A guide extension catheter includes a push member and a distal shaft coupled to and extending distally from the push member. The distal shaft includes a shaft wall and a passageway. The shaft wall includes a helically-shaped proximal end and a distal end. The helically-shaped proximal end coils helically about a first central longitudinal axis of the passageway of the distal shaft. The helically-shaped proximal end of the shaft wall defines a helically-shaped entry port of the distal shaft. The distal shaft may include a helically-shaped collar coupled to the helically-shaped proximal end of the distal shaft, the collar defining the helically-shaped entry port of the distal shaft.