Ultrasound measurements of fetal breathing movements in the rat.

The goal of this study was to determine when fetal breathing movements (FBMs) commence in the rat and to characterize age-dependent changes of FBMs in utero. These data provide a frame of reference for parallel in vitro studies of the cellular, synaptic, and network properties of the perinatal rat respiratory system. Ultrasound recordings were made from unanesthetized Sprague-Dawley rats from embryonic (E) day 15 (E15) to E20. Furthermore, the effects of respiratory stimulants (doxapram and aminophylline) and hypoxia on FBMs were studied. Single FBMs, occurring at a very low frequency (approximately 8 FBMs/h), commenced at E16. The incidence of single FBMs increased to approximately 80 FBMs/h by E20. Episodes of clustered rhythmic FBMs were first observed at E18 (approximately 40 FBMs/h). The incidence of episodic clustered FBMs increased to approximately 300 FMBs/h by E20, with the duration of each episode ranging from approximately 40 to 180 s. Doxapram, presumably acting to stimulate carotid body receptors, did not increase FBMs until E20, when the incidence of episodic clustered FBMs increased twofold. Aminophylline, a central-acting stimulant, caused an increase in episodic clustered FBMs after E17, reaching significance at E20 (3-fold increase). Exposing the dam to 10% O(2) caused a rapid, marked suppression of FBMs (5-fold decrease) that was readily reversed on exposure to room air.

High-flow nasal cannulae in the management of apnea of prematurity: a comparison with conventional nasal continuous positive airway pressure.

Apnea of prematurity (AOP) is frequently managed with nasal continuous positive airway pressure (NCPAP). Nasal cannula (NC) are used at low flows (<0.5 L/min) to deliver supplemental oxygen to neonates. A number of centers use high-flow nasal cannula (HFNC) in the management of AOP without measuring the positive distending pressure (PDP) generated. Objective. To determine the NC flow required to generate PDP equal to that provided by NCPAP at 6 cm H(2)O and to assess the effectiveness of HFNC as compared NCPAP in the management of AOP. Method. Forty premature infants, gestation 28.7 +/- 0.4 weeks (mean +/- standard error of mean), postconceptual age at study 30.3 +/- 0.6 weeks, birth weight 1256 +/- 66 g, study weight 1260 +/- 63 g who were being managed with conventional NCPAP for at least 24 hours for clinically significant apnea of prematurity, were enrolled in a trial of ventilator-generated conventional NCPAP versus infant NC at flows of up to 2.5 L/min. End expiratory esophageal pressure was measured on NCPAP and on NC, and the gas flow on NC was adjusted to generate an end expiratory esophageal pressure equal to that measured on NCPAP. Two 6-hour periods were continuously recorded and the data were stored on computer. Results. The flow required to generate a comparable PDP with NC varied with the infant's weight and was represented by the equation: flow (L/min) = 0.92 + 0.68x, x = weight in kg, R = 0.72. There was no difference in the frequency and duration of apnea, bradycardia or desaturation per recording between the 2 systems. Conclusion. NC at flows of 1 to 2.5 L/min can deliver PDP in premature neonates. HFNC is as effective as NCPAP in the management of AOP.

Structure of the primordial diaphragm and defects associated with nitrofen-induced CDH.

The goals of this study were to further our understanding of diaphragm embryogenesis and the pathogenesis of congenital diaphragmatic hernia (CDH). Past work suggests that the pleuroperitoneal fold (PPF) is the primary source of diaphragmatic musculature. Furthermore, defects associated with an animal model of CDH can be traced back to the formation of the PPF. This study was designed to elucidate the anatomic structure of the PPF and to determine which regions of the PPF malform in the well-established nitrofen model of CDH. This was achieved by producing three-dimensional renderings constructed from serial transverse sections of control and nitrofen-exposed rats at embryonic day 13.5. Renderings of left- and right-sided defects demonstrated that the malformations were always limited to the dorsolateral portions of the caudal regions of the PPF. These data provide an explanation of why the holes in diaphragmatic musculature associated with CDH are characteristically located in dorsolateral regions. Moreover, these data provide further evidence against the widely stated hypothesis that a failure of pleuroperitoneal canal closure underlies the pathogenesis of nitrofen-induced CDH.

