Regional distribution of cerebral blood volume and cerebral blood flow in newborn piglets--effect of hypoxia/hypercapnia

Cerebral haemodynamics in patients with glutaryl-coenzyme A dehydrogenase deficiency

In glutaric aciduria type 1, glutaryl-coenzyme A and its derivatives are produced from intracerebral lysine and entrapped at high concentrations within the brain, where they interfere with energy metabolism. Biochemical toxicity is thought to trigger stroke-like striatal degeneration in susceptible children under 2 years of age. Here, we explore vascular derangements that might also contribute to brain damage. We studied injured and non-injured Amish glutaric aciduria type 1 patients using magnetic resonance imaging (n = 26), transcranial Doppler ultrasound (n = 35) and perfusion computed tomography (n = 6). All glutaric aciduria type 1 patients had wide middle cerebral, internal carotid and basilar arteries. In non-injured patients, middle cerebral artery velocities were 18-26% below control values throughout late infancy and early childhood, whereas brain-injured children had an early velocity peak (18 months) and low values thereafter. Perfusion scans from six patients showed that tissue blood flow did not undergo a normal developmental surge. We observed four different perfusion patterns. (i) Three children (two non-injured) had low cerebral blood flow, prolonged mean transit time, elevated cerebral blood volume and high mean transit time/cerebral blood flow and cerebral blood volume/cerebral blood flow ratios. This pattern optimizes substrate extraction at any given flow rate but indicates low perfusion pressure and limited autoregulatory reserve. (ii) Ten hours after the onset of striatal necrosis in an 8-month-old infant, mean transit time and cerebral blood volume were low relative to cerebral blood flow, which varied markedly from region to region. This pattern indicates disturbed autoregulation, regional perfusion pressure gradients, or redistribution of flow from functional capillaries to non-exchanging vessels. (iii) In an infant with atrophic putaminal lesions, striatal flow was normal but mean transit time and cerebral blood volume were low, consistent with perfusion in excess of metabolic demand. (iv) Finally, a brain-injured adult with glutaric aciduria type 1 had regional perfusion values within the normal range, but the putamina, which normally have the highest regional perfusion, had cerebral blood flow values 24% below cortical grey matter. Although metabolic toxicity appears central to the pathophysiology of striatal necrosis, cerebrovascular changes probably also contribute to the process. These changes may be the primary cause of expanded cerebrospinal fluid volume in newborns, intracranial and retinal haemorrhages in infants and interstitial white matter oedema in children and adults. This pilot study suggests important new areas for clinical investigation.

Gray matter blood flow change is unevenly distributed during moderate isocapnic hypoxia in humans

Hypoxia increases cerebral blood flow (CBF), but it is unknown whether this increase is uniform across all brain regions. We used H(2)(15)O positron emission tomography imaging to measure absolute blood flow in 50 regions of interest across the human brain (n = 5) during normoxia and moderate hypoxia. Pco(2) was kept constant ( approximately 44 Torr) throughout the study to avoid decreases in CBF associated with the hypocapnia that normally occurs with hypoxia. Breathing was controlled by mechanical ventilation. During hypoxia (inspired Po(2) = 70 Torr), mean end-tidal Po(2) fell to 45 +/- 6.3 Torr (means +/- SD). Mean global CBF increased from normoxic levels of 0.39 +/- 0.13 to 0.45 +/- 0.13 ml/g during hypoxia. Increases in regional CBF were not uniform and ranged from 9.9 +/- 8.6% in the occipital lobe to 28.9 +/- 10.3% in the nucleus accumbens. Regions of interest that were better perfused during normoxia generally showed a greater regional CBF response. Phylogenetically older regions of the brain tended to show larger vascular responses to hypoxia than evolutionary younger regions, e.g., the putamen, brain stem, thalamus, caudate nucleus, nucleus accumbens, and pallidum received greater than average increases in blood flow, while cortical regions generally received below average increases. The heterogeneous blood flow distribution during hypoxia may serve to protect regions of the brain with essential homeostatic roles. This may be relevant to conditions such as altitude, breath-hold diving, and obstructive sleep apnea, and may have implications for functional brain imaging studies that involve hypoxia.

