Left atrial mechanical and biochemical adaptation to pacing induced heart failure.

OBJECTIVE: To determine the left atrial mechanical and biochemical adaptations to congestive heart failure, 10 dogs with rapid atrial and ventricular pacing and seven control dogs were studied.
METHODS: Animals were instrumented with left atrial sonomicrometers and micromanometers and left atrial pressure-volume relationships were generated by phenylephrine boluses for maximum elastance (Emax) and end systolic elastance (Ees) calculations. Left atrial maximum volume, ejection fraction, and mean circumferential fibre shortening (Vcf) were compared at matched left atrial pressure. At necropsy, myosin heavy chain (MHC) isoforms from the left atrial body and appendage were separated with SDS-PAGE, stained with monoclonal antibodies to alpha and beta MHC, and quantified with laser densitometry.
RESULTS: Left atrial ejection fraction and Vcf were significantly lower and maximum atrial volume and atrial systolic stroke volume were significantly greater in heart failure than in control. Emax was not significantly altered in heart failure, at 5.9(SD 2.9) v 4.5(1.6) mm Hg.ml-1 in controls. However, Vcf was lower (P < 0.05) and the A loop pressure-volume area (an index of eternal mechanical work performed by the left atrium) was greater (P < 0.05) in heart failure than in control dogs. The percent beta MHC in the left atrial body was greater in dogs with heart failure than in controls, at 42.6(9.8) v 17.3(9.0)%, P < 0.05. By contrast there was no significant beta MHC isoform switch in the left atrial appendage [14.4(7.6) v 17.9(9.7)%].
CONCLUSIONS: In this model of left atrial pressure and volume overload, there is significant upregulation of beta MHC in the left atrial body but not in the appendage and this isoform switch is associated with decreased velocity of left atrial contraction, increased atrial mechanical work, and unchanged force generation.