Evaluation of parameters for the assessment of regional myocardial contractile function during asynchronous left ventricular contraction

Summary The primary purpose of this study was to evaluate parameters used for the measurement of regional myocardial contractile function in the setting of left ventricular (LV) asynchrony. Secondarily, we tested whether the peak negative value of left ventricular dP/dt (-dP/dt) can be used to estimate global LV end-systole during asynchrony. In seven anesthetized (Isoflurane) swine the left anterior descending coronary artery was cannulated and perfused at constant blood flow rates. To produce LV asynchrony, dobutamine (D) was infused into the perfusion system. This was repcated later during coronary hypoperfusion (HYPO) sufficient to produce regional contractile dysfunction. The amount of LV wall thickening during systole (% WT, sonomicrometry) was calculated using either - dP/dt or the closure of the aortic valve (AO, electromagnctic flow probc) for estimating the timing of global LV end-systole. % WT was compared to other paramcters which are not dependent upon the timing of global LV end-systole, including the amplitude of the first harmonic of the Fourier transform (AMP) and regional myocardial work (WI) estimated from the left ventricular pressure-wall thickness relationship. A close correlation between global LV end-systole defined by the AO or - dP/dt existed during control. D or HYPO. During HYPO+D no such relationship was found (r=.22, NS), and % WT calculated using - dP/dt as an estimate of end-systole was underestimated when comparcd to % WT calculated by use of the AO to estimate end-systole (2.9±6.8% vs 6.3±6.6%, p<.05). % WT, AMP, and WI showed similar results during control, D and HYPO. However, D during HYPO increased the AMP from .59±.23 mm to .76±.32 mm and WI from 67±20 mm Hg*mm to 95±24 mm Hg*mm (p<.05), respectively. This increase in regional myocardial function, however, was not detected by % WT (10.5±6.4% vs 6.3±6.6%). Thus, during left ventricular asynchrony, the measurement of LV-dP/dt to estimate the timing of global LV end-systole is inappropriate and can lcad to inaccuracies in the measurement of regional contractile function. Parameters such as AMP or WI are advantageous since global LV end-systole docs not need to be accurately defined.Supported in part by the American Heart Association California Affiliate grant-in-aid #86-S105, and by the German Research Foundation (He 1320/3-2). Dr. Guth is the recipient of a Research Fellowship from the Alexander von Humboldt-Stiftung, Jean-Paul-Straße 12, D-5300 Bonn 2     

Effect of oxygen-derived free radical scavengers on infarct size following six hours of permanent coronary artery occlusion: salvage or delay of myocyte necrosis?

Summary Oxygen-derived free radicals (O ₂ ^– and ·OH) have been implicated in myocardial injury associated with coronary artery occlusion followed by reperfusion. While these cytotoxic oxygen species are predominantly produced upon reintroduction of molecular oxygen to previously ischemic tissue, they may also be generatedthroughout coronary occlusion in species (such, as dog and man) in which native collateral vessels permit residual blood flow into the ischemic bed. To test this theory, 20 anesthetized, open-chest dogs underwent 6 h of permanent left anterior descending coronary artery occlusion: ten dogs were treated with the potent free radical scavenging enzymes superoxide dismutase (SOD: 5 mg/kg per hour) plus catalase (5 mg/kg per hour), while the remaining ten animals received saline. Infusion of drug or saline solution was begun 15 min prior to occlusion, and maintained throughout occlusion. Infusion of SOD+catalase did not significantly affect the extent of the area at risk of infarction (19.5±1.8% vs 24.0±1.4% of the left ventricle for the treated vs control group;P=NS), did not reduce myocardial oxygen demand (heart rate and arterial pressures were comparable for both groups), and did not alter collateral blood flow to the ischemic myocardium. However, mean infarct size in dogs treated with SOD+catalase (39.6±6.6% of the area at risk; 8.4±2.1% of the left ventricle) was significantly smaller than that observed in the saline controls (73.0±6.3% of the area at risk,P<0.01; 19.8±2.2% of the left ventricle,P<0.01). Thus, infusion of SOD+catalase prior to and during 6 h of coronary occlusion significantly reduced infarct size assessed at 6 h postocclusion. To determine whether this reduction in infarct size represented long-term salvage of ischemic myocardium, an additional 14 dogs (seven treated and seven controls) underwent the same procedure as described above; in this case the artery was reperfused after the 6-hour occlusion period, infusion of the SOD+catalase or saline solution stopped at 5–10 min postreperfusion, and the hearts examined at 30–48 h postocclusion. In contrast to the results at 6 h postocclusion, the necrosis at 30–48 h postocclusion was large, confluent and transmural in all dogs (infarct size=21.2±2.5% vs 22.7±4.4% of the left ventricle for treated vs controls;P=NS). These results suggest that infusion of SOD+catalase in this model may delay, but not prevent, the development of ischemic necrosis.Presented in part at the X World Congress of Cardiology, Washington, DC, September 1986.Performed during Dr. Kloner's tenure as an Established Investigator of the American Heart Association.     

Open chest and open pericardium affect the distribution of myocardial blood flow in the right ventricle

Summary We have investigated the effects of open chest and open pericardium on the distribution of myocardial blood flow assessed with the radioactive microsphere technique (15 m). Five dogs with intact thorax served as controls (group I) and six dogs were studied after a right-sided thoracotomy and pericardiotomy (group II). Global myocardial blood flow (mean±S.D.) was 0.73±0.17 ml·min^–1·g^–1 in group I and 1.22±0.09 ml·min^–1·g^–1 in group II (p<0.05). Analysis of transmural blood flow distribution revealed that flow was 44% higher in the right and 60% higher in the left ventricular endocardial layers in the open-chest animals, whereas epicardial flow increased by 105% and 90%, respectively. As a result of the preferential blood flow to the epicardial layers of the right ventricle, the endo/epi ratio was reduced from 1.30±0.26 in group I to 0.86±0.11 in the open-chest group (p<0.05). Left ventricular endo/epi ratio was 1.27±0.16 and 1.06±0.11 (n.s.), respectively. External work and diastolic filling pressure of the right ventricle did not differ between the two groups and therefore cannot account for the redistribution of myocardial blood flow. It is concluded that the distribution of myocardial blood flow, particularly in the RV, is severely disturbed by thoracotomy and pericardiotomy. This is an important aspect for the planning and evaluation of studies under open-chest/open-pericardium conditions.Supported by Deutsche Forschungsgemeinschaft grant SFB 320     

Superoxide dismutase and catalase do not improve recovery of regional myocardial contractile function when given at the time of reