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Fig. 6A. —83-year-old woman with known atherosclerotic disease of left subclavian artery. Doppler sonogram shows conversion of presteal waveform to complete vertebral steal. Spectral tracings from left vertebral artery show waveform characterized by pronounced mid systolic cleft (arrow) and transient reversal of flow during systole. This wave pattern indicates bidirectional flow, represented above and below baseline. This transient reversal represents progression of mid systolic cleft noted in early presteal waveform illustrated in Figure 5A, 5B. Note that flow in artery is antegrade for most of cardiac cycle.