Segmental Pulmonary Vascular Resistance Research Articles

Levosimendan reduces segmental pulmonary vascular resistancein isolated perfused rat lungs and relaxes human pulmonary vessels.

IntroductionLevosimendan is approved for acute heart failure. Within this context, pulmonary hypertension represents a frequent co-morbidity. Hence, the effects of levosimendan on segmental pulmonary vascular resistance (PVR) are relevant. So far, this issue has been not studied. Beyond that the relaxant effects of levosimendan in human pulmonary vessel are unknown. We addressed these topics in rats’ isolated perfused lungs (IPL) and human precision-cut lung slices (PCLS).Material and methodsIn IPL, levosimendan (10 μM) was perfused in untreated and endothelin-1 pre-contracted lungs. The pulmonary arterial pressure (PPA) was continuously recorded and the capillary pressure (Pcap) was determined by the double-occlusion method. Thereafter, segmental PVR, expressed as precapillary (Rpre) and postcapillary resistance (Rpost) and PVR were calculated. Human PCLS were prepared from patients undergoing lobectomy. Levosimendan-induced relaxation was studied in naïve and endothelin-1 pre-contracted PAs and PVs. In endothelin-1 pre-contracted PAs, the role of K+-channels was studied by inhibition of KATP-channels (glibenclamide), BKCa2+-channels (iberiotoxin) and Kv-channels (4-aminopyridine). All changes of the vascular tone were measured by videomicroscopy. In addition, the increase of cAMP/GMP due to levosimendan was measured by ELISA.ResultsLevosimendan did not relax untreated lungs or naïve PAs and PVs. In IPL, levosimendan attenuated the endothelin-1 induced increase of PPA, PVR, Rpre and Rpost. In human PCLS, levosimendan relaxed pre-contracted PAs or PVs to 137% or 127%, respectively. In pre-contracted PAs, the relaxant effect of levosimendan was reduced, if KATP- and Kv-channels were inhibited. Further, levosimendan increased cGMP in PAs/PVs, but cAMP only in PVs.DiscussionLevosimendan reduces rats’ segmental PVR and relaxes human PAs or PVs, if the pulmonary vascular tone is enhanced by endothelin-1. Regarding levosimendan-induced relaxation, the activation of KATP- and Kv-channels is of impact, as well as the formation of cAMP and cGMP. In conclusion, our results suggest that levosimendan improves pulmonary haemodynamics, if PVR is increased as it is the case in pulmonary hypertension.

Effect of increase of pulmonary blood flow on segmental pulmonary vascular resistance--with special emphasis on the pulmonary capillary bed

The decrease of pulmonary vascular resistance with increase of pulmonary blood flow has been attributed to the function of the pulmonary vascular bed, consisting of capillary recruitment or dilatation. However, experimental evidence of a decrease of capillary resistance is lacking. To clarify this phenomenon, we perfused isolated cat lungs with diluted autologous blood and measured pulmonary microvascular pressure while changing pulmonary blood flow rate. We set the initial flow rate to achieve Ppa of 20 cmH2O and changed the flow rate while keeping left atrial pressure (Pla) and alveolar pressure (Palv) constant at 10 cmH2O and 9 cmH2O, respectively. Thereafter, pulmonary flow rate was increased 2 to 3 fold, and pulmonary arterial pressure (Ppa) and venular pressure (Pvo) were measured by venous occlusion method. The initial flow rate was 150 to 270 ml/min and the achieved maximum flow rate was from 300 to 543 ml/min. There was a linear correlation between Ppa, Pvo, and flow rate, in which the slope (rate of increase of pressure with flow) of Ppa was slightly steeper than that of Pvo. We calculated total and segmental pulmonary vascular resistance from the mean pressure-flow relationship. The total and upstream resistance involving arterial and capillary segments decreased hyperbolically with increase in flow rate. The mean upstream resistance decreased about 54%, from 35.20 x 10(-3) (cmH2O/ml/min) at a flow rate of 150 ml/min to 16.27 x 10(-3) (cmH2O/ml/min) at a flow rate of 450 ml/min. The degree of this decrease was similar to the result observed by micropuncture method (Nagasaka, 1990). Downstream resistance, which represents venous resistance, did not change significantly with the flow rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Relative effects of cyclooxygenase and nitric oxide synthase inhibition on vascular resistances in neonatal lamb lungs.

