Left ventricular performance and contractility before and after volume infusion: a comparative study of preterm and full-term newborn lambs

Role of Volume Replacement during Neonatal Resuscitation in the Delivery Room

Volume expanders are indicated in the delivery room when an asphyxiated neonate is not responding to the steps of neonatal resuscitation and has signs of shock or a history of acute blood loss. Fetal blood loss (e.g., feto-maternal hemorrhage) may contribute to perinatal asphyxia. Cord compression or a tight nuchal cord can selectively occlude a thin-walled umbilical vein, resulting in feto-placental transfusion and neonatal hypovolemia. For severe bradycardia or cardiac arrest secondary to fetal blood loss, Neonatal Resuscitation Program (NRP) recommends intravenous volume expanders (crystalloids such as normal saline or packed red blood cells) infused over 5 to 10 min. Failure to recognize hypovolemia and subsequent delay in volume replacement may result in unsuccessful resuscitation due to lack of adequate cardiac preload. However, excess volume load in the presence of myocardial dysfunction from hypoxic–ischemic injury may precipitate pulmonary edema and intraventricular hemorrhage (especially in preterm infants). Emergent circumstances and ethical concerns preclude the performance of prospective clinical studies evaluating volume replacement during neonatal resuscitation. Translational studies, observational data from registries and clinical trials are needed to investigate and understand the role of volume replacement in the delivery room in term and preterm neonates. This article is a narrative review of the causes and consequences of acute fetal blood loss and available evidence on volume replacement during neonatal resuscitation of asphyxiated neonates.

Effects of Early Myocardial Postnatal Maturation on Tolerance to Atrial Tachycardia With Altered Loading Conditions: An in vivo Swine Model

Post-natal maturation of the myocardium starts shortly after birth and could affect how clinicians should provide hemodynamic support during this transition. Our aim was to assess the impact of post-natal maturation on tolerance to tachycardia with altered loading condition in a piglet model. Methods: We report three series of experimentations. Six groups of landrace cross neonatal piglets (NP) (1-3 days) and young piglets (YP) (14-17 days) were assigned to tachycardia (NP, YP), tachycardia and hypervolemia (NPV, YPV) or tachycardia and increased afterload (NPA, YPA) groups (n = 7/group). Under anesthesia, a pressure catheter was placed in the left ventricle and pacing wire in the right atrium. NPV and YPV groups had 60 ml/kg of normal saline infused over 20 min. NPA and YPA had balloon sub-occlusion of the descending aorta. Heart rate was increased by 10 bpm increments to 300 bpm. Left ventricular output was measured by echocardiography. Results: NP maintained left ventricular output throughout the pacing protocol but it decreased in the YP (p < 0.001). With volume loading both NPV and YPV maintained their output with tachycardia. Although increased afterload resulted in reduced output during tachycardia in NPA (p = 0.005), there was no added impact on output in YPA. Interestingly, 4 of 7 NPV had significant desaturation at 300 bpm (baseline 99.7% vs. 300 bpm 87.9%, p = 0.04), associated with a right to left shunt through the patent foramen ovale which resolved immediately on cessation of pacing. Conclusions: Early post-natal maturation is associated with improved myocardial tolerance to increased afterload and poor tolerance of tachycardia, the latter of which may be alleviated by increasing intravascular volume. These data could translate into the development of better strategies to optimize cardiac output at these early development ages.

Hemodynamic coherence in critically ill pediatric patients

Differences in physiology and pathophysiology make the treatment of developing, critically ill children particularly challenging as compared to that of adults. Significant differences in the cardiovascular system of neonates and children in size, weight, body proportions, and metabolism should be considered. Hemodynamic monitoring is crucial for early warning of pending deterioration and to guide therapy. Current monitoring is limited to the macrocirculation, but an adequately functioning macrocirculation does not guarantee a well-functioning microcirculation. Research in children revealed loss of hemodynamic coherence, i.e., microcirculatory alterations despite normal systemic hemodynamics. Implementing the framework of hemodynamic coherence in microcirculatory monitoring in children can aid physicians in titrating therapy on both macrocirculatory and microcirculatory effects to assure optimal oxygen delivery. Monitoring the microcirculation at the bedside requires further technical development. Although more research is necessary to validate the concept of hemodynamic coherence in children, the possibilities of applying this concept in children seem promising.

