- Email: [email protected]
Pharmacologically induced myocardial ischemia: A comparison of dobutamine and dipyridamole
Pharmacologically Induced Myocardial Ischemia: A Comparison of Dobutamine and Dipyridamole Douglas S. Segar, MD, Thomas Ryan, MD, Stephen G. Sawada, MD, Michael Johnson, and Harvey Feigenbaum, MD, Indianapolis, Indiana
The purpose of our study was to compare the ability of dobutamine and dipyridamole infusion to induce myocardial ischemia. In a population of 16 anesthetized open-chest swine, a coronary artery stenosis sufficient to abolish the hyperemic response to a 15-second total occlusion was created. Heart rate, systolic blood pressure, and d P / d t were recorded. Myocardial segment shortening was determined by sonomicrometry in all animals. In a subset of seven animals regional myocardial blood flow was measured by injection o f radiolabeled microspheres. Dipyridamole was infused according to a high-dose protocol. After a washout period and reestablishment of a baseline state, dobutamine was infused incrementally. There was no significant difference between the baseline states. Dipyridamole did not affect heart rate but did significantly decrease blood pressure and rate-pressure product. Myocardial segment shortening decreased in the ischemic zone by 0.07 +- 0.08 (p = 0.004). Dobutamine infusion significantly increased heart rate, blood pressure, and rate-pressure product. Myocardial segment shortening in the ischemic zone decreased by 0.17 +-. 0.09 (p < 0.001). Dobutamine decreased blood flow in the ischemic zone relative to baseline. Both dobutamine and dipyridamole infusion resulted in myocardial ischemia. The magnitude of the ischemic response is greater for dobutamine than for dipyridamole. (J AM Soc ECHOCmmlOGV, 1995;8:9-14.)
P h a r m a c o l o g i c stress testing is being used increasingly in the evaluation o f patients with known or suspected coronary artery disease. The most commonly employed pharmacologic stress agents are dipyridamole and dobutamine. The mechanism o f action o f these two drugs differs substantially. Although both agents are widely used clinically, little work has been done to investigate the physiology o f myocardial ischemia resulting from their pharmacologic effects? q~ From the Department of Medicine, Indiana University School of Medicine, The Krannert Institute of Cardiology, and the Wishard Memorial Hospital. Supported in part by the Herman C. Krannert Fund, Indianapolis, grants HL-06308 and HL-07182 from the National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland, and the American Heart Association, Indiana Affiliate, Indianapolis. Presented in part at the Fourth Annual Meeting of the American Society of Echocardiography, June 1993, Orlando, Florida. Reprint requests: Douglas S. Segar, MD, Krannert Institute of Cardiology, 1111 W. Tenth St., Indianapolis, IN 46202-4800. Copyright 9 1995 by the AmericanSocietyof Echocardiography. 0894-7317/95 $3.00 + 0 27/1/58982
T o date, w o r k in most animals has been in a canine model. The canine is known to have a coronary artery distribution that includes extensive collateral vessels. Swine have a coronary artery distribution that is more similar to that o f humans and is therefore a more representative model for investigation o f pharmacologically induced myocardial ischemia. 11 The purpose o f this study was to compare the ability o f dipyridamole and dobutamine to induce regional left ventricular dysfunction (myocardial ischemia) with a swine model o f fixed coronary artery stenosis.
METHODS
Twenty-six domestic swine o f either sex (18 to 27 kg; mean 21.5 kg) were used in this study. Ten animals were eliminated from analysis either because o f death during the study or inadequate data collection (usually a resuk o f sonomicrometer loss). The remaining 16 animals completed the study and are reported. The study protocol was approved by the In-
10
Segar et al.
diana University Laboratory Animals Review Committee for humane treatment of animals. Anesthesia was induced by injection of thiopental sodium (25 mg/kg). The animals were then intubated and placed on mechanical ventilation with 100% oxygen (Ohio Medical). Anesthesia was maintained with inhaled isoflurane (1 to 1.75 vol%). At the end of the experiment, the animals were given additional thiopental and killed with a lethal injection of saturated potassium chloride. Animal Preparation Catheters were placed in the femoral artery and vein for pressure measurement , blood withdrawal, and fluid :and drug administration. The femoral artery catheter was connected to an appropriately calibrated fluid-filled pressure transducer. An electrocardiogram was recorded in standard fashion with needle electrodes. A left lateral thoracotomy was performed and the heart was suspended in a pericardial cradle. A Millar catheter was placed into the left ventricle through a stab incision in the left atrial appendage passing antegrade through the mitral valve. A largebore catheter was placed into the left atrium through the same incision for radioactive microsphere injection. Paired 5 M H z sonomicrometer crystals were positioned in the subendocardial layer of the myocardium along the minor axis of the ventricle to measure myocardial segment shortening. One set was placed in the distribution o f the left anterior descending coronary artery (designated the ischemic zone) and the other was placed in the distribution of the left circumflex coronary artery (the nonischemic
zone). An area of the mid-left anterior descending coronary artery was dissected free. A Doppler flow probe (Transonic Technology, Ithaca, N.Y.) was placed around the artery. A rigid plastic collar was then placed around the artery. An appropriately sized commercially available angioplasty balloon was placed between the plastic collar and the artery. The angioplasty balloon was inflated with water, creating a stenosis in the coronary artery. The degree of balloon inflation was adjusted to abolish the hyperemic response (as measured by the Doppler flow probe) to a 15-second complete vessel occlusion. The inflation pressure was monitored and maintained constant throughout the experiment. All hemodynamic and sonomicrometric data were recorded on a Gould strip chart recorder (Gould Inc., Cleveland, Ohio). In addition, all data were digitized on line and recorded on a personal computer-based analysis system (Macintosh II FX, Lab View Run
Journal of the American Society of Echocardiography January-February 1995
Time; National Instrument Co., Inc., Baltimore, Md.).
