Heterogeneity of cerebral vasoreactivity in preterm infants supported by mechanical ventilation

The reaction of cerebral blood flow to acute changes in arterial carbon dioxide pressure (PaCO2) and mean arterial blood pressure was determined in 57 preterm infants supported by mechanical ventilation (mean gestational age 30.1 weeks) during the first 48 hours of life. All infants had normal brain sonograms at the time of the investigation. In each infant, global cerebral blood flow was determined by xenon-133 clearance two to five times within a few hours at different levels of PaCO2. Changes in PaCO2 followed adjustments of the ventilator settings. Arterial oxygen pressure was intended to be kept constant, and mean arterial blood pressure fluctuated spontaneously between measurements. The data were analyzed by stepwise multiple regression, with changes in global cerebral blood flow, PaCO2, mean arterial blood pressure, and postnatal age or intracranial hemorrhage used as variables. In infants with persistently normal brain sonograms, the global cerebral blood flow-carbon dioxide reactivity was markedly lower during the first day of life (mean 11.2% to 11.8%/kPa PaCO2) compared with the second day of life (mean 32.6/kPa PaCO2), and pressure-flow autoregulation was preserved. Similarly, global cerebral blood flow-carbon dioxide reactivity and pressure-flow autoregulation were present in infants in whom mild intracranial hemorrhage developed after the study. In contrast, global cerebral blood flow reactivity to changes in PaCO2 and mean arterial blood pressure was absent in infants in whom ultrasonographic signs of severe intracranial hemorrhage subsequently developed. These infants also had about 20% lower global cerebral blood flow before hemorrhage, in comparison with infants whose sonograms were normal, a finding that suggests functional disturbances of cerebral blood flow regulation. Several perinatal factors were tested, but only birth after abruptio placentae was related to subsequent periventricular hemorrhage (p = 0.037).

Tetrahydrobiopterin and Parkinson's disease

Two patients with Parkinson's disease were treated with 1 g tetrahydrobiopterin (BH4) for 5 days. Clinical improvement was not observed. In the cerebrospinal fluid (CSF) a 4-8 fold increase in the concentration of homovanillic acid (HVA), and a 3-fold increase in the concentration of 5-hydroxyindole acetic acid (5-HIAA) was measured. However, the concentration of HVA reached, was only approximately half as high, as that of patients treated with madopar (DOPA + benserazid). In urine, the excretion of HVA increased 13-37 fold, when the patients were treated with madopar, whereas no increase in the HVA excretion was measured after the BH4 administration. Additionally, 2 patients with Parkinson's disease were treated with 1 g BH4 in combination with 15 g tyrosine for 3 days, and 1 parkinsonian patient was treated with 15 g tyrosine daily for 7 weeks. No increase in the CSF concentrations of HVA or 5-HIAA was observed. The results suggest, the BH4 in the dosage used, is not effective in the treatment of Parkinson's disease.

Ossification of the cricothyroid membrane following minitracheotomy

We describe a case in which a repeat minitracheotomy (MT) could not be performed 2 months after the first. The cricothyroid membrane seemed calcified and histological examination showed scar formation with dystrophic calcification and heterotopic bone formation. Scar formation following MT may have caused these alterations making repeat access to the airway through the membrane impossible.

Poor reversibility of EEG abnormality in hypotensive, preterm neonates

Twenty-four infants, 32 weeks of gestation or less, were studied with continuous recording of amplitude integrated electroencephalogram (aEEG) and repeated Doppler ultrasound determination of the mean blood flow velocity in the internal carotid artery (cMFV). The recording was started after the initiation of mechanical ventilation for respiratory distress. Of twelve infants receiving blood or albumin transfusion to expand the intravascular volume and in whom adequate data were available, both mean arterial blood pressure and cMFV increased in eight, and cMFV but not blood pressure in further two. In the present circumstances we consider the cMFV increases to represent true increases in cerebral blood flow. aEEG burst rate increased distinctly in five of the twelve infants during or immediately following transfusion, but did only approach the level of the four non-transfused infants after several hours, indicating a dysfunction of neural tissue not readily reversible by improved blood flow.

EEG depression and germinal layer haemorrhage in the newborn

Amplitude integrated EEG (aEEG) recordings from 32 mechanically ventilated infants, gestational age 32 weeks or less, were analysed. All recordings were started within 24 h of birth and continued for at least 50 h. Germinal layer haemorrhage (GLH) was diagnosed by repeated ultrasonography. In six infants neither GLH nor hypocalcaemia were diagnosed; aEEG in these infants rapidly became more active after birth: at 30 h of age continuous background activity was present for more than 20% of the time, and a seizure-like pattern was exceptional. In seven infants without GLH but with hypocalcaemia and other signs of metabolic derangement, continuous background activity appeared later and seizure-like activity was frequent. In the infants with GLH, depression of the background activity was apparent. This finding was particularly distinct in the presence of severe haemorrhages. Four infants developed GLH after 30 h of age. All these infants had depressed aEEG before the development of GLH, with less than 20% continuous activity at 30 h of age. In ten infants an analysis of the aEEG during the occurrence of GLH was possible. In six of these, cortical electrical activity decreased. Due to the limitation of GLH timing, it was not possible to decide whether this decrease closely preceded or followed GLH. We suggest that GLH primarily occurs in brains with a preceding metabolic and neurophysiologic abnormality, and that further functional deterioration is caused by the most severe haemorrhages.

