Mixed venous oxygen saturation in the newborn. Can we and should we measure it?

Conventional measurements of oxygenation used in the critical care of sick newborn infants are limited to arterial blood. This approach fails to describe fully the physiological economy of oxygen in terms of supply (systemic oxygen transport), demand (oxygen consumption), or functional reserve (mixed venous oxygen content). The relationship between supply and demand can be described by the fractional oxygen uptake, which can be determined from arterial (SaO2) and mixed venous (Sv-O2) oxygen saturations. Fractional oxygen uptake = (SaO2 - Sv-O2)/SaO2. Review of the literature shows that conventional methods of continuous monitoring of arterial oxygen tension or content have had little measurable effect on the incidence of retinopathy of prematurity or on the survival of low birthweight infants. In hypoxic states, oxygen can be almost completely extracted by the tissues. However, a critical value for fractional oxygen extraction can be recognised which reflects a critical value for systemic oxygen transport. While the pulmonary artery is the ideal site for the measurement of mixed venous oxygen content, blood sampled from the mid-lateral site of the right atrium is higher in mean oxygen saturation by only 0.7 (SE 0.1) vol.%, indicating that the effect of incomplete mixing is very small. The difference in oxygen saturation between right atrium and pulmonary artery remains variable, and this may reflect either true variation of central venous saturation or instrument error. Because of this, central venous oxygen saturation may be of limited use for the measurement of oxygen consumption, cardiac output or fractional oxygen extraction. The level of central venous oxygen saturation as a direct measure of blood oxygen reserve is worthy of further examination.(ABSTRACT TRUNCATED AT 250 WORDS)