Nitroglycerin Enhances the Ability of Dobutamine Stress Echocardiography to Detect Hibernating Myocardium
Background A biphasic response of wall thickening with initial improvement and subsequent deterioration during dobutamine stress echocardiography (DSE) has been increasingly used for detection of hibernating myocardium. However, the improvement of wall thickening at low-dose DSE may be limited in hibernating myocardium by severe hypoperfusion. Nitroglycerin (NTG) improves myocardial perfusion, reduces oxygen demand, and may enhance low-dose dobutamine to improve wall thickening.
Methods and Results A pig model of myocardial hibernation of 24 hours to 7 days was created through severe left anterior descending coronary artery stenosis with coronary flow reductions of ≈40%, producing severe regional left ventricular dysfunction but no infarction in seven pigs. Myocardial infarction was produced in five pigs with occlusion of the artery. DSE was performed with incremental doses with and without an NTG infusion of 50 to 100 μg/min. In the hibernating group, NTG alone improved wall thickening in the hibernating region modestly from 11.4±7.2% at baseline to 19.1±7.0%. The improvement was associated with increased regional coronary flow from 0.46±0.12 to 0.55±0.13 mL · beat−1 · 100 g myocardium−1 (P<.05). There was an additive effect of NTG to low-dose (2.5 to 5 μg · kg−1 · min−1) dobutamine on wall thickening in the hibernating region. The improvement of wall thickening of hibernating myocardium with NTG and dobutamine, from 23.7±11.1% to 31.1±8.9% (P<.001), was associated with an increase in regional coronary flow (P<.01). NTG did not prevent high doses of dobutamine from inducing deterioration of wall thickening in hibernating myocardium. In the infarcted group, no improvement in wall thickening was observed in infarcted regions during NTG infusion, dobutamine infusion, or the combination.
Conclusions NTG enhances the improvement of wall thickening at low-dose dobutamine and does not prevent high-dose dobutamine from inducing ischemia in hibernating myocardium. Thus, NTG augments the biphasic response of wall thickening and improves the accuracy of DSE for detecting viable myocardium.
Dobutamine stress echocardiography is used as a diagnostic tool to noninvasively detect coronary artery stenoses and differentiate viable from infarcted myocardium in patients with LV dysfunction.1–6 It has been demonstrated that incremental doses of dobutamine characteristically induce a biphasic response with an improvement in wall thickening initially at low doses and deterioration subsequently at high doses in LV regions with hibernating myocardium subtending a critical coronary stenosis.7–9 However, this characteristic biphasic pattern was observed in only 60% to 76% of hibernating myocardial regions with low-flow perfusion with severe coronary artery stenosis in both human9 and animal7 studies.
In a pig model of short-term myocardial hibernation subtending a severe coronary stenosis, the degree of the initial improvement in wall thickening with dobutamine has been shown to correlate with an increase in regional coronary flow.7 Therefore, the failure of dobutamine to improve regional wall thickening may result from severely reduced perfusion that does not improve adequately with low-dose dobutamine. We hypothesized that agents that improve coronary flow and the oxygen supply/demand balance may enhance the sensitivity of dobutamine stress echocardiography for detection of hibernating myocardium. Nitroglycerin can improve the oxygen supply-to-demand ratio by increasing coronary flow in hypoperfused regions with severe coronary stenosis,10 redistributing flow favorably to the subendocardium,11–13 and by decreasing ventricular preload and afterload through reduction in venous blood return to the heart.14 Thus, nitroglycerin may magnify the initial improvement of wall thickening by low doses of dobutamine and enhance the sensitivity of detecting hibernating myocardium with the use of dobutamine stress echocardiography. However, whether nitroglycerin protects hibernating myocardium against ischemia with a deterioration of wall thickening at high doses of dobutamine is not known. Accordingly, this study was designed to test these hypotheses in a pig model of myocardial hibernation: Specific questions that we addressed were (1) whether nitroglycerin can improve myocardial perfusion in regions supplied by a severe coronary stenosis and enhance wall thickening of hibernating myocardium; (2) whether dobutamine and nitroglycerin have additive effects to increase wall thickening in hibernating myocardium; and (3) whether nitroglycerin can protect against the deterioration of wall thickening at high doses of dobutamine, altering the biphasic response of wall thickening to incremental doses of dobutamine in hibernating myocardium.