Sonomicrometry Sonomicrometer crystals were connected to a commercially available multichannel sonomicrometcr system (Triton Technology, San Diego, Calif.). Calibration of the sonomicrometcr was performed before each study and the calibration standards were transferred to the computer analysis system. End diastole was defined as the onset of the rapid upstroke of the dP/dt (determined by the left ventricular catheter). End systole was defined as 25 mscc before the peak negative dP/dr. The segment shortening was calculated according to a standard formula: SS = (EDL - ESL)/EDL, where SS = segment shortening, EDL = end-diastolic length, and ESL = end-systolic length. At the end of the experiment, placement of the sonomicrometer crystal was confirmed by visual inspection.
Radioactive Microspheres Myocardial blood flow was determined with radioactive microsphercs (STCo, 113Sn, l~ and l~ New England Nuclear, Boston, Mass.). 12 Microspheres (3.66 x 10s/rag) were suspended in 10% dextran with Twecn 80 and agitated for a minimum of 5 minutes before 50 p~Ciwas injected into the left atrium. Blood was withdrawn at 8 ml/min with a Harvard pump (Harvard Apparatus Co., Inc., S. Natick, Mass.) beginning 20 seconds before injection and continuing for a total of 2 minutes. Segments of the myocardium in the ischemic and nonischemic zones were divided into epicardial and endocardial halves and approximately 1 gm tissue was placed into a scintillation counter (Packard Instrument Co., Inc., Downers Grove, Ill.). Spillover was corrected for and myocardial and blood activity was recorded. Myocardial blood flow was calculated according to the a standard formula: MBF = (C,/TW) • (RBW/Cb) • 100, whereMBF = myocardial blood flow in ml/min/100 gm tissue, C~ = tissue radioactive counts, TW = tissue weight in grams, RBW = reference blood withdrawal rate in milliliters per minute, and CD = total radioactive counts of reference blood sample. Experimental Protocol After creation of a flow-limiting stenosis, baseline measurements of heart rate, systolic blood pressure, left ventricular pressure and dP/dt, myocardial blood flow, and segment shortening ill both the ischemic and nonischemic zones were recorded. Dipyridamole
Journal of the American Society of Echocardiography Volume 8 Number 1
S e g a r et al.
11
Table 1 Hemodynamic changes during dipyridamole and dobutamine infusions Heart rate (beats/min)
Baseline Dipyridamole Baseline Dobutaminc
78 79 78 148
• 11" • 1P • 13? _+ 3 3 t
Systolic blood pressure ( m m Hg)
110 86 94 117
• 16t _+ 2 1 t • 16t • 26~-
Rate-pressure product (man Hg/min)
8661 6879 7496 17264
• 18707 _+ 2233? • 2387~ • 5788t
*Difference not significant. tp < 0.001.
was infused (Harvard pump) according to a highdose protocol (0.56 mg/kg in 4 minutes followed 4 minutes later by an additional 0.28 mg/kg given in 2 minutes). Measurements were obtained 3 minutes after completion of the dipyridamole infusion. The animals was then allowed to recover for 2 hours, after which a second set of baseline measurements was obtained. The dobutamine was infused in an incremental fashion beginning at a dose of 5 ~g/kg/min and increasing at 3-minute intervals to 10, 20, 30, and 40 wg/kg/min. Measurements were obtained on evidence of myocardial ischemia or reaching the peak dose. Statistical Analysis Paired data in each animal were analyzed with a paired t test. Repeated-measures analysis of variance was used when comparing more than one variable in each animal.
RESULTS Comparison o f Baseline States There was no significant difference in the heart rates between the two baseline states (78 • 11 and 78 + 13 beats/min; difference not signifcant). Systolic blood pressure and rate-pressure product were lower at the second compared with the first baseline state (both p < 0.05; Table 1), which is most likely a result of the cffkcts of anesthesia. Importantly, there was no change in segment shortening in either the ischemic zone (0 • 0.03) or the nonischemic zone ( - 0 . 0 1 + 0.03) between the two baseline states.
Effect o f Dipyridamole The dipyridamole infusion was completed in all animals. Heart rate did not change significantly during the dipyridamole infusion. Both systolic blood pressure and rate-pressure product decreased during di-
pyridamole (p < 0.001; Table 1). There was a significant decrease in myocardial systolic segment shortening relative to baseline in the ischemic zone of 0.07 + 0.08 (p = 0.004) during drug infusion. Twelve (75%) of the 16 animals had an ischemic response to dipyridamole. The magnitude of the change in segment shortening varied from + 0.04 to - 0 . 1 6 . In the nonischemic zone there was a small increase in segment shortening of 0.02 +_ 0.03 (p = 0.01).
