An amount of fluid in a thoracic or other region of a subject may be monitored by internally injecting an electrical energy stimulus (e.g., constant voltage source) through the region, detecting voltage resulting from the electrical energy stimulus, and calculating a fluid volume indicative signal. The injected energy stimulus creates a first lead field. The responsive voltage is detected using an electrode configuration that defines a second lead field, which is arranged in a negative sensitivity configuration with respect to the first lead field at the region being monitored.