Seven pigs weighing 45.7±7.3 kg were studied. The study protocol was approved by the Committee on Animal Care at Hartford Hospital, and the animal care guidelines of the American Heart Association were followed. A pig model of 24-hour to 7-day myocardial hibernation in regions subtending severe LAD stenosis was created and has been previously described.15 Briefly, general anesthesia was applied with isoflurane (0.5% to 1.5%) and a 40% oxygen/60% nitrous oxide mixture. A midline thoracotomy was performed, and the heart was suspended in a pericardial cradle. A 3F coronary perfusion catheter was introduced retrograde via the coronary sinus to the cardiac vein parallel to the stenotic LAD segment for monitoring oxygen content, lactate, and pH. Full anticoagulation was achieved and maintained with 200 IU/kg heparin IV, followed by 30 IU/kg per hour. The proximal LAD was dissected free over 1 to 2 cm to accept a coronary flowmeter probe (Transonic) for measuring of LAD flow.
To produce myocardial hibernation, a LAD stenosis was created by gradually filling the hydraulic occluder with saline (n=2) or by the use of a silk tie (n=5) to partially occlude the LAD to reduce resting LAD flow to ≈60% of baseline.15 The stenosis was maintained for 24 hours (n=2) or 7 days (n=5) to produce myocardial hibernation of different durations because myocardial hibernation of different durations may have different features.16,17
To produce myocardial infarction, in another five pigs, the LAD was totally occluded for 15 to 25 minutes, and a LAD stenosis was created as described above to reduce coronary flow by 30% to 40%. The residual stenosis was maintained for 24 hours in two pigs and 7 days in three pigs.
Baseline measurements of heart rate, aortic pressure, left atrial pressure and regional coronary flow, coronary venous lactate, pH, and oxygen content were obtained under stable conditions, defined as two consecutive measurements at 5-minute intervals with a difference in pH of ≤0.02, coronary flow of ≤3 mL, and mean blood pressure of ≤5 mm Hg.
In the hibernating group of seven pigs, dobutamine stress echocardiography was first performed with incremental doses of dobutamine infusion from 2.5, 5, 10, and 20 to 30 μg · kg−1 · min−1 at 3-minute intervals. The dobutamine stress test was terminated if a significant regional wall motion abnormality was noted through online visual evaluation. After normalization from the previous stages for 15 to 20 minutes, a nitroglycerin infusion was started with the dose titrated to reduce systolic blood pressure by 5 to 10 mm Hg or to increase heart rate by 5 to 10 bpm. The nitroglycerin dose ranged from 50 to 100 μg/min. After recording of hemodynamic and echocardiographic changes with nitroglycerin, the same dobutamine stress protocol was repeated with simultaneous nitroglycerin infusion. Measurements of coronary flow, heart rate, blood pressure, and left atrial pressure were obtained at the end of each experimental stage. Coronary venous blood samples were obtained at low doses of dobutamine when the improvement of regional wall thickening in the hibernating region was maximal and at the maximal dose of dobutamine.
In the infarcted group of five pigs, nitroglycerin infusion was initiated first and followed by a dobutamine stress protocol as outlined above. After 30 minutes of normalization from the previous stages, the dobutamine stress protocol was repeated without nitroglycerin.
Two-dimensional epicardial echocardiography was performed at the short-axis view at midpapillary muscle level of the LV. Images were obtained from the epicardial surface of the right ventricle and recorded on a videotape for offline analysis of wall thickening. The end-diastolic frame of the echocardiographic images was selected using the onset of the Q wave of the ECG; the frame with the smallest left ventricular cavity was defined as end systole. The endocardial and epicardial borders of each frame were manually traced according to the technique recommended by the American Society of Echocardiography with a cursor and digitizing trackball that were directly interactive with echocardiographic images on a commercially available computer system (Nova Microsonics). The papillary muscles were excluded. Premature beats and the first beat after an extrasystole were excluded. The tracings were digitized and stored. The LV was divided into 100 equal chords, and regional wall thickening was quantified according to a centerline method,8,18 as shown in Fig 1⇓. The left ventricle was then divided into eight equal segments, starting from the conjunction of the LV inferior wall and the right ventricular or inferior ventricular septum, midseptum, anterior septum, anterior wall, anterolateral wall, posterolateral wall, posterior wall, and inferior wall.
In the hibernating group, the anterior septum and anterior wall were considered the regions supplied by LAD. Although LAD also supplied the anterolateral wall, this segment was not included in the analysis for wall thickening in the LAD region because of the possibility of overlapping perfusion to this segment from the circumflex artery and the LAD. The inferior wall was considered the normal control region. Wall thickening from 12 chords in each region was averaged for comparisons.
