The jugular venous pressure (JVP, sometimes referred to as jugular venous pulse) is the indirectly observed pressure over the venous system via visualization of the internal jugular vein. It can be useful in the differentiation of different forms of heart and lung disease. Classically three upward deflections and two downward deflections have been described. The upward deflections are the "a" (atrial contraction), "c" (ventricular contraction and resulting bulging of tricuspid into the right atrium during isovolumetric systole) and "v" (venous filling). The downward deflections of the wave are the "x" descent (the atrium relaxes and the tricuspid valve moves downward) and the "y" descent (filling of ventricle after tricuspid opening). Method Visualization The patient is positioned at a 45° incline, and the filling level of the external jugular vein determined. The internal jugular vein is visualised when looking for the pulsation. In healthy people, the filling level of the jugular vein should be less than 4 centimetres vertical height above the sternal angle. A pen-light can aid in discerning the jugular filling level by providing tangential light. The JVP is easiest to observe if one looks along the surface of the sternocleidomastoid muscle, as it is easier to appreciate the movement relative to the neck when looking from the side (as opposed to looking at the surface at a 90 degree angle). Like judging the movement of an automobile from a distance, it is easier for an observer to see the movement of an automobile when it is crossing the observer's path at 90 degrees (i.e., moving left to right or right to left), as opposed to coming towards the observer. Pulses in the JVP are rather hard to observe, but trained cardiologists do try to discern these as signs of the state of the right atrium. Differentiation from the carotid pulse The JVP and carotid pulse can be differentiated several ways: multiphasic – the JVP "beats" twice (in quick succession) in the cardiac cycle. In other words, there are two waves in the JVP for each contraction-relaxation cycle by the heart. The first beat represents that atrial contraction (termed a) and second beat represents venous filling of the right atrium against a closed tricuspid valve (termed v) and not the commonly mistaken 'ventricular contraction'. These wave forms may be altered by certain medical conditions; therefore, this is not always an accurate way to differentiate the JVP from the carotid