Severe hypocarbia in preterm infants and neurodevelopmental deficit

We report significant neurological abnormality at 18 months of age in 3 of 7 very low birth weight infants (less than or equal to 1,500 g), who during mechanical ventilation inadevertently became severely hypocarbic (arterial carbondioxide tension less than 2.0 kPa (15 mmHg)) at some time during the first 24 h of life. Although the number is small the outcome was significantly worse than the outcome in two fairly similar groups of infants selected as controls (p = 0.026). The infants in one of the control groups were also mechanically ventilated but remained normocapnic. Germinal layer haemorrhage (GLH) was more frequent among these infants compared with the severely hypocarbic infants (p = 0.022). The infants in the other control group was not mechanically ventilated. In all the severely hypocarbic infants the Bayley mental developmental index uncorrected for prematurity was at or below the median for the total sample (p = 0.01). The results suggest that neonatal cerebral ischaemia, for instance due to hypocarbia, is of greater prognostic significance than GLH.

Impairment of cerebral blood flow autoregulation in the newborn lamb by hypoxia

Autoregulation of cerebral blood flow has been demonstrated in both fetal and newborn animal models under normoxic conditions. In the present experiments we have attempted to define the minimal hypoxic insult which impairs autoregulation in the newborn lamb and to assess the time to recovery. We measured cerebral blood flow by the intracarotid 133Xe method in fifteen 4- to 9-day-old lambs and tested autoregulation of cerebral blood flow by increasing blood pressure 20-30% with phenylephrine. Autoregulation was tested in the control state and at successive time intervals after an hypoxic stress (PaO2 of 30 mm Hg) of 10 or 20 min. We found that cerebral autoregulation was abolished after 20 min of hypoxia and recovered within 7 h. Since this model identifies the minimal hypoxic insult to abolish autoregulation it might be used to study means to protect autoregulation or to hasten its recovery after hypoxia.

Dietary problems of phenylketonuria: effect on CNS transmitters and their possible role in behaviour and neuropsychological function

Thirty years ago it was observed that the synthesis of serotonin, dopamine and norepinephrine was impaired in untreated phenylketonuria (PKU) as judged either by a decreased concentration in the blood or decreased excretion in the urine of these neurotransmitters, or of their metabolites, 5-hydroxyindoleacetic acid (5-HIAA) and homovanillic acid (HVA). Fifteen years later, when early treatment of PKU with a phenylalanine restricted diet was routinely introduced, an inverse relationship was found between phenylalanine levels and the urinary excretion of dopamine and serotonin. An inverse relationship between blood phenylalanine levels and cerebrospinal fluid (CSF) concentrations of HVA and 5-HIAA has repeatedly been reported during the past 10 years. Recently, the effect of the discontinuation of diet in PKU on the synthesis of dopamine, norepinephrine and serotonin has been examined, and the possible relationship between low levels of these neurotransmitters and impaired performance on neuropsychological tests has been evaluated. In some PKU patients the performance on neuropsychological tests of higher integrative function is impaired after discontinuation of diet, especially when blood phenylalanine values exceed 1200 mumol/L, and the patients often complain of lack of concentration and emotional instability. When these patients return to a 'relaxed' phenylalanine restricted, tyrosine enriched diet, the impaired neuropsychological and behavioural functions appear to be reversible. One mechanism may involve an impaired synthesis of dopamine and serotonin, as the improvement is accompanied by an increase in dopamine and serotonin excretion and a significant increase in CSF concentrations of HVA and 5-HIAA.(ABSTRACT TRUNCATED AT 250 WORDS)

Sleep-waking shifts and cerebral blood flow in stable preterm infants

Cerebral blood flow was estimated on 60 occasions in 15 well infants, 29-34 wk of gestational age, 5-17 days after birth, using 133-Xenon clearance after intravenous injection. The sleep state of the infants was determined by biparietal electroencephalography, clinical observation, and tracings of heart rate and respiration. Blood flow was 22% higher in the 11 estimations made during wakefulness, when compared to the 17 estimations made during quiet sleep. There was no difference between blood flow in active and quiet sleep. Also there was no difference between blood flow during periods of trace alternant and blood flow during periods of continuous electroencephalographic activity. It is suggested that flow-metabolism coupling is present in stable, preterm infants. The absence of an increase in cerebral blood flow during active sleep as compared with quiet sleep suggests that the neurophysiologic and neurometabolic mechanisms of rapid eye movement sleep are not yet fully developed in preterm infants.

