I never thought that modest increase in PEEP would have a real effect on ductal shunting. As the size of the ductal shunt depends on the pressure gradient across the PDA and the resistance of the vessel, in order to really have an effect on the size of the shunt pulmonary vascular resistance would have to significantly increase, and lead to an increase in PA pressure. Increasing PEEP from 4 to 6 or 7 seemed to me to be unlikely to have an effect. I assumed that most PVR is due to arteriolar resistance vessels and that the main effect of PEEP would probably be to compress the capillary bed.

I may have to think again.

This study evaluating hemodynamics by ultrasound (Fajardo MF, Claure N, Swaminathan S, Sattar S, Vasquez A, D’Ugard C, Bancalari E: Effect of Positive End-Expiratory Pressure on Ductal Shunting and Systemic Blood Flow in Preterm Infants with Patent Ductus Arteriosus Neonatology 2013, 105(1):9-13) in 16 preterm infants with fairly large PDA examined the effects of either increasing the PEEP by 3 (from 5 to 8 in all but one infant, who started at 4) or decreasing the PEEP by 3 (to a minimum of 2). The unblinded study showed a very small decrease in Left Ventricular Output with PEEP of 8, compared to baseline, and no change in SVC flow. Which means that ductal shunting was reduced a bit. A couple of provisos, the median LVO was actually lowest at 2 cmH2O of PEEP, although this was not significantly different, presumably because the direction of change was more variable, and they were using non-parametric statistics (appropriately).  The change in LVO was actually less than 5%, which is probably within the margin of error of the measurement.

The babies in the study had fairly good lung function, with a mean FiO2 of about 25%. Babies with stiffer lungs would be likely to have even less impact of PEEP on the PVR.

So it looks possible that increasing PEEP to 8 might indeed have a very small effect on ductal shunting, if the infant has fairly good lung function. There was no impact on cerebral oxygenation, and a very tiny increase in TcPCO2.