Failure of autoregulation of cerebral blood flow in neonates studied by pulsed Doppler ultrasound of the internal carotid artery

Surfactant and neonatal hemodynamics during the postnatal transition

Surfactant replacement therapy (SRT) has revolutionized the management of respiratory distress syndrome (RDS) in premature infants, leading to improved survival rates and decreased morbidity. SRT may, however, be associated with hemodynamic changes, which can have both positive and negative effects on the immature cardiovascular system, during the transitional adaptation from fetal to extrauterine environment. However, there is a relative paucity of evidence in this domain, with most of them derived from small heterogeneous observational studies providing conflicting results. In this review, we will discuss the hemodynamic changes that occur with surfactant administration during this vulnerable period, focusing on available evidence regarding changes in pulmonary and systemic blood flow, cerebral circulation and their clinical implications.

Cerebral autoregulation in infants during sevoflurane anesthesia for craniofacial surgery

BACKGROUND

Data on cerebral pressure-flow autoregulation in the youngest children are scarce. We studied the correlation between mean arterial pressure and cerebral tissue oxygen saturation (rSO₂ ) by near-infrared spectroscopy (NIRS) in patients undergoing nose, lip, and palate surgery.

AIM

We tested the hypothesis that cerebral pressure-flow autoregulation is impaired in children less than 1 year undergoing surgery and general anesthesia with sevoflurane under controlled mechanical ventilation.

METHOD

After approval from the Ethical board, 15 children aged <1 year were included. Before anesthesia induction, a NIRS sensor (INVOS^TM , Medtronic, Minneapolis, USA) was placed over the cerebral frontal lobe. Frontal rSO₂ , a surrogate for cerebral perfusion, mean arterial pressure, end-tidal CO₂ - and sevoflurane concentration, and arterial oxygen saturation were sampled every minute after the induction. A repeated measures correlation analysis was performed to study correlation between mean arterial pressure and cerebral rSO₂ , and the repeated measures correlation coefficient (r_rm ) was calculated.

RESULTS

Fifteen patients, aged 7.7 ± 1.9 months, were studied. rSO₂ showed a positive correlation with mean arterial pressure ([95% CI: 9.0-12.1], P < 0.001) with a moderate to large effect size (r_rm  = 0.462), indicating an impaired cerebral pressure-flow autoregulation. The slopes of the rSO₂ -mean arterial pressure correlations were steeper in patients who were hypotensive (mean arterial pressure <50 mm Hg) compared to patients having a mean arterial pressure ≥50 mm Hg, indicating that at lower mean arterial pressure, the cerebral pressure dependence of cerebral oxygenation is even more pronounced.

CONCLUSION

During sevoflurane anesthesia in the youngest pediatric patients, cerebral perfusion is pressure-dependent, suggesting that the efficiency of the cerebral blood flow autoregulation is limited.

Intracranial dynamics in pre-term infants and neonates: implications for physiotherapists

The control of cerebral circulation and intracranial dynamics differs markedly in the pre-term and full term neonate from that in the adult. Immaturity can combine with several clinical conditions and iatrogenic factors to predispose the neonate to cerebral lesions, which may subsequently increase morbidity. As physiotherapists play an increasingly important role in neonatology, it is important to appreciate the immaturity of the nervous system and to recognise the risk factors for such conditions as peri-intraventricular haemorrhage and periventricular leucomalacia. This paper describes intracranial dynamics in the pre-term and full term infant and illustrates how these factors may interact with clinical conditions to cause cerebral lesions. Studies which examine the effect of respiratory physiotherapy on intracranial dynamics are reviewed and suggestions made for further research.

Observational study of cerebral hemodynamics during dopamine treatment in hypotensive ELBW infants on the first day of life

OBJECTIVE

To evaluate cerebral hemodynamics during dopamine treatment in hypotensive (mean arterial blood pressure (MABP)<gestational age) extremely low birth weight (ELBW) infants.

STUDY DESIGN

Continuous monitoring of cerebral blood flow velocity (CBFv), MABP and PCO2 was performed in hypotensive ELBW infants on the first day of life, beginning with an ∼15-min baseline reading and continued during advancing dopamine infusion until MABP was optimized. Physiological variables and CBFv reactivity were compared before and after MABP was optimized.

RESULT

Fifteen hypotensive ELBW infants (625±174 g; 24 (23 to 24.8) weeks) were studied. Mean CBFv increased from 10.9±3.7 to 15.7±5.7 cm s(-1) (P=0.001) simultaneously as MABP increased from 22.3±2.8 to 35.2±9.7 mm Hg (P<0.001). Mean CBFv reactivity (95% confidence interval (CI)) was 3.9 (1.6 to 6.2) %mm Hg(-1). Nine infants died and/or developed severe intraventricular hemorrhage (IVH).

CONCLUSION

Mean CBFv increased pressure-passively as MABP was optimized by dopamine treatment in very sick hypotensive ELBW infants on the first day of life.

Effect of antihypotensive treatment on cerebral oxygenation of preterm infants without PDA

BACKGROUND

Preterm infants with hypotension (mean arterial blood pressure [MABP] < gestational age [GA]) are treated with volume expansion and/or dopamine to ensure adequate cerebral perfusion/oxygenation. We used near-infrared spectroscopy to analyze the effects of volume expansion and dopamine on cerebral oxygenation in hypotensive preterm infants without patent ductus arteriosus (PDA).

PATIENTS AND METHODS

Among 390 infants, 71 (GA < 32 weeks) were hypotensive and eligible for inclusion. Thirty-three infants received volume expansion only (NaCl 0.9%; 20 mL/kg), and 38 received additional dopamine (5 μg/kg per minute). Nine and 11 infants initially treated with dopamine subsequently needed 7.5 and 10 μg/kg per minute, respectively. Seventy-one infants without hypotension were individually matched to serve as controls. MABP, regional cerebral oxygen saturation (rSco(2)), fractional tissue oxygen extraction (cFTOE), and arterial saturation (Sao(2)) were monitored 15 minutes before and 30 and 60 minutes after volume or dopamine and at comparable postnatal ages in controls.

RESULTS

No changes in MABP, rSco(2), or cFTOE were found 30 minutes after volume expansion. MABP increased 60 minutes after 5 μg/kg per minute dopamine (median [range]: 28 [19-32] vs 33 [23-46] mm Hg; P < .001). There was a small increase and decrease, respectively, in rSco(2) (63 [43-84] vs 66 [46-87]%; P < .05) and cFTOE (0.33 [0.14-0.56] vs 0.31 [0.07-0.54]1/1; P < .05). However, no differences were found at any time point between controls and infants treated with volume or additional dopamine (5, 7.5, and 10 μg/kg per minute) for rSco(2) or cFTOE.

CONCLUSIONS

Volume expansion and additional dopamine do not cause any significant change in rSco(2) or cFTOE in hypotensive preterm infants without PDA. We speculate that very preterm infants with hypotension but without signs of a compromised cerebral oxygenation and systemic perfusion might not be in need of antihypotensive therapy.

Changes in cerebral oxygenation and hemodynamics during cranial ultrasound in preterm infants

OBJECTIVES

To evaluate whether application of a transducer on the anterior fontanelle during cranial ultrasound (US) examination effects cerebral hemodynamics and oxygenation in preterm infants. STUDY DESIGN*: During cranial US examination, changes in cerebral blood oxygenation (cHbD) and cerebral blood volume (CBV) were assessed using near infrared spectrophotometry (NIRS) in 76 infants (GA 30.7 (4.1)wk, BW 1423 (717)g) within two days after birth. Ten of these infants (GA 29.1 (1.6)wk, BW 1092 (455)g) were studied again at a postnatal age of one week. RESULTS*: We obtained stable and consistent NIRS registrations in 54 infants within the first two days after birth. Twenty-eight of these infants showed a decrease in cHbD (0.59 (0.54) micromol/100g) during the scanning procedure while CBV did not change. Twenty-four infants showed no changes in NIRS and 2 infants showed an atypical NIRS response during cranial US examination. At the postnatal age of one week, stable and consistent NIRS registrations were obtained in 7 infants. None of these infants showed changes in NIRS variables during cranial US examination.

