Jugular Venous Pressure (JVP). Systemic venous pressure is much lower than arterial pressure. Although venous pressure ultimately depends on left ventricular contraction, much of this force is dissipated as blood passes through the arterial tree and the capillary bed. Walls of veins contain less smooth muscle than walls of arteries, which reduces venous vascular tone and makes veins more distensible. Other important determinants of venous pressure include blood volume and the capacity of the right heart to eject blood into the pulmonary arterial system. Cardiac disease may alter these variables, producing abnormalities in central venous pressure. For example, venous pressure falls when left ventricular output or blood volume is significantly reduced; it rises when the right heart fails or when increased pressure in the pericardial sac impedes the return of blood to the right atrium. These venous pressure changes are reflected in the height of the venous column of blood in the internal jugular veins, termed the jugular venous pressure or JVP.
Pressure in the jugular veins reflects right atrial pressure, giving clinicians an important clinical indicator of cardiac function and right heart hemodynamics. Assessing the JVP is an essential, though challenging, clinical skill. The JVP is best estimated from the internal jugular vein, usually on the right side, since the right internal jugular vein has a more direct anatomic channel into the right atrium.
The internal jugular veins lie deep to the sternomastoid muscles in the neck and are not directly visible, so the clinician must learn to identify the pulsa-
tions of the internal jugular vein that are transmitted to the surface of the neck, making sure to carefully distinguish these venous pulsations from pulsations of the carotid artery. If pulsations from the internal jugular vein cannot be identified, those of the external jugular vein can be used, but they are less reliable.
To estimate the level of the JVP, you will learn to find the highest point of oscillation in the internal jugular vein or, if necessary, the point above which the external jugular vein appears collapsed. The JVP is usually measured in vertical distance above the sternal angle, the bony ridge adjacent to the second rib where the manubrium joins the body of the sternum.
Study the illustrations below very carefully. Note that regardless of the patient's position, the sternal angle remains roughly 5 cm above the right atrium. In this patient, however, the pressure in the internal jugular vein is somewhat elevated.
■ In Position A, the head of the bed is raised to the usual level, about 30°, but the JVP cannot be measured because the meniscus, or level of oscillation, is above the jaw and therefore not visible.
■ In Position B, the head of the bed is raised to 60°. The "top" of the internal jugular vein is now easily visible, so the vertical distance from the sternal angle or right atrium can now be measured.
■ In Position C, the patient is upright and the veins are barely discernible above the clavicle, making measurement untenable.
Note that the height of the venous pressure as measured from the sternal angle is the same in all three positions, but your ability to measure the height of the column of venous blood, or JVP, differs according to how you position the patient. Jugular venous pressure measured at more than 4 cm above the sternal angle, or more than 9 cm above the right atrium, is considered elevated or abnormal. The techniques for measuring the JVP are fully described in Techniques of Examination on pp. - .
Jugular Venous Pulsations. The oscillations that you see in the internal jugular veins (and often in the externals as well) reflect changing pressures within the right atrium. The right internal jugular vein empties more directly into the right atrium and reflects these pressure changes best.
Careful observation reveals that the undulating pulsations of the internal jugular veins (and sometimes the externals) are composed of two quick peaks and two troughs.
The first elevation, the a wave, reflects the slight rise in atrial pressure that accompanies atrial contraction. It occurs just before the first heart sound and before the carotid pulse. The following trough, the x descent, starts with atrial relaxation. It continues as the right ventricle, contracting during systole, pulls the floor of the atrium downward. During ventricular systole, blood continues to flow into the right atrium from the venae cavae. The tricuspid valve is closed, the chamber begins to fill, and right atrial pressure begins to rise again, creating the second elevation, the v wave. When the tricuspid valve opens early in diastole, blood in the right atrium flows passively into the right ventricle and right atrial pressure falls again, creating the second trough or y descent. To remember these four oscillations in a somewhat oversimplified way, think of the following sequence: atrial contraction, atrial relaxation, atrial filling, and atrial emptying. (You can think of the a wave as atrial contraction and the v wave as venous filling.)
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