Effects of external constraint on the fetal left ventricular function curve

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.

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.

Achieving and maintaining lung volume in the preterm infant: from the first breath to the NICU

The main goal for the neonatologist is to facilitate the adaptation to extra-uterine life during initial transition, while minimizing lung injury opening and protecting the premature lung from the first breath onwards. An appropriate management from birth should lead to the achievement of an early functional residual capacity (FRC), and the following steps should aim at maintaining an adequate lung volume. To date, different strategies are available to optimize fetal-neonatal transition and promote lung recruitment. New ventilation approaches, such as sustained lung inflation (SLI) and "open lung strategy", well-established ventilation techniques with a more tailored application and less invasive modalities to administer surfactant have been recently introduced in clinical practice with promising results.

CONCLUSIONS

given the current status of neonatal care, it seems that lung injury and BPD could be reduced with multiple strategies starting early in the delivery room. Literature underlines the importance of a respiratory tailored management of preterm infants from birth and during the whole NICU stay. What is Known: • Experimental and clinical studies have shown that the transition from fetal to adult type cardiorespiratory circulation needs an adequate lung ventilation. An appropriate management in the delivery room should lead to the achievement of an early FRC, and through the following steps, the neonatologist should aim at maintaining an adequate lung volume. • Literature underlines the importance of a respiratory tailored management of preterm infants during the whole NICU stay to maintain the benefits of a successful postnatal adaption. What is New: • Herewith, we describe the most relevant and recent interventions which can be performed from the delivery room to the NICU stay to guarantee an adequate tradition to postnatal life and an effective cardiorespiratory stability.

Biphasic changes in fetal heart rate variability in preterm fetal sheep developing hypotension after acute on chronic lipopolysaccharide exposure

Perinatal exposure to infection is highly associated with adverse outcomes. Experimentally, acute, severe exposure to gram-negative bacterial lipopolysaccharide (LPS) is associated with increased fetal heart rate variability (FHRV). It is unknown whether FHRV is affected by subclinical infection with or without acute exacerbations. We therefore tested the hypothesis that FHRV would be associated with hypotension after acute on chronic exposure to LPS. Chronically instrumented fetal sheep at 0.7 gestation were exposed to a continuous low-dose LPS infusion (n = 12, 100 ng/kg over 24 h, followed by 250 ng·kg(-1)·24 h(-1) for a further 96 h) or the same volume of saline (n = 10). Boluses of either 1 μg LPS or saline were given at 48, 72, and 96 h. Low-dose infusion was not associated with hemodynamic or FHRV changes. The first LPS bolus was associated with tachycardia and suppression of nuchal electromyographic activity in all fetuses. Seven of twelve fetuses developed hypotension (a fall in mean arterial blood pressure ≥5 mmHg). FHRV was transiently increased only at the onset of hypotension, in association with increased cytokine induction and electroencephalogram suppression. FHRV then fell before the nadir of hypotension, with transient suppression of short-term FHRV. After the second LPS bolus, the hypotension group showed a biphasic pattern of a transient increase in FHRV followed by more prolonged suppression. These findings suggest that infection-related hypotension in the preterm fetus mediates the transient increase in FHRV and that repeated exposure to LPS leads to progressive loss of FHRV.

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.

Renal sympathetic nerve activity during asphyxia in fetal sheep

The sympathetic nervous system (SNS) is an important mediator of fetal adaptation to life-threatening in utero challenges, such as asphyxia. Although the SNS is active well before term, SNS responses mature significantly over the last third of gestation, and its functional contribution to adaptation to asphyxia over this critical period of life remains unclear. Therefore, we examined the hypotheses that increased renal sympathetic nerve activity (RSNA) is the primary mediator of decreased renal vascular conductance (RVC) during complete umbilical cord occlusion in preterm fetal sheep (101 ± 1 days; term 147 days) and that near-term fetuses (119 ± 0 days) would have a more rapid initial vasomotor response, with a greater increase in RSNA. Causality of the relationship of RSNA and RVC was investigated using surgical (preterm) and chemical (near-term) denervation. All fetal sheep showed a significant increase in RSNA with occlusion, which was more sustained but not significantly greater near-term. The initial fall in RVC was more rapid in near-term than preterm fetal sheep and preceded the large increase in RSNA. These data suggest that although RSNA can increase as early as 0.7 gestation, it is not the primary determinant of RVC. This finding was supported by denervation studies. Interestingly, chemical denervation in near-term fetal sheep was associated with an initial fall in blood pressure, suggesting that by 0.8 gestation sympathetic innervation of nonrenal vascular beds is critical to maintain arterial blood pressure during the rapid initial adaptation to asphyxia.

