Figure 92b

Trial of external transthoracic pacing in a patient with 2:1 AV block (A). In B the first three paced beats are captured as noted by RS and T wave. The fourth paced beat failed to produce either a QRS or a T wave while number 5 captured again. Paced QRS morphology changes when pacing nears threshold. Arrows represent the pacing stimulus.

Fletcher, R.D., MD, Wish, M., MD, Alexander, E.P., MD, Temporary Cardiac Pacing, Hurst's The Heart, 8th edition, p 809. (modified)