An invasive medical device (10) is disclosed comprising a device portion (15) for inserting into a patient, the device portion including a functional module (20); an electrically conductive path (40)extending through the invasive medical device and connecting to the functional module; and a deformable switch (30) within the device portion arranged to alter the electrical impedance of the conductive path as a function of a degree of deformation of the switch, the deformation being caused by a blood parameter of the patient's blood that varies during the cardiac cycle of the patient.