The effect of hypercapnia and hypercapnia associated with central cooling on breathing in unanesthetized fetal lambs

A novel technique for prediction of preterm birth: fetal nasal flow Doppler

OBJECTIVES

Absence of fetal breathing movements (FBM) has been found to be a good predictor of preterm delivery in symptomatic patients. However, analysis of FBM patterns and Doppler measurement of them for preterm birth prediction have not been performed before. In this study, we aimed to investigate and analyze FBM patterns in symptomatic preterm labor patients by fetal ultrasonography and nasal Doppler.

METHODS

This was a multicenter, prospective cohort study. Singleton pregnant patients between 24 and 37 gestational weeks diagnosed with preterm labor were included in the study. Patients were evaluated in three groups: no FBM (Group 1), regular FBM (Group 2), irregular FBM (Group3).

RESULTS

Seventy-three patients were available for the final analysis after exclusion. Preterm delivery rate in 24 h in groups were 91.7, 32.7 and 100%, respectively. The absence of FBM (Group 1) was statistically significant for preterm delivery in for both 24 (91.7 vs. 42.6%, p=0.002) and 48 h (91.7 vs. 49.2%, p=0.006) when compared with fetal breathing positive Group 2 and 3. In fetal nasal Doppler analyses in Group 2, the inspiration/expiration number rate was significantly lower in the patients who delivered in 24 h (0.98±0.2 vs. 1.25±0.57, p=0.015). By using fetal nasal Doppler, combination of absence of FBM or irregular FBM or regular FBM with inspiration number/expiration number (I/E) <1.25 detects 94.6% of patients who will eventually deliver in the first 24 h after admission.

CONCLUSIONS

Examining FBM patterns and using nasal Doppler may help the clinician to differentiate those who will deliver preterm and may be an invaluable tool for managing preterm labor patients.

Control of breathing activity in the fetus and newborn

Breathing movements have been demonstrated in the fetuses of every mammalian species investigated and are a critical component of normal fetal development. The classic sheep preparations instrumented for chronic fetal monitoring determined that fetal breathing movements (FBMs) occur in aggregates interspersed with long periods of quiescence that are strongly associated with neurophysiological state. The fetal sheep model also provided data regarding the neurochemical modulation of behavioral state and FBMs under a variety of in utero conditions. Subsequently, in vitro rodent models have been developed to advance our understanding of cellular, synaptic, network, and more detailed neuropharmacological aspects of perinatal respiratory neural control. This includes the ontogeny of the inspiratory rhythm generating center, the preBötzinger complex (preBötC), and the anatomical and functional development of phrenic motoneurons (PMNs) and diaphragm during the perinatal period. A variety of newborn animal models and studies of human infants have provided insights into age-dependent changes in state-dependent respiratory control, responses to hypoxia/hypercapnia and respiratory pathologies.

The role of CO(2) and central chemoreception in the control of breathing in the fetus and the neonate

Central chemoreception is active early in development and likely drives fetal breathing movements, which are influenced by a combination of behavioral state and powerful inhibition. In the premature human infant and newborn rat ventilation increases in response to CO(2); in the rat the sensitivity of the response increases steadily after ∼P12. The premature human infant is more vulnerable to instability than the newborn rat and exhibits periodic breathing that is augmented by hypoxia and eliminated by breathing oxygen or CO(2) or the administration of respiratory stimulants. The sites of central chemoreception active in the fetus are not known, but may involve the parafacial respiratory group which may be a precursor to the adult RTN. The fetal and neonatal rat brainstem-spinal-cord preparations promise to provide important information about central chemoreception in the developing rodent and will increase our understanding of important clinical problems, including The Sudden Infant Death Syndrome, Congenital Central Hypoventilation Syndrome, and periodic breathing and apnea of prematurity.

The importance of ‘awareness’ for understanding fetal pain

Our understanding of when the fetus can experience pain has been largely shaped by neuroanatomy. However, completion of the cortical nociceptive connections just after mid-gestation is only one part of the story. In addition to critically reviewing evidence for whether the fetus is ever awake or aware, and thus able to truly experience pain, we examine the role of endogenous neuro-inhibitors, such as adenosine and pregnanolone, produced within the feto-placental unit that contribute to fetal sleep states, and thus mediate suppression of fetal awareness. The uncritical view that the nature of presumed fetal pain perception can be assessed by reference to the prematurely born infant is challenged. Rigorously controlled studies of invasive procedures and analgesia in the fetus are required to clarify the impact of fetal nociception on postnatal pain sensitivity and neural development, and the potential benefits or harm of using analgesia in this unique setting.