Effective attenuation of pulmonary vasoconstriction is essential during early postnatal development when increased pulmonary vascular resistance (PVR) may lead to a resumption of right-to-left shunting across fetal channels. In addition, modulation of venous resistance contributes to normal lung fluid balance. This study was designed to identify the relative modulating effects of endothelium-derived nitric oxide (EDNO) and dilator prostaglandins (PG) on normoxic and hypoxic pulmonary vasomotor tone in young newborns. Total and segmental PVR were measured using inflow-outflow and double occlusion techniques in isolated lungs of 6-h-old lambs studied under control conditions or after blocking PG and/or EDNO synthesis with indomethacin and/or N omega-nitro-L-arginine, respectively. During normoxia, both indomethacin and N omega-nitro-L-arginine were required to increase total PVR, but EDNO appeared to have the greater modulating effect. Indomethacin markedly enhanced hypoxic pulmonary vasoconstriction of large and small arteries and small veins, whereas N omega-nitro-L-arginine caused a lesser, but significant, increase in hypoxic pulmonary vasoconstriction of small arteries and veins, suggesting that dilator PG played the dominant modulating role during hypoxia. In addition, PG synthesis appeared to be enhanced after inhibition of EDNO synthesis. In contrast, indomethacin caused a decrease in venous resistance, suggesting that constrictor prostanoids had a greater effect than dilator PG on this segment. EDNO had a modest modulating effect on venous resistance in these lungs. These data suggest that dilator PG and EDNO exert complementary effects in attenuating total and segmental PVR during normoxia and hypoxia in 6-hold lamb lungs.

Effect of alveolar hypoxia on segmental pulmonary vascular resistance and lung fluid balance in dogs

In a biventricular bypass preparation with constant-flow perfusion, pulmonary arterial pressure (Ppa), average pulmonary capillary pressure (Ppc), venous pressure (Pv), extravascular lung water volume (EVWd) and capillary permeability-surface area product for urea (PS) were determined in control animals and in animals subjected to alveolar hypoxia. During hypoxia, Ppa increased in a biphasic manner, the site of hypoxic pulmonary vasoconstriction being located in the arterial upstream segment. At baseline, Ppc values were identical in control and experimental animals (3.4 +/- 0.4 vs. 3.6 +/- 0.2 mmHg). During 150 min of airway hypoxia, the rise in Ppc (5.1 +/- 0.3 mmHg) did not exceed the rise in Ppc (4.9 +/- 0.5 mmHg) recorded in control animals at same time interval during normoxic ventilation. EVWd increased during hypoxia to values significantly higher than those obtained in control animals (0.559 +/- 0.036 vs. 0.466 +/- 0.027 mL water g-1 lung). PS remained unchanged at baseline level throughout experiments in both groups of animals. Present data suggest that lung oedema formation during alveolar hypoxia may be caused by increased transcapillary fluid loss preferentially through transcellular hydraulic pathways in capillary endothelial cells.

