Figure.

A) example of a normal popliteal vein Doppler pattern during prone position (PP) with only antegrade velocities. Note the correlation with the electrocardiogram and respiratory variation. Echocardiography (only apical views) at the same time revealed normal right ventricular (RV) function. B) 58-yr old male (patient #1) with central pulmonary embolism and acute RV failure and multiorgan dysfunction. B-mode and spectral Doppler profile of the left common femoral vein in supine position shows absence of respiratory variation and a bidirectional velocity profile with a significant retrograde component (> 10 cm·sec⁻¹). C) 70-yr old woman (patient #2) with cor pulmonale from long standing ARDS. Spectral Doppler profile of the left common femoral vein in supine position. Note the abnormal, pulsatile signal without respiratory modulation. Echocardiography showed RV dilatation and an estimated systolic pulmonary artery pressure of 62 mmHg using echocardiography and central venous pressure of 17 mmHg. D) 65-yr old male (patient #4) with COVID-19 pneumonia complicated by severe ARDS. Duplex ultrasound on day 6 in PP showed only slightly abnormal Doppler profile of the popliteal vein: note the only mild retrograde velocities (retrograde velocities < 5 cm·sec⁻¹) along with absence of respiratory modulation. E) Accordingly, a normal spectral Doppler profile of the portal vein was observed with a portal venous pulsatility index (PVPI) of 20% (PI = [maximal–minimal velocity]/mean velocity). Six hours later back in supine position, the Doppler profile differed clearly, now showing a strong pulsatile velocity pattern in the F femoral vein, G portal vein (PVPI 64%) and H popliteal vein, suspicious of RV dysfunction. CI = cardiac index; EtCO_{2 =} end-expiratory carbon dioxide; F_iO₂ = fraction of inspired oxygen; HR = heart rate; MAP = mean arterial pressure; paO₂ = arterial partial pressure of oxygen; paCO₂ = arterial partial pressure of carbon dioxide; PEEP = positive end-expiratory pressure; PVPI = portal venous pulsatility index; RR = respiratory rate