DRUG ACTION ON PULMONARY CIRCULATION OF UNANESTHETIZED DOGS

Improved right ventricular-vascular coupling during active pulmonary hypertension

Right ventricular adaptation to pulmonary hypertension (PH) is an important prognostic factor. Pulmonary artery (PA) smooth muscle activation attenuates arterial dysfunction during acute PH. We investigated the role of the pulmonary artery vascular smooth muscle activation on the right ventricular-vascular coupling during acute PH. PA flow, pressure, and diameter, right ventricular and aortic pressures were recorded in six anesthetized sheep. Acute PH was induced by phenylephrine (APH) and PA mechanical constriction (PPH). We calculated the PA buffering function, the incremental elastic modulus and pulmonary vascular compliance. Pulmonary vascular impedance and right ventricular hydraulic power were calculated through Fourier approach. We also quantified the magnitude and timing of the reflected wave. Right ventricular-vascular coupling was assessed by the energy transmission ratio. Pulmonary buffering function and vascular compliance increased (P<0.05) and arterial wall stiffness decreased (P<0.05) during APH with respect to PPH. Although total input resistance increased and reflected wave came back earlier during PH states (P<0.05), only PPH produced a rightward shift of the pulmonary impedance and a more prominent reflected wave. Accordingly, APH determined a minor increase of total hydraulic power with a smaller pulsatile to total power ratio and energy transmission ratio (P<0.05). In conclusion, isobaric PA vasoconstriction prevents the pulsatile hydraulic load to increase by preserving the PA buffering function and the reflected wave magnitude. Thus, vascular smooth muscle activation of the main PA improves the energy transfer from the right ventricle to the hypertensive pulmonary circulation, and this may play relevant role in the right ventricular adaptation to acute PH.

Pulmonary vasodilator responses to vagal stimulation and acetylcholine in the cat

Responses to vagal stimulation and acetylcholine were investigated in the feline pulmonary vascular bed under conditions of controlled pulmonary blood flow and constant left atrial pressure. Under baseline conditions, electrical stimulation of vagal efferent fibers increases lobar arterial pressure. However, when vasoconstrictor tone was increased, a depressor response was unmasked. The pressor response under baseline conditions and the depressor response under enhanced tone conditions were blocked by phenoxybenzamine and atropine. These data suggest that, in the cat, the vagus is composed of efferent fibers from both the sympathetic and parasympathetic systems. After treatment with 6-hydroxydopamine to destroy the integrity of the sympathetic system, vagal stimulation caused significant frequency-dependent decreases in lobar arterial pressure when lobar vascular tone was increased by infusion of a stable prostaglandin endoperoxide analog or ventilatory hypoxia. Injections of acetylcholine also caused significant dose-related decreases in lobar arterial pressure when lobar vascular resistance was elevated. Depressor responses to vagal stimulation and acetylcholine in 6-hydroxydopamine-treated animals were blocked by atropine and enhanced by physostigmine. Decreases in lobar arterial pressure in response to vagal stimulation in 6-hydroxydopamine-treated animals with enhanced tone were blocked by hexamethonium, whereas responses to injected acetylcholine were not altered by the ganglionic blocking agent. Decreases in lobar arterial pressure in response to vagal stimulation and acetylcholine were similar when the lung was ventilated and when the left lower lobe bronchus was obstructed. In addition, responses to vagal stimulation were similar when systemic arterial pressure was decreased to the level of pressure in the perfused lobar artery. Responses to acetylcholine were not altered after treatment with 5,8,11,14-eicosatetraynoic acid, a lipoxygenase inhibitor. The present data suggest that the feline pulmonary vascular bed is functionally innervated by cholinergic nerves and that vagal stimulation dilates the pulmonary vascular bed by releasing acetylcholine which acts on muscarinic receptors in pulmonary vessels.

Adrenergic mechanisms in the effects of histamine in the pulmonary circulation of the cat

Histamine constricts airways and pulmonary blood vessels in isolated lungs, but is said to dilate pulmonary blood vessels in intact animals. The present experiments test the hypothesis that histamine only constricts pulmonary blood vessels, and that dilatation depends on epinephrine released by histamine from the adrenal glands. In cats anesthetized with pentobarbital, the difference in pressure between the pulmonary artery (PA) and left atrium (LA) was related to pulmonary blood flow measured by dye dilution before, and at intervals of 0.5 to 2 minutes after PA injection of histamine. In intact or vagotomized cats histamine increased PA-LA pressure difference and pulmonary blood flow, but calculated pulmonary vascular resistance (PVR) decreased significantly at 1 minute; control values were restored in 4 minutes. In adrenalectomized cats, histamine (10 to 40 µg) increased PA-LA pressure and PVR, but did not change pulmonary blood flow. After beta-receptor blockade with propranolol, histamine (10 to 20 µg) had the same effect as in adrenalectomized cats. After adrenalectomy or propranolol, the histamine-induced increase in PVR was prolonged. When histamine had increased PVR in adrenalectomized cats, epinephrine (5 µg iv) reduced PA-LA pressure and PVR. Histamine infusion (5 µg/kg/min iv) decreased PVR in intact cats, increased PVR in adrenalectomized cats, and when epinephrine (5 µ/kg/min) was also infused, decreased PVR in adrenalectomized cats. The results support the proposed hypothesis. Release of histamine locally in the lungs may encourage distribution of both blood and gas flows away from damaged and toward normal respiratory tissue.

Effects of aminophylline on the pulmonary circulation and left ventricular performance in patients with valvular heart disease

The effects of aminophylline given intravenously on the pulmonary circulation and left ventricular performance were studied during combined right and transseptal left heart catheterization in 32 patients with mitral and aortic valvular disease. We recorded complete data on the changes of the pulmonary vascular distending pressure (P D ) and the pulmonary blood volume (PBV) before and during aminophylline infusion in 28 patients. In 19 patients a fall in the P D was associated with a rise in the PBV. In eight patients a decline in the P D was accompanied by a fall (five patients) or no change (three patients) in the PBV. In a single case, both P D and the PBV rose. In the majority of cases, observation of decrease in P D and a discordant rise in PBV is new evidence suggestive of a direct active vasodilating effect of aminophylline on the pulmonary vasculature.

Although the cardiac index was unaffected, a positive inotropic effect on the myocardium was documented by a decrease in left ventricular end-diastolic pressure accompanied by an increase in the maximum rate of ventricular pressure rise (LV dp/dt). These findings contrast with those in certain patients with heart failure of nonvalvular origin in whom increases in cardiac index are usually prominent. The salutory effect of aminophylline in acute pulmonary edema is thought to be due to synergistic effects on the systemic circulation, pulmonary circulation, and left ventricular function, all of which tend to produce a fall in P D .

Response of the pulmonary vasculature to hypoxia and H+ ion concentration changes

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