Normal lower limb venous Doppler flow phasicity: is it cardiac or respiratory?

OBJECTIVE: The purposes of this study were to determine the origin and nature of normal lower limb venous Doppler flow phasicity and to assess normal and respiratory variations. SUBJECTS AND METHODS: The common femoral veins of 12 healthy volunteers (three men and nine women; age range, 21-50 years; mean, 29 years) were evaluated by detailed spectral Doppler examinations with simultaneous ECG and respirometric tracings. The examinations were performed using a 5- or 7-MHz linear-array transducer with breath held in mid respiration, at the end of deep expiration, at the end of deep inspiration, during Valsalva's maneuver, and during quiet and deep breathing. The tracing obtained during breath-hold in mid respiration was considered the baseline. Tracings obtained during the other respiratory phases were analyzed for changes from the baseline. Doppler tracings were analyzed for phasicity, waveform frequency, components, velocities, velocity ratios, and presence of retrograde flow, all in correlation with simultaneous ECG and respirometric tracings. Tracings were analyzed independently by two observers to assess interobserver variability.
RESULTS: With breath-hold in mid respiration, the common femoral vein Doppler tracings consisted of multiphasic waveforms that had a frequency similar to that of the heart rate. Each waveform consisted of systolic, v, diastolic, and a waves. The systolic wave occurred 0.4 sec later than the QRS complex of the ECG and was always antegrade. The v wave was always retrograde without flow reversal. The diastolic wave was always antegrade. The a wave was always retrograde but showed flow reversal in nine of 12 subjects. The systolic:diastolic velocity ratio ranged from 0.9 to 1.5 (mean, 1.1). The minimum:maximum velocity ratio ranged from -0.4 to 0.2 (mean, -0.15). With breath-hold at the end of expiration, the waveforms became slightly damped, becoming biphasic in five subjects and remaining multiphasic in seven. With breath-hold at the end of inspiration, the waveforms became nonphasic or biphasic in nine and decreased in velocity in 12. With Valsalva's maneuver, flow stopped. With normal respiration, cardiac waveforms were modulated by higher amplitude and less frequent biphasic respiratory waves. The plasticity was equal in two, dominantly cardiac in six, and dominantly respiratory in four. Flow velocity increased with expiration and decreased with inspiration. With deep breathing, the respiratory waves further increased, while the cardiac ones decreased in amplitude. The latter continued to modulate the respiratory phasicity in 10 subjects.
CONCLUSION: During quiet respiration, lower limb venous Doppler tracings consisted of both cardiac and respiratory waveforms. Although respiratory waveforms disappeared when patients held their breath, Doppler tracings continued to be multiphasic and cardiac. Therefore, cardiac phasicity in lower limb venous Doppler tracings does not necessarily indicate cardiac disease. Other respiratory phases can modulate this basic cardiac pattern. Decrease in or loss of phasicity in these waveforms does not always mean proximal obstruction, because it can be caused by respiratory factors. Finally, the presence of minimal cyclic retrograde flow that is 5 cm/sec or less does not necessarily indicate cardiac disease.