Effect o f Dobutamine The dobutamine infusion protocol was completed in all animals. The mean peak infusion rate was 26.5 ~g/kg/min. Dobutamine caused a mean increase in heart rate from 78 _+ 13 to 148 • 33 beats/min (p < 0.001). Both systolic blood pressure and ratepressure product increased significantly with dobutamine (both p < 0.001; Table 1). In the ischemic zone dobutamine infusion caused a mean decrease in segment shortening of 0.17 +_ 0.09 (p < 0.001). Sixteen (100%) out of 16 animals had a decrease in segment shortening during dobutanlinc infusion. The range of the decrease in segment shortening was from - 0.08 to - 0.42. In all animals the magnitude of the change in segment shortening in the ischemic zone was greater with dobutamine compared with dipyridamole (p < 0.001) (Figure 1). Six of the 16 animals had dyskinctic segment shortening during dobutamine; none of the animals had a dyskinetic response with dipyridamole. In the nonischemic zone dobutamine increased myocardial segment shortening by 0.05 _+ 0.06 (p = 0.03).
Regional Myocardial Blood Flow Regional myocardial blood flow was determined in a subset of seven animals. There were no differences in myocardial blood flow at baseline. Dipyridamole infusion did not change blood flow in the ischemic zone relative to baseline flow (Figure 2). In the nonischemic zone dipyridamole significantly increased
12
Journal of the American Societyof Echocardiography January-February 1995
Segar et al.
INDIVIDUAL ANIMALS 0,06 0
,,
-0.05 -0.1 -0.15 -0.2 -0.25 , -0.3. -0.35. -0.4, -0.45.
Figure 1 Change in segment shortening for each animal during infusion of diwridamole or dobutamine. Change in segment shortening is on vertical axis and each individual animal is found on horizontal axis. Black column is response to dipyridamole; white column is response to dobutamine. SS, Change in segment shortening; DIP, dipyridamole; DOB, dobutamine.
myocardial blood flow in both the endocardium and epicardium (p < 0.001; Figure 2). In the ischemic zone, dobutamine infusion caused a significant decrease in myocardial blood flow in both the endocardium and epicardium (both p = 0.03; Figure 3). In the nonischemic zone blood flow increased in both the epicardium and endocardium (p < 0.001; Figure 2). A significant difference in the endocardial/epicardial blood flow ratio did not occur (Figures 2 and 3).
DISCUSSION
In 1935 Tennant and Wiggers la established that myocardial ischemia results in a predictable decrease in regional myocardial function. Their seminal work has become the foundation for stress echocardiography. Although clinical experience in pharmacologic stress echocardiography has been well documented, there exists a relative paucity of basic physiologic investigation into the mechanism of myocardial ischemia that results from dobutamine or dipyridamole infusion. Our study attempted to investigate and compare the effects of dobutamine and dipyridamole infusion in the induction of myocardial ischemia. Our experimental model was developed to mimic as closely as possible a single-vessel coronary artery stenosis of 75%, 14 as is commonly seen in humans. The swine model was chosen instead of the canine model because the coronary artery circulation is more similar to that of humans. The protocol allowed us
to compare in the same animal and with the same degree of stenosis the effect of dobutamine and dipyridamole infusion with dosing protocols that are commonly used in clinical practice. Each animal served as its own control, and the equivalence of the baseline states before drug infusion was established. The major finding of the study was that both dipyridamole and dobutamine infusion resulted in myocardial ischemia as demonstrated by a reduction in regional myocardial segment shortening. However, the magnitude of the ischemic response was significantly different. Dobutamine infusion resulted in a decrease in segment shortening that was 242% greater than that seen with dipyridamole. All animals had myocardial ischemia during dobutamine infusion but only 75% did so when given dipyridamole. Six animals had dyskinetic wall motion during dobutamine administration, whereas none had a dyskinetic response to dipyridamole. Both agents resulted in a small increase in segment shortening (hyperdynamic response) in the nonischemic zone. Dobutamine augmented segment shortening in the nonischemic zone to a greater extent than did dipyridamole, although the absolute difference in effect was small. As expected, myocardial blood flow increased significantly in the nonischemic area during both dobutamine and dipyridamole infusion. Interestingly, dobutamine decreased blood flow in the ischemic zone, whereas dipyridamole had no effect. We were unable to demonstrate any differences between endocardial and epicardial blood flow. Dobutamine and dipyridamole are thought to
Journal of the AmericanSocietyof Echocardiography Volume 8 Number 1
Segar et al.
Mvocardiai
Blood
Flow .
Dinvridamole
~40~
320 300200-
240240220 mm O O
200180-
-...o---
Epicardium Ischemic Zone
160-
---0---
Epicardium Nonlschemic Zone
140.
.... 0----
Endocardium Ischemic Zone
---h---
Endocardium
..= E
120.
Nonlschemic Zone
100806040200
t
i
Baseline
Dipyridamole
F i g u r e 2 Change in myocardial blood flow at baseline and during dipyridamole infusion. Myocardial blood flow is plotted o n vertical axis and stage o n horizontal axis. Area of blood flow determination is depicted.
Myocardial
Flow
Blood
- Dobutamine
320 300280260240220 200-
9~
180---C)---
~ l~-
Epicardium
Ischemic Zone
E#cardinm Nonlschemlc Zone
"~ 140-
Endocnrdium lschemic Zone 120 -
uA
100-
Endocardium Nonlschemic Zone
-~176176176176176
0
Figure 3
i
i
Baseline
Dobutamine
Same graph as in Figure 2 except agent is n o w dobutamine instead ofdipyridamole.