CONCLUSIONS

Application of an US transducer on the anterior fontanelle causes changes in cerebral oxygenation and hemodynamics in a substantial number of preterm infants. ( *values are expressed as median (interquartile range)).

The effect of changes in tPCO2 on the fractional tissue oxygen extraction--as measured by near-infrared spectroscopy--in neonates during the first days of life

The cerebral fractional oxygen extraction (FOE) reflects the balance between cerebral oxygen delivery (OD) and consumption (VO(2)). PCO(2) affects the cerebral blood flow (CBF): hypocapnia decreases CBF and OD and increases FOE. We recently showed that the fractional tissue oxygen extraction (FTOE) reflects FOE and hypothesized that a decrease in tPCO(2) increases FTOE. In this study we looked at the effect of changes in tPCO(2) on FTOE. We analysed 23 measurements in 13 neonates with birth weight below 1500 g and need for intensive care. Exclusion criteria were congenital malformations or cerebral complications. The tissue oxygenation index (TOI), tPCO(2), mean arterial blood pressure (MABP), heart rate (HR) and peripheral oxygen saturation (SaO(2)) were continuously recorded for 4h during the first days of life and FTOE was calculated. Over the whole group we found a significant negative (r=-0.227) correlation between tPCO(2) and FTOE and a significant positive (r=0.258) correlation between tPCO(2) and TOI. After correction for MABP these correlations remained significant. Over the whole group we found a significant positive correlation between tPCO(2) and TOI and a significant negative correlation between tPCO(2) and FTOE, which remained significant after correction for MABP. This implies that tPCO(2) influences the cerebral oxygenation independently of MABP. We therefore believe that for the interpretation of cerebral oxygenation in mechanically ventilated neonates during the first days of life continuous measurements of tPCO(2) are needed. Moreover we suggest FTOE to become a continuous parameter in the clinical setting for the non-invasive measurement of the neonatal brain oxygenation.

Relationship between blood pressure, cerebral electrical activity, cerebral fractional oxygen extraction, and peripheral blood flow in very low birth weight newborn infants

There is uncertainty about the level of systemic blood pressure required to maintain adequate cerebral oxygen delivery and organ integrity. This prospective, observational study on 35 very low birth weight infants aimed to determine the mean blood pressure (MBP) below which cerebral electrical activity, peripheral blood flow (PBF), and cerebral fractional oxygen extraction (CFOE) are abnormal. Digital EEG, recorded every day on the first 4 d after birth, were analyzed a) by automatic spectral analysis, b) by manual measurement of interburst interval, and c) qualitatively. CFOE and PBF measurements were performed using near-infrared spectroscopy and venous occlusion. MBP was measured using arterial catheters. The median (range) of MBP recorded was 32 mm Hg (16-46). The EEG became abnormal at MBP levels below 23 mm Hg: a) the relative power of the delta (0.5-3.5 Hz) frequency band was decreased, b) interburst intervals were prolonged, and c) all four qualitatively abnormal EEG (low amplitude and prolonged interburst intervals) from four different patients were recorded below this MBP level. The only abnormally high CFOE was measured at MBP of 20 mm Hg. PBF decreased at MBP levels between 23 and 33 mm Hg. None of the infants in this study developed cystic periventricular leukomalacia. One infant (MBP, 22 mm Hg) developed ventricular dilatation after intraventricular hemorrhage. The EEG and CFOE remained normal at MBP levels above 23 mm Hg. It would appear that cerebral perfusion is probably maintained at MBP levels above 23 mm Hg.

Effects of open vs. closed system endotracheal suctioning on cerebral blood flow velocities in mechanically ventilated extremely low birth weight infants

BACKGROUND

Endotracheal (ET) suctioning causes cardiovascular side effects and may impair cerebral hemodynamics. Subjectively, these effects are worse if patients are disconnected from the ventilator (open system suctioning, OSS) than if they remain connected to the ventilator during suctioning (closed system suctioning, CSS). It is uncertain whether the response to ET suctioning is similar in conventionally (CV) and high frequency (HF) ventilated patients.

OBJECTIVES

To investigate if the mode of suctioning or of mechanical ventilation influences cerebral blood flow velocities (CBFVs) in extremely low birth weight (ELBW) infants.

METHODS

Transcranial Doppler sonography in the middle cerebral artery during OSS and CSS in CV and HF ventilated ELBW infants.

RESULTS

Forty-one measurements were performed in 19 infants within the first two weeks of life. Mean CBFVs decreased during suctioning from baseline 18.8 to 14.3 cm/s (-24%), increased thereafter to 24.7 cm/s (73%) and then returned to baseline (20.8 cm/s). Changes in CBFV were less pronounced in infants with higher baseline CBFVs. Heart rate decreased during ET suctioning and thereafter returned to baseline values. The alterations in CBFV and heart rate were both independent of the mode of ventilation or suctioning.

CONCLUSIONS

The mode of suctioning or ventilation does not influence CBFVs in ELBW infants.

The effects of hypercapnia on cerebral autoregulation in ventilated very low birth weight infants

Permissive hypercapnia, a strategy allowing high Pa(CO2), is widely used by neonatologists to minimize lung damage in ventilated very low birth weight (VLBW) infants. While hypercapnia increases cerebral blood flow (CBF), its effects on cerebral autoregulation of VLBW infants are unknown. Monitoring of mean CBF velocity (mCBFv), Pa(CO2), and mean arterial blood pressure (MABP) from 43 ventilated VLBW infants during the first week of life was performed during and after 117 tracheal suctioning procedures. Autoregulation status was determined during tracheal suctioning because it perturbs cerebral and systemic hemodynamics. The slope of the relationship between mCBFv and MABP was estimated when Pa(CO2) was fixed at 30, 35, 40, 45, 50, 55, and 60 mm Hg. A slope near or equal to 0 suggests intact autoregulation, i.e. CBF is not influenced by MABP. Increasing values >0 indicate progressively impaired autoregulation. Infants weighed 905 +/- 259 g and were 26.9 +/- 2.3 wk gestation. The autoregulatory slope increased as Pa(CO2)) increased from 30 to 60 mm Hg. While the slopes for Pa(CO2) values of 30 to 40 mm Hg were not statistically different from 0, slopes for Pa(CO2) > or = 45 mm Hg indicated a progressive loss of cerebral autoregulation. The autoregulatory slope increases with increasing Pa(CO2), suggesting the cerebral circulation becomes progressively pressure passive with hypercapnia. These data raise concerns regarding the use of permissive hypercapnia in ventilated VLBW infants during the first week of life, as impaired autoregulation during this period may be associated with increased vulnerability to brain injury.

