CLINICS 2012;67(5):503-508 to protect vital organs and reduce tissue O2 needs and subsequent hypoxia (11-15). Despite the activation of these protective mechanisms, it was shown that hypoxia modifies the NMDA receptor/ion channel complex (16) and increases protein tyrosine kinase activity in cortical cell membranes (17), whereas hypoxia decreases the activity of nitric oxide synthase in every region of the brain except the cortex (18). In the heart, perinatal hypoxia was reported to induce permanent changes in the structure and function of the left ventricle (19,20). Moreover, significant changes in left ventricular gene expression were reported in newborns, with physiologic implications for the adult myocardium (21). Rodents, sheep, and primates have been used as animal models of perinatal hypoxia (22-24). The pig is used more frequently because its heart is anatomically and physiolo- gically similar to that of the human (25,26). The coronary circulation and heart conduction system are similar in humans and swine (27,28). Consequently, humans and pigs have comparable histology under normal and pathologic conditions (29,30). Thirteen animals required more aggressive resuscitation with adrenaline and chest compressions. Although adrena- line may enhance myocardial damage due to its beta- adrenergic effects (31), the fact that adrenaline was administered in a subset of animals in all of the groups makes a possible association between adrenaline and the observed cardiac findings unlikely. To the best of the authors’ knowledge, chest compressions have never been associated with cardiac damage (32). Histological examination of the heart specimens of our asphyxiated piglets revealed pathological changes in all of the specimens. The most important changes were interstitial edema leading to disarrangement of the cardiac architecture, the wavy arrangement of cardiomyocytes and hypereosino- philia/coagulative necrosis among the cardiomyocytes. Although myocardial histological changes were observed in all four groups and observed to correlate with the fiO2 used for resuscitation, differences were not found between the groups but were detected among piglets within each group. In addition, the extreme O2 concentrations (18% and 100%) appear to be associated with higher apoptosis, whereas room air (21%) and moderate (40%) O2 concentra- tions appear to protect against apoptosis. These findings, taken together, suggest the existence of an individual sus- ceptibility to hypoxia, allowing each piglet to organize a personal adaptive response to oxygen deprivation, tailored to its biochemical and enzymatic profile. Moreover, reoxygena- tion and, in particular, the concentration of O2 used, do not represent key factors in the determination of cardiac injury in our experimental model. Additional interesting data on histological myocardial changes were obtained when the piglets were grouped according to resuscitation time. The analysis of the patholo- gical changes observed in group A animals allowed us to identify early and late lesions. Edema and hypereosinophilia were the typical early lesions, observed in animals with a very short recovery time. On the other extreme of the spectrum of recovery time, we found that cytoplasmic vacuoles were absent in all animals in group A; each animal also exhibited coagulative necrosis, hemorrhage, apoptosis, and neutrophil infiltrates, which are signs of mature lesions. The coagulative necrosis of cardiomyocytes deserves parti- cular attention, as this severe heart lesion was very rare in 507 Cardiac changes following hypoxia Faa A et al. animals in group A; its incidence progressively increased to 57% in animals with a medium recovery time, reaching 100% in piglets with slow and very slow recovery times. The association between the incidence of coagulative necrosis and recovery time post-asphyxia clearly indicates coagulative necrosis to be the most important pathological change occurring in the hearts of newborn piglets following hypoxia. The low incidence of coagulative necrosis in deceased animals might be attributed to the short survival time, as coagulative necrosis requires more than 15 min. Another lesion worth mentioning is the presence of vacuoles in the cardiomyocyte cytoplasm. Heart cell vacuolization was not detected in any of the animals in the very fast recovery group, whereas cytoplasmic vacuoles were present in cardiomyocytes in 57% of the animals in group B. This finding suggests that cardiac cell vacuolization may be a marker of hypoxic insult lasting more than 15 min. This finding apparently runs counter to data obtained from the investigation of renal lesions in the same animals (unpublished data), which demonstrate the presence of vacuoles in proximal and/or distal tubular cells in more than 50% of the animals in group A, indicating renal cell vacuolization as the cause of the kidney lesions that develop soon after acute hypoxia. This difference may be attributed to the adaptive mechanisms of mammals following hypoxic insults, a situation in which a life support system should be adopted. These decisions must be made quickly and prioritize the heart over the kidney. In conclusion, our data clearly indicate Landrace/Large White piglets as a favorable swine breed for experimenta- tion on neonatal hypoxia. The hemodynamic fluctuations at baseline, as reported by our group, during normocapnic hypoxia and reoxygenation in this animal model resulted in important myocardial histological changes, indicating the reliability of this model for the study of human neonatal hypoxia (10). Cardiac lesions appear to play an important role in asphyxiated newborns, with coagulative necrosis being the principal and most severe lesion that is strictly associated with recovery time, which differs widely among animals in the same group. This study suggests an inter- individual variability in the response of cardiac cells to asphyxia that is independent from the fiO2 used during resuscitation. Further studies are needed in the field of hypoxia-reoxygenation before the application of these findings in clinical practice. ACKNOWLEDGMENTS The authors would like to thank Miss. Patrizia Baire and Mr. Ignazio Ferru for secretarial assistance. The technical work of Mrs. Sandra Serra is also acknowledged. The financial support of ‘‘Fondazione Banco di Sardegna’’ (Cagliari, Italy) and ELPEN Research-Experimental Center (Athens, Greece) is kindly acknowledged. AUTHOR CONTRIBUTIONS Faa A analyzed and interpreted the data and prepared the manuscript. Iacovidou N and Xanthos T performed the experiment and critically revised the manuscript. Locci A processed the heart tissues. Pampaloni P provided assistance to the manuscript preparation and assembled the photos. Aroni F performed the experiment. Papalois A performed the experiment. Faa G conceived and designed the study. Fanos V critically revised the final manuscript version.