The vulnerable microcirculation in the critically ill pediatric patient

In neonates, cardiovascular system development does not stop after the transition from intra-uterine to extra-uterine life and is not limited to the macrocirculation. The microcirculation (MC), which is essential for oxygen, nutrient, and drug delivery to tissues and cells, also develops. Developmental changes in the microcirculatory structure continue to occur during the initial weeks of life in healthy neonates. The physiologic hallmarks of neonates and developing children make them particularly vulnerable during critical illness; however, the cardiovascular monitoring possibilities are limited compared with critically ill adult patients. Therefore, the development of non-invasive methods for monitoring the MC is necessary in pediatric critical care for early identification of impending deterioration and to enable the initiation and titration of therapy to ensure cell survival. To date, the MC may be non-invasively monitored at the bedside using hand-held videomicroscopy, which provides useful information regarding the microcirculation. There is an increasing number of studies on the MC in neonates and pediatric patients; however, additional steps are necessary to transition MC monitoring from bench to bedside. The recently introduced concept of hemodynamic coherence describes the relationship between changes in the MC and macrocirculation. The loss of hemodynamic coherence may result in a depressed MC despite an improvement in the macrocirculation, which represents a condition associated with adverse outcomes. In the pediatric intensive care unit, the concept of hemodynamic coherence may function as a framework to develop microcirculatory measurements towards implementation in daily clinical practice.

Postnatal neonatal myocardial adaptation is associated with loss of tolerance to tachycardia: a simultaneous invasive and noninvasive assessment

Doppler studies at rest suggest left ventricular (LV) diastolic function rapidly improves from the neonate to infant. Whether this translates to its response to hemodynamic challenges is uncertain. We sought to explore the impact of early LV maturation on its ability to tolerate atrial tachycardia. As tachycardia reduces filling time, we hypothesized that the neonatal LV would be less tolerant of atrial tachycardia. Landrace cross piglets of two age groups (1-3 days; NPs; 14-17 days, YPs; n = 7/group) were instrumented for an atrial pacing protocol (from 200 to 300 beats/min) and assessed by invasive monitoring and echocardiography. NPs maintained their LV output and blood pressure, whereas YPs did not. Although negative dP/dt in NPs at baseline was lower than that of YPs (-1,599 ± 83 vs. -2,470 ± 226 mmHg/s, respectively, P = 0.007), with increasing tachycardia negative dP/dt converged between groups and was not different. Both groups had similar preload reduction during tachycardia; however, NPs maintained shortening fraction while YPs decreased (NPs: 35.4 ± 1.4 vs. 31.8 ± 2.2%, P = 0.35; YPs: 31.4 ± 0.8 vs. 22.9 ± 0.8%, P < 0.001). Contractility measures did not differ between groups. Peak LV twist and untwisting rate also did not differ; however, NPs tended to augment LV twist through increased apical rotation and YPs through increasing basal rotation (P = 0.009). The NPs appear more tolerant of atrial tachycardia than the YPs. They have at least similar diastolic performance, enhanced systolic performance, and different LV twist mechanics, which may contribute to improved tachycardia tolerance of NPs.

Advanced hemodynamic monitoring in critically ill children

Circulatory shock is an important cause of pediatric morbidity and mortality and requires early recognition and prompt institution of adequate treatment protocols. Unfortunately, the hemodynamic status of the critically ill child is poorly reflected by physical examination, heart rate, blood pressure, or laboratory blood tests. Advanced hemodynamic monitoring consists, among others, of measuring cardiac output, predicting fluid responsiveness, calculating systemic oxygen delivery in relation to oxygen demand, and quantifying (pulmonary) edema. We discuss here the potential value of these hemodynamic monitoring technologies in relation to pediatric physiology.

