The capnogram is a direct monitor of the inhaled and exhaled concentration or partial pressure of CO 2, and an indirect monitor of the CO 2 partial pressure in the arterial blood. In healthy individuals, the difference between arterial blood and expired gas CO 2 partial pressures is very small. In the presence of most forms of lung disease, and some forms of congenital heart disease (the cyanotic lesions) the difference between arterial blood and expired gas increases and can exceed 1 kPa.