Methemoglobin levels in umbilical cord blood of women with intrauterine growth restriction and preeclampsia.

OBJECTIVE: To measure nitric oxide (NO) levels, as assessed by methemoglobin (MetHb), in the umbilical cord arterial blood of women with preeclampsia (PE), normotensive pregnancies with intrauterine growth restriction (IUGR, <10th percentile for birth weight) and normotensive pregnancies with appropriate for gestational age (AGA) infants. DESIGN/METHODS: Prospective analysis of MetHb levels in umbilical arterial blood of pregnancies with PE, normotensive IUGR and normotensive AGA infants using spectrophotometry. Exclusion criteria included cigarette smoke exposure, hemolytic disorders, chronic hypertension, any significant medical illness and medications affecting NO or MetHb levels. RESULTS: There were 42 women in the PE, 42 in the AGA and 37 in the IUGR group. Maternal age, mode of delivery, gravidity, parity and infant gender were similar in the three groups. MetHb levels were significantly lower in the IUGR group compared to the AGA group (p < 0.01). MetHb levels, while higher in the PE group compared to the AGA group, did not reach statistical significance. CONCLUSION: Our data suggests that NO may play a key role in the pathogenesis of IUGR. We also, for the first time, provide information on umbilical arterial MetHb levels in PE, normotensive IUGR and normotensive AGA pregnancies.

Bilateral cataracts associated with glucose-6-phosphate dehydrogenase deficiency.

Glucose-6-phosphate dehydrogenase (G6PD) has an essential role in the defense against cellular oxidative injury. In neonates, the most common manifestation of G6PD deficiency is jaundice and hemolysis due to factors causing oxidative stress. Less known are the ocular associations described with G6PD deficiency, including cataracts. Oxidative injury is involved in the pathogenesis of almost all forms of cataracts, causing the lens proteins to undergo modifications, denaturation and form insoluble aggregates resulting in cataracts. Although cataracts in adult males have been reported in several studies, there are few reports of cataracts in infants with G6PD deficiency. We describe a preterm male neonate with G6PD deficiency who developed bilateral cataracts following an episode of neonatal sepsis and severe hemolysis necessitating an exchange blood transfusion.

Effects of vagal denervation on cardiorespiratory and behavioral responses in the newborn lamb.

Recently, Wong et al. (Wong KA, Bano A, Rigaux A, Wang B, Bharadwaj B, Schurch S, Green F, Remmers JE, and Hasan SU, J Appl Physiol 85: 849-859, 1998) demonstrated that fetal lambs that have undergone vagal denervation prenatally do not establish adequate alveolar ventilation shortly after birth. In their study, however, vagal denervation was performed prenatally and the deleterious effects of vagal denervation on breathing patterns and gas exchange could have resulted from the prenatal actions of the neurotomy. To quantify the relative roles of pre- vs. postnatal vagal denervation on control of breathing, we studied 14 newborn lambs; 6 were sham operated, and 8 were vagally denervated below the origin of the recurrent laryngeal nerve. Postoperatively, all denervated animals became hypoxemic and seven of eight succumbed to respiratory failure. In vagally denervated lambs, expiratory time increased, whereas respiratory rate, minute ventilation, and lung compliance decreased compared with the sham-operated animals. In the early postoperative period, the frequency of augmented breaths was lower but gradually increased over time in the denervated vs. sham-operated group. The dynamic functional residual capacity was significantly higher than the passive functional residual capacity among the sham-operated group compared with the denervated group. No significant differences were observed in the prevalence of various sleep states and in the amount of total phospholipids or large- and small-aggregate surfactants between the two groups. We provide new evidence indicating that intrauterine actions of denervation are not required to explain the effects of vagal denervation on postnatal survival. Our data suggest that vagal input is critical in the maintenance of normal breathing patterns, end-expiratory lung volume, and gas exchange during the early neonatal period.

Prenatal cigarette smoke exposure selectively alters protein kinase C and nitric oxide synthase expression within the neonatal rat brainstem.