The role of the neural sympathetic and parasympathetic systems in diurnal and sleep state-related cardiovascular rhythms in the late-gestation ovine fetus

The efferent mechanisms mediating the well-known diurnal cardiovascular rhythms in the late-gestation fetus are only partially understood. In the present study, we evaluated the contribution of the parasympathetic and sympathetic nervous systems (SNS) to these rhythms. Chronically instrumented fetal sheep at a mean (SE) of 122 ( 1 ) days gestation (term is 147 days) underwent either chemical sympathectomy with 6-hydroxydopamine the day after surgery ( n = 8), vagotomy at surgery ( n = 8), or were sham controls ( n = 8). Fetal heart rate (HR), fetal HR variability (HRV), mean arterial blood pressure (MAP), carotid blood flow (CaBF), electrocorticogram (ECoG) activity, and nuchal activity were measured continuously for 24 h. Changes between sleep states were determined in a 6-h interval. Control fetal sheep showed consistent diurnal rhythms in fetal HR, HRV, MAP, and CaBF, with maximal activity in the evening, but not in nuchal activity. Sympathectomy was associated with a significant reduction of both fetal HR and HRV, while vagotomy was associated with a fall in fetal HRV ( P < 0.05) but no change in HR. Despite this, most animals in the two intervention groups still showed diurnal rhythms for fetal HR, HRV, MAP, and CaBF, although peak HR may have been delayed in the sympathectomy group (mean 02:22 vs. 23:54 h in controls, P = 0.06). There was no effect of either intervention on sleep state cycling, although state-related cardiovascular rhythms were significantly modulated. These data indicate that, neither the SNS nor vagal activity, in isolation at least, is essential for generating cardiovascular diurnal rhythms in the late-gestation fetus.

A 2D FE model of the heart demonstrates the role of the pericardium in ventricular deformation

During pulmonary artery constriction (PAC), an experimental model of acute right ventricular (RV) pressure overload, the interventricular septum flattens and inverts. Finite element (FE) analysis has shown that the septum is subject to axial compression and bending when so deformed. This study examines the effects of acute PAC on the left ventricular (LV) free wall and the role the pericardium may play in these effects. In eight open-chest anesthetized dogs, LV, RV, aortic, and pericardial pressures were recorded under control conditions and with PAC. Model dimensions were derived from two-dimensional echocardiography minor-axis images of the heart. At control (pericardium closed), FE analysis showed that the septum was concave to the LV; stresses in the LV, RV, and septum were low; and the pericardium was subject to circumferential tension. With PAC, RV end-diastolic pressure exceeded LV pressure and the septum inverted. Compressive stresses developed circumferentially in the septum out to the RV insertion points, forming an arch-like pattern. Sharp bending occurred near the insertion points, accompanied by flattening of the LV free wall. With the pericardium open, the deformations and stresses were different. The RV became much larger, especially with PAC. With PAC, the arch-like circumferential stresses still developed in the septum, but their magnitudes were reduced, compared with the pericardium-closed case. There was no free wall inversion and flattening was less. From these FE results, the pericardium has a significant influence on the structural behavior of the septum and the LV and RV free walls. Furthermore, the deformation of the heart is dependent on whether the pericardium is open or closed.

Current concepts in fetal cardiovascular disease

This article discusses the unique properties of the fetal cardiovascular system and patterns of blood flow in congenital heart disease. It also explores the complex interactive dependency between the developing heart and pulmonary vasculature, with particular attention to hypoplastic left heart syndrome. The article goes on to highlight some recent advances in the understanding of fetal cardiovascular medicine, specifically the possibilities and prognosis for fetal cardiac interventions. An understanding of the enigmatic process of twin-twin transfusion syndrome may help in elucidating one of the mechanisms for development of cardiac structural abnormalities in the fetus.

