A wrist-worn heart monitoring device , comprising: two or more electrodes configured to output an electrical signal indicating a wearer's heart has been commanded to contract, comprising at least a first electrode located on a user-adjacent surface of the wrist-worn heart monitoring device, and a second electrode located on an externally-facing surface of the wrist-worn heart monitoring device; an electrode-positioning sensor subsystem configured to determine a three-dimensional position of the second electrode respective to a second measurement location; a controller configured to receive a voltage difference from the first and second electrodes and to receive an indication of the three-dimensional position of the second electrode respective to the second measurement location, the voltage difference being indicative of an electrical potential of the wearer's heart, wherein the controller is configured to output an electrical potential of the wearer's heart if the second electrode is located within a threshold distance of the second measurement location, and to control a user interface to communicate an instruction to adjust the three-dimensional position of the second electrode if the second electrode is located more than a threshold distance from the second measurement location; a radial tonometer configured to output a pressure signal indicating a pulse pressure wave at a wearer's wrist; a microphone configured to output an audio signal indicating a closing of a wearer's aortic valve; and a pulse transit time monitor configured to calculate a pre-ejection period of the wearer's heart based on at least the pressure, electrical, and audio signals, and calculate a pulse transit time based on at least the pre-ejection period, the pressure signal, and the electrical signal. two or more electrodes configured to output an electrical signal indicating a wearer's heart has been commanded to contract, comprising at least a first electrode located on a user-adjacent surface of the wrist-worn heart monitoring device, and a second electrode located on an externally-facing surface of the wrist-worn heart monitoring device; an electrode-positioning sensor subsystem configured to determine a three-dimensional position of the second electrode respective to a second measurement location; a controller configured to receive a voltage difference from the first and second electrodes and to receive an indication of the three-dimensional position of the second electrode respective to the second measurement location, the voltage difference being indicative of an electrical potential of the wearer's heart, wherein the controller is configured to output an electrical potential of the wearer's heart if the second electrode is located within a threshold distance of the second measurement location, and to control a user interface to communicate an instruction to adjust the three-dimensional position of the second electrode if the second electrode is located more than a threshold distance from the second measurement location; a radial tonometer configured to output a pressure signal indicating a pulse pressure wave at a wearer's wrist; a microphone configured to output an audio signal indicating a closing of a wearer's aortic valve; and a pulse transit time monitor configured to calculate a pre-ejection period of the wearer's heart based on at least the pressure, electrical, and audio signals, and calculate a pulse transit time based on at least the pre-ejection period, the pressure signal, and the electrical signal. <EOS>