Estimation of Coherence Between Blood Flow and Spontaneous EEG Activity in Neonates

Blood flow to the brain responds to changes in neuronal activity and, thus, metabolic demand. In earlier work, we observed correlation between cerebral blood flow and spontaneous electroencephalogram (EEG) activity in neonates. Using coherence, we now found that during Tracé Alternant EEG activity in quiet sleep of normal term neonates, this correlation is strongest at frequencies around 0.1 Hz, reaching statistical significance (p < 0.05) in six of the nine subjects studied (p < 0.07 in eight subjects). Due to noise, artifact, and spontaneous changes in the subjects' EEG patterns, the signals investigated included epochs of missing samples. We, therefore, developed a novel algorithm for the estimation of coherence in such data and applied a Monte Carlo (surrogate data) method for its statistical analysis. This process provides a test for the statistical significance of the maximum coherence within a selected frequency band. In addition to permitting further insight into the mechanisms of cerebral blood flow control, these algorithms are potentially of great benefit in a wide range of biomedical applications, where interrupted (gapped) recordings are often a problem.

The ratio of flow velocities in the middle cerebral and internal carotid arteries for the prediction of cerebral palsy in term neonates

OBJECTIVE

This study evaluated whether the ratio of the mean flow velocities in the middle cerebral artery (MCA) and the internal carotid artery (ICA) of neonates in the first days of life can be used to identify future neurodevelopmental disabilities.

METHODS

We observed 127 term neonates without congenital malformations, chromosomal aberrations, intracranial hemorrhage, or early onset sepsis. The mean cerebral blood flow velocities were measured in the right and left ICAs and in the right and left MCAs with a Doppler flowmeter once from day 1 to day 3. The Vm ratio was defined as the mean velocity in the right and left MCAs/mean velocity in the right and left ICAs. Neurologic examinations were performed at 12 months of age in the outpatient follow-up clinic to detect cerebral palsy (CP), and the subjects were divided into 4 groups according to the diagnosis of hypoxic-ischemic encephalopathy (HIE) and neurologic prognosis: HIE- and normal neurologic function, HIE- and CP, HIE+ and normal neurologic function, and HIE+ and CP.

RESULTS

The Vm ratio in infants with the HIE- diagnosis and CP was significantly lower than that in infants with the HIE- diagnosis without CP (P < .05). There was no significant difference between the Vm ratios in infants with the HIE+ diagnosis without CP and infants with the HIE+ diagnosis and CP.

CONCLUSIONS

The Vm ratio might be a useful index in estimating neurologic outcome at birth, especially in neonates without the diagnosis of HIE.

Identification of pressure passive cerebral perfusion and its mediators after infant cardiac surgery

Cerebrovascular pressure autoregulation (CPA) regulates cerebral blood flow (CBF) in relation to changes in mean arterial blood pressure (MAP). Identification of a pressure-passive cerebral perfusion and the potentially modifiable physiologic factors underlying it has been difficult to achieve in sick infants. We previously validated the near-infrared spectroscopy-derived hemoglobin difference (HbD) signal (cerebral oxyhemoglobin - deoxyhemoglobin) as a reliable measure of changes in CBF in animal models. We now sought to determine whether continuous measurements of DeltaHbD would correlate to middle cerebral artery flow velocity (CBFV), allow identification and quantification of pressure-passive state, and help to delineate potentially modifiable factors. We enrolled 43 infants (2 d to 7 mo old) who were undergoing open cardiac surgery and cardiopulmonary bypass. At 6 and 20 h after surgery, we measured changes in HbD, CBFV (by transcranial Doppler), and MAP at different end-tidal CO(2) levels. We assigned a pressure-passive index (PPI) to each study on the basis of the relative duration of significant coherence between DeltaMAP and DeltaHbD. We found a significant relationship between DeltaHbD and DeltaCBFV at both time points. At 6 h after surgery, we showed high concordance (coherence > 0.5; PPI > or = 41%) between DeltaMAP and DeltaHbD, consistent with disturbed CPA in 13% of infants. End-tidal CO(2) values > or = 40 mm Hg and higher MAP variability both were associated with increased odds (p < 0.001) of autoregulatory failure. This approach provides a means to identify and quantify disturbances of CPA. High CO(2) levels and fluctuating MAP are two important preventable factors associated with disturbed CPA.

Hypotensive extremely low birth weight infants have reduced cerebral blood flow

OBJECTIVES

Whether extremely low birth weight (ELBW) infants are at risk of cerebral hypoperfusion is uncertain because key issues concerning their cerebral blood flow (CBF) and mean arterial pressure (MAP) are unresolved: (1) whether CBF is pressure-passive or autoregulated; (2) the normal level of MAP; and (3) whether inotropic drugs used to increase MAP might inadvertently impair CBF. We addressed these issues in ELBW infants undergoing intensive care.

METHODS

CBF (measured by near-infrared spectroscopy) and MAP were measured in 17 infants aged 1.5 to 40.5 hours.

RESULTS

Five infants remained normotensive (MAP 37 +/- 2 mm Hg, [mean +/- SEM]); twelve became hypotensive (MAP 25 +/- 1 mm Hg) and were treated with dopamine (10-30 mug x kg(-1) per min). CBF of hypotensive infants (14 +/- 1 mL x 100 g(-1) per min) was lower than the CBF of normotensive infants (19 +/- mL x 100 g(-1) per min). After commencement of dopamine in hypotensive infants, MAP increased (29 +/- 1 mm Hg) and CBF also increased (18 +/- 1 mL x 100g(-1) per min). CBF was correlated with MAP in hypotensive infants before (R = 0.62) and during (R = 0.67) dopamine, but not in normotensive infants. A breakpoint was identified in the CBF versus MAP autoregulation curve of untreated infants at MAP = 29 mm Hg; no breakpoint was evident in dopamine-treated infants.

CONCLUSIONS

In ELBW infants (1) cerebral autoregulation is functional in normotensive but not hypotensive infants; (2) a breakpoint exists at approximately 30 mm Hg in the CBF-MAP autoregulation curve; and (3) dopamine improves both MAP and CBF.

Cerebral ischaemia during cardiac surgery in children detected by combined monitoring of BIS and near-infrared spectroscopy

BACKGROUND

Children frequently suffer transient cerebral ischaemia during cardiac surgery. We measured cerebral ischaemia in children during cardiac surgery by combining two methods of monitoring.

METHODS

We studied 65 children aged between 5 months and 17 yr having surgery to correct non-cyanotic heart disease using hypothermic cardiopulmonary bypass (CPB). During surgery, we measured the Bispectral Index (BIS) and regional cerebral haemoglobin oxygen saturation (SrO2) with near-infrared spectroscopy (NIRS). Cerebral ischaemia was diagnosed if both SrO2 and BIS decreased abruptly when acute hypotension occurred. In each patient, the relationship between SrO2 and arterial blood pressure (AP) was indicated by a plot of mean SrO2 against simultaneous mean AP.

RESULTS

We noted 72 episodes of cerebral ischaemia in 38 patients. Sixty-three ischaemic events were during CPB. Cerebral ischaemia was less frequent in older patients. Cerebral ischaemia was more common and more frequent in children under 4 yr old. Haematocrit during CPB was lower and SrO2 was more dependent on AP in children under 4 yr.

CONCLUSIONS

Children less than 4 yr of age are more likely to have cerebral ischaemia caused by hypotension during cardiac surgery. Ineffective cerebral autoregulation and haemodilution during CPB may be responsible.

Is hypotension a major risk factor for neurological morbidity at term age in very preterm infants?

OBJECTIVE

To investigate the influence of perinatal risk factors, especially hypotension, on neuromotor status at term in surviving preterm infants born before 32 weeks of gestation.

METHODS

This study is part of the Leiden Follow-Up Project on Prematurity: a prospective, regional study of 266 live born infants with a gestational age (GA) < 32 weeks born in 1996-1997. Twenty-eight infants died before term age. Two hundred and eleven infants were examined neurologically at term according to Prechtl. The findings were classified as normal (N), mildly abnormal (MA) or definitely abnormal (DA). Hypotension was defined as a mean arterial blood pressure (MABP) < 30 mm Hg on at least two occasions.

