Four months after the Nashoba Valley Medical Center in North Central Massachusetts shuttered, emergency medical services in the surrounding communities are "on the verge of collapse," 13 local fire chiefs wrote in a recent letter to the state. The letter, sent Dec. 27 from the chiefs, their towns' leaders, and state lawmakers, urged Governor Maura Healey's administration to include in the state's budget proposals $9.6 million over the next two fiscal years to increase emergency response staffing and sustain overtime pay. The increasingly dire situation described in the letter highlights the continuing impact of the bankruptcy of Steward Health Care, the for-profit company that closed both the Nashoba Valley hospital in Ayer and Carney Hospital in Dorchester on Aug. 31, while in the process of selling its other Massachusetts hospitals to new operators.