13
14
Segar et al.
cause demand and supply type ischemia, respectively, is Most previous studies have used a canine model} s'16 In the canine the network of collateral coronary artery vessels is more developed, which may favor a "steal" type phenomenon. Our study differs from the previous work of Fung et al}6 in several respects. First, our study was performed in swine rather than canine and the left anterior descending coronary artery was used instead of the circumflex coronary artery. Second, we were able to demonstrate an ischemic effect of dipyridamole in 75% of animals instead of the 55% reported in their work. Third, the affect of dobutamine on myocardial blood flow in the ischemic zone was different. The differences in animal model, dosing protocols, and our use of sonomicrometry rather than wall thickening likely accounts for the experimental differences. Several clinical studies have compared dobutamine with either dipyridamole or adenosine. 17-19Marwick et al.19 found dobutamine stress echocardiography to be more accurate than adenosine stress echocardiography. Our results offer one possible physiologic explanation for their findings. The combined actions ofdobutamine on both the ischemic and nonischemic areas of the heart (hypokinetic in the ischemic zone and hyperkinetic in the nonischemic zone) lead to a greater disparity in myocardial wall motion. It would follow that this greater differentiation in wall motion should allow for more accurate discrimination between normal and abnormal areas. Our study has several limitations. A relatively small number of animals underwent radiolabeled microsphere determination of myocardial blood flow. The effects of anesthesia and the fact that the experiments were performed on an open-chest animal may hamper the applicability of this study to the human population. It is not known if the dose of dipyridamole that was used is optimal for swine. Finally, dipyridamole was always given first rather than randomized. The equivalence of the baseline states likely negates this limitation. We thank Naomi Fineberg, PhD, for her expert statistical assistance.
REFERENCES 1. Berthe C, Pi6rard LA, Hiemaux M, et al. Predicting the extent and location of coronary artery disease in acute myocardial infarction by echocardiography during dobutamine infusion. Am J Cardiol 1986;58:1167-72. 2. Mannering D, Cripps T, Leech G, et al. The dobutamine stress test as an alternative to exercise testing after acute myocardial infarction. Br Heart J 1988;59:521-6.
Journal of the American Societyof Echocardiography January-February 1995
3. Sawada SG, Segar DS, Ryan T, et al. Echocardiographic detection of coronary artery disease during dobutamine infusion. Circulation 1991;83:1605-14. 4. Segar DS, Brown SE, Sawada SG, Ryan T, Feigenbaurn H. Dobutamine stress echocardiography: correlation with coronary lesion severity as determined by quantitative angiography. J Am Coil Cardiol 1992;19:1197-202. 5. Mazeika PK, Nadazdin A, Oakley CM. Dobutamine stress echocardiography for detection and assessment of coronary artery disease. J Am Coil Cardiol 1992;19:1203-11. 6. Zwehl W, Gueret P, Meerbaum S, Holt D, Corday E. Quantitative two dimensional echocardiography during bicycle exercise in normal subjects. Am J Cardiol 1981;47:866-73. 7. Picano E, Lattanzi F, Masini M, Distante A, L'Abbate A. High dose dipytidamole echocardiography test in effort angina pectoris. J Am Coil Cardiol 1986;8:848-54. 8. Mazeika P, Nihoyannopoulos P, Joshi J, Oakley CM. Uses and limitations of high dose dipyridamole stress echocardiography for evaluation of coronary artery disease. Br Heart J 1992;67:144-9. 9. Kern MI, Pearson AC, Labovitz Aj', Deligonul U, Vandormad M, Gudipati C. Effects of pharmacologic coronary hyperemia on echocardiographic left ventricular function in patients with single vessel coronary artery disease. J Am Coil Cardiol 1989;13:1042-51. 10. Bolognese L, Sarasso G, Aralda D, Bondo AS, Rossi L, Rossi P. High dose dipyridamole echocardiography early after uncomplicated acute myocardial infarction: correlation with exercise testing and coronary angiography. J Am Coil Cardiol 1989; 14: 357-63. 11. Pashkow F, Holland R, Brooks H. Early changes in contractility and coronary blood flow in the normal areas of the ischemfc porcine heart. Am Heart J 1977;93:349-57. 12. Heymann MA, Payne BD, Hoffman JIE, Rudolph AM. Blood flow measurements with radionuclide-labelled particles. Prog Cardiovasc Dis 1977;20:55-79. 13. Tennant R, Wiggers CJ'. The effects of coronary artery occlusion on myocardial contraction. Am J Physiol 1935;112: 351-61. 14. Folts ID, Gallagher K, Rowe GG. Hemodynanlic effects of controlled degrees of coronary artery stenosis in short-term and long-term studies in dogs. J Thorac Cardiovasc Surg 1977;73:722-7. 15. Carlson RE, Kavanaugh KM, Buda AJ. The ef[~ct of different mechanisms of myocardial ischemia on left ventricular function. Am Heart J 1988;116:536-45. 16. Fung AY, Gallagher KP, Buda AJ. The physiologic basis of dobutamine as compared with dipyridamole stress interventions in the assessment of critical coronary stenoses. Circulation 1987;76:943-51. 17. Martin TW, Seaworth JF, Johns JP, Pupa LE, Condos WR. Comparison of adenosine, dipyridamole, and dobutamine in stress echocardiography. Ann Intern Med 1992; 116:190-6. 18. Previtali M, Lanzarini L, Ferario M, Tortorici M, Mussini A, Montemartini C. Dobutamine versus dipyridamole echocardiography in coronary artery disease. Circulation 1991;83:27-31. 19. Marwick T, Willemart B, D'Hondt A, et al. Selection of the optimal nonexercise stress tbr the evaluation of ischemic regional myocardial dysfimction and malperfusion. Circulation 1993;87:345-54.