RESULTS

One hundred and six (50%) infants were classified as neurologically N, 92 (44%) infants were classified as MA and 13 (6%) infants as DA. Hypotension, bronchopulmonary dysplasia (BPD), flaring and cystic periventricular leucomalacia (PVL) were risk factors for neurological morbidity. Of the 68 infants with hypotension, 33 (49%) were classified as MA and 7 (10%) as DA. Of the 141 infants without hypotension, 58 (41%) were MA, and 5 (4%) were DA. The odds ratio of hypotension for neurological morbidity was 1.9 (95% CI 1.06-3.40), adjusted for gestational age, birth weight, small for gestational age (SGA) and gender, it was 1.96 (95% CI 1.02-3.77). The adjusted odds ratio of PVL was 18.6 (4.4-78.5), of flaring was 2.37 (1.18-4.74) and of BPD was 2.44 (1.08-5.5).

CONCLUSIONS

Apart from gestational age, periventricular leucomalacia, and bronchopulmonary dysplasia, hypotension in preterm infants is a major risk factor for neurological morbidity at term.

Brain Imaging Studies of the Anatomical and Functional Consequences of Preterm Birth for Human Brain Development

Premature birth can have devastating effects on brain development and long-term functional outcome. Rates of psychiatric illness and learning difficulties are high, and intelligence on average is lower than population means. Brain imaging studies of infants born prematurely have demonstrated reduced volumes of parietal and sensorimotor cortical gray matter regions. Studies of school-aged children have demonstrated reduced volumes of these same regions, as well as in temporal and premotor regions, in both gray and white matter. The degrees of these anatomical abnormalities have been shown to correlate with cognitive outcome and with the degree of fetal immaturity at birth. Functional imaging studies have shown that these anatomical abnormalities are associated with severe disturbances in the organization and use of neural systems subserving language, particularly for school-aged children who have low verbal IQs. Animal models suggest that hypoxia-ischemia may be responsible at least in part for some of the anatomical and functional abnormalities. Increasing evidence suggests that a host of mediators for hypoxic-ischemic insults likely contribute to the disturbances in brain development in preterm infants, including increased apoptosis, free-radical formation, glutamatergic excitotoxicity, and alterations in the expression of a large number of genes that regulate brain maturation, particularly those involved in the development of postsynaptic neurons and the stabilization of synapses. The collaboration of both basic neuroscientists and clinical researchers is needed to understand how normal brain development is derailed by preterm birth and to develop effective prevention and early interventions for these often devastating conditions.

CBF reactivity in hypotensive and normotensive preterm infants

Perinatal distress in the preterm neonate, and the consequent loss of cerebrovascular autoregulation, has been implicated in the pathogenesis of neonatal cerebral lesions. A component of this distress is thought to be hypotension. We examined the autoregulatory capacity of hypotensive and normotensive infants using the 133Xe technique to measure cerebral blood flow. Global CBF was measured during only normotension in 5 infants, and during both hypotension and normotension in 11 infants. All the infants were ventilated and blood pressure was measured using an intra-arterial catheter. Fourteen CBF measurements were made on the normotensive infants. Forty-seven CBF measurements were made on the hypotensive infants, 34 measurements during hypotension and 13 during normotension. The global CBF of the normotensive and hypotensive infants were 13.3 and 13.6 mL/100 g/min, respectively. The mean arterial blood pressure (MABP)-CBF reactivity (95% CI) of the normotensive and hypotensive infants were 1.9% (-0.8% to 4.7%)/mm Hg and 1.9% (0.8% to 3.0%)/mm Hg, respectively. The CO2-CBF reactivity (95%CI) of the normotensive and hypotensive infants was 11.1% (6.8% to 15.5%)/KPa deltaPaCO2 and 4.1% (-5.0% to 14.1%)/KPa deltaPaCO2. The implications of these calculated CBF reactivities is that normotensive infants may have intact autoregulation but with a diminished response to fluctuations in PaCO2. The hypotensive infants appear to have attenuated or absent autoregulation with little or no response in CBF to changes in PaCO2.

Racemic ketamine does not abolish cerebrovascular autoregulation in the pig

BACKGROUND

Little is known about the influence of racemic ketamine on autoregulation of cerebral blood flow (CBF), and available reports regarding its influence on cerebral hemodynamics are contradictory. This study was designed to evaluate cerebrovascular responses to changes in the mean arterial pressure (MAP) during ketamine anesthesia.

METHODS

In eight normoventilated pigs anesthesia was induced with propofol and maintained by i.v. infusion of ketamine (15.0 mg kg(-1) x h(-1)) during measurements. The intra-arterial xenon clearance technique was used to calculate CBF. Balloon-tipped catheters were introduced in the inferior caval vein and mid-aorta, and increases or decreases by up to 40% in mean arterial pressure (MAP) in random order were achieved by titrated inflation of these balloon catheters. Cerebral blood flow was determined at each MAP level. Regression coefficients of linear pressure-flow curves were calculated in all animals.

RESULTS

From the mean baseline level (101 mmHg) MAP was reduced by 20% and 40%, and increased by 26% and 43%. The maximal mean increase and decrease in MAP induced a 12% increase and a 15% decrease, respectively, of CBF from the mean baseline level (52.6 ml.100 g(-1) x min1). The 95% confidence interval (-0.02; 0.38) of the mean regression coefficient of individual pressure-flow curves does not include the regression coefficient (0.64) of a linear correlation between MAP and CBF including origo (correlation coefficient 0.99), which indicates complete lack of cerebrovascular autoregulation.

CONCLUSIONS

We conclude that autoregulation of CBF is not abolished during continuous ketamine infusion in normoventilated pigs and that previous divergent conclusions are unlikely to be associated with severe impairment of cerebrovascular autoregulation.

Dynamic cerebral autoregulation in healthy adolescents

BACKGROUND

: There is little information on the limits of cerebral autoregulation and the autoregulatory capacity in children. The aim of this study was to compare dynamic cerebral autoregulation between healthy adolescents and adults.

METHODS

: Seventeen healthy volunteers 12-17 years (n = 8) and 25-45 years (n = 9) were enrolled in this study. Bilateral mean middle cerebral artery flow velocities (Vmca; (cm/s)) were measured using transcranial Doppler ultrasonography (TCD). Mean arterial blood pressure (MAP) and end-tidal carbon dioxide were measured continuously during dynamic cerebral autoregulation studies. Blood pressure cuffs were placed around both thighs and inflated to 30 mmHg above the systolic blood pressure for 3 min and then rapidly deflated, resulting in transient systemic hypotension. The change of Vmca to change in MAP constitutes the autoregulatory response, and the speed of this response was quantified using computer model parameter estimation. The dynamic autoregulatory index (ARI) was averaged between the two sides.

RESULTS

: Adolescents had significantly lower ARI (3.9 +/- 2.1 vs. 5.3 +/- 0.8; P=0.05), and higher Vmca (75.2 +/- 15.2 vs. 57.6 +/- 15.0; P<0.001) than adults.

CONCLUSION

: The autoregulatory index is physiologically lower in normal adolescents 12-17 years of age than in adults.

Variations in prevalence of hypotension, hypertension, and vasopressor use in NICUs

OBJECTIVE

Very low birth weight infants are vulnerable to hypotension and its associated complications. Vasopressors are used to raise blood pressure (BP), but indications for use are uncertain. Our objectives were (1) to study variations in BP stability among NICUs, (2) to investigate inter-NICU differences in vasopressor use, and (3) to address the association between intraventricular hemorrhage (IVH) and abnormal BPs.

