As with other types of shunts, pulse oximetry with ambulation as well as at rest may identify patients with increased pulmonary arterial resistance and dynamic shunt reversal in- duced by exercise. A subset of patients with rest- ing systemic oxygen saturation >90% will have a decrease in oxygen saturation with activity to <90%, emphasizing the importance of per- forming resting and ambulatory pulse oximetry assessment. 2. Invasive hemodynamic assessment still has an im- portant role as a confirmatory tool and for the evaluation of pulmonary vasoreactivity, which car- ries prognostic significance.S4.1.5-1,S4.1.5-4 3. When signs of volume overload are indicative of significant left-to-right shunt, closing the PDA is likely to prevent further left atrial or LV enlarge- ment, progression or development of PAH, and pulmonary hypertension secondary to left HF and will possibly provide symptom relief if symp- toms are present. Closure is typically performed percutaneously with good success and minimal Pulmonary blood flow and thus Qp:Qs can be difficult to calculate accurately because of differences in right/left PA blood flow caused by the flow from the PDA. Invasive he- modynamics including pulmonary vascular resist- ance are generally relied on for decision-making. Surgical closure can be performed but is poten- tially hazardous in adults because of calcification and tissue fragility. 4. Even with elevated pulmonary pressure and el- evated pulmonary vascular resistance, closure of a PDA may improve clinical status in some patients with persistent left-to-right shunting and prevent further progression of PAH.S4.1.5-3,S4.1.5-4 Consultation with ACHD and pulmonary hyper- tension providers is important given the low fre- quency of this circumstance and the complexity of decision-making. 5. Morbidity and mortality are high when closure of a shunt is attempted in patients with Eisenmenger physiology with elevated pulmonary pressure and net right-to-left shunting.S4.1.5-5 Table 16. 