Myocardial contractility in premature neonates with and without patent ductus arteriosus

Controversy exists as to whether a hemodynamically significant left-to-right shunt due to a patent ductus arteriosus (PDA) affects ventricular contractility. Load-dependent indices such as ejection fraction and shortening fraction have traditionally been used to assess contractility, but the relationship between the rate-corrected velocity of fiber shortening (MVCFc) and wall stress may be more suitable, as it is a preload-independent, afterload-adjusted method of assessing ventricular contractility. Age-related differences have been established for these variables in normal adults and children and it has been recommended for use in the premature neonate. The study was performed to assess left ventricular contractility in premature neonates with a significant left-to-right shunt due to a PDA. Using echocardiography, we measured the relationship of MVCFc to stress at peak systole (SPS) in two groups of premature infants. Group 1 consisted of 15 controls (680-1495 g, 25-32 weeks' gestation), and Group 2 of 15 neonates with hemodynamically significant PDA (840-1635 g, 26-33 weeks' gestation). In both groups, MVCFc was linearly and inversely related to SPS ( p < 0.001). The regression equations were as follows: Group 1, MVCFc = -0.0153SPS + 1.70 ( R(2) = 0.68); and Group 2, MVCF = - 0.019SPS + 1.89 ( R(2) = 0.76). There was no significant difference in the relationship between the two groups, but their slopes were significantly steeper and had a higher Y-intercept than the relationship we previously reported for older children. This preliminary study establishes the normal MVCFc/SPS relationship in the premature neonate (25-33 weeks' gestation) and suggests that premature infants function at a higher resting contractile state than older children. A hemodynamically significant PDA has no effect on contractility. These data will be useful in assessing left ventricular contractility in premature neonates with other types of ventricular loading and noncardiac stress.

Effect of ligation of patent ductus arteriosus on left ventricular performance and its determinants in premature neonates

OBJECTIVES

The purpose of this study was to determine in preterm newborn infants the effects of ductal ligation on ventricular performance and its determinants: preload, afterload and contractility.

BACKGROUND

Neonatal ventricular performance is highly sensitive to afterload. Therefore, the increase in systemic vascular resistance associated with ligation of a patent ductus arteriosus might worsen ventricular performance in the preterm infant.

METHODS

All 14 premature infants undergoing patent ductus arteriosus ligation in a 1-year period at our institution underwent echocardiography at three times: before, immediately after and 24 h after ligation. Indexes studied included ventricular performance (fractional area change), preload (left ventricular end-diastolic dimension), afterload (end-systolic wall stress) and contractility (the difference between the measured and predicted velocity of circumferential fiber shortening). Blood pressure was measured; systemic resistance was calculated. These data were compared with those of 14 preterm infants without patent ductus arteriosus.

RESULTS

The infants with patent ductus arteriosus had higher values for ventricular performance (mean +/- SD fractional area change 60 +/- 9% vs. 52 +/- 11%, p < 0.05) and lower values for wall stress (22 +/- 6 vs. 44 +/- 17 g/cm2, p < 0.05) before ligation than did the control group. At 24 h after ligation, ventricular performance was not significantly changed (fractional area change 60 +/- 9% to 57 +/- 12%). There were significant increases in blood pressure and systemic vascular resistance but no changes in wall stress or contractility.

CONCLUSIONS

Ventricular performance is higher in premature infants with than in those without patent ductus arteriosus because afterload is lower in the former group. Although ductal ligation increases blood pressure and systemic resistance, wall stress and ventricular performance are maintained. Our results suggest that the premature newborn maintains ventricular performance during stress, at least in part, by manipulating afterload.

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.

The neonatal heart has a relatively high content of total collagen and type I collagen, a condition that may explain the less compliant state

OBJECTIVES

This study evaluated the extent of the collagen network in neonatal heart muscle and whether the type I/type III collagen ratio is the same as in the adult heart.

BACKGROUND

The functional integrity and the stress-strain relation of heart muscle depends largely on the extracellular collagen matrix. The question therefore arises whether the altered compliance of the neonatal heart could relate to the developmental state of collagen and, in particular, the distribution of types I and III collagen. Type I collagen mainly provides rigidity and type III collagen elasticity.

