A number of serious medical conditions may be treated in a minimally invasive manner with various kinds of catheters designed to reach treatment sites internal to a patient's body. One such medical condition is atrial fibrillation—a condition that results from abnormal electrical activity within the heart. This abnormal electrical activity may originate from various focal centers of the heart and generally decreases the efficiency with which the heart pumps blood. It is believed that some of these focal centers reside in the pulmonary veins of the left atrium. It is further believed that atrial fibrillation can be reduced or controlled by structurally altering or ablating the tissue at or near the focal centers of the abnormal electrical activity.
One method of ablating tissue of the heart and pulmonary veins to treat atrial fibrillation is cryotherapy—the extreme cooling of body tissue. Cryotherapy may be delivered to appropriate treatment sites inside a patient's heart and circulatory system by a cryotherapy catheter. A cryotherapy catheter generally includes a treatment member at its distal end, such as an expandable balloon having a cooling chamber inside. A cryogenic fluid may be provided by a source external to the patient at the proximal end of the cryotherapy catheter and delivered distally through a lumen to the cooling chamber where it is released. Release of the cryogenic fluid into the chamber cools the chamber (e.g., through the Joule-Thomson effect), and correspondingly, the balloon's outer surface, which is in contact with tissue that is to be ablated. Gas resulting from release of the cryogenic fluid may be exhausted proximally through an exhaust lumen to a reservoir or pump external to the patient.