Cerebral hemodynamics and metabolism during infant cardiac surgery. Mechanisms of injury and strategies for protection

Noninvasive cerebral perfusion imaging in high-risk neonates

Advances in medical and surgical care of the high-risk neonate have led to increased survival. A significant number of these neonates suffer from neurodevelopmental delays and failure in school. The focus of clinical research has shifted to understanding events contributing to neurological morbidity in these patients. Assessing changes in cerebral oxygenation and regulation of cerebral blood flow (CBF) is important in evaluating the status of the central nervous system. Traditional CBF imaging methods fail for both ethical and logistical reasons. Optical near infrared spectroscopy (NIRS) is increasingly being used for bedside monitoring of cerebral oxygenation and blood volume in both very low birth weight infants and neonates with congenital heart disease. Although trends in CBF may be inferred from changes in cerebral oxygenation and/or blood volume, NIRS does not allow a direct measure of CBF in these populations. Two relatively new modalities, arterial spin-labeled perfusion magnetic resonance imaging and optical diffuse correlation spectroscopy, provide direct, noninvasive measures of cerebral perfusion suitable for the high-risk neonates. Herein we discuss the instrumentation, applications, and limitations of these noninvasive imaging techniques for measuring and/or monitoring CBF.

Postmortem intracranial neuropathology in children following cardiac transplantation

At Loma Linda University Children's Hospital, the medical information of 405 pediatric patients who received orthotopic cardiac transplantation were reviewed. Of those who died (n=136), 86% (n=117) underwent postmortem examinations, and the brain was examined in 61% (n=82, male=39). The number and type of intracranial lesions present were compiled, and these were matched to underlying functional cardiac disease categories. Intracranial abnormalities were present in 87%. Infarct was the most common primary central nervous system pathology in hypoplastic left heart syndrome (41%) but was also observed frequently in children with obstructive lesions (37%), cyanotic disease (31%), or cardiac shunting (29%). Secondary findings included extraparenchymal hemorrhage in obstructive lesions (31%); hypoxic changes occurred in 15% of patients with cyanotic disease and in 14% of those with cardiac shunting. Thirty-three percent of children with restrictive lesions had no neuropathology reported. Postmortem examination brain weights were matched against age and sex norms, with 29% of females and 36% of males below two standard deviations. These findings revealed that intracranial pathology was present in the majority of transplanted children who underwent postmortem examination, and that infarctive changes constituted the most common neuropathologic abnormality. Additionally, a number of children had significantly reduced brain weight.

Hypoxic-ischemic brain injury in infants with congenital heart disease dying after cardiac surgery

Cardiac surgery for congenital heart disease is performed increasingly earlier in infancy, including in the neonatal period. With increased survival of infants, there is growing concern about the long-term neurological sequelae of hypoxic-ischemic injury due to congenital heart disease itself prior to surgery, corrective surgery with the use of low-flow cardiopulmonary bypass (CPB) and/or deep hypothermic circulatory arrest (DHCA), and/or unstable hemodynamic factors postoperatively. In analyzing the neuropathology of 38 infants dying after cardiac surgery, we tested a set of questions related to the severity and patterns of brain injury, CPB, DHCA, and age of the infants at the time of surgery. In all infants dying after cardiac surgery, irrespective of the modality, cerebral white matter damage [periventricular leukomalacia (PVL) or diffuse white matter gliosis] was the most significant lesion in terms of severity and incidence, followed by a spectrum of gray matter lesions. There was no significant association between the duration of deep hypothermic circulatory arrest and the degree of severity of overall brain injury, and the pattern of brain injury was similar irrespective of the modality of cardiac surgery. There was no significant association between the age at the time of surgery (neonatal versus postneonatal) and the severity of overall brain injury. The patterns of brain injury were not age-related in the limited time-frame analyzed, except that infants who developed acute PVL after both closed and DHCA/CPB surgery (14/38 infants, 34%) were significantly younger at death (median age 13.0 days) compared to unaffected infants (median age at death 42.5 days) (P=0.031). This observation suggests that neonatal (<30 postnatal days), but not postneonatal (>30 postnatal days), brains are at risk for acute PVL, and likely reflects the vulnerability of immature (pre-myelinating) white matter to hypoxia-ischemia.

Systemic venous oxygen saturation after the Norwood procedure and childhood neurodevelopmental outcome

OBJECTIVE

Neonates with hypoplastic left heart syndrome have impaired systemic oxygen delivery and also have a high risk of hypoxic ischemic brain injury with resultant neurodevelopmental impairment. We hypothesized that decreased postoperative oxygen delivery, as measured on the basis of systemic venous oxyhemoglobin saturation, would be related to persistent neurodevelopmental abnormality assessed in childhood.

METHODS

Early perioperative hemodynamic data, prospectively acquired from neonates undergoing staged palliation of hypoplastic left heart syndrome by using deep hypothermic circulatory arrest with uniform perioperative management, were tested for relationship to later neurodevelopmental outcome assessed at age 4 years.

RESULTS

Complete hemodynamic and neurodevelopmental data were available in 13 patients aged 7 +/- 8 days at the time of the Norwood procedure and aged 4.5 +/- 0.7 years at follow-up assessment. The subjects scored significantly below the population mean for motor, visual-motor integration, and composite neurodevelopmental outcomes. The 5 (38%) patients with abnormal outcomes had significantly lower postoperative systemic venous oxygen saturation values than those with normal outcomes (46% +/- 8% vs 56% +/- 6%, P = .024). Standard hemodynamic parameters did not differentiate patient outcomes. The risk of abnormal outcome increased with increasing time at a systemic venous oxygen saturation of less than 40% (P < .001). A multivariate model of deep hypothermic circulatory arrest time, systemic venous oxygen saturation, blood pressure, and carbon dioxide tension accounted for 79% of the observed variance (P < .001).

CONCLUSIONS

Decreased systemic oxygen delivery in the neonatal postoperative period is associated with hypoxic-ischemic brain injury and childhood neurodevelopmental abnormality. Measures of systemic oxygen delivery should be used to guide perioperative strategies to reduce the risk of hypoxic-ischemic brain injury.