How to update a contact details using a custom field?

We are planning to use the Active Campaign for tracking all our clients (patients). We are preloading all our client data, using .csv. Planning to use API to update the patient data in Active Campaign using the APIs.
In our we are not tracking the contact_id of Active Campaign, we only have the patient_id. So we wanna update the patient details in AC using the patient_id.
How we can do this effectively.

There is always a contact_id. Map your endpoint and then use an automation to screen out if they are a patient for any update or map that as a secondary foreign key.