In the infarcted group, the anatomic infarcted location on TTC staining was defined through the use of natural markers of the LV such as distance from the junction between interventricular septum and the anterior wall and the lateral papillary muscles. The infarction was typically located centrally in the area at risk (LAD regions). The infarcted zone involved approximately one third of the area at risk (see “Results”) in the infarcted group in this study. A total of 36 chords were assigned to the LAD region, including anterior septum, anterior wall, and anterolateral wall. Therefore, the central third of the total regions supplied by the LAD or 12 chords in the center of the LAD region were defined as infarcted zone. In visual assessment, the corresponding infarcted center usually presented the most severe wall thickening abnormalities when the echocardiographically defined infarcted center was aligned to the pathologically confirmed infarcted center through the use of natural anatomic markers such as papillary muscles and the junction of interventricular septum and anterior wall. The adjacent 12 chords (6 chords on each side) of wall thickening to the infarcted zone were considered to be the peripheral viable (hibernating/stunned) region around the infarction, which was supplied by the stenotic LAD. To avoid the tethering effect of wall motion from adjacent normal regions, the most peripheral 12 chords in the LAD regions (the area at risk) that were adjacent to the midseptum and lateral wall were excluded in the analysis of wall thickening.
All echocardiographic measurements were performed by two observers who were blinded to each other’s results. Their results were averaged. Interobserver and intraobserver variabilities of measurement of regional wall thickening in our laboratory have been reported previously.8 The LV end-diastolic and -systolic volumes were determined according to a two-dimensional echocardiographic area-length method in which the area was measured from LV cross section at papillary muscle level and length was obtained from a long-axis view.19
Regional Myocardial Blood Flow Measurements
Regional myocardial blood flow was measured with a cuff flow probe connected to a Transonic flowmeter. At the conclusion of each experiment, the flowmeter was calibrated against a known rate of blood flow to ensure the accuracy of the measurements. After completion of the experiment, the pigs were killed, and methylene blue was injected into the LAD to stain the myocardium supplied by the stenotic vessel. The stained tissue was dissected and weighed to determine the regional myocardial mass perfused by the stenotic LAD. Regional coronary blood flow was expressed as mL · min−1 · g of wet tissue−1.8
Regional Myocardial Metabolic Measurements
To inhibit glycolysis, arterial and coronary venous blood samples were obtained anaerobically in cold, dry syringes containing heparin fluoride. Samples were divided for blood gases and lactate analysis, stored in ice, and processed immediately after the experiment. Blood gases were analyzed in duplicate, and the values were averaged. Plasma for lactate content was deproteinated with perchloric acid, neutralized with potassium hydroxide and imidazole buffer, and then analyzed according to the enzymatic method. Regional myocardial oxygen consumption was calculated by subtracting the coronary venous oxygen content from the arterial oxygen content and then multiplying the regional transmural blood flow supplied by the LAD. Lactate consumption/production was calculated by subtracting the coronary venous lactate from arterial lactate and then multiplying the regional transmural blood flow.
Pathological and Histological Examinations
At the conclusion of the experiment, the pigs were killed with an overdose of isoflurane, and the heart was arrested with an intravenous injection of 10 mL of 10% KCl. Methylene blue was injected distally into the stenotic LAD to delineate the area at risk as described previously. The LV was cut into cross sections at 0.5-cm intervals from apex to base. The area at risk was dissected and weighed. The LV sections (both the normal portion and the area at risk) were stained with 1.0% TTC to identify gross myocardial necrosis.7,15 In pigs with infarction (necrosis, not stained by TTC), total surface area at risk and necrotic area were traced onto transparent paper. The infarcted size for each pig was calculated by integrating necrotic areas and normal areas from all LV sections. All LV sections, including area with infarction, were then fixed with 10% formalin for ≥48 hours, embedded in paraffin, sliced into 5-μm sections, and stained with hematoxylin and eosin and Mason’s trichrome for light microscopic examinations.
All parameters were expressed as mean±SD. Repeated-measures ANOVA was used for parametric data to examine the difference between stages. Least-squares linear regression was applied to test the correlation between two parameters. Fisher’s exact test was used to examine nonparametric data. A value of P<.05 was considered statistically significant.
