Zoll’s stock has worst day since 2004

It was a heart-stopper of a day for Zoll Medical Corp.: Shares of the Chelmsford company plunged 27 percent, the most since April 2004. The maker of defibrillators for hospitals and ambulances said its LifeVest product, which is worn by patients at risk for sudden cardiac arrest and accounts for about 22 percent of the company’s revenue, could face limits on Medicare reimbursements.