Segmental pulmonary vascular resistance following wood smoke inhalation

To locate the specific site (i.e., pulmonary arteries, veins, or capillaries) of increased pulmonary vascular resistance after wood smoke inhalation and to demonstrate whether the prostanoids, thromboxane B2 or 6-keto-prostaglandin F1 alpha, play a role in these vascular resistance changes. Prospective, randomized, controlled trial. Laboratory at a university medical center. Five mongrel dogs. The isolated canine left lower lobe preparation was used to measure changes in the pressure drop across the pulmonary arteries, veins, and capillaries. The left lower lobe was surgically isolated and perfused by a pump primed with autologous blood. The arterial and venous occlusion technique and the vascular pressure-flow relationship were used to assess changes in pulmonary vascular resistance. After baseline measurements, the left lower lobe was exposed to wood smoke for 2.5 mins and measurements were repeated. Smoke exposure caused an immediate (5 mins post-inhalation) increase in the total pressure gradient across the lobe (baseline = 9.8 +/- 0.5 torr [1.3 +/- 0.06 kPa]); smoke inhalation = 24.3 +/- 3.9 torr [3.24 +/- 0.5 kPa]; p < .05). Total pressure drop was partitioned longitudinally into pressure drops across arteries, veins, and the middle vessels. The increase in total pressure drop was associated with a moderate increase in the pressure drop across the middle vessels (baseline = 1.1 +/- 0.2 torr [0.14 +/- 0.02 kPa]; smoke inhalation = 5.2 +/- 1.1 torr [0.69 +/- 0.14 kPa]; p < .05); a large increase in the pressure drop across the veins (baseline = 4.8 +/- 1.3 torr [0.64 +/- 0.17 kPa]; smoke inhalation = 20.7 +/- 3.4 torr [2.7 +/- 0.45 kPa]; p < .05), and no significant change in the pressure drop across the arteries (baseline = 3.7 +/- 0.4 torr [0.49 +/- 0.05 kPa]; smoke inhalation = 4.8 +/- 0.5 torr [0.64 +/- 0.06 kPa]; p = NS). Increases in the pressure drop across the middle and venous vessels were transient and no longer significantly different from baseline 15 mins after smoke inhalation. Similarly, analysis of the pulmonary artery/blood flow data demonstrated that the mean slope and pressure intercept were greater than baseline only at 5 mins postsmoke inhalation (p < .05). Thromboxane B2 did not significantly change from baseline values after smoke exposure and prostaglandin F1 alpha demonstrated a slight but significant decrease 30 mins postsmoke. Pulmonary edema was measured gravimetrically (wet/dry weight ratio) and smoke significantly increased lung water in the left lower lobe (wet/dry weight ratio = 6.55 +/- 0.4) as compared with the normal left upper lobe (wet/dry weight ratio = 4.97 +/- 0.2). We conclude that smoke causes an intense but transient increase in the pressure drop across the venous segment that may accelerate the formation of pulmonary edema, which is not mediated by changes in thromboxane B2 or prostaglandin F1 alpha.

Segmental pulmonary vascular resistance during oleic acid lung injury in rabbits

We studied in isolated lungs the effects of oleic acid (OA) injury on the segmental distribution of vascular resistance. Vascular occlusion pressures were measured in control and OA-injured preparation over 90 min. Capillary filtration coefficient K F, C increased from 0.61(±0.10) to 0.91(±0.14) g·min −1·mmHg −1·(100 g) −1 in OA-injured lungs whereas it remained constant in control lungs. Total pulmonary vascular resistance changed little in both control and OA-injured lungs. OA injur resulted in a 15% increase of the double occlusion capillary pressure. In addition, the contribution of the microvasculature to the total vascular resistance rose from 8% to 22%. The increase in microvascular resistance was significant 15 min after OA on the arteriolar side and became significant 30 min later on the venular side. Oleic acid injury does not change the total pulmonary vascular resistance but alters the distribution of segmental resistances in the isolated rabbit lung, thereby contributing to the accumulation of lung water in this model of low pressure permeability edema.

Effects of N omega-nitro-L-arginine on total and segmental vascular resistances in developing lamb lungs.

