to VA(co2), obtained with the steady-state CO(2) method (r = 0.87), while in critically ill patients the xenon measurement was significantly lower, averaging 54% of VA(co2). In theory, underestimates in VA(Xe) and decrease in the ratio VA(Xe)/VA(co2) relate to nonuniformity in regional ventilation and perfusion. The effect is greatest for the slightly soluble gas, xenon. The significant inverse correlation between VA(Xe)/VA(co2) and the physiologic shunt is consistent with this postulate.FRC(Xe) was similar to the predicted FRC in animals but was 76% of the helium measured FRC in patients. FRC(Xe) was significantly lower than the xenon measured air volumes during breath-holding when nonuniformity of ventilation was not operative. Lung tissue volumes in animals were 83% of gravimetric lung weights, while in patients the volumes were much lower than predicted. Nonhomogeneous lung function, including failure to perfuse the entire capillary bed, with resultant incomplete penetration of tracers into all segments of lung air and tissue, may explain these findings. The resultant errors can be significant in sick patients, and may themselves be used to study nonhomogeneities in the distribution of ventilation and volume.

Indicator dilution measurements of extravascular water in the lungs

Multiple indicator dilution studies of the pulmonary circulation were carried out in conscious, resting and exercising, and anesthetized dogs under conditions where there was no pulmonary edema. Labeled red cells, water, and albumin were injected together into the pulmonary artery, and effluent dilution patterns were obtained from the descending thoracic aorta. The product of the mean transit time differences between labeled water and red cells, and the pulmonary water flow was used to estimate extravascular parenchymatous water; and this was expressed as a proportion of the water content of the blood-drained lung at postmortem examination. These estimates of the proportional water content were found to increase with flow, and to approach an asymptotic value. Reconsideration of the flow patterns in capillaries, however, led to the postulate that extravascular water should be calculated, utilizing as the appropriate vascular reference a substance that uniformly labels the water in red cells and plasma, and which is confined to the circulation, rather than a tracer that only labels red cells. The mean transit time of this substance is approximated by the sum of the mean transit times of labeled red cells and albumin, each weighted according to the proportion of the water content of blood present in that phase. The values for lung water content so computed also increased with flow, and appeared to approach an asymptote that corresponded to approximately two-thirds of the wet lung weight. The estimated values for the water space after pentobarbital anesthesia corresponded to the lower values obtained in the resting conscious animals. When the anesthetized animals were also bled, the estimated water space was disproportionately large, in relation to the previous values. These experimental results support the hypothesis that dilutional estimates of the lung water space reflect pulmonary capillary filling; that this filling increases with exercise; and that a relative increase in filling also occurs as part of the response to hemorrhage.