Frontiers in fetal cardiovascular disease

This article reviews the state of the art in prenatal cardiovascular imaging, the elucidation of regional vascular flow patterns in the developing fetus and what can be learned from these flow patterns, the new clinical practice systems being that are being implemented to deal with the fetal patient and its family, and the prospects for fetal surgery.

Fetal cardiovascular physiology

The cardiovascular system of the fetus is physiologically different than the adult, mature system. Unique characteristics of the myocardium and specific channels of blood flow differentitate the physiology of the fetus from the newborn. Conditions of increased preload and afterload in the fetus, such as sacrococcygeal teratoma and twin-twin transfusion syndrome, result in unique and complex pathophysiological states. Echocardiography has improved our understanding of human fetal cadiovasvular physiology in the normal and diseased states, and has expanded our capability to more effectively treat these disease processes.

Right atrial pressure as a measure of ventricular constraint arising from positive end-expiratory pressure during mechanical ventilation of the neonatal lamb

OBJECTIVE

By constraining the heart, the chest wall, lungs, and pericardium limit diastolic filling and thus have a major role in determining cardiac output. Although intermittent positive pressure ventilation and the application of positive end-expiratory pressure amplifies this constraint, no clinical method exists to assess the impact that positive end-expiratory pressure has on ventricular constraint in the newborn. In this study, we tested the hypothesis that a change in right atrial pressure (Pra) reflects the change in ventricular constraint associated with a change in airway pressure.

DESIGN

Experimental, comparative animal study.

SETTING

Ritchie Centre for Baby Health Research cardiovascular laboratory. SUBJECTS Neonatal (4-wk-old, n = 6) and newborn (3-day-old, n = 6) Merino/Border-Leicester cross lambs.

INTERVENTIONS

Lambs were anesthetized (alpha-chloralose and ketamine), ventilated, and instrumented to record Pra, thoracic inferior vena caval pressure (Pivc, saline-filled catheters), and pericardial pressure (Pper, liquid-containing balloon).

MEASUREMENTS AND MAIN RESULTS

Changes (Delta) in Pra, Pivc, and Pper were assessed while airway pressure was rapidly reduced from four set levels of continuous positive airway pressure (2.5, 5, 7.5, and 15 cm H2O) to atmospheric pressure. A strong linear relationship was observed between DeltaPra and DeltaPper (DeltaPra = 0.90 DeltaPper - 0.02, r =.98), and between DeltaPivc and DeltaPper (DeltaPivc = 0.86 DeltaPper - 0.02, r =.98) in both the 4-wk-old lambs and the 3-day-old lambs.

CONCLUSIONS

Our experiments demonstrate that, in the newborn and neonatal lamb, DeltaPra provides an accurate measure of the change in ventricular constraint that accompanies a change in airway pressure, and thus may provide a means of quantifying the magnitude of ventricular constraint imposed by positive end-expiratory pressure and mechanical ventilation during neonatal intensive care.

Left ventricular stroke volume in the fetal sheep is limited by extracardiac constraint and arterial pressure

1. Extracardiac constraint and sensitivity to arterial pressure may be critical factors that limit the functional reserves of the developing fetal heart in utero. We hypothesise that extracardiac constraint is the predominant factor that limits fetal stroke volume (SV). To test this hypothesis we studied six chronically instrumented fetal sheep to determine the relative roles that extracardiac constraint and arterial pressure play in determining left ventricular (LV) function. 2. Pregnant ewes (128-131 days gestation, term = 147 days) were anaesthetised (5 mg kg(-1) Propofol I.V., then 1.5 % halothane, 50 % O(2), balance N(2)O by inhalation) and instrumented using sterile surgical techniques to record LV end-diastolic pressure (P(lved)), aortic pressure (P(ao)), pericardial pressure (P(per)), and LV SV. 3. After a minimum of 72 h recovery, LV function was assessed by altering fetal blood volume to vary P(lved). Ventricular function curves were generated using two measures of ventricular function, SV and stroke work index (SWI = SV x P(ao)), and two measures of ventricular filling, P(lved) and LV end-diastolic transmural pressure (P(lved,tm) = P(lved) - P(per)). 4. Although decreasing P(lved) from the resting level decreased SV, increasing P(lved) from the resting level did not increase SV because the ventricular function curve plateaued. This plateau was not explained solely by an increase in aortic pressure, as the plateau remained present in the SWI versus P(lved) curve. When extracardiac constraint was accounted for (SV against P(lved,tm)), the plateau was largely eliminated (approximately 80 %). The remaining portion of the plateau (approximately 20 %) was eliminated when both extracardiac constraint and arterial pressure were accounted for (SWI versus P(lved,tm)). 5. Thus, the major limitation upon LV function in the near-term fetus results from extracardiac constraint limiting ventricular filling while, at the same time, a much smaller limitation arises from increasing arterial pressure.

