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

Reducing lung liquid volume in fetal lambs decreases ventricular constraint

BACKGROUND

This study evaluated whether an increased left ventricular (LV) pump function accompanying reduction of lung liquid volume in fetal lambs was related to increased LV preload, augmented LV contractility, or both.

METHODS

Eleven anesthetized preterm fetal lambs (gestation 128 ± 2 days) were instrumented with (1) an LV micromanometer-conductance catheter to obtain LV end-diastolic volume (EDV) and end-diastolic pressure (EDP), the maximal rate of rise of LV pressure (dP/dt_max), LV output, LV stroke work, and LV end-systolic elastance (E_es), a relatively load-independent measure of contractility; (2) an endotracheal tube to measure mean tracheal pressure and to reduce lung liquid volume. LV transmural pressure was calculated as LV EDP minus tracheal pressure.

RESULTS

Reducing lung liquid volume by 16 ± 4 ml kg^-1 (1) augmented LV output (by 16%, P = 0.001) and stroke work (29%, P < 0.001), (2) increased LV EDV (12%, P < 0.001), (3) increased LV transmural pressure (2.2 mmHg, P < 0.001), (4) did not change LV dP/dt_max normalized for EDV (P > 0.7), and (5) decreased LV E_es (20%, P < 0.01).

CONCLUSION

These findings suggest a rise in LV pump function evident after reduction of lung liquid volume in fetal lambs was related to increased LV preload secondary to lessening of external LV constraint, without any associated rise in LV contractility.

IMPACT

This study has shown that reducing the volume of liquid filling the fetal lungs lessens the degree of external constraint on the heart. This lesser constraint permits a rise in left ventricular dimensions and thus greater cardiac filling that leads to increased left ventricular pumping performance. This study has defined a mechanism whereby a reduction in lung liquid volume results in enhanced pumping performance of the fetal heart. These findings suggest that a reduction in lung liquid volume which occurs during the birth transition contributes to increases in left ventricular dimensions and pumping performance known to occur with birth.

Brief asphyxial state following immediate cord clamping accelerates onset of left-to-right shunting across the ductus arteriosus after birth in preterm lambs

Reversal of shunting across the ductus arteriosus from right-to-left to left-to-right is a characteristic feature of the birth transition. Given that immediate cord clamping (ICC) followed by an asphyxial cord clamp-to-ventilation (CC-V) interval may augment left ventricular (LV) output and central blood flows after birth, we tested the hypothesis that an asphyxial CC-V interval accelerates the onset of postnatal left-to-right ductal shunting. High-fidelity central blood flow signals were obtained in anesthetized preterm lambs (gestation 128 ± 2 days) after ICC followed by a nonasphyxial (∼40 s, n = 9) or asphyxial (∼90 s, n = 9) CC-V interval before mechanical ventilation for 30 min after birth. Left-to-right ductal flow segments were related to aortic isthmus and descending aortic flow profiles to quantify sources of ductal shunting. In the nonasphyxial group, phasic left-to-right ductal shunting was initially minor after birth, but then rose progressively to 437 ± 164 ml/min by 15 min (P < 0.001). However, in the asphyxial group, this shunting increased from 24 ± 21 to 199 ± 93 ml/min by 15 s after birth (P < 0.001) and rose further to 471 ± 190 ml/min by 2 min (P < 0.001). This earlier onset of left-to-right ductal shunting was supported by larger contributions (P < 0.001) from direct systolic LV flow and retrograde diastolic discharge from an arterial reservoir/windkessel located in the descending aorta and its major branches, and associated with increased pulmonary arterial blood flow having a larger ductal component. These findings suggest that the duration of the CC-V interval after ICC is an important modulator of left-to-right ductal shunting, LV output and pulmonary perfusion at birth.NEW & NOTEWORTHY This birth transition study in preterm lambs demonstrated that a brief (∼90 s) asphyxial interval between umbilical cord clamping and ventilation onset resulted in earlier and greater left-to-right shunting across the ductus arteriosus after birth. This greater shunting 1) resulted from an increased left ventricular output associated with a higher systolic left-to-right ductal flow and increased retrograde diastolic discharge from a lower body arterial reservoir/windkessel, and 2) was accompanied by greater lung perfusion after birth.

