Arterial, capillary, and venous transit times and dispersion measured in isolated rabbit lungs

A perpetual switching system in pulmonary capillaries

Of the 300 billion capillaries in the human lung, a small fraction meet normal oxygen requirements at rest, with the remainder forming a large reserve. The maximum oxygen demands of the acute stress response require that the reserve capillaries are rapidly recruited. To remain primed for emergencies, the normal cardiac output must be parceled throughout the capillary bed to maintain low opening pressures. The flow-distributing system requires complex switching. Because the pulmonary microcirculation contains contractile machinery, one hypothesis posits an active switching system. The opposing hypothesis is based on passive switching that requires no regulation. Both hypotheses were tested ex vivo in canine lung lobes. The lobes were perfused first with autologous blood, and capillary switching patterns were recorded by videomicroscopy. Next, the vasculature of the lobes was saline flushed, fixed by glutaraldehyde perfusion, flushed again, and then reperfused with the original, unfixed blood. Flow patterns through the same capillaries were recorded again. The 16-min-long videos were divided into 4-s increments. Each capillary segment was recorded as being perfused if at least one red blood cell crossed the entire segment. Otherwise it was recorded as unperfused. These binary measurements were made manually for each segment during every 4 s throughout the 16-min recordings of the fresh and fixed capillaries (>60,000 measurements). Unexpectedly, the switching patterns did not change after fixation. We conclude that the pulmonary capillaries can remain primed for emergencies without requiring regulation: no detectors, no feedback loops, and no effectors-a rare system in biology. NEW & NOTEWORTHY The fluctuating flow patterns of red blood cells within the pulmonary capillary networks have been assumed to be actively controlled within the pulmonary microcirculation. Here we show that the capillary flow switching patterns in the same network are the same whether the lungs are fresh or fixed. This unexpected observation can be successfully explained by a new model of pulmonary capillary flow based on chaos theory and fractal mathematics.

Functional imaging: CT and MRI

Numerous imaging techniques permit evaluation of regional pulmonary function. Contrast-enhanced CT methods now allow assessment of vasculature and lung perfusion. Techniques using spirometric controlled multi-detector row CT allow for quantification of presence and distribution of parenchymal and airway pathology; xenon gas can be employed to assess regional ventilation of the lungs, and rapid bolus injections of iodinated contrast agent can provide a quantitative measure of regional parenchymal perfusion. Advances in MRI of the lung include gadolinium-enhanced perfusion imaging and hyperpolarized gas imaging, which allow functional assessment, including ventilation/perfusion, microscopic air space measurements, and gas flow and transport dynamics.

State of the Art. A structural and functional assessment of the lung via multidetector-row computed tomography: phenotyping chronic obstructive pulmonary disease

With advances in multidetector-row computed tomography (MDCT), it is now possible to image the lung in 10 s or less and accurately extract the lungs, lobes, and airway tree to the fifth- through seventh-generation bronchi and to regionally characterize lung density, texture, ventilation, and perfusion. These methods are now being used to phenotype the lung in health and disease and to gain insights into the etiology of pathologic processes. This article outlines the application of these methodologies with specific emphasis on chronic obstructive pulmonary disease. We demonstrate the use of our methods for assessing regional ventilation and perfusion and demonstrate early data that show, in a sheep model, a regionally intact hypoxic pulmonary vasoconstrictor (HPV) response with an apparent inhibition of HPV regionally in the presence of inflammation. We present the hypothesis that, in subjects with pulmonary emphysema, one major contributing factor leading to parenchymal destruction is the lack of a regional blunting of HPV when the regional hypoxia is related to regional inflammatory events (bronchiolitis or alveolar flooding). If maintaining adequate blood flow to inflamed lung regions is critical to the nondestructive resolution of inflammatory events, the pathologic condition whereby HPV is sustained in regions of inflammation would likely have its greatest effect in the lung apices where blood flow is already reduced in the upright body posture.

