Plasma protein binding of lidocaine during hypothermic conditions

The effect of hypothermia on the protein binding of lidocaine in plasma from six healthy volunteers was studied in vitro. A significant increase (p <0.001) in the unbound fraction of lidocaine was found at 24°C (38.3 [SD 7.0] %) compared to 37°C (30.7 [SD 6.0] %). A further reduction in temperature to 10°C resulted in a slight but not statistically significant increase in the free fraction of the drug (39.9 [SD 10.2] %). As a conclusion, hypothermia enhances the transfer of lidocaine over the blood-brain barrier. In order to avoid unwanted side effects or frank toxicity, caution is recommended when administering highly protein-bound drugs during hypothermic conditions.

Effects of pronounced haemodilution on the plasma protein binding of lidocaine

The effects of pronounced haemodilution on the protein binding of lidocaine was investigated in vitro in plasma from five healthy adult volunteers. The plasma was diluted with a phosphate buffer to reach a plasma protein concentration normally seen during paediatric cardiopulmonary bypass (CPB) and protein binding was determined at a low (1.5 micrograms/ml) and a moderate (4 micrograms/ml) total plasma concentration of lidocaine. The effects of different haematocrits on plasma protein binding was also determined over the haematocrit range 20-60%. The binding of lidocaine was found to be inversely related to the degree of dilution, i.e. the free fraction increased significantly with increasing dilution (p < 0.0001). Furthermore, the binding was dependent on the total plasma concentration of lidocaine, since a significantly higher percentage of free drug was found at the higher total lidocaine level (4 micrograms/ml) compared with the lower level (1.5 micrograms/ml) (p < 0.05). No significant difference in the free fraction of lidocaine could be found over the studied haematocrit range. The results of the present study indicate that plasma protein levels commonly associated with CPB in neonates and infants are associated with a significant increase in the free, unbound and pharmacologically active fraction of lidocaine compared with normal conditions. The use of commonly recommended dosages of lidocaine might result in toxic-free concentration in this setting.