Maternal smoking is a major risk factor for sudden infant death syndrome. Protein kinase C (PKC) and neuronal nitric oxide synthase (NOS) activities within the dorsocaudal brainstem (DB) mediate critical components of respiratory drive and could be implicated in SIDS. Thus, exposure to smoking during fetal life could modify the expression of these kinases in the DB. Rats were exposed to cigarette smoke or room air (Sham) from day 2 to 22 of pregnancy. Immunoblots of DB lysates at 2 days postnatally revealed no differences in PKC-alpha, PKC-beta, and endothelial NOS expression. However, PKC-gamma, PKC-delta, and neuronal NOS immunoreactivities were reduced in the cigarette smoke group. We conclude that gestational smoking is associated with selective reductions in PKC and NOS isoforms within the DB, which could decrease respiratory drive and lead to enhanced hypoxic vulnerability in infants of smoking mothers.

Significance of vagal innervation in perinatal breathing and gas exchange.

The mechanisms responsible for the establishment of continuous breathing at birth remain unknown. Several studies have shown that postnatal vagal denervation produces deleterious effects on ventilation as well as breathing patterns during the newborn period. However, the validity of these studies was compromised by anesthesia, tracheostomy or possible secondary laryngeal obstruction. We have recently developed an unanesthetized lamb model in which vagal denervation was performed antenatally and below the recurrent laryngeal nerves thereby avoiding anesthesia, tracheostomy and laryngeal paralysis. The denervated animals developed life-threatening respiratory failure shortly after birth, implying that vagal innervation of the lungs plays an essential role in establishing adequate gas exchange in the first hours after birth. We have subsequently investigated various mechanisms of respiratory failure in denervated animals. Our results show that the surfactant system dysfunction and loss of vagally mediated volume feedback likely contributed to the respiratory failure observed in the intrathoracically denervated animal model.

Target cell contact suppresses neurite outgrowth from soma-soma paired Lymnaea neurons.

Neurite extension from developing and/or regenerating neurons is terminated on contact with their specific synaptic partner cells. However, a direct relationship between the effects of target cell contact on neurite outgrowth suppression and synapse formation has not yet been demonstrated. To determine whether physical/synaptic contacts affect neurite extension from cultured cells, we utilized soma-soma synapses between the identified Lymnaea neurons. A presynaptic cell (right pedal dorsal 1, RPeD1) was paired either with its postsynaptic partner cells (visceral dorsal 4, VD4, and Visceral dorsal 2, VD2) or with a non-target cell (visceral dorsal 1, VD1), and the interactions between their neurite outgrowth patterns and synapse formation were examined. Specifically, when cultured in brain conditioned medium (CM, contains growth-promoting factors), RPeD1, VD4, and VD2 exhibited robust neurite outgrowth within 12-24 h of their isolation. Synapses, similar to those seen in vivo, developed between the neurites of these cells. RPeD1 did not, however, synapse with its non-target cell VD1, despite extensive neuritic overlap between the cells. When placed in a soma-soma configuration (somata juxtaposed against each other), appropriate synapses developed between the somata of RPeD1 and VD4 (inhibitory) and between RPeD1 and VD2 (excitatory). Interestingly, pairing RPeD1 with either of its synaptic partner (VD4 or VD2) resulted in a complete suppression of neurite outgrowth from both pre- and postsynaptic neurons, even though the cells were cultured in CM. A single cell in the same dish, however, extended elaborate neurites. Similarly, a postsynaptic cell (VD4) contact suppressed the rate of neurite extension from a previously sprouted RPeD1. This suppression of the presynaptic growth cone motility was also target cell contact specific. The neurite suppression from soma-soma paired cells was transient, and neuronal sprouting began after a delay of 48-72 h. In contrast, when paired with VD1, both RPeD1 and this non-target cell exhibited robust neurite outgrowth. We demonstrate that this neurite suppression from soma-soma paired cells was target cell contact/synapse specific and Ca(2+) dependent. Specifically, soma-soma pairing in CM containing either lower external Ca(2+) concentration (50% of its control level) or Cd(2+) resulted in robust neurite outgrowth from both cells; however, the incidence of synapse formation between the paired cells was significantly reduced. Taken together, our data show that contact (physical and/or synaptic) between synaptic partners strongly influence neurite outgrowth patterns of both pre- and postsynaptic neurons in a time-dependent and cell-specific manner. Moreover, our data also suggest that neurite outgrowth and synapse formation are differentially regulated by external Ca(2+) concentration.