In seven pigs, no myocardial infarction was detected by TTC; these seven pigs were included in the hibernating group (Table 1⇓). Microscopic examination showed several areas of minimal (0.5 to 2 mm) patchy replacement fibrosis with granular tissue consistent with myocardial necrosis in the subendomyocardium in two pigs with 7 days of LAD stenosis. LAD coronary flow was reduced from 1.00±0.14 mL · g−1 · min−1 at baseline to 0.58±0.20 mL · g−1 · min−1 after creation of the stenosis or from 0.86±0.16 mL · g−1 · min−1 at baseline to 0.46±0.12 mL · 100 g−1 · beat−1 with the stenosis after correction for heart rate. Wall thickening in regions (anterior and anteroseptal) supplied by the stenotic artery was reduced from 38.4±4.0% to 11.7±7.2% (P<.001). Mean blood pressure was not different before and after the LAD stenosis.
In five pigs, myocardial infarction was detected with TTC, confirmed with microscopic examination, and included in the infarcted group. Mean infarcted area was 26.7±6.4% of the area at risk (LAD regions including anterior septum, anterior wall, and anterolateral wall). The infarction was transmural in three pigs as defined by infarcted area involving >50% of LV wall thickness. In two pigs, the infarction was located only in the subendomyocardium. In the infarcted group, LAD flow was reduced from 1.06±0.13 to 0 during the temporal total occlusion of the LAD for 15 to 25 minutes. After release of the occlusion and placement of the LAD stenosis, LAD flow was 0.67±0.19 mL · g−1 · min−1 in this group (Table 2⇓). Regional wall thickening was reduced from 38.6±4.5% to −0.8±2.5%. Other parameters are included in Table 2⇓.
Effects of Nitroglycerin and Dobutamine on Hibernating Myocardium
Table 2⇑ shows that coronary flow increased significantly from 0.58±0.20 mL · g−1 · min−1 at the stenosis baseline to 0.69±0.20 mL · g−1 · min−1 after infusion of nitroglycerin (P<.01); similarly, coronary flow per beat increased from 0.46±0.12 to 0.55±0.13 mL · 100 g−1 · beat−1 (P<.05), a 20% increase on average, whereas LV end-diastolic volume and left atrial pressure decreased after nitroglycerin (P<.01 and <.05, respectively). Wall thickening in the LAD region improved from 11.4±7.2% before nitroglycerin to 19.1±7.0% after nitroglycerin (P<.01, Figs 1⇑, 2⇓, and 3⇓), and an improvement of wall thickening of ≥7%, which was beyond the intraobserver variability,8 was observed in three of seven pigs. In contrast, the improvement of wall thickening in the control (inferior) region was not statistically significant (40.2±6.0% before and 43.1±4.1% after nitroglycerin, P>.05). Heart rate increased slightly (P<.05). Mean aortic pressure tended to decrease with nitroglycerin, but the difference was not statistically significant.
Effects of Nitroglycerin During Dobutamine Stress (Table 1⇑)
Additive Effect of Nitroglycerin and Dobutamine at Low Doses. The dose used for the maximal improvement of wall thickening (stages of low-dose dobutamine) was 2.86±0.94 μg · kg−1 · min−1 with nitroglycerin and 3.93±1.34 μg · kg−1 · min−1 without nitroglycerin. The heart rate was higher at dobutamine doses of 10 and 20 μg · kg−1 · min−1 with nitroglycerin than without nitroglycerin (Fig 3A⇑). Rate-pressure production, mean left atrial pressure, and end-diastolic volume tended to be lower with nitroglycerin than without for the same dose of dobutamine, but the differences did not reach statistical significance. With low-dose dobutamine, coronary flow per beat was 0.49±0.14 with dobutamine alone but 0.60±0.14 mL · 100 g−1 · beat−1 (P<.01) with nitroglycerin and dobutamine (Fig 3B⇑). Coronary flow increased from 0.55±0.13 with nitroglycerin alone to 0.60±0.14 mL · 100 g−1 · beat−1, a 9% increase on average, with the combination of nitroglycerin and dobutamine (P<.05, Fig 3B⇑).
Anterior wall thickening improved significantly with low-dose dobutamine or nitroglycerin alone (Figs 2⇑ and 3C⇑). This improvement was more obvious with dobutamine and nitroglycerin (31.1±8.9%) than with dobutamine alone (23.7±11.1%, P<.01, Fig 3C⇑). The improvement of ≥7% in regional wall thickening was observed in four of seven pigs with dobutamine alone and in all seven pigs with the combination of nitroglycerin and dobutamine. Inferior wall thickening did not differ between stages with and without nitroglycerin (56.0±2.6% and 51.8±6.4%, respectively, P=NS).
