Vascular pressure effects on lung edema formation after glass bead embolism

The canine lung lobe was embolized with 100-micron glass beads before lobectomy and blood anticoagulation. The lobe was isolated, ventilated, and pump-perfused with blood at an arterial pressure (Pa) of about 50 (high pressure, HP, n = 9) or 25 Torr (low pressure, LP, n = 9). Rus/PVR, the ratio of upstream (Rus) to total lobar vascular resistance (PVR), was determined by venous occlusion and the isogravimetric capillary pressure technique. The capillary filtration coefficient (Kf), an index of vascular permeability, was obtained from rate of lobe weight gain during stepwise capillary pressure (Pc) elevation. The embolized lobes became more edematous than nonembolized controls, (C, n = 11), (P less than 0.05), with Kf values of 0.20 +/- 0.04, 0.25 +/- 0.06, and 0.07 +/- 0.01 ml X min-1 X Torr-1 X 100 X g-1 in LP, HP, and C, respectively (P less than 0.05). The greater Rus/PVR in embolized lobes (P less than 0.05) protected the microvessels and, although Pc was greater in HP than in controls (P less than 0.05), Pc did not differ between HP and LP (P greater than 0.05). Although indexes of permeability did not differ between embolized groups (P greater than 0.05), HP became more edematous than LP (P less than 0.05). The greater edema in HP did not appear due to a greater imbalance of Starling forces across the microvessel wall or to vascular recruitment. At constant Pc and venous pressure, elevating Pa from 25 to 50 Torr in embolized lobes resulted in greater edema to suggest fluid filtration from precapillary vessels.     

Venous occlusion pressure and vascular permeability in the dog lung after air embolization

Pulmonary edema has frequently been associated with air embolization of the lung. In the present study the hemodynamic effects of air emboli (AE) were studied in the isolated mechanically ventilated canine right lower lung lobe (RLL), pump perfused at a constant blood flow. Air was infused via the pulmonary artery (n = 7) at 0.6 ml/min until pulmonary arterial pressure (Pa) rose 250%. While Pa rose from 12.4 +/- 0.6 to 44.6 +/- 2.0 (SE) cmH2O (P less than 0.05), venous occlusion pressure remained constant (7.0 +/- 0.5 to 6.8 +/- 0.6 cmH2O; P greater than 0.05). Lobar vascular resistance (RT) increased from 2.8 +/- 0.3 to 12.1 +/- 0.2 Torr.ml-1.min.10(-2) (P less than 0.05), whereas the venous occlusion technique used to determine the segmental distribution of vascular resistance indicated the increase in RT was confined to vessels upstream to the veins. Control lobes (n = 7) administered saline at a similar rate showed no significant hemodynamic changes. As an index of microvascular injury the pulmonary filtration coefficient (Kf) was obtained by sequential elevations of lobar vascular pressures. The Kf was 0.11 +/- 0.01 and 0.07 +/- 0.01 ml.min-1.Torr-1.100 g RLL-1 in AE and control lobes, respectively (P less than 0.05). Despite a higher Kf in AE lobes, total lobe weight gains did not differ and airway fluid was not seen in the AE group. Although air embolization caused an increase in upstream resistance and vascular permeability, venous occlusion pressure did not increase, and marked edema did not occur.     

Oleic acid dose-related edema in isolated canine lung perfused at constant pressure