Maternal fever and neonatal depression: preliminary observations

The objectives of this study were to determine in term infants: (1) the importance of maternal fever (maternal temperature > 38 degrees C) as a risk factor for neonatal depression and (2) the clinical course of infants admitted to the Neonatal Intensive Care Unit (NICU) born to mothers with fever. For 2 years, 59 (0.24%) of 25,000 term infants had a 5-minute Apgar score < or = 5 and 22 (0.08%) infants were administered chest compressions with or without epinephrine as part of cardiopulmonary resuscitation (CPR) in the delivery room. The perinatal event most commonly associated with a 5-minute Apgar score < or = 5 was maternal fever in 19 infants (32%), with meconium + fetal heart rate (FTHR) abnormalities in 15 (25%), and FTHR abnormalities only in 13 (22%), additional associations (n = 13). By stepwise linear regression analysis, a 5-minute Apgar < or = 5 was related only to the initial infant temperature (p = 0.009, r = 0.33). Maternal fever noted in six infants (27%) was also commonly associated with CPR, as was the presence of meconium + FTHR abnormalities in seven (32%), and FTHR abnormalities only in four (18%). One hundred thirteen (7.5%) of the approximately 1,500 term infants born to mothers with maternal fever were admitted to the NICU. In addition to fever, the labor was complicated by meconium (in 16 infants), meconium + FTHR abnormalities (in 19 infants), and FTHR abnormalities only (in 11 infants). Resuscitative interventions in the delivery room included oxygen only in 43 infants, bag and mask ventilation in 38, continuous positive airway pressure in 10, intubation in 16, and CPR in six infants. Forty-nine infants (43%) had an initial temperature > 38 degrees C including 13 (11%) with an initial temperature > 39 degrees C. Twelve (10%) infants remained intubated on admission and five required ventilator support > 24 hours. One blood culture was positive although all mothers were pretreated with antibiotics. One infant developed hypoxic ischemic encephalopathy including seizures. Maternal fever is the perinatal event most frequently associated with a 5-minute Apgar score < or = 5 and a common association with the need for CPR. Clinicians attending the delivery of a mother with fever should anticipate the potential for neonatal depression; such awareness should facilitate appropriate preparation before delivery and potentially reduce the need for more intensive resuscitation.

Abolition of breathing rhythmicity in lambs by CO2 unloading in the first hours of life

The mechanisms responsible for the maintenance of regular breathing after initiation of breathing at birth are still poorly understood. This study was designed to test the hypothesis that removing the chemical CO2 drive would abolish breathing rhythmicity in lambs in the first hours of life. A technique of graded CO2 removal through a veno-venous extracorporeal circuit was used in five unanesthetized lambs aged from 4 to 12 hours. In all lambs, CO2 unloading invariably resulted in sustained central apnea, after a decrease in Paco, of 6.9 +/- 5.7 Torr. We were unable to find a significant relationship between the decrease in PaCO2 and PaO2 (range 35-275 Torr) at onset of apnea. During apnea, the passage from behavioral quiet sleep to arousal or to active sleep was marked by transient and weak breathing movements. We conclude that the CO2 drive, but not the behavioral states, is a major factor for maintaining breathing rhythmicity in lambs in the first hours of life.

Initiation and maintenance of continuous breathing at birth

Changes in arterial PCO2 (PaCO2) and body temperature normally occurring at the moment of birth may play a role in the initiation and maintenance of continuous breathing. To clarify these mechanisms, five chronically instrumented fetal lambs were connected to an extracorporeal membrane oxygenation (ECMO) system. ECMO was initiated in utero at a flow rate sufficient to support the fetus totally, the umbilical cord was occluded, and the fetuses were delivered into a warm isotonic saline bath. Breathing activity was present periodically before connection to the ECMO system and on ECMO during fetal normocapnia and normoxia. Near delivery there were no breathing movements, because all ewes were in labor. After delivering the fetuses into the warm saline bath, breathing movements remained episodic, being absent during high voltage electrocortical activity, whereas fetal PaCO2 remained constant. However, after 36-192 min, breathing activity became present continuously in all animals, at a time when fetal central temperature decreased. Once initiated, continuous breathing could be stopped by reducing the PaCO2. We conclude that maintenance of fetal PaCO2 and a slow decrease in central temperature after cord occlusion delays the establishment of continuous breathing, and that the level of PaCO2 is important in the maintenance of breathing activity during early postnatal life.