Sevoflurane induced suppression of inhibitory synaptic transmission between soma-soma paired Lymnaea neurons.

The cellular and synaptic mechanisms by which general anesthetics affect cell-cell communications in the nervous system remain poorly defined. In this study, we sought to determine how clinically relevant concentrations of sevoflurane affected inhibitory synaptic transmission between identified Lymnaea neurons in vitro. Inhibitory synapses were reconstructed in cell culture, between the somata of two functionally well-characterized neurons, right pedal dorsal 1 (RPeD1, the giant dopaminergic neuron) and visceral dorsal 4 (VD4). Clinically relevant concentrations of sevoflurane (1-4%) were tested for their effects on synaptic transmission and the intrinsic membrane properties of soma-soma paired cells. RPeD1- induced inhibitory postsynaptic potentials (IPSPs) in VD4 were completely and reversibly blocked by sevoflurane (4%). Sevoflurane also suppressed action potentials in both RPeD1 and VD4 cells. To determine whether the anesthetic-induced synaptic depression involved postsynaptic transmitter receptors, dopamine was pressure applied to VD4, either in the presence or absence of sevoflurane. Dopamine (10(-]5) M) activated a voltage-insensitive K(+) current in VD4. The same K(+) current was also altered by sevoflurane; however, the effects of two compounds were nonadditive. Because transmitter release from RPeD1 requires Ca(2+) influx through voltage-gated Ca(2+) channels, we next tested whether the anesthetic-induced synaptic depression involved these channels. Individually isolated RPeD1 somata were whole cell voltage clamped, and Ca(2+) currents were analyzed in control and various anesthetic conditions. Clinically relevant concentrations of sevoflurane did not significantly affect voltage-activated Ca(2+) channels in RPeD1. Taken together, this study provides the first direct evidence that sevoflurane-induced synaptic depression involves both pre- and postsynaptic ion channels.

Pulmonary vagal innervation is required to establish adequate alveolar ventilation in the newborn lamb.

To investigate the effects of bilateral intrathoracic vagotomy on the establishment of continuous breathing and effective gas exchange at birth, we studied 8 chronically instrumented, unanesthetized, sham-operated and 14 vagotomized newborn lambs after a spontaneous, unassisted vaginal delivery. Fetal lambs were instrumented in utero to record sleep states, diaphragmatic electromyogram, blood pressure, arterial pH, and blood-gas tensions. Six of eight sham-operated lambs established effective gas exchange within 10 min of birth, whereas 12 of 14 vagotomized animals developed respiratory acidosis and hypoxemia (P = 0.008). Breathing frequency in vagotomized newborns was significantly lower during the entire postnatal period compared with sham-operated newborns. Vagotomized subjects also remained hypothermic during the entire postnatal period (P < 0.05). Bronchoalveolar lavage indicated an increased minimum surface tension, whereas lung histology showed perivascular edema and partial atelectasis in the vagotomized group. We conclude that stimulation of breathing and effective gas exchange are critically dependent on intact vagal nerves during the transition from fetal to neonatal life.

Inhaled NO as a viable antiadhesive therapy for ischemia/reperfusion injury of distal microvascular beds.

