|Abstract or Summary
- Linear M-mode (1D) and later 2-dimensional (2D) measurements of the left atrium (LA) are commonly been used to assess LA size in veterinary studies despite the fact that studies in human subjects have shown that LA volume (LAV) more accurately reflects LA size than linear measurements due to the complex geometry of the LA. The primary goal of this study was to assess and compare LA size in a population of normal dogs using transthoracic M-mode (1D), 2D, and three-dimensional (3D) echocardiographic techniques. In addition, this study was designed to assess left atrial size at three different points in the cardiac cycle in order to characterize left atrial function in terms of dimensional changes at the beginning and end of ventricular and atrial systole.
Forty clinically normal dogs of various breeds were stratified into one of four groups (10 dogs <10 kg, 10 dogs=10-25 kg, 10 dogs=25-40 kg, and 10 dogs>40 kg). Complete echocardiographic examinations were obtained on all dogs; dogs with exceedingly poor image quality precluding measurement were excluded from analysis. LAV was estimated by converting linear measurements (average of 3 beats) derived from M-mode and 2D images into LAV using cube and sphere formulas from the right parasternal short axis view at the heart base. LAV was also estimated by the monoplane Simpson’s method of disks (MOD), monoplane and biplane area-length method (ALM) from left apical 4- and 2-chamber views. Three-dimensional volume data was derived from 3 different image sets obtained over 4 cardiac cycles from both the right parasternal and left apical imaging planes. Each cardiac cycle was further stratified into 3 conventionally defined LAV; maximum LAV (reservoir phase), minimal LAV (systolic phase), and pre-atrial contraction LAV (conduit phase). Using these volumes, various functional indices were obtained. These included the LA expansion index, total LA emptying volume/fraction, passive LA emptying volume/fraction, and active LA emptying volume/fraction.
The results of this study indicate that different methods of LA measurement are not all comparable. The M-mode derived estimates of LAV were significantly smaller than all 2D and 3D-derived LAV. With regard to the different methods employed for LAV assessment, the 3D LAP has the highest correlation with body weight for all phases of the LA cycle. The mean left atrial ejection fraction (LAEF) was approximately 50% with range of 24-76% in our population. The functional indices are method-dependent and may not be used interchangeably; the only functional index that correlated with BW in our population was total LA emptying volume. The interobserver variability for all LAV measurements was acceptable.
The results of our study support the hypothesis that 3D echocardiography is a feasible, noninvasive method to measure LAV in a population of normal dogs. The utility of 3D echocardiography in cardiac dysfunction also merits further investigative process.