An undulating waveform observed in the cardiograph channel during PDD testing. During breathing vasoconstrictor neurons are activated in the inspiratory phase leading to rhythmic vasoconstriction of blood vessels controlling blood pressure. Increased vasomotor constriction results in increased blood pressure. Additionally, deep breathing results in negative pressure in the venae cavae resulting in increased blood flow. Increased blood flow results in a larger “pre-load” (the amount of blood returning to the right side of the heart) or end diastolic volume which leads to increased blood pressure. Respiratory Sinus Arrhythmia (RSA) results in increased heart rate during the inspiration cycle. Increased heart rate results in increased cardiac output which, in turn, results in increased blood pressure. When an examinee engages in a deliberate attempt to control his or her breathing one can expect to see an exacerbation of any cyclic waveform in the cardiovascular channel. The synchronous rise and fall of the relative blood pressure is quite possibly a result of any combination of these physiological factors. The image below shows the undulations in the cardiograph tracing and its relationship with breathing. Note that the timing of cardiograph undulations follows the breathing by about 2 – 3 seconds. See: Handler & Reicherter (2008).