Heart rate is the speed of the heartbeat measured by the number of poundings of the heart per unit of time — typically beats per minute (bpm). The heart rate is commonly measured by observing the patient’s pulse, which is the physical expansion of the artery. The pulse commonly taken is from the radial artery at the wrist. Sometimes the pulse cannot be taken at the wrist and is taken at the elbow (brachial artery), at the neck against the carotid artery (carotid pulse), behind the knee (popliteal artery), or in the foot dorsalis pedis or posterior tibial arteries. In addition, the heart rate may be measured directly electrocardiography (adapted from Wikipedia). This class represents the measurement (or “clinical observation”) of the patient’s heart rate. This class re-defines the generic clinical Observation class to fix the observed characteristic to LOINC 8867-4 and to set the data type of the observation value to a Physical Quantity Ratio. It also redefines the bodySite and method properties such that they are bound to the Heart Rate Measurement Site and Heart Rate Measurement Method valuesets, respectively.