In Reply In their letter, Nitzan et al raise several important concepts that merit clarification. The first is that oxygen consumption (VO2) is a more valuable metric of tissue oxygenation than oxygen content, suggesting that VO2 is easily calculated from known cardiac output and the arteriovenous oxygen content difference. While we agree that VO2 is a useful marker of limited oxygen delivery, it is in reality difficult to measure accurately, particularly in newborn infants. To suggest that it is useful to calculate VO2 from cardiac output and oxygen content difference is misleading because cardiac output is not itself routinely measured at the bedside.