Pearson correlation between plasma arginine and kyotorphin levels (μM) for all available time point values. A strong correlation exists between plasma arginine and plasma kyotorphin concentrations (r = 0.72; P < .0001). When an outlier time point with a peak kyotorphin level of 5.0 μM was excluded from the analysis, the correlation was even stronger (r = 0.77; P < .0001). Filled circles represent data at 1, 1.5, and 2 hours after the initiation of arginine infusion, reflective of the significant acute increase in plasma kyotorphin levels. Unfilled circles represent time 0 (predose), 4 and 8 hours after initiation of arginine infusion, and daily values for patients remaining in the hospital.