Right atrial pressure as measure of ventricular constraint in newborn lambs

Although the lungs and pericardium constrain the heart and limit cardiac output, no method exists to assess this constraint in sick newborns. We hypothesize that a useful estimate of ventricular constraint may be obtained by measuring right atrial pressure (P(RA)) in the newborn. To test this hypothesis, we measured P(RA), thoracic inferior vena caval pressure (P(IVC); saline-filled catheters), and ventricular constraint (pericardial pressure, P(PER); liquid-containing balloon) in 4-wk-old (neonatal, n = 12) and 3-day-old (newborn, n = 6) anesthetized lambs. The measurements were made while LV filling pressure was altered (0-20 mmHg) and while positive end-expiratory pressure (PEEP) was maintained at 2.5 or 15 cmH2O. In all of the lambs, a strong linear relationship (r) existed between P(RA) and P(PER) (P(RA) = 1.19 P(PER) + 0.0, r = 0.99) and between P(IVC) and P(PER) (P(IVC) = 1.24 P(PER) + 0.1, r = 0.99; PEEP of 2.5 cmH2O). Similar relationships were also observed with increased PEEP (P(RA) = 1.29 P(PER)-1.2, r = 0.98 and P(IVC) = 1.32 P(PER)-1.2, r = 0.97). Because P(RA) provides an accurate measure of ventricular constraint in the normal lamb, it may be a useful measure of ventricular constraint in the sick newborn.

Age-related differences in the distortion of the sheep lung in response to localised pleural stress

In order for diastolic filling to occur, the heart must displace the lung. Given the changes in lung structure and compliance that follow birth, we sought to determine whether the neonatal lung resists neighbouring structures encroaching into its space more than the adult lung and whether the lung surface making up the cardiac fossa resists distortion more than the lateral surface does. Pleural distortions, induced by applied pressures (Pappl) of 20-120 g cm(-2) at airway pressures (Paw) of 2.5-15 cm H2O, were recorded in isolated lungs of adult, neonatal (4-week-old) and newborn (1-week-old) sheep. The depth of pleural distortion increased (P < 0.05, ANOVA) with increasing Pappl in all lungs. Adult lungs were significantly more distortable than newborn and neonatal lungs (P < 0.05). As Paw increased, the distortability of the adult lung decreased progressively (P < 0.05) while the distortability of the newborn and neonatal lung remained constant at Paw of 2.5 and 5 cm H2O. Adult lungs also differed from newborn and neonatal lungs in that the cardiac fossal surface was significantly less distortable than the lateral surface. As newborn and neonatal lungs are less easily distorted than adult lungs, the potential for the lungs to limit cardiac filling is greater in the newborn and neonate than in the adult.

Interactions between the right ventricle and pulmonary vasculature in the fetus

A midsystolic plateau differentiates the pattern of fetal pulmonary trunk blood flow from aortic flow. To determine whether this plateau arises from interactions between the left (LV) and right ventricle (RV) via the ductus arteriosus or from interactions between the RV and the lung vasculature, we measured blood flows and pressures in the pulmonary trunk and aorta of eight anesthetized (ketamine and alpha-chloralose) fetal lambs. Wave-intensity analysis revealed waves of energy traveling forward, away from the LV and the RV early in systole. During midsystole, a wave of energy traveling back toward the RV decreased blood flow velocity from the RV and produced the plateau in blood flow. Calculations revealed that this backward-traveling wave originated as a forward-traveling wave generated by the RV that was reflected from the lung vasculature back toward the heart and not as a forward-traveling wave generated by the LV that crossed the ductus arteriosus. Elimination of this backward-traveling wave and its associated effect on RV flow may be an important component of the increase in RV output that accompanies birth.