STUDY DESIGN

A total of 1288 infants with birth weight <1500 g were admitted to six NICUs in Massachusetts and Rhode Island over 21 months. The lowest and highest mean BPs were collected within the first 12 hours. Also recorded were the use of vasopressors within the first 24 hours and the occurrence of IVH. Logistic regressions were used to model outcomes, controlling for gestational age and illness severity using the Score for Neonatal Acute Physiology.

RESULTS

Two of the six NICUs had significantly higher percentages of infants with at least one hypotensive BP, with prevalences of 24% to 45%. Percentages of infants treated with vasopressors ranged from 4% to 39%. This range of vasopressor use could not be explained by inter-NICU differences in birth weight, illness severity, or rates of hypotension. We found a borderline association between severe IVH and hypotension (odds ratio 1.6, p=0.055), but not between severe IVH and hypertension.

CONCLUSION

Wide differences exist in the prevalence of hypotension, hypertension, and vasopressor use among NICUs. We also found an association between hypotension and IVH, but not between hypertension and IVH.

Grading of cerebral autoregulation in preterm and term neonates

The response of cerebral blood flow velocity to a single spontaneous transient rise in blood pressure was studied to grade the cerebral autoregulatory response of newborns. Blood pressure was measured continuously through an umbilical or peripheral arterial catheter; continuous flow velocity recordings were taken from the middle cerebral artery using continuous wave Doppler ultrasound. From a cohort of 62 healthy term and preterm neonates, 325 transients in mean arterial blood pressure and mean cerebral blood flow velocity were identified for analysis using a foot-seeking algorithm. An initial classification of active or impaired autoregulation was given to each transient using a self-clustering technique. The grading of the transients was studied by examining the slope of the return of the cerebral blood flow velocity to baseline. Negative slopes indicate a normal autoregulation; slopes of 0 or greater indicate an absence of autoregulation. This classification was in agreement with the self-clustering method (Cohen's kappa = 0.94, P<0.0001). The relationship between the autoregulatory response assessed by the grading method and gestational age, postnatal age, and PCO(2) was examined using linear regression analysis. A significant relationship with gestational age (P = 0.002) but not PCO(2) (P = 0.06) or postnatal age (P = 0.14) was evident.

Dynamic cerebral autoregulation in sick newborn infants

The sick newborn infant is vulnerable to brain injury and impaired cerebral autoregulation is thought to contribute to this. Coherent averaging is a method of measuring the dynamic cerebral autoregulatory response that is particularly suitable for neonates. We used this method in combination with a measure of the gradient of the cerebral blood flow velocity (CBFV) response following transient blood pressure (BP) peaks to study dynamic autoregulation in infants undergoing intensive care. Term and preterm infants at high risk of neurologic injury were compared with a control group of infants, also undergoing intensive care. Simultaneous video-EEG, CBFV (using transcranial Doppler), and arterial blood pressure measurements were obtained intermittently during a study period of at least 2 h. Cerebral autoregulatory response curves were constructed for high risk and control groups. Intact cerebral autoregulation produces a characteristic response consisting of a brief period when CBFV follows arterial blood pressure but quickly returns to baseline value. An impaired autoregulatory response shows CBFV mirroring the arterial blood pressure curve closely. Thirteen high-risk infants, who also had seizures (10 term and 3 preterm) and 12 control infants (6 term and 6 preterm) were studied. Autoregulation was absent in high-risk term and preterm infants. It was also absent in preterm control infants. Term, neurologically healthy infants undergoing intensive care have an intact autoregulatory response. The constant passive response seen in high-risk infants may reflect the severity of the underlying neurologic disease.

Cerebral blood flow velocity during neonatal seizures

AIM

To determine if cerebral blood flow velocity increases during all types of neonatal seizure, and whether the effect is due solely to an increase in blood pressure, transmitted to the cerebral circulation when autoregulation is impaired.

METHODS

Seizures were diagnosed in 11 high risk neonates using cotside 16 channel video-EEG polygraphy. EEG, cerebral blood flow velocity (CBFV) using transcranial Doppler ultrasound, and arterial blood pressure (ABP) measurements were made. At least two 5-10 minute epochs of simultaneous measurements were performed on each infant. These epochs were then reviewed to eliminate artefacts, and one minute data periods containing a clear seizure onset were created. Each period contained 20 seconds before the seizure. Data periods without seizures from the same infants were also analysed and compared with seizure periods.

RESULTS

Four infants had purely electrographic seizures-without clinical manifestations. Six infants had electroclinical seizures. One infant displayed both seizure types. A random effects linear regression analysis was used to determine the effect of seizures on CBFV and ABP. A significant increase was found in mean CBFV in those periods containing seizures. The mean percentage change in velocity for all infants was 15.6%. Three infants showed a significant increase in mean ABP after seizures but the overall increase in ABP for all infants was not significant.

CONCLUSION

Electroclinical and electrographic neonatal seizures produce an increase in CBFV. In some infants the increase is not associated with an increase in blood pressure. These preliminary results suggest that electrographic seizures are associated with disturbed cerebral metabolism. Treatment of neonatal seizures until electrographic seizure activity is abolished may improve outcome for these infants.

Cerebral blood flow is independent of mean arterial blood pressure in preterm infants undergoing intensive care

OBJECTIVE

Preterm infants are often presumed to have a pressure passive cerebral circulation implying that a low mean arterial blood pressure (MABP) results in reduced cerebral perfusion. The aim of this study was to determine whether cerebral blood flow (CBF) was compromised in preterm infants whose MABP fell below 30 mm Hg (4 kPa).

METHODS

Thirty preterm infants undergoing intensive care were studied within the first 24 hours of life. CBF was measured using near infrared spectroscopy. The infants were analyzed in two groups on the basis of their MABP at the time of study: group 1 had a MABP below 30 mm Hg and group 2 more than 30 mm Hg. CBF in the two groups was compared.

RESULTS

There was no significant difference in the mean CBF between the two groups. In group 1 the median MABP was 27.2 mm Hg (range, 23.7-29.9 mm Hg) and CBF was 13.9 (standard deviation, +/-6.9) mL . 100 g-1 . min-1. In group 2 the median MABP was 35.3 mm Hg (range, 30.1-39.3 mm Hg) and CBF was 12.3 (standard deviation, +/-6.4) mL . 100 g-1 . min-1. Mortality and incidence of cranial ultrasound scan abnormalities were also not significantly different.

CONCLUSION

These results indicate that preterm infants undergoing intensive care are able to maintain adequate cerebral perfusion at a MABP in the range of 23.7 to 39.3 mm Hg.

