INTRODUCTION

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Pulmonary hypertension occurs frequently in the acute respiratory distress syndrome (ARDS), but right ventricular failure is relatively uncommon, and rarely identified as a cause of death (1). This apparent contradiction may be explained not only by a generally moderate increase in pulmonary artery pressure (Ppa), but also by the increase in pulmonary vascular resistance (Rpva) occurring peripherally. In both clinical (6) and experimental (7) ARDS pulmonary artery pressure is affected very little by changes in pulmonary blood flow (), which is probably due to an increased closing pressure of small extraalveolar vessels (8). Partitioning of Rpva by the occlusion method to calculate an effective capillary pressure (Pc') has shown arterial and venous components that were not greatly different from normal in clinical (9) and in experimental (10) ARDS, in keeping with an increase in small vessel resistance. Experimental ARDS induced by the injection of small glass beads has been shown to affect pulmonary vascular impedance (Zpva) spectra by decreased or unchanged characteristic impedance (Zc) and a minimal effect on wave reflection, thereby limiting right ventricular afterload (11). However, until now there has been no study combining all these methodological approaches to evaluate the functional state of the pulmonary circulation in oleic acid-induced acute lung injury, which is one of the most commonly used experimental animal models of ARDS (14).

Therefore, to test the hypothesis that right ventricular failure is uncommon in ARDS not only because of the limited severity of pulmonary hypertension, but also because of specific characteristics of both steady and pulsatile flow hemodynamic alterations, we investigated pulmonary arterial function in intact dogs before and after oleic acid lung injury by concomitant determinations of multipoint Ppa/ plots, partitioning of Rpva, and Zpva.

	METHODS

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Preparation

Eight mongrel dogs (mean weight, 25 kg; range, 16 to 35 kg) were included in the present study, which was approved by the Committee on the Care and Use of Animals in Research of the Brussels Free University School of Medicine. The dogs were anesthetized with morphine (0.1 mg kg) and -chloralose (80 mg/kg), followed by a continuous infusion of -chloralose at the rate of 20 mg/h supplemented with hourly boluses of morphine (0.1 mg/kg). Paralysis was obtained with pancuronium bromide (0.2 mg/kg per hour). The dogs were ventilated (Elema 900 B servo-ventilator, Siemens Elema, Solna, Sweden) via a cuffed endotracheal tube with an inspired O₂ fraction (FI_O2) of 0.4 to 1.0 to maintain arterial oxygen saturation (Sa_O2) > 90%, a respiratory rate of 10 breaths/min, a tidal volume (VT) of 15-25 ml/kg adjusted to maintain arterial PCO₂ between 35 and 45 mm Hg, and a positive end-expiratory pressure (PEEP) of 5 cm H₂O. Periodic deep inspirations were administered to prevent atelectasis formation. Body temperature was maintained at 36-38° C, using an electric heating pad. When metabolic acidosis occurred, it was corrected by a slow infusion of sodium bicarbonate. Throughout the experiments 0.9% NaCl was infused at about 10 ml/kg per hour to maintain a left atrial pressure (Pla) between 5 and 10 mm Hg.

A balloon-tipped flow-directed pulmonary catheter (model 131H-7F; Baxter Edwards, Irvine, CA) was inserted through the left external jugular vein and positioned by the pressure wave form into a branch of the pulmonary artery for measurements of occluded Ppa (Ppao), Pc' computed from the Ppa decay curve after arterial balloon occlusion, , and central temperature, and for mixed venous blood sampling. A polyethylene catheter was inserted in the abdominal aorta via the right femoral artery for measurements of systemic arterial pressure (Psa) and for arterial blood sampling. A balloon catheter (Percor Stat-DL 10.5F; Datascope, Paramus, NJ) was advanced into the inferior vena cava through a right femoral venotomy. Inflation of the balloon produced a titratable decrease in by reducing venous return. Thrombus formation along the balloon catheter was prevented by intravenous administration of sodium heparin (100 U/kg) just before the insertion. A large-bore polyethylene cannula was inserted into the left femoral artery and vein to act as an arteriovenous fistula, in order to increase by opening the fistula and increasing venous return.

