A normal spectral waveform from the hepatic veins is characterized as:

The normal spectral waveform from the hepatic veins is characterized as bidirectional because it reflects the unique flow dynamics influenced by both the cardiac cycle and respiratory changes. In healthy individuals, the blood flow in the hepatic veins typically exhibits a pattern of flow that varies with different phases of the heartbeat. This results in a waveform that shows both positive and negative flow components—positive during atrial contraction and negative during ventricular contraction, which corresponds to the respiratory changes in thoracic pressure.

This bidirectional nature is crucial for understanding the hemodynamics of the liver and indicates a normal physiological state. A unidirectional waveform would suggest a continuous flow in one direction, which is not representative of the normal pulsatile nature of venous flow in the hepatic veins. Meanwhile, triphasic waveforms are generally associated with arterial flow, indicating healthy arterial circulation, while a flat waveform suggests stagnant or low-velocity flow and can indicate pathological conditions. Thus, the bidirectional waveform accurately reflects the normal function of the hepatic veins and their relationship with the heart and respiratory system.