Assessment of Nonuniformity of Transmural Myocardial Velocities by Color-Coded Tissue Doppler Imaging
Characterization of Normal, Ischemic, and Stunned Myocardium
Background—Transmural myocardial contractile performance is nonuniform across the different layers of the left ventricular wall. We evaluated the accuracy of color M-mode tissue Doppler imaging (TDI) to assess the transmural distribution of myocardial velocities and to quantify the severity of dysfunction induced by acute ischemia and reperfusion in the inner and outer myocardial layers.
Methods and Results—Thirteen open-chest dogs underwent 15 minutes of left anterior descending coronary artery occlusion followed by 120 minutes of reperfusion. M-mode TDI was obtained from an epicardial short-axis view. Systolic velocities were calculated within endocardium and epicardium of the anterior and posterior walls. Regional myocardial blood flow was assessed by radioactive microspheres. Segment shortening was measured by sonomicrometry in endocardium and epicardium of both the anterior and posterior walls. At baseline, endocardial velocities were higher than epicardial velocities, resulting in an inner/outer myocardial velocity gradient. Ischemia caused a significant and comparable reduction in endocardial and epicardial systolic velocities in the anterior wall with the disappearance of the velocity gradient. Systolic velocities significantly correlated with segment shortening in both endocardium and epicardium during ischemia and reperfusion. In the first minutes after reflow, endocardial velocities showed a greater improvement than epicardial velocities, and the velocity gradient resumed although to a limited extent, indicative of stunning.
Conclusions—TDI is an accurate method to assess the nonuniformity of transmural velocities and may be a promising new tool for quantifying ischemia-induced regional myocardial dysfunction.
Experimental studies have demonstrated that circumferential fiber thickening varies across the different layers of myocardial walls and is more pronounced in endocardium than in epicardium.1 2 3 4 This transmural inhomogeneity is important to take into account in the setting of ischemic cardiomyopathy to differentiate the various patterns of contractile abnormalities that may occur during acute ischemia, hibernation, or stunning. Conventional assessment of contractile function is based on the measurement of transmural thickening. Previous ultrasonic studies of myocardial contractility based on M-mode or 2-dimensional images have assessed the differences between end-systolic and end-diastolic myocardial wall thicknesses or have involved digitization of endocardial and epicardial echoes.5 6 However, neither of these approaches provides information regarding the transmural distribution of contractile performance.
Tissue Doppler imaging (TDI) is a recent ultrasound technique that enables quantification of intramural myocardial velocities by detection of consecutive phase shifts of the ultrasound signal reflected from the contracting myocardium.7 8 9 TDI may display velocities with B-mode, M-mode, or pulsed Doppler. M-mode TDI overcomes the temporal resolution problems inherent in the B-mode approach, analyzes in real time endocardial and epicardial velocities, and provides new indexes of myocardial function such as the myocardial velocity gradient (MVG).10 11 Recent experimental studies using 2-dimensional and pulsed TDI have demonstrated that TDI can quantify ischemia-induced regional myocardial dysfunction, but there has been no report that M-mode TDI can characterize transmural distribution of velocities during ischemia and reperfusion.12 13
Therefore, the objectives of this study, performed in the open-chest canine model of ischemia-reperfusion, were (1) to assess the ability of M-mode TDI to quantify endocardial and epicardial velocities with sonomicrometry as a reference method, (2) to analyze the variations of the transmural distribution of systolic myocardial velocities induced by ischemia and reperfusion, and (3) to investigate whether M-mode TDI may help to differentiate ischemia- as opposed to reperfusion-induced contractile dysfunction.
Thirteen dogs were entered into the present study. In 2 dogs, contractile function could not be analyzed during reperfusion because of sustained ventricular arrhythmia. Overall data are presented for 81 matched systolic measurements of TDI velocities and segment shortening (SS).
