ADT_A48 - Change alternate patient id

Segment structure for the ADT_A48 message in HL7 v2.5.1. Cardinality is shown as [min..max]; groups are in bold.

• MSH Message header [1..1]
• EVN Event type [1..1]
• PID Patient identification [1..1]
• PD1 Patient additional demographic [0..1]
• MRG Merge patient information [1..1]