Impedance plethysmography (IPG)

In 1939, Nyboer introduced the concept of impedance plethysmography (IPG), and in the 1970′s several IPG devices became commercially available particularly for the diagnosis of PVDs [36, 58]. IPG uses electrical impedance to derive changes in blood volume to determine hemodynamic functionality. In this method, circumferential electrodes are placed on the leg (Fig. 1d), a weak high-frequency alternating current passes through the leg, and voltage changes in the electrodes are monitored to measure blood volume changes in the test area [31]. This plethysmography method is reported to be less cumbersome than fluid displacement based plethysmographic methods [53, 54]. Additionally, as discussed in the following paragraphs, IPG can be employed to detect DVT and to evaluate both arterial and venous competence [36, 59, 60].

In a study of 33 legs with arteriography confirmed arterial obstruction greater than 50% diameter, compared to 28 healthy control legs, IPG waveform analysis (resting arterial pulse wave amplitude and maximum systolic slope) provided a sensitivity and specificity greater than 90% [36]. Threshold pulse wave amplitude of 0.06% dR (resistance change due to blood volume change) and 0.60% dR/s for systolic slope were used [36]. When compared to ABI diagnosed PVD in a study of 66 patients, a sensitivity of 73.2% and a specificity of 96% was reported, using crest time thresholds of 180 ms [61].

Anderson evaluated IPG for detecting CVI in 44 subjects compared to Doppler ultrasound [36]. Venous refilling time > 11 s was chosen to represent a competent set of venous valves, while < 11 s indicated venous reflux [36]. Reported accuracy was 90%, however, sensitivity and specificity values were not provided [36].

Impedance plethysmography to detect the presence of DVT evaluates the patient in supine position with the leg elevated above the level of the heart (Fig. 2). A thigh cuff is inflated above venous pressure (50 mmHg) with calf impedance monitored until plateau before cuff deflation. The presence of DVT is inferred by measuring maximum venous capacitance (venous volume after 50 mmHg pressure application) and venous outflow rate (venous volume decrease in the first 3 s of deflation) [36, 62]. Six medical centres were cited to independently confirm the accuracy (> 94%) of IPG to diagnose a recent DVT proximal to the knee [36]. Sinton et al. compared IPG with venography in 85 legs and reported that IPG was successful in detecting proximal DVT in 20 of 22 subjects [63]. However, IPG was less successful in distinguishing been healthy and pathological groups for distal DVT’s [63]. A review of venous disorder diagnosis reported acceptable sensitivity (87–98%) for IPG compared to venography [62, 64–69]. In contrast, sensitivity (12–64%) was less acceptable in asymptomatic populations [62, 70–73].

Schematic view of the use of impedance plethysmography for detection of DVT

(adapted from [36])