METHODS

Specimens from the left lateral wall of the left ventricle of human hearts (immature to full term, n = 23; 3 weeks to 12 years, n = 17) were used to determine the total collagen amount, using the hydroxyproline assay. Similar left ventricular specimens of human hearts (fetal to mature, n = 20; 2 months to 1.5 years, n = 6) were fixed in formalin, paraffin embedded and stained with Sirius red F3BA for total collagen. The ratio of total collagen to total protein was quantified spectrophotometrically. Frozen sections of left ventricular myocardium (immature to mature, n = 17; 4 months to 12 years, n = 10) were stained with antibodies raised against types I and III collagen. Antibody titration was done on human leiomyoma tissue with a known type I/type III collagen ratio. The endomysial collagen types were quantified using a spectrophotometer and expressed as a ratio. Adult human myocardium (n = 10) was used as reference.

RESULTS

The study showed that the total amount of collagen increases with age. However, the ratio of total collagen to total protein and the ratio of type I to type III collagen were very high in hearts of the very young. During development, a gradual decrease occurred, with the total collagen/total protein ratio reaching normal levels at approximately 5 months after birth and the type I/type III collagen ratio stabilizing at a much later age.

CONCLUSIONS

These findings suggest that the relative high content of collagen, related to the myocytes, and the high ratio of type I to type III collagen provide the substrate for a rigid, less compliant heart in neonates.

Left ventricular mechanics in the preterm infant and their effect on the measurement of cardiac performance

The effects of the transition from fetal to postnatal circulation on left ventricular geometry, wall motion, and echocardiographic measurements of function in the human preterm infant are largely unknown. To determine whether abnormalities in left ventricular geometry are present in the normal preterm infant after birth and, if so, for how long, and to examine possible contributing factors and their effect on the measurement of cardiac performance, we obtained serial echocardiograms of 14 healthy preterm infants (gestational age, 33 +/- 2 weeks; birth weight, 1940 +/- 470 gm) at 9.5 +/- 3.5 days of age (time 1) and again at 51 +/- 16 days (time 2). Left ventricular shape and wall motion were measured and estimates of wall stress and mass were made. Performance was assessed by standard M-mode shortening fraction and by transverse two-dimensional area shortening. At time 1 septal flattening caused distortion of left ventricular shape. As the patients grew older, septal flattening resolved and the left ventricle tended to assume a circular cross-sectional shape. Wall-motion analysis demonstrated poor motion of the midseptum and anterior free wall at time 1, which improved at time 2 (p = 0.06). Left ventricular mass increased from 24 +/- 5 to 41 +/- 7 gm/m2 (p = 0.0001) and wall stress decreased from 49 +/- 21 to 38 +/- 13 gm/cm2 (p = 0.005) between time 1 and time 2. Shortening fraction was lower at time 1 than at time 2 (18% +/- 7% vs 28% +/- 8%; p = 0.001; normal limit = 28% to 45%); however, there was no significant difference in area shortening between time 1 and time 2 (49% +/- 10% vs 53% +/- 8%; normal limit = 45% to 65%). We conclude that the preterm newborn infant has distorted left ventricular shape and abnormal wall motion, which alter measurements of shortening fraction and persist for the first weeks of life. Area shortening may be necessary to assess left ventricular performance during the first weeks of life in the premature infant.

Control of blood pressure and the distribution of blood flow

Systemic blood pressure (BP) is the product of cardiac output and total peripheral resistance. Cardiac output is controlled by the heart rate, myocardial contractility, preload, and afterload. Vascular resistance (vascular hindrance × viscosity) is under local autoregulation and general neurohumoral control through sympathetic adrenergic innervation and circulating catecholamines. Sympathetic innovation predominates in organs receivingflowin excess of their metabolic demands (skin, splanchnic organs, kidney), while innervation is poor and autoregulation predominates in the brain and heart. The distribution of blood flow depends on the relative resistances of the organ circulations. During stress (hypoxia, low cardiac output), a raise in adrenergic tone and in circulating catecholamines leads to preferential vasoconstriction in highly innervated organs, so that blood flow is directed to the brain and heart. Catecholamines also control the levels of the vasoconstrictors renin, angiotensin II, and vasopressin. These general principles also apply to the neonate.