The purpose of our study was to compare the ability of dobutamine and dipyridamole infusion to induce myocardial ischemia. In a population of 16 anesthetized open-chest swine, a coronary artery stenosis sufficient to abolish the hyperemic response to a 15-second total occlusion was created. Heart rate, systolic blood pressure, and d P / d t were recorded. Myocardial segment shortening was determined by sonomicrometry in all animals. In a subset of seven animals regional myocardial blood flow was measured by injection o f radiolabeled microspheres. Dipyridamole was infused according to a high-dose protocol. After a washout period and reestablishment of a baseline state, dobutamine was infused incrementally. There was no significant difference between the baseline states. Dipyridamole did not affect heart rate but did significantly decrease blood pressure and rate-pressure product. Myocardial segment shortening decreased in the ischemic zone by 0.07 +- 0.08 (p = 0.004). Dobutamine infusion significantly increased heart rate, blood pressure, and rate-pressure product. Myocardial segment shortening in the ischemic zone decreased by 0.17 +-. 0.09 (p < 0.001). Dobutamine decreased blood flow in the ischemic zone relative to baseline. Both dobutamine and dipyridamole infusion resulted in myocardial ischemia. The magnitude of the ischemic response is greater for dobutamine than for dipyridamole. (J AM Soc ECHOCmmlOGV, 1995;8:9-14.)
P h a r m a c o l o g i c stress testing is being used increasingly in the evaluation o f patients with known or suspected coronary artery disease. The most commonly employed pharmacologic stress agents are dipyridamole and dobutamine. The mechanism o f action o f these two drugs differs substantially. Although both agents are widely used clinically, little work has been done to investigate the physiology o f myocardial ischemia resulting from their pharmacologic effects? q~ From the Department of Medicine, Indiana University School of Medicine, The Krannert Institute of Cardiology, and the Wishard Memorial Hospital. Supported in part by the Herman C. Krannert Fund, Indianapolis, grants HL-06308 and HL-07182 from the National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland, and the American Heart Association, Indiana Affiliate, Indianapolis. Presented in part at the Fourth Annual Meeting of the American Society of Echocardiography, June 1993, Orlando, Florida. Reprint requests: Douglas S. Segar, MD, Krannert Institute of Cardiology, 1111 W. Tenth St., Indianapolis, IN 46202-4800. Copyright 9 1995 by the AmericanSocietyof Echocardiography. 0894-7317/95 $3.00 + 0 27/1/58982
T o date, w o r k in most animals has been in a canine model. The canine is known to have a coronary artery distribution that includes extensive collateral vessels. Swine have a coronary artery distribution that is more similar to that o f humans and is therefore a more representative model for investigation o f pharmacologically induced myocardial ischemia. 11 The purpose o f this study was to compare the ability o f dipyridamole and dobutamine to induce regional left ventricular dysfunction (myocardial ischemia) with a swine model o f fixed coronary artery stenosis.
METHODS
Twenty-six domestic swine o f either sex (18 to 27 kg; mean 21.5 kg) were used in this study. Ten animals were eliminated from analysis either because o f death during the study or inadequate data collection (usually a resuk o f sonomicrometer loss). The remaining 16 animals completed the study and are reported. The study protocol was approved by the In-
10
Segar et al.
diana University Laboratory Animals Review Committee for humane treatment of animals. Anesthesia was induced by injection of thiopental sodium (25 mg/kg). The animals were then intubated and placed on mechanical ventilation with 100% oxygen (Ohio Medical). Anesthesia was maintained with inhaled isoflurane (1 to 1.75 vol%). At the end of the experiment, the animals were given additional thiopental and killed with a lethal injection of saturated potassium chloride. Animal Preparation Catheters were placed in the femoral artery and vein for pressure measurement , blood withdrawal, and fluid :and drug administration. The femoral artery catheter was connected to an appropriately calibrated fluid-filled pressure transducer. An electrocardiogram was recorded in standard fashion with needle electrodes. A left lateral thoracotomy was performed and the heart was suspended in a pericardial cradle. A Millar catheter was placed into the left ventricle through a stab incision in the left atrial appendage passing antegrade through the mitral valve. A largebore catheter was placed into the left atrium through the same incision for radioactive microsphere injection. Paired 5 M H z sonomicrometer crystals were positioned in the subendocardial layer of the myocardium along the minor axis of the ventricle to measure myocardial segment shortening. One set was placed in the distribution o f the left anterior descending coronary artery (designated the ischemic zone) and the other was placed in the distribution of the left circumflex coronary artery (the nonischemic
zone). An area of the mid-left anterior descending coronary artery was dissected free. A Doppler flow probe (Transonic Technology, Ithaca, N.Y.) was placed around the artery. A rigid plastic collar was then placed around the artery. An appropriately sized commercially available angioplasty balloon was placed between the plastic collar and the artery. The angioplasty balloon was inflated with water, creating a stenosis in the coronary artery. The degree of balloon inflation was adjusted to abolish the hyperemic response (as measured by the Doppler flow probe) to a 15-second complete vessel occlusion. The inflation pressure was monitored and maintained constant throughout the experiment. All hemodynamic and sonomicrometric data were recorded on a Gould strip chart recorder (Gould Inc., Cleveland, Ohio). In addition, all data were digitized on line and recorded on a personal computer-based analysis system (Macintosh II FX, Lab View Run
Journal of the American Society of Echocardiography January-February 1995
Time; National Instrument Co., Inc., Baltimore, Md.).
