Intrinsic tone and distensibility of in vitro and in situ cat pulmonary arteries.

This study was designed to determine the in vitro and in situ diameter vs pressure relationship of 200- to 1,200-microns diameter pulmonary arteries in the cat. Diameter vs pressure relationships of these arteries were obtained using two methods, microscopic observation of in vitro cannulated and pressurized arteries and X-ray angiography of in situ arteries. Both in vitro and in situ arteries were studied first under normal conditions and then after reducing tone with Ca(2+)-free solution (in vitro) or papaverine (in situ). In vitro arteries commonly increased their tone in response to elevated transmural pressure, and in some cases, the diameter actually decreased as pressure increased. This behavior was not observed in the in situ arteries. The major difference between in vitro and in situ arteries was that when the in vitro arteries were relaxed, the slope of the diameter vs pressure curves increased, whereas the slope was not altered significantly by relaxation of the in situ arteries. This difference is emphasized by the increased distensibility with relaxation of the in vitro arteries but the decreased distensibility with relaxation of the in situ arteries. The results of this study suggest that, at least in the cat, small pulmonary arteries possess a mechanism that is dormant in the in situ environment within the normal lung. However, the potential for pressure-induced constriction may be unmasked by changing the vessel history and/or environment. Extrapolating results obtained from in vitro pulmonary arteries to the in situ situation should therefore be done with caution. Studies directed at what factors contribute to differences in the responses of in vitro and in situ arteries might help in understanding pulmonary vascular pathophysiology.

Regional transit time estimation from image residue curves.

Methods for estimating regional flow from digital angiography or dynamic computed tomography images require determination of indicator mean transit time (t) through a region-of-interest (ROI). We examine how the ROI kinematics and input dispersion influence the recovery of t using a computer-simulated vessel network representing that which might occur in a real organ. The network simulates flow through a large artery branching into two small arteries, each feeding a system of smaller vessels intended to represent capillaries and small vessels below the resolution of the imaging system. The capillaries are drained by a similar system of veins. Concentration curves measured over the inlet to the network and microvascular ROI residue curves are simulated. When the area-height ratio of the microvascular ROI curve is used and all of the indicator is contained within the ROI for at least one time point, t is recovered exactly. As the size of the ROI is reduced or the inlet concentration curve becomes more dispersed, the error in the recovery of t grows. By first deconvolving the inlet concentration curve from the microvascular ROI curve, and then calculating the area-height ratio, t is recovered accurately. If the inlet concentration curve becomes more dispersed between its measured site and the actual inlet to the ROI, or if the flow distribution within the ROI is changed, the estimation of t can be degraded. To put the simulations in perspective relative to an example of image data, the methods were applied to microfocal x-ray angiography data obtained from a approximately 700 micron canine pulmonary artery and vein, the surrounding microvasculature and the inlet lobar arterial cannula.

Influence of hypoxia and serotonin on small pulmonary vessels.

X-ray angiograms obtained from isolated perfused dog lungs were used to measure changes in the internal diameter of small intraparenchymal pulmonary arteries (150-1,600 microns) and veins (200-1,000 microns) in response to hypoxia or intra-arterial serotonin [5-hydroxytryptamine (5-HT)] infusion. The diameter changes in response to the two stimuli were measured over a range of stimulus-induced increases (delta Pa) in the total arteriovenous pressure drop. When the resulting delta Pa was small, all arteries in the diameter range studied constricted in response to either stimuli. The maximum decrease in diameter was approximately 25% with hypoxia and 36% with 5-HT. However, when delta Pa was large, arteries with a control diameter larger than approximately 800 microns distended with hypoxia. On the other hand, 5-HT constricted all the arteries in the size range studied regardless of the resulting magnitude of delta Pa. Hypoxia caused a small (approximately 9%) constriction in all veins in the diameter range studied independent of diameter or the magnitude of delta Pa, whereas in the concentration range studied 5-HT had no significant influence on these veins. An analysis of the potential impact of these vessels on total pulmonary vascular resistance suggested that although vessels in the size range studied contributed significantly to the total response to these two stimuli, vessels smaller than those studied also made a major contribution to the total response.

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.

Distensibility of small veins of the dog lung.

To determine the distensibility of the intrapulmonary veins (250-2,900 microns diam) of the dog lung, we obtained X-ray angiograms from isolated lung lobes over a vascular pressure range of approximately 0-30 Torr. Over this pressure range the diameter vs. pressure curves tended to flatten out at the high pressures. In the pressure range of 0-19 Torr, we characterized the vessel distensibility by alpha (the ratio of the slope, beta, of the graph of diameter vs. intravascular pressure to the intercept, Do). The average value of alpha was approximately 1.2%/Torr. There was a weak negative correlation (r = -0.32) between alpha and Do. Infusion of enough norepinephrine to produce approximately 50% increase in total lobar vascular resistance produced a decrease in Do and alpha of approximately 33 and 32%, respectively.

Distensibility of small arteries of the dog lung.

To obtain in situ measurements of the distensibility of small (100- to 1,000-microns-diam) pulmonary arterial vessels of the dog lung, X-ray angiograms were obtained from isolated lung lobes with the vascular pressure adjusted to various levels. The in situ diameter-pressure relationships were compared with the diameter-pressure relationships for small arteries that were dissected free from the lungs and cannulated with small glass pipettes for the measurement of diameter and transmural pressure. The diameter-vascular or diameter-transmural pressure curves from both in situ and cannulated vessels were sufficiently linear in the pressure range studied (0-30 Torr) that they could be characterized by linear regression to obtain estimates of D0, the diameter at zero vascular pressure, and beta, the change in diameter (micron) per Torr change in pressure. The vessel distensibility coefficient (alpha) was defined as alpha = beta/D0. The mean values of alpha were approximately 2.0 +/- 0.8%/Torr (SD) for the in situ vessels and 1.7 +/- 0.6%/Torr for the cannulated vessels, with no statistically significant difference between the two methods. The influence of vasoconstriction elicited by serotonin was evaluated in the in situ vessels. Serotonin-induced vasoconstriction caused a decrease in D0 and little change in alpha.

An algorithm for angiographic estimation of blood vessel diameter.

This study was carried out in an attempt to develop an objective and robust method for measuring changes in the diameters of small blood vessels from X-ray angiographic images. Recognizing potential problems with edge detection methods applied to cylindrical vessels in which the contrast diminishes as the boundary is approached, we have attempted to utilize the X-ray absorbance data across the entire cross section of the vessel. Then, assuming a cylindrical geometry, the absorbance data are fit to the cylindrical absorbance function by use of nonlinear regression analysis. The method was tested and calibrated using glass tubes filled with various concentrations of contrast medium. The diameters of small pulmonary arteries were estimated by applying the method of angiograms obtained from an isolated dog lung lobe. The structure of the residuals obtained after the fitting procedure was analyzed to test the appropriateness of the model for use with images of vessels. The results suggest that this approach will have utility for systematically quantifying vessel dimensions.