Respiratory pacemaker cells responsive to CO(2) in the upper medulla: dose response and effects of mediators.

We previously reported on the presence of respiratory pacemaker cells that are highly sensitive to CO(2), in a region of the medulla oblongata in the fetal rat, 2 mm rostral to the obex. We now report on the CO(2) dose responses of these cells, as well as their responsiveness to certain chemical agents known to affect breathing in the fetus. Twenty-day-old fetal Sprague Dawley rats were block-dissected, and the cells of target areas were dissociated as previously described. Neuronal cells were plated on a medullary background and placed in the incubator with 10% CO(2) for 2-3 weeks. Cells were then studied using patch-clamp techniques. Pacemaker cells with single or bursting potentials showed responsiveness to CO(2) starting with pulses of 10 msec. Irregular beating or silent cells had poor or absent responsiveness to CO(2). Pacemaker cells responded to norepinephrine with increased firing potential; this action was blocked by metropolol. PGE(2) had no effect on pacemaker-cell activity, but indomethacin increased the spike frequency from 336+/-41 to 384+/- 65 spikes/min. Morphine stimulated the pacemaker cells from 205+/-25 to 272+/-29 spikes/min; this was blocked by naloxone. Finally, a placental extract, which inhibited breathing in the unanesthetized fetal sheep preparation, increased the activity of pacemaker cells from 301+/-35 to 452+/-52 spikes/min. In all of the above, irregular beating cells responded poorly and silent cells did not respond. The findings indicate that these pacemaker cells are uniquely designed to respond to CO(2) and have some properties which allow them to respond to certain chemical mediators in a manner similar to that of the whole respiratory system in vivo.

Quantification of recirculation by thermodilution during venovenous extracorporeal membrane oxygenation.

PURPOSE: The aim of this study was to determine whether recirculation could be quantified by a thermodilution technique during venovenous (VV) extracorporeal membrane oxygenation (ECMO) in a rabbit model. METHODS: Five New Zealand white rabbits, mean weight, 4.5 (range, 3.7 to 5.7) kg, were anesthetized, instrumented, cannulated with a double-lumen catheter, and placed on VV ECMO. Serial injections of ice-cold saline were performed at the arterial arm of the circuit, and the resultant temperature change at various pump flows was measured at the venous arm of the circuit using a thermistor-tipped catheter and a cardiac output computer. Results were compared with the respective 100% recirculation measured with all the circuit flow passing through the bridge. RESULTS: Using linear regression, recirculation percentage could be calculated as: 19 + 0.1 x pump flow (R2 = 0.81, P < .005). Recirculation correlated positively with pump flow. Variability between results at each flow was less than 10%. CONCLUSIONS: Recirculation can be quantified during VV ECMO by measuring the change in temperature in the venous arm using a cardiac output computer after injection of a known quantity of ice-cold saline in the arterial side of the circuit. The effect of interventions to reduce recirculation can be assessed conveniently and reliably.

Recent advances in understanding the pathogenesis of nitrofen-induced congenital diaphragmatic hernia.

In this review, we discuss recent advances in the study of the pathogenesis of congenital diaphragmatic hernia (CDH). Much of the research has involved the use of an animal model of CDH in which diaphragmatic defects are produced in fetal rats by administering the herbicide nitrofen to dams during mid-gestation. The animal model is described and the relevance to the human condition is discussed. The data derived from the animal studies are critically assessed in the context of commonly cited hypotheses proposed for the pathogenesis of CDH. Finally, experimental strategies are proposed for systematically examining the normal and pathological formation of the pleuroperitoneal fold. We conclude that a malformation of the primordial diaphragm, the pleuroperitoneal fold, underlies the muscle defects associated with CDH.

