Mechanistic Insights into Cell-free Hemoglobin-induced Injury During Septic Shock

Cell-free hemoglobin (CFH) levels are elevated in septic shock and higher in non-survivors. Whether CFH is only a marker of sepsis severity or is involved in pathogenesis is unknown. To investigate whether CFH worsens sepsis-associated injuries and to determine potential mechanisms of harm. Methods&Results:Fifty-one, 10-12kg purpose-bred beagles were randomized to receive Staphylococcus aureus intrapulmonary challenges or saline followed by CFH infusions (oxyhemoglobin >80%) or placebo. Animals received antibiotics, and intensive care-support for 96h. CFH significantly increased mean pulmonary arterial pressures and right ventricular afterload in both septic and non-septic animals, effects that were significantly greater in non-survivors. These findings are consistent with CFH-associated nitric oxide (NO) scavenging and were associated with significantly depressed cardiac function, and worsened shock, lactate levels, metabolic acidosis and multi-organ failure. In septic animals only, CFH administration significantly increased mean alveolar-arterial oxygenation gradients, also to a significantly greater degree in non-survivors. CFH associated iron levels were significantly suppressed in infected animals, suggesting that bacterial iron uptake worsened the pneumonia. Notably, cytokine levels were similar in survivors and non-survivors and not predictive of outcome. In the absence and presence of infection, CFH infusions resulted in pulmonary hypertension, cardiogenic shock and multi-organ failure, likely through NO scavenging. In the presence of infection alone, CFH infusions worsened oxygen exchange and lung injury, presumably by supplying iron that promoted bacterial growth. CFH elevation, a known consequence of clinical septic shock, adversely impacts sepsis outcomes through more than one mechanism and is a biologically plausible, non-antibiotic, non-cytokine target for therapeutic intervention.