Critical closure in the canine pulmonary vasculature

Application of a novel in vivo imaging approach to measure pulmonary vascular responses in mice

Noninvasive imaging of the murine pulmonary vasculature is challenging due to the small size of the animal, limits of resolution of the imaging technology, terminal nature of the procedure, or the need for intravenous contrast. We report the application of laboratory-based high-speed, high-resolution x-ray imaging, and image analysis to detect quantitative changes in the pulmonary vascular tree over time in the same animal without the need for intravenous contrast. Using this approach, we detected an increased number of vessels in the pulmonary vascular tree of animals after 30 min of recovery from a brief exposure to inspired gas with 10% oxygen plus 5% carbon dioxide (mean ± standard deviation: 2193 ± 382 at baseline vs. 6177 ± 1171 at 30 min of recovery; P < 0.0001). In a separate set of animals, we showed that the total pulmonary blood volume increased (P = 0.0412) while median vascular diameter decreased from 0.20 mm (IQR: 0.15-0.28 mm) to 0.18 mm (IQR: 0.14-0.26 mm; P = 0.0436) over the respiratory cycle from end-expiration to end-inspiration. These findings suggest that the noninvasive, nonintravenous contrast imaging approach reported here can detect dynamic responses of the murine pulmonary vasculature and may be a useful tool in studying these responses in models of disease.

Pulmonary artery wave propagation and reservoir function in conscious man: impact of pulmonary vascular disease, respiration and dynamic stress tests

KEY POINTS

Wave travel plays an important role in cardiovascular physiology. However, many aspects of pulmonary arterial wave behaviour remain unclear. Wave intensity and reservoir-excess pressure analyses were applied in the pulmonary artery in subjects with and without pulmonary hypertension during spontaneous respiration and dynamic stress tests. Arterial wave energy decreased during expiration and Valsalva manoeuvre due to decreased ventricular preload. Wave energy also decreased during handgrip exercise due to increased heart rate. In pulmonary hypertension patients, the asymptotic pressure at which the microvascular flow ceases, the reservoir pressure related to arterial compliance and the excess pressure caused by waves increased. The reservoir and excess pressures decreased during Valsalva manoeuvre but remained unchanged during handgrip exercise. This study provides insights into the influence of pulmonary vascular disease, spontaneous respiration and dynamic stress tests on pulmonary artery wave propagation and reservoir function.

ABSTRACT

Detailed haemodynamic analysis may provide novel insights into the pulmonary circulation. Therefore, wave intensity and reservoir-excess pressure analyses were applied in the pulmonary artery to characterize changes in wave propagation and reservoir function during spontaneous respiration and dynamic stress tests. Right heart catheterization was performed using a pressure and Doppler flow sensor tipped guidewire to obtain simultaneous pressure and flow velocity measurements in the pulmonary artery in control subjects and patients with pulmonary arterial hypertension (PAH) at rest. In controls, recordings were also obtained during Valsalva manoeuvre and handgrip exercise. The asymptotic pressure at which the flow through the microcirculation ceases, the reservoir pressure related to arterial compliance and the excess pressure caused by arterial waves increased in PAH patients compared to controls. The systolic and diastolic rate constants also increased, while the diastolic time constant decreased. The forward compression wave energy decreased by ∼8% in controls and ∼6% in PAH patients during expiration compared to inspiration, while the wave speed remained unchanged throughout the respiratory cycle. Wave energy decreased during Valsalva manoeuvre (by ∼45%) and handgrip exercise (by ∼27%) with unaffected wave speed. Moreover, the reservoir and excess pressures decreased during Valsalva manoeuvre but remained unaltered during handgrip exercise. In conclusion, reservoir-excess pressure analysis applied to the pulmonary artery revealed distinctive differences between controls and PAH patients. Variations in the ventricular preload and afterload influence pulmonary arterial wave propagation as demonstrated by changes in wave energy during spontaneous respiration and dynamic stress tests.

Lung Circulation

The circulation of the lung is unique both in volume and function. For example, it is the only organ with two circulations: the pulmonary circulation, the main function of which is gas exchange, and the bronchial circulation, a systemic vascular supply that provides oxygenated blood to the walls of the conducting airways, pulmonary arteries and veins. The pulmonary circulation accommodates the entire cardiac output, maintaining high blood flow at low intravascular arterial pressure. As compared with the systemic circulation, pulmonary arteries have thinner walls with much less vascular smooth muscle and a relative lack of basal tone. Factors controlling pulmonary blood flow include vascular structure, gravity, mechanical effects of breathing, and the influence of neural and humoral factors. Pulmonary vascular tone is also altered by hypoxia, which causes pulmonary vasoconstriction. If the hypoxic stimulus persists for a prolonged period, contraction is accompanied by remodeling of the vasculature, resulting in pulmonary hypertension. In addition, genetic and environmental factors can also confer susceptibility to development of pulmonary hypertension. Under normal conditions, the endothelium forms a tight barrier, actively regulating interstitial fluid homeostasis. Infection and inflammation compromise normal barrier homeostasis, resulting in increased permeability and edema formation. This article focuses on reviewing the basics of the lung circulation (pulmonary and bronchial), normal development and transition at birth and vasoregulation. Mechanisms contributing to pathological conditions in the pulmonary circulation, in particular when barrier function is disrupted and during development of pulmonary hypertension, will also be discussed.

