Diurnal heart rate patterns in inappropriate sinus tachycardia

Inappropriate sinus tachycardia (IAST) is a supraventricular tachycardia originating from the sinus node. Proposed etiologies for this symptom complex include autonomic dysfunction, abnormal automaticity, or hypersensitivity of the sinus node.

Patients with IAST were identified by symptomatic tachycardia, with P-wave morphology consistent with origination from a sinus location. A matched set of control subjects was included. Hourly heart rate (HR) was measured as the average HR during each one hour period on an ambulatory 24-hour Holter monitor. Patients were further divided into two subgroups based on average daily HR < or = 80 and >80 bpm. Harmonic analysis was used to evaluate diurnal variation.

The mean HR was 86.0 +/- 12.8 beats per minute (bpm) in the IAST group and 73.9 +/- 8.6 bpm in the control group (P = 0.056). There was an increased overall heart rate for the IAST group, which appeared to be more prominent in the morning hours. In the IAST subgroup with average daily HR < or = 80, hourly HR appears similar to controls for the period 8 pm-8 am. However, in the late AM, the IAST group had an increase in HR not seen in the control subjects. In the IAST subgroup with average HR > 80, there appeared to be a fixed difference in HR compared to the control group, without hourly change.

Patients with IAST and elevated average daily HR exhibit normal diurnal variation around a higher mean HR. In contrast, patients with IAST and lower average daily HR had an exaggerated morning rise in HR. These diurnal patterns may be useful to classify the pathophysiology of IAST.