Hemodynamic Data and Regional Myocardial Blood Flow
All dogs had comparable heart rates and blood pressures at baseline and throughout the experiment (Table 1⇓). Baseline regional myocardial blood flow (RMBF) was comparable in endocardium and epicardium in both the ischemic and nonischemic zones. As expected, left anterior descending coronary artery (LAD) occlusion resulted in a dramatic decrease in both endocardial and epicardial RMBF from 0.88±0.05 to 0.12±0.04 and 0.94±0.04 to 0.27±0.07 mL · min−1 · g−1, respectively (P<0.01 versus baseline for both). At 30 minutes after reflow, endocardial and epicardial RMBF in the anterior wall averaged 3.81±0.93 and 3.18±0.84 mL · min−1 · g−1, respectively, indicative of hyperemia (P<0.01 versus baseline) (Table 1⇓). In nonischemic myocardium, RMBF increase during occlusion was not statistically significant. At 30 minutes after reflow, RMBF increased significantly in the nonischemic zone although to a lower extent than in the ischemic zone (P<0.01 versus baseline, P<0.05 versus ischemic zone).
Normal Pattern of Myocardial Velocities
Pericardial opening induced a significant decrease in diastolic velocities but did not significantly alter systolic velocities (Table 2⇓). Myocardial velocities recorded after pericardial opening were used as baseline values for further comparison during ischemia/reperfusion. Velocity profiles derived from M-mode TDI traces indicated that distribution of transmural velocities was inhomogeneous across the myocardial wall. Velocities significantly and progressively increased from epicardium to endocardium (Figure 1A⇓). In the anterior wall, baseline endocardial and epicardial systolic velocities (Vs) averaged −4.9±0.7 and −1.7±0.4 cm/s, respectively (Figure 1B⇓). In the posterior wall, endocardial and epicardial Vs averaged 6.8±0.6 and 3.2±0.2 cm/s, respectively. Thus, a MVG could be measured (Figure 1C⇓). This MVG profile displayed 2 distinct negative peaks during systole and 2 positive peaks during diastole. During systole, the first peak was brief and occurred during isovolumic contraction, whereas the second peak was more prolonged, of smaller amplitude, and occurred during the ejection phase. The first diastolic peak occurred during isovolumic relaxation; the second, during the early ventricular filling phase. MVG was abolished during atrial contraction, indicating that during this time period, inner and outer myocardial layers were contracting at a similar speed.
Myocardial Velocity Changes During Ischemia-Reperfusion
Anterior wall thickening decreased from 55±9% at baseline to 3±1% during ischemia (P<0.0001). As depicted in Figure 2⇓, Vs dramatically decreased to a similar extent in the inner and outer layers. Vs averaged 0.4±0.1 cm/s in endocardium (P<0.0001 versus baseline) and 0.2±0.2 cm/s in epicardium (P<0.0001 versus baseline) (P=NS between endocardium and epicardium). Consequently, the MVG during the ejection phase was significantly reduced from 3.2±0.5 at baseline to 0.3±0.1 s−1 during ischemia (P<0.0001). The whole MVG profile was dramatically altered, with only 1 brief and small negative peak persisting during isovolumic contraction and mean MVG back to nearly zero during the ejection phase. As opposed to systolic velocities, diastolic velocities failed to change significantly (Table 2⇑).
Thirty minutes after reflow, anterior wall thickening remained severely depressed, averaging 11±5% (P<0.0001 versus baseline) (Figure 3⇓). Similarly, epicardial Vs remained dramatically low and not significantly different from the preceding ischemic values. In contrast, endocardial Vs tended to increase yet failed to fully recover. At 30 minutes of reperfusion, Vs recovered to 42±21% of baseline in endocardium (P<0.05 versus occlusion) but only to 9.5±12% of baseline in epicardium (P=NS versus occlusion). At that same time, MVG averaged 2.1±0.3 s−1, a value lower than baseline but significantly higher than the ischemic values (P<0.01).
During occlusion, wall thickening slightly but not significantly increased in the nonischemic territory to 115% of baseline values. Endocardial Vs and SS increased to 121% and 119% of control values, respectively (P=NS), whereas epicardial Vs and SS remained unchanged. After 30 minutes of reperfusion, all parameters returned to baseline values.
