The Development and Performance of an Ambulance Stretcher Suspension

Abstract

The deterioration of the medical condition of stretchered patients during ambulance transport is discussed. The dominant vibration is generally low frequency (below 10 Hz) and in the vertical direction. Assessment of the sensitivity of supine patients to front-to-back vibration is referred to. Methods of alleviating the vibration experienced by stretchered patients are discussed and particular types of stretcher suspension described. This suspension has a constant natural frequency of vertical oscillations over a wide range of patient mass. A stretcher ride height (above the ambulance floor) independent of patient mass is achieved. The stretcher suspension was tested over four types o surface (one ofl-highway), using a commercial vehicle having a chassis similar to that of a commonly used ambulance. Vibration of the vehicle floor and of the stretcher in both the vertical and the pitch senses was measured using accelerometers. Transmissibility of the stretcher suspension, the stretcher frame and the stretcher mattress was obtained. The vertical vibration level (averaged over the four types of surface) experienced by the patient is compared with vibration tolerance levels for supine patients. Vibration inputs are closest in magnitude to the vibration exposure limit in the range 1.5–4 Hz. The transmissibility of the suspension in this range is between −7 and −9 dB. Subjective evaluation of the suspension has been favourable.