TABLE IV.
Factors changing in the postnatal environment (non-exhaustive)
| Oxygen tension |
| Before birth, the arterial oxygen tension is only = 30 mmHg and increases around 3-fold after birth to 100 mm Hg. This has a large impact on metabolism, and the heart transitions from a relatively hypoxic environment where glycolysis is the primary form of energy production to oxidative phosphorylation after birth.97,109 This also creates reactive oxygen species affecting cardiac maturation and additionally has widespread consequences throughout the body.110 |
| Catecholamines (norepinephrine, epinephrine, and dopamine) |
| There is a release of catecholamines following birth to: (1) increase cardiac output, (2) stimulate gluconeogenesis and glycogenolysis in the liver, (3) release free fatty acids, and (4) regulate blood pressure.111 |
| Metabolic substrates |
| Energy production undergoes a switch, as there is a shift from a carbohydrate based to a fatty acid dominated metabolism the newborn starts feeding on breast milk. This induces widespread physiological adaptations such as induction of mitochondrial biogenesis, gluconeogenesis, glycogenolysis, and ketogenesis in the liver to supply other metabolic substrates.112 |
| Serum proteome |
| The serum proteome undergoes major changes after birth.113 Considerable work is required to determine the impact of these changes and which factors are important for cardiac maturation. |
| Cellular composition |
| The cellular composition in the myocardium changes during postnatal maturation. There are varying estimates in the percentage of cells that are cardiomyocytes; however, the general consensus is that the fraction of cardiomyocytes decreases during the maturation period.114–116 The stromal fraction also changes during postnatal maturation;117 in some papers, this has been shown to change from 52:41:6.5 to 52:25:20 endothelial cells:fibroblasts:leukocytes during postnatal maturation and is thus a major change in the fibroblast to leukocyte/macrophage ratio.65 It is still unclear how this influences cardiac maturation and how the different cell populations interact with each other, but there is increasing evidence of complex interplay between the cell types.118 |