Persistent pulmonary hypertension of the newborn presenting as the primary manifestation of intracranial arteriovenous malformation of the Vein of Galen.

Arteriovenous malformations of the Vein of Galen continue to present diagnostic and therapeutic challenges in the neonatal period. Approximately 40-50% of all malformations of the Vein of Galen present in the neonatal period, usually with congestive heart failure. These neonates represent the most severe cases and are also the most difficult to manage. We report a case of a neonate with a Vein of Galen Malformation who presented with cyanosis, a cardiac murmur, and severe persistent pulmonary hypertension of the newborn. Cardiac failure developed later in the patient's course. The degree of pulmonary hypertension on echocardiography was used to time endovascular embolization of the Vein of Galen Malformation. Following embolization, his pulmonary hypertension subsided dramatically. We speculate that pulmonary hypertension associated with Vein of Galen Malformations has been underestimated in the morbidity and demise of these neonates, and should be more aggressively monitored and treated.

An overview of phrenic nerve and diaphragm muscle development in the perinatal rat.

In this overview, we outline what is known regarding the key developmental stages of phrenic nerve and diaphragm formation in perinatal rats. These developmental events include the following. Cervical axons emerge from the spinal cord during embryonic (E) day 11. At approximately E12.5, phrenic and brachial axons from the cervical segments merge at the brachial plexi. Subsequently, the two populations diverge as phrenic axons continue to grow ventrally toward the diaphragmatic primordium and brachial axons turn laterally to grow into the limb bud. A few pioneer axons extend ahead of the majority of the phrenic axonal population and migrate along a well-defined track toward the primordial diaphragm, which they reach by E13.5. The primordial diaphragmatic muscle arises from the pleuroperitoneal fold, a triangular protrusion of the body wall composed of the fusion of the primordial pleuroperitoneal and pleuropericardial tissues. The phrenic nerve initiates branching within the diaphragm at approximately E14, when myoblasts in the region of contact with the phrenic nerve begin to fuse and form distinct primary myotubes. As the nerve migrates through the various sectors of the diaphragm, myoblasts along the nerve's path begin to fuse and form additional myotubes. The phrenic nerve intramuscular branching and concomitant diaphragmatic myotube formation continue to progress up until E17, at which time the mature pattern of innervation and muscle architecture are approximated. E17 is also the time of the commencement of inspiratory drive transmission to phrenic motoneurons (PMNs) and the arrival of phrenic afferents to the motoneuron pool. During the period spanning from E17 to birth (gestation period of approximately 21 days), there is dramatic change in PMN morphology as the dendritic branching is rearranged into the rostrocaudal bundling characteristic of mature PMNs. This period is also a time of significant changes in PMN passive membrane properties, action-potential characteristics, and firing properties.

Airway closure during mixed apneas in preterm infants: is respiratory effort necessary?

Airway closure during mixed apneas in preterm infants may be due to lack of tone in the upper airway followed by collapse and obstruction or diaphragmatic action inducing obstruction. We examine whether respiratory efforts are necessary for airway closure using a new method of detecting airway obstruction, based on the disappearance of an amplified cardiac pulse observed on the respiratory flow tracing. We analyzed 198 episodes of mixed apnea of various lengths (> or = 3 seconds) observed in 33 preterm infants (birth weight, 1.4 +/- 0.1 kg [mean +/- SEM]; study weight, 1.7 +/- 0.1 kg; gestational age, 29 +/- 1 weeks; post-natal age, 33 +/- 4 days). The great majority of these episodes (88%) had a central, followed by an obstructive, component. Infants were studied by using a nosepiece and a flow-through system. Respiratory efforts (abdominal and chest movements) were recorded. Of the apneas, 20 were < 5 seconds; 78, 5 to < 10 seconds; 45, 10 to < 15 seconds; 27, 15 to < 20 seconds; and 28, > or = 20 seconds. Of the 198 mixed apneas, 151 (76%) occurred in the absence of any respiratory effort; 43 (22%) showed a simultaneous cessation of the cardiac oscillation and respiratory effort; and 4 (2%) showed diaphragmatic activity appearing after cessation of the cardiac oscillation (airway occlusion). Respiratory efforts never preceded the cessation of the cardiac oscillation. The findings suggest that diaphragmatic action is not needed to occlude the airway in mixed apneas. The simultaneous cessation of cardiac oscillations (airway occlusion) and onset of respiratory efforts may indicate that such effort contributes to closure or is induced by the same stimulus that closes the airway. We speculate that the mechanism for airway closure in mixed apneas is most likely a lack of upper airway tone, which normally occurs with the cessation of a central drive to breathe.

