THE article appearing in this issue of Anesthesiology by Ditsworth et al. 1reports that cell death in the brains of piglets subjected to 90 min of deep hypothermic circulatory arrest (DHCA) is largely apoptotic, accompanied by activation of caspases 3 and 8, as well as early release of cytochrome c and the presence of Fas. This article raises several issues of interest to anesthesiologists. These include 1) the potential danger of DHCA to the developing brain and whether we can do anything to better protect the brains of these infants during surgery, 2) the role of development in determining the mechanisms of brain injury, and 3) the role of development in susceptibility to brain injury and sensitivity to neuroprotective interventions.
Although DHCA clearly provides significant protection to the brain and other organs from ischemia, the results of this article and prior reports 2–4demonstrate that DHCA for a sufficient duration does result in brain injury. This finding is not surprising, because the extent of brain injury resulting from ischemia must in part depend on the duration of the ischemic insult, even in the presence of hypothermia. This work and that of others has shown that 60–90 min of DHCA is sufficient to cause brain cell death. Further investigation of the effects of deep hypothermia alone are needed, as well as investigation into the use of low-flow perfusion versus intermittent perfusion as a way to protect the brain but still permit adequate surgical conditions.
The results reported by Ditsworth et al. 1focus on the mechanism of brain cell death following DHCA. Their observations strengthen the argument that much of the cell death is apoptotic by demonstrating activation of caspases 3 and 8, as well as cytochrome c release from mitochondria, a step often necessary for activation of the caspase proteases that kill the cell. Making the distinction between different types of cell death is not a purely academic one, since different mechanisms of cell death may suggest different strategies for protection. Necrotic cell death involves swelling rather than condensation of the cell and internal organelles, random DNA fragmentation, early disruption of organelles without formation of apoptotic bodies, and early loss of plasma membrane integrity.
In contrast, apoptosis is a type of cell death with a distinct morphology consisting of nuclear condensation, early preservation of nuclear and cytoplasmic membranes, and relative preservation of cellular organelles. 5Apoptotic cell death plays a key role in the normal development of the central nervous system. 6As each region develops, the number of cells is reduced from the number initially generated so that the number of different types of neurons is appropriate, 7and the number of astrocytes and oligodendrocytes is matched to the number of neurons and axons. This process results in developmentally determined vulnerable periods for specific cell populations. For example, cerebral white matter injury consisting of periventricular leukomalacia and hypomyelination are the anatomic correlates of cerebral palsy. These forms of brain injury are thought to be due to the specific vulnerability of premyelinating oligodendrocytes in the mid to late third trimester of human pregnancy when ischemia or infection/cytokine exposure may result in excessive loss of oligodendrocytes and a reduced number of mature myelinating oligodendrocytes. 8,9This response to ischemia is not seen in older patients or animals, suggesting that the tendency of a cell to undergo apoptosis is developmentally regulated. This concept is further supported by the recent observation that expression of caspase 3 decreases during postnatal development, 10but it increases in very old animals. 11In addition, the pro-apoptotic Bax molecule and associated release of cytochrome c is increased in brain mitochondria from immature compared to mature animals, further indicating that early postnatal brain cells are primed to undergo apoptosis. 12
Thus an important aspect of understanding brain injury due to cerebral ischemia requires understanding the role played by development. Although several investigators argue against a role for apoptosis in adult brain ischemia, there is much better agreement that apoptosis plays an important role in the response to ischemia in the perinatal period. Work from several laboratories studying normothermic ischemia clearly suggests that apoptotic cell death in the brain is developmentally regulated, with apoptosis being readily detected after models of perinatal hypoxia/ischemia, but less prominent in adult models of cerebral ischemia. 13–22
Many genes involved in the cell death process have been identified. Many biochemical changes and specific signaling pathways have been shown to participate in this process. Apoptosis may result from imbalances in signaling pathways (such as lack of growth factors), may be initiated by activation of membrane receptors, and has several potential pathways for execution. These include (1) activation of proteases called caspases that carry out the cell death, 23–26(2)participation of mitochondria in the release of proapoptotic proteins, 27,28and (3) regulation by the bcl-2 family of proteins. 29
Several steps in the apoptosis cascade have provided new ways to reduce ischemic cell death in models of cerebral ischemia. Caspase inhibitors and overexpression of antiapoptotic regulatory proteins (such as bcl-2) have been shown to be effective at reducing ischemic brain injury in animal models. 30–35Despite this type of evidence, due to the heterogeneity of the morphologic picture in cerebral ischemia, there is still disagreement about the extent to which cell death during stroke involves apoptosis, necrosis, or a combination of both. 36–40
Complicating our understanding of the mechanisms of cell death are recent findings that suggest that there are multiple methods of genetically controlled cell death and that the morphologic picture of apoptosis does not always correlate with activation of caspases nor does the appearance of a necrotic death rule out an active genetically determined type of cell death. Genetically determined types of cell death independent of caspase activation have been described, 41,42which may still display the cellular morphology of apoptosis. Cell death in which activation of caspases is important but results in necrosis-like morphology has also been reported. 43Recent data suggest that both caspase-dependent and caspase-independent forms of cell death are involved in cerebral ischemia. 44
In addition to changes in the mechanisms of brain cell loss with development, the effect of the same insult changes with age. Vulnerability or the extent of injury observed in response to an ischemic insult increases as a function of age. Vulnerability to ischemia changes rapidly with age in the perinatal period as demonstrated in a study of combined focal ischemia–hypoxia in rat between the ages of postnatal days 1 and 7. 45Brains of postnatal day 5 rats showed markedly less injury than did brains of postnatal day 7 animals. Similar changes in response to injury have also been seen in brain cell cultures. 46Thus, although a given duration of ischemia may result in less severe injury in an infant, because this deficit will be present throughout life, it is still a matter of great concern.
Understanding differences in the mechanisms of brain injury provoked by ischemia in neonates compared to adults will lead to the development of age-specific protective strategies. At this time, a great deal remains to be learned about age-specific responses to cerebral ischemia, and the efficacy of potential protective strategies should be evaluated in both perinatal and adult models of cerebral ischemia.
This Editorial View accompanies the following article: Ditsworth D, Priestley MA, Loepke AW, Ramamoorthy C, McCann J, Staple L, Kurth D: Apoptotic neuronal death following deep hypothermic circulatory arrest in piglets. Anesthesiology 2003; 98:1119–27.