Correlations Between Vs, SS, and Myocardial Blood Flow
To evaluate whether the severity of regional contractile dysfunction induced by ischemia could be accurately evaluated by TDI, Vs was plotted versus SS (both expressed as percentage of baseline values) within endocardium and epicardium. VS% was significantly correlated to SS% within both endocardium (r=0.94, P<0.0001) and epicardium (r=0.91, P<0.0001) (Figure 4⇓).
The relationship between Vs and RMBF in the anterior wall at baseline, during ischemia, and after 30 minutes of reperfusion is summarized in Figure 5⇓. During occlusion, the decrease in endocardial and epicardial RMBF was accompanied by a dramatic reduction in endocardial and epicardial Vs Thirty minutes after reflow, despite hyperemia in both layers of the anterior wall, Vs remained significantly depressed to 42±21% and 9.5±12% of baseline values in endocardium and epicardium, respectively.
The present study demonstrates that M-mode TDI can accurately assess the nonuniformity of the transmural distribution of myocardial contractile performance. Specifically, M-mode TDI was able to differentiate and quantify ischemia- and reperfusion-induced endocardial and epicardial contractile dysfunction and assess the transmural MVG.
Nonuniformity of Left Ventricular Circumferential Thickening in Normal Myocardium
Conventional quantitative echocardiographic methods classically address transmural but not inner or outer layer myocardial function. Their principle is to analyze the displacement of the endocardial border, which does not take into account the nonuniformity of wall thickening.14 15 16 In a previous study, we used pulsed-wave TDI to analyze septal wall velocity resulting from left ventricular (LV) long-axis shortening and its variations after LAD occlusion.13 We demonstrated that pulsed-wave TDI is accurate to quantify online ischemia-induced dysfunction. But we were unable to discriminate endocardial and epicardial velocities.
In the present study, M-mode TDI allowed interrogation of intramural velocities to quantify LV circumferential contraction. This TDI modality is the first noninvasive method that can quantify in the in situ heart the velocity of myocardial thickening that has been recognized as an index of regional contractility in isolated papillary muscle preparations.17 Theoretical considerations based on various models of the LV and numerous in vivo experiments have clearly demonstrated that wall thickening is not uniform and the ratio of inner to outer half-thickening approximates 2.0:1.0.1 3 4 The present M-mode TDI data are consistent with these previous studies. As depicted in Figure 7⇓, the ratio of endocardium to epicardium systolic velocities was close to 2.0:1.0. This progressive increase in velocities from inner to outer layer created, under baseline conditions, a velocity gradient that may represent an interesting new index of regional myocardial function.10 18
Detection and Quantification of Ischemia- and Reperfusion-Induced Wall Motion Abnormalities by M-Mode TDI
During LAD occlusion, endocardial and epicardial SS was replaced by passive bulging consistent with a severe reduction in myocardial blood flow. TDI data were closely related to sonomicrometric measurements, indicating that M-mode TDI allows accurate quantification of contractile function during ischemia and reperfusion. Importantly, this accuracy of M-mode TDI applied for any severity of regional dysfunction exhibited by endocardium and epicardium. Both endocardial and epicardial velocities were markedly and uniformly decreased during ischemia and resulted in the disappearance of MVG. This absence of MVG across the anterior wall during severe flow reduction is congruent with previous investigations that reported the abolition of the transmural thickening gradient during dramatic flow deprivation.19 20 It is worth noting that no significant decrease occurred in diastolic velocities during ischemia. This may be related to the fact that the pericardial opening had already significantly reduced diastolic velocities, thereby possibly blunting further reduction related to ischemia.13
After reperfusion, wall motion in the distribution of the LAD remained severely depressed, indicative of stunning. Conventional M-mode imaging failed to detect any significant improvement in transmural wall thickening. In contrast, M-mode TDI was able to detect a slight but significant increase in endocardial (but not epicardial) velocities, resulting in the resumption of a MVG. This MVG, however, was short lived because endocardial and epicardial velocities were no longer different at 90 minutes of reperfusion. The greater improvement in endocardial velocities early after reflow was likely a consequence of the hyperemic response to the preceding ischemic insult, as suggested by Figure 4⇑. Despite hyperemia, epicardium failed to recover early after reperfusion, suggesting the development of severe stunning and possible tethering to endocardium.21 These data are in close agreement with those from a study by Bolli et al20 that reported comparable time course of nonuniform transmural functional recovery after reflow in dogs submitted to 15 minutes of LAD occlusion followed by 7 days of reperfusion. In that study, dogs exhibited a transmural systolic thickening gradient at baseline that disappeared during ischemia. On reperfusion, the inner/outer gradient first resumed, was maximal during the first hour after reflow, and decreased thereafter.20
During reperfusion, RMBF and TDI velocities were simultaneously measured 30 minutes after reflow. At that time point, as previously demonstrated,22 there is considerable variability in the response of both RMBF and myocardial wall velocity to reperfusion and no relationship between wall velocity and myocardial perfusion (Figure 5⇑).
