Impedance phlebography
Encyclopedia
Impedance phlebography, or impedance plethysmograph
y (IPG), is a non-invasive medical test that measures small changes in electrical resistance
of the chest, calf or other regions of the body. These measurements reflect blood volume
changes, and can indirectly indicate the presence or absence of venous thrombosis
. This procedure provides an alternative to venography
, which is invasive and requires a great deal of skill to execute adequately and interpret accurately.
For the chest, the technique was developed by NASA
to measure the split second impedance changes within the chest, as the heart beats, to calculate both cardiac output and lung water content. This technique has progressed clinically (often now called BioZ, i.e. biologic impedance, as promoted by the leading manufacturer in the US) and allows low cost, non-invasive estimations of cardiac output and total peripheral resistance, using only 4 skin electrodes, oscillometric blood pressure
measurement and lung water volumes with minimal removal of clothing in physician offices having the needed equipment.
For leg veins, the test measures blood volume in the lower leg due to temporary venous obstruction. This is accomplished by inflating a pneumatic cuff around the thigh to sufficient pressure to cut off venous flow but not arterial flow, causing the venous blood pressure to rise until it equals the pressure under the cuff. When the cuff is released there is a rapid venous runoff and a prompt return to the resting blood volume. Venous thrombosis will alter the normal response to temporary venous obstruction in a highly characteristic way, causing a delay in emptying of the venous system after the release of the tourniquet
. The increase in blood volume after cuff inflation is also usually diminished.
Plethysmograph
A plethysmograph is an instrument for measuring changes in volume within an organ or whole body .-Lungs:...
y (IPG), is a non-invasive medical test that measures small changes in electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
of the chest, calf or other regions of the body. These measurements reflect blood volume
Blood volume
Blood volume is the volume of blood in the circulatory system of an individual.-Humans:A typical adult has a blood volume of approximately between 4.7 and 5 liters, with females generally having less blood volume than males....
changes, and can indirectly indicate the presence or absence of venous thrombosis
Venous thrombosis
A venous thrombosis is a blood clot that forms within a vein. A venous thrombosis is a blood clot that forms within a vein. A venous thrombosis is a blood clot that forms within a vein. (Thrombosis is a medical term for blood clotting (Haemostasis) occurring in the wrong place, i.e...
. This procedure provides an alternative to venography
Venography
Venography is a procedure in which an x-ray of the veins, a venogram, is taken after a special dye is injected into the bone marrow or veins. The dye has to be injected constantly via a catheter, making it an invasive procedure...
, which is invasive and requires a great deal of skill to execute adequately and interpret accurately.
For the chest, the technique was developed by NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
to measure the split second impedance changes within the chest, as the heart beats, to calculate both cardiac output and lung water content. This technique has progressed clinically (often now called BioZ, i.e. biologic impedance, as promoted by the leading manufacturer in the US) and allows low cost, non-invasive estimations of cardiac output and total peripheral resistance, using only 4 skin electrodes, oscillometric blood pressure
Blood pressure
Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...
measurement and lung water volumes with minimal removal of clothing in physician offices having the needed equipment.
For leg veins, the test measures blood volume in the lower leg due to temporary venous obstruction. This is accomplished by inflating a pneumatic cuff around the thigh to sufficient pressure to cut off venous flow but not arterial flow, causing the venous blood pressure to rise until it equals the pressure under the cuff. When the cuff is released there is a rapid venous runoff and a prompt return to the resting blood volume. Venous thrombosis will alter the normal response to temporary venous obstruction in a highly characteristic way, causing a delay in emptying of the venous system after the release of the tourniquet
Tourniquet
An emergency tourniquet is a tightly tied band applied around a body part sometimes used in an attempt to stop severe traumatic bleeding. Tourniquets are also used during venipuncture and other medical procedures. Severe bleeding means the loss of more than 1,000 ml of blood. This flow of blood...
. The increase in blood volume after cuff inflation is also usually diminished.