A left lateral thoracotomy was performed. A balloon tipped flow-directed pulmonary catheter (model 131H-7F; Baxter Edwards) was inserted in the left atrium via the atrial appendage to measure left atrial pressure (Pla). A 16- to 24-mm nonconstricting ultrasonic flow probe (T101; Transonic Systems, Ithaca, NY) was positioned around the main pulmonary artery for the measurement of instantaneous pulmonary . The Transonic flowmeter system is linear to 60 Hz, with a flat amplitude response to 35 Hz. A 5F high-fidelity manometer-tipped catheter (model SPC 350; Millar Instruments, Houston TX) was introduced through the right ventricle into the main pulmonary artery, and its tip was positioned just distal to the flow probe for the measurement of instantaneous Ppa. The frequency response of the micromanometer system is flat beyond 200 Hz. The chest was tightly closed, pleural air was evacuated, and the lungs reexpanded with several large volume inspirations.

Measurements

Heart rate (HR) was determined from a continuous electrocardiogram. Psa, Ppa, Ppao, Pla, and Pc' were measured with Statham P50 transducers (Gould, Oxnard, CA). The vascular pressure and flow signals were displayed on a monitor (Sirecust 404; Siemens, Erlangen, Germany) and recorded on a six-channel recorder (model 2600S; Gould, Instruments Division, Cleveland, OH). The pressure transducers of the fluid-filled catheters were zero referenced at midchest, and vascular pressures were recorded at end expiration. was measured by thermodilution as a mean of at least three successive measurements (CO-set; Baxter Edwards, Santa Ana, CA). The zero from the ultrasonic flow probe was adjusted to the end-diastolic value, assumed to be zero. The instantaneous pulmonary pressures and flow signals were sampled at 200 Hz with an analog/digital converter (DAS 8-PGA; Keithley-Metrabyte, Taunton, MA), and stored and analyzed on a personal computer. Zpva was calculated from the Fourier series expressions for pressure and flow signals as previously reported (15). Five end-expiratory heartbeats were analyzed for each data collection interval. Pressure and flow harmonics with amplitude of < 1% of pressure and of flow pulse amplitude were considered as noise and excluded from Zpva calculations. The Zpva modulus was computed as the ratio between pressure and flow moduli, and its phase computed as the difference between flow and pressure phases. The impedance at 0 Hz (Z₀) was taken as the total resistance (Ppa/) and the characteristic impedance (Zc) was calculated as the average of impedance moduli between 2 and 15 Hz. The first harmonic modulus (Z₁) and the first harmonic phase angle (Ph₁) were also derived from Zpva spectra. Total hydraulic power (Wtot) was calculated as the integral of the instantaneous product of pressure multiplied by flow. Steady hydraulic power (Ws) was calculated as the product of mean pressure and mean flow, and oscillatory power (Wosc) as the difference between total and steady power.

To quantify wave reflection, the recorded instantaneous pressure waves were separated into their forward and backward components, according to:

where P and are the recorded pressure and flow waves, Pm and Qm the mean pressure and flow, and Pf and Pb are forward and backward waves (15). The equations show that P is the sum of Pf and Pb. The backward or reflected wave was characterized by its amplitude (the difference between the maximal and minimal values) and by the time intervals between the electrocardiographic R wave and the following events: the foot of the wave (i.e., the starting inflection point), the upward zero crossing, the peak, and the downward zero crossing of the wave (15, 16). The energy transmission ratio (ETR) was calculated as the ratio between the hydraulic power in the measured wave and the hydraulic power in the forward wave (17). Pulmonary vascular compliance was estimated by the ratio of stroke volume (SV) to pulse pressure (PP) (18). Stroke volume was calculated as /HR and PP was calculated as the difference between maximum and minimal values of instantaneous Ppa.

Pc' was computed three times from the Ppa decay curve after inflation of the balloon of the pulmonary artery catheter. For this measurement the dogs were disconnected from the ventilator at end expiration for 15 s. The Ppa decay curve was analyzed by a dual exponential fitting procedure, which includes a rapidly decreasing exponential (filling of the capillary compartment from the arterial compartment) and a slowly decreasing exponential (emptying of the capillary compartment into the venous compartment) (19). The resulting compartmental resistance and compliance values were used to generate a capillary pressure decay curve and estimate Pc' at the instant of occlusion (19). Pc' was normalized to mean Ppa (20). The arterial component of Rpva (Ra) was calculated as (Ppa  Pc')/ and expressed as the percentage of Rpva calculated as (Ppa  Ppao)/. Arterial and mixed venous blood gases were measured immediately after drawing the samples by an automated analyzer (ABL 2; Radiometer, Copenhagen, Denmark) and corrected for temperature.