Assessment of cerebral pressure autoregulation in humans--a review of measurement methods

Assessment of cerebral autoregulation is an important adjunct to measurement of cerebral blood flow for diagnosis, monitoring or prognosis of cerebrovascular disease. The most common approach tests the effects of changes in mean arterial blood pressure on cerebral blood flow, known as pressure autoregulation. A 'gold standard' for this purpose is not available and the literature shows considerable disparity of methods and criteria. This is understandable because cerebral autoregulation is more a concept rather than a physically measurable entity. Static methods utilize steady-state values to test for changes in cerebral blood flow (or velocity) when mean arterial pressure is changed significantly. This is usually achieved with the use of drugs, shifts in blood volume or by observing spontaneous changes. The long time interval between measurements is a particular concern in many of the studies reviewed. Parallel changes in other critical variables, such as pCO2, haematocrit, brain activation and sympathetic tone, are rarely controlled for. Proposed indices of static autoregulation are based on changes in cerebrovascular resistance, on parameters of the linear regression of flow/velocity versus pressure changes, or only on the absolute changes in flow. The limitations of studies which assess patient groups rather than individual cases are highlighted. Newer methods of dynamic assessment are based on transient changes in cerebral blood flow (or velocity) induced by the deflation of thigh cuffs, Valsalva manoeuvres, tilting and induced or spontaneous oscillations in mean arterial blood pressure. Dynamic testing overcomes several limitations of static methods but it is not clear whether the two approaches are interchangeable. Classification of autoregulation performance using dynamic methods has been based on mathematical modelling, coherent averaging, transfer function analysis, crosscorrelation function or impulse response analysis. More research on reproducibility and inter-method comparisons is urgently needed, particularly involving the assessment of pressure autoregulation in individuals rather than patient groups.

Frequency-domain analysis of cerebral autoregulation from spontaneous fluctuations in arterial blood pressure

The dynamic relationship between spontaneous fluctuations of arterial blood pressure (ABP) and corresponding changes in cerebral blood flow velocity (CBFV) is studied in a population of 83 neonates. Static and dynamic methods are used to identify two subgroups showing either normal (group A, n = 23) or impaired (group B, n = 21) cerebral autoregulation. An FFT algorithm is used to estimate the coherence and transfer function between CBFV and ABP. The significance of the linear dependence between these two variables is demonstrated by mean values of squared coherence > 0.50 for both groups in the frequency range 0.02-0.50 Hz. However, group A has significantly smaller coherences than group B in the frequency ranges 0.02-0.10 Hz and 0.33-0.49 Hz. The phase response of group A is also significantly more positive than that of group B, with slopes of 9.3 +/- 1.05, and 1.80 +/- 1.2 rad Hz-1, respectively. The amplitude frequency response is also significantly smaller for group A in relation to group B for the frequency range 0.25-0.43 Hz. These results suggest that transfer function analysis may be able to identify different components of cerebral autoregulation and also provide a deeper understanding of recent findings by other investigators.

Hemodynamic effects of intravenous morphine infusion in ventilated preterm babies

BACKGROUND

the importance of sedation and analgesia of newborn babies in intensive care is only now receiving recognition in many neonatal units.

OBJECTIVE

to evaluate the hemodynamic effects of morphine on Cerebral Blood Flow velocities (CBFv), Cardiac Output (CO), Stroke Volume (SV), Mean Arterial Blood Pressure (MABP) and Heart Rate (HR) in ventilated preterm infants, before and during the infusion of a loading dose.

DESIGN

prospective, open, non-randomized, before-after intervention study with hemodynamic measurements made by Doppler ultrasound.

SETTING

neonatal Intensive Care Unit, Tertiary Care Center.

PATIENTS

sequential sample of 30 ventilated preterm newborns (gestational age (GA) 29 +/- 2 wks, range 27-31, birth weight (BW) 1240 +/- 440 g, range 800-1680).

INTERVENTION

each subject received an intravenous loading dose of morphine (100 mcg/Kg/h) for 2 h, followed by a continuous infusion of 25 mcg/kg/h.

MEASUREMENTS

the following Doppler parameters of the anterior cerebral artery were estimated: Peak systolic flow velocity (Vs), end-diastolic flow velocity (Vd), mean flow velocity (Vm) and Pourcelot' Resistance Index (RI). Measurements of CBFv, CO and SV (by Doppler ultrasound), MABP and HR were made 30 min before (baseline values) and at 15 (M15), 30 (M30), 60 (M60) and 120 min (M120), during the morphine loading infusion. Statistical evaluation analysis of variance, significance was calculated by Student-Newman-Kenfeld test.

RESULTS

there were no statistically significant changes in the cerebral and cardiac Doppler parameters before or during the 120 min of morphine loading infusion. There was a non-significant fall in MABP (MABP: Baseline value = 44 +/- 6 mmHg, M120 = 42 +/- 4 mmHg; reduction = 4%) and HR (HR = Baseline value = 148 +/- 12 beats/min., M120 = 140 +/- 16 beats/min.; reduction = 5%).

CONCLUSIONS

a loading dose of morphine over 2 h did not have any significant effect on MABP or cerebral and cardiac hemodynamics. No adverse effects were noted that could be attributed to morphine therapy.

Analysis of cerebral blood flow autoregulation in neonates

The dynamic response of cerebral autoregulation to spontaneous changes in arterial blood pressure (ABP) is described by the relationship between cerebral blood flow velocity (CBFV) and resistance-area product (RAP). CBFV was measured with Doppler ultrasound in the middle cerebral artery and ABP with an intra-arterial catheter in 66 neonates. Spontaneous changes in mean ABP were automatically detected and the maximum derivative was used to synchronize the coherent averaging of corresponding CBFV and RAP transients. These were classified into two groups corresponding to intact (group A) or impaired (group B) autoregulation. The cross correlation between RAP and CBFV indicates a significant relationship with a time delay of 5 s for group A. The frequency response of RAP was estimated by the cross spectra with CBFV. Groups A and B present a similar amplitude spectra but the phase spectra of group A lags that of group B. The impulse responses of the two groups are also markedly different and were used to simulate the velocity response to a 5% step change in ABP. Impulse responses were also obtained for four different levels of pCO2 showing that hypercapnia leads to an impulse response similar to that of group B (impaired autoregulation). This method can be used to extend the usual dichotomic classification adopted in clinical studies of autoregulation.

Haemodynamic effects of differing blood transfusion rates in infants less than 1500 g

OBJECTIVE

To investigate whether the haemodynamic effects of the standard 2-3 h blood transfusion increases the risk for intraventricular haemorrhage (IVH) and patent ductus arteriosus (PDA) in very low birthweight infants.

METHODOLOGY

In a randomized controlled study, haemodynamic changes using slow and rapid transfusion were compared. Twenty-seven very low birthweight infants were divided between 12h (n = 14) and 3h (n = 13) transfusion groups. Blood pressure, ejection fraction (EF), anterior cerebral artery pulsatility index (PI), blood gases, serum electrolytes and haematocrit were measured pre- and post-transfusion. Infectious status was also monitored.

RESULTS

Blood pressure (48.1/25.5 vs 55.7/30.2 mmHg) and EF (0.68 vs 0.73) increased significantly during rapid transfusion (P<0.01) but remained stable with slow transfusion. Serum potassium, base excess and incidence of infection did not increase in either group.

CONCLUSIONS

Slow transfusion causes less haemodynamic disturbance than rapid transfusion, thereby preventing the potential risk for IVH and PDA.

Effects of fentanyl administration on general and cerebral haemodynamics in sick newborn infants

Despite the wide use of fentanyl for analgesia in newborns, concerns have been raised about potential haemodynamic side-effects. Since sick newborns may lose their cerebral blood flow autoregulation, a drug-induced haemodynamic instability could lead to brain injury. We assessed the effects of a 15-min infusion of fentanyl (3 micrograms/kg) on the general and cerebral haemodynamics in 15 newborns (median gestational age 29 weeks, 25th-75th percentile, range 28-31 weeks; birthweight 1170 g, range 955-1790 g). The heart rate and mean arterial blood pressure were continuously recorded. Mean cerebral blood flow velocity and pulsatility index were measured using pulsed Doppler ultrasound before, during and up to 60 min after the onset fentanyl administration. No significant modification of general or cerebral haemodynamics was observed. In conclusion, the infusion of 3 micrograms/kg of fentanyl did not lead to any deleterious effect on the general or cerebral haemodynamics in sick normovolaemic newborns.

