As mentioned above, we add bicarbonate to those drugs prepared at a particularly acidic pH (lidocaine, chloroprocaine) to speed onset of anesthesia (it also reduces burning when making a skin wheal).
We might add epinephrine to the local anesthetic solution to (i) prolong the dur-ation of anesthesia, particularly for vasodilating local anesthetics such as lido-caine; (ii) reduce peak plasma concentration of the local anesthetic, also more important for vasodilating agents; (iii) increase the density of regional anesthetic
blocks (by an unknown mechanism); and (iv) as a marker for intravascular injec-tion. Because of epinephrine instability in an alkaline environment, commercial local anesthetic preparations containing epinephrine are highly acidic. We can add bicarbonate, and/or use plain local anesthetics to which we add epinephrine ourselves. Remember that 1:200 000 epinephrine is only 5 mcg/mL – use a tuber-culin syringe and measure carefully! Important note: because we fear necrosis of the tip we do not add epinephrine to blocks placed at an “end organ,” e.g., digits, penis, nose, ears.
Through unclear mechanisms, small doses of clonidine enhance and prolong regional anesthesia. One mcg/kg added to the local anesthetic for a Bier block appears to delay the onset of tourniquet pain. In epidural and spinal anesthesia, 50 to 75 mcg clonidine has been found to augment the effect of both local anesthetics and opioids.
We add opioids to neuraxial anesthetics to prolong the analgesic effect. Man-ageable side effects include itching, nausea, and vomiting. Respiratory depres-sion, though less common, concerns us greatly, and we usually employ pulse oximetry on the post-surgical ward. Neuraxial morphine carries a risk of delayed respiratory depression, so we continue to monitor about 24 hours after the last dose.