Effect of Nitroglycerin at High Doses of Dobutamine. Heart rate with nitroglycerin (180±25 bpm) was higher than without (173±27 bpm, P<.05). Mean blood pressure was not different. Coronary flow was higher with (0.77±0.24 mL · g−1 · min−1) than without (0.68±0.26 mL · g−1 · min−1 P<.01) nitroglycerin at the maximal dobutamine dose, as was coronary flow per beat (0.43±0.09 with versus 0.38±0.12 mL · 100 g−1 · beat−1 without nitroglycerin, P<.05; Fig 3B⇑). Wall thickening in the anterior region deteriorated at both conditions compared with low-dose dobutamine (each P<.001). At the maximal dose of dobutamine, there was no difference in wall thickening between stages with and without nitroglycerin (5.6±4.9% with versus 7.1±5.2% without, P=NS; Fig 3C⇑). Wall thickening in the inferior regions was also similar (63.2±4.9% with versus 63.9±4.0% without nitroglycerin, P=NS). Left ventricular end-diastolic volume was similar with or without nitroglycerin (48.4±19.8 and 47.7±25.1 cm3, P>.05), as was regional lactate production (P=NS).
The lowest dobutamine dose that produced the maximal improvement of wall thickening was 2.5 μg · kg−1 · min−1 in six of seven pigs and 5 μg · kg−1 · min−1 in one pig with nitroglycerin, somewhat lower than the stage without nitroglycerin (2.5 μg · kg−1 · min−1 in three of seven pigs and 5 μg · kg−1 · min−1 in four of seven pigs, P=NS). The maximal dobutamine dose that induced the maximal deterioration of wall thickening was 13±7 μg · kg−1 · min−1 with nitroglycerin, slightly lower than the dose (19±11 μg · kg−1 · min−1) used without nitroglycerin, but this did not reach statistical significance (P=NS), probably because of the small number of animals included. A maximal dose of 30 μg · kg−1 · min−1 was used in four of seven pigs with dobutamine infusion only, whereas no pig with the combination of dobutamine and nitroglycerin received 30 μg · kg−1 · min−1. When we compared regional wall thickening with and without nitroglycerin (Fig 3⇑) at each dose of dobutamine (2.5, 5, 10, and 20 μg · kg−1 · min−1), there was a significant difference between regional wall thickening with and without nitroglycerin during dobutamine infusion at some stages with the same dose. With a low dobutamine dose of 2.5 μg · kg−1 · min−1, wall thickening was significantly greater with nitroglycerin than without nitroglycerin. With a high dobutamine dose of 20 μg · kg−1 · min−1, regional wall thickening was significantly lower with nitroglycerin than without nitroglycerin. There was no difference in wall thickening with or without nitroglycerin at intermediate dobutamine doses of 5 and 10 μg · kg−1 · min−1.
Factors Related to the Improvement of Regional Wall Thickening
During low doses of dobutamine infusion with nitroglycerin, regional coronary flow per beat (r=.48, P<.05) and left ventricular end-diastolic volume (r=.58, P<.01) correlated with wall thickening in regions supplied by LAD. During high doses of dobutamine with nitroglycerin, regional coronary flow per beat (r=.60, P<.01) and myocardial lactate production (r=.64. P=.01) correlated significantly with the deterioration of regional wall thickening in regions supplied by the stenotic LAD. If data were pooled from low and high doses of dobutamine with nitroglycerin, correlation between the wall thickening and regional coronary flow per beat was similarly significant (r=.53, P<.01; Fig 4⇓).
Effects of Nitroglycerin and Dobutamine on Infarcted Myocardium and Viable Myocardium Peripheral to the Infarction
Table 2⇑ presents data illustrating the effects of nitroglycerin with and without the addition of dobutamine in the infarcted group. Although baseline LAD coronary flow reduction was similar (P=NS) compared with the hibernating group, regional wall thickening was more severely reduced in the infarcted group than in the hibernating group. There was no significant improvement of regional wall thickening in the infarcted region either after nitroglycerin alone (P=NS) or after the combination of nitroglycerin and dobutamine (P=NS). One pig (pig 88) with subendomyocardial infarction involving approximately one third of the wall thickness showed a significant improvement (8%) in regional wall thickening with the nitroglycerin and dobutamine combination. In all pigs, wall thickening in the peripheral regions of the infarction that was supplied by the stenotic LAD improved significantly, from 3.6±2.0% at baseline to 6.6±2.1% with nitroglycerin and to 16.4±5.8% with the combination of nitroglycerin and dobutamine (P<.05). The wall thickening in the peripheral region of the infarction also improved from 3.6±2.0% to 11.3±6.2% with low-dose dobutamine alone. Dobutamine alone improved wall thickening of ≥8% in three of five pigs, and the combination of nitroglycerin and dobutamine improved wall thickening of ≥8% in all five pigs in the peripheral regions of the infarction.