Autoregulation of cochlear blood flow in young and aged mice

Autoregulation is the capacity of an organ system to maintain organ blood flow constant in response to changes in arterial blood pressure (BP). The current study was carried out to investigate the effect of age on autoregulation of cochlear blood flow (CBF) in mice. CBF was measured using a laser-Doppler flowmeter while BP was increased by angiotensin II injections and decreased by exsanguination in 2-month-old, 10-month-old and 18-month-old CBA mice. Autoregulation of CBF was significantly weaker in the 2-month-old mice when compared to the older mice. Although CBF autoregulation was weaker in the 18-month-old mice compared to the 10-month-old mice, this difference was not statistically significant. These results suggest that autoregulation changes with maturation and age. Findings are discussed in relationship to the possible development of presbycusis.

Vagal response to feeding tube insertion in preterm infants: has the key been found?

We prospectively studied the vagal response to feeding tube insertion in eight healthy preterm infants, on three occasions in each infant during the first three weeks of life. Heart rate, oxygen saturation, respiration and cerebral blood flow velocities were assessed before, during and immediately after insertion of an orogastric feeding tube. The whole procedure was recorded on video. The duration and quality of tube insertion and the behaviour of the infant were evaluated from the recordings. Twenty-one measurements in eight infants were evaluated. The heart-rate decrease observed immediately after tube insertion correlated significantly with the duration of tube insertion, the quicker the manipulation, the greater the heart-rate decrease (P = 0.000). The maximal decrease of oxygen saturation after tube insertion correlated with the degree of heart rate deceleration (P = 0.009). Significant alterations of the flow velocities were observed only when the heart rate fell below 80 beats/min. We speculate that such episodes of bradycardia can be avoided by carefully inserting the feeding tube over a period of at least 15 s.

Cerebral autoregulation dynamics in premature newborns

BACKGROUND AND PURPOSE

Autoregulation of cerebral blood flow is easily disrupted, and loss of this normal physiological reflex may worsen the neurological outcome for patients undergoing intensive care. We studied the response of cerebral blood flow velocity to changes in mean arterial blood pressure.

METHODS

Cerebral blood flow velocity was measured with Doppler ultrasonography in one middle cerebral artery for 5-minute periods in 33 babies of gestational age < 33 weeks admitted to a neonatal intensive care unit. Two methods of evaluating autoregulation were developed. The first used linear regression analysis of blood flow velocity on blood pressure. Records were classified as showing loss of autoregulation if the regression slope was greater than a critical value. A minimum change in mean arterial blood pressure of 5 mm Hg and a critical slope of 1.5%/mm Hg were found to be adequate criteria for the classification of records by the regression method. The second method used coherent averaging, a technique similar to that used in recording evoked potentials. Spontaneous transient increases in blood pressure were automatically detected, and the instant corresponding to its maximum rate of rise was used to synchronize averages of the blood pressure and blood velocity transients. The resulting coherent averages were classified into two groups based on the morphology of the cerebral blood flow velocity average.

RESULTS

Whereas the regression method allowed the classification of only 51 of 106 records, the coherent average method classified 101 of 106 (95.3%) of the records available. For 51 records that were classified by both methods, there was agreement in 42 cases (82.3%). The coherent average of all records classified as having an active autoregulation showed cerebral blood flow velocity returning to baseline much earlier than blood pressure, suggesting that autoregulation was taking place within 1 to 2 seconds. This pattern was absent in records in which autoregulation was classified as absent.

CONCLUSIONS

Computerized coherent averaging of the cerebral blood flow velocity response to spontaneous blood pressure transients offers a promising new method for noninvasive bedside assessment of autoregulation in patients undergoing intensive care. The time course for autoregulation, when present, is in agreement with that reported in adults.

Neonatal cerebral blood flow velocity responses to changes in posture

Maintaining a constant cerebral blood flow during a change in cerebral perfusion pressure is known as autoregulation. The integrity of this phenomenon is considered to be important in preventing cerebral lesions in preterm infants. A study was carried out using Doppler ultrasound measurements of cerebral blood flow velocities (CBFV) as an indicator of alterations in cerebral haemodynamics. CBFV were recorded on a beat to beat basis over 60 second epochs, during which time the cerebral perfusion pressure was changed by rapidly altering the infants' posture from horizontal to either 20 degrees head up or head down. An informative response in CBFV was considered to be either (a) a uniphasic, immediate, passive alteration in velocity occurring with the change in posture and without a subsequent change or (b) a biphasic response of an initial change in CBFV followed within 20 seconds by a second response. This latter response is considered to be consistent with autoregulatory activity. A total of 501 epochs in 60 neonates of gestational age 24-41 weeks was analysed. It was shown that any one infant can make either response, but the reliability of making an active, biphasic response increases with increasing gestational age.

Adaptive multipurpose probe fixation device for use on newborns

Detailed information on the construction and assembly of a noncommercial hood-like multipurpose probe fixation device is given. A vacuum technique is used to individually mold the device to the skull of newborn infants and to attach sensors firmly with minimum pressure. By design, in the range of 22 to 38 cm head circumference for transfontanellar Doppler monitoring, no more than four different sizes of the hood are required to match individual positions of the anterior fontanelle. A ball-and-socket joint facility is integrated. The flexibility of the design allows for an easy adaptation of the device to different probe requirements. This provides the opportunity to investigate cerebral circulation by using various methods simultaneously.

Internal carotid arterial blood flow velocity in infants

The differences in the velocity and pulsatility indexes in the internal carotid artery were evaluated in 62 normal controls, 42 infants with cerebral palsy, and 22 infants with mental retardation, all within the first year of life. In the normal controls, the average maximal blood flow velocity (A/L), and the maximal end-diastolic flow velocity increased during the first year of life. Pulsatility index decreased significantly between the ages of newborn to 2 mos and 3-5 mos, and remained constant thereafter. Compared with normal controls, the average maximal blood flow velocity and the maximal end-diastolic flow velocity values were significantly reduced in infants with cerebral palsy during the first 6 months of life, while no differences in these values were observed in infants with mental retardation. There were no differences in the pulsatility index values in the 3 subject groups throughout the first year of life. Flow velocity in the internal carotid artery could reflect the status of the cerebral circulation in infants within the first year of life.

Control of cerebral circulation in the high-risk neonate

A knowledge of neonatal cerebrovascular physiology is essential to the understanding of diseases that frequently affect the subsequent development of the newborn brain. Recent observations indicate that the cerebral vessels of the healthy newborn infant, even the very preterm, respond to physiological stimuli in the same manner as in the mature organism. Thus, cerebral blood flow changes with changes in arterial carbon dioxide tension (PaCO2), oxygen concentration (CaO2), or glucose concentration, whereas cerebral blood flow remains constant at minor fluctuations in arterial blood pressure. In pathological states, pressure autoregulation may become impaired, and in severe cases the vessels do not react to chemical or metabolic stimuli. These infants are at high risk for developing cerebral lesion, and they may be candidates for new "brain-protecting regimens."

Cerebrovascular regulation and neonatal brain injury

Neuropathology occurring as a result of hemodynamic injury occurs in up to 25% of preterm newborns of less than 1,500 gm birth weight and in a much smaller, but nonetheless meaningful, proportion of more mature infants. Abnormalities in cerebrovascular regulation have been proposed as major contributing factors to both ischemic and hemorrhagic injuries in the newborn brain. In this review we explore several factors that play a role in cerebrovascular regulation in the immature brain and relate them to what is known about vascular regulation in the mature brain and to the types of pathology that occur in the newborn brain. One goal in this "decade of the brain" should be to increase our basic and clinical knowledge about the cerebrovasculature of the newborn in order to enhance our ability to predict and prevent perinatal brain injury.

