Models of lung transvascular fluid and protein transport

Transient transcapillary exchange of water driven by osmotic forces in the heart

Osmotic transient responses in organ weight after changes in perfusate osmolarity have implied steric hindrance to small-molecule transcapillary exchange, but tracer methods do not. We obtained osmotic weight transient data in isolated, Ringer-perfused rabbit hearts with NaCl, urea, glucose, sucrose, raffinose, inulin, and albumin and analyzed the data with a new anatomically and physicochemically based model accounting for 1) transendothelial water flux, 2) two sizes of porous passages across the capillary wall, 3) axial intracapillary concentration gradients, and 4) water fluxes between myocytes and interstitium. During steady-state conditions approximately 28% of the transcapillary water flux going to form lymph was through the endothelial cell membranes [capillary hydraulic conductivity (Lp) = 1.8 +/- 0.6 x 10-8 cm. s-1. mmHg-1], presumably mainly through aquaporin channels. The interendothelial clefts (with Lp = 4.4 +/- 1.3 x 10-8 cm. s-1. mmHg-1) account for 67% of the water flux; clefts are so wide (equivalent pore radius was 7 +/- 0.2 nm, covering approximately 0.02% of the capillary surface area) that there is no apparent hindrance for molecules as large as raffinose. Infrequent large pores account for the remaining 5% of the flux. During osmotic transients due to 30 mM increases in concentrations of small solutes, the transendothelial water flux was in the opposite direction and almost 800 times as large and was entirely transendothelial because no solute gradient forms across the pores. During albumin transients, gradients persisted for long times because albumin does not permeate small pores; the water fluxes per milliosmolar osmolarity change were 200 times larger than steady-state water flux. The analysis completely reconciles data from osmotic transient, tracer dilution, and lymph sampling techniques.

Optical measurement of isolated canine lung filtration coefficients at normal hematocrits

In this study, lung filtration coefficient (Kfc) values were measured in eight isolated canine lung preparations at normal hematocrit values using three methods: gravimetric, blood-corrected gravimetric, and optical. The lungs were kept in zone 3 conditions and subjected to an average venous pressure increase of 10.24 +/- 0.27 (SE) cmH2O. The resulting Kfc (ml . min-1 . cmH2O-1 . 100 g dry lung wt-1) measured with the gravimetric technique was 0.420 +/- 0.017, which was statistically different from the Kfc measured by the blood-corrected gravimetric method (0.273 +/- 0.018) or the product of the reflection coefficient (sigmaf) and Kfc measured optically (0. 272 +/- 0.018). The optical method involved the use of a Cellco filter cartridge to separate red blood cells from plasma, which allowed measurement of the concentration of the tracer in plasma at normal hematocrits (34 +/- 1.5). The permeability-surface area product was measured using radioactive multiple indicator-dilution methods before, during, and after venous pressure elevations. Results showed that the surface area of the lung did not change significantly during the measurement of Kfc. These studies suggest that sigmafKfc can be measured optically at normal hematocrits, that this measurement is not influenced by blood volume changes that occur during the measurement, and that the optical sigmafKfc agrees with the Kfc obtained via the blood-corrected gravimetric method.

A model of fluid, erythrocyte, and solute transport in the lung

A mathematical model of fluid, solute, and red cell transport in the lung has been developed that includes the effects of simultaneous changes in lung vascular and interstitial volumes. The model provides separate arterial, microvascular, and venous pulmonary regions and a systemic vascular region in addition to a pulmonary interstitial compartment. Pressure, volume, hematocrit, flow, and concentration of up to 12 solutes and tracers can be computed in each compartment. Computer code is written in the C programming language, with Microsoft Excel serving as a user interface. Implementation is currently on PC-486 microcomputer systems, but the core program can easily be moved to other computer systems. The user can select different models for the blood-interstitial barrier (e.g., multiple pore, nonlinear Patlak equation), osmotic pressure-concentration relationships (e.g., Nitta, Navar-Navar), solute reflection coefficients interstitial macromolecule exclusion, or lymph barrier characteristics. Each model parameter or a combination of parameters can be altered with time in a predetermined fashion. The model is particularly useful in interpreting lung experimental data where simultaneous changes occur in vascular and extravascular compartments. Several applications are presented and discussed, including interpretation of optical filtration experiments, venous occlusion experiments, external detection of macromolecular exchange, and blood-lymph studies that use exogenous tracers. A number of limitations of the model are identified and improvements are proposed. A major strength of the model is that it is specifically designed to incorporate newly discovered relationships as the field of lung physiology expands.

Optical measurements of lung microvascular filtration coefficient using polysulfone fibers

Lung fluid balance, which is governed by the product of net transvascular pressure difference and lung filtration coefficient, can be altered in pulmonary diseases. A simple measurement of the lung filtration coefficient (Kfc) would be clinically useful and has been examined by several researchers. Current methods of determining Kfc include gravimetric measurement in isolated lungs and lymph node cannulation, neither of which can be extended to human use. Optical measurements of protein concentration changes in venous blood can be combined with pressure measurements to calculate Kfc. Blood, though, contains red corpuscles, which tend to absorb and scatter light, obscuring these optical measurements. In this study, an optical system was developed in which a polysulfone filter cartridge was used to remove red blood cells before the filtrate was passed through a spectrophotometer. Absorbance changes caused by changes in concentration of albumin labeled with Evans Blue were monitored at 620 nm after venous pressure was elevated by about 13 cm H2O. Optical measurements of Kfc averaged 0.401 +/- 0.074 (ml/min cm H2O 100 g DLW) for an isolated canine lung. Optical measurements of Kfc (0.363 +/- 0.120 ml/min cm H2O 100 g DLW) were made for the first time in an intact, closed chest sheep in which pulmonary pressure was altered by inflating a Foley balloon in the left atrium. We conclude that absorbance and scattering artifacts introduced by red blood cells can be eliminated by first filtering the blood through polysulfone fibers. Kfc measurements using the optical method are similar to values obtained by others using gravimetric methods. Finally, we have demonstrated that the technique can be used to estimate Kfc in an intact animal.

Pressure and flow changes in the pulmonary circulation in exercising sheep: evidence for elevated microvascular pressure

The purpose of this study was to measure hemodynamic and transvascular filtration changes in the lung during strenuous exercise in sheep. The specific goals were (1) to determine the nature of the reduction in pulmonary arterial pressure (Ppa) after its initial peak rise with onset of exercise; (2) to use a pulmonary artery catheter distal wedge technique ("microwedge") to better assess longitudinal changes in resistance in the pulmonary circulation with exercise; and (3) to compare lung lymph flow and protein concentration changes at comparable estimated microvascular pressure (Pmv) (mean Ppa - mean left atrial pressure) (Pla) x 0.4 + mean Pla) during exercise versus passive left atrial hypertension to determine whether exercise causes a higher than expected lymph flow. We found that cardiac output rises quickly and thereafter remains constant with constant-rate exercise, and, thus, the secondary reduction in Ppa was due to vasodilation and/or recruitment. The microwedge pressure rose more than did Pla, suggesting that actual Pmv was probably higher than that estimated. With hypoxia, most of the change in pulmonary vascular resistance was in upstream vessels (arteries and capillaries), as was most of the exercise-induced vasodilation. Lymph flow rose more quickly and was much higher during brief exercise than during left atrial hypertension at a comparable calculated Pmv, and lymph protein content decreased more quickly. The data point to a Pmv with strenuous exercise that is higher than expected.