Identifying the Late Systolic Shoulder and its Determinants

The late systolic shoulder is an important point in the cardiac cycle. It is caused by the interaction between the forward and reflected waves within the arterial system. Important cardiovascular metrics, such as augmentation index, are calculated using the late shoulder. Furthermore, it is also used as an important input to many cardiovascular models. However, despite its importance, its definition throughout cardiovascular literature remains inconsistent. The late systolic shoulder is often observed as a late peak during systole, and this is widely accepted as its general definition. However, there is often no visible second peak, and identifying the late shoulder using this definition is not always possible. The shape of an arterial pressure wave is heavily dependant on the timing, magnitude and shape of both the forward and reflected waves. The patient specific nature of the late shoulder thus requires a patient specific approach for identification. Currently, there is no consensus algorithm which can reliably and accurately identify it. A few general algorithms have been proposed and accepted by the literature, however, they are known to provide inaccurate and inconsistent results. Given its clinical importance, a clearer definition of the late shoulder is required, and a reliable method for identification needs to be developed. This work aims to evaluate the currently used algorithms for identifying the late shoulder. Furthermore, waveform decomposition of the aortic pressure and flow wave is used to explore the relationship between the late shoulder and the forward and reflected waves, in an attempt to better understand the determinants of the late shoulder.