(Reuters) - U.S. hospital admissions in November were the weakest in more than a decade, under pressure from a change in reimbursement rules for Medicare patients and confusion tied to the problem-ridden rollout of Obamacare, according to a survey by Citi Research.

New billing rules for the Medicare program for the elderly and disabled require hospitals to treat patient stays lasting less than "two midnights" as an outpatient visit.

"In addition, it is reasonable to conclude that the cumulative impact of changing physician employment and payment models is beginning to play a role, as well as the paralyzing effect of the impotent Obamacare rollout," Citi analyst Gary Taylor said in a report.

Hospital inpatient admissions in November fell to their weakest level in more than a decade, based on responses to the bank's monthly survey of 98 hospitals, Taylor said.

In October and November combined, admissions were down 4 to 5 percent from a year ago, which will likely weigh on hospital operators' fourth-quarter earnings and 2014 forecasts, he said.

Shares of hospital operators, which have climbed throughout the year on an expected increase in demand for healthcare when the health reform law kicks in, slumped on Tuesday afternoon. HCA Holdings Inc fell 2.9 percent to $45.81, Community Health Systems Inc declined 2.9 percent to $38.11, and Tenet Healthcare Corp was down 1.9 percent at $39.80.

(Reporting by Susan Kelly in Chicago; Editing by Leslie Adler)