A piezoelectric mechanical energy harvesting (MEH) technique was recently demonstrated through in vivo experiment by harvesting energy from the motion of porcine left ventricle (LV) myocardial wall. This provides a new strategy of energy supply for operating implantable biomedical devices so as to avoid various shortcomings associated with battery energy. This paper resorts to an analytical electromechanical model for evaluating the efficiency of the piezoelectric MEH device especially of that used in closed chest environment. A nonlinear compressive spring model is proposed to account for the impeding effect of surrounding tissues on the device. Inputting the periodic variation of the LV volume as a loading condition to the device, numerical predictions for the electric outputs are obtained and compare well with experiments. A simple scaling law for the output electric power is established in terms of combined material, geometrical, circuit, and LV motion parameters. The results presented here may provide guidelines for the design of in vivo piezoelectric energy harvesting from motions of biological organs.