Hypoxic brain and heart injury thresholds in piglets.

Both heart and brain are at risk for damage from asphyxia. However, these 2 organs' relative injury-thresholds have remained poorly defined. The present study using 16 anesthetized newborn piglets attempts to separate brain and heart damaging exposures from those that leave these organs unaffected. The hypoxic exposure (mean PaO2 = 3.6 +/- 0.6 kPa (27 mmHg) lasted for an average duration of 40 minutes and was associated with hypotension less than 4.7 kPa (35 mmHg) MABP. For brain damage assessment, 9 piglets that survived greater than 12 hours following resuscitation permitting histologic evaluation were used. For heart outcome assessment, those piglets that developed a postexposure, secondary hypotension to less than 4.7 kPa (35 mmHg) were compared to those without excluding 3 with uncertain cause of death. BRAIN-RESULTS: Six piglets remained brain intact while 3 exhibited brain edema and diffuse neuronal damage. The damaged animals' exposures differed from those that remained brain intact in sustaining significantly lower lowest MABPs (1.6 +/- 0.1 vs 3.3 +/- 0.4 kPa (12 vs 25 mmHg] and longer durations of MABP below 3.3 kPa (25 mmHg): 6 vs 1 min. and below 2.7 kPa (20 mmHg): 4 vs 0 min. HEART-RESULTS: Six of 13 animals developed marked delayed post-exposure hypotension requiring 5 to be killed prior to complete cardiovascular collapse. The only significant difference observed during exposure differentiating the two outcome groups (blood pressure maintenance vs cardiogenic shock) was the latter's more marked systemic acidosis (lowest mean arterial blood pH: 6.61 +/- 0.10 vs 6.91 +/- 0.10). These results suggest the brain is at risk for damage during hypoxia only when accompanied by an extreme lowering of blood pressure and the heart when accompanied by a severe acidosis. Further, the heart and brain need not both be damaged by the same hypoxic exposure. Contrary to common belief, the brain is not readily damaged from hypoxia alone absent marked circulatory changes.