Dominant frequency increase rate predicts transition from paroxysmal to long-term persistent atrial fibrillation

Abstract

Background: Little is known about the mechanisms underlying the transition from paroxysmal to persistent atrial fibrillation (AF). In an ovine model of long-standing persistent AF we tested the hypothesis that the rate of electric and structural remodeling, assessed by dominant frequency (DF) changes, determines the time at which AF becomes persistent.

Methods and results: Self-sustained AF was induced by atrial tachypacing. Seven sheep were euthanized 11.5±2.3 days after the transition to persistent AF and without reversal to sinus rhythm; 7 sheep were euthanized after 341.3±16.7 days of long-standing persistent AF. Seven sham-operated animals were in sinus rhythm for 1 year. DF was monitored continuously in each group. Real-time polymerase chain reaction, Western blotting, patch clamping, and histological analyses were used to determine the changes in functional ion channel expression and structural remodeling. Atrial dilatation, mitral valve regurgitation, myocyte hypertrophy, and atrial fibrosis occurred progressively and became statistically significant after the transition to persistent AF, with no evidence for left ventricular dysfunction. DF increased progressively during the paroxysmal-to-persistent AF transition and stabilized when AF became persistent. Importantly, the rate of DF increase correlated strongly with the time to persistent AF. Significant action potential duration abbreviation, secondary to functional ion channel protein expression changes (CaV1.2, NaV1.5, and KV4.2 decrease; Kir2.3 increase), was already present at the transition and persisted for 1 year of follow up.

Conclusions: In the sheep model of long-standing persistent AF, the rate of DF increase predicts the time at which AF stabilizes and becomes persistent, reflecting changes in action potential duration and densities of sodium, L-type calcium, and inward rectifier currents.

Keywords: atrial fibrillation; electrophysiological; fibrosis; ion channels; refractory period.

Figure 1

Time-course of AF development. A: representative 3D plot of percentage of AF episodes in a given week (Y-axis) vs episode duration (X-axis) and weeks of follow-up after initiation of pacing (Z-axis). The first paroxysmal episode occurred 3 weeks after initiation of pacing. Duration of episodes progressively increased until persistent AF developed (week 12). B: summary of temporal measurements. AF: Atrial fibrillation; LS-PAF: Long-Standing Persistent AF.

Figure 2

AF-induced changes in extracellular matrix. A: Mean±SEM values for patchy fibrosis (left) and interstitial fibrosis (right) in right atrium (RA), left atrium (LA) and posterior left atrium (PLA) of sham-operated (N=6), transition (N=7) and LS-PAF (N=7). Twenty pictures per slide were randomly selected and analyzed; *p<0.05; **p<0.001 vs. sham. B: Representative picrosirius red staining of PLA of sham-operated, transition and LS-PAF. C and D: Western blots of α-smooth muscle actin (SMA) and Collagen III (Col III) in LA and RA tissue homogenates relative to GAPDH. N=6 for each group. *p<0.05, **p<0.01 vs. sham. N = number of animals.

Figure 3

Dominant frequency increases in RA and LA (A) and surface ECG (B) during progression of AF. N=14 for RA, N=8 for LA. #p<0.001 for RA vs. LA, **p<0.001 vs. sham. N= number of animals.

Figure 4

Rate of increase in DF during paroxysmal AF predicts transition to persistent AF. A: Representative graphs for three animals. Left, sheep with the highest dDF/dt (0.14 Hz/day, time to transition 19 days); middle, intermediate dDF/dt (0.03 Hz/day, time to transition 46 days); right, lowest dDF/dt (0.003 Hz/day, time to transition 346 days); left and right from transition group, middle from LS-PAF group. B: log-log plots of time from first episode to onset of self-sustained persistent AF versus dDF/dt for the RA (intracardiac electrode), LA (loop recorder) and ECG (surface Lead 1). Each point represents an animal. dDF/dt correlated with time to develop self-sustained persistent AF. N=14 for RA and ECG, N=8 for LA.

Figure 5

APD and frequency dependence in myocytes from sham, transition, and persistent AF. A: Action potential duration (APD₉₀ at 1Hz) is reduced in both atria at transition from paroxysmal to persistent AF. For RA: N=3/n=13 (sham), N=3/n=13 (transition), N=3/n=14 (LS-PAF); for LA: N=3/n=18 (sham), N=3/n=14 (transition), N=3/n=18 (LS-PAF). *p<0.05. Right: Representative LA APs are superimposed. B: Cycle length (CL) dependence of APD₉₀. For RA: N=3/n=13 (sham), N=3/n=13 (transition), N=3/n=14 (LS-PAF); for LA: N=3/n=18 (sham), N=3/n=14 (transition), N=3/n=18 (LS-PAF). *p<0.05 Transition and LS-PAF vs. sham at 1000 ms CL. #p<0.05 Sham at 300ms vs. sham at v1000ms CL. N= number of animals; n= number of cells.

Figure 6

Sustained AF reduces functional expression of Na⁺ and L-type Ca²⁺ channels. A: Current-voltage relationships for I_Na in myocytes from LA (left) and RA (right). For LA: N=3/n=12 (sham), N=4/n=21 (transition), N=5/n=21 (LS-PAF); for RA: N=3/n=10 (sham), N=4/n=18 (transition), N=5/n=18 (LS-PAF). *p<0.05 vs. sham, # p<0.05 vs. transition. B: Current-voltage relationships for I_CaL in myocytes from LA (left) and RA (right). For the LA: N=3/n=13 (sham), N=4/n=17 (transition), N=4/n=11 (LS-PAF); for the RA: N=3/n=12 (sham), N=4/n=16 (transition), N=4/n=14 (LS-PAF). *p<0.05 vs. sham. C: Representative traces for I_Na (upper) and I_CaL (lower) in myocytes from LA of sham-operated and LS-PAF animal. D–E: Western blot analysis of Na_V1.5 and Ca_V1.2 protein expression in LA tissue homogenates (D) and RA tissue homogenates (E). Top, Representative blots; bottom, Quantification of protein expression relative to GAPDH. N=6. F–G: Real time RT-PCR analysis of SNC5A and CACNA1C gene expression in tissue homogenates from LA (F) and RA (G); quantification of gene expression relative to GAPDH. N=6. **p< 0.01 vs. sham. N= number of animals; n= number of cells.

Figure 7

Sustained AF increases functional expression of Kir2.3. A: Current-voltage relationships for I_K1 in myocytes from LA (top) and RA (bottom). For LA: N=3/n=7 (sham), N=5/n=10 (transition), N=2/n=4 (LS-PAF); for RA: N=3/n=6 (sham), N=3/n=10 (transition), N=3/n=9 (LS-PAF). *p<0.05 vs. sham. B: Western blots for Kir2.3 in LA tissue homogenates. Top: representative blots of 2 different groups; bottom: quantification of protein expression relative to GAPDH. N=6. *p<0.05 vs. sham. N= number of animals; n= number of cells.

Figure 8

Simulations predict consequences of ion channel remodeling on rotor frequency. A: Action potential traces for sham, paroxysmal and transition AF predicted by experimentally derived ion channel changes (Figures 6–7). APD₉₀ was abbreviated in both paroxysmal and transition AF compared to sham. Resting membrane potential was hyperpolarized −2 mV. B: Rotor in paroxysmal (left) had lower frequency than transition AF. C: Rotors in paroxysmal AF meandered considerably and eventually self-terminated upon collision with boundary. In transition AF, the rotor was stable, had higher frequency and persisted throughout the simulation.