Nurses at St. Vincent Hospital are planning to walk off the job Friday because of stalled negotiations for a new contract at the downtown medical center, but their dispute goes beyond Worcester and reflects a disagreement between nurses' unions and hospital managers nationally about staffing. Unions in Maine, Illinois, California and Minnesota have raised complaints about staffing levels in contract negotiations recently in what hospital officials describe as a national union agenda and what nurses call concern for their patients. The issue generally boils down to ratios. Nurses' unions have argued that the way to guarantee safe staffing is to limit the number of patients handled by nurses on a unit. Hospital officials say ratios are too rigid and fail to account for complexities such as the skills of nurses assigned to particular units and the severity of patients' illnesses. The Massachusetts Hospital Association collects voluntary filings on staffing levels from hospitals and posts them on its website, but it opposes fixed ratios at hospitals.