Inhaled nitric oxide (NO) is being used more and more in intensive care units as a modality to improve the outcome of patients with pulmonary complications. Our objective was to demonstrate that inhaled NO could impact upon a distally inflamed microvasculature-improving perfusion, leukocyte adhesive interactions, and endothelial dysfunction. Using intravital microscopy to visualize ischemia/reperfusion of postcapillary venules, we were able to demonstrate that the reduction in perfusion, the dramatic increase in leukocyte rolling, adhesion, and emigration, and the endothelial dysfunction could all be significantly abrogated with 80 ppm, but not 20 ppm inhaled NO. Perfusing whole blood directly over an inert P-selectin and CD18 ligand substratum incorporated in a flow chamber recruited the same number of rolling and adhering leukocytes from NO-ventilated and non-NO-ventilated animals, suggesting that inhaled NO was not directly affecting leukocytes. To demonstrate that inhaled NO was actually reaching the peripheral microvasculature in vivo, we applied a NO synthase inhibitor locally to the feline mesentery and demonstrated that the vasoconstriction, as well as leukocyte recruitment, were essentially abolished by inhaled NO, suggesting that a NO-depleted peripheral microvasculature could be replenished with inhaled NO in vivo. Finally, inhaled NO at the same concentration that was effective in ischemia/reperfusion did not affect vascular alterations, leukocyte recruitment, and endothelial dysfunction associated with endotoxemia in the feline mesentery. In conclusion, our data for the first time demonstrate a role for inhaled NO as a therapeutic delivery system to the peripheral microvasculature, showing tremendous efficacy as an antiadhesive, antivasoconstrictive, and antipermeabilizing molecule in NO-depleted tissues, but not normal microvessels or vessels that have an abundance of NO (LPS-treated). The notion that blood borne molecules have NO carrying capacity is conceptually consistent with our observations.

De novo protein synthesis in isolated axons of identified neurons.

Neurons are highly polarized cells that contain a wealth of cytoplasmic and membrane proteins required for neurotransmission, synapse formation and various forms of neuronal plasticity. Typically, these proteins are differentially distributed over somatic, dendritic and axonal compartments. Until recently, it was believed that all proteins destined for various neuronal sites were synthesized exclusively in the somata and were subsequently targeted to appropriate extrasomal compartments. The discovery of various messenger RNA molecules in both dendrites and axons is suggestive of de novo protein synthesis in extrasomatic regions. The latter process has been demonstrated in few neuronal svrstems, but direct proof for the axonal transcription of a specific protein from a given messenger RNA is still lacking. This lack of fundamental knowledge in the field of cellular and molecular neurobiology is due primarily to both anatomical and experimental difficulties encountered in most animal preparations studied thus far. In this study we developed a neuronal experimental system comprising of individually identified neurons and their isolated axons from the mollusc Lymnaea stagnalis. We injected a foreign messenger RNA encoding a peptide precursor into the isolated axons of cultured neurons; and utilizing cellular, molecular and immunocytochemical techniques, we provide direct evidence for specific protein synthesis in isolated axons. The Lymnaea model provides us with an opportunity to examine the role and specificity of de novo protein synthesis in the extrasomal regions.

Umbilical arterial blood flow and plasma prostaglandin E2 concentrations during arousal and breathing movements in fetal sheep.

To investigate the effects of lung distension and oxygenation on umbilical blood flow (UBF) and plasma prostaglandin E2 (PGE2) in relation to arousal and stimulation of breathing movements, we studied eight chronically instrumented, unanesthetized fetal sheep between 137 and 143 d of gestation. Electrocorticogram, electro-oculogram, nuchal and diaphragmatic electromyograms, arterial pH and blood gas tensions, Hb oxygen saturation, body temperature, and UBF were recorded in each fetus. Electrocorticogram, electro-oculogram, and nuchal electromyograms were used to define sleep states. No sooner than 4 d after surgery, fetal lungs were distended with 100% O2 or N2 in a randomized order via an in situ Y-endotracheal tube. PGE2 concentrations were analyzed by RIA. A significant increase in fetal arousal and stimulation of breathing during nonrapid eye movement sleep was observed during lung distension with O2 as compared with control periods and lung distension with nitrogen. In all sleep states, UBF significantly decreased during oxygenation as compared with the control values. However, no significant correlation was observed between the time of the onset of arousal and the decrease in UBF. Lung distension with N2 resulted in increased plasma PGE2 concentrations, whereas, no change was observed during oxygenation. Our data suggest that an increase in fetal partial pressure of arterial O2 leads to a decrease in UBF. However, the onset of arousal and stimulation of breathing during lung distension and oxygenation were not dependent on a decrease in plasma PGE2 concentrations.