An effective model of blood flow in capillary beds

In this article we derive applicable expressions for the macroscopic compliance and resistance of microvascular networks. This work yields a lumped-parameter model to describe the hemodynamics of capillary beds. Our derivation takes into account the multiscale nature of capillary networks, the influence of blood volume and pressure on the effective resistance and compliance, as well as, the nonlinear interdependence between these two properties. As a result, we obtain a simple and useful model to study hypotensive and hypertensive phenomena. We include two implementations of our theory: (i) pulmonary hypertension where the flow resistance is predicted as a function of pulmonary vascular tone. We derive from first-principles the inverse proportional relation between resistance and compliance of the pulmonary tree, which explains why the RC factor remains nearly constant across a population with increasing severity of pulmonary hypertension. (ii) The critical closing pressure in pulmonary hypotension where the flow rate dramatically decreases due to the partial collapse of the capillary bed. In both cases, the results from our proposed model compare accurately with experimental data.

Gas exchange and pulmonary hypertension following acute pulmonary thromboembolism: has the emperor got some new clothes yet?

Patients present with a wide range of hypoxemia after acute pulmonary thromboembolism (APTE). Recent studies using fluorescent microspheres demonstrated that the scattering of regional blood flows after APTE, created by the embolic obstruction unique in each patient, significantly worsened regional ventilation/perfusion (V/Q) heterogeneity and explained the variability in gas exchange. Furthermore, earlier investigators suggested the roles of released vasoactive mediators in affecting pulmonary hypertension after APTE, but their quantification remained challenging. The latest study reported that mechanical obstruction by clots accounted for most of the increase in pulmonary vascular resistance, but that endothelin-mediated vasoconstriction also persisted at significant level during the early phase.

Pulmonary Vascular Response Patterns to Exercise: Is there a Role for Pulmonary Arterial Pressure Assessment during Exercise in the Post-Dana Point Era?

Pulmonary hypertension (PH) is often diagnosed late in its course when it purports a particularly poor prognosis. Exercise effectively unmasks early forms of several cardiopulmonary diseases but the role of performing pulmonary arterial pressure measurements during exercise in the evaluation of PH remains unclear. Whether pulmonary arterial pressure-flow relationships during exercise may provide a window into earlier diagnosis of functionally significant pulmonary arterial hypertension and left ventricular dysfunction,¹ or add incrementally to our armentarium of diagnostic tests and prognostic indicators in PH, is the topic of active ongoing investigation. Evidence is emerging that abnormal pulmonary arterial pressure response patterns to exercise, when properly indexed to increased blood flow, may help to identify early forms of heart failure and pulmonary arterial hypertension. This article will discuss approaches to performing hemodynamic measurements during exercise as well as the potential clinical utility of identifying normal and abnormal pulmonary vascular response patterns to exercise.

Cerebral blood flow velocity during mental activation: interpretation with different models of the passive pressure-velocity relationship

The passive relationship between arterial blood pressure (ABP) and cerebral blood flow velocity (CBFV) has been expressed by a single parameter [cerebrovascular resistance (CVR)] or, alternatively, by a two-parameter model, comprising a resistance element [resistance-area product (RAP)] and a critical closing pressure (CrCP). We tested the hypothesis that the RAP+CrCP model can provide a more consistent interpretation to CBFV responses induced by mental activation tasks than the CVR model. Continuous recordings of CBFV [bilateral, middle cerebral artery (MCA)], ABP, ECG, and end-tidal CO(2) (EtCO(2)) were performed in 13 right-handed healthy subjects (aged 21-43 yr), in the seated position, at rest and during 10 repeated presentations of a word generation and a constructional puzzle paradigm that are known to induce differential cortical activation. Due to its small relative change, the CBFV response can be broken down into standardized subcomponents describing the relative contributions of ABP, CVR, RAP, and CrCP. At rest and during activation, the RAP+CrCP model suggested that RAP might reflect myogenic activity in response to the ABP transient, whereas CrCP was more indicative of metabolic control. These different influences were not reflected by the CVR model, which indicated a predominantly metabolic response. Repeated-measures multi-way ANOVA showed that CrCP (P = 0.025), RAP (P = 0.046), and CVR (P = 0.002) changed significantly during activation. CrCP also had a significant effect of paradigm (P = 0.045) but not hemispheric dominance. Both RAP (P = 0.039) and CVR (P = 0.0008) had significant effects of hemispheric dominance but were not sensitive to the different paradigms. Subcomponent analysis can help with the interpretation of CBFV responses to mental activation, which were found to be dependent on the underlying model of the passive ABP-CBFV relationship.