This study demonstrates that nitroglycerin confers a limited improvement of wall thickening detectable by two-dimensional echocardiography in hibernating myocardium subtended by severe flow-limiting coronary stenoses. The addition of nitroglycerin to low-dose dobutamine further improves wall thickening in hibernating regions and enhances echocardiographic detection of recruitable contractile reserve in viable hibernating myocardium. This additive effect of nitroglycerin and dobutamine at low doses did not prevent high doses of dobutamine from inducing ischemia with deterioration of wall thickening in hibernating regions. Therefore, the addition of nitroglycerin to dobutamine stress testing augments the characteristic biphasic response of initial improvement and subsequent deterioration of hibernating myocardial wall thickening during incremental doses of dobutamine infusion. Because the biphasic response is being used to an increasing degree clinically as a marker of both viability and inducible ischemia in the regions with viable myocardium, these results imply that nitroglycerin may improve the sensitivity and accuracy of dobutamine stress echocardiography for the detection of viable, hibernating myocardium clinically.
Study Design Limitations and Advantages
A major limitation of this study was the use of a model with a one-vessel (LAD) stenosis. This study ideally would have been conducted in a model of myocardial hibernation with multivessel coronary artery disease. Unfortunately, such a model has not yet been successfully developed. The fact that nitroglycerin can also improve coronary flow in multivessel disease10,13,24–27 suggests the possibility that results obtained in this study may be extrapolated to multivessel coronary artery disease. Radioactive microspheres were not applied to measure ratio of the subendocardial and subepicardial myocardial blood flow distribution; therefore, the effect of nitroglycerin on the redistribution of myocardial blood flow in the hibernating region could not be evaluated in this study.
Nitroglycerin was used intravenously in this study. Whether sublingual nitroglycerin can be used as a substitute was not evaluated; however, it is reasonable to assume that similar hemodynamic and functional effects can be achieved with sublingual nitroglycerin, although an appropriate substitute dose needs to be determined. There is concern that the protocol used in the hibernating group in which the dobutamine stress test was performed first and the stress test combining nitroglycerin and dobutamine was performed second might have aided nitroglycerin in inducing the improvement of wall thickening. However, baseline wall thickening and coronary flow were similar at the first baseline study before the dobutamine stress test only and the second baseline study at 20 minutes after a full run of dobutamine up to a maximal dose. This suggests that there was no significant residual ischemia or postischemic hyperemia (increase in coronary flow) after 20 minutes of dobutamine stress test. This is further ensured by the result of a similar enhancement in the LAD region of dysfunctional but viable hibernating/stunned myocardium around the infarcted regions in the infarcted group in which a different sequence of tests was used: nitroglycerin infusion was first used and followed by the combined dobutamine stress test, 30 minutes of normalization, and then dobutamine stress test alone. Because intravenous nitroglycerin has a half-time of 1 to 3 minutes, residual effects of nitroglycerin may be minimal with this protocol. This is supported by the fact that there were no differences in baseline hemodynamic data, coronary flow, and wall thickening before and after the first run of nitroglycerin and dobutamine stress test (Table 1⇑).
Different durations (24 hours and 7 days) of myocardial hibernation may have different features and degrees of coronary flow and contractile reserves. Although this study included pigs with myocardial hibernation of 24 hours and 7 days, the limited number of animals studied prevents further analysis of the different effects of nitroglycerin or dobutamine on hibernating myocardium of different durations. Myocardial infarction defined as positive TTC staining may miss minimal microscopic myocardial necrosis, which can occur in patients with clinical unstable angina. Indeed, two of seven pigs had a mild degree of patchy replacement fibrosis with granular tissue, indicative of microscopic infarction. However, the vast majority of myocardium supplied by LAD was not infarcted in these two pigs; therefore, we attribute them to the hibernating group.