Intrauterine growth restriction improves cerebral O2 utilization during hypercapnic hypoxia in newborn piglets

Data are scant regarding the capacity of cerebrovascular regulation during asphyxia for prevention of brain oxygen deficit in intrauterine growth-restricted (IUGR) newborns. We tested the hypothesis that IUGR improves the ability of neonates to withstand critical periods of severe asphyxia by optimizing brain oxygen supply. Studies were conducted to examine the effects of IUGR on cerebral blood flow (CBF) regulation and oxygen consumption (cerebral metabolic rate for oxygen, CMRO(2)) at different stages of asphyxia (hypercapnic hypoxaemia) in comparison to pure hypoxia (normocapnic hypoxaemia). We used 1-day-old anaesthetized and ventilated piglets. Animals were divided into normal weight (NW) piglets (n = 47; aged 11-26 h, body weight 1481 +/- 121 g) and IUGR piglets (n = 48; aged 13-28 h, body weight 806 +/- 42 g) according to their birth weight. Different stages of hypoxaemia were induced for 1 h by appropriate lowering of the inspired fraction of oxygen (moderate hypoxia: = 31-34 mmHg; severe hypoxia: = 20-22 mmHg). Fourteen NW and 16 IUGR piglets received additionally 9% CO(2) in the breathing gas, so that a of 74-80 mmHg resulted (hypoxia/hypercapnia groups). Eight NW and nine IUGR animals served as untreated controls. Furthermore, affinity of haemoglobin for oxygen was measured under hypoxic and asphyxic conditions. During asphyxia cerebral oxygen extraction was markedly increased in IUGR animals (P < 0.05). This resulted in a significantly diminished CMRO(2)-related increase of CBF at gradually reduced arterial oxygen content (P < 0.05). Therefore, an enhanced effectivity in oxygen availability appeared in newborn IUGR piglets under graded asphyxia by improved cerebral oxygen utilization (P < 0.05). This was not supported by related O(2) affinity of haemoglobin. Thus, IUGR newborns are more capable to ensure brain O(2) demand during asphyxia (hypercapnic hypoxia) than NW neonates.

Near-infrared spectroscopy measurement of the pulsatile component of cerebral blood flow and volume from arterial oscillations

We describe a near-infrared spectroscopy (NIRS) method to noninvasively measure relative changes in the pulsate components of cerebral blood flow (pCBF) and volume (pCBV) from the shape of heartbeat oscillations. We present a model that is used and data to show the feasibility of the method. We use a continuous-wave NIRS system to measure the arterial oscillations originating in the brains of piglets. Changes in the animals' CBF are induced by adding CO(2) to the breathing gas. To study the influence of scalp on our measurements, comparative, invasive measurements are performed on one side of the head simultaneously with noninvasive measurements on the other side. We also did comparative measurements of CBF using a laser Doppler system to validate the results of our method. The results indicate that for sufficient source-detector separation, the signal contribution of the scalp is minimal and the measurements are representative of the cerebral hemodynamics. Moreover, good correlation between the results of the laser Doppler system and the NIRS system indicate that the presented method is capable of measuring relative changes in CBF. Preliminary results show the potential of this NIRS method to measure pCBF and pCBV relative changes in neonatal pigs.

Distribution of cardiac output and oxygen delivery in an acute animal model of single-ventricle physiology

BACKGROUND

When single-ventricle physiology is established acutely (ie, after a Norwood procedure), the combination of limited cardiac output and hypoxemia could result in limited oxygen transport to systemic organs. This study investigates the regional distribution of cardiac output and oxygen delivery after creation of single-ventricle physiology.

METHODS

Single-ventricle physiology was created in 8 piglets, and 8 other piglets served as sham control animals. Aortopulmonary shunt, echocardiography-guided atrial septostomy, tricuspid valve avulsion, and pulmonary artery occlusion allowed the left ventricle to support systemic and pulmonary circulations. Physiologic parameters and regional blood flow were determined at baseline and at 30 and 120 minutes after conversion to single-ventricle physiology. Parameters were compared by means of 1-way and 2-way analysis of variance.

RESULTS

Single-ventricle physiology resulted in lower diastolic arterial pressure, oxygen saturation, and arterial oxygen saturation (P < .05), whereas hemoglobin was unchanged. Cerebral blood flow increased markedly in control animals (P = .04). In contrast, in single-ventricle physiology regional blood flow was unchanged in the brain, higher in the myocardium (P = .1), and mildly reduced in low-priority organs (liver, kidneys, and bowel). Cerebral oxygen delivery increased in control animals, whereas in animals with single-ventricle physiology, oxygen delivery decreased in the brain, liver, kidneys, and bowel (P < .05) and was unchanged in the myocardium. Total-body oxygen delivery decreased in animals with single-ventricle physiology (P < .001) but not in control animals. Total-body oxygen consumption was unchanged in both groups.

