Editor—The paper of Zhang et al 1 on the effects of dopamine relies on the assumption that left ventricular output (LVO) is a measure of systemic blood flow (SBF). This is not true in the presence of any shunt through the ductus arteriosus where LVO becomes the sum of SBF and volume of blood shunting back into the lungs through the duct, and so overestimates SBF. While Zhang et al 1 define the ducts in their study babies as insignificant, we would question the validity of the criteria used to make this definition. We have never seen shunting only in diastole, chamber enlargement is an inconsistent sign of ductal significance which develops after day one,2 and shunt velocity has little relation to shunt size during week one.3 In fact, the left to right velocity often increases as the duct constricts and the shunt diminishes in size.3 Bidirectional shunting is also usually predominantly left to right.

There is much in the data presented by Zhang et al 1 to suggest that these ducts were highly significant. Firstly, they selected a population at high risk for a significant duct,4 and secondly they measured a mean diameter of 2.9 mm. We showed that this measure is the most accurate predictor of early haemodynamic significance and that preterm ducts over 2 mm in diameter, with very few exceptions, have a haemodynamic impact.3 , …