Role of endogenous nitric oxide in endotoxin-induced alteration of hypoxic pulmonary vasoconstriction in mice

Pulmonary vasoconstriction in response to alveolar hypoxia (HPV) is frequently impaired in patients with sepsis or acute respiratory distress syndrome or in animal models of endotoxemia. Pulmonary vasodilation due to overproduction of nitric oxide (NO) by NO synthase 2 (NOS2) may be responsible for this impaired HPV after administration of endotoxin (LPS). We investigated the effects of acute nonspecific (N(G)-nitro-L-arginine methyl ester, L-NAME) and NOS2-specific [L-N6-(1-iminoethyl)lysine, L-NIL] NOS inhibition and congenital deficiency of NOS2 on impaired HPV during endotoxemia. The pulmonary vasoconstrictor response and pulmonary vascular pressure-flow (P-Q) relationship during normoxia and hypoxia were studied in isolated, perfused, and ventilated lungs from LPS-pretreated and untreated wild-type and NOS2-deficient mice with and without L-NAME or L-NIL added to the perfusate. Compared with lungs from untreated mice, lungs from LPS-challenged wild-type mice constricted less in response to hypoxia (69 +/- 17 vs. 3 +/- 7%, respectively, P < 0.001). Perfusion with L-NAME or L-NIL restored this blunted HPV response only in part. In contrast, LPS administration did not impair the vasoconstrictor response to hypoxia in NOS2-deficient mice. Analysis of the pulmonary vascular P-Q relationship suggested that the HPV response may consist of different components that are specifically NOS isoform modulated in untreated and LPS-treated mice. These results demonstrate in a murine model of endotoxemia that NOS2-derived NO production is critical for LPS-mediated development of impaired HPV. Furthermore, impaired HPV during endotoxemia may be at least in part mediated by mechanisms other than simply pulmonary vasodilation by NOS2-derived NO overproduction.

The critical closing pressure of the cerebral circulation

The critical closing pressure (CrCP) of the cerebral circulation indicates the value of arterial blood pressure (ABP) at which cerebral blood flow (CBF) approaches zero. Measurements in animals and in humans, have shown that the CrCP is significantly greater than zero. A simple mathematical model, incorporating the effects of arterial elasticity and active wall tension, shows that CrCP can be influenced by several structural and physiological parameters, notably intracranial pressure (ICP) and active wall tension. Due to the non-linear shape of the complete ABP-CBF curve, most methods proposed for estimation of CrCP can only represent the linear range of the pressure-flow (or velocity) relationship. As a consequence, only estimates of apparent CrCP can be obtained, and these tend to be significantly higher than the true CrCP. Estimates of apparent CrCP have been shown to be influenced by arterial PCO2, ICP, cerebral autoregulation, intra-thoracic pressure, and mean ABP. There is a lack of investigation, under well-controlled conditions, to assess whether CrCP is altered in disease states. Studies of the cerebral circulation need to take CrCP into account, to obtain more accurate estimates of cerebrovascular resistance changes, and to reflect the correct dynamic relationship between instantaneous ABP and CBF.

In vivo pressure-flow curve in unilateral rat lung ischemia-reperfusion injury

The pressure-flow (P-Q) curve has been widely used in many studies to describe the effects of various factors on vascular hemodynamics. It is not clear, however, whether unilateral ischemia-reperfusion (IR) alters the P-Q curve of the rat lung. In this study, we developed an in vivo P-Q curve using the unilateral (left) rat lung before and after IR. Animals were divided into two groups: sham and IR. The protocol of the IR group consisted of three periods: baseline, ischemia, and reperfusion. P-Q curves were obtained by altering blood flow of the left lung during the baseline and the reperfusion periods. The sham group received the same operation without IR procedure. An additional group was used to compare pulmonary blood flow measured by the microsphere and the ultrasonic methods. IR treatment rotated the P-Q curve toward the left, indicating an increase in resistance in the left lung. However, this rotation was not found in the sham group. A significant correlation (r = 0.87, P < 0.01) between percentages of blood flow obtained by the microsphere and ultrasonic methods in both right and left lungs was demonstrated. Therefore, we demonstrated a simple and useful technique to evaluate changes in the P-Q curves caused by IR in the unilateral rat lung model.

