EFFECTS OF AUTONOMIC STIMULATION AND BLOCKADE ON SIGNAL-AVERAGED P-WAVE DURATION

Objectives. This study sought to evaluate the effects of autonomic sti mulation and blockade on the signal-averaged P wave duration. Backgrou nd. Signal averaged P wave duration has been shown to have prognostic implications for patients prone to develop atrial fibrillation, but au tonomic influences on the signal-averaged P wave duration have not bee n studied. Methods. In 14 healthy volunteers (8 men, 6 women; mean [+/ -SD] age 28.5 +/- 4.8 years, range 22 to 38), signal-averaged P wave d uration was measured on day 1 at baseline, during sympathetic stimulat ion with infusions of epinephrine (50 ng/kg body weight per min) and i soproterenol (50 ng/kg per min), beta-blockade with propranolol (0.2 m g/kg) and autonomic blockade with propranolol followed by atropine (0. 04 mg/kg). On a second day, 10 of the 14 subjects returned for repeat baseline recordings and recordings during parasympathetic blockade wit h atropine (0.04 mg/kg), Signal averaging was performed using a P wave template, Both unfiltered and filtered (least-squares fit filter with 100-ms window) P wave durations were measured, Day to day and interob server variability were assessed by calculation of intraclass correlat ion coefficients. Results. The mean (+/-SD) baseline filtered P wave d uration on day 1 was 141 +/- 10 ms. Isoproterenol infusion significant ly shortened the P wave duration to 110 +/- 16 ms (p < 0.001), and epi nephrine resulted in significant prolongation to 150 +/- 10 ms (p < 0. 05). Beta-adrenergic blockade increased the P wave duration to 153 +/- 10 ms (p < 0.005), Autonomic blockade shortened the P wave duration t o 143 +/- 16 ms (p < 0.05 vs, beta-blockade), On the second day, the m ean baseline P wave duration was slightly longer (144 +/- 10 ms, p < 0 .02), Parasympathetic blockade with atropine resulted in mild shorteni ng of the P wave duration to 136 +/- 15 ms (p < 0.1), Interobserver re producibility was excellent (intraclass correlation coefficient 0.99), Day to day reproducibility was good (intraclass correlation coefficie nt 0.56). Conclusions. The signal-averaged P wave duration is not a fi xed variable because it may change significantly under different auton omic conditions, This has important implications for the application o f this test to the heterogeneous population susceptible to atrial fibr illation.