Medical device and treatment method

A treatment method, comprising:

• forming a through-hole in an atrial septum to allow a right atrium and a left atrium of a heart failure patient to communicate with each other;
• expanding the through-hole;
• and
• performing maintenance treatment for maintaining the through-hole with a medical device after expanding the through-hole, the medical device comprising a shaft portion, an expandable and contractible expansion body disposed distal of the shaft portion, the expansion body including a concave portion recessed in a direction intersecting with an axial direction of the shaft portion, the concave portion including a bottom portion, a distal side standing portion disposed distal of the bottom portion and rising in a radial direction intersecting with the axial direction from the bottom portion, and a proximal side standing portion disposed proximal of the bottom portion and rising in the radial direction intersecting with the axial direction from the bottom portion, and the medical device further comprising a maintenance treatment element disposed on at least one of the bottom portion, the distal side standing portion and the proximal side standing portion,
• wherein the expansion body includes a circulation portion inside of the expansion body in the radial direction, the circulation portion including a portion defined by the bottom portion in the radial direction, and
• wherein during the maintenance treatment, the circulation portion allows blood flow between the right atrium and the left atrium via the through-hole in a state where the concave portion is disposed in the through-hole.