Evidence of a critical period of airway instability during central apneas in preterm infants.

The timing and magnitude of airway narrowing in central apneas is unknown. We have developed a method of apnea classification that relies on the transmission of cardiac airflow oscillation to indicate airway patency. Using a theoretical model, we showed that the amplitude of the cardiac airflow oscillation is proportional to airway diameter for small lumens. While in the majority of central apneas the amplitude of the cardiac airflow oscillation remains nearly constant, in a subset of events the waveform decreases with time, suggesting airway narrowing. We hypothesized that this is not a random occurrence but reflects a critical period of airway instability during central apnea. To test this hypothesis we studied 41 preterm infants. Of 4,456 central apneas, 585 had a decrease in the amplitude of the cardiac oscillation. The amplitude of the cardiac airflow oscillation during an apnea was recorded to provide a dynamic measure of changes in airway diameter with time. To allow for comparisons between patients the amplitude of each cardiac airflow oscillation was expressed as a proportion of the maximum amplitude observed in each infant. We then compared the amplitude at multiple successive 0.5 s intervals with the amplitude of the cardiac airflow oscillation observed at the apnea outset using ANOVA. We found a significant decrease in cardiac airflow oscillation after only 1 s irrespective of the apnea duration (3 to 16 s). We conclude that airway narrowing during central apnea is not a random occurrence but appears shortly after the onset of the apnea. We speculate that the phenomenon is secondary to passive airway relaxation.

Prolonged apnea in the preterm infant is not a random event.

We tested the hypothesis that in preterm infants, prolonged apneas (apneas > or = 20 sec) are not random events but are preceded by frequent and progressively longer respiratory pauses associated with changes in ventilatory variables. We studied 36 preterm infants with apnea [birth weight 1190 +/- 60 g (mean +/- SEM), study weight 1300 +/- 60 g, gestational age 28 +/- 1 weeks, and postnatal age 23 +/- 2 days]. A nosepiece with a flow-through system was used to measure ventilation and alveolar gases. Throughout the monitoring period for each infant we established 10-min moving "window of observation" followed by a 1-min interval examined for the detection of a prolonged apnea. Within the 10-min window, three variables were defined: the number of apneic episodes, the maximum length of a single apneic episode, and the total duration of apneic time. During the following minute (eleventh) the presence or absence of a prolonged apnea was determined. Chi-square test for a linear-trend in the rate of prolonged apnea and multiple logistic regression analysis showed that the relative risk of a prolonged apnea increases significantly from preceding periods without apnea to preceding periods containing the potential predictors of prolonged apnea. The strongest predictor was total duration of apneic time in the previous 10 min. When the 1 min before prolonged apnea was compared with the 1 min of similar sleep state not having prolonged apnea, minute ventilation decreased, primarily due to a decrease in respiratory frequency. Oxygen saturation decreased and alveolar PCO2 did not change. These findings suggest that prolonged apnea is not a random event but is preceded by a disturbance of the respiratory control system characterized by (1) frequent apneas of progressive duration, (2) decrease in respiratory minute volume and frequency, and (3) decreased O2 saturation.

Preliminary characterization of a placental factor inhibiting breathing in fetal sheep.

