Deep Venous Thrombosis Examination by Plethysmography

The four plethysmographs just described have been used for the identification and monitoring of DVT. For purposes of this text a generic procedure for DVT will be described.

Deep venous thrombosis is a life-threatening disease; for that reason alone accurate diagnosis and therapy are essential. The deep venous system is not only a conduit for returning blood to the right side of the heart; it is also a storage or capacitant system. This means its volume changes rapidly as pressure within the deep system changes. If one examines a vein at low pressure the walls are nearly fully collapsed and only a small flow channel is present. It takes very little increase in internal fluid pressure to expand the flow channel of a vein. Finally, if there is obstruction in a segment of deep vein, despite rich venous collateral channels, venous pressure distal to the obstruction will increase. Examination by plethysmograph makes use of these two principles (i.e., volume change with increased pressure and resistance).

Typically a plethysmograph transducer is placed at the calf or distal thigh with the patient lying supine on a table. In the case of APG the transducer is an air bladder inflated to 5 mmHg; in the case of PPG the transducer is a light emitting diode. Proximal to the transducer a method of rapidly occluding the deep system must be used. For all transducers this can be a thigh cuff inflated rapidly by hand bulb or automatic inflator.

With the transducer recording a stable venous signal at 5 mm/second chart speed, the pressure in the proximal occluding cuff is rapidly elevated to 50 mmHg. The transducer is measuring absolute levels of volume.

With the increased pressure in the proximal cuff, venous blood in the deep system cannot pass under the cuff until the venous pressure reaches approximately occluding cuff pressure. This increase in venous pressure (i.e., pooling) develops because the proximal cuff does not obstruct the arterial inflow. After about 20 to 40 seconds, pressure in the distal venous system reaches the pressure in the occluding cuff and venous volume reaches a plateau. Once the plateau has been reached, the operator rapidly releases the pressure in the occluding cuff. The pooled venous blood can then return to the right side of the heart via the larger veins upstream. Two measures of venous hemodynamics are taken during this test. First, there is the volume increase from the baseline to the plateau. This is known as Segmental Venous Capacitance (SVC) and represents the blood storage capacity of the segment vein. This generally is quoted in mm of deflection or ml if the system is calibrated to volume. The second measurement is the slope of the volume-time curve immediately after the pressure in the occluding cuff is released. This is known as Maximum Venous Outflow (MVO) and represents resistance to blood flow in the deep system. This may be quoted in mm of deflection/second or ml/second if the system is calibrated to volume. The next two sections define the diagnostic use of these parameters.

Segmental Venous Capacitance (SVC)

With experience, vascular technologists and physicians are able to identify a normal range of SVC with their specific plethysmography equipment. With the subject supine, normal veins have significant capacitance. If proximal deep venous obstruction is present, pressure distal to the obstruction increases and SVC is markedly reduced. Therefore, if SVC reduces more than 25% when compared to normal levels, venous abnormality is suggested.11 It is recommended that SVC always be measured bilaterally. In the case of unilateral disease, the normal limb can serve as a control, which increases both sensitivity and specificity.

Maximum Venous Outflow (MVO)

As in the case of SVC, vascular technologists and physicians are able to identify a normal range of MVO with their specific plethysmographic equipment. Normal veins exhibit a very rapid decrease in volume on deflation of the occluding cuff. When deep system resistance is increased due to deep venous obstruction, the reduction in MVO is dramatic. Again, in the case of unilateral disease, the normal limb can serve as a control. A difference in MVO between limbs of 25% is abnormal.11

When Continuous-wave Venous Doppler measurements, SVC, and MVO are performed as a diagnostic package, sensitivity and specificity of the combined testing reach 85%, respectively.11 It should be acknowledged Duplex Venous Doppler Ultrasonic Imaging, which requires more expensive equipment, clearly demonstrates a higher sensitivity and specificity. Further, ultrasound is able to more accurately localize obstruction and age thrombus. For this reason, plethysmographic methods have limited diagnostic use. There is one area in venous disease where SVC and MVO provide unique and important information. This is in the determination of venous collaterization following a DVT. Patients that normalize SVC and MVO rapidly have an improved prognosis when compared to subjects in which normalization is prolonged.

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