Fatty acid embolism of the lung results in pulmonary edema. Isolated lung lobes ventilated and blood perfused at constant pressure were treated with 1 (n = 6) or 45 microliter/kg body wt (n = 6 oleic acid or saline (n = 7). Lobe weight increase linearly over 1-3 h following oleic with regression slopes indicating a more rapid rate of weight gain at the higher oleic acid dosage. Total lobe weight gain was greater in the 45 than in the 1 microliter/kg group (0.60 +/- 0.10 vs. 0.31 +/- 0.07 g/g initial lobe wt) and greater in the acid-treated lobes than in the controls (0.13 +/- 0.05 g/g initial lobe wt). Pulmonary vascular resistance increased 79% after 45 microliter/kg oleic acid but appeared unchanged following 1 microliter/kg oleic acid or saline. The decrease in arterial O2 partial pressure was greater in the 45 microliter/kg group than in the controls, 47 vs 22 Torr. High vascular pressures and increased flow velocities in patent vessels are not essential for oleic acid-associated edema, since weight increased at constant pressure perfusion. Weight gain related to oleic acid dosage suggests that oleic acid increases permeability by affecting the vascular endothelium either directly or through biochemical intermediates endogenous to the lung or blood.     

Pulmonary vascular reactivity and permeability to alveolar hypoxia in the dog

Hemodynamics and vascular permeability were studied during acute alveolar hypoxia in isolated canine lung lobes perfused at constant flow with autogenous blood. Hypoxia was induced in the presence (COI + Hypox, n = 6) or absence (Hypox, n = 6) of cyclooxygenase inhibition (COI) with indomethacin or meclofenamate. Hypoxic ventilation reduced blood PO2 from 143 to 25-29 Torr without a change in PCO2. During hypoxia a capillary filtration coefficient (Kf) was obtained gravimetrically as an index of vascular permeability to water. In COI + Hypox, pulmonary arterial pressure (Pa) increased from 11.5 +/- 0.7, post-COI normoxia, to a peak of 22.1 +/- 2.3 during hypoxia (P less than 0.01) without a change in capillary pressure (Pc). In contrast, hypoxia changed neither Pa nor Pc in Hypox relative to an untreated normoxic control group (Normox, n = 6, P greater than 0.05). Kfs (means +/- SE in ml.min-1.Torr-1.100 g-1) for Normox (0.070 +/- 0.014), Hypox (0.082 +/- 0.024), and COI + Hypox (0.057 +/- 0.017) did not differ from one another (P greater than 0.05). Although COI markedly enhanced the pressor response to acute alveolar hypoxia, hypoxia increased neither Pc nor vascular permeability regardless of COI.     

PO2 modulation of paraquat-induced microvascular injury in isolated dog lungs

We determined the effects of paraquat (PQ) concentrations ranging from 10(-3) to 10(-2) M and three levels of venous PO2 [hypoxia (41 +/- 3 Torr), normoxia (147 +/- 8 Torr), and hyperoxia (444 +/- 17 Torr)] in the presence of 4 x 10(-3) M PQ on microvascular permeability in isolated blood-perfused dog lungs. Capillary filtration coefficient (Kf,c) increased and isogravimetric capillary pressure (Pc,i) decreased 3 h after perfusion with 10(-2) M PQ (n = 7) and 5 h after perfusion with 4 x 10(-3) M PQ (n = 6) but not with 10(-3) M PQ (n = 4). In hyperoxic lungs perfused with 4 x 10(-3) M PQ, Kf,c increased to nine times the base-line value 5 h after PQ [0.15 +/- 0.01 to 1.35 +/- 0.25 (SE) ml.min-1.cmH2O-1.100 g-1]. Pc,i significantly decreased from a base-line value of 9.4 +/- 0.2 to 7.1 +/- 0.4 cmH2O at 3 h. In hypoxic lungs perfused with 4 x 10(-3) M PQ (n = 5), Pc,i and Kf,c changes were not significantly different from those in normoxic lungs treated with PQ. Thus both hyperoxia and an increased dose of PQ shortened the latent period and increased the severity of the PQ-induced microvascular permeability lesion, but hypoxia failed to prevent the PQ damage.     

