Angiographic evidence of pulmonary vasomotion in the dog

Analytic Reviews : Hypoxic Pulmonary Vasoconstriction: Physiology and Pathophysiology

Hypoxic pulmonary vasoconstriction is the unique re sponse of the lung circulation to ventilation with hy poxic gas, resulting in an increase in pulmonary arterial pressure and vascular resistance. The site of hypoxic vasoconstriction is the small pulmonary arteries. The hypoxic pressor response optimizes ventilation and per fusion matching and thus preserves arterial oxygen ten sion. The mechanism of this response is not clear, al though considerable knowledge has been gained about its modulation. We review the current state of under standing of the physiology and pathophysiology of hy poxic pulmonary vasoconstriction.

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.

An experimental model for simultaneous quantitative analysis of pulmonary micro- and macrocirculation during unilateral hypoxia in vivo

An experimental model was developed for quantitative analysis of pulmonary microcirculation using in vivo fluorescence videomicroscopy during unilateral hypoxia induced by one-lung ventilation (1 LV). In five white New Zealand rabbits, pulmonary arterioles on the surface of the right lung were visualized by means of intra-arterial injection of FITC-labeled erythrocytes and FITC-Dextran. During 1 LV of the left lung, the mean airway pressure in the right lung was kept at the level of two-lung ventilation (2 LV) by means of N2-CPAP. Arteriolar diameters as well as parameters of macrocirculation (AP, CVP, PAP, LAP, CO) and gas exchange (paO2, Qs/Qt) were measured simultaneously during 2 LV and 1 LV. FiO2 was kept constant at 1.0 during both experimental phases. Macrohemodynamic parameters during 1 LV did not differ from those measured during 2 LV. 1 LV induced a significant decrease in paO2 (213 +/- 105 versus 427 +/- 22 mm Hg, P < 0.05) and a significant increase in Qs/Qt (22 +/- 7 versus 13 +/- 2%, P < 0.05). During 2 LV (baseline), the pulmonary arteriolar diameters ranged from 15-120 microns. 1 LV resulted in a significant decrease of arteriolar diameters to 89.0 +/- 9.3% of baseline (P < 0.05). Relative changes in arteriolar diameters were similar for vessels with baseline diameters of 0-40, 40-60, and 60-120 microns (88.4 +/- 9.9%, 89.6 +/- 9.4%, and 88.4 +/- 8.7%, respectively). The present model is the first one allowing in-vivo investigation of HPV during 1 LV and 2 LV on the basis of simultaneous measurement of pulmonary arteriolar diameters and macrocirculatory parameters in vivo. Although PAP and PVR did not change significantly, a reduction of pulmonary arteriolar diameters was proven in response to alveolar hypoxia during 1 LV. We suggest the model to be useful in studying the physiological effects of HPV on macro- and microcirculation as well as investigating pathophysiological and pharmacological influences on HPV.

Pulmonary Vascular Impedance in the Dog

Pulmonary vascular hydraulic input impedance was measured in 13 anesthetized openchest dogs with normal sinus rhythm, and 2 dogs with surgically induced atrioventricular block, by means of electromagnetic flowmeters and strain gauge manometers of known frequency response. The linearity of the pulmonary bed was evaluated by measuring impedance while the heart rate, and hence the pulsatile input to the bed, was varied.

The pulmonary bed behaved as a quasilinear system, within the limits of accuracy of the methods employed and the range of frequencies tested. The use of input impedance, or oscillatory pressure/flow ratio, to describe some characteristics of the bed is therefore justifiable, and analogies with linear models like the simple transmission line are not unreasonable.

The characteristic input impedance averaged 3,094 dyne sec cm -5 kg, or about one-third the magnitude of the pulmonary vascular resistance, and was therefore a significant part of the total opposition that must be overcome in moving blood through the lungs. This impedance to pulsatile flow is not included in calculations of resistance from mean pressure and flow measurements.

The pattern of the impedance spectrum suggested that reflections originating from arteries 1.0 mm and less in diameter play a large role in determining input impedance and its variations with frequency. Pulmonary vasoconstriction by 5-hydroxytryptamine (serotonin) altered the impedance pattern in a manner consistent with increased wave reflection and displacement of the dominant reflecting sites to positions nearer the main pulmonary artery. Capillary compression by increased intra-alveolar pressure also increased reflection, but did not alter the sites of reflection significantly, providing further evidence that the normal impedance pattern and its modification by serotonin are both determined by the characteristics of the arterial part of the bed.