Insulin protects cognitive function in experimental stroke

There is evidence from in vitro systems that the extent of neuronal loss in acute central nervous system ischaemia can be reduced by manoeuvres which restrict availability of glucose to the ischaemic area. Experiments were designed to test whether hypoglycaemia induced with insulin is associated with improved behavioural outcome in a recovery model of stroke. Rats learned a maze task as a test of working memory, believed to be subserved by the hippocampus, and then had a period of cerebral ischaemia, followed by reperfusion. After an interval of 14 days they were tested on the same maze, where lesioned animals had very significant (p less than 0.0001) impairment of working memory, whereas lesioned and treated (2.0 u/kg-1 insulin, minimum single plasma glucose value: 3.1 mmol/l-1) animals were indistinguishable from control animals. It is concluded that a striking degree of protection can be obtained with levels of mild hypoglycaemia which may be acceptable and practicable for use in humans.

Long-term Doppler ultrasound measurement through the anterior fontanel in neonates: choice of parameter for computer supported analysis

There is no easy comparison of the Doppler spectra. Therefore, determining parameters is necessary for long-term measurements. A manual analysis is laborious, limits the number of possible examinations, and considerations are restricted to the envelope curve (maximal cerebral blood flow velocity). The applied ultrasonic apparatus (a modified HP 77020AC) determines three waveforms besides the Doppler spectrum: V max as maximal, V means as average, and V mode as the most frequently measured velocity. These data are directly transferred to a PC. Using low Doppler intensity V max resulted in values up to 39% lower than determined by manual analysis because of the apparatus' algorithm. By use of higher Doppler intensity this difference was reduced to 19%. Regarding clinical validity, the V s mean seemed to be the best parameter for further processing. Determining peak systolic velocity (V mean), end-diastolic velocity (V d mean), and mean velocity over time (V mean) were selected for standard analysis. The Pourcelot index was not taken into account, first for lack of definition for V mean and secondly for not missing simultaneous changes of the peak systolic and the end-diastolic velocity.

Continuous cerebral electrical impedance monitoring in sick preterm infants

Cerebral electrical impedance (dZ) and intra-arterial blood pressure were measured continuously during the first 48 h after birth in 26 sick ventilated preterm infants with a birth weight less than 1500 g. The aim was to establish whether any patterns of dZp or the variability of either blood pressure or dZp would allow identification of those infants who developed intraventricular haemorrhage (IVH), periventricular leucomalacia (PVL) or a poor neurological outcome. IVH and PVL were diagnosed by ultrasound image obtained every 6 h. Cerebral electrical impedance recordings were unsuitable for analysis in three patients and a further three died within 14 h of birth. In the remaining 20 patients, no step changes that may have been related to the onset of IVH or PVL were evident and whilst three patterns of dZp were identified, they were not useful in distinguishing between normal infants or those who developed IVH, PVL or had a poor neurological outcome. Using multiple linear regression, the coefficient of variation of dZp was significantly associated with both IVH and outcome, as was the coefficient of variation of blood pressure. Continuous measurement of cerebral electrical impedance, whilst technically feasible in sick preterm infants, was not found useful as a method of identifying those who developed IVH, PVL or had a poor neurological outcome.

Acoustic power measurements of Doppler ultrasound devices used for perinatal and infant examinations

Acoustic power output levels were measured in four different pulsed Doppler systems (transcranial, Duplex mode, colour mode, miniature for continuous monitoring) currently used for examination of fetal and infant blood flow velocities. The frequencies of the transducers ranged from 2 to 8 MHz. The devices were studied at three to five different intensity settings. The measurements were performed using the radiation force balance of the Fraunhofer-Institut, which was especially adapted for this study. Each of the four devices was tested while running in its commonly used mode, and comparison showed that their acoustic power values varied widely: 96.8 mW (2 MHz, EME TC2-64B), 8.7 mW (5 MHz, ATL Mk 500), 61.9 mW (3.5 MHz, Acuson 128) and 13.5 mW (5 MHz, HP 77020). All transducers had total power output levels below the limits recommended by the American Institute for Ultrasound in Medicine and Biology in the conclusions on a thermal bioeffects mechanism, which were approved in October 1987.

Major pitfalls in Doppler investigations with particular reference to the cerebral vascular system. Part I. Sources of error, resulting pitfalls and measures to prevent errors

Major pitfalls in Doppler investigations are presented based on 340 evaluated cerebral Doppler examinations in infants. Substantial pitfalls may result from: A. Physics of sound waves and Doppler instruments (errors due to high pass filter cut off, aliasing, rapid image update). B. Quality and adjustment of the Doppler instrument (errors due to low sensitivity, inappropriate adjustment of Doppler controls, inadequate wall filter). C. Examination technique (errors due to an unfavourable angle of incidence or due to transducer-induced pressure: decrease predominantly in diastolic flow velocity-increase in maximum flow velocity in the straight sinus). D. Hemodynamics (errors due to spatial or temporal variations of the flow profile, pulsatility, non-uniform distribution of cerebral blood flow/CBF). E. Cerebral vascular anatomy (errors due to an unfavourable probe position as related to the three-dimensional arrangement of vessels, inadequate separation of closely adjacent vessels). F. Interpretation (flow velocity or Resistance Index/RI is taken to equal CBF, RI is taken to equal peripheral vascular resistance, one artery is taken to represent the cerebral circulation). Pitfalls may be avoided by using adequate means (low wall filter adjustment, high Doppler frequency, critical assessment of velocity spectra) to reduce the likelihood of errors occurring.

Acute and protracted effects of intratracheal surfactant application on internal carotid blood flow velocity, blood pressure and carbondioxide tension in very low birth weight infants

As a part of a multicentre clinical trial of prophylactic treatment with bovine surfactant (SF-RI 1) given to immature infants below 31 gestational weeks, short term and protracted effects on cerebral haemodynamics were assessed by Dopplersonographic measurements of the right internal carotid artery. Measurements were performed every 10 min for 1 h after intratracheal application of the surfactant in ten treated infants. The results of additional measurements every 12 h up to the age of 100 h were compared with a control group. In single cases there were changes of time averaged mean maximum velocity (Vmax) of as much as 100% immediately after intratracheal surfactant application, although the mean short term and protracted variability of Vmax was the same as the protracted variability in the control group. Variability of mean arterial blood pressure and transcutaneous carbondioxide tension (tcpCO2) was even less. With proper adjustment of ventilatory settings intratracheal treatment with surfactant does not affect variability or absolute values of internal carotid Vmax, mean arterial blood pressure and transcutaneous pCO2 in low birth weight infants within 100 h after application.

Cerebral blood flow in newborn infants with and without mechanical ventilation

The influence of mechanical ventilation with low mean airway pressure (MAP) on cerebral blood flow (CBF) veolocity in newborn infants was assessed in fifteen ventilated infants by Duplex Doppler Sonography (Duplex DS). As a control, CBF velocities were examined in 15 age and weight matched non-ventilated infants. For quantitation, maximal systolic velocity, enddiastolic velocity and the semiquantitative Pourcelot index were determined as representative flow variables. There was no significant difference of these flow variables between ventilated and non-ventilated infants. The pH, pO2 and pCO2 did not differ significantly between the two groups and there was no correlation between the flow variables, pH, pO2, pCO2 or MAP. Mechanical ventilation with low MAP is not associated with adverse effects on cerebral hemodynamics in newborn infants when significant alterations of the blood gases are avoided.