Cerebral physiological and metabolic effects of hyperventilation in the neonatal dog

Hyperventilation-induced nystagmus in vestibular neuritis: pattern and clinical implication

BACKGROUND

It was the aim of this study to investigate the pattern of evolution of hyperventilation-induced nystagmus (HIN) in vestibular neuritis (VN) and to determine whether HIN influences the dizziness outcome at the last follow-up visit.

METHODS

Fifty-three consecutive patients with VN underwent a quantitative vestibular function test including hyperventilation and the Korean version of the Dizziness Handicap Inventory during the acute period and the follow-up visit.

RESULTS

The incidence of HIN was higher in the acute (62%, 33/53) than in the chronic (17%, 9/53) stages of VN. Approximately 70% (6/9) of patients who continued to have persistent HIN at the last follow-up reported dizziness compared to only 27% (12/44) of patients who had no HIN. Patients who complained of persistent dizziness were significantly more likely to have persistent HIN and high Korean Dizziness Handicap Inventory scores at the last follow-up compared with patients who did not suffer from dizziness. In terms of the degree of recovery of dizziness, patients with HIN initially beating toward the contralesional side exhibited significantly more improvement than patients with HIN initially beating toward the ipsilesional side.

CONCLUSIONS

The presence of either HIN beating toward the ipsilesional side at the acute stage of VN or persistent HIN at the follow-up visit is associated with persistent dizziness.

Permissive hypercapnia to decrease lung injury in ventilated preterm neonates

Lung injury in ventilated premature infants occurs primarily through the mechanism of volutrauma, often due to the combination of high tidal volumes in association with a high end-inspiratory volume and occasionally end-expiratory alveolar collapse. Tolerating a higher level of arterial partial pressure of carbon dioxide (PaCO2) is considered as 'permissive hypercapnia' and when combined with the use of low tidal volumes may reduce volutrauma and lead to improved pulmonary outcomes. Permissive hypercapnia may also protect against hypocapnia-induced brain hypoperfusion and subsequent periventricular leukomalacia. However, extreme hypercapnia may be associated with an increased risk of intracranial hemorrhage. It may therefore be important to avoid large fluctuations in PaCO2 values. Recent randomized clinical trials in preterm infants have demonstrated that mild permissive hypercapnia is safe, but clinical benefits are modest. The optimal PaCO2 goal in clinical practice has not been determined, and the available evidence does not currently support a general recommendation for permissive hypercapnia in preterm infants.

Brain changes to hypocapnia using rapidly interleaved phosphorus-proton magnetic resonance spectroscopy at 4 T

Substantial controversy persists in the literature concerning the physiologic consequences hypocapnia, or low partial pressure of carbon dioxide (PaCO(2)). Invasive animal studies have demonstrated large pH increases (>0.25 U), phosphocreatine (PCr) decreases (>30%), and adenosine triphosphate (ATP) decreases (>10%) after hyperventilation (HV) (20 mm Hg PaCO(2)). However, using magnetic resonance spectroscopy, HV studies in awake humans have demonstrated only small pH changes ( approximately 0.05 U) and no changes in PCr or ATP. It remains important to ascertain whether this failure to detect PCr changes in human studies reflects a true absence of changes, or a limitation in data fidelity. The present study used a rapidly interleaved phosphorus-proton spectroscopy acquisition from large samples at high magnetic field (4 T), to measure pH, PCr, inorganic phosphate, beta-ATP, and lactate changes with high temporal and signal sensitivity. Five of six subjects had usable data. During 20 mins HV, PaCO(2) reached a minimum at 16 mins (17 mm Hg); however, the maximum pH change (+0.047) peaked earlier (14 mins). Maximal lactate increases were measured at 15 mins. By 10 mins, maximum changes were observed for PCr (-3.4%) and inorganic phosphate (+6.4%). No changes in beta-ATP were observed. The peak in pH, despite continued decreases in PaCO(2), suggests active buffering during HV. These data, and the small magnitude of early PCr and inorganic phosphate changes, do not support substantial energy compromise during HV. Other mitigating factors, such as anesthesia-induced deregulation of the cerebrovasculature, might have contributed to the exaggerated metabolic changes observed in previous animal investigations.