Although we can conclude that nitroglycerin does not protect high doses of dobutamine from inducing ischemia in hibernating myocardium subtending severe coronary stenosis, whether nitroglycerin potentiates high-dose dobutamine to induce ischemia is suggested but cannot be conclusively determined on the basis of this study. At a dobutamine dose of 20 μg · kg−1 · min−1, wall thickening was slightly more reduced by the addition of nitroglycerin to dobutamine protocol than by the use of dobutamine alone. This was associated with an increase in lactate production and a lower coronary venous pH at a dobutamine dose of 20 μg · kg−1 · min−1 compared with baseline. Unfortunately, no lactate or pH measurements were done at a dobutamine dose of 20 μg · kg−1 · min−1 for dobutamine infusion alone without nitroglycerin by study design where coronary venous blood was drawn at low-dose dobutamine when maximal improvement of wall thickening was observed by online visual estimation and at the maximal dose of dobutamine. The reduction in wall thickening of 20 μg · kg−1 · min−1 with nitroglycerin added to dobutamine was slightly less but statistically not different from the reduction in wall thickening at a dobutamine dose of 30 μg · kg−1 · min−1 without nitroglycerin. Unfortunately, by study design, the end point of termination of dobutamine infusion was defined by a clinically used protocol in which dobutamine infusion is terminated when there was deterioration of regional wall thickening by visual evaluation of wall thickening. According to this protocol, a maximal dobutamine dose of 30 μg · kg−1 · min−1 was used in four pigs without nitroglycerin, whereas no pigs had a maximal dobutamine dose of 30 μg · kg−1 · min−1 with the dobutamine-plus-nitroglycerin combination. Therefore, it is not known whether regional myocardial lactate production would have been higher and whether coronary venous pH would have been lower if a maximal dobutamine dose of 30 μg · kg−1 · min− 1 had been used with nitroglycerin.
Angiographic studies in patients have demonstrated that nitroglycerin improves regional ischemic dysfunction and global LV ejection fraction, although the mechanism has not been well elucidated.20–22 Improved regional wall motion with nitroglycerin predicts recovery of LV function after coronary artery bypass graft surgery. However, because only a small number of patients have been included in these angiographic studies with nitroglycerin, the exact sensitivity, specificity, and predictive accuracy of nitroglycerin are unknown. In a preliminary report by Stadius et al,1,23 the sensitivity and accuracy were 50% to 65% and 70 to 73% with angiography, respectively. With echocardiography, the diagnostic value of applying nitroglycerin to enhance wall thickening or motion to predict functional recovery after revascularization has not been systematically studied.14
In myocardial perfusion imaging studies, nitroglycerin enhances myocardial blood flow during 201Tl reinjection and redistribution, and the combination of an isosorbide dinitrate infusion and rest 99mTc-sestamibi tomography increases tracer uptake in some chronically hypoperfused asynergic territories.24–27 The increased tracer uptake was associated with postrevascularization functional recovery.23 Therefore, nitroglycerin combined with nuclear perfusion studies at rest or during redistribution also appears promising for the noninvasive detection of viable hibernating myocardium.
Mechanism of the Effect of Nitroglycerin on Regional Wall Thickening in Hibernating Myocardium
A significant correlation between improvement of wall thickening and reduced LV end-diastolic volume with nitroglycerin suggests that reduced wall stress may contribute to increased wall thickening with nitroglycerin. However, the correlation is only modest, implying that other factors may be related. The improvement of myocardial perfusion or coronary flow in regions with severe coronary stenosis has been demonstrated by perfusion imaging studies with 201Tl or sestamibi and by regional coronary flow measurement using the 131Xe technique in patients10 and by the microsphere technique or Rb-86 clearance method in canine studies.11,12 In this study, the observation that the magnitude of improvement of coronary flow correlated significantly with improved wall thickening supports the notion that the two phenomena are related. Nitroglycerin improves regional coronary flow, providing fuel for low-dose dobutamine to enhance wall thickening. However, that depressed wall thickening may not recover for days or weeks after reperfusion of the hibernating myocardial regions15 suggests that the increase in coronary flow by nitroglycerin may not proportionally improve wall thickening. Dobutamine enhances hibernating myocardial contraction proportional to the increase in coronary flow by nitroglycerin.
