Investigating the relationship between intrathoracic blood volume index and cardiac index

To determine whether cardiac index and intrathoracic blood volume index are "mathematically coupled" under euvolaemic conditions with increasing levels of inotropic support. Prospective case series. A 14-bed general intensive care unit in a university-affiliated hospital. Twelve mechanically ventilated patients, monitored with the COLD system and receiving dobutamine as inotropic support. After measuring cardiac index and intrathoracic blood volume index the rate of dobutamine infusion was increased until cardiac index rose by at least 20%. A further measurement of intrathoracic blood volume index was made at the new cardiac index. The mean increase in cardiac index was 31.7%, compared with a mean increase in intrathoracic blood volume index of only 2.84%. Under euvolaemic conditions, raising cardiac index by increasing inotropic support does not alter intrathoracic blood volume index significantly, thus demonstrating that the two measurements are not 'mathematically coupled' under these conditions.