Permissive hypercapnia

Although lifesaving, mechanical ventilation can result in lung injury and contribute to the development of bronchopulmonary dysplasia. The most critical determinants of lung injury are tidal volume and end-inspiratory lung volume. Permissive hypercapnia offers to maintain gas exchange with lower tidal volumes and thus decrease lung injury. Further physiologic benefits include improved oxygen delivery and neuroprotection, the latter through both avoidance of accidental hypocapnia, which is associated with a poor neurologic outcome, and direct cellular effects. Clinical trials in adults with acute respiratory failure indicated improved survival and reduced incidence of organ failure in subjects managed with low tidal volumes and permissive hypercapnia. Retrospective studies in low birth weight infants found an association of bronchopulmonary dysplasia with low PaCO(2). Randomized clinical trials of low birth weight infants did not achieve sufficient statistical power to demonstrate a reduction of BPD by permissive hypercapnia, but strong trends indicated the possibility of important benefits without increased adverse events. Herein, we review the mechanisms leading to lung injury, the physiologic effects of hypercapnia, the dangers of hypocapnia, and the available clinical data.

Hypocapnia under hypotension induces apoptotic neuronal cell death in the hippocampus of newborn rabbits

We investigated the adverse effect of hypocapnia on the neonatal rabbit brain. Two-week-old Japanese white rabbits were assigned to three groups, hyperventilation (H group), ischemia (I group), or hypocapnia with ischemia (HI group) and then subjected for 1.5 h with simultaneous measurement of the mean arterial blood pressure (MABP) and intracranial Hb concentration changes. Marked reductions of PaCO2 and MABP were induced in the hyperventilation-loaded groups and the ischemia-loaded groups, respectively. The intracranial oxyhemoglobin and total Hb concentrations decreased slightly in the H group and markedly in the I and HI groups after the start of experimental protocols, although there were no statistical differences between the I and HI groups. Animals were killed at 24 h after experiments and then subjected to pathologic examination. Damaged neurons with shrunken cell bodies and nuclear changes were found on light microscopic examination, mainly in the pyramidal cell layer of the subiculum and cornu ammonis 1. The numerical density of damaged neurons was significantly higher in the HI group than those in the H or I groups (p < 0.05). These damaged neurons were positive on DNA nick end labeling. A DNA ladder was detected on electrophoresis with a DNA sample extracted from hippocampal tissue in the HI group, but not in the other two groups. On electron microscopic examination, not only condensation of the nucleus but also disruption of mitochondria and the cell membrane were detected. These results suggested that hypocapnia under hypotension might cause neuronal cell death in the hippocampus of neonatal rabbit. Not only ischemia but also a metabolic change induced by hypocapnia might contribute to this apoptotic neuronal cell damage.

Lactate attenuates neuron specific enolase elevation in newborn rats

This study was undertaken to investigate the protective role of lactate on the hypoxic brain in newborn rats. A total of 107 7-day-old Wistar rats were divided into three groups. The lactate accumulation group was given 5% oxygen and 95% nitrogen for 30 minutes. The lactate elimination group was given 5% oxygen, a concentration of 7.5% carbon dioxide, and 87.5% nitrogen for 30 minutes. The control rats were placed in room air. Lactate levels in the brain tissue were higher in the lactate accumulation group than in those of the control group (control: 1.78 +/- 0.91, lactate accumulation: 11.42 +/- 1.64 mmol/kg) and significantly decreased in the lactate elimination group (4.10 +/- 1.73 mmol/kg). Blood pH remained at the same levels in the two groups. Neuron specific enolase in the cerebrospinal fluid, which is the initial neurocyte damage marker, was significantly elevated in the lactate elimination group (control: 18.3 +/- 7.5, lactate accumulation: 18.8 +/- 7.9, lactate elimination: 63.1 +/- 61.3 ng/mL). Brain adenosine 5'-triphosphate levels were significantly decreased in the lactate elimination group. Histologic findings of the brain at 72 hours after the load revealed no abnormal changes in any of the groups examined. The authors conclude that lactate accumulation plays a protective role on the hypoxic brain in newborn rats.

