How does PPHN cause low systemic blood pressure? 

I think about the connection between PPHN and low BP as related to several possible mechanisms: cardiac (impacted left ventricular function due to other increased to right sided pressures) and vascular (vasodilatation from PPHN treatment). As PPHN is also rather a symptom, the underlying etiology as such may also impact cardiac function. For example, in the case of asphyxia/mec-aspiration, reduced myocardial function may be more related to that than the actual PPHN as such.

Here a nice review of PPHN! https://neoreviews.aappublications.org/content/16/12/e680

Rv dysfunction and with hypoxia ultimately lv dysfunction.  Reduced blood flow to the lungs leading to reduced blood return from the lungs to the left atrium and then ultimately low LVOT.

 

Having a PDA with right to left shunting supports systematic flow. If it’s becoming restrictive or closes then hypotension can occur