A compartmental capillary, convolution integration model to investigate nutrient transport and metabolism in vivo from paired indicator/nutrient dilution curves

Thirty-three paired indicator/nutrient dilution curves across the mammary glands of four cows were obtained after rapid injection of para-aminohippuric acid (PAH) plus glucose into the external iliac artery. For the measurement of extracellular volume and kinetics of nutrient uptake from indicator dilution curves, several models of solute dispersion and disappearance have been proposed. The Crone-Renkin models of exchange in a single capillary assume negligible washout of solutes from the extracellular space and do not describe entire dilution curves. The Goresky models include a distribution of capillary transit times to generate whole system outflow profiles but require two indicators to parametize extracellular behavior. A compartmental capillary, convolution integration model is proposed that uses one indicator to account for the extracellular behavior of the nutrient after a paired indicator/nutrient injection. With the use of an iterative approach to least squares, unique solutions for nonexchanging vessel transit time t(mu) and its variance sigma were obtained from all 33 PAH curves. The average of heterogeneous vascular transit times was approximated as 2sigma = 8.5 s. The remainder of indicator dispersion was considered to be due to washout from a well-mixed compartment representing extracellular space that had an estimated volume of 5.5 liters or 24% of mammary gland weight. More than 99% of the variation in the time course of venous PAH concentration after rapid injection into the arterial supply of the mammary glands was explained in an unbiased manner by partitioning the organ into a heterogeneous nonexchanging vessel subsystem and a well-mixed compartmental capillary subsystem.

Transit time dispersion in the pulmonary arterial tree

Knowledge of the contributions of arterial and venous transit time dispersion to the pulmonary vascular transit time distribution is important for understanding lung function and for interpreting various kinds of data containing information about pulmonary function. Thus, to determine the dispersion of blood transit times occurring within the pulmonary arterial and venous trees, images of a bolus of contrast medium passing through the vasculature of pump-perfused dog lung lobes were acquired by using an X-ray microfocal angiography system. Time-absorbance curves from the lobar artery and vein and from selected locations within the intrapulmonary arterial tree were measured from the images. Overall dispersion within the lung lobe was determined from the difference in the first and second moments (mean transit time and variance, respectively) of the inlet arterial and outlet venous time-absorbance curves. Moments at selected locations within the arterial tree were also calculated and compared with those of the lobar artery curve. Transit times for the arterial pathways upstream from the smallest measured arteries (200-micron diameter) were less than approximately 20% of the total lung lobe mean transit time. Transit time variance among these arterial pathways (interpathway dispersion) was less than approximately 5% of the total variance imparted on the bolus as it passed through the lung lobe. On average, the dispersion that occurred along a given pathway (intrapathway dispersion) was negligible. Similar results were obtained for the venous tree. Taken together, the results suggest that most of the variation in transit time in the intrapulmonary vasculature occurs within the pulmonary capillary bed rather than in conducting arteries or veins.

Effects of hypoxia, blood P(CO2) and flow on O2 transport in excised rabbit lungs

In previous studies using isolated perfused rabbit lungs, an O2 deficit measured by an alveolar gas-to-end capillary blood P(O2) difference (A-aD(O2)) was absent at blood flows (Q) consistent with severe exercise. Thus factors such as VA/Q heterogeneity, shunt and diffusion limitation that contribute to an O2 deficit in vivo were absent. Here we attempted to increase diffusion limitation to O2 transport by reducing the equilibration coefficient D/(betaQ), the ratio of the diffusing capacity (D) to the product of Q and the capacitance coefficient (beta, the slope of the blood O2 content-P(O2) curve). First, we used hypoxic (10% O2) ventilation in conjunction with a low PV(O2) (approximately 25 mmHg) because beta is largest in this region of the O2 dissociation curve. Second, we increased beta by decreasing blood P(CO2) which shifts the O2 dissociation curve to the left (Bohr effect). Third, we increased Q to three times control to reduce D/Q. CO diffusing capacity was measured as a function of blood flow and blood P(O2). A deficit in O2 transport as measured by a significant A-aD(O2) was measured only under conditions of hypoxia and high blood flow. The measured O2 deficit matched the predictions from the equilibration coefficients D/(betaQ) based on measurements of beta, D and Q.

