Sinus Node Recovery Time (SNRT)

The simplest pacing protocol, the S1 train, delivers fixed-rate pulses to test the automaticity of the SA node. To measure the SNRT, the HRA is paced at a rate slightly higher than the intrinsic sinus rate for a sustained period of 30 to 40 seconds. Cycle lengths typically range from 700 ms down to 300 ms across multiple attempts. Upon the abrupt cessation of pacing, the clinician measures the exact time it takes for the SA node to recover from overdrive suppression and generate its first spontaneous impulse. A prolonged SNRT is a primary indicator of sinus node dysfunction. To streamline this process across patients with different baseline heart rates, the corrected SNRT (CSNRT = SNRT minus baseline sinus cycle length) is calculated instead of the raw value. Normal CSNRT is generally <525-550 ms in adults; values above this threshold are considered abnormal and support a diagnosis of sinus node dysfunction.