Measurement of capillary pressure in humans using a venous occlusion method

The venous occlusion technique was used to measure capillary pressure in the forearm and foot of man over a wide range of venous pressures. In six recumbent subjects venous pressure (Pv) in the forearm (mean +/- SE) was 9.3 +/- 1.4 mmHg and the venous occlusion estimate of capillary pressure (Pc) was 17.0 +/- 1.6 mmHg, whereas in another six subjects Pv in the foot was 17.1 +/- 1.2 mmHg and Pc was 23.4 +/- 2.5 mmHg. Venous pressure in the limbs was increased either by changes in posture or by venous congestion with a sphygmomanometer cuff. On standing Pv in the foot increased to 95.2 +/- 1.5 mmHg and Pc rose to 112.8 +/- 3.1 mmHg. The relationship established between venous pressure and capillary pressure in the forearm is Pc = 1.16 Pv + 8.1, whereas in the foot the relationship is Pc = 1.2 Pv + 1.6. The magnitude and duration of the changes in capillary pressure were also recorded during reactive hyperemia. The venous occlusion method of measuring capillary pressure is simple and easily applied to studies in humans.     

Relationship of fluid filtration to lung vascular pressure during edema

Effect of edema on the relationship between rate of fluid filtration and vascular pressure was studied in ventilated isolated dog lung lobes blood-perfused at constant flow. Constant rate of lobe weight gain (S), representing transvascular fluid flux, was obtained at different venous pressures (Pv) as Pv was increased stepwise from 2 to 40 and then similarly decreased from 40 to 2 Torr (n = 6). In another group (n = 6), edema was maximized by reversing the sequence of Pv change; S was obtained during similar Pv steps as Pv was decreased from 40 to 2 and then returned to 40 Torr. In both groups, delta S was disproportionately greater for delta Pv at higher Pv's, with S vs. Pv fit by an exponential curve (P less than 0.001). The exponential relationship was independent of lung hydration inasmuch as greater edema on the second limb of Pv change did not alter the curve (P greater than 0.05). At 144% weight gain, interstitial compliance was 55.5 +/- 26.8 ml.100 g-1.Torr-1 (n = 10). Interstitial pressure reportedly remains constant, i.e., fails to increase to further buffer fluid filtration, after transition of the lung interstitium from low to high compliance at approximately 40% lung weight gain. If so, then the exponential S vs. Pv relationship observed in the present study at elevated interstitial compliance does not appear related to tissue pressure-buffering effects.     

Effects of arachidonate on permeability and resistance distribution in canine lungs

In this study, 14 canine lung lobes were isolated and perfused with autologous blood at constant pressure (CP) or constant flow (CF). Pulmonary capillary pressure (Pc) was measured via venous occlusion or simultaneous arterial and venous occlusions. Arterial and venous pressures and blood flow were measured concurrently so that total pulmonary vascular resistance (RT) as well as pre- (Ra) and post- (Rv) capillary resistances could be calculated. In both CP and CF perfused lobes, 5-min arachidonic acid (AA) infusions (0.085 +/- 0.005 to 2.80 +/- 0.16 mg X min-1 X 100 g lung-1) increased RT, Rv, and Pc (P less than 0.05 at the highest dose), while Ra was not significantly altered and Ra/Rv fell (P less than 0.05 at the highest AA dose). In five CP-perfused lobes, the effect of AA infusion on the pulmonary capillary filtration coefficient (Kf,C) was also determined. Neither low-dose AA (0.167 +/- 0.033 mg X min-1 X 100 g-1) nor high-dose AA (1.35 +/- 0.39 mg X min-1 X 100 g-1) altered Kf,C from control values (0.19 +/- 0.02 ml X min-1 X cmH2O-1 X 100 g-1). The hemodynamic response to AA was attenuated by prior administration of indomethacin (n = 2). We conclude that AA infusion in blood-perfused canine lung lobes increased RT and Pc by increasing Rv and that microvascular permeability is unaltered by AA infusion.     

