Increased right ventricular output and central pulmonary reservoir function support rise in pulmonary blood flow during adenosine infusion in the ovine fetus

Reducing lung liquid volume increases biventricular outputs and systemic arterial blood flows despite decreased cardiac filling pressures in fetal lambs

As prior work has shown that reducing lung liquid volume 1) increases pulmonary arterial (PA) blood flow, 2) augments right ventricular (RV) output/power, and 3) decreases left atrial (LA) pressure, we tested the hypothesis that this perturbation has global cardiovascular effects. Ten anesthetized, open-chest fetal lambs (128 ± 2 days gestation, full term = 147 days) were acutely instrumented with 1) LA and right atrial (RA) catheters, 2) aortic and pulmonary trunk catheters, 3) brachiocephalic trunk, aortic isthmus, ductal, and left PA flow probes to obtain left ventricular (LV) and RV outputs and hydraulic power and flow in the descending thoracic aorta, and 4) an endotracheal tube to remove lung liquid. A 17 ± 7 ml/kg reduction of lung liquid volume 1) decreased LA and RA pressures similarly (1.5-1.6 mmHg, P < 0.001), 2) augmented LV and RV outputs (21-24%, P < 0.001) and total power (27-28%, P < 0.005), 3) increased systolic flows in the brachiocephalic trunk (18%, P < 0.001), aortic isthmus (29%, P < 0.005), ductus (12%, P < 0.005), and descending thoracic aorta (16%, P < 0.001), 4) increased mean PA flow via a higher systolic inflow (37%, P < 0.001) and lower diastolic backflow (-16%, P < 0.05), and 5) did not change systemic vascular conductance or arterial compliance but increased both pulmonary vascular conductance and arterial compliance (1.8-fold, P < 0.001). These data suggest that hemodynamic effects of lung liquid volume reduction are not confined to the lungs but extend to all cardiac chambers via rises in LV and RV outputs and power, despite falls in cardiac filling pressures, as well as the systemic circulation, via downstream increases in systolic flows of major central arteries.

Increased right ventricular power and ductal characteristic impedance underpin higher pulmonary arterial blood flow after betamethasone therapy in fetal lambs

BACKGROUND

The glucocorticosteroid betamethasone is routinely administered prior to anticipated preterm birth to enhance lung maturation. While betamethasone also increases fetal pulmonary blood flow and reduces pulmonary vascular resistance (PVR), we investigated whether alterations in right ventricular (RV) function and ductal characteristic impedance (Z_c) additionally contributed to rises in pulmonary flow.

METHODS

Anesthetized preterm fetal lambs with (n = 10) or without (n = 8) betamethasone pretreatment were instrumented with a pulmonary trunk micromanometer and ductus arteriosus and left pulmonary artery (PA) flow probes to calculate Z_c, and obtain RV output and hydraulic power.

RESULTS

Betamethasone (1) increased systolic and pulse arterial pressures (P ≤ 0.04), heart rate (P = 0.02), and lowered PVR (P = 0.04), (2) increased mean (P = 0.008) and systolic (P = 0.004), but not diastolic PA flow or PA Z_c, (3) increased ductal Z_c (P < 0.05), but not ductal flow, (4) increased RV output (P = 0.03) and the proportion of PT flow distributed to the lungs (P = 0.02), and (5) increased RV power (P ≤ 0.002).

CONCLUSION

An increased fetal PA blood flow after betamethasone therapy was confined to the systole and underpinned not only by decreased PVR, but also greater RV power and preferential distribution of an augmented RV systolic outflow to the lungs due to higher ductal Z_c.

Major contribution of central pulmonary reservoir discharge to increased pulmonary arterial diastolic blood flow after birth in near-term lambs

Recent fetal lamb data have suggested that the pulmonary trunk (PT) region displays a reservoir function and that a pharmacologically induced fall in pulmonary vascular resistance (PVR) increases and redistributes diastolic discharge from this central pulmonary reservoir toward the lungs, thereby producing a positive diastolic offset in the pulmonary arterial (PA) blood flow profile. As a similar offset in PA flow characteristically occurs after birth, this study tested the hypotheses that 1) central pulmonary reservoir discharge is both redistributed toward the lungs and increased in magnitude during the birth transition and 2) discharge from this reservoir constitutes a major component of increased PA diastolic blood flow after birth. Six anesthetized near-term fetal lambs were instrumented with PT, ductal and left PA transit-time flow probes, and aortic, PT and left atrial catheters. Hemodynamic data were recorded in fetuses and at regular intervals during 2-h mechanical ventilation following cesarean section delivery. Diastolic PA blood flow rose from near zero in fetuses to 468 ± 188 ml/min by 15 min ( P < 0.001). Central pulmonary reservoir discharge in fetuses (99 ± 44 ml/min) passed primarily right-to-left across the ductus. However, this reservoir discharge redistributed entirely to the lungs by 1 min after birth, and then doubled to a peak of 214 ± 167 ml/min at 15 min ( P < 0.001). Reservoir discharge subsequently stabilized at 151 ± 60 ml/min at 30–120 min, which comprised ∼50% of diastolic and ∼20% of mean PA blood flow. These findings suggest that enhanced diastolic central pulmonary reservoir discharge plays a major role in supporting an increased pulmonary perfusion after birth.

Enhanced ventricular pump function and decreased reservoir backflow sustain rise in pulmonary blood flow after reduction of lung liquid volume in fetal lambs

Although a reduction in lung liquid volume increases fetal pulmonary blood flow, the changes in central flow patterns that sustain this increased pulmonary perfusion are unknown. To address this issue, eight anesthetized late-gestation fetal sheep were instrumented with pulmonary trunk (PT), ductus arteriosus (DA), and left pulmonary artery (PA) micromanometer catheters and transit-time flow probes, with blood flow profile and wave intensity analyses performed at baseline and after withdrawal of lung liquid via an endotracheal tube. Reducing lung liquid volume by 19 ± 6 ml/kg (mean ± SD) augmented right ventricular power by 34% (P < 0.001), with distribution of an accompanying increase in mean PT blood flow (245 ± 63 ml/min, P < 0.001) to the lungs (169 ± 91 ml/min, P = 0.001) and across the DA (77 ± 92 ml/min, P = 0.04). However, although PT and DA flow increments were confined to systole and were related to an increased magnitude of flow-increasing, forward-running compression waves, the rise in PA flow spanned both systole (108 ± 66 ml/min) and diastole (61 ± 32 ml/min). Flow profile analysis showed that the step-up in PA diastolic flow was associated with diminished PA diastolic backflow and accompanied by a lesser degree of diastolic right-to-left DA shunting. These data suggest that an increased pulmonary blood flow after reduction of lung liquid volume is associated with substantial changes in PT-DA-PA interactions and underpinned by two main factors: 1) enhanced right ventricular pump function that increases PA systolic inflow and 2) decreased PA diastolic backflow that arises from a fundamental change in PA reservoir function, thereby resulting in greater passage of systolic inflow through the lungs.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.