Non-invasive assessment of technetium-99m albumin transit time distribution in the pulmonary circulation by first-pass angiocardiography

This study describes a non-invasive method for assessment of the lung transit time distribution of a tracer, using first-pass technetium-99m albumin angiocardiography and a model-free method of deconvolution. Ten patients received a first injection of 1 MBq kg-1 in the external jugular vein to position a gamma camera in the left anterior oblique position and two additional injections (5 MBq kg-1) to record first-pass angiocardiographic data. Right and left ventricular time-activity curves were derived from regions of interest every 0.5 s over a 1-min period. The left ventricular curve was deconvoluted by the right ventricular curve to obtain the lung transport function. The deconvolution procedure was based on a modified version of the Kalman filtering technique. The procedure was repeated at an interval of 30 min in eight patients. Two patients were re-examined up to 2 years later. Skewness, kurtosis and relative dispersion of the distributions did not change over time. We also found that the distribution, once normalized by its first moment, was independent of isolated changes in heart rate or cardiac output. Comparison of curve shapes at an interval of 30 min by point by point analysis demonstrated the reproducibility of the technique. We conclude that computation of the pulmonary transit time distribution of 99mTc-albumin from a standard angiocardiography procedure by model-free deconvolution is reliable and reproducible over time. We suggest that it may be a valuable tool for the non-invasive follow-up of the pulmonary circulation.

Effect of inflation on interstitial cuff and pressure in liquid-filled rabbit lung

Degassed rabbit lungs were inflated to 15 cmH2O pressure with 3% albumin solution. To study cuff growth, lungs were frozen in liquid N2 at several (20-120 min) inflation times. Cuff-to-vessel area ratio measured from frozen lung pieces increased with time reaching a maximum value (0.5-0.9) by 1 h. Time constants (to) of cuff growth were similar to those (28 min) of the interstitial pressure (Pi) response measured by micropuncture at the lung hilum. Pi response was slower with saline (to = 84 min) than with albumin. Compared to saline, positively charged protamine sulphate increased the Pi response (to = 44 min). Time constants for cuff growth and Pi response were smaller at 15 cmH2O than at 5 cmH2O inflation pressure (30 vs. 60-120 min). Electrical analog models indicated a doubling of interstitial resistance with a four-to eight-fold decrease in interstitial specific compliance at the higher inflation pressure, the latter attributed to nonlinear elastic behavior of lung parenchyma.

Effect of blood flow on capillary transit time and oxygenation in excised rabbit lung

We used an isolated perfused lung preparation of the rabbit to study the effect of increasing blood flow on pulmonary capillary transit time by two methods. In one method, capillary transit time was measured from fluorescent dye dilution curves from arterioles and venules of the subpleural microcirculation. Values of transit time were similar to those for the whole lung determined by dividing capillary blood volume by blood flow. Capillary transit times averaged 0.50-0.62 sec at a control blood flow of 80 ml min-1 kg-1 and decreased to 0.14-0.18 sec as blood flow increased to 6 times control. To determine whether the reduced transit time would limit O2 transport, we studied the effect of blood flow on oxygenation. Two isolated rabbit lungs were perfused in series. Blood from one lung deoxygenated by ventilation with a N2-CO2 mixture was oxygenated by the test lung ventilated with air. Ventilation was matched to blood flow. PO2 and PCO2 were measured in blood flowing into and out of the test lung. At all flows, no significant alveolar gas-to-end-capillary blood PO2 gradient (A-aDO2) was measured. The isolated perfused rabbit lung showed no transit time limitation to oxygenation for blood flows that are consistent with heavy exercise in vivo.