A method including obtaining a first electrocardiography [“ECG”] reading from a test subject when the test subject is believed to not suffer from blood volume loss and obtaining a second ECG reading from the test subject when the test subject is in an unknown condition regarding blood volume loss. The first ECG reading and the second ECG reading are obtained using the same electrode positions on the test subject and include sets of ECG signals which, based on experimental data, respond to small decrease in blood volume by a statistically significant strength decrease. If the second ECG reading exhibits a statistically significant strength decrease compared with the first ECG reading, an alert condition is raised, which indicates potential small decrease in blood volume. The method can be embodied as a stand-alone ECG apparatus or as an add-on unit to an ECG apparatus.
