Venous Pressure Physiology Notes

Venous Pressure Definition And Normal Values

Venous pressure is the pressure exerted by the contained blood in the veins. The pressure in the vena cava and right atrium is called central venous pressure.

And the pressure in peripheral veins is called peripheral venous pressure. Pressure is not the same in all the veins. It varies in different veins in the extremities of the body and also varies from central veins to peripheral veins.

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Venous Pressure In The Extremities Of The Body

Venous pressure is less in the parts of the body above the level of the heart and it is more in parts below the level of the heart.

Pressure in jugular vein: 5.1 mm Hg (6.9 cm H20).

Pressure in the dorsal venous arch of the foot: 13.2 mm Hg (17.9 cm H20) (1 mm Hg pressure = 1.359 cm H20 pressure)

1. Venous Pressure In Central And Peripheral Veins
2. Pressure is greater in peripheral veins than in central veins.
3. Pressure in antecubital vein: 7.1 mm Hg (9.6 cm H20).
4. Pressure in superior vena cava: 4.6 mm Hg (6.2 cm H20).

Variations Of Venous Pressure

Venous pressure is altered both in physiological and pathological conditions.

Physiological conditions when venous pressure increases:

1. Changing from standing to a supine position
2. Tilting the body
3. Forced expiration (Valsalva maneuver)
4. Contraction of abdominal and limb muscles
5. Effect of gravity during prolonged traveling or standing
6. Excitement.

Pathological conditions when venous pressure increases:

1. Low cardiac output
2. Congestive heart failure
3. Venous obstruction
4. Failure of valves in veins
5. Paralysis of muscles
6. Immobilization of parts of the body
7. Renal failure.

Pathological conditions when venous pressure decreases:

1. Severe hemorrhage
2. Surgical shock.

Measurement Of Venous Pressure M Direct Method

Central venous pressure is measured by a catheter introduced through the median cubital vein of the forearm.

The position of the tip of the catheter is checked by fluoroscopy. The other end of the catheter is connected to a manometer which measures the pressure.

Peripheral venous pressure is measured by using a needle connected to a manometer.

Indirect Method

It is done by using an apparatus designed by By this apparatus, the collapse of the vein is noticed by the reflection of light through a transparent device.

The pressure required to cause the collapse of periph&sa! vein denotes the pressure in the particular vein.

Factors Regulating Venous Pressure

1. Vis A Tergo Or Force From Behind

Vis a tergo or force from behind is the left ventricular contraction, which forces the blood through the arteries, arterioles, capillaries, and veins to the right atrium.

The venous pressure is directly proportional to left ventricular pressure.

By the time blood passes through capillaries and reaches the venules, the pressure becomes less than 8 mm Hg and, when it reaches the right atrium, the pressure may be less than 1 mmHg.

2. Vis A Fronte Or Force From Front

Vis a front or force from the front is the right atrial pressure which determines the venous return.

It is also called central venous pressure, which in turn regulates peripheral venous pressure. Normal right atrial pressure is 0 mm Hg.

3. Vis A Latre Or Force From Side

Vis a later or force from the side is the resistance offered to blood flow through the veins. The venous pressure is directly proportional to the resistance, which is due to the venous tone and extravascular factors.

Because of the thin-walled nature, veins and venules are compressed by extravascular factors such as:

1. Compression of arm vein while passing over the first rib
2. Compression of neck veins in erect posture due to fall in pressure and atmospheric pressure
3. Compression of abdominal veins by increased intra-abdominal pressure
4. Compression of veins while passing in between the muscles.

4. Volume Of Venous Blood

The venous pressure is directly proportional to the volume of blood in the venous system.

5. Peripheral Resistance

Venous pressure is inversely proportional to peripheral resistance. When peripheral resistance is more, arterioles constrict and the veins are filled with less blood.

Vence, the pressure decreases. When peripheral insistence is less, the veins are filled with more blood to pressure increases.

Gravity And Posture

Pressure is more in the veins below the level of the heart and the pressure is less in veins above the level of the heart.

The weight of the column of blood in veins influences the venous pressure. During prolonged standing, the pressure in the lower extremities is more (90 cm H20).

It is because of the pooling of blood in the legs due to gravity. It increases the weight of the column of blood leading to an increase in pressure. During the movement, the venous pressure in the foot decreases.

In the head region, the venous pressure is -10 cm H20 because of the hydrostatic suction below the skull.

So, there is always a negative venous pressure in the head.

Effect Of Respiration On Venous Pressure

During normal quiet breathing, the central venous pressure is altered in accordance with intrathoracic pressure. Thus, during inspiration, the central venous pressure decreases because of decreased intrathoracic pressure.

During expiration, it increases because of increased intrathoracic pressure.

The effect of respiration on venous pressure is demonstrated by some procedures which exaggerate these effects on venous pressure. Such procedures are the Valsalva maneuver and Mueller’s maneuver.

Valsalva Maneuver Or Valsalva Experiment

Valsalva maneuver is the forced expiratory effort with closed glottis. It is performed by attempting to exhale forcibly while keeping the mouth and nose closed.

During this maneuver, the intrathoracic pressure increases greatly and causes the following effects:

1. Compression of the central vein in the thorax
2. Accumulation of blood in peripheral veins like veins of the neck, face, and limbs leads to increase in peripheral venous pressure to about 30 cm H20
3. Decrease in the venous return to the right atrium
4. Decrease in central venous pressure.

Uses of Valsalva Maneuver

Valsalva maneuver is used as a diagnostic tool to evaluate cardiovascular disorders.

30 seconds endurance test: The subject is asked to blow against a sphygmomanometer in which the pressure is maintained at 40 mm Hg for 30 seconds.

Then the changes in heart rate, blood pressure, or murmurs are observed to evaluate the cardiovascular disorders.

Sometimes it is practiced to relieve chest pain and correct abnormal heart rhythms.

Mueller’S Maneuver Or Mueller’S Experiment

Mueller’s maneuver or experiment is the forced inspiratory effort with closed glottis.

It is performed by attempting to inhale forcibly while keeping the mouth and nose closed. It is also called the reverse Valsalva maneuver.

During this maneuver, the intrathoracic pressure decreases greatly (becomes more negative) and causes the following effects:

1. Dilatation of right atrium and central vein because of increase in negative intrathoracic pressure
2. Rapid emptying of blood from peripheral veins into the central veins
3. Increase in central venous pressure and decrease in peripheral venous pressure. The peripheral venous pressure falls below 3-4 cm H20.

Uses of Mueller’s Maneuver

Mueller’s maneuver is used to evaluate upper respiratory tract problems and sleep apnea syndrome.