Microdialysate nitrate/nitrite levels following severe head injury

Nitric oxide (NO) has important regulatory functions within the central nervous system. The purpose of this study was to measure the concentration of nitric oxide in the brain after severe traumatic brain injury. NO is oxidized in vivo to nitrate and nitrite. Measurement of these products gives an index of NO production. Laboratory studies have shown a good correlation between NO measured directly with an electrode, and indirectly by microdialysis nitrate/nitrite. Using chemiluminescence method we measured nitrate/nitrite levels in 2024 microdialysate samples obtained from 24 patients during the first five days following severe head injury. We used CMA 70 probe (AB Microdialysis, Sweden), perfused by normal saline at a rate of 2 μl/min. The median values of nitrate/nitrite for the whole group were highest on day 1 and gradually decreased over the 5 day monitoring period (day 1 - 19.2 μmol/l, day 5 - 12.7 μmol/l). Average values were lowest in the patients that died of their injury (14.3 μmol/l), and highest in patients who recovered by 3 months after injury with a moderate or severe disability (25.8 μmol/l or 31.9 μmol/l). In addition, there was a strong interaction between the severity of neurological injury and the change in dialysate nitrate/nitrite over time. The results suggest that nitric oxide may have a role in secondary injury mechanisms, but that this role is complex and varies as the injury evolves over time.