To determine whether endothelium-derived nitic oxide (EDNO), like dilator prostaglandins, attenuates pulmonary vasomotor tone more in younger than in older newborns, we examined the effects of a nitric oxide synthase inhibitor, N omega-nitro-L-arginine (L-NA), on total and segmental pulmonary vascular resistance (PVR) in isolated blood-perfused cyclooxygenase-inhibited lungs of < 2-day-old (2D) and 1-mo-old (1M) lambs. Total PVR was determined both from steady-state pressure-flow curves and total pressure gradients (delta PT) measured at constant flow (100 ml.kg-1 x min-1). Pressure gradients across arterial (delta Pa), middle (delta Pm), and venous (delta Pv) segments were determined by inflow-outflow occlusion. In 1M lungs (n = 6), L-NA increased delta PT, delta Pa, and delta Pv during normoxia and hypoxia. However, delta Pm increased only during hypoxia, suggesting that EDNO attenuates resistance of small vessels more when tone is high. The response to L-NA in 2D lungs was variable. In four "responders" (2D"R"), normoxic and hypoxic delta PT and all segmental resistances increased markedly after L-NA, but in five "nonresponders" (2D"NR"), L-NA had an insignificant effect on delta PT. Moreover, control delta PT values were higher in 2D"NR" than in 2D"R" lungs, suggesting that basal EDNO activity was minimal in some young newborns. Nonetheless, EDNO appears to attenuate venous resistance in newborns, because L-NA increased delta Pv in all groups. The significance of and mechanism(s) responsible for lesser modulation of PVR by EDNO in some young newborns remain to be determined.

Effect of increased alveolar surface tension on segmental pulmonary vascular resistance.

The site of change in pulmonary vascular resistance (PVR) after surfactant displacement with the detergent diocytl sodium sulfosuccinate (OT) was studied in the isolated canine left lower lobe preparation. Changes in PVR were assessed using the arterial and venous occlusion technique and the vascular pressure-flow relationship. Changes in alveolar surface tension were confirmed from measurements of pulmonary compliance as well as from measurements of surface tension of extracts from lung homogenates. After surfactant depletion (the perfusion rate constant) the total pressure gradient (delta PT) across the lobe increased from 13.4 +/- 1 to 17.1 +/- 0.8 mmHg. This increase in delta PT was associated with a significant increase in the arterial and venous gradients (3.7 +/- 0.3 to 4.9 +/- 0.4 and 5.7 +/- 0.5 to 9.4 +/- 0.6 mmHg, respectively) and a decrease in middle pressure gradient (4.1 +/- 0.8 to 2.9 +/- 0.6 mmHg). The vascular pressure-flow relationship supported these findings and showed that the mean slope increased by 52% (P less than 0.05), whereas the pressure intercept decreased slightly but not significantly (3.7 +/- 0.7 to 3.2 +/- 0.8 mmHg). These results suggest that the resistance of arteries and veins increases, whereas the resistance of the middle segment decreases after surfactant depletion. These effects were apparently due to surface tension that acts directly on the capillary wall. Direct visualization of subpleural capillaries supported the notion that capillaries become distended and recruited as alveolar surface tension increases. In the normal lung (perfused at constant-flow rate) changes in alveolar pressure (Palv) were transmitted fully to the capillaries as suggested by equal changes in pulmonary arterial pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Segmental pulmonary vascular resistance in progressive hydrostatic and permeability edema.

We used the in situ blood-perfused left lower lobe preparation of the dog to examine the effect of hydrostatic and permeability edema on the slope and intercept of the vascular pressure-flow (P/Q) relationship and on the longitudinal distribution of vascular resistance with the arterial and venous occlusion technique. Hydrostatic edema (HE) was induced by raising the venous pressure, and permeability edema (PE) was induced with alpha-naphthylthiourea. When the hematocrit (Hct) of the perfusate was kept normal (approximately 40%), HE had no significant effect on either the slope or the intercept of the P/Q relationship or on the distribution of vascular resistance. PE caused a small increase in the intercept of the P/Q relationship and a small rise in the resistance of the vessels in the middle segment. In another series of HE experiments in which Hct was allowed to increase during edema formation, there was a marked increase in vascular resistance. We conclude that edema per se does not increase vascular resistance significantly and that the increases in vascular resistance which were observed previously by other investigators in the isolated lungs may be due to increases in blood hematocrit.