PEEP-induced pulmonary vasoconstriction in neonatal piglets: analysis by pressure-flow curves

We analyzed the effect of two levels of positive end-expiratory pressure (PEEP: 10 and 15 cm H2O) on pulmonary hemodynamics in neonatal piglet lungs isolated in situ and perfused extracorporeally using pulmonary artery pressure-flow (Pa/Q) relationships. Pulmonary artery pressure (Pa) was measured at flow rates of 50, 75, 100, 125, and 150 mL/kg/min. Pa/Q relationship was evaluated by the slope of the Pa/Q plot and the zero-flow intercept pressure (Pi). Pa/Q relationship with PEEP was studied before and after verapamil. Both levels of PEEP increased the slope of the Pa/Q plot and Pi. PEEP of 15 cm H2O resulted in a steeper slope and a higher Pi compared to 10 cm H2O of PEEP (P < 0.05). Verapamil abolished the increase in slope of the pulmonary artery Pa/Q plot but did not affect the increase in Pi with PEEP. The increase in Pi was equal to the increase in mean airway pressure. Verapamil did not affect changes in ventilatory parameters. PEEP increased pulmonary vascular resistance (PVR) both by increasing the Pi, which reflects the weighted average of the critical closing pressure, and represents a "Starling resistor" phenomenon, and an increase in the slope of the P-Q plot, reflecting an increase in pulmonary vascular tone. This response may be unique to the neonatal pulmonary circulation.

Efficacy of inhaled prostanoids in experimental pulmonary hypertension

OBJECTIVES

To evaluate the effects of inhaled prostacyclin (PGI2) and inhaled as well as intravenous prostaglandin E1 (PGE1) on thromboxane A2 mimetic-induced pulmonary vasoconstriction. Active pulmonary vasoconstriction was to be distinguished from passive resistance to blood flow.

DESIGN

Prospective, randomized, crossover study.

SETTING

Experimental animal laboratory.

SUBJECTS

Eight anesthetized and paralyzed sheep.

INTERVENTIONS

The stable thromboxane A2 mimetic, U46619, was infused in increasing dosage to obtain a stable pulmonary hypertension of approximately 30 mm Hg. Subsequently, PGE1 aerosol (0.6, 6, 58, 259 ng/kg/min), intravenous PGE, (0.5 microg/kg/min), or PGI2 aerosol (27 ng/kg/min) were administered in randomized order.

MEASUREMENTS AND MAIN RESULTS

Active pulmonary vasoconstriction was assessed by determining the pulmonary pressure-flow relationship (PPFR). For measurement of pulmonary artery flow, an ultrasound flow probe was placed around the pulmonary artery after a sternotomy. Pulmonary arterial pressure was measured with a pulmonary artery flotation catheter. Flow was varied by partial occlusion of the inferior vena cava or incremental opening of an arterio-venous fistula between the large neck vessels. The primary end points were the slope of the resulting linear pressure-flow relationship, and pulmonary vascular resistance (PVR). Infusion of U46619 increased the slope of the PPFR (2.9+/-0.7 vs. 4.2+/-1.2 mm Hg/L/min [median+/-semi-interquartile range]; p < or = .05), and PVR (221+/-20 vs. 424+/-57 dyne x sec/cm5) (p < .05). Neither dose of PGE1 aerosol induced changes of the slope of PPFR or PVR. In contrast, intravenous administration of the same drug reduced the slope of the PPFR (4.0+/-1.0 vs. 3.1+/-0.4) (p < .05) but left PVR unchanged. Inhalation of PGI2 reduced both the slope of the PPFR, slightly but significantly, and PVR (424+/-98 vs. 323+/-26 dyne x sec/cm5) (p < .05).

CONCLUSIONS

This study is the first to show reduction of active pulmonary vasoconstriction by PGI2 aerosol. Neither inhalation nor intravenous administration of PGE1 reduced PVR but the latter reduced the slope of PPFR. We conclude that PGE1 has potential for pulmonary vasodilation, but that it is ineffective as an aerosol, even in high doses, in sheep. PVR may fail to reflect drug-induced pulmonary vasodilation.

A blind, randomized comparison of the circulatory effects of dopamine and epinephrine infusions in the newborn piglet during normoxia and hypoxia

OBJECTIVE

To determine the hemodynamic responses to dopamine and epinephrine infusions in newborn piglets during normoxia and hypoxia.

DESIGN

Prospective, randomized, blind cross-over study.

SUBJECTS

Newborn piglets (n = 7).

INTERVENTIONS

Animals were acutely instrumented for measurements of cardiac output, pulmonary and systemic pressures, carotid and coronary artery blood flow, and coronary artery oxygen consumption. Dopamine at infusion rates of 2 to 16 micrograms/kg/min and epinephrine 0.2 to 1.6 micrograms/kg/min were administered during normoxia. Six piglets were similarly prepared and were then made hypoxic to an arterial O2 saturation of 45% to 50%. Epinephrine at infusion rates of 0.2 to 3.2 micrograms/kg/min and dopamine at rates of 2 to 32 micrograms/kg/min were administered in random order during hypoxia.

