Techniques and devices for extending a piston, for example connected to a medical device such as a mechanical CPR device, to accommodate different sized patients, are described herein. In some cases, a piston of a mechanical CPR device may include an inner piston at least partially slidable into an external piston sleeve. In one aspect, an external piston spacer may be attached to an outward surface of the inner piston to extend the length of the piston. In another aspect an internal bayonet sleeve may contact one or more locking rods at various positions, enabling adjustment of the length of the inner piston. In yet another aspect, a piston adapter may be removably attached to the end of the piston. In all aspects, the change in length of the piston may be detected and used to modify movement of the piston, for example to more safely perform mechanical CPR.