Our data demonstrate that M-mode TDI is a sensible technique that can detect and quantify mild changes in regional wall motion that may occur during ischemia or reperfusion. The present study has potential important clinical implications, ie, identification and quantification of the regional endocardial or epicardial contractile dysfunction that may arise during or as a consequence of acute coronary syndromes. However, further studies are needed to determine whether these data can apply to human patients.
All experiments performed in this study conformed to the Guiding Principles in the Care and Use of Animals approved by the American Physiological Society. Thirteen adult mongrel dogs weighing 20 to 39 kg were instrumented as previously described.13
Echocardiography was performed by use of a SEQUOIA system (ACUSON) with a 7-MHz transducer. Measurement of myocardial velocities resulting from the left ventricular (LV) circumferential contraction was performed with the beam positioned on the midanterior wall, from an epicardial short-axis view at the level of the papillary muscles (Figure 6⇓). A first series of velocity measurements was performed before the pericardium was opened to determine whether this might induce some change. Thereafter, all measurements were performed with the heart suspended in a pericardial cradle. Gray-scale receive gain was set to optimize the clarity of the endocardial and epicardial boundaries. Doppler receive gain was adjusted to maintain optimal coloring of the myocardium. Doppler velocity range was set as low as possible to avoid aliasing occurrence. The angle of interrogation of the M-mode beam was carefully aligned to be perpendicular to the LV walls. Freeze-frame images were then downloaded to a magneto-optic disk and transferred to an IBM-compatible computer for offline analysis. Custom-made software was designed to analyze myocardial velocities from M-mode TDI traces. This computer program converted the digital representation of colors into velocity values with the use of color values obtained from the velocity scale bar. This color bar displays a linear representation of velocities (64 colors representing 32 positive and 32 negative velocities) and was used as a lookup table for the conversion. Thus, a velocity value was determined for each pixel by finding the best matching color value stored in the lookup table. By convention, velocities were encoded “positive” and “negative” when the displacement of the myocardium was directed toward or away from the transducer, respectively.
To provide validation of this computer program, we specifically set up an in vitro model to measure velocities in a continuous hydraulic, nonturbulent jet. We used an electrical syringe (Harvard Apparatus, model 55–1119) generating a forward and backward flow (from 1.59 to 31.8 mL/min in both directions) through a 4.8-mm-diameter catheter. Fluid was composed of 70% water and 30% glycerol to obtain a viscosity comparable to that of blood. Fluid velocity within the catheter was calculated by the following formula: Velocity=[flow (mL/min)/60]×ΠD2/4. Four positive and 4 negative velocity values were obtained: 2.9, 1.18, 0.58, and 0.3 cm/s.These velocities were then plotted against the mean velocity measured within the catheter by use of TDI technology with a 5-MHz probe at a 0.27- and 0.34-m/s Nyquist limit. As depicted in Figure 7⇓, the correlation between fluid and TDI velocities was excellent (r=0.97). In addition, the Bland-Altman test confirmed that both methods provided comparable results.