MEASUREMENTS AND MAIN RESULTS

During normoxia, cardiac output increased similarly with both drugs and was significantly increased by > or = 0.2 micrograms/kg/min of epinephrine and significantly increased by 8 or 16 micrograms/kg/min of dopamine. Mean arterial blood pressure was not affected by dopamine but was significantly increased by epinephrine at a rate of 1.6 micrograms/kg/min. The relative effects of the drugs on pulmonary and systemic vascular resistance differed, the pulmonary/systemic vascular resistance ratio was reduced at the higher doses of epinephrine (i.e., 0.8 and 1.6 micrograms/kg/min) and was unaffected by dopamine. Coronary artery oxygen consumption and coronary blood flow increased significantly with both medications at rates > 0.4 and 4 micrograms/kg/min, respectively. Increases of both variables were greater with epinephrine than with dopamine. Myocardial extraction ratio was unaffected by dopamine and reduced at 0.2 and 1.6 micrograms/kg/min of epinephrine. Hypoxia caused significant increases in cardiac index, systemic blood pressure, pulmonary arterial pressure, carotid artery blood flow, coronary artery blood flow, coronary oxygen consumption, coronary oxygen extraction ratio, and the pulmonary/systemic vascular resistance ratio. Mean systemic arterial blood pressure increased significantly with 1.6 and 3.2 micrograms/kg/min of epinephrine, but was not significantly affected by dopamine at any infusion rate. Cardiac index was not affected significantly by either of the medications. Thus, there was a significant increase in the calculated systemic vascular resistance index with the highest dose of epinephrine, in contrast to the slight, statistically significant, decrease in calculated systemic vascular resistance index with the highest dose of dopamine. Epinephrine significantly reduced pulmonary arterial pressures at 0.2, 0.4, and 0.8 microgram/kg/min. Dopamine had no effect on this variable. The pulmonary/systemic vascular resistance ratio was significantly reduced by epinephrine at doses of 0.2 and 3.2 micrograms/kg/min, whereas the highest dose of dopamine caused a significant increase in the pulmonary/systemic vascular resistance ratio.

CONCLUSIONS

Epinephrine infusion during normoxia increases systemic pressure more than pulmonary arterial pressure at doses > or = 8 micrograms/kg/min, and furthermore, produces a more appropriate hemodynamic profile in the presence of hypoxic pulmonary hypertension than dopamine infusion, in the acutely operated anesthetized piglet.

Effects of progressive vascular occlusion on slope and intercept of the pulmonary artery pressure-flow relationship

Pulmonary hemodynamics may be described by mean pulmonary arterial pressure (PAP)-cardiac output (CO) plots. The slope of the PAP-CO relationship may define the incremental resistance (IR), and the extrapolated pressure intercept (P(I)), the effective outflow pressure. The authors investigated the effects of progressive pulmonary vascular occlusion on the IR and P(I) of the PAP-CO plot. Nine experimental and nine time control dogs were studied. In the former group, PAP-CO plots were obtained in three conditions: (1) Baseline, (2) following occlusion of the right pulmonary artery, and (3) following occlusion of the right pulmonary artery and blood flow to the left upper lobe. Following progressive occlusion, there was a corresponding increase in the IR of the PAP-CO plot, from 1.95 to 3.62 to 5.16 mmHg.1-1.min (all P < 0.05). In contrast to the increase in IR, P(I) remained constant. Over the same interval, there were no changes in IR or P(I) in the time control group. These findings indicate that changes in the slope of the PAP-CO plot correspond to changes in the number of parallel vascular units.

Effect of hydralazine on vascular mechanics in a canine lobar preparation of pulmonary embolism

We studied the effect of hydralazine (H) on pulmonary vascular mechanics in an isolated, in situ, canine lobe model of normal and increased pulmonary vascular resistance (Rp) produced by Gelfoam embolization (GE). Pulmonary pressure-flow (P-Q) curves from 24 lobes were obtained at baseline and after each intervention. Hemodynamic parameters for analysis included: the mean critical closing pressure (Ppai), vascular conductance (1/Rp), lobar flow (QL), and the pulmonary inflow pressure (Ppa) at different levels (50, 100, 200, 400, and 600 ml/min) of a fixed flow. After the preparation was stabilized, the 24 lobes were classified into 2 groups. For group 1 (n = 8) we studied the effect of H on the normal pulmonary vasculature. In group 2 (n = 16) we studied the effect of GE. Following GE, this group was further divided in half. For group 2A (n = 8) we followed the natural history of GE with measurements at 15 and 60 min. For group 2B (n = 8) measurements were done 15 min after GE and repeated again 15 min after the infusion of H. For group 1 lobes, H promoted a significant decrease (p less than 0.001) in Ppa at fixed flows of 200, 400, and 600 ml/min compared to baseline, with no change in Ppa for flows below 100 ml/min. QL and 1/Rp increased (p less than 0.01), and there was not any significant change in Ppai. In group 2A lobes, GE produced an increase in Ppa at all levels of flow (p less than 0.01), QL and 1/Rp decreased (p less than 0.05), and there was an increase in Ppai (p less than 0.05). These changes remained stable over the 60 min of observation. For group 2B lobes, GE produced the same hemodynamic changes as in group 2A, and the infusion of H caused a decrease in Ppa at flows between 100 and 300 ml/min. (p less than 0.01) with no change in Ppa at flows below 100 ml/min. QL and 1/Rp increased (p less than 0.01) and Ppai did not change compared to 15 min after GE. We conclude that in the normal canine pulmonary vasculature as well as in the model of GE, H decreased Rp and did not affect mean critical closing pressure, all of which may be explained by an increase in vascular conductance due to an increase in vascular distensibility.

