Watch the Example 3 online guitar lesson by Brad Carlton from Guitar Lab: Boogie-Woogie
This is a three voice passage which can be viewed as sixth intervals out of A mixolydian played over an A pedal tone. The 1st voice moves 3, 4, 5, 4; the 2nd voice moves 5, 6, b7, 6 and the 3rd voice pedals the root. The chords that are outlined are: A, D/A and A7 (no 3). This lick involves some stretching because the top voice is being played on the 3rd string.