Interaction of chemical and high vascular pressure injury in isolated canine lung

Because both chemical and mechanical insults to the lung may occur concomitantly with trauma, we hypothesized that the pressure threshold for vascular pressure-induced (mechanical) injury would be decreased after a chemical insult to the lung. Normal isolated canine lung lobes (N, n = 14) and those injured with either airway acid instillation (AAI, n = 18) or intravascular oleic acid (OA, n = 25) were exposed to short (5-min) periods of elevated venous pressure (HiPv) ranging from 19 to 130 cmH2O. Before the HiPv stress, the capillary filtration coefficient (Kf,c) was 0.12 +/- 0.01, 0.27 +/- 0.03, and 0.31 +/- 0.02 ml.min-1.cmH2O-1 x 100 g-1 and the isogravimetric capillary pressure (Pc,i) was 9.2 +/- 0.3, 6.8 +/- 0.5, and 6.5 +/- 0.3 cmH2O in N, AAI, and OA lungs, respectively. However, the pattern of response to HiPv was similar in all groups: Kf,c was no different from the pre-HiPv value when the peak venous pressure (Pv) remained less than 55 cmH2O, but it increased reversibly when peak Pv exceeded 55 cmH2O (P less than 0.05). The reflection coefficient (sigma) for total proteins measured after pressure exposure averaged 0.60 +/- 0.03, 0.32 +/- 0.04, and 0.37 +/- 0.09 for N, AAI, and OA lobes respectively. However, in contrast to the result expected if pore stretching had occurred at high pressure, in all groups the sigma measured during the HiPv stress when Pv exceeded 55 cmH2O was significantly larger than that measured during the recovery period.(ABSTRACT TRUNCATED AT 250 WORDS)     

Splenic baroreceptors control splenic afferent nerve activity

Stenosis of either the portal or splenic vein increases splenic afferent nerve activity (SANA), which, through the splenorenal reflex, reduces renal blood flow. Because these maneuvers not only raise splenic venous pressure but also reduce splenic venous outflow, the question remained as to whether it is increased intrasplenic postcapillary pressure and/or reduced intrasplenic blood flow, which stimulates SANA. In anesthetized rats, we measured the changes in SANA in response to partial occlusion of either the splenic artery or vein. Splenic venous and arterial pressures and flows were simultaneously monitored. Splenic vein occlusion increased splenic venous pressure (9.5 +/- 0.5 to 22.9 +/- 0.8 mmHg, n = 6), reduced splenic arterial blood flow (1.7 +/- 0.1 to 0.9 +/- 0.1 ml/min, n = 6) and splenic venous blood flow (1.3 +/- 0.1 to 0.6 +/- 0.1 ml/min, n = 6), and increased SANA (1.7 +/- 0.4 to 2.2 +/- 0.5 spikes/s, n = 6). During splenic artery occlusion, we matched the reduction in either splenic arterial blood flow (1.7 +/- 0.1 to 0.7 +/- 0.05, n = 6) or splenic venous blood flow (1.2 +/- 0.1 to 0.5 +/- 0.04, n = 5) with that seen during splenic vein occlusion. In neither case was there any change in either splenic venous pressure (-0.4 +/- 0.9 mmHg, n = 6 and +0.1 +/- 0.3 mmHg, n = 5) or SANA (-0.11 +/- 0.15 spikes/s, n = 6 and -0.05 +/- 0.08 spikes/s, n = 5), respectively. Furthermore, there was a linear relationship between SANA and splenic venous pressure (r = 0.619, P = 0.008, n = 17). There was no such relationship with splenic venous (r = 0.371, P = 0.236, n = 12) or arterial (r = 0.275, P = 0.413, n = 11) blood flow. We conclude that it is splenic venous pressure, not flow, which stimulates splenic afferent nerve activity and activates the splenorenal reflex in portal and splenic venous hypertension.     