Antenatal betamethasone augments early rise in pulmonary perfusion at birth in preterm lambs: role of ductal shunting and right ventricular outflow distribution

The glucocorticosteroid betamethasone is routinely administered via maternal intramuscular injection to enhance fetal lung maturation before anticipated preterm birth. Although antenatal betamethasone increases fetal pulmonary arterial (PA) blood flow, whether this agent alters the contribution of 1) right ventricular (RV) output or 2) left-to-right shunting across the ductus arteriosus to rises in PA blood flow after preterm birth is unknown. To address this question, anesthetized control (n = 7) and betamethasone-treated (n = 7) preterm fetal lambs (gestation 127 ± 1 days, means ± SD) were instrumented with aortic, pulmonary, and left atrial catheters as well as ductus arteriosus and left PA flow probes to calculate RV output, with hemodynamics measured for 30 min after cord clamping and mechanical ventilation. Mean PA blood flow was higher in betamethasone-treated than in control lambs over the initial 10 min after birth (P < 0.05). This higher PA flow was accompanied by 1) a greater pulmonary vascular conductance (P ≤ 0.025), 2) a larger proportion of RV output passing to lungs (P ≤ 0.01), despite a fall in this output, and 3) earlier reversal and a greater magnitude (P ≤ 0.025) of net ductal shunting, due to the combination of higher left-to-right (P ≤ 0.025) and lesser right-to-left phasic shunting (P ≤ 0.025). These results suggest that antenatal betamethasone augments the initial rise in PA blood flow after birth in preterm lambs, with this augmented rise supported by the combination of 1) a greater redistribution of RV output toward the lungs and 2) a faster and larger reversal in net ductal shunting underpinned not only by greater left-to-right, but also by lesser right-to-left phasic shunting.

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.

Onset of asphyxial state in nonrespiring interval between cord clamping and ventilation increases hemodynamic lability of birth transition in preterm lambs

Experimentally, a typical ∼2-min cord clamp-to-ventilation interval in preterm lambs is accompanied by increased hemodynamic lability of the birth transition. However, whether this lability is related to development of asphyxia after cord clamping, or can be avoided with a shorter clamp-to-ventilation interval, is unknown. To address these questions, anesthetized preterm fetal lambs (gestation 127 ± 2 days) were instrumented with ductus arteriosus and left pulmonary artery flow probes to obtain right ventricular (RV) output, brachiocephalic trunk and aortic isthmus flow probes to measure left ventricular (LV) output, and aortic trunk catheters for pressure measurement and blood gas analysis. With hemodynamics recorded continuously, fetuses were delivered onto the ewe's abdomen and the cord clamped for 1.5 min before ventilation (n = 8), with aortic sampling at 15, 30, 45, and 60 s, or for 0.5 min, with sampling at 15 s (n = 4). With 1.5-min cord clamping, an asphyxial state (Po2 < 10 mmHg) was evident at ≥45 s, with bradycardia and marked falls in LV and RV outputs (by 60% and 50%, P < 0.001), followed after ventilation onset by tachycardia and LV and RV output surges (4- and 3-fold, P < 0.001). By contrast, heart rate and outputs remained stable after 0.5-min cord clamping, with no postventilation change in heart rate or RV output, and a lesser rise in LV output (22%, P < 0.005). In preterm lambs, rapid development of an asphyxial state within 45 s in the cord clamp-to-ventilation interval increased hemodynamic lability of the birth transition, which was reduced with a shorter (∼0.5 min) cord clamp-to-ventilation interval.