An apparatus, comprising: (a) a shaft ; and (b) an end effector at a distal end of the shaft (10, 210, 216, 310, wherein the end effector is sized to fit in an anatomical passageway within a subject's cardiovascular system, wherein the end effector comprises at least one electrode (85, 232, 234, 402, 402"', 404, 404"', 406, 406', 408) pair, wherein the at least one electrode (85, 232, 234, 402, 402"', 404, 404"', 406, 406', 408) pair is configured to contact cardiovascular tissue and thereby pick up electrocardiogram signals, each electrode (85, 232, 234, 402, 402"', 404, 404"', 406, 406', 408) pair includes first and second electrodes (85, 232, 234, 402, 402"', 404, 404"', 406, 406', 408) spaced apart along a longitudinal axis from each other by a gap area (Ag) located longitudinally between the electrodes (85, 232, 234, 402, 402"', 404, 404"', 406, 406', 408), the gap area (Ag) having a longitudinal gap length (Lg) with respect to the longitudinal axis such that: a length of one of the electrodes (85, 232, 234, 402, 402′′′, 404, 404"', 406, 406', 408) (L) along the longitudinal axis is equal to or greater than the longitudinal gap length; and a ratio of an area defined by the gap area (Ag) to one electrode (85, 232, 234, 402, 402′′′, 404, 404"', 406, 406', 408) surface area (Ae) is equal to or less than one; characterized in that : the longitudinal gap length (Lg) is determined by a product of the one electrode (85, 232, 234, 402, 402′′′, 404, 404"', 406, 406', 408) surface area (Ae) of one electrode up to one millimeter squared and a conversion factor of about 1.25 mm -1 or less. (a) a shaft ; and (b) an end effector at a distal end of the shaft (10, 210, 216, 310, wherein the end effector is sized to fit in an anatomical passageway within a subject's cardiovascular system, wherein the end effector comprises at least one electrode (85, 232, 234, 402, 402"', 404, 404"', 406, 406', 408) pair, wherein the at least one electrode (85, 232, 234, 402, 402"', 404, 404"', 406, 406', 408) pair is configured to contact cardiovascular tissue and thereby pick up electrocardiogram signals, each electrode (85, 232, 234, 402, 402"', 404, 404"', 406, 406', 408) pair includes first and second electrodes (85, 232, 234, 402, 402"', 404, 404"', 406, 406', 408) spaced apart along a longitudinal axis from each other by a gap area (Ag) located longitudinally between the electrodes (85, 232, 234, 402, 402"', 404, 404"', 406, 406', 408), the gap area (Ag) having a longitudinal gap length (Lg) with respect to the longitudinal axis such that: a length of one of the electrodes (85, 232, 234, 402, 402′′′, 404, 404"', 406, 406', 408) (L) along the longitudinal axis is equal to or greater than the longitudinal gap length; and a ratio of an area defined by the gap area (Ag) to one electrode (85, 232, 234, 402, 402′′′, 404, 404"', 406, 406', 408) surface area (Ae) is equal to or less than one; characterized in that : the longitudinal gap length (Lg) is determined by a product of the one electrode (85, 232, 234, 402, 402′′′, 404, 404"', 406, 406', 408) surface area (Ae) of one electrode up to one millimeter squared and a conversion factor of about 1.25 mm -1 or less. a length of one of the electrodes (85, 232, 234, 402, 402′′′, 404, 404"', 406, 406', 408) (L) along the longitudinal axis is equal to or greater than the longitudinal gap length; and a ratio of an area defined by the gap area (Ag) to one electrode (85, 232, 234, 402, 402′′′, 404, 404"', 406, 406', 408) surface area (Ae) is equal to or less than one; characterized in that : the longitudinal gap length (Lg) is determined by a product of the one electrode (85, 232, 234, 402, 402′′′, 404, 404"', 406, 406', 408) surface area (Ae) of one electrode up to one millimeter squared and a conversion factor of about 1.25 mm -1 or less. <EOS>