Left ventricular systolic function, arterial elastance, and ventricular-vascular coupling: a developmental study in piglets

Circulatory changes occur during perinatal life that increase cardiac output and left ventricular contractile reserve. To examine postnatal changes in left ventricular systolic function and ventricular-vascular coupling, piglets underwent cardiac catheterization at 1, 2, 4, and 6 wk of age. We measured end-systolic elastance (Ees), preload-recruitable stroke work, dP/dt(max), the dP/dt(max) end-diastolic volume relation, cardiac index, heart rate, arterial elastance (Ea), and the ratio Ea/Ees, at rest, during isoproterenol infusions (0.05-1.0 microg/kg/min), and after propranolol (1 mg/kg i.v.). Resting heart rate and cardiac index decreased between 1 and 6 wk. In 1 wk olds, resting Ees was at maximum and was unchanged during isoproterenol infusion; isoproterenol increased other contractility indices. Two, 4, and 6 wk olds demonstrated reserve using all contractility indices. Contractile efficiency was not different between ages. In 1 wk olds, Ea decreased during isoproterenol infusion; isoproterenol did not change Ea at 6 wk. Ea/Ees was higher at rest at 6 wk than at 1 wk, and fell significantly on isoproterenol; isoproterenol did not change Ea/Ees at 1 wk. With beta-adrenergic stimulation, 1 wk olds increased cardiac index by increasing heart rate and decreasing afterload, 6 wk olds increased cardiac index by increasing heart rate and contractility; no change in contractile efficiency was found in either group. In summary, contractile reserve is limited at 1 wk when measured by Ees, but other indices demonstrated reserve. Indexed Ea falls in response to beta-adrenergic stimulation in all ages but 6 wk. Ventricular-vascular coupling is optimized at 1 wk even under baseline conditions.

Perinatal left ventricular performance in fetal sheep: interaction between oxygen ventilation and contractility

Left ventricular (LV) output nearly triples at birth, in association with increases in serum catecholamines. Similar increases in catecholamines in utero, however, do not increase output. We hypothesized that catecholamines increase contractility in utero, but that output cannot increase until LV loading conditions are changed by oxygen ventilation. To address this hypothesis, we studied nine fetal sheep acutely placed in a warm water bath (40 degrees C). Conductance and manometric catheters were placed in the LV to generate pressure-volume loops during caval occlusion in the nonventilated and oxygen-ventilated states, each under control, dobutamine, and propranolol conditions. Contractility was estimated by the end-systolic pressure-volume relationship, preload by end-diastolic volume, and afterload by arterial elastance. Oxygen ventilation increased LV output 1.4-fold, despite a decrease in contractility to about three-fourths of the nonventilated value. Heart rate remained constant, whereas preload increased and afterload decreased significantly. During oxygen ventilation, dobutamine increased output to 2.3 times the control, nonventilated value, associated with increases in contractility and heart rate and no change in preload and afterload. Although dobutamine increased contractility and heart rate similarly in the nonventilated and oxygen ventilated states, output increased significantly more during ventilation. Similarly, propranolol decreased contractility and heart rate equally in both states, but output decreased far more during ventilation. Thus, oxygen ventilation is associated with advantageous changes in LV load such that the positive inotropic and chronotropic effects of dobutamine are translated into greater increases in LV output.

The heart and development

The perspective from which the developing heart is viewed can lead to differing conclusions about the effects of development on cardiac function. The hearts of the embryo, fetus and adult, viewed from a global perspective, sustain the circulation through the same basic mechanisms of developing pressure and ejecting blood. The failure of the embryonic heart to perform these tasks results in growth failure, edema, and embryonic death, just as in the infant and adult such failure results in premature death. Furthermore, from the viewpoint of gross anatomy, following embryonic morphogenesis, the developing and adult hearts appear in general to be structurally similar, differing only in size and mass. However, a closer view shows, in the molecular and structural makeup of the myocardium, richly complex changes that can modulate the basic physiological properties of the cardiac myocyte. This article focuses on how these changes and the effects of birth and development alter ventricular function.