Hyperbaric oxygenation increases arousal and breathing movements in fetal lambs.

Oxygenation produced by distending the lungs with 100% O2 increases the occurrence of arousal and fetal breathing movements (FBM), particularly during non-rapid-eye-movement (NREM) sleep, in fetal sheep of > or = 135 days of gestation. We studied the breathing and behavioral responses to a rise in arterial PO2 (PaO2) without lung distension in fetuses between 128 and 132 days of gestation. Twelve fetuses were chronically instrumented to record FBM, behavioral state, blood pressure, arterial blood gas tensions, and pH. Fetal PaO2 was raised by having the ewe breathe 100% O2 at 3 atmosphere absolute pressure spontaneously (group 1, n = 5, 129 +/- 1 days of gestation) or with mechanical ventilation to control fetal arterial PCO2 (group 2, n = 7, 131 +/- 1 days of gestation). Hyperbaric oxygenation raised fetal PaO2 by 20 Torr in both groups. During hyperbaric oxygenation, the occurrence of arousal increased severalfold in both groups. The occurrence of FBM increased during arousal in both groups, during rapid-eye-movement sleep in group 1, and during NREM sleep in group 2. The timing of diaphragmatic activity during arousal and the variability of diaphragmatic activity during NREM sleep were different than those in rapid-eye-movement sleep. We conclude that oxygenation without lung distension increases the occurrence of arousal and of FBM, principally during arousal and NREM sleep, in fetuses of < or = 135 days of gestation.

Vagal nerve maturation in the fetal lamb: an ultrastructural and morphometric study.

The maturation of the left vagal nerve was studied in the fetal lamb by transmission electron microscopy and by computer-assisted morphometry of sections of the entire nerve at seven gestational ages between 79 and 145 days (term is 147 days) and in the adult ewe. The number of unmyelinated axons per Schwann cell progressively decreased from 25 to 55 at 79 days to 1 to 5 at near-term. Unmyelinated axons of various sizes were enclosed within a single Schwann cell at all ages, but the mean axonal diameter increased in inverse relation to the number of unmyelinated axons. A few Schwann cells enclosed two myelinated axons, but in most instances myelination did not begin until a 1:1 ratio was achieved; some single axons with a Schwann cell remained unmyelinated in the adult. Myelinated fibers were rare at 79 days but myelination progressed rapidly thereafter until the adult ratio of myelinated: unmyelinated fibers was reached at about 100 days; myelinated axons were not uniformly distributed. The myelin sheaths and axons of small fibers progressively increased in diameter in late gestation, but new large fibers were not added. Early myelinating fibers and immature unmyelinated axons contained more microtubules than neurofilaments; neurofilaments predominated in mature axons with or without myelin. Cross-linkages between neurofilaments were already evident by 79 days. Maturation of the vagal nerve thus occurs first by an increase in number of myelinated fibers and then by an increase in the size of each fiber in this fixed population. The bimodal distribution in the size histogram of myelinated fibers is not achieved until 134 days gestation and correlates well with physiological maturation of respiratory patterns.

Hyperoxemia profoundly alters breathing pattern and arouses the fetal sheep.