Nitroglycerin did not prevent hibernating myocardium subtending severe coronary stenosis from ischemia with deterioration of regional wall thickening at high doses of dobutamine infusion. This result contrasts with the observation that nitroglycerin protects against exercise and pacing-induced myocardial ischemia in regions with severe coronary stenoses.28–30 The explanation for this is only speculative. In patients with coronary stenoses, marked increases in LV diastolic pressure and LV volume are seen during exercise-induced ischemia, and the mechanism by which nitroglycerin protects against ischemia has been demonstrated to be a reduction in LV diastolic pressure and volume by nitroglycerin during exercise.26 In contrast, during dobutamine stress testing, in a normal ventricle without ischemia, LV end-diastolic volume and end-diastolic pressure decreased significantly at high doses of dobutamine infusion; even in ischemic LVs, end-diastolic volume did not change or decrease, and end-diastolic pressure increased only mildly at high doses of dobutamine in this study and in our previous studies in one-vessel coronary disease.7,8 Intracoronary dobutamine infusion in a pig model of short-term myocardial hibernation has been demonstrated to significantly decrease myocardial ATP and creatine phosphate content.31 Whether the addition of nitroglycerin with initially enhanced regional wall thickening accelerates the exhaustion of myocardial ATP and other high-energy compounds in the hibernating region, leading to deterioration of wall thickening in hibernating myocardium during dobutamine stress test, cannot be determined in this study. This study showed that there was a tendency toward an early deterioration of wall thickening during incremental doses of dobutamine infusion by the addition of nitroglycerin. However, the difference between maximal doses of dobutamine, which were used in this study with and without the addition of nitroglycerin, did not reach a statistical significance. Obviously, a large number of animals are required to conclusively determine whether nitroglycerin accelerates ischemia in the hibernating region during high doses of dobutamine infusion.
A biphasic response of wall thickening with initial improvement and subsequent deterioration was similarly observed in the peri-infarcted viable myocardial region supplied by a significant residual coronary stenosis, although myocardium in the peri-infarcted region may be stunned, hibernating, or both. The mechanism for the biphasic response in the stunned/hibernating region supplied by a significant coronary stenosis is likely similar in the hibernating myocardium. The deterioration of wall thickening of peri-infarcted myocardium supplied by a significant residual stenosis is most likely due to myocardial ischemia rather than a tethering effect from the dyskinetic infarcted region because at low-dose dobutamine, there was an initial improvement in wall thickening in the peri-infarcted viable myocardium, whereas wall thinning or dyskinetic wall motion already occurred in the infarcted center with necrotic myocardium. If the deterioration of wall thickening at high doses of dobutamine were due to the tethering effect, it would have developed at the initial low doses of dobutamine and there would not have been the initial improvement of wall thickening in peri-infarcted viable myocardium with low doses of dobutamine.
Because it is characteristic of viable (hibernating/stunned) myocardial regions supplied by a significant coronary stenosis and the best parameter to predict functional recovery after revascularization,7,9 the biphasic response of wall thickening with initial improvement (recruitable contractile reserve) and subsequent deterioration (inducible ischemia) during incremental dobutamine infusion has been increasingly used clinically as a marker of viable myocardium with inducible ischemia. This study showed that the combination of low-dose dobutamine and nitroglycerin augments wall thickening to a significantly greater degree than either drug alone in hibernating myocardium subtending severe coronary stenosis or viable (stunned/hibernating) myocardium around the infarcted (peri-infarcted) region supplied by a significant residual stenosis. Furthermore, this study demonstrated that nitroglycerin did not prevent hibernating myocardium from ischemia with deterioration of regional wall thickening induced by high doses of dobutamine infusion. Therefore, nitroglycerin augments the biphasic response of wall thickening in the hibernating or stunned region supplied by a significant coronary stenosis, and the combination of nitroglycerin and dobutamine should be tested clinically to improve the sensitivity and accuracy of dobutamine stress echocardiography for the detection of hibernating or stunned myocardium with a significant residual coronary stenosis.
Selected Abbreviations and Acronyms
|LAD||=||left anterior descending coronary artery|
|LV||=||left ventricle, ventricular|
This study is supported by Hartford Hospital Research Fund. The authors appreciate the excellent technical assistance provided by William Dyckman and Edward Hall.
This study was presented in part at the 69th Scientific Sessions of American Heart Association, November 15, 1995, Anaheim, Calif, and in part at the 46th Scientific Session of American College of Cardiology, March 24, 1996, Orlando, Fl.
- Received February 6, 1997.
- Revision received August 18, 1997.
- Accepted August 28, 1997.
- Copyright © 1997 by American Heart Association
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