Preconditioning Against Periventricular Leukomalacia Associated with Neonatal Cardiac Surgery

Doctor's Name: 
Edward Hickey, M.D.
Doctor's Name 2: 
Glen Van Arsdell, M.D.
The Hospital for Sick Children

“The use of LPS to induce sustained protection against white matter brain injury in neonatal heart surgery.”

Brain surgery is alarmingly common following the surgical repair of congenital heart disease in infancy. Neurobehavorial abnormalities occur in as many as 25-30% and sub-normal IQ is the norm following complex repair. Severe spasticity ( cerebral palsy ) and coma are fortunately less common in the present era, but movement abnormalities are nevertheless present in over a third of patients one week following repair using heart-lung bypass ( cardiopulmonary bypass, CPB ). CPB involves two main insults: periods of reduced oxygen ( H-I ) and high levels of inflammatory mediators ( TNFa). Therefore there is considerable interest in developing methods of protecting against brain injury during infant heart surgery.

Preconditioning is an experimental technique that provides robust tolerance against brain injury caused by H-I ( lack of oxygen ) in animals. There are several methods of inducing preconditioning , but among the best characterized is a derivative of bacteria, known as lipopolysaccharide ( LPS ). If LPS is administered in extremely low doses ( below that which causes overt ill health ), robust protection ( size of stroke is reduced by approx. 50% ) is afforded within 24 hours that lasts in excess of 7 days. LPS preconditioning is therefore attractive as a therapeutic technique because protection might be achieved that spans the entire clinical episode. This is important because we now realize that brain injury following infant heart surgery is attributable to events within the entire intensive care period and not just following the surgical window.

It has been demonstrated that LPS preconditioning induces robust protection against brain injury occurring as a result of deep hypothermic circulatory arrest ( DCHA, a type of CPB strategy involved H-I ) on CPB. Dr’s Van Arsdell & Hickey blindly preconditioned neonatal piglets with either intravenous saline or low-dose LPS. Three days later they were placed on CPB and subjected to a clinical perfusion strategy involving DCHA. After separation from CPB they were supported, anesthetized and ventilated in a carefully controlled laboratory intensive care environment for 24 hours. The brains were then extracted and analyzed for evidence of neuronal cell death in the grey matter. Significantly reduced injury was observed in those animals that were preconditioned with LPS. These preliminary efforts confirmed that 1) LPS preconditioning can be successfully applied to CPB-related brain injury, and 2) neonatal large mammals are amenable to LPS preconditioning.

However, we are increasingly recognizing that brain injury following neonatal cardiac surgery is white matter injury, otherwise known as PVL. PVL is a specific type of infant brain injury that arises because of a specific vulnerability of immature white cells – known as preOLs – to H-I and inflammation. In fact, inflammation alone ( in the absence of H-I ) can trigger PVL in vitro in neonatal brain tissue.

The researchers propose to examine where the very immature neonatal brain can be preconditioned against PVL using LPS. They plan to examine 2 very young developmental time windows in newborn rats ( known as P1 & P7 models ).

The first part of the proposal aims to explore any differential susceptibility to LPS preconditioning between the P1 & P7 brain models. Because the P1 model will have a higher proportion of preOLs, the researchers suspect that these brains may require a particularly low dose of LPS to precondition. It is possible that the P1 brains will NOT be amenable to preconditioning, or instead that the very low-dose LPS actually precipitates injury in these brains.

The second part of the proposal aims to explore the effect of exogenous TNFa on PVL injury caused by H-I in vivo. Because CPB liberates high levels of circulating and intra-cerebral TNFa, the consequences of this on white matter injury in the neonate are extremely important. In particular, the researchers wish to explore the ability of LPS preconditioning to protect against TNFa-induced amplification of PVL. The researchers hypothesize that LPS preconditioning  renders TNFa-signaling axis refractory to activation by the exogenous TNFa load in the neonatal brain. This possibility has exciting implications because TNFa is likely an important instigator of CPB-related injury in the neonate.

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