Shafts with pressure relief in cryotherapeutic catheters and associated devices, systems, and methods

A cryotherapeutic catheter comprising:

• an elongate shaft having a distal end portion, a pressure-relief portion proximal to the distal end portion, and a collapsible portion proximal to the pressure-relief portion, wherein the shaft is configured to move longitudinally within a guide passage defined by a tubular guide catheter, wherein the distal end portion of the shaft is configured to sealingly contact the guide catheter and thereby occlude an annular gap between the shaft and the guide catheter;
• a supply lumen carried by the shaft;
• an exhaust passage carried by the shaft;
• and
• a balloon at the distal end portion of the shaft wherein the balloon is configured to receive refrigerant from the supply lumen and to exhaust refrigerant to the exhaust passage,
• wherein the pressure-relief portion of the shaft is configured to rupture and thereby release refrigerant therethrough from the exhaust passage when a pressure of refrigerant within the exhaust passage exceeds a threshold pressure less than a pressure rating of the balloon, and
• wherein the collapsible portion of the shaft configured to collapse in response to pressure from refrigerant released from the exhaust passage via the pressure-relief portion of the shaft.