A leadless cardiac pacemaker (LCP) configured to sense cardiac activity and to pace a patient's heart, the LCP comprising: a housing having a proximal end and a distal end; a power source provided within the housing ; a first electrode secured relative to the housing and exposed to the environment outside of the housing ; a second electrode secured relative to the housing and exposed to the environment outside of the housing ; the housing having a diaphragm that is exposed to the environment outside of the housing , the diaphragm is responsive to a pressure applied to the diaphragm by the environment outside of the housing ; a pressure sensor within the housing , the pressure sensor having a pressure sensor diaphragm that is responsive to a pressure applied to the pressure sensor diaphragm and provides a pressure sensor output signal that is representative of the pressure applied to the pressure sensor diaphragm ; a fluid filled cavity in fluid communication with both the diaphragm of the housing and the pressure sensor diaphragm of the pressure sensor , the fluid filled cavity configured to communicate a measure related to the pressure applied by the environment to the diaphragm of the housing to the pressure sensor diaphragm of the pressure sensor ; circuitry in the housing in operative communication with the pressure sensor , the circuitry configured to determine a pressure exterior to the housing based on the pressure sensor output signal, wherein the circuitry is further operatively coupled to the first electrode and the second electrode , and is further configured to provide pacing pulses via the first and second electrodes based on the pressure sensor output signal; and a fixation member at the distal end of the housing for fixing the distal end of the housing to the patient's heart, and wherein the diaphragm of the housing is adjacent the proximal end of the housing . a housing having a proximal end and a distal end; a power source provided within the housing ; a first electrode secured relative to the housing and exposed to the environment outside of the housing ; a second electrode secured relative to the housing and exposed to the environment outside of the housing ; the housing having a diaphragm that is exposed to the environment outside of the housing , the diaphragm is responsive to a pressure applied to the diaphragm by the environment outside of the housing ; a pressure sensor within the housing , the pressure sensor having a pressure sensor diaphragm that is responsive to a pressure applied to the pressure sensor diaphragm and provides a pressure sensor output signal that is representative of the pressure applied to the pressure sensor diaphragm ; a fluid filled cavity in fluid communication with both the diaphragm of the housing and the pressure sensor diaphragm of the pressure sensor , the fluid filled cavity configured to communicate a measure related to the pressure applied by the environment to the diaphragm of the housing to the pressure sensor diaphragm of the pressure sensor ; circuitry in the housing in operative communication with the pressure sensor , the circuitry configured to determine a pressure exterior to the housing based on the pressure sensor output signal, wherein the circuitry is further operatively coupled to the first electrode and the second electrode , and is further configured to provide pacing pulses via the first and second electrodes based on the pressure sensor output signal; and a fixation member at the distal end of the housing for fixing the distal end of the housing to the patient's heart, and wherein the diaphragm of the housing is adjacent the proximal end of the housing . <EOS>