A testing apparatus for evaluating cardiovascular condition of a patient, the testing apparatus comprising: one or more electrodes ; one or more processors ; a computer-readable memory storing non-transient instructions that when executed by the one or more processors cause the testing apparatus to: (a) use the one or more electrodes to record a first impedance of a neck of the patient at a first time in response to receiving a first impedance reading from a plurality of sensors on the neck; (b) after the occurrence of an event modulating blood return to the heart via the venous system of the patient, use the one or more electrodes to record a second impedance of the neck at a second time in response to receiving a second impedance reading from a plurality of sensors on the neck, wherein the first impedance and the second impedance each correspond to a volume of blood flowing within the neck; and (c) determine a change in venous blood volume between the first time and the second time by comparing the first impedance and the second impedance to determine a change in volume of blood; and (d) determine how the patient will hemodynamically respond to one or more of an addition of cardiovascular fluid or removal of cardiovascular fluid. one or more electrodes ; one or more processors ; a computer-readable memory storing non-transient instructions that when executed by the one or more processors cause the testing apparatus to: (a) use the one or more electrodes to record a first impedance of a neck of the patient at a first time in response to receiving a first impedance reading from a plurality of sensors on the neck; (b) after the occurrence of an event modulating blood return to the heart via the venous system of the patient, use the one or more electrodes to record a second impedance of the neck at a second time in response to receiving a second impedance reading from a plurality of sensors on the neck, wherein the first impedance and the second impedance each correspond to a volume of blood flowing within the neck; and (c) determine a change in venous blood volume between the first time and the second time by comparing the first impedance and the second impedance to determine a change in volume of blood; and (d) determine how the patient will hemodynamically respond to one or more of an addition of cardiovascular fluid or removal of cardiovascular fluid. (a) use the one or more electrodes to record a first impedance of a neck of the patient at a first time in response to receiving a first impedance reading from a plurality of sensors on the neck; (b) after the occurrence of an event modulating blood return to the heart via the venous system of the patient, use the one or more electrodes to record a second impedance of the neck at a second time in response to receiving a second impedance reading from a plurality of sensors on the neck, wherein the first impedance and the second impedance each correspond to a volume of blood flowing within the neck; and (c) determine a change in venous blood volume between the first time and the second time by comparing the first impedance and the second impedance to determine a change in volume of blood; and (d) determine how the patient will hemodynamically respond to one or more of an addition of cardiovascular fluid or removal of cardiovascular fluid. <EOS>