Fast and slow components of cerebral blood flow response to step decreases in end-tidal PCO2 in humans

This study examined the dynamics of the middle cerebral artery (MCA) blood flow response to hypocapnia in humans (n = 6) by using transcranial Doppler ultrasound. In a control protocol, end-tidal PCO2 (PETCO2) was held near eucapnia (1.5 Torr above resting) for 40 min. In a hypocapnic protocol, PETCO2 was held near eucapnia for 10 min, then at 15 Torr below eucapnia for 20 min, and then near eucapnia for 10 min. During both protocols, subjects hyperventilated throughout and PETCO2 and end-tidal PO2 were controlled by using the dynamic end-tidal forcing technique. Beat-by-beat values were calculated for the intensity-weighted mean velocity (VIWM), signal power (P), and their instantaneous product (P.VIWM). A simple model consisting of a delay, gain terms, time constants (tauf,on, tauf, off) and baseline levels of flow for the on- and off-transients, and a gain term (gs) and time constant (taus) for a second slower component was fitted to the hypocapnic protocol. The cerebral blood flow response to hypocapnia was characterized by a significant (P < 0.001) slow progressive adaptation in P.VIWM, with gs = 1.26 %/Torr and taus = 427 s, that persisted throughout the hypocapnic period. Finally, the responses at the onset and relief of hypocapnia were asymmetric (P < 0.001), with tauf,on (6.8 s) faster than tauf,off (14.3 s).

Blood flow distribution in the normal human preterm brain

Disturbances in cerebral blood flow (CBF) are a major factor in the etiology and pathogenesis of cerebral damage in the neonate. As most animals are more mature at birth than man, extrapolation from animal studies to the human is questionable. Therefore, we have measured regional CBF (rCBF) in preterm infants. rCBF flow was measured in 12 normotensive and normoxic preterm infants [mean birth weight 915 g (range 550 to 2680 g), mean gestational age 27.7 wk (25 to 32 wk)]. All infants had a normal cerebral ultrasound examination. rCBF was measured using a mobile brain dedicated fast-rotating four-head multidetector system specially designed for neonatal studies. The tracer was 99mTc-labeled D,L-hexamethylpropylenamine oxime in a dose of 4 Mbq/kg. rCBF of the subcortical white matter was 0.53 (0.48-0.58) of the global CBF. After correction for scattered radiation, the estimate of rCBF to the white matter was reduced to 0.39 (0.36-0.42). The flow to the basal ganglia was 2.33 (2.08-2.59) times the global CBF. After correction for partial volume effect, the cortical flow was higher than the flow to the basal ganglia and highest in the frontotemporal cortex (motor cortex). The flow to the cerebellum was of the same magnitude as the flow to the basal ganglia, but with a significantly higher variation. rCBF in 12 preterm infants showed a flow distribution similar to flow in other newborn mammals. The gray-white matter contrast, however, was greater. This new information, combined with existing data showing low global CBF, suggests that blood flow to the white matter in the preterm human neonate is extremely low.

Effect of cerebral blood flow and cerebrovascular autoregulation on the distribution, type and extent of cerebral injury

Global cerebral blood flow (GCBF) is low in the human neonate compared to the adult. It is even lower in mechanically ventilated, preterm infants: 10-12 ml/100 g/minute, a level associated with brain infarction in adults. The reactivity, however, of global CBF to changes in cerebral metabolism, PaCO2, and arterial blood pressure is normal, except following severe birth asphyxia, or in mechanically ventilated preterm infants, who subsequently develop major germinal layer hemorrhage. The low level of cerebral blood flow (CBF) matches a low cerebral metabolism of glucose and a relatively small number of cortical synapses in the perinatal period. It has not been possible to define a threshold for GCBF below which electrical dysfunction or brain damage occurs (such as white matter and thalamic-basal ganglia necrosis). Three explanations for the lack of clear relation between GCBF and electrical brain activity of the preterm infant must be examined more closely: 1) low levels of CBF are adequate; 2) GCBF does not adequately reflect critically low perfusion of the white matter, and 3) acute white matter ischemia does not result in electrical silence. Two clinical patterns of brain damage following asphyxia may be explained by changes in the blood flow distribution induced by asphyxia: brainstem sparing and parasagittal cerebral injury. Hours to days after severe asphyxia, a state of marked global hyperperfusion may prevail. It is associated with poor neurological outcome and may be an entry point for trials of interventions aiming sat blocking the translation of asphyctic injury to cellular death and tissue damage.