CONCLUSIONS

This study shows that in acute single-ventricle physiology hypoxemia and limited regional blood flow reduce oxygen transport to low-priority organs and partly to the brain. These findings might contribute to the understanding of gastrointestinal and neurologic complications in children with single-ventricle physiology.

Single-Ventricle Physiology Reduces Cerebral Oxygen Delivery in a Piglet Model

BACKGROUND

In single-ventricle physiology, cerebral blood flow and oxygen (O2) delivery may be inadequate. This study tests the hypotheses that in acute univentricular physiology (1) cerebral blood flow increases inadequately to maintain O2 delivery, (2) the brain is incapable of increasing O2 extraction due to hypoxemia, and (3) cerebral O2 delivery diminishes selectively in different brain regions.

MATERIAL AND METHODS

Univentricular physiology was created in 8 piglets, while 8 animals were sham controls. Aortopulmonary shunt, echocardiography-guided atrial septostomy, tricuspid valve avulsion, and pulmonary artery occlusion were performed to allow the left ventricle to support systemic and pulmonary circulations. Cerebral blood flow (microspheres), cerebral O2 and lactate metabolism, and cerebral O2 saturation were measured at baseline, 30 minutes, and 120 minutes after conversion to univentricular physiology.

RESULTS

Cerebral blood flow increased in the cerebrum and subtentorium in controls (p < 0.05), whereas it remained unchanged in univentricular piglets. Cerebral O2 delivery at 30 and 120 minutes was lower in univentricular physiology than in controls (p = 0.05). Fractional oxygen extraction was unchanged in both groups. Cerebral O2 consumption trended lower in univentricular physiology (p = not significant), while it was unchanged in controls. Lactate cerebral metabolic rate (CMRLactate) increased at 30 and 120 minutes in both groups. The decline in O2 delivery was variable, but present in nearly all brain regions.

CONCLUSIONS

This study confirms the hypothesis that, in univentricular physiology, hypoxemia and limited cerebral blood flow reduce cerebral O2 availability in nearly all regions. These findings contribute to the understanding of brain abnormalities in infants with univentricular physiology.

Pathophysiology of traumatic injury in the developing brain: an introduction and short update

Current understanding about the main peculiarities in pathophysiology of immature brain traumatic injury involves marked developmental discrepancy of biomechanical properties, aspects of altered features in water and electrolyte homeostasis as well as maturation dependent differences in structural and functional responses of major transmitter systems. Based on the fact that traumatic brain injury (TBI) is one of the major causes of morbidity and mortality in infants and children, the currently available epidemiological data are reviewed in order to gain insights about scope and dimension of health care engagement and derive the requirements for reinforced pathogenetic research. To this end, the main aspects of peculiarities in primary and secondary TBI mechanisms in the immature/developing brain are discussed, including structural and functional conditions resulting in a markedly diminished shear resistance of the immature brain tissue. As such, the immature brain tissue appears to be more susceptible to mechanical alterations, because similar mechanical load induces a more intense brain tissue displacement. Furthermore, available indications for increased incidence of brain swelling in the immature brain after TBI are reviewed, focusing on the interrelationship between the age-dependent differences in extracellular space and aquaporin-4 expression during brain maturation. The developmental differences of TBI induced cerebrovascular response as well as some relevant aspects of altered neurotransmission following TBI of the immature brain in regard to the glutamatergic and dopaminergic transmitter system are assessed. Thus, this mini-review highlights some progress but also an increased necessity for expanded pathogenetic research on a clinical scale in order to develop a solid foundation for adequate therapeutic strategies for the different life-threatening consequences of TBI in infancy and childhood, which mainly have failed up to now.