The effect of hypocarbia on the cardiovascular system of puppies

Respiratory alkalosis (hypocapnia) is used to treat persistent pulmonary hypertension in newborns. With the exception of the brain, there are no data on the effects of hypocapnia on organ blood flow in the neonate. Therefore, we examined the effects of 2 h of hypocapnia on blood pressure, oxygen consumption, cardiac output and organ blood flows in puppies. In six animals (group I), reducing the PaCO2 to 21.8 +/- 1.5 mm Hg (pH 7.62 +/- 0.04) caused an immediate and sustained reduction in cerebral blood flow (40%) and in myocardial blood flow (25%). There were no significant changes in arterial blood pressure, total body oxygen consumption, cardiac output, right and left ventricular rate-pressure product, or blood flow to the gut, liver, muscle, and kidneys. In four control animals (group II)(PaCO2 39.8 +/- 3.0, pH 7.38 +/- 0.04), there were no changes in any of the measured variables (arterial blood pressure, total body oxygen consumption, cardiac output, or blood flow to any organ, including brain and heart) during 2 h of normocarbic ventilation. We did not determine whether the reductions in cerebral and myocardial blood flows were detrimental. We suspect that they were not because the animals did not develop metabolic acidosis and they had normal cardiac outputs, and ventricular rate-pressure products throughout the study.

Combined tetrahydrobiopterin-phenylalanine loading test in the detection of partially defective biopterin synthesis

Deficiency in the synthesis of biopterin causes neonatal hyperphenylalaninemia. We report a 10-year-old girl of normal appearance with a partial defect in biopterin synthesis, normal intelligence and normal serum phenylalanine levels (95 mumol/l) (1.6 mg/dl). During her 1st year of life serum phenylalanine levels were 250 mumol/l (4 mg/dl) and phenylalanine loading performed at 6 months and 1 year of age was not followed by an increase in serum tyrosine. At 9 years of age she had developed a severely abnormal EEG with focal spike activity but no observable clinical abnormalities. Determination of urinary pterins showed abnormal low levels of biopterin and high levels of neopterin. Phenylalanine loading combined with oral administration of tetrahydrobiopterin (BH4) was followed by a normal increase in serum tyrosine and a normal decrease in serum phenylalanine. Considering the importance of BH4 for the synthesis of dopamine, catecholamines, and serotonin we suggest that these cases should be followed carefully. If neurological symptoms appear, e.g., epilepsy, it may be worthwhile to consider treatment with BH4 and neurotransmitter precursors.

Perinatal brain ischaemia: impact at four years of age

A group of 15 children for whom cerebral blood flow (CBF) had been determined during the first few hours of life were examined between the ages of 3 years 9 months and four years. Clinical neurological and psychological (Stanford-Binet) tests were carried out and in three cases a computerised tomographic examination was made. Poor articulation, dysphasia, attention deficits and low IQ were found significantly more often among the five children with a documented episode of ischaemia (CBF less than 20 ml/100g/min) in the neonatal period. By summarizing the number of abnormal findings, the difference between the groups was found to be highly significant. It is concluded that ischaemia during the first hours of life is a critical determinant for the development of congitive disorders.

Arterial oxygenation determines autoregulation of cerebral blood flow in the fetal lamb

We examined autoregulation of cerebral blood flow (CBF) over the range of oxygen tension commonly observed in the chronic fetal lamb preparation. Seventeen animals were surgically prepared under general anesthesia for chronic in utero studies. Based on measured resting arterial PO2 and calculated % saturation 24-48 h after surgery, two groups were defined: a normoxic group of eight with saturation of 57% or higher (our laboratory normal for physiologically stable preparations) and an hypoxic group of nine with saturation less than 57%. Regional CBF was measured with radiolabeled 15-microns microspheres. Autoregulation of CBF was assessed by measuring the change in CBF when fetal mean arterial blood pressure (MABP) was acutely decreased and increased by withdrawal and reinfusion of fetal blood. In normoxic animals CBF was constant over an MABP range of 42-61 torr in all four areas of the brain examined: cerebral hemispheres, basal ganglia, cerebellum, and brain stem. In hypoxic animals CBF was pressure dependent in all areas over an MABP range of 46-73 torr, i.e., autoregulation was abolished. These studies demonstrate that the mechanism of autoregulation is functionally developed in the mature fetal lamb, but operationally dependent upon arterial oxygen concentration. Below a saturation of approximately 50-60% CBF varies directly with perfusion pressure.