After baseline measurements, the left anterior descending (LAD) coronary artery was occluded for 15 minutes and reperfused for 120 minutes. Echographic and sonomicrometric recordings and regional myocardial blood flow (RMBF) measurements were performed sequentially at the following time points: at baseline, during occlusion, and 30 minutes after reperfusion. Echographic measurements were also performed at 5, 15, 30, 60, and 90 minutes after reflow.
At the end of each experiment, the LAD was briefly reoccluded, and 0.5 mg/kg Unisperse Blue Pigment (Ciba-Geigy) was injected intravenously to delineate the in vivo area at risk, as previously described.23 Under deep anesthesia, the heart was stopped by intravenous injection of potassium chloride, excised, and cut into 5 to 7 (10-millimeter-thick) slices parallel to the AV groove. We verified that the anterior wall at the level of the papillary muscles was unstained, ie, that the TDI interrogation was well in the ischemic area. The correct position of the 2 pairs of ultrasonic crystals was checked within both the area at risk and the remote nonischemic zone. Each slice was then incubated for 15 minutes in triphenyltetrazolium chloride at 37°C to exclude any necrosis.
Heart rate and arterial and LV blood pressures were averaged over 5 continuous cardiac cycles in sinus rhythm at baseline, during occlusion, and after reperfusion.
Conventional echographic measurements (LV wall thickness, wall thickening, and LV cavity dimensions) were obtained from gray-scale M-mode tracings according to the criteria of the American Society of Echocardiography. The anterior and posterior walls were arbitrarily divided from the endocardial to epicardial borders into 2 layers of equal thickness by manual tracing of endocardial and epicardial boundaries. This allowed calculation of endocardial and epicardial velocities by M-mode TDI and further comparison with sonomicrometry. Endocardial and epicardial mean velocities were defined as the average values of the velocity estimates measured along each M-mode scan line throughout the thickness of the inner and outer layers of myocardial walls. Peak mean systolic velocity (Vs) was defined as the maximum value of the mean velocity during the ejection phase. MVG was defined as the difference between endocardial and epicardial velocities divided by wall thickness. Three beats were averaged for each of these measurements. During occlusion or after reperfusion, velocity was expressed as a percentage of baseline values. Myocardial velocities and MVG were measured by 2 independent observers in 10 animals to determine interobserver and intraobserver variabilities. Interobserver variability was low: 0.27±0.2 cm/s for Vs and 0.12±0.15 s−1 for MVG. Intraobserver variability was low: 0.24±0.2 cm/s for Vs and 0.1±0.12 s−1 for MVG.
Regional Myocardial Function
Segment shortening (SS), used as an index of systolic function, was defined as follows: SS=[(EDL−ESL)/EDL]×100%, where ESL and EDL are end-systolic and end-diastolic lengths, respectively. ESL and EDL were obtained from 3 cardiac cycles in each sample period. EDL was measured at the onset of the rapid increase in LV dP/dt, whereas ESL was measured at peak negative LV dP/dt. SS during each sample period was expressed as percentage of baseline values (SS%).
Measurement of RMBF
RMBF (mL · min−1 · g−1) was assessed by injection of radioactive microspheres labeled with 141Ce, 95Nb, or 103Ru (Du Pont–New England Nuclear), as previously described.23 RMBF in the ischemic area was expressed as a percentage of RMBF in the nonischemic region. RMBF was measured at baseline (n=12), during occlusion (n=12), and reperfusion (n=10).
Baseline and subsequent echographic and sonomicrometry measurements were compared by use of repeated-measures ANOVA. Standard linear regression analysis was used to relate changes in systolic velocities to either SS or RMBF. All values are presented as mean±SEM. A value of P<0.05 was considered statistically significant.
The Methods section of this article can be found at http://www.circulationaha.org
- Received June 2, 1999.
- Revision received October 7, 1999.
- Accepted October 20, 1999.
- Copyright © 2000 by American Heart Association
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