Experimental Protocol

As soon as the animals were in steady state conditions (stable HR, Psa, Ppa, and for 20 min) a baseline set of hemodynamic and blood gas measurements was obtained, Pc' was recorded, and instantaneous Ppa and flow signals were sampled for Zpva calculations. A first Ppa/ plot was obtained by a rapid inflation of the inferior vena cava balloon. This fast flow-pressure curve was obtained by filling the caval balloon in order to reduce flow by approximately 50% in less than 10 s to prevent sympathetic activation (21).

The same procedure was repeated 90 min after the administration of oleic acid (0.07 ml/kg; Sigma, St. Louis, MO) as a slow injection (5 min) into the right atrium.

Statistical Analysis

Results are expressed as means ± SEM. Linear regression analysis was performed on the Ppa/ coordinates, obtained by the rapid inflation of the inferior vena cava balloon to compute a slope and an extrapolated pressure intercept (Pi) for each of them. To obtain composite Ppa/ plots, Ppa was recalculated from the regression analysis from individual dogs and interpolated at the of 2 and 5 L min¹ m². Hemodynamic data and blood gas results were analyzed by paired t test (22).

	RESULTS

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The Ppa/ relationships were linear in all experimental situations with correlation coefficients of 0.98 ± 0.01 at baseline and 0.97 ± 0.01 after induction of oleic acid lung injury.

Oleic acid lung injury was associated with a decrease in Pa_O2/FI_O2 from 397 ± 41 to 86 ± 18 mm Hg (p < 0.01). Arterial PCO₂ increased from 39 ± 1 to 47 ± 2 mm Hg (p < 0.01), with a decrease in arterial pH from 7.40 ± 0.01 to 7.36 ± 0.03 (p < 0.05) and a decrease in mixed venous PO₂ from 48 ± 2 to 33 ± 2 mm Hg (p < 0.01).

As shown in Table 1, oleic acid lung injury induced moderate increases in Ppa and decreases in , decreased Psa, and slightly increased Pla. Effective capillary pressure increased from 12 ± 1 to 15 ± 1 mm Hg (p < 0.01), but the partitioning of Rpva was unchanged (Figure 1). Both the extrapolated pressure intercept Pi and the slope of the Ppa/ plots were significantly increased after oleic acid administration (Table 1, Figure 2).

View larger version (20K): [in this window] [in a new window]   Figure 1.   Pressure drop across the arterial (open columns) and the venous (hatched columns) segments at: Base = baseline; OA = 90 min after oleic acid administration (n = 8). Numbers inside the arterial columns are the percentages of the pressure drop across the arterial segment (arterial component of the pulmonary vascular resistance, Rpva). Oleic acid lung injury did not affect the partitioning of Rpva.

View larger version (15K): [in this window] [in a new window]   Figure 2.   Composite plots of mean pulmonary artery pressure (Ppa) versus pulmonary blood flow () at: Base = baseline; OA = 90 min after oleic acid administration (n = 8). Ppa was interpolated at the of 2 and 5 L/min 1 m2 and is presented as the mean ± SEM. *p < 0.001 between oleic acid-induced lung injury and baseline. Oleic acid lung injury was associated with an increase in both slope and pressure intercept of Ppa/ plots.

The effects of oleic acid lung injury on indices of pulsatile pulmonary hemodynamics are shown in Table 2. Z₀ increased by 60%, with no change in Z₁ and a decrease in Zc. The first minimum frequency (fmin) of the Zpva spectrum shifted toward higher frequencies and the phase angle decreased. SV/ PP remained unchanged. Wtot did not change, but Wosc/Wtot decreased, along with a decreased ETR. A typical Zpva spectrum in a dog before and after injection of oleic acid is shown in Figure 3.

View larger version (23K): [in this window] [in a new window]   Figure 3.   Representative pulmonary vascular impedance (Zpva) spectra in a dog at baseline (solid line) and after oleic acid (OA) lung injury (dashed line). After OA administration impedance at 0 Hz increased from 344 to 423 dyn s cm5; characteristic impedance decreased from 115 to 83 dyn s cm5; the frequency of the first minimum shifted from 3.4 to 6.1 Hz and the low-frequencies phase angle decreased.

As shown in Table 3, the time-domain indices of wave reflection were unaffected by oleic acid lung injury. Typical pulmonary artery pressure waves decomposed into forward and backward components, before and after oleic acid lung injury, are shown in Figure 4.

