Reversible ischemic injury means the myocyte can recover normal structure and function when flow is restored. Irreversible injury means it cannot, and the earliest reliable marker of that point is loss of sarcolemmal integrity.
Cellular criteria that separate the two states
Reversible injury
- Sarcolemma intact; ionic gradients can be re-established
- Mitochondria swollen but structurally intact; ATP synthesis can resume
- Nucleus and contractile proteins largely preserved
- Cytosolic enzymes retained inside the cell
- Function returns after reperfusion
Irreversible injury
- Sarcolemma breached; gradients cannot be restored
- Mitochondria contain amorphous dense deposits; permanently uncoupled
- Nucleus and myofilaments degraded by calcium-activated enzymes
- Cytosolic enzymes leak into interstitium and blood
- No recovery even with full reperfusion
Why the threshold is not a single number
The transition depends on how far the ATP deficit and calcium load have progressed, which varies with the metabolic demand of the region and with collateral flow. Subendocardial myocytes, already the most vulnerable to reduced perfusion, cross the threshold sooner than epicardial myocytes. This is why the same duration of occlusion produces different infarct sizes in different patients and why the boundary between reversible and irreversible tissue is a moving front rather than a fixed line.