In an increasingly mobile world, many citizens and professionals are frequent travellers. Access during unplanned care to their patient summary, their most essential health information in a form physicians in another country can understand can impact not only their safety, but also the quality and effectiveness of care. International health information technology (HIT) standards such as HL7 CDA have been developed to advance interoperability. Implementation guides (IG) and IHE profiles constrain standards and make them fit for the purpose of specific use cases. A joint effort between HL7, IHE, and HealthStory created Consolidated CDA (C-CDA), a set of harmonized CDA IGs for the US that is cited in the Meaning Use II (MU-II) regulation. In the EU, the Patient Summary (PS) Guideline recently adopted, cites the epSOS IG also based on HL7 CDA, to support cross-border care in the EU and inform national eHealth programs. Trillium Bridge project supports international standards development by extending the EU PS Guideline and MU-II in the transatlantic setting. This paper presents preliminary findings from comparing patient summaries in the EU and US and reflects on the challenge of implementing interoperable eHealth systems in the cross-border or transatlantic setting.