Diastolic pressure determines autonomic responses to pressure perturbation in humans

Arterial baroreceptors reflexly regulate sympathetic and heart rate responses to alteration of blood pressure. The primary mechanical determinant of arterial baroreceptor activity in humans remains unclear. We examined the influence of systolic, diastolic, pulse, and mean arterial pressures on efferent muscle sympathetic nerve activity (MSNA, microneurography) and heart rate responses during perturbation of arterial pressure in 10 normal human subjects [age 25 +/- 2 (SE) yr]. We directly measured arterial pressure, heart rate, and MSNA during intravenous vasodilator infusion (nitroprusside, 6 +/- 1 micrograms.kg-1.min-1, n = 6; or hydralazine, 16 +/- 2 mg, n = 4) while central venous pressure was held constant by simultaneous volume expansion. Changes in arterial pressures were compared with changes in heart rate and MSNA over 3-min periods of vasodilator infusion during which we observed increases in systolic and pulse pressures with simultaneous decreases in mean and diastolic pressures. During vasodilator infusion, there were increases in systolic (124.2 +/- 2.1 to 131.7 +/- 2.9 Torr, P less than 0.001) and pulse pressures (57.0 +/- 2.2 to 72.7 +/- 2.7 Torr, P less than 0.001) although mean arterial pressure fell (88.0 +/- 2.6 to 80.4 +/- 2.7 Torr, P less than 0.001) because of decreases in diastolic pressure (67.2 +/- 3.0 to 59.0 +/- 2.7 Torr, P less than 0.001). The changes in arterial pressures were accompanied by simultaneous increases in heart rate (66.4 +/- 3.0 to 92.6 +/- 4.8 beats/min, P less than 0.001) and MSNA (327 +/- 59 to 936 +/- 171 U, P less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)     

Lung expansion and the perialveolar interstitial pressure gradient

We have determined the combined effects of lung expansion and increased extravascular lung water (EVLW) on the perialveolar interstitial pressure gradient. In the isolated perfused lobe of dog lung, we measured interstitial pressures by micropuncture at alveolar junctions (Pjct) and in adventitia of 30- to 50-microns microvessels (Padv) with stopped blood flow at vascular pressure of 3-5 cmH2O. We induced edema by raising vascular pressures. In nonedematous lobes (n = 6, EVLW = 3.1 +/- 0.3 g/g dry wt) at alveolar pressure of 7 cmH2O, Pjct averaged 0.5 +/- 0.8 (SD) cmH2O and the Pjct-Padv gradient averaged 0.9 +/- 0.5 cmH2O. After increase of alveolar pressure to 23 cmH2O the gradient was abolished in nonedematous lobes, did not change in moderately edematous lobes (n = 9, EVLW = 4.9 +/- 0.6 g/g dry wt), and increased in severely edematous lobes (n = 6, EVLW = 7.6 +/- 1.4 g/g dry wt). Perialveolar interstitial compliance decreased with increase of alveolar pressure. We conclude that increase of lung volume may reduce perialveolar interstitial liquid clearance by abolishing the Pjct-Padv gradient in nonedematous lungs and by compressing interstitial liquid channels in edematous lungs.     