[Acid-base equilibrium and the brain]

In physiological conditions, the regulation of acid-base balance in brain maintains a noteworthy stability of cerebral pH. During systemic metabolic acid-base imbalances cerebral pH is well controlled as the blood/brain barrier is slowly and poorly permeable to electrolytes (HCO3- and H+). Cerebral pH is regulated by a modulation of the respiratory drive, triggered by the early alterations of interstitial fluid pH, close to medullary chemoreceptors. As blood/brain barrier is highly permeable to Co2, CSF pH is corrected in a few hours, even in case of severe metabolic acidosis and alkalosis. Conversely, during ventilatory acidosis and alkalosis the cerebral pH varies in the same direction and in the same range than blood pH. Therefore, the brain is better protected against metabolic than ventilatory acid-base imbalances. Ventilatory acidosis and alkalosis are able to impair cerebral blood flow and brain activity through interstitial pH alterations. During respiratory acidosis, [HCO3-] increases in extracellular fluids to control cerebral pH by two main ways: a carbonic anhydrase activation at the blood/brain and blood/CSF barriers level and an increase in chloride shift in glial cells (HCO3- exchanged for Cl-). During respiratory alkalosis, [HCO3-] decreases in extracellular fluids by the opposite changes in HCO3- transport and by an increase in lactic acid synthesis by cerebral cells. The treatment of metabolic acidosis with bicarbonates may induce a cerebral acidosis and worsen a cerebral oedema during ketoacidosis. Moderate hypocapnia carried out to treat intracranial hypertension is mainly effective when cerebral blood flow is high and vascular CO2 reactivity maintained. Hypocapnia may restore an altered cerebral blood flow autoregulation. Instrumental hypocapnia requires a control of cerebral perfusion pressure and cerebral arteriovenous difference for oxygen, to select patients for whom this kind of treatment may be of benefit, to choose the optimal level of hypocapnia and to avoid any deleterious effect. If hypocapnia is maintained over several days, an adaptation of CSF pH may limit the therapeutic effect on the cerebral blood flow and the intracranial pressure.

Quantitative EEG during progressive hypocarbia and hypoxia. Hyperventilation-induced EEG changes reconsidered

To investigate the role of cerebral hypoxia as a causative factor in the alteration of the qEEG during hyperventilation, qEEG changes caused by progressive hypocapnia were compared with qEEG changes due to progressive normobaric hypoxia in two parallel groups of 12 and 10 healthy male subjects (age 20-27 years), respectively. In the first group, qEEG records were obtained before and during hyperventilation to pCO2 levels of 4.0, 3.0 and 2.0 kPa. In the second group, the qEEG samples were taken before and during hypoxia with hemoglobin oxygen saturations of 80, 70 and 60%. In both groups, blood flow velocity in the middle cerebral artery was also recorded. Hyperventilation caused an exponential increase in slow activity and a decrease in alpha power. No shift in the alpha mean frequency and alpha peak frequency was observed, except with the pCO2 level of 4.0 kPa, which caused an increase in both variables. Hypoxia with a hemoglobin oxygen saturation of 60% caused a much less pronounced increase in slow activity. No change in total power in the alpha band was found, but both the alpha peak frequency and alpha mean frequency decreased. Lesser degrees of hypoxia caused only minimal EEG changes. Blood flow velocity was decreased by hyperventilation but increased by hypoxia. It is concluded that the EEG changes observed during hyperventilation must mainly or totally be attributed to factors other than cerebral hypoxia.

Cerebral blood flow and metabolism during and after prolonged hypocapnia in newborn lambs

We studied the effects of prolonged (6 hours) hypocapnia and the abrupt termination thereof on cerebral blood flow and metabolism in six paralyzed, sedated (but not anesthetized) newborn lambs. Thirty minutes after institution of hyperventilation to an arterial carbon dioxide pressure of 15 +/- 2 torr, hyperventilation, cerebral blood flow had returned to baseline. Abrupt termination of hyperventilation after 6 hours resulted in a 110 +/- 71% increase in cerebral blood flow over baseline after 30 minutes of normocapnia. This cerebral hyperemia persisted for at least 90 minutes after hyperventilation was discontinued. Cerebral oxygen consumption did not change throughout the study. The posthypocapnia hyperemia noted in these animals after abrupt normalization of arterial carbon dioxide pressure may contribute to the increased risk of intracranial hemorrhage in newborn infants who are treated similarly in the management of pulmonary hypertension.

1H and 31P NMR measurement of cerebral lactate, high-energy phosphate levels, and pH in humans during voluntary hyperventilation: associated EEG, capnographic, and Doppler findings

In order to explore the sensitivity of spatially resolved 1H and 31P NMR spectroscopy on a whole-body NMR instrument, cerebral metabolic changes in human volunteers were measured during hyperventilation provocation. During hyperventilation the flow velocity in the middle cerebral artery decreased significantly and the EEG showed a marked increase in slow activity. 1H NMR spectra revealed an increase in cerebral lactate concentration. 31P NMR spectra showed no changes in ATP or PCr peak heights, but a shift toward tissue alkalosis was derived from changes in Pi chemical shift. During subsequent recovery, lactate concentration decreased and a slight intracellular acidosis was detected. In three experiments broadening of the lactate resonance peak resulted in separation into two components at 1.32 and 1.48 ppm, in which the latter signal possibly arose from alanine.