Sonomicrometry Sonomicrometer crystals were connected to a commercially available multichannel sonomicrometcr system (Triton Technology, San Diego, Calif.). Calibration of the sonomicrometcr was performed before each study and the calibration standards were transferred to the computer analysis system. End diastole was defined as the onset of the rapid upstroke of the dP/dt (determined by the left ventricular catheter). End systole was defined as 25 mscc before the peak negative dP/dr. The segment shortening was calculated according to a standard formula: SS = (EDL - ESL)/EDL, where SS = segment shortening, EDL = end-diastolic length, and ESL = end-systolic length. At the end of the experiment, placement of the sonomicrometer crystal was confirmed by visual inspection.
Radioactive Microspheres Myocardial blood flow was determined with radioactive microsphercs (STCo, 113Sn, l~ and l~ New England Nuclear, Boston, Mass.). 12 Microspheres (3.66 x 10s/rag) were suspended in 10% dextran with Twecn 80 and agitated for a minimum of 5 minutes before 50 p~Ciwas injected into the left atrium. Blood was withdrawn at 8 ml/min with a Harvard pump (Harvard Apparatus Co., Inc., S. Natick, Mass.) beginning 20 seconds before injection and continuing for a total of 2 minutes. Segments of the myocardium in the ischemic and nonischemic zones were divided into epicardial and endocardial halves and approximately 1 gm tissue was placed into a scintillation counter (Packard Instrument Co., Inc., Downers Grove, Ill.). Spillover was corrected for and myocardial and blood activity was recorded. Myocardial blood flow was calculated according to the a standard formula: MBF = (C,/TW) • (RBW/Cb) • 100, whereMBF = myocardial blood flow in ml/min/100 gm tissue, C~ = tissue radioactive counts, TW = tissue weight in grams, RBW = reference blood withdrawal rate in milliliters per minute, and CD = total radioactive counts of reference blood sample. Experimental Protocol After creation of a flow-limiting stenosis, baseline measurements of heart rate, systolic blood pressure, left ventricular pressure and dP/dt, myocardial blood flow, and segment shortening ill both the ischemic and nonischemic zones were recorded. Dipyridamole
Journal of the American Society of Echocardiography Volume 8 Number 1
S e g a r et al.
11
Table 1 Hemodynamic changes during dipyridamole and dobutamine infusions Heart rate (beats/min)
Baseline Dipyridamole Baseline Dobutaminc
78 79 78 148
• 11" • 1P • 13? _+ 3 3 t
Systolic blood pressure ( m m Hg)
110 86 94 117
• 16t _+ 2 1 t • 16t • 26~-
Rate-pressure product (man Hg/min)
8661 6879 7496 17264
• 18707 _+ 2233? • 2387~ • 5788t
*Difference not significant. tp < 0.001.
was infused (Harvard pump) according to a highdose protocol (0.56 mg/kg in 4 minutes followed 4 minutes later by an additional 0.28 mg/kg given in 2 minutes). Measurements were obtained 3 minutes after completion of the dipyridamole infusion. The animals was then allowed to recover for 2 hours, after which a second set of baseline measurements was obtained. The dobutamine was infused in an incremental fashion beginning at a dose of 5 ~g/kg/min and increasing at 3-minute intervals to 10, 20, 30, and 40 wg/kg/min. Measurements were obtained on evidence of myocardial ischemia or reaching the peak dose. Statistical Analysis Paired data in each animal were analyzed with a paired t test. Repeated-measures analysis of variance was used when comparing more than one variable in each animal.
RESULTS Comparison o f Baseline States There was no significant difference in the heart rates between the two baseline states (78 • 11 and 78 + 13 beats/min; difference not signifcant). Systolic blood pressure and rate-pressure product were lower at the second compared with the first baseline state (both p < 0.05; Table 1), which is most likely a result of the cffkcts of anesthesia. Importantly, there was no change in segment shortening in either the ischemic zone (0 • 0.03) or the nonischemic zone ( - 0 . 0 1 + 0.03) between the two baseline states.
Effect o f Dipyridamole The dipyridamole infusion was completed in all animals. Heart rate did not change significantly during the dipyridamole infusion. Both systolic blood pressure and rate-pressure product decreased during di-
pyridamole (p < 0.001; Table 1). There was a significant decrease in myocardial systolic segment shortening relative to baseline in the ischemic zone of 0.07 + 0.08 (p = 0.004) during drug infusion. Twelve (75%) of the 16 animals had an ischemic response to dipyridamole. The magnitude of the change in segment shortening varied from + 0.04 to - 0 . 1 6 . In the nonischemic zone there was a small increase in segment shortening of 0.02 +_ 0.03 (p = 0.01).