The pulmonary circulation in acute lung injury: a review of some recent advances

1. According to the Starling resistor model of the pulmonary circulation, the pulmonary hypertension of oleic acid lung injury, an experimental model close to the early stage of clinical ARDS, primarily results from an increased vascular closing pressure which exceeds Pla and becomes the effective outflow pressure of the pulmonary circulation. Therefore, calculated pulmonary vascular resistance should be interpreted cautiously during haemodynamic investigations in patients with ARDS. 2. Part of this increased vascular closing pressure is functional. During acute lung injury pulmonary vasomotor tone can be reduced by vasodilators, or increased by cyclooxygenase inhibitors and almitrine. 3. Pulmonary vasodilation due to infused vasodilators usually impairs gas exchange in ARDS. 4. There is evidence that HPV is altered during ARDS. Drugs capable of enhancing the efficacy of HPV could improve gas exchange. If proven safe in the future, cyclooxygenase inhibitors and almitrine are interesting compounds to be tested in ARDS.

Vasodilators do not abolish pulmonary vascular critical closing pressure

To examine whether the critical closing pressure (Pcrit) of the pulmonary vasculature is dependent upon vasomotor tone, we measured Pcrit in six dog lobes before and after the administration of vasodilators. We evaluated the pressure-flow (P-Q) relationship in zone 2 flow conditions in situ perfused dog lobe (control period). We calculated Pcrit as the mean extrapolated zero-flow pressure intercepts for the P-Q relationship. We also used arterial and venous occlusions under zone 3 conditions to partition pulmonary vascular resistance into arterial, middle and venous segment resistances. We then repeated all measurements following administration of papaverine (150 micrograms/ml) and sodium nitroprusside (200 micrograms/min) into the venous reservoir (vasodilator period). Resistance in all three vascular segments was significantly reduced during vasodilator conditions. Pcrit decreased from 3.68 +/- 0.76 cm H2O to 2.53 +/- 0.92 cm H2O during control and vasodilator periods respectively (P less than 0.05). The slopes of the P-Q relationships were similar during both conditions. Our data support a model in which vasomotor tone normally sets Pcrit but in which the pulmonary vasculature can exhibit the phenomenon of critical closure even with vasomotor tone pharmacologically ablated.

Problems associated with the determination of pulmonary vascular resistance

The presence of critical pressures in the pulmonary circulation complicates the traditional use of pulmonary vascular resistance (PVR). The recruitable nature of the pulmonary circulation violates a basic assumption of the PVR formula, that is, that the involved vessels are rigid-walled. Flow through collapsible blood vessels is subject to the influence of critical opening pressures in addition to inflow and outflow pressures. As a result, PVR has a variable relationship to the Poiseuille resistance, approximating it better when zone 3 conditions predominate. In addition to being flow-dependent, PVR cannot easily distinguish among vasodilation, recruitment, and rheologic changes. PVR may be viewed as an index of steady-state power dissipation by the circulation, describing the relationship between power dissipation and flow, but it will still underestimate power dissipation by as much as 50%, since it cannot express oscillatory and kinetic power components. Laboratory data regarding the pulmonary circulation are predicted and explained by positing the existence of critical pressures in the pulmonary circulation and allow estimation of Poiseuille resistance. Unfortunately, clinical application of this approach is difficult owing to the necessity of generating pressure-flow plots under very stringent conditions. The clinical use of both pressure-flow and PVR-flow plots is impaired by shifting to different curves during hemodynamic manipulation. PVR must be interpreted in light of its considerable limitations.

