Footnotes

• Contributors KPP, AB: conception and design, data analysis/interpretation, drafting the manuscript, critical revision of the manuscript and final approval of the manuscript. Both authors agree to be accountable for all aspects of this manuscript.

• Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

• Competing interests KPP has research grant from Edwards Lifesciences.

• Provenance and peer review Commissioned; internally peer reviewed.

• Author note References which include a * are considered to be key references.