Modification of lidocaine protein binding with CO2.

Using new, specially designed ultrafiltration devices and an enzyme immunoassay technique, the authors determined the effect of carbon dioxide tension on the fractional binding of lidocaine to human plasma proteins. Identical samples of serum at therapeutic (2.2 micrograms X ml-1) and toxic (6.8 micrograms X ml-1) lidocaine concentrations were tonometered at 37 degrees C to CO2 tensions between 0.13 and 10.7 kPa (1.0 to 80.5 mmHg). The fraction of unbound lidocaine increased linearly with increasing pCO2 (r = 0.93, p less than 0.001). These changes help to explain the increased central nervous system toxicity of lidocaine associated with hypercarbia.