Effect o f Dobutamine The dobutamine infusion protocol was completed in all animals. The mean peak infusion rate was 26.5 ~g/kg/min. Dobutamine caused a mean increase in heart rate from 78 _+ 13 to 148 • 33 beats/min (p < 0.001). Both systolic blood pressure and ratepressure product increased significantly with dobutamine (both p < 0.001; Table 1). In the ischemic zone dobutamine infusion caused a mean decrease in segment shortening of 0.17 +_ 0.09 (p < 0.001). Sixteen (100%) out of 16 animals had a decrease in segment shortening during dobutanlinc infusion. The range of the decrease in segment shortening was from - 0.08 to - 0.42. In all animals the magnitude of the change in segment shortening in the ischemic zone was greater with dobutamine compared with dipyridamole (p < 0.001) (Figure 1). Six of the 16 animals had dyskinctic segment shortening during dobutamine; none of the animals had a dyskinetic response with dipyridamole. In the nonischemic zone dobutamine increased myocardial segment shortening by 0.05 _+ 0.06 (p = 0.03).
Regional Myocardial Blood Flow Regional myocardial blood flow was determined in a subset of seven animals. There were no differences in myocardial blood flow at baseline. Dipyridamole infusion did not change blood flow in the ischemic zone relative to baseline flow (Figure 2). In the nonischemic zone dipyridamole significantly increased
12
Journal of the American Societyof Echocardiography January-February 1995
Segar et al.
INDIVIDUAL ANIMALS 0,06 0
,,
-0.05 -0.1 -0.15 -0.2 -0.25 , -0.3. -0.35. -0.4, -0.45.
Figure 1 Change in segment shortening for each animal during infusion of diwridamole or dobutamine. Change in segment shortening is on vertical axis and each individual animal is found on horizontal axis. Black column is response to dipyridamole; white column is response to dobutamine. SS, Change in segment shortening; DIP, dipyridamole; DOB, dobutamine.
myocardial blood flow in both the endocardium and epicardium (p < 0.001; Figure 2). In the ischemic zone, dobutamine infusion caused a significant decrease in myocardial blood flow in both the endocardium and epicardium (both p = 0.03; Figure 3). In the nonischemic zone blood flow increased in both the epicardium and endocardium (p < 0.001; Figure 2). A significant difference in the endocardial/epicardial blood flow ratio did not occur (Figures 2 and 3).
DISCUSSION
In 1935 Tennant and Wiggers la established that myocardial ischemia results in a predictable decrease in regional myocardial function. Their seminal work has become the foundation for stress echocardiography. Although clinical experience in pharmacologic stress echocardiography has been well documented, there exists a relative paucity of basic physiologic investigation into the mechanism of myocardial ischemia that results from dobutamine or dipyridamole infusion. Our study attempted to investigate and compare the effects of dobutamine and dipyridamole infusion in the induction of myocardial ischemia. Our experimental model was developed to mimic as closely as possible a single-vessel coronary artery stenosis of 75%, 14 as is commonly seen in humans. The swine model was chosen instead of the canine model because the coronary artery circulation is more similar to that of humans. The protocol allowed us
to compare in the same animal and with the same degree of stenosis the effect of dobutamine and dipyridamole infusion with dosing protocols that are commonly used in clinical practice. Each animal served as its own control, and the equivalence of the baseline states before drug infusion was established. The major finding of the study was that both dipyridamole and dobutamine infusion resulted in myocardial ischemia as demonstrated by a reduction in regional myocardial segment shortening. However, the magnitude of the ischemic response was significantly different. Dobutamine infusion resulted in a decrease in segment shortening that was 242% greater than that seen with dipyridamole. All animals had myocardial ischemia during dobutamine infusion but only 75% did so when given dipyridamole. Six animals had dyskinetic wall motion during dobutamine administration, whereas none had a dyskinetic response to dipyridamole. Both agents resulted in a small increase in segment shortening (hyperdynamic response) in the nonischemic zone. Dobutamine augmented segment shortening in the nonischemic zone to a greater extent than did dipyridamole, although the absolute difference in effect was small. As expected, myocardial blood flow increased significantly in the nonischemic area during both dobutamine and dipyridamole infusion. Interestingly, dobutamine decreased blood flow in the ischemic zone, whereas dipyridamole had no effect. We were unable to demonstrate any differences between endocardial and epicardial blood flow. Dobutamine and dipyridamole are thought to
Journal of the AmericanSocietyof Echocardiography Volume 8 Number 1
Segar et al.
Mvocardiai
Blood
Flow .
Dinvridamole
~40~
320 300200-
240240220 mm O O
200180-
-...o---
Epicardium Ischemic Zone
160-
---0---
Epicardium Nonlschemic Zone
140.
.... 0----
Endocardium Ischemic Zone
---h---
Endocardium
..= E
120.
Nonlschemic Zone
100806040200
t
i
Baseline
Dipyridamole
F i g u r e 2 Change in myocardial blood flow at baseline and during dipyridamole infusion. Myocardial blood flow is plotted o n vertical axis and stage o n horizontal axis. Area of blood flow determination is depicted.
Myocardial
Flow
Blood
- Dobutamine
320 300280260240220 200-
9~
180---C)---
~ l~-
Epicardium
Ischemic Zone
E#cardinm Nonlschemlc Zone
"~ 140-
Endocnrdium lschemic Zone 120 -
uA
100-
Endocardium Nonlschemic Zone
-~176176176176176
0
Figure 3
i
i
Baseline
Dobutamine
Same graph as in Figure 2 except agent is n o w dobutamine instead ofdipyridamole.