Echo Doppler assessment of cardiac output and its relation to growth in normal infants

In a study of 38 normal infants, serial measurements of systemic (n = 169) and pulmonary (n = 143) blood flow were undertaken from the ages of 2 weeks to 12 months by 2-dimensional, M-mode and pulsed Doppler echocardiography. Cardiac output changed linearly (cardiac output = 0.3 X height -0.99 liter/min), and cardiac index was validated as a means for standardizing cardiac output in infants younger than 10 to 13 months of age. Infants younger than 2 months had lower cardiac indexes and stroke volume indexes (2.6 +/- 0.7 liters/min/m2 and 19 +/- 5 ml/m2, respectively) compared with those aged 12 months (3.2 +/- 0.7 liter/min/m2 and 25 +/- 5 ml/m2, respectively). Changes in cardiac output in individual infants over time suggest nonmorphometric modulating factors for cardiovascular function.

Changes of the cardiovascular system during the perinatal period

After describing the particular features of the fetal circulation, changes in the pattern of blood flow at the time of birth and during early neonatal life are explained. From animal studies it is wellknown that during the first hours and weeks after birth newborns are characterized by an extremely high cardiac output due to high metabolic demands. In order to meet this marked volume loading, already under resting conditions the neonatal heart appears to be operating nearly at its full capacity without reserves in contractility, preload and afterload. Consequently the newborn heart has less ability to cope with additional acute afterload and/or preload stress. Few investigations on cardiac output and myocardial performance in healthy human newborns provide presumptive evidence that the postnatal human heart performs probably as well as the heart of other species. These observations may influence the therapeutic approach in clinical situations with additional alterations in loading conditions.

Effect of fetal adrenalectomy on catecholamine release and physiologic adaptation at birth in sheep

Plasma catecholamine levels increase dramatically at birth. To determine the contribution of adrenal catecholamine secretion to the surge in catecholamines at birth and the role in newborn adaptation, we performed surgical adrenalectomy or sham operation on near-term ovine fetuses. After recovery in utero, the animals were delivered and supported by mechanical ventilation. Plasma catecholamine levels, heart rate, blood pressure, cardiac output, pulmonary function, surfactant secretion, and release of free fatty acids (FFA) and glucose were compared in control and adrenalectomized animals. Plasma epinephrine increased rapidly at birth in controls but was undetectable in adrenalectomized animals. Norepinephrine levels were not statistically different. Heart rate, blood pressure, cardiac output and contractility increased abruptly after cord cutting in controls but did not increase in adrenalectomized animals. Lung compliance, pulmonary function, surfactant pool size, glucose and FFA levels were significantly decreased in adrenalectomized animals. These results suggest that adrenal epinephrine secretion is vital to many of the adaptive events at birth.

Increased right ventricular afterload alters left ventricular function in newborn lambs

The effects of acutely increased right ventricular afterload induced by mechanical constriction of the main pulmonary artery or with alveolar hypoxia on function of the left ventricle were assessed in six and nine lambs, respectively (aged 1 to 3 days). Mechanical constriction of the main pulmonary artery in newborn lambs compromised left ventricular function with significant decreases in systemic blood flow and the peak first derivative of left ventricular pressure when considered simultaneously and as single variables. In contrast, alveolar hypoxia augmented left ventricular function with significant increases in systemic blood flow and the peak first derivative of left ventricular pressure when considered simultaneously or as single variables. Interaction appears to exist between the right and left ventricles during the newborn period. The mechanically increased afterload may have compromised left ventricular function by altering its end-diastolic size, inotropic state, or both. On the other hand, the augmented left ventricular function in the presence of hypoxia may have been due to an increase in inotropic background. The clinical implications in some infants with pulmonary hypertension and left ventricular dysfunction are raised.