Analysis of the jugular venous pressure and pulse provides information regarding hemodynamic changes in the right side of the heart.
INTERNAL VERSUS EXTERNAL JUGULAR VEIN
It is preferable to examine the internal rather than external jugular veins since the internal jugular veins are in a direct line with the superior vena cava and right atrium, whereas the external jugular veins are not in a direct line with the superior vena cava and connect with it after negotiating two almost 90 degree angles . There are valves between the superior vena cava and both internal and external jugular veins; however, elevated venous pressure can be transmitted through the venous valves. The external jugular venous bulb is a site for thrombus formation, which can cause partial obstruction of the external jugular veins.
Partial compression of the left innominate vein is usually relieved during modest inspiration as the diaphragm and the aorta descend and the pressure in the two internal jugular veins becomes equal. However, partial obstruction of the left innominate vein from compression by the aorta may persist, particularly in relatively elderly patients, impairing transmission of right atrial pressure to the left internal jugular vein; this is also the most common cause of unequal pressures between right and left internal jugular veins. There is better transmission of right atrial pressures and pulses to the right internal jugular vein since the right innominate and internal jugular veins are in a direct line with the superior vena cava. Thus, examination of the right internal jugular venous pulse is preferable for assessing the hemodynamic changes in the right side of the heart. (See "Cardiac catheterization techniques: Normal hemodynamics".)
The normal jugular venous pulse wave or right atrial pressure wave recordings usually consist of three positive waves, a, c and v, and two negative waves, x and y  (figure 1). (See "Cardiac catheterization techniques: Normal hemodynamics" and "Pulmonary artery catheterization: Interpretation of tracings".)
●The positive a wave is caused by the right atrial pressure transmitted to the jugular veins during right atrial systole. The a wave peaks just before or during the first heart sound (S1) and before the onset of ventricular ejection (carotid pulse upstroke).