13
14
Segar et al.
cause demand and supply type ischemia, respectively, is Most previous studies have used a canine model} s'16 In the canine the network of collateral coronary artery vessels is more developed, which may favor a "steal" type phenomenon. Our study differs from the previous work of Fung et al}6 in several respects. First, our study was performed in swine rather than canine and the left anterior descending coronary artery was used instead of the circumflex coronary artery. Second, we were able to demonstrate an ischemic effect of dipyridamole in 75% of animals instead of the 55% reported in their work. Third, the affect of dobutamine on myocardial blood flow in the ischemic zone was different. The differences in animal model, dosing protocols, and our use of sonomicrometry rather than wall thickening likely accounts for the experimental differences. Several clinical studies have compared dobutamine with either dipyridamole or adenosine. 17-19Marwick et al.19 found dobutamine stress echocardiography to be more accurate than adenosine stress echocardiography. Our results offer one possible physiologic explanation for their findings. The combined actions ofdobutamine on both the ischemic and nonischemic areas of the heart (hypokinetic in the ischemic zone and hyperkinetic in the nonischemic zone) lead to a greater disparity in myocardial wall motion. It would follow that this greater differentiation in wall motion should allow for more accurate discrimination between normal and abnormal areas. Our study has several limitations. A relatively small number of animals underwent radiolabeled microsphere determination of myocardial blood flow. The effects of anesthesia and the fact that the experiments were performed on an open-chest animal may hamper the applicability of this study to the human population. It is not known if the dose of dipyridamole that was used is optimal for swine. Finally, dipyridamole was always given first rather than randomized. The equivalence of the baseline states likely negates this limitation. We thank Naomi Fineberg, PhD, for her expert statistical assistance.
REFERENCES 1. Berthe C, Pi6rard LA, Hiemaux M, et al. Predicting the extent and location of coronary artery disease in acute myocardial infarction by echocardiography during dobutamine infusion. Am J Cardiol 1986;58:1167-72. 2. Mannering D, Cripps T, Leech G, et al. The dobutamine stress test as an alternative to exercise testing after acute myocardial infarction. Br Heart J 1988;59:521-6.
Journal of the American Societyof Echocardiography January-February 1995
3. Sawada SG, Segar DS, Ryan T, et al. Echocardiographic detection of coronary artery disease during dobutamine infusion. Circulation 1991;83:1605-14. 4. Segar DS, Brown SE, Sawada SG, Ryan T, Feigenbaurn H. Dobutamine stress echocardiography: correlation with coronary lesion severity as determined by quantitative angiography. J Am Coil Cardiol 1992;19:1197-202. 5. Mazeika PK, Nadazdin A, Oakley CM. Dobutamine stress echocardiography for detection and assessment of coronary artery disease. J Am Coil Cardiol 1992;19:1203-11. 6. Zwehl W, Gueret P, Meerbaum S, Holt D, Corday E. Quantitative two dimensional echocardiography during bicycle exercise in normal subjects. Am J Cardiol 1981;47:866-73. 7. Picano E, Lattanzi F, Masini M, Distante A, L'Abbate A. High dose dipytidamole echocardiography test in effort angina pectoris. J Am Coil Cardiol 1986;8:848-54. 8. Mazeika P, Nihoyannopoulos P, Joshi J, Oakley CM. Uses and limitations of high dose dipyridamole stress echocardiography for evaluation of coronary artery disease. Br Heart J 1992;67:144-9. 9. Kern MI, Pearson AC, Labovitz Aj', Deligonul U, Vandormad M, Gudipati C. Effects of pharmacologic coronary hyperemia on echocardiographic left ventricular function in patients with single vessel coronary artery disease. J Am Coil Cardiol 1989;13:1042-51. 10. Bolognese L, Sarasso G, Aralda D, Bondo AS, Rossi L, Rossi P. High dose dipyridamole echocardiography early after uncomplicated acute myocardial infarction: correlation with exercise testing and coronary angiography. J Am Coil Cardiol 1989; 14: 357-63. 11. Pashkow F, Holland R, Brooks H. Early changes in contractility and coronary blood flow in the normal areas of the ischemfc porcine heart. Am Heart J 1977;93:349-57. 12. Heymann MA, Payne BD, Hoffman JIE, Rudolph AM. Blood flow measurements with radionuclide-labelled particles. Prog Cardiovasc Dis 1977;20:55-79. 13. Tennant R, Wiggers CJ'. The effects of coronary artery occlusion on myocardial contraction. Am J Physiol 1935;112: 351-61. 14. Folts ID, Gallagher K, Rowe GG. Hemodynanlic effects of controlled degrees of coronary artery stenosis in short-term and long-term studies in dogs. J Thorac Cardiovasc Surg 1977;73:722-7. 15. Carlson RE, Kavanaugh KM, Buda AJ. The ef[~ct of different mechanisms of myocardial ischemia on left ventricular function. Am Heart J 1988;116:536-45. 16. Fung AY, Gallagher KP, Buda AJ. The physiologic basis of dobutamine as compared with dipyridamole stress interventions in the assessment of critical coronary stenoses. Circulation 1987;76:943-51. 17. Martin TW, Seaworth JF, Johns JP, Pupa LE, Condos WR. Comparison of adenosine, dipyridamole, and dobutamine in stress echocardiography. Ann Intern Med 1992; 116:190-6. 18. Previtali M, Lanzarini L, Ferario M, Tortorici M, Mussini A, Montemartini C. Dobutamine versus dipyridamole echocardiography in coronary artery disease. Circulation 1991;83:27-31. 19. Marwick T, Willemart B, D'Hondt A, et al. Selection of the optimal nonexercise stress tbr the evaluation of ischemic regional myocardial dysfimction and malperfusion. Circulation 1993;87:345-54.