View larger version (28K): [in this window] [in a new window]   Figure 4.   Representative pulmonary artery pressure waves (measured, forward and backward waves) in the same dog as in Figure 3 at baseline and after oleic acid lung injury. There was a decrease in the amplitude of the reflected wave.

	DISCUSSION

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The present results show that oleic acid lung injury is associated with a moderate increase in Rpva but no change in compliance or in wave reflection. Characteristic impedance is decreased, resulting in an improved energy transfer from the right ventricle to the pulmonary circulation.

Pulmonary Vascular Pressure-Flow Relationships

In the initial report of pulmonary hypertension in patients with ARDS, pulmonary hemodynamic measurements were presented as Ppa versus and Rpva versus , which disclosed an apparent independence of pulmonary vascular pressures and flows, and a flow dependency of Rpva (6). These observations were tentatively explained by a closing pressure higher than Pla accounting for increased Ppa in clinical ARDS (6). Studies on experimental ARDS induced in intact dogs by the injection of oleic acid have confirmed a relative independence of Ppa and , with a positive extrapolated pressure intercept of linear (Ppa  Pla)/ relationships (7). This observation, together with the finding that upstream transmission to Ppa of progressively increased Pla occurred at values higher than the extrapolated pressure intercept of Ppa/ plots, confirmed the hypothesis that pulmonary hypertension in oleic acid lung injury is accounted for by an increased closing pressure (7). In the same experimental model, it could be shown that alveolar pressure does not affect Ppa, measured at constant and Pla, until it exceeds the pressure intercept of Ppa/ plots, suggesting that the site of closure would be at the small extraalveolar vessels (8). In the present study, Ppa/ plots were generated with rapid, less than 10-s changes in flow, to prevent low -mediated sympathetic nervous system-induced pulmonary vasoconstriction, which could have increased the extrapolated pressure intercept and decreased the slope of Ppa/ plots (21). The present results based therefore on passive, autonomic nervous system-independent, hemodynamic measurements confirm that the pressure intercept of Ppa/ plots is increased in oleic acid ARDS, compatible with increased small vessel closure accounting for pulmonary hypertension.

However, the concept of closing pressure derives from a Starling resistor model of the circulation, which is difficult to prove exclusively on the basis of Ppa and Pla measurements at variable flow (23, 24). It has been shown that viscoelastic models, which do not postulate vascular closure, may provide valid alternative explanations (23, 24). Alternatively, pulmonary hypertension in the present experiments was due not only to an increased pressure intercept but also to an increased slope of Ppa/ plots. We therefore used the occlusion method as an independent approach to locate the site of increased Rpva.

Partitioning of Rpva

A variety of methods based on more or less complex reference electrical analogs have been previously reported for the estimation of Pc' by the analysis of Ppa decay curves after pulmonary arterial occlusion (25). We applied a biexponential fitting and recalculation of a derived Pc' decay curve based on assumptions of changes in arterial and venous resistances and compliances (19). We found a baseline arterial resistance accounting for 60% of total Rpva, in keeping with previous studies, which used a biexponential fitting (26). Oleic acid injury increased both components of Rpva without affecting its partitioning, suggesting unchanged longitudinal distribution of resistances. It has been previously shown, using different techniques including micropuncture, small retrograde catheter, and arterial and venous occlusion in isolated perfused dog lungs, that arterial occlusion measures pressures in vessels that are > 50 µm and < 1,000 µm in diameter, and probably close to 100 to 150 µm in diameter (27). Thus, oleic acid lung injury increases Rpva at the periphery of the pulmonary arterial tree. This interpretation is compatible with the major arteriolar and capillary structural damage found on microscopic examination of the lungs of dogs with oleic acid lung injury (28) as well as of patients with ARDS (29).

Pulmonary Vascular Impedance

Previous studies of acute lung injury induced by the injection of small (150- to 200-µm) glass beads in dogs have reported an increase in Z₀, a shift in the first minimum of Ppa/ moduli to higher frequencies, and an increase in low-frequency phase angle negativity, with either no change or a decrease in Zc (11- 13, 17). Similar changes have been observed in dogs with small (< 3-mm-diameter) blood clot pulmonary embolic pulmonary hypertension (15). Pulmonary hypertension is less severe in canine oleic acid lung injury (14) and in patients with ARDS (1). The present results, showing that oleic acid lung injury increases Z₀ but decreases Zc, are in keeping with previous studies of acute microembolic lung injury in dogs (11). Both Z₀ and Zc increase in pigs with acute lung injury induced by bronchoalveolar lavage (30), or with small blood clot pulmonary embolism (15), but these results are explained by more muscularized and reactive large porcine pulmonary arteries (15).