Halothane inhibits hypoxic pulmonary vasoconstriction in the presence of cyclooxygenase blockade

Using an isolated lung the effects of halothane on hypoxic pulmonary vasoconstriction (HPV) were studied in the presence of cyclooxygenase blockade. The pulmonary vasculature can be divided into arterial, middle and venous segment resistances. Analysis of the vascular pressure-flow relationship further separates resistance into a flow dependent resistance (1/slope) and a zero-flow pressure intercept (PCRIT). We ventilated six lobes with control (35 per cent O2) and hypoxic (three per cent O2) gas mixtures with the addition of either 0, 0.5, 1.0, or 2.0 per cent halothane. We found that after addition of indomethacin (5 mg.kg-1), ventilation with three per cent O2 increased total resistance by 87 per cent over baseline with the increase primarily in the middle vascular segment. During normoxic ventilation PCRIT was 7.9 cm H2O and this increased significantly with hypoxia to 11.5 cm H2O). Only 2.0 per cent halothane blocked the increases in middle segment resistance and in PCRIT. We conclude that following cyclooxygenase blockade, halothane inhibits HPV by acting on middle segment vessels.

Vascular properties of canine lungs perfused with Eurocollins solution and prostacyclin

Although Eurocollins solution (ECS) is commonly used for lung preservation, its vascular effects and their time course and response to pharmacological interventions are not well understood. The effect of 4 degrees C ECS on the pulmonary circulation was assessed in excised canine left lower lobes. The roles of static oxygen inflation and prostacyclin infusion during ECS perfusion were also examined. The lobes were divided into five groups: time control (A), ECS with oxygen (B), ECS without oxygen (C), ECS with glycine buffer (D), and ECS with prostacyclin (E); group D was the control for E. Eurocollins solution had no effect on gas exchange but had a marked effect on the pulmonary circulation. Vascular conductance decreased from 22.6 to 18.9 mL/min/cm H2O and from 21.3 to 14.1 mL/min/cm H2O with average vascular closure increasing by 1.2 and 2.1 cm H2O in groups B and C, respectively. The decreased vascular conductance and increased vascular closure was associated with a reduction in vascular compliance from 1.63 to 1.25 mL/cm H2O. When prostacyclin was added to ECS, the reduction in vascular closure was much less and was associated with a decrease in vascular closure and no loss of vascular compliance. Eurocollins solution increases pressure cost for perfusion by causing both vascular obstruction and increased tone, especially when oxygen is not provided. This is significantly overcome by addition of prostacyclin infusion during ECS perfusion.

The effect of ketanserin on serotonin-induced vascular responses in the isolated perfused rat lung

Based on a vasoconstrictor role for serotonin (5-HT) in various types of cardiopulmonary conditions the effect of the 5-HT2 receptor antagonist, ketanserin, on 5-HT-induced responses in the intact, isolated rat lung was investigated. 5-HT produced biphasic responses consisting of weak vasodilator and predominant, dose-dependent constrictor components. Ketanserin showed no direct vasodilator effects but could completely antagonise the 5-HT-induced vasoconstrictor responses, suggesting that this response was 5-HT2 receptor-mediated.

The pulmonary circulation in essential systemic hypertension

Seventy-one men, ages 16 to 59 years, were referred for systemic hypertension, which was without detectable cause and with limited organ damage (World Health Organization stages I to II). They performed a graded exercise test on the bicycle ergometer in the sitting position. Mean brachial intraarterial pressure, mean pulmonary artery and wedge pressures and cardiac output (Fick method) were measured. At rest mean brachial artery pressure ranged from 72 to 168 mm Hg. Mean pulmonary wedge pressure was significantly (p less than 0.05) related to mean brachial artery pressure at rest, at submaximal work (50 watts) and at the end of exercise (161 +/- 42 [standard deviation] watts). In each subject pulmonary vascular resistance was calculated as the slope of the relation between the pressure gradient across the pulmonary circulation and cardiac output from data at rest, at 50 watts and at the end of exercise; mean critical closing pressure was calculated as the intercept of this relation. Pulmonary vascular resistance averaged 0.63 +/- 0.37 mm Hg/liter/min and was significantly related to age (r = 0.28, p less than 0.05) but not to rest brachial artery pressure (r = 0.14) or pulmonary wedge pressure (r = 0.09, difference not significant for both). The mean critical closing pressure averaged 6.1 +/- 4.0 mm Hg and was not related to brachial artery pressure (r = -0.08) or to age (r = -0.18, difference not significant for both). It is concluded that there is neither a primary nor a secondary effect of systemic hypertension on the pulmonary vasculature in patients with World Health Organization stages I to II essential hypertension.