Response of the cerebral circulation to profound hypocarbia in neonatal lambs

Hyperventilation to extremely low arterial carbon dioxide tension (PaCO2) has been used in the management of persistent pulmonary hypertension in newborn infants. With progressive hypocarbia, cerebral vasoconstriction occurs, raising the concern that extreme hypocarbia may result in cerebral oxygen deprivation. Therefore, I evaluated regulation of the cerebral circulation during acute hypocarbia in 10 newborn lambs. Whole-brain and regional blood flows measured using radioactive microspheres, arterial and venous (sagittal sinus) blood gases, and oxygen contents were measured in each lamb at four arterial carbon dioxide tensions. Whole-brain oxygen delivery, oxygen consumption, and fractional oxygen extraction were calculated. Finally, arterial and venous lactate concentrations were measured to assess cerebral lactate production. Whole-brain blood flow (CBF) decreased in a nonlinear fashion as PaCO2 ranged from 46 to 12 mm Hg [In(CBF) = 0.025(PaCO2) + 3.38; r = 0.70, p less than 0.001]. Similar responses were demonstrated for all regional blood flows examined. Cerebral fractional oxygen extraction (E) increased in a nonlinear fashion [In(1-E) = 0.023(PaCO2)-1.37; r = 0.80, p less than 0.001], and cerebral metabolic rate for oxygen was unchanged with hypocarbia. Cerebral venous lactate concentration increased significantly (3.49 +/- 0.23 vs. 2.01 +/- 0.22 mM, p less than 0.001) during severe hypocarbia (PaCO2 of less than 22 mm Hg), and the arterial-venous lactate concentration difference became negative. These results demonstrate uniform responses of whole-brain and regional blood flows and stable cerebral oxygen consumption during moderate and severe hypocarbia. Although there is evidence for cerebral lactate production during severe hypocarbia, this is not likely to indicate cerebral hypoxia as oxygen consumption does not change.

Brain cellular and mitochondrial respiration in media of altered pH

This study was designed to investigate the effects of altered pH on cellular aerobic energy metabolism in the immature and adult rat cerebral cortex. Cerebral cortical slice respiration was measured polarographically in acid and alkaline media. In separate experiments, the extracellular pH was changed by altering the HCO3- concentration or the intracellular pH and extracellular pH were changed by altering the CO2. Respiratory rates and oxidative phosphorylation in adult rat cerebral mitochondria also were measured in media with an altered pH. Increased intracellular pH inhibited respiratory rates in cortical slices from immature rats more than in tissue from adults. Decreasing the pH to 6.7 produced no changes in respiration in mature cortical slices and moderate inhibition of immature tissue respiration. In cerebral mitochondria, altered pH caused inhibition of State 3 respiration, respiratory control ratios, and ADP/O ratios. These changes were greater and occurred with smaller pH changes in the alkaline compared to the acid direction. From the results of these studies, we conclude that brain cellular respiration is not affected by moderate decreases in intracellular pH. With increased pH, there is inhibition of cellular and mitochondrial respiration, which may be the mechanism for the rise in lactic acid previously observed to result from hypocarbia in vivo.

