Artery-to-vein differences in nitric oxide metabolites are diminished in sepsis

Abstract

Objective: Nitric oxide deficiency may contribute to microvascular dysfunction in sepsis. Current physiologic paradigms contend that nitrite and/or S-nitrosohemoglobin mediate intravascular delivery of nitric oxide. These nitric oxide metabolites are purportedly consumed during hemoglobin deoxygenation, producing nitric oxide and coupling intravascular nitric oxide delivery with metabolic demand. Systemic nitrite and S-nitrosohemoglobin consumption can be assessed by comparing their concentrations in arterial vs. venous blood. We hypothesized that arterial vs. venous differences in nitrite and S-nitrosohemoglobin are diminished in sepsis and associated with mortality.

Design: Case-control and prospective cohort study.

Setting: Adult intensive care units of an academic medical center.

Patients and subjects: Eighty-seven critically ill septic patients and 52 control subjects.

Interventions: None.

Measurements and main results: Nitrite and S-nitrosohemoglobin were measured using tri-iodide-based reductive chemiluminescence. In control subjects, arterial plasma, whole blood, and red blood cell nitrite levels were higher than the corresponding venous levels. In contrast, S-nitrosohemoglobin was higher in venous compared to arterial blood. In septic patients, arterial vs. venous red blood cell nitrite and S-nitrosohemoglobin differences were absent. Furthermore, the plasma nitrite arterial vs. venous difference was absent in nonsurvivors.

Conclusions: In health, nitrite levels are higher in arterial vs. venous blood (suggesting systemic nitrite consumption), whereas S-nitrosohemoglobin levels are higher in venous vs. arterial blood (suggesting systemic S-nitrosohemoglobin production). These arterial vs. venous differences are diminished in sepsis, and diminished arterial vs. venous plasma nitrite differences are associated with mortality. These data suggest pathologic disruption of systemic nitrite utilization in sepsis.

Figure 1

Enrollment flow-diagram for sepsis patients

Figure 2

Arterial and venous nitrite and SNOHb concentrations in control subjects vs. sepsis patients. Panel A: plasma nitrite; Panel B: WB nitrite; Panel C: RBC nitrite; Panel D: SNOHb. Concentrations are plotted on the log₁₀ scale. Dot plots show individual data points and corresponding summary plots show mean values with error bars representing standard deviation. Significance values above the bars refer to artery vs. vein comparisons. *p < 0.05 compared to corresponding venous concentration in control subjects.

Figure 3

Arterial and venous nitrite and SNOHb concentrations in sepsis survivors vs. non-survivors. Panel A: plasma nitrite; Panel B: WB nitrite; Panel C: RBC nitrite; Panel D: SNOHb. Concentrations are plotted on the log₁₀ scale. Dot plots show individual data points and corresponding summary plots show mean values with error bars representing standard deviation. Significance values above the bars refer to artery vs. vein comparisons. *p < 0.05 compared to corresponding venous concentration in survivors.

Figure 4

Proposed model for explaining diminished artery-to-vein NO metabolite concentration differences in sepsis. Panel A. Normal physiology. In resistance arterioles, RBCs become partially deoxygenated and the concentration of deoxyHb increases. DeoxyHb catalyzes the reduction of RBC nitrite (RBCNO₂⁻) to form N₂O₃ and FeNOHb (22, 25). N₂0₃ S-nitrosates Hb forming SNOHb, then exits the RBC through a membrane-associated protein complex where it decomposes to NO (and nitrogen dioxide, not shown), augmenting microvascular blood flow (28). Plasma nitrite (PNO₂⁻) enters the RBC to replenish RBCNO₂⁻ (42, 43). This metabolic pathway is reflected by NO metabolite concentrations changes in the draining vein. The net result is A-V differences in NO metabolite concentrations, with higher plasma and RBC nitrite (PNO₂⁻ and RBCNO₂⁻) in artery vs. vein, but lower SNOHb and FeNOHb in artery vs. vein (10, 12, 13, 22). Panel B. Sepsis. RBC nitrite metabolism is impaired either because of sepsis-associated derangements of the RBC membrane-associated proteins involved in nitrite reduction and NO export (gray color of membrane protein) or lower concentration of deoxyHb (smaller font) (30, 31). The production of SNOHb and FeNOHb decline (dashed arrows and smaller font), and there is diminished RBC NO release (dashed arrows and smaller font), impairing microvascular blood flow (smaller arrow). The defects in membrane-associated proteins and RBCNO₂⁻ metabolism are not yet severe enough to affect PNO₂⁻ uptake. The net result is diminished A-V differences in all metabolites except PNO₂⁻. Panel C. Lethal sepsis. The membrane-associated protein and metabolic defects are more severe, eliminating NO export (black color of membrane protein complex) and critically reducing blood flow (smaller arrow). The defects are now severe enough to impair PNO₂⁻ uptake by the RBC (gray color of membrane channel, larger PNO₂⁻ font). The net result is diminished A-V differences for all NO metabolites shown.