Atrial arrhythmia in ageing spontaneously hypertensive rats: unraveling the substrate in hypertension and ageing

Abstract

Background: Both ageing and hypertension are known risk factors for atrial fibrillation (AF) although the pathophysiological contribution or interaction of the individual factors remains poorly understood. Here we aim to delineate the arrhythmogenic atrial substrate in mature spontaneously hypertensive rats (SHR).

Methods: SHR were studied at 12 and 15 months of age (n = 8 per group) together with equal numbers of age-matched normotensive Wistar-Kyoto control rats (WKY). Electrophysiologic study was performed on superfused isolated right and left atrial preparations using a custom built high-density multiple-electrode array to determine effective refractory periods (ERP), atrial conduction and atrial arrhythmia inducibility. Tissue specimens were harvested for structural analysis.

Results: COMPARED TO WKY CONTROLS, THE SHR DEMONSTRATED: Higher systolic blood pressure (p<0.0001), bi-atrial enlargement (p<0.05), bi-ventricular hypertrophy (p<0.05), lower atrial ERP (p = 0.008), increased atrial conduction heterogeneity (p = 0.001) and increased atrial interstitial fibrosis (p = 0.006) & CD68-positive macrophages infiltration (p<0.0001). These changes resulted in higher atrial arrhythmia inducibility (p = 0.01) and longer induced AF episodes (p = 0.02) in 15-month old SHR. Ageing contributed to incremental bi-atrial hypertrophy (p<0.01) and atrial conduction heterogeneity (p<0.01) without affecting atrial ERP, fibrosis and arrhythmia inducibility. The limited effect of ageing on the atrial substrate may be secondary to the reduction in CD68-positive macrophages.

Conclusions: Significant atrial electrical and structural remodeling is evident in the ageing spontaneously hypertensive rat atria. Concomitant hypertension appears to play a greater pathophysiological role than ageing despite their compounding effect on the atrial substrate. Inflammation is pathophysiologically linked to the pro-fibrotic changes in the hypertensive atria.

Figure 1. Atrial Refractoriness.

Atrial ERP was significantly lower in the SHR as compared to the WKY controls at both time points (Mean ERP from LAFW shown in the top row). Although ERP was not affected by ageing or pacing cycle length (S1), higher ERP was seen in the LA as compared to the RA (Mean ERP with S1 @200 ms shown in the bottom row). Error bars denote standard error of the mean.

Figure 2. Representative Atrial Activation Maps.

Activation maps from representative left atrium of both WKY (top row) and SHR (bottom row) during pacing with S1 at 200 ms are shown here. Isochronal lines of 2 ms are superimposed with fixed time range from 0 to 30 ms. Overall plaque activation times are similar indicating comparable atrial conduction velocity. However, the SHR demonstrated higher conduction heterogeneity as compared to the WKY, which were further increased with ageing. Corresponding phase histograms of these activation maps are also shown on the right panel of this figure.

Figure 3. Atrial Conduction.

Neither hypertension nor ageing had any significant effect on atrial conduction velocity (top row). In contrast, the SHR atria demonstrated higher absolute conduction heterogeneity (P5–95) and conduction heterogeneity index (P5–95/P50) than the WKY controls (middle & bottom rows). Both P5–95 and P5–95/P50 also increased significantly in tandem with ageing. In addition, increased pacing rate was associated with slower conduction velocity and higher P5–95 (top & middle rows). Significant differences were seen in all three conduction parameters between the left and right atrium.

Figure 4. Atrial Arrhythmia Inducibility and Duration.

Hypertension resulted in greater arrhythmia inducibility (left panel) and more sustained induced AT/AF episodes in the SHR as compared to normotensive WKY controls at 15 months (right panel). Horizontal lines denote the median values in the scatter plot with the proportion of animals with inducible arrhythmia shown in brackets. However, ageing had no significant effect on both parameters in both groups.

Figure 5. Atrial Structural Changes.

Increased atrial myocyte dimension (Left, top panel), collagen deposition (Left, middle panel) and CD68-positive macrophages (Left, bottom panel) were seen with hypertension. Although ageing had no effect on atrial fibrosis, atrial myocyte hypertrophy was evident with lower CD68-positive macrophages count in both SHR and WKY with ageing. Representative H&E (Right, A & B, magnification×400), picrosirius red (Right, C & D, magnification×200) and CD68 immuno-stained sections (Right, E & F, magnification×200) are shown to demonstrate the atrial myocyte hypertrophy, increased amount of interstitial collagen deposition and CD68-positive macrophages in the 15 month-old SHR vs. WKY atria.