In vivo diffusion of lidocaine through tissue expanders

It has been suggested that the addition of lidocaine to the saline used to fill tissue expanders will reduce the pain often associated with the expansion process. In vitro experiments have shown that lidocaine as it is normally supplied will diffuse through an expander only at a very slow rate, which would probably be inadequate for a clinical effect. We found that the addition of sodium bicarbonate resulted in a substantial increase in the rate of diffusion. Studies in rabbits demonstrated that at a pH of 8.0, 75 percent of the lidocaine dose crossed the silicone elastomer membrane at 24 hours and greater than 95 percent had left the expander at 1 week. We have concluded that intraluminal lidocaine can be effective only when the pH is close to the pKa of lidocaine.