Effect of mesenteric vascular congestion on reflex control of renal blood flow

Portal hypertension initiates a splenorenal reflex, whereby increases in splenic afferent nerve activity and renal sympathetic nerve activity cause a decrease in renal blood flow (RBF). We postulated that mesenteric vascular congestion similarly compromises renal function through an intestinal-renal reflex. The portal vein was partially occluded in anesthetized rats, either rostral or caudal to the junction with the splenic vein. Portal venous pressure increased (6.5 +/- 0.1 to 13.2 +/- 0.1 mmHg; n = 78) and mesenteric venous outflow was equally obstructed in both cases. However, only rostral occlusion increased splenic venous pressure. Rostral occlusion caused a fall in RBF (-1.2 +/- 0.2 ml/min; n = 9) that was attenuated by renal denervation (-0.5 +/- 0.1 ml/min; n = 6), splenic denervation (-0.2 +/- 0.1 ml/min; n = 11), celiac ganglionectomy (-0.3 +/- 0.1 ml/min; n = 9), and splenectomy (-0.5 +/- 0.1 ml/min; n = 6). Caudal occlusion induced a significantly smaller fall in RBF (-0.5 +/- 0.1 ml/min; n = 9), which was not influenced by renal denervation (-0.2 +/- 0.2 ml/min; n = 6), splenic denervation (-0.1 +/- 0.1 ml/min; n = 7), celiac ganglionectomy (-0.1 +/- 0.3 ml/min; n = 8), or splenectomy (-0.3 +/- 0.1 ml/min; n = 7). Renal arterial conductance fell only in intact animals subjected to rostral occlusion (-0.007 +/- 0.002 ml.min(-1).mmHg(-1)). This was accompanied by increases in splenic afferent nerve activity (15.0 +/- 3.5 to 32.6 +/- 6.2 spikes/s; n = 7) and renal efferent nerve activity (32.7 +/- 5.2 to 39.3 +/- 6.0 spikes/s; n = 10). In animals subjected to caudal occlusion, there were no such changes in renal arterial conductance or splenic afferent/renal sympathetic nerve activity. We conclude that the portal hypertension-induced fall in RBF is initiated by increased splenic, but not mesenteric, venous pressure, i.e., we did not find evidence for intestinal-renal reflex control of the kidneys.     

Alveolar pressure in fluid-filled occluded lung segments during permeability edema

In a model of increased hydrostatic pressure pulmonary edema Parker et al. (J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 44: 267-276, 1978) demonstrated that alveolar pressure in occluded fluid-filled lung segments was determined primarily by interstitial fluid pressure. Alveolar pressure was subatmospheric at base line and rose with time as hydrostatic pressure was increased and pulmonary edema developed. To further test the hypothesis that fluid-filled alveolar pressure is determined by interstitial pressure we produced permeability pulmonary edema-constant hydrostatic pressure. After intravenous injection of oleic acid in dogs (0.01 mg/kg) the alveolar pressure rose from -6.85 +/- 0.8 to +4.60 +/- 2.28 Torr (P less than 0.001) after 1 h and +6.68 +/- 2.67 Torr (P less than 0.01) after 3 h. This rise in alveolar fluid pressure coincided with the onset of pulmonary edema. Our experiments demonstrate that during permeability pulmonary edema with constant capillary hydrostatic pressures, as with hemodynamic edema, alveolar pressure of fluid-filled segments seems to be determined by interstitial pressures.     

Prostacyclin production with serotonin, increased flow, or elevated venous pressure in dog lung

The lung may release prostacyclin (PGI2) in response to humoral or mechanical stimuli. We measured 6 keto-PGF1 alpha as an index of PGI2 production during serotonin (5-HT) infusion, elevated venous pressure (Pv), or increased blood flow (Q) in the isolated canine lower left lung lobe (LLL). Lobar vascular resistance (LVR) was partitioned into arterial (Ra), middle (Rm), and venous (Rv) components by arterial and venous occlusions. The infusion of 55-210 micrograms/min 5-HT (n = 9) was associated with concomitant increases in PGI2 production and dose-related increases in pulmonary arterial pressure (Pa) and LVR. 5-HT increased Ra at each infusion rate, whereas Rm was not changed and Rv was increased only at the highest infusion rate. When Pa was increased by stepwise elevations in Pv from 3.7 to 19.1 cmH2O (n = 8) or by increases in Q from 250 to 507 ml/min (n = 5) to match the Pa increase observed during 5-HT infusion, PGI2 production was not altered. Increases in Pv reduced LVR largely by decreasing Ra, whereas increases in Q reduced LVR without changing Ra, Rm, or Rv. Infusion of 5-HT when Pa was held constant by reduction in blood flow (n = 6) did not increase PGI2. Thus infusion of 5-HT at a normal blood flow rate increased PGI2 formation in the isolated blood-perfused dog lung lobe. The results also suggest that sustained mechanical effects related to increased venous pressure or elevated blood flow are not associated with a sustained elevation of PGI2 formation.     

