Renal Function

Renal function has a wide physiologic reserve. Despite predictable decreases in renal blood flow accompanying neuraxial block, the decrease is of little physiologic importance.^[56] One aspect of genitourinary function that is of clinical importance is the belief that neuraxial blocks are a frequent cause of urinary retention, which delays discharge of outpatients and necessitates bladder catheterization in inpatients. Lower concentrations of local anesthetic are necessary for paralysis of bladder function than for motor nerves to lower extremities. However, some studies do not support this belief. For example, in orthopedic patients undergoing hip replacement, it was demonstrated that bladder catheterization was no more frequent after neuraxial (spinal or epidural) block than after general anesthesia and narcotic analgesics.^[57] In any case, it is prudent to avoid administration of excessive volumes of crystalloid solutions intravenously to patients undergoing spinal anesthesia.