Multi-sensor volume index

A method of controlling operation of an ambulatory medical device, the method comprising:

• sensing a plurality of sensor signals using a plurality of physiological sensors, including a heart sound signal representative of mechanical cardiac activation of a subject and an impedance signal representative of physiological impedance of the subject;
• determining a value of at least one heart sound parameter using the heart sound signal and determining a value of at least one physiological impedance parameter value using the impedance signal;
• calculating a volume index representative of fluid volume status of the subject using the at least one heart sound parameter value and the at least one physiological impedance parameter value;
• comparing a metric of the calculated volume index to one or more high threshold metric values and one or more low threshold metric values;
• and
• generating, with the ambulatory medical device, an indication of a fluid volume status of the subject according to the comparison.

A system comprising an ambulatory medical device and a second device, wherein the ambulatory medical device includes:

• a plurality of physiologic sensors, including a heart sound sensor configured to generate a heart sound signal representative of mechanical cardiac activation of a subject and a respiration sensor configured to generate a respiration signal representative of respiration of the subject;
• and
• a first communication circuit configured to communicate information with the second device;
• wherein the second device includes:
• a second communication circuit configured to communicate information with the ambulatory medical device;
• a display;
• and
• a processor circuit including a volume index module configured to:
• determine a value of at least one heart sound parameter using the heart sound signal and determine a value of at least one respiration parameter using the respiration signal;
• calculate a volume index representative of fluid volume status of the subject using the at least one heart sound parameter value and the at least one respiration parameter value;
• and
• compare the calculated volume index to one or more high threshold index values and one or more low threshold index values, and wherein the processor circuit is configured to display a recommended change to a treatment for at least one of hypervolemia or hypovolemia of the subject according to the comparison.