Detection of acute myocardial ischemia during pharmacological stress in graded coronary artery stenosis: analysis of left ventricular asynchrony using Fourier phase imaging in pigs.

Background: Stress echocardiography is increasingly used to identify coronary artery disease, but quantitative techniques are required to improve the accuracy of this method. Current algorithms used to analyze wall motion usually neglect motion asynchrony that is found in acute ischemia. Fourier phase imaging of echocardiographic images may offer the possibility to detect asynchrony, but its feasibility with dobutamine stress echocardiography (DSE) is undefined. The aim of this study was to investigate whether the extent of left ventricular asynergy can be used to quantify the severity of regional myocardial dysfunction and to detect functionally significant coronary artery stenoses during DSE.

Methods: Regional wall motion abnormalities were induced by graded coronary stenoses (mild and severe) of the left anterior descending coronary artery (LAD) in seven open-chest anesthetized pigs. DSE (10-40 microg/kg/min) was performed under control conditions and during sustained ischemia. Coronary flow was measured under resting conditions and during maximal hyperemic response due to intravenous infusion of adenosine. Functional significance of stenoses was defined as mild when coronary flow reserve (CFR) was reduced but > 1.5 and severe when CFR was < or = 1.5. Echocardiographic cine loops were mathematically transformed using a first-harmonic Fourier algorithm displaying the sequence of wall motion as phase angles in parametric images and regional phase histograms. The phase difference (PD) of the first Fourier harmonic of posterior vs. anterior myocardial wall motion was calculated as an index of left ventricular asynchrony. Segmental fractional area shortening (FAS) and wall thickening (WT) as ratio of stress to rest served as a reference method of regional wall motion.

Results: The increase in FAS (1.62 +/- 0.6 vs. 0.42 +/- 0.2, p = 0.0002) and WT (1.92 +/- 0.5 to 0.3 +/- 1.1; p = 0.004) in anterior regions during DSE was significantly higher in the control group compared to severe ischemia but not compared to mild ischemia. During graded ischemia, profiles of phase angles were consistently modified, showing a delayed onset in regional contraction. The mean PD during DSE in the control group was 10.4 +/- 7 degrees. PD rose in mild ischemic segments (CFR > 1.5) to 28.9 +/- 10 degrees (p = 0.003) and to 89.6 +/- 25 degrees (p = 0.0002) in severely ischemic segments (CFR < or = 1.5). There was a significant inverse correlation between the PDs and WT (r = -0.87, p < 0.0001). Normal WT ratios yielded low phase angles while segmental phase angles increased with decreased WT. The intraobserver variability of phase analysis was 2.7 +/- 24 degrees (mean +/- 2SD).

Conclusions: These results suggest that left ventricular asynchrony is an indicator of acute ischemia. Echocardiographic Fourier phase imaging is feasible to quantify wall motion displaying contraction sequence in a simple and objective format and is a promising approach for the clinical interpretation of stress echocardiograms.