Operation of a patient's heart or lungs may be analyzed by transmitting ultrasound energy into the patient's lung, and detecting Doppler shifts of reflected ultrasound induced by moving borders between blood vessels in the lung and air filled alveoli that surround the blood vessels. Movement of the border is caused by pressure waves in the blood vessels that result in changes in diameter of those blood vessels. The detected Doppler shifts are used to obtain power and velocity data at each of a plurality of different air pressure levels, and the pulmonary blood pressure of the patient is then estimated based on the obtained data.