1979 George Simon Memorial Fellowsip Award. A radiologic and physiologic investigation into hypoxic pulmonary vasoconstriction in the dog

Analysis of flow resistance in the pulmonary arterial circulation: implications for hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) plays an essential role in distributing blood in the lung to enhance ventilation-perfusion matching and blood oxygenation. In this study, a theoretical model of the pulmonary vasculature is used to predict the effects of vasoconstriction over specified ranges of vessel diameters on pulmonary vascular resistance (PVR). The model is used to evaluate the ability of hypothesized mechanisms of HPV to account for observed levels of PVR elevation during hypoxia. The vascular structure from pulmonary arteries to capillaries is represented using scaling laws. Vessel segments are modeled as resistive elements and blood flow rates are computed from physical principles. Direct vascular responses to intravascular oxygen levels have been proposed as a mechanism of HPV. In the lung, significant changes in oxygen level occur only in vessels less than 60 μm in diameter. The model shows that observed levels of hypoxic vasoconstriction in these vessels alone cannot account for the elevation of PVR associated with HPV. However, the elevation in PVR associated with HPV can be accounted for if larger upstream vessels also constrict. These results imply that upstream signaling by conducted responses to engage constriction of arterioles plays an essential role in the elevation of PVR during HPV.NEW & NOTEWORTHY A theoretical model of the pulmonary vasculature is used to predict the effects of vasoconstriction over specified ranges of vessel diameters on pulmonary vascular resistance (PVR). The model shows that observed levels of hypoxic vasoconstriction in terminal vessels cannot account for the elevation of PVR associated with hypoxic pulmonary vasoconstriction (HPV). Upstream signaling by conducted responses to engage constriction of arterioles, therefore, plays an essential role in the elevation of PVR during HPV.

Hypoxic pulmonary vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Effects of hypoxia on pulmonary microvascular volume

To determine the effects of alveolar hypoxia on pulmonary microvascular volume, X-ray microfocal angiographic images of isolated perfused dog lung lobes were obtained during passage of a bolus of radiopaque contrast medium during both normoxic (alveolar gas, 15% O(2), 6% CO(2), and 79% N(2)) and hypoxic (3% O(2), 6% CO(2), and 91% N(2)) conditions. Regions of interest (ROIs) over the lobar artery and vein at low magnification and a feeding artery ( approximately 500 microm diameter) and the nearby microvasculature (vessels smaller than approximately 50 microm) at high magnification were identified, and X-ray absorbance vs. time curves were acquired under both conditions from the same ROIs. The total pulmonary vascular volume was calculated from the flow and the mean transit time for the contrast medium passage from the lobar artery to lobar vein. The fractional changes in microvascular volume were determined from the areas under the high-magnification X-ray absorbance curves. Hypoxia decreased lobar volume by 13 +/- 3% (SE) and regional microvascular volume by 26 +/- 4% (SE). Given the morphometry of the lung vasculature, these results suggest that capillary volume was decreased by hypoxia.

Hydrostatic versus oleic acid-induced pulmonary edema: high-resolution computed tomography findings in the pig lung

RATIONALE AND OBJECTIVES

We evaluated the differences between combined hydrostatic and hypervolemic edema and oleic acid-induced edema on high-resolution computed tomography (HRCT) scans.

METHODS

Twelve anesthetized and ventilated pigs were studied. Hydrostatic edema was induced by ligation of the abdominal aorta and infusion of normal saline (n = 4); permeability edema was induced by intravenous injection of oleic acid (n = 4). Four pigs were studied as normal controls. Serial scans were obtained before and after induction of edema at a constant position in the caudal lobe of the lung. The distribution of edema was assessed visually. Cross-sectional areas (CSAs) of the pulmonary artery and vein were measured both at the lobar and segmental levels.

RESULTS

Gravity-dependent opacity, peribronchovascular fluid collection, prominent centrilobular core, thickening of the interlobular septa, and air-space consolidation at the dependent site were the sequential HRCT findings of hydrostatic edema. Randomly distributed, diffuse patchy high attenuation areas with a tendency for predilection in the subpleural and peripheral areas of the secondary lobule were the findings of oleic acid-induced edema. Hydrostatic edema increased the mean CSAs of the lobular vein by 137.8% +/- 78.7, but oleic acid edema decreased the mean CSAs by 33.2% +/- 22.7. Changes in the mean CSAs of the pulmonary arteries were not significant. The mean vein-to-artery ratio increased significantly in hydrostatic edema but decreased in oleic acid edema.

CONCLUSION

HRCT findings for hydrostatic and oleic acid-induced pulmonary edema differed both in distribution of edema and in pulmonary vascular response.

The pulmonary circulation: the radiology of adaptation

In this George Simon Lecture, I have tried to present a simplistic yet philosophical approach to the physiological processes of adaptation of circulatory and respiratory function in response to pathophysiological challenge. The objective of the body is to achieve a normal ventilation-perfusion ratio of 1 in order to optimise blood-gas exchange in a wide variety of pathological conditions. It is all a question of the right ingredients (oxygen in atmospheric air and reduced haemoglobin in desaturated blood), in the right copious amounts (100 ml/s), in the right proportion (1:1) and in the right place (alveolar capillary membrane). In order to demonstrate the above concepts, a variety of well known conditions, congenital and acquired, have been discussed in the developing infant lung and the adult mature lung; some primarily cardiovascular, others primarily respiratory; some conditions resulting in permanent and others resulting in temporary and reversible major readjustments of the blood supply to the lung. The emphasis has been to illustrate that simple, unsophisticated radiological techniques may provide a substantial amount of valuable information concerning the remarkable capacity of the pulmonary circulation to adapt to varying physiological or pathological demands in order to conserve our precious respiratory resources. George Simon, like all good doctors, had a tremendous respect for the ability of the body to respond to these challenges. He always emphasised the important basic principles rather than fine minutiae. It is hoped that this lecture in his honour follows these principles which he preached so well.