Sentence:  The reinforcement of anesthesia by an epidural “<WSD>top-up</WSD>” in combined spinal-epidural anesthesia may be explained by a dual mechanism: a volume effect compressing the dural sac and a local anesthetic effect. 
Meaning of the word marked with <WSD> tags:  A dose of epidural anesthetic added to previously injected spinal anesthetic in combined spinal-epidural anesthesia