A volumetric screening procedure for the Göttingen minipig brain

A screening procedure was developed to provide quantitative estimates of structural parameters, regional volumes and neuron number, in a neurotoxicologic study of the Göttingen minipig brain. The study material consisted of normal controls and brains collected from young minipigs which had been exposed in utero to the mitotic inhibitor methylazoxymethanol acetate (MAM). Based on stereological principles and systematic sampling techniques, volumetric data from pre-selected regions of the pig brain was obtained using Cavalieri's principles and point-counting. Secondarily, estimates of total hemispheric neocortical cell numbers were obtained from pre-selected groups to test the potential effect of MAM on neuron number. No significant differences were observed in volume of the pre-selected regions of MAM intoxicated pigs nor in estimates of total neocortical neuron number.

The influx of neutral amino acids into the porcine brain during development: a positron emission tomography study

Pigs of three different age groups (newborns, 1 week old, 6 weeks old) were used to study the transport of the large neutral amino acids 6-[18F]fluoro-L-DOPA ([18F]FDOPA) and 3-O-methyl-6-[18F]fluoro-L-DOPA ([18F]OMFD) across the blood-brain barrier (BBB) with positron emission tomography (PET). Compartmental modeling of PET data was used to calculate the blood-brain clearance (K1) and the rate constant for the brain-blood transfer (k2) of [18F]FDOPA and [18F]OMFD after i.v. injection. A 40-70% decrease of K1(OMFD), K1(FDOPA) and k2(OMFD) from newborns to juvenile pigs was found whereas k2(FDOPA) did not change. Generally, K1(OMFD) and k2(OMFD) are lower than K1(FDOPA) and k2(FDOPA) in all regions and age groups. The changes cannot be explained by differences in brain perfusion because the measured regional cerebral blood flow did not show major changes during the first 6 weeks after birth. In addition, alterations in plasma amino acids cannot account for the described transport changes. In newborn and juvenile pigs, HPLC measurements were performed. Despite significant changes of single amino acids (decrease: Met, Val, Leu; increase: Tyr), the sum of large neutral amino acids transported by LAT1 remained unchanged. Furthermore, treatment with a selective inhibitor of the LAT1 transporter (BCH) reduced the blood-brain transport of [18F]FDOPA and [18F]OMFD by 35% and 32%, respectively. Additional in-vitro studies using human LAT1 reveal a much lower affinity of FDOPA compared to OMFD or L-DOPA. The data indicate that the transport system(s) for neutral amino acids underlie(s) developmental changes after birth causing a decrease of the blood-brain barrier permeability for those amino acids during brain development. It is suggested that there is no tight coupling between brain amino acid supply and the demands of protein synthesis in the brain tissue.

Regional response of cerebral blood volume to graded hypoxic hypoxia in rat brain

BACKGROUND

The response of cerebral blood flow to hypoxic hypoxia is usually effected by dilation of cerebral arterioles. However, the resulting changes in cerebral blood volume (CBV) have received little attention. We have determined, using susceptibility contrast magnetic resonance imaging (MRI), changes in regional CBV induced by graded hypoxic hypoxia.

METHODS

Six anaesthetized rats were subjected to incremental reduction in the fraction of inspired oxygen: 0.35, 0.25, 0.15, and 0.12. At each episode, CBV was determined in five regions of each hemisphere after injection of a contrast agent: superficial and deep neocortex, striatum, corpus callosum and cerebellum. A control group (n = 6 rats) was studied with the same protocol without contrast agent, to determine blood oxygenation level dependent (BOLD) contribution to the MRI changes.

RESULTS

Each brain region exhibited a significant graded increase in CBV during the two hypoxic episodes: 10-27% of control values at 70% SaO2, and 26-38% at 55% SaO2. There was no difference between regions in their response to hypoxia. The mean CBV of all regions increased from 3.6 (SD 0.6) to 4.1 (0.6) ml (100 g)-1 and to 4.7 (0.7) ml (100 g)-1 during the two hypoxic episodes, respectively (Scheffé F-test; P < 0.01). Over this range, CBV was inversely proportional to SaO2 (r2 = 0.80). In the absence of the contrast agent, changes due to the BOLD effect were negligible.

CONCLUSIONS

These findings imply that hypoxic hypoxia significantly raises CBV in different brain areas, in proportion to the severity of the insult. These results support the notion that the vasodilatory effect of hypoxia is deleterious in patients with reduced intracranial compliance.

