## 1 Introduction  
Whole-body periodic acceleration (WBPA), or periodic acceleration (pGz), is a method that applies repetitive headward to footward periodic motion to the body in the horizontal position [1]. The periodic motion of WBPA has been reported to induce arterial pulsatile shear stress on the endothelium and to stimulate endothelial nitric oxide release [2]. WBPA is regarded as a noninvasive approach for cardiovascular protection, as shown in animal studies [3], via enhancing antioxidative capacity [4], releasing cardiovascular protective mediators [5], and improving coronary microcirculation [6]. WBPA has been shown to improve flow-mediated vasodilation and brachial artery endothelial function in 20 healthy volunteers [7] and improve coronary microcirculation in healthy subjects and in patients with coronary artery disease [8] via upregulating endothelial nitric oxide synthase. WBPA increased vasodilatation and reduced the dicrotic notch in healthy adults and adult patients with inflammatory disease, which is similar to the response seen after giving nitric oxide-donor drugs to patients [2]. The effects of WBPA, when in the horizontal supine position, on blood pressure are still controversial, potentially due to the various designs of WBPA devices with different moving distances from headward to footward, and different frequencies. In WBPA design, the key component to induce pulsatile shear stress is the abdominal compartments moving headward to footward to produce intrathoracic pressure gradients for oscillatory periodic acceleration in the horizontal supine position [9]. WBPA manufactured by Non-Invasive Monitoring Systems, Inc. (NIMS) was designed to move headward to footward by about 2.4 cm to generate enough acceleration to stimulate endothelial nitric oxide release, and this WBPA was approved by FDA as medical device [10]. WBPA at a constant frequency of 4 Hz for the oscillatory period produced an inspiratory, inward displacement of the rib cage volume along with an outward displacement of abdominal volume and vice versa during expiration [9]. It is unknown whether a 160-degree V-shape WBPA (WBPA-V-160) could lower blood pressure more easily than WBPA in the horizontal supine position. It is not known whether a WBPA-V-160 in a semi-supine position with a minor moving distance, from headward to footward, of 2 mm can improve cardiovascular hemodynamics or not. Peripheral and central blood pressure (BP) parameters can be used to predict cardiovascular risks and hypertension for clinical practice [11]. Some studies have reported that WBPA did not change arterial BP and heart rate in animals [9] and in diabetic patients [6], whereas some have reported that 60 min of WBPA lowered BP in piglets [12]. Hemodynamic analysis of the cardiovascular system includes peripheral and central BP parameters (e.g., systolic BP, diastolic BP, and heart rate) and central BP parameters (e.g., end systolic pressure, end diastolic pressure, and pulse pressure). DynaPulse Noninvasive and Quantitative Hemodynamic Profile Analysis can be used to evaluate central BP parameters and cardiovascular hemodynamics [13; 14]. The Pulse Dynamic noninvasive method has been validated and the central aortic BP, vascular compliance and waveform can be measured without catheterization [14]. Cardiac output is the product of the stroke volume, which is the volume of blood pumped from the left ventricle per beat, and the heart rate, i.e., the number of heart beats per minute. In an anesthetized model of severe hemorrhagic shock in pigs after 40 mL/kg blood loss, cardiac output, BP, and oxygen delivery were dropped similarly between WBPA (1 h before and during) and non-WBPA conditions, but the regional blood flow was much better preserved in the case of WBPA than in non-WBPA [15]. Neither cardiac output nor stroke volume was reported after WBPA in humans. Systemic arterial compliance is calculated as the ratio of the stroke volume index to arterial pulse pressure [16].