We have recently shown that hyperoxemia alone or combined with umbilical cord occlusion causes continuous breathing and arousal in the fetal sheep (Baier, Hasan, Cates, Hooper, Nowaczyk & Rigatto, 1990). We have not however analyzed the changes in the pattern of breathing associated with these events. To do this, we measured the changes in breathing pattern, electrocortical activity and behaviour on 29 occasions in 15 fetal sheep in late gestation. Fetuses were studied during rest, and during lung distention (about 30 cm H2O) with 100% nitrogen (control), 17% oxygen, 100% oxygen and umbilical cord occlusion. Lung distention was obtained using a high frequency oscillator (Senko Co) and in some fetuses a stroke volume of 0 to 20 cm H2O was used to keep PaCO2 near-constant. We found that lung distention with nitrogen or 17% oxygen did not alter the pattern of breathing or behaviour. In 12 out of 34 (35%) experiments 100% oxygen induced continuous breathing, PaO2 increasing to about 250 torr. In the remaining 22 experiments, PaO2 increased to about 100 torr only and breathing was not continuous but it became continuous upon cord occlusion; with occlusion there was a further increase in PaO2 to 190 torr. The increased breathing with oxygen and occlusion was associated with an increase in breathing output (integral of EMGdi x f), an increase in inspiratory drive (integral of EMGdi/Ti), and a decrease in inspiratory (Ti) and expiratory (Te) times. In ten experiments PaCO2 was kept near-constant and the magnitude of the changes remained.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of bilateral vagotomy on oxygenation, arousal, and breathing movements in fetal sheep.

To investigate the effects of bilateral cervical vagotomy on arousal and breathing responses, we studied eight sham-operated and eight chronically instrumented unanesthetized vagotomized sheep fetuses between 136 and 144 days of gestation (term approximately 147 days). Each fetus was instrumented to record sleep states, diaphragmatic electromyogram, blood pressure, pH, and blood gas tensions. In a randomized order, fetal lungs were distended with four different O2 concentrations: 0 (100% N2), 21, 50, and 100% at a continuous positive airway pressure of 30 cmH2O via an in situ Y-endotracheal tube. Under control conditions, inspiratory time and the duration of the single longest breathing episode decreased from 598 +/- 99 (SD) ms and 24 +/- 10 min in sham group to 393 +/- 162 ms and 11.0 +/- 3.0 min in vagotomized group (P = 0.04 and 0.033), respectively. In response to lung distension with 100% N2, breathing time decreased from 44 +/- 17 to 20 +/- 18% (P = 0.045) in sham-operated fetuses, whereas it remained unchanged in the vagotomized group. In response to 100% O2, fetal arterial PO2 increased in five of eight fetuses sham-operated from 18.2 +/- 5.1 to 227 +/- 45 Torr (P = 0.0001) and in six of eight vagotomized fetuses from 18.5 +/- 4.4 to 172 +/- 39 Torr (P < 0.001). Although arousal was observed in all oxygenated fetuses at the onset of breathing, the duration of arousal was markedly attenuated in vagotomized fetuses (14 +/- 10 vs. 46 +/- 29 min in sham group; P = 0.024). Frequency and amplitude of breathing and respiratory output (frequency x amplitude) increased only in sham group (P = 0.02, 0.004, and 0.0002, respectively). We conclude that in response to lung distension and oxygenation, arousal and stimulation of breathing during active and quite sleep are critically dependent on intact vagal nerves.

Arterial oxygen tension threshold range for the onset of arousal and breathing in fetal sheep.

Mechanisms for the control of episodic fetal breathing movements or the onset of continuous breathing at birth remain unknown. Lung distension with 100% O2 at a continuous positive airway pressure of 30 cm H2O may induce arousal and continuous breathing. To investigate 1) the threshold range of arterial oxygen tension (PaO2) for the onset of arousal and breathing and 2) the graded response of breathing to various levels of PaO2, we studied 10 fetal sheep between 135 and 142 d of gestation (term = 147 +/- 2 d). Each fetus was instrumented to record sleep states, diaphragmatic electromyogram, arterial pH, and blood gas tensions. PaO2 threshold was determined through an indwelling O2 sensor catheter. Fetal lungs were distended at a continuous positive airway pressure of 40 cm H2O with 100% N2 or with O2 ranging from 40 to 100% via an in situ endotracheal tube. At the onset of arousal (n = 10), PaO2, arterial carbon dioxide tension, and Hb O2 saturation increased from control values of 21.7 +/- 0.75 torr (2.9 +/- 0.09 kPa), 41.8 +/- 1.1 torr (5.47 +/- 0.15 kPa), and 52.9 +/- 2.6% to 65.6 +/- 9.6 torr (8.74 +/- 1.28 kPa), 46.9 +/- 1.3 torr (6.25 +/- 0.17 kPa), and 92.9 +/- 2.06%, respectively, whereas the pH decreased from 7.31 +/- 0.006 to 7.27 +/- 0.009 (mean +/- SEM; p = 0.001, 0.04, 0.002, and 0.001, respectively). Seven of 10 fetuses breathed continuously. In these fetuses, PaO2 and arterial carbon dioxide tension further increased and pH decreased; however, no further significant increase in Hb O2 saturation was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of lung distension, oxygenation, and gestational age on fetal behavior and breathing movements in sheep.

