Inhibition of endotoxin-induced pulmonary vascocontriction in dogs by alpha-methyl dopa

The demonstration in a previous study of the effectiveness of an antihistaminic drug in blocking some of the systemic but not the pulmonary vascular effects of endotoxin led to the study of the effect of an inhibitor of serotonin synthesis, alpha-methyl 3,4-dihydroxyphenylalanine (α-m dopa). One group of seven dogs was pretreated with a single dose of 250 mg, and a second group of six animals with three doses of 250 mg, each given at 10-min intervals. Results in these two groups were compared with those in six control animals. Purified E. coli endotoxin, 1 mg/kg, was administered intravenously in all 19 experiments. Intravenous administration of α-m dopa alone had no effect on measured physiologic parameters. Compared with the endotoxin response in control animals, pretreatment with either dose level appeared to have no effect on the magnitude or duration of systemic arterial hypotension, portal venous hypertension, or drop in cardiac output. However, pretreatment with 250-mg and 750-mg doses was associated with significant reduction and abolition, respectively, of pulmonary arterial hypertension. The results are consistent with the interpretation that the pulmonary vasoconstrictive response to endotoxin is mediated through the release of serotonin and that α-m dopa blocks this response by interfering with the synthesis of this intermediary.

Pulmonary hemodynamics following acute atelectasis

Locally induced alterations in pulmonary lobar blood flow and vascular resistance produced by atelectasis, hypoxia, or hypercarbia were studied in the autoperfused, innervated, separately ventilated right lower lobes of 14 dogs. Systemic effects of hypoxia or hypercarbia were excluded by shunting the lobar blood back to the vena cava; a second shunt between the internal mammary artery and the lobar artery permitted perfusion with either oxygenated or desaturated blood. Lobar blood flow was monitored with a magnetic flowmeter, and pressures were measured in the lobar artery, lobar vein, and femoral artery. Oxygen saturation was measured at these sites in nine of the dogs with a cuvette oximeter. Although systemic arterial pressure and oxygen saturation remained normal throughout these studies, hypoxia secondary to nitrogen ventilation produced average flow decreases of 19% and 14% with desaturated and saturated blood, respectively, and resistance increases of 26% and 15%. Atelectasis decreased flow by 37% and 23% and caused resistance to rise 93% and 65%. Ventilation of the lobe with 5% and 30% carbon dioxide in oxygen was without apparent effect.

Cardiorespiratory effects of large pneumothorax in conscious and anesthetized dogs

Ventilation, arterial blood gases, diffusing capacity for carbon monoxide, and flow and pressure in the pulmonary circulation were measured in conscious and pentobarbital-anesthetized dogs at intervals up to 5 hr after intrapleural injection of air. Pneumothorax of twice the functional residual capacity increased the frequency of breathing and reduced physiological dead space in both conscious and anesthetized dogs, but conscious dogs were otherwise unaffected. In contrast, the arterial blood oxygen saturation fell, pulmonary artery pressure rose, and carbon monoxide diffusing capacity was reduced in anesthetized dogs despite greatly increased respiratory minute volume and alveolar ventilation. Ventilation was depressed by anesthesia alone in controls and improved gradually during the experiments. Pentobarbital anesthesia appeared to augment cardiorespiratory depression or abolish compensatory reactions evoked by pneumothorax to produce effects in anesthetized dogs which did not occur in their conscious counterparts. The potent effects of pentobarbital anesthesia on ventilation, pulmonary circulation, and carbon monoxide diffusing capacity, emphasize the advantage of conscious subjects in cardiorespiratory studies.

Submitted on July 25, 1962

PULMONARY EXCHANGE AS RELATED TO ALTERED PULMONARY MECHANICS IN ANESTHETIZED DOGS

Pulmonary mechanics and gas exchange were studied in supine anesthetized dogs, with and without periodic hyperinflations of the lungs, and after a single forced deflation. After a period without hyperinflations the lung compliance decreased to 66% of control values and patchy atelectasis was observed, but no changes were observed in diffusing capacity (Dl) or per cent venous admixture ( Qva/ Qt). Following a single forced deflation the compliance was 40% of control. Dl was 40% and Qva/ Qt 180% of control values, respectively. These data could be accounted for if there was an accompanying shift of blood flow from atelectatic to nonatelectatic areas, the compensation being very nearly complete for moderate, but less than complete for marked reductions in compliance. Alternative explanations of the apparent compensation are: 1) preferential initial atelectasis of regions with high ventilation-perfusion ratios (as proposed by Farhi) and 2) substantial reductions in compliance without closure of air space, as might result from changes in surface forces with time.

aerotonometer; atelectasis; venous admixture; mechanics of breathing; compliance of dog lungs; diffusion capacity of dog lungs; ventilation perfusion; relationships of dog lungs

Submitted on October 28, 1963

SYSTEMIC BLOOD PRESSURE RESPONSE TO CHANGES IN RIGHT VENTRICULAR FUNCTION

The relationship between right ventricular and aortic pressure was studied in open-chest anesthetized dogs. A simultaneous increase in both pressures was observed in animals with an intact circulation and animals with the systemic circulation perfused at a constant rate and stroke volume by an extracorporeal pump. With gradual partial constriction of the main pulmonary artery, the increase in right ventricular systolic and aortic pressures occurred without consistent change in other measured parameters. Bilateral cervical vagotomy abolished the systemic vascular response.

These observations are consistent with a pressor reflex which is mediated over vagal afferent fibers from the right ventricle. It is suggested that in animals under varying physiologic stimuli an increase in right ventricular pressure is associated with a reflex increase in systemic vascular resistance. In abnormal circumstances with increased resistance to right ventricular emptying, the increased total peripheral vascular resistance would be important in maintaining systemic blood pressure and blood flow.

DRUG ACTION ON PULMONARY CIRCULATION OF UNANESTHETIZED DOGS

A preparation is described which overcomes most of the difficulties previously encountered in the assessment of vasomotor responses of the pulmonary vessels to pharmacologic agents. The use of electromagnetic flow transducers on the main and left pulmonary arteries allows for the observation of immediate local effects following unilateral drug administration before general systemic effects occur. The opposite lung serves as a control, since it is subjected to the same changes of left artrial pressure, pulmonary arterial pressure, and pulmonary blood flow as the test lung. Our studies have demonstrated a direct pulmonary vasoconstrictor response to 5-hydroxytryptamine and acetylcholine in both the conscious and anesthetized dog. Epinephrine was also shown to have a direct pulmonary vasoconstrictor activity, but this could be elicited in only three of the eight dogs studied. Isoproterenol, methoxamine, aminophylline, phenylephrine, γ-amino butyric acid and pentobarbital had no apparent direct pulmonary vasomotor effect.