Long-standing pulmonary hypertension increases both Z₀ and Zc (31). Acute proximal obstruction of the pulmonary arterial tree increases Zc (11, 13, 17). Acute increases in Zc may be humorally mediated. The administration of norepinephrine decreases pulmonary artery distensibility at normal as well as at high intravascular pressures (32). Stimulation of the stellate ganglion in dogs increases Zc without changing Rpva (33). The serotonin antagonist ketanserin (which also has some ₁-adrenergic blocking affects) blocks the Zc increase induced by ensnarement of the left main pulmonary artery (17). Whether the absence of an increase or even decrease in Zc in acute lung injury might have an active component is unclear. It would be of interest to use a pharmacological tool to test for reversibility of decreased Zc. Obvious candidate mediators of an active decrease in Zc would be nitric oxide (NO) and prostacyclin, but inhibition of NO synthase or cyclooxygenase increases Rpva in oleic acid lung injury (34), so that associated changes in Zc would be of uncertain interpretation.

Characteristic impedance is a ratio between the inertance and compliance of the proximal pulmonary arterial tree (31). The increase in Ppa in oleic acid lung injury is moderate, and would not therefore be expected to change Zc by a major effect on proximal arterial dimensions. On the other hand, any increase in Ppa would tend to passively decrease compliance. In spite of unchanged SV/PP calculations, it is thus possible that an active increase in compliance contributed to the decreased Zc in the present experiments. An indirect argument in favor of increased compliance is given by the observed decrease in the amplitude of the reflected wave. Absence of other time-related indices of wave reflection has already been observed in acute small blood clot pulmonary embolism (15) and may simply be related to the distal nature of pulmonary vascular obstruction (11, 13).

In the present experiments, oleic acid lung injury did not increase heart rate, which could have decreased the pulsatile component of hydraulic load (35). Therefore, unchanged or increased compliance, together with unchanged or decreased wave reflection, were the only possible causes of a decreased pulsatile component of the hydraulic load, in agreement with previous studies on acute canine microembolic lung injury (11). A decrease in the pulsatile component of hydraulic load decreases pulmonary artery pulse pressure, and therefore decreases right ventricular systolic wall tension, or afterload (31). Decreased ETR, also previously reported in acute canine blood clot embolic pulmonary hypertension (15, 17), can be interpreted as a reduction in the amount of energy necessary for a given amount of forward flow and thus also a decrease in the amount of hydraulic work imposed on the right heart.

Limitations of the Present Study

Oleic acid lung injury is not a perfect representation of clinical ARDS. Both conditions have a large number of pathological and physiological similarities, but oleic acid causes the initial lung injury directly, without requiring inflammatory cells or their products to mediate the initial damage, while ARDS is most often related to sepsis and associated inflammation (14).

In addition to this difference in etiology, oleic acid lung injury also tends to improve after several hours (14), so that it cannot be used to evaluate the effects of pulmonary hypertension on right ventricular function after one to several day's duration of ARDS. It is possible that in most patients with ARDS, right ventricular remodeling has already occurred at the time of diagnosis of pulmonary hypertension, improving right ventriculovascular coupling. This could be another reason why right heart failure is a relatively rare cause of mortality in ARDS (1).

Clinical Implications

The pulsatile components normally make up about one-third of total hydraulic power output of the right ventricle, as compared with only one-tenth of left ventricular output (31, 35, 36). The right ventricle is thin-walled and has a crescent shape, which limit its capacity to maintain stroke volume in the presence of even moderate increases in pulmonary artery pressures. Right ventricular ejection thus carries a higher component of reactance (i.e., inertance and elastance) than that of the left ventricle, making it more sensitive to relatively smaller changes in Zpva (36).

That mean Ppa or Rpva is a poor predictor of outcome in patients with ARDS (2, 3) may be understandable since steady flow hemodynamic measurements do not take right ventricular pulsatile hydraulic load into consideration. It is of interest that a large-scale study showed systolic Ppa, not mean Ppa, to be a predictor of mortality (3). It is possible that those patients with ARDS who present with increased mortality due to right heart failure also present with increased Zc, due to progression of disease or other causes that decrease proximal pulmonary arterial compliance and/or increase wave reflection.