Lung distension with 100% O2 at a continuous positive airway pressure of 30 cm H2O may induce continuous fetal breathing movements (FBM) in sheep. The objectives of this study were 1) to investigate the relative roles of lung distension and oxygenation and 2) to test the hypothesis that FBM can be induced during labor, when normally they are greatly reduced or absent. We studied 13 chronically instrumented, unanesthetized fetal sheep between 128 and 144 d of gestation (term = 147 +/- 2 d). Each fetus was instrumented to record sleep states, diaphragm electromyogram, blood pressure, arterial pH, and blood gas tensions. The fetal lungs were distended via an in situ endotracheal tube with four different concentrations of O2 (0, 21, 50 and 100%) at a continuous positive airway pressure of 10, 20 and 30 cm H2O in a randomized order. No change in any recorded physiologic variable was observed at 129 +/- 1 or 132 +/- 1 d of gestation. At 135 +/- 1 and 138 +/- 1 d, in response to a continuous positive airway pressure of 30 cm H2O and 100% O2, pH decreased (p = 0.0004 and 0.005, respectively) and arterial O2 tension increased (p = 0.004 and 0.02, respectively). However, increases in 1) breathing time, 2) breathing time/low-voltage electrocortical activity ratio, 3) duration of arousal, and 4) length of single breathing epochs were observed only at 138 +/- 1 d. Lung distension with N2 resulted in a decrease in FBM. Six fetuses were studied during labor.(ABSTRACT TRUNCATED AT 250 WORDS)

The ventilatory response to endogenous CO2 in preterm infants.

The measurement of the ventilatory response to inhaled CO2 is unphysiologic because the CO2 that normally stimulates breathing is endogenous (tissue or venous CO2). We took advantage of the spontaneous changes in alveolar PCO2 and ventilation occurring in preterm infants during periodic breathing to calculate the ventilatory response to endogenous CO2. This response was obtained in 20 infants and compared with those obtained using the more conventional methods of steady-state inhalation of CO2 (12 infants) and rebreathing of CO2 (11 infants); it was also compared with a transient change in alveolar CO2 obtained by inhalation of 7% CO2 in air for 10 s (CO2 "bolus"; 11 infants). All groups of infants had similar birth weight and gestational ages. To calculate the response to endogenous CO2, delta PACO2 was measured as the difference between lowest and highest PaCO2 and delta VE was the difference between the corresponding instantaneous ventilation. To adjust for circulation time, values for PACO2 were made lowest for the last breath before apnea and highest for the first breath after apnea. The coefficient of variation of the method was 8%. The slope of the ventilatory response to endogenous CO2 was 0.067 +/- 0.009 (mean +/- SE) L.min-1.kg-1.mm Hg PACO2(-1), a value greater than that using steady-state and rebreathing methods (0.038 +/- 0.004 and 0.040 +/- 0.006 L.min-1.kg-1.mm Hg PACO2(-1), respectively), but similar to that of infants inhaling a CO2 "bolus" (0.051 +/- 0.009 L.min-1.kg-1.mm Hg PACO2(-1)).(ABSTRACT TRUNCATED AT 250 WORDS)