Cerebral effects of extended hyperventilation in unanesthetized goats

Thirty-six adult, male unanesthetized goats were hyperventilated to a PaCO2 level of 16-18 mm Hg for 6 hours. Arterial and sagittal sinus blood and cerebrospinal fluid were analyzed for pH, blood gases, bicarbonate, lactate, and pyruvate before hyperventilation, during hyperventilation, and after the termination of hyperventilation. Total cerebral blood flow, regional brain blood flows, and cerebral metabolic rate for oxygen were calculated from the distribution of radioactive microspheres. Intracranial pressure was measured in either the right or left cerebral ventricle. With the initiation of hyperventilation, cerebral blood flow and cerebral metabolic rate for oxygen fell significantly (64 +/- 5 ml/100 g/min to 41 +/- 3; 4.6 +/- 0.3 ml O2/100 g/min to 3.6 +/- 0.2), but both returned to prehyperventilation values within 6 hours of hyperventilation. With termination of hyperventilation, cerebral blood flow and cerebral metabolic rate for oxygen increased significantly above control levels (64 +/- 5 vs. 105 +/- 9; 4.6 +/- 0.3 vs. 5.4 +/- 0.4). Intracranial pressure was unaffected by hyperventilation or its termination. Arterial and sagittal sinus blood and cerebrospinal fluid pH increased with hyperventilation but returned to control values by 6 hours. However, pH was still significantly elevated at 6 hours. Lactate and pyruvate followed a similar pattern except in the cerebrospinal fluid, where both increased throughout the course of hyperventilation. There were no significant differences in the lactate:pyruvate ratio. On termination of hyperventilation, pH of the arterial and sagittal sinus blood and cerebrospinal fluid fell below control levels. Bicarbonate values decreased in all fluid compartments and were still below control values 2 hours after the cessation of hyperventilation.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain intracellular pH and metabolism during hypercapnia and hypocapnia in the new-born lamb

1. The effects of hypercapnia and hypocapnia on brain intracellular pH (pHi) and metabolism were investigated in new-born lambs under barbiturate anaesthesia. 2. 31P nuclear magnetic resonance (n.m.r.) spectroscopy was used to determine brain pHi and the relative concentrations of compounds containing mobile phosphorus nuclei including phosphocreatine (PCr), nucleoside triphosphates (NTP) and inorganic phosphate (Pi). Simultaneous measurements were made of the molar ratio of glucose to oxygen uptake by the brain. 3. During normocapnia (arterial partial pressure of CO2 Pa, CO2, 39 +/- 1 mmHg mean +/- S.E. of mean, n = 9) brain pHi was 7.13 +/- 0.02. Hypercapnia (Pa, CO2, 98 +/- 3 mmHg) was associated with a fall in brain pHi to 6.94 +/- 0.03 (n = 19, P less than 0.001), whereas no significant change in brain pHi occurred during hypocapnia (Pa, CO2, 16 +/- 1 mmHg; brain pHi 7.15 +/- 0.01). 4. During hypercapnia there was an increase in the ratio of Pi to NTP from 1.09 +/- 0.08 to 1.47 +/- 0.06 (P less than 0.001) and a decrease in the ratio PCr/Pi from 1.60 +/- 0.08 to 0.93 +/- 0.04 (P less than 0.001). There was a linear correlation between Pi/NTP and brain pHi. 5. Alterations in arterial PCO2 had no significant effect on the molar ratio of glucose to oxygen uptake by the brain, which remained close to unity. 6. The change in brain pHi observed during hypercapnia can be accounted for by the known physico-chemical buffering capacity of brain tissue. Homoeostasis of brain pHi during hypocapnia provides further evidence that additional regulatory mechanisms operate in these circumstances. 7. The observed changes in PCr and Pi can be accounted for in part by the [H+] dependence of the creatine kinase reaction.

Effects of hyperventilation on pattern-reversal visual evoked potentials in patients with demyelination

The effects of hyperventilation on the pattern-reversal visual evoked potential (VEP) were studied in seven normal subjects and 13 multiple sclerosis patients with visual pathway involvement. Significantly greater reductions in P100 latency occurred in the multiple sclerosis patients than in controls and normalisation of the half-field response topography occurred in one patient after hyperventilation. The VEP changes are attributed to improved impulse transmission in demyelinated fibres in the visual pathway as a result of the alkalosis and changes in ionised calcium levels induced by hyperventilation.

31P NMR study of cerebral metabolism during prolonged seizures in the neonatal dog

The effects of prolonged bicuculline-induced seizures on cerebral blood flow and metabolism were determined in paralyzed, mechanically ventilated neonatal dogs. Transient changes occurring early in the course of status epilepticus included significant arterial hypertension, hypocarbia, elevation of plasma norepinephrine levels, and decline in brain glucose concentration. Cerebral blood flow remained elevated throughout the 45 minutes of seizure. Determination of cerebral metabolite values by in vivo phosphorus 31 nuclear magnetic resonance spectroscopy and by in vitro enzymatic analysis of frozen brain samples showed significant decreases in the level of phosphocreatine and relatively less change in ATP values. Progressive intracellular acidosis occurred, coincident with elevation of brain lactate concentrations. We conclude that the physiological and metabolic alterations that occur during prolonged seizures are not uniform, but change with time. Any hypothesis advanced to explain the mechanism of neuronal injury during prolonged seizures must take into account these temporally related changes.