Precise measurement of cerebral blood flow in newborn piglets from the bolus passage of indocyanine green

Indocyanine green (ICG) is a near-infrared dye that has the potential to be used as a tracer for the minimally invasive measurement of cerebral blood flow (CBF). In order to examine the technique, the arterial and cerebral concentrations of ICG were measured in newborn piglets during the bolus passage of ICG at normocapnia and two levels of mild hypercapnia. The results were analysed by applying the Fick principle in both integral and differential forms using a linear regression technique to improve the precision of calculated values of CBF. It was found that the integral method, which has been used previously, is particularly sensitive to errors in the time registration between the arterial and tissue signals whereas the differential method is less so. In addition, the differential method allows the venous outflow to be calculated which gives further information on the state of the capillary bed. CBF was 39.7 +/- 4.6 ml 100 g(-1) min(-1) at an arterial carbon dioxide tension (PaCO2) of 33.0+/-2.2 mmHg and increased to 53.7+/-9.1 and 75.4+/-15.2 ml 100 g(-1) min(-1) at a PaCO2 of 42.1 +/- 2.6 and 54.2 +/- 3.1 mmHg respectively (mean +/- SD, n = 7). There was no significant change in cerebral metabolic rate for oxygen, validating the value of blood flow to an arbitrary scaling factor. When the inspired CO2 fraction was returned to zero, calculated CBF returned to baseline with a variation of 7% of the mean, indicating that this technique is highly precise.

Relation between brain tissue pO2 and dopamine synthesis of basal ganglia--a 18FDOPA-PET study in newborn piglets

Perinatal hypoxic-ischemic cerebral injury is a major determinant of neurologic morbidity and mortality in the neonatal period and later in childhood. There is evidence that the dopaminergic system is sensitive to oxygen deprivation. However, the respective enzyme activities have yet not been measured in the living neonatal brain. In this study, we have used 18F-labelled 6-fluoro-L-3,4-dihydroxyphenylalanine (FDOPA) together with positron emission tomography (PET) to estimate the activity of the aromatic amino acid decarboxylase (AADC), the ultimate enzyme in the synthesis of dopamine, in the brain of newborn piglets under normoxic and moderate asphyxial conditions. The study was performed on 8 newborn piglets (2-5 days old). In each piglet PET studies were performed under control conditions and during 2-hour asphyxia. Simultaneously, brain tissue pO2 was recorded, cerebral blood flow (CBF) was measured with colored microspheres and cerebral metabolic rate of oxygen (CMRO2) was determined. Asphyxia was induced by lowering the inspired fraction of oxygen from 0.35 to 0.10 and adding about 6% CO2 to the inspired gas. Asphyxia elicited a more than 3-fold increase of the CBF (p < 0.01) so that CMRO2 remained unchanged throughout the asphyxial period. Despite this, brain tissue pO2 was reduced from 19 +/- 4 mm Hg to 6 +/- 3 mm Hg (p < 0.01). Blood-brain transfer of FDOPA as well as permeability-surface area product (PS) from striatum were unchanged. Striatal synthesis rate of fluoro-dopamine from FDOPA (k3) was, however, significantly increased (p < 0.01). This increase of the AADC activity is associated with reduced brain tissue pO2. Asphyxia-induced CBF increase impedes an alteration of brain oxidative metabolism.

Upregulation of the aromatic amino acid decarboxylase under neonatal asphyxia

Perinatal hypoxic-ischemic cerebral injury is a major determinant of neurologic morbidity and mortality in the neonatal period and later in childhood. There is evidence that the dopaminergic system is sensitive to asphyxia. However, the respective enzyme activities have not yet been measured in the living neonatal brain. In this study, we have used 18F-labeled 6-fluoro-L-3,4-dihydroxyphenylalanine (FDOPA) together with positron-emission tomography (PET) to estimate the activity of the aromatic amino acid decarboxylase (AADC), the ultimate enzyme in the synthesis of dopamine (DA), in the brain of newborn piglets. Simultaneously, the cerebral blood flow (CBF) was measured with colored microspheres. Asphyxia elicited an up to threefold increase of the CBF. Despite this, the blood-brain transfer of FDOPA as well as the clearance rate constants from brain were unchanged. However, the synthesis rate of FDA from FDOPA was significantly increased in frontal cortex, striatum, and midbrain. This increase of the AADC activity and the decrease of monoamine oxidase activity may contribute to the increase of extracellular DA during asphyxia which is expected to be involved in severe disturbances of neuronal metabolism, e.g., by generating free radicals.