ITBV: intrathoracic blood volume

This chapter is relevant to Section G6(iii) of the 2023 CICM Primary Syllabus, which expects the exam candidate to "describe the methods of measurement of cardiac output, including limitations, potential sources of error, the need for calibration and the values obtained". 

Relationship of Global End-diastolic Volume (GEDV) and Intra-Thoracic Blood Volume (ITBV)

Experimentally, it has been shown that the total amount of blood in the cardiopulmonary circulation is related to the GEDV, and is usually 1.25 times the total volume of the blood in the ventricles.
ITBV diagram
ITBV equation

The ITBV calculation does not take into account any ort of hideous structural abnormalities in the heart, like shunts, space occupying lesions, or massive pulmonary emboli.

ITBV will be inaccurate in patients with a saddle embolus or pulmonectomy.


From Bersten and Soni’s” Oh's Intensive Care Manual”, 6th Edition, as well as who are sadly the best source for this sort of information.

Endo, Tomoyuki, et al. "Limitations of global end-diastolic volume index as a parameter of cardiac preload in the early phase of severe sepsis: a subgroup analysis of a multicenter, prospective observational study." Journal of Intensive Care 1.1 (2013): 1-8.'

Marik, Paul E., et al. "Dynamic changes in arterial waveform derived variables and fluid responsiveness in mechanically ventilated patients: a systematic review of the literature*." Critical care medicine 37.9 (2009): 2642-2647.