OKY-046 prevents increases in LTB4 and pulmonary edema in phorbol ester-induced lung injury in dogs.

Thromboxanes (Txs) were implicated as possible participants in the altered microvascular permeability of acute lung injury when the Tx synthase inhibitor, OKY-046, was reported to prevent pulmonary edema induced by phorbol myristate acetate (PMA). Recently, however, we found that OKY-046, at a dose just sufficient to block Tx synthesis in intact dogs, did not prevent PMA-induced pulmonary edema but rather merely reduced it modestly. The present study was designed to explore other mechanisms whereby OKY-046 might prevent PMA-induced pulmonary edema. The finding that 5-lipoxygenase (5-LO) metabolites of arachidonic acid were increased within the lung after PMA administration, coupled with the report that OKY-046 inhibited slow-reacting substance of anaphylaxis formation, permitted formulation of the hypothesis that OKY-046, at a dose in excess of that required to inhibit Tx synthesis, inhibits the formation of a product(s) of 5-LO and, thereby, prevents edema formation. In vehicle-pretreated pentobarbital-anesthetized male mongrel dogs (n = 4), PMA (20 micrograms/kg i.v.) increased pulmonary vascular resistance (PVR) from 4.4 +/- 0.3 to 26.3 +/- 8.8 mmHg.l-1 x min (P < 0.01) and extravascular lung water from 6.7 +/- 0.5 to 19.1 +/- 6.2 ml/kg body wt (P < 0.05). Concomitantly, both TxB2 and leukotriene B4 (LTB4) were significantly increased in the lung. Pretreatment with OKY-046 (100 mg/kg i.v., n = 8) prevented PMA-induced increases in TxB2, LTB4, and pulmonary edema formation but did not prevent the increase in PVR.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of portal hypertension on splenic blood flow,intrasplenic extravasation and systemic blood pressure

We have previously shown that intrasplenic fluid extravasation is important in controlling blood volume. We proposed that, because the splenic vein flows in the portal vein, portal hypertension would increase splenic venous pressure and thus increase intrasplenic microvascular pressure and fluid extravasation. Given that the rat spleen has no capacity to store/release blood, intrasplenic fluid extravasation can be estimated by measuring the difference between splenic arterial inflow and venous outflow. In anesthetized rats, partial ligation of the portal vein rostral to the junction with the splenic vein caused portal venous pressure to rise from 4.5 +/- 0.5 to 12.0 +/- 0.9 mmHg (n = 6); there was no change in portal venous pressure downstream of the ligation, although blood flow in the liver fell. Splenic arterial flow did not change, but the arteriovenous flow differential increased from 0.8 +/- 0.3 to 1.2 +/- 0.1 ml/min (n = 6), and splenic venous hematocrit rose. Mean arterial pressure fell (101 +/- 5.5 to 95 +/- 4 mmHg). Splenic afferent nerve activity increased (5.6 +/- 0.9 to 16.2 +/- 0.7 spikes/s, n = 5). Contrary to our hypothesis, partial ligation of the portal vein caudal to the junction with the splenic vein (same increase in portal venous pressure but no increase in splenic venous pressure) also caused the splenic arteriovenous flow differential to increase (0.6 +/- 0.1 to 1.0 +/- 0.2 ml/min; n = 8). The increase in intrasplenic fluid efflux and the fall in mean arterial pressure after rostral portal vein ligation were abolished by splenic denervation. We propose there to be an intestinal/hepatic/splenic reflex pathway, through which is mediated the changes in intrasplenic extravasation and systemic blood pressure observed during portal hypertension.     

