There is insufficient evidence to recommend CRT in patients with NYHA class I limitation or for nonambulatory class IV NYHA symptoms because these patients were largely not included in the major CRT studies. For this same reason, there is insufficient data to recommend CRT in patients with QRS durations < 130 ms. Patients with LBBB and QRS duration ≥ 150 ms appear more likely to benefit from CRT than patients with non-LBBB conduction and/or less QRS prolongation (see the Patient Selection chapter).