Previous studies have revealed a placental extract that inhibits breathing in fetal sheep. In the present study of 29 chronically instrumented sheep at 132+/-1 days of gestation, infusion of the 1-10 kDa extract inhibited breathing in 76% of the experiments whereas Krebs' solution inhibited it in 24%. It retained this activity after 6 months of freezing, after lyophilization, and upon lowering the pH during purification from 8.0 to 4.0, but it inhibited breathing in only 35% when the pH was lowered to 2.0. A significant dose-dependent effect was observed from a 16-fold dilution to a 4-fold concentration. Treatment of the extract with proteinase K or boiling reduced the activity to 30% or 26% inhibition, respectively. The activity was not adsorbed to an ion-exchange column at pH 7.0 or 8.0, but it was at pH 9.0 and it eluted with increasing NaCl concentrations. On a polyacrylamide gel the activity was eluted at a K(av) of 0.66 (82% inhibition), corresponding to between 2.5 and 4.5 kDa. These findings suggest that a peptide produced by the placenta, with a molecular mass between 2.5 and 4.5 kDa, inhibits fetal breathing.

Use of tolazoline to counteract vasospasm in peripheral arterial catheters in neonates.

Arterial access for blood sampling and continuous blood pressure monitoring is the cornerstone of modern neonatal intensive care. Although umbilical arterial catheters have traditionally been utilized for arterial access, they are associated with potentially devastating complications. Consequently, there has been an increase in the use of peripheral arterial catheters. Unfortunately, these catheters have a limited useful lifespan secondary to vasospasm, intimal damage and/or thrombus formation. In this report, we describe the use of tolazoline (0.02 to 0.2 mg/kg/h) to counteract local arterial vasospasm in five critically ill neonates, where arterial access was vital for care but difficult to maintain.

Spinal cord infarct after arterial switch associated with an umbilical artery catheter.

Paraplegia after an open heart operation in a neonate is a rare complication. We report a case of a infant in whom paraplegia developed after a successful arterial switch operation for transposition of the great arteries. The infant was monitored and resuscitated in the preoperative period with umbilical arterial and venous catheter tips located in the midthoracic region. He likely suffered a clinically silent thromboembolic event predisposing him to a localized hemorrhagic infarction during the repair.

Use of a magnified cardiac airflow oscillation to classify neonatal apnea.

Currently the classification of neonatal apnea relies upon an inference of airway closure based upon the presence of breathing efforts against such an obstruction. In this study we evaluate a new method of classification which utilizes the presence or absence of cardiac airflow oscillation to detect airway closure. Specifically, this evaluation consisted of an examination of the transmission characteristics of an artificially produced airflow oscillation through discrete airway narrowing in a model system; a confirmation that voluntary upper airway occlusion in adult volunteers uniformly induces complete loss of the oscillation; and a comparison of the cardiac oscillation method with the traditional method of apnea classification in a cohort of 4,309 apneas in 32 infants. We determined that the amplitude of the oscillation is negatively correlated with resistance (r = 0.97) and positively with the radius (r = 0.98) of narrowing in a model system, and that voluntary airway obstruction in adult subjects uniformly results in loss of transmitted cardiac oscillations. Moreover, although there was similarity in the frequency distribution of central, obstructive, and mixed apneas in our infants, there were statistically significantly greater obstructive events detected by the cardiac oscillation method. In addition, the cardiac oscillation method had the additional advantage of providing information regarding the timing of airway obstruction during apnea.

Subarachnoid fluid collections: a cause of macrocrania in preterm infants.

We report the outcome of 12 very low birth weight infants with macrocrania caused by subarachnoid fluid collections. By the age of 15 to 18 months, head growth had stabilized along a curve above and parallel to the 95th percentile. No infant required neurosurgical intervention, nor was cerebral palsy or mental retardation diagnosed in any of the infants.

A case of aplasia cutis congenita in dizygotic twins.

We report a newborn infant with extensive aplasia cutis congenita of the flanks and thighs, associated with a co-twin fetus papyraceus. DNA restriction fragment polymorphism analysis of the twins proved dizygosity. The association of these two conditions in twins is reviewed.