Effects of abdominal pressure on venous return: abdominal vascular zone conditions

The effects of changes in abdominal pressure (Pab) on inferior vena cava (IVC) venous return were analyzed using a model of the IVC circulation based on a concept of abdominal vascular zone conditions analogous to pulmonary vascular zone conditions. We hypothesized that an increase in Pab would increase IVC venous return when the IVC pressure at the level of the diaphragm (Pivc) exceeds the sum of Pab and the critical closing transmural pressure (Pc), i.e., zone 3 conditions, but reduce IVC venous return when Pivc is below the sum of Pab and Pc, i.e., zone 2 conditions. The validity of the model was tested in 12 canine experiments with an open-chest IVC bypass. An increase in Pab produced by phrenic stimulation increased the IVC venous return when Pivc-Pab was positive but decreased the IVC venous return when Pivc - Pab was negative. The value of Pivc - Pab that separated net increases from decreases in venous return was 1.00 +/- 0.72 (SE) mmHg (n = 6). An increase in Pivc did not influence the femoral venous pressure when Pivc was lower than the sum of Pab and a constant, 0.96 +/- 0.70 mmHg (n = 6), consistent with presence of a waterfall. These results agreed closely with the predictions of the model and its computer simulation. The abdominal venous compartment appears to function with changes in Pab either as a capacitor in zone 3 conditions or as a collapsible Starling resistor with little wall tone in zone 2 conditions.     

Increases in lung expansion alter pulmonary hemodynamics in fetal sheep.

Prolonged increases in fetal lung expansion stimulate fetal lung growth and development, but the effects on pulmonary hemodynamics are unknown. Our aim was to determine the effect of increased fetal lung expansion, induced by tracheal obstruction (TO), on pulmonary blood flow (PBF) and vascular resistance (PVR). Chronically catheterized fetal sheep (n = 6) underwent TO from 120 to 127 days of gestational age (term approximately 147 days); tracheas were not obstructed in control fetuses (n = 6). PBF, PVR, and changes to the PBF waveform were determined. TO significantly increased lung wet weight compared with control (166.3 +/- 20.2 vs. 102.0 +/- 18.8 g; P < 0.05). Despite the increase in intraluminal pressure caused by TO (5.0 +/- 0.9 vs. 2.4 +/- 1.0 mmHg; P < 0.001), PBF and PVR were similar between groups after 7 days (TO 28.1 +/- 3.2 vs. control 34.1 +/- 10.0 ml.min(-1).100 g lung wt(-1)). However, TO markedly altered pulmonary hemodynamics associated with accentuated fetal breathing movements, causing a reduction rather than an increase in PBF at 7 days of TO. To account for the increase in intraluminal pressure, the pressure was equalized by draining the lungs of liquid on day 7 of TO. Pressure equalization increased PBF from 36.8 +/- 5.2 to 112.4 +/- 22.8 ml/min (P = 0.01) and markedly altered the PBF waveform. These studies provide further evidence to indicate that intraluminal pressure is an important determinant of PBF and PVR in the fetus. We suggest that the increase in PBF associated with pressure equalization following TO reflects an increase in growth of the pulmonary vascular bed, leading to an increase in its cross-sectional area.     

Validation of double vascular occlusion method for Pc,i in lung and skeletal muscle

Capillary pressures in isogravimetric lung and skeletal muscle measured with the double vascular occlusion technique (Pdo) were compared to those measured using the traditional gravimetric technique (Pc,i). Pressures were measured using both techniques in isolated blood-perfused canine lungs (n = 18), blood-perfused rat hindquarters before (n = 8) and after (n = 6) maximal dilatation with papaverine and in rat hindquarters perfused with an artificial plasma (n = 6). In both organs, regardless of vascular tone, the double vascular occlusion isogravimetric pressure was the same as the gravimetric Pc,i, and the two measurements were highly correlated. Lung: Pdo = -0.22 + 1.06 Pc,i (r = 0.85, P less than 0.01); hindquarter: Pdo = -1.03 + 0.99 Pc,i (r = 0.91, P less than 0.01). In addition, Pdo was the same at every combination of isogravimetric arterial and venous pressures tested. The results indicate that the more rapidly applied double vascular occlusion pressure yields an accurate measure of isogravimetric capillary pressure in isolated organs over a wide range of isogravimetric pressures.