Determinants of pulmonary blood volume. Effects of acute changes in airway pressure

To examine the effects of airway pressure (AWP) on pulmonary blood volume (PBV) at various pulmonary vascular pressures and flows, experiments were performed in anaesthetized, open-chest dogs. The AWP was raised by elevating end-expiratory pressure, and PBV was calculated as the product of electromagnetic aortic flow and pulmonary mean transit time for ascorbate (polarographic method). When AWP was raised from 3 to 13 mmHg, changing lung conditions from zone 3 [left atrial pressure (LAP) higher than AWP] to zone 2 (AWP higher than LAP), PBV decreased by 14.5 +/- 6.2%. When LAP was raised above 7 mmHg at constant pulmonary arterial pressure (PAP), PBV increased under zone 2 but not under zone 3 conditions. During blood volume expansion to LAP 15 mmHg, PBV rose by 30-50% and became equal at AWP of 4 and 14 mmHg, whereas the pulmonary vascular resistance remained 40% higher at high AWP. These data suggest that PAP, LAP and AWP regulate PBV by acting on compliant vessels surrounding the alveoli. Under zone 2 conditions with collapsed aveolar capillaries, elevation of LAP results in re-expansion of the alveolar capillaries, and PBV is restored without a rise in PAP. Under zone 3 conditions, a rise in LAP cannot increase PBV without raising PAP, explaining why PBV remains constant when PAP is kept constant.

The rectilinear pressure-flow relationship in the pulmonary vasculature: zones 2 and 3

To elucidate the significance of the rectilinear pressure-flow relationship of the pulmonary vasculature under zone 2 and 3 conditions, isolated left lower lobes of dog lungs were perfused at different flow rates under the two zone conditions. The lobar arterial pressure (Pa) was measured directly, whereas the pressures in the peripheral end of the arteries (Pa') and veins (Pv') were measured with the arterial and venous occlusion (AVO). We thus obtained three pressure-flow relationships: Pa-Q, Pa'-Q and Pv'-Q. From these relationships the contribution of each of the three segments (defined by the arterial and venous occlusion) to the slope and intercept of the Pa-Q relationship was determined. To a first approximation, in zone 3 the ohmic resistance was mainly in the arterial and venous segments, whereas the Starling resistance, although small, was equal in the three segments. In zone 2 (Palv = 15 mm Hg) the Starling resistance increased markedly in the middle segment. Because zone 3 and 2 conditions can co-exist within the lung, a simple three-segment model to represent the pulmonary vasculature was inadequate to explain all the pressure-flow data and therefore was modified to include two parallel channels one of which contains a critical closing pressure (Pc). Such a model implies that two different driving pressures determine the total flow: in vascular channels where Pc greater than Pv (outflow venous pressure) flow is determined by Pa-Pc gradient, and in channels were Pc less than Pv flow would be determined by the Pa-Pv gradient.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of acute pulmonary hypertension on pressure/flow in the canine pulmonary vascular bed

The relationship of pulmonary blood flow and vascular resistance to pulmonary artery pressure was examined in pentobarbital-anesthetized open-chest dogs. The pulmonary artery perfusion pressure was varied in the range 0-100 mm Hg. This was achieved by perfusion of the lobar artery supplying the left upper lobe of the lung from the subclavian artery. Perfusion pressure was varied by a clamp on the external shunt. The relationship was rectilinear in the lower pressure range, from zero flow at 5 mm Hg, to 45-55 mm Hg. Above this, curvilinearity developed, concave toward the pressure axis. Possible participation of the autonomic nervous system (ANS) in the P/F relationship was examined by treatment with the ganglionic blocking agent, hexamethonium. Although the drug was successful in lowering systemic arterial pressure, it had no effect on the pulmonary P/F relationship. Possibly, the sympathetic nervous system (SNS) tone to pulmonary vessels was low or absent in our preparation. Changes in compliance of the pulmonary arterial supply, resulting from changes in transmural pressure, may explain the results. A myogenic type of response is favored.

The effect of lung edema on pulmonary vasoactivity of furosemide

Previous data suggest that furosemide improves gas exchange in pulmonary edema by preferential perfusion of nonedematous lung units. To test whether this is a direct effect of furosemide on the pulmonary vasculature as opposed to a secondary phenomenon resulting from the known peripheral effects of this drug, the effect of furosemide on the pressure-flow characteristics of the pulmonary vasculature was studied in six isolated perfused canine lungs with different degrees of gravimetrically determined edema. Furosemide shifted the pressure-flow curve by decreasing the mean intercept or average closing pressure of the pulmonary vascular bed from 13.8 +/- 5.3 to 9.5 +/- 5.4 cm H2O and the zero-flow critical closing pressure from 9.3 +/- 4.3 to 4.7 +/- 3.5 cm H2O (P less than 0.05). The slopes of these curves did not change between control and furosemide treatment. The decrease in intercept and the decrease in zero-flow critical closing pressures were closely correlated with the increase in edema (r = 0.895 for average closing pressure and r = -0.928 for critical closing pressure) (P less than 0.05). Furosemide doubled the pulmonary blood flow in the isolated lobe for the same driving pressure and the greater the amount of lobar edema the less pronounced was this furosemide-associated increase in blood flow. This direct effect of furosemide on the pulmonary vasculature could explain the improved gas exchange seen before a decrease in pulmonary edema, since this pulmonary vasoactivity would result in preferential perfusion of nonflooded alveolar units.