What is the test for discharge of a juror in a civil case?

Penal Code section 1089 provides, in pertinent part: "If at any time, whether before or after the final submission of the case to the jury, a juror dies or becomes ill, or upon other good cause shown to the court is found to be unable to perform his duty, or if a juror requests a discharge and good cause appears therefor, the court may order the juror to be discharged and draw the name of an alternate, who shall then take a place in the jury box, and be subject to the same rules and regulations as though he had been selected as one of the original jurors." (See Code Civ. Proc., 233, 234.) "We review for abuse of discretion the trial court's determination to discharge a juror and order an alternate to serve. If there is any substantial evidence supporting the trial court's ruling, we will uphold it. [Citation.] We also have stated, however, that a juror's inability to perform as a juror ' "must appear in the record as a demonstrable reality." ' " (People v. Marshall (1996) 13 Cal.4th 799, 843.)