|AN EXERCISE IN THE MEASUREMENT OF VENOUS PRESSURE
In a previous exercise, you were able to determine the arterial blood pressure using a sphygmomanometer. In this exercise, you will learn how to estimate the venous blood pressure by direct observation of the degree of distention of the veins. Naturally, the venous pressure is much lower than the arterial pressure. Using the method outlined in this exercise, the normal values are approximately 6 torr to 15 torr. Compare this with the MAP (Mean Arterial Pressure) of approximately 100 mm Hg seen with arterial blood pressure (see previous Exercise).
Measurements of the venous pressure will be taken under both normal conditions of venous return and abnormal conditions of venous return (Valsalva maneuver). Performing the Valsalva maneuver will reduce venous return and cause the damming of blood in the peripheral veins and will allow us to simulate an what an elevated central venous pressure would look like in a patient.
For this exercise, the only materials needed are a table and a meter stick.
Have the subject sit on one of the laboratory chairs and remain quiet. Observe the peripheral veins in the neck region. Think of how these veins are emptying blood into the superior vena cava for transport to the right atrium of the heart. In the sitting position, the neck veins should not be distended if the venous return is normal and the CVP (Central Venous Pressure-from lecture) is not elevated. However, when the CVP is elevated to about 10 mm Hg, the venous return is reduced (why?) and the blood begins to back-up in the venous system. This will cause the venous pressure to rise and the neck veins to protrude.
Have the subject recline on the table in a horizontal position. The subject should have their arms to their sides. Place the meter stick so that it rests on the table about level with the subject's heart. Have the subject slowly raise their arm from the resting position. Using the meter stick, measure in millimeters the vertical distance, how far the arm is raised until the veins in the hand suddenly collapse. Since this offers no discomfort to the subject, repeat the procedure as often as necessary to get an accurate recording. You need to get the measurement, in millimeters, of the distance from the resting position (level with the heart) to the point where the veins collapse. To estimate the venous pressure from this measurement, you need to divide the millimeter measurement by 13.6, which is the density of mercury to convert the millimeter height to mm of Hg:
Venous pressure = Distance in mm
13.6 mmHg (torr)
To observe a change in the venous pressure, have the subject perform a Valsalva maneuver. Be sure that the subject is in good physical condition and does not have a history of heart condition or vascular problems. Have the subject relax in the reclining position and breathe normally for a minute or so. Then have the subject take a deep forced inhalation (inspiration) and HOLD the breath. While the air is trapped in the lungs the subject should make a strong prolonged expiratory effort which will build-up pressure in the thoracic cavity. Do not let the air out during this forced expiratory effort. (Do not overdo this action.) While the subject is in this Valsalva position, repeat the above procedure to estimate the venous pressure. Again measure the distance from the resting position to the point where the hand veins collapse. Let the subject breathe normally again. Repeat if necessary.
The Valsalva maneuver increased the air pressure in the thoracic cavity. By exerting the expiratory force in the thoracic cavity, pressure was put upon the inferior vena cava. This pressure partially compressed the vena cava and therefore reduced venous return. The venous blood began to "dam up" in the venous system. This maneuver should, therefore, INCREASE peripheral venous pressure. Make sure you understand this.
How did you perform this experiment?
What are the values for venous pressure vs. arterial pressure.
Why are veins considered to be capacitance vessels?
Why did elevating your arm cause the veins to collapse? What does this represent?
What is central venous pressure (CVP)?
Why does an elevated CVP cause venous return to decrease? What is happening?
Why did the Valsalva maneuver cause your PVP to “rise”? How did you know this was happening based on the height of your arm?
What happens to aortic pressure during the phases of the Valsalva maneuver? A very nice review of this can be found at: http://www.cvphysiology.com/Hemodynamics/H014.htm
Why could a Valsalva maneuver be used to convert a patient with SVT (supra-ventricular tachycardia- a rapid heart-beat)?
What is the Valsalva maneuver?