Left atrial access apparatus and methods are described herein. Different parameters, such as oxygen saturation difference, between the left and right atrial chambers is utilized to guide a needle or catheter into a desired position within the heart. Various sensing elements can be utilized to detect the physiological parameter difference, such as oxygen levels, in the left atrium. The sensor can be carried by the needle, at its tip or along its body, and can measure the physiological parameter levels contained in the blood, fluid, or tissue.