Pleural and pericardial pressures limit fetal right ventricular output

BACKGROUND

The chest wall, lungs, and pericardium limit diastolic filling of the left ventricle in the fetus, neonate, and adult. To determine the effect that these tissues have on the fetal right ventricle (RV), we studied six fetal lambs (142 days of gestation)

METHODS AND RESULTS

Pregnant ewes were anesthetized (ketamine and alpha-chloralose), and the fetuses were partially delivered by cesarean section. Fetuses were instrumented to record RV stroke volume, RV end-diastolic pressure (Prved), intrapericardial pressure (Pip), and pleural pressure. Prved was varied between 2 and 20 mm Hg under three conditions: initially with a closed chest and a closed pericardium (CCCP); subsequently with an open chest (chest wall and lungs retracted) and a closed pericardium (OCCP); and finally after the chest wall, lungs, and pericardium were retracted (OCOP). At equal Prved, stroke volume increased substantially when the chest wall and lungs were retracted from the heart and increased further on subsequent retraction of the pericardium (eg, at Prved of 9 mm Hg, stroke volume increased from 1.2 +/- 0.2 mL [mean +/- SEM] in the CCCP condition to 2.9 +/- 0.4 and 4.2 +/- 0.3 mL in the OCCP and OCOP conditions, respectively, P < or = .05). The limitation of stroke volume in the CCCP and OCCP conditions occurred because Pip increased in an almost one-to-one fashion as Prved increased; as a consequence, RV preload (RV end-diastolic transmural pressure, Prved minus Pip) was relatively unchanged.

CONCLUSIONS

The chest wall-lung combination and the pericardium each significantly constrain the fetal RV and together limit RV stroke volume.

Left ventricular preload reserve in preterm infants with patent ductus arteriosus

The left ventricular Frank-Starling response was studied in 15 preterm infants, less than 1500 g birth weight, and in 16 fullterm infants with patent ductus arteriosus. Left ventricular end diastolic volume (LVEDV), stroke volume, and cardiac output were calculated from biplane echocardiographic images with a modified Simpson's rule, and the left ventricular function curve was obtained by standardising with birth weight and body length. In the relationship between LVEDV and stroke volume, the slope of the regression line was significantly milder in preterm than in fullterm infants; however, there was no significant difference in the relationship between LVEDV and cardiac output. The heart rate was significantly higher in preterm than in fullterm infants. Our data indicated that the premature infants had less left ventricular reserve capacity to respond to the increased preload through the left-to-right ductal shunting than the mature ones, and that the high pulse rate made it possible to generate adequate cardiac output in premature infants.

Changes in pericardial pressure during the perinatal period

BACKGROUND

To determine how the tissues that surround the heart affect diastolic and systolic function during the perinatal period, we studied the pressure-diameter relation of the left ventricle in partially delivered fetal lambs.

METHODS AND RESULTS

We anesthetized (1.5-2.0% halothane, balance O2) and ventilated six pregnant ewes (142-144 days of gestation) and then partially delivered each lamb by cesarean section. Each lamb was instrumented to record left ventricular anteroposterior diameters (endocardial ultrasonic transducers), pericardial pressure (liquid-containing balloon), and left ventricular pressure (transducer-tipped catheter). Left ventricular pressure-diameter relations were recorded under three conditions: initially, with a closed chest and closed pericardium (before ventilation); second, after interruption of the umbilical circulation and 1 hour of ventilation; and finally, when the lungs and the pericardium were retracted from the heart. Pericardial pressure (recorded at a common diameter, i.e., the maximal end-diastolic diameter recorded before ventilation) decreased by 48% after 1 hour of ventilation (p < 0.05). After ventilation, left ventricular anteroposterior diameters were 4-5% greater (p < 0.05) at each end-diastolic pressure compared (12.5, 15.0, 17.5, and 20 mm Hg). Thus, ventilation appeared to increase left ventricular diastolic compliance. Contractility also appeared to increase after ventilation when evaluated using ventricular stroke work as a function of end-diastolic pressure as preload. When we used a more appropriate measure of preload (i.e., transmural end-diastolic pressure), ventilation did not change left ventricular diastolic compliance or contractility. Thus, left ventricular systolic function increased because of an increase in preload.

CONCLUSIONS

The tissues surrounding the fetal heart significantly augment pericardial pressure and limit left ventricular preload. The initiation of ventilation reduces pericardial pressure, increases left ventricular preload, and increases left ventricular systolic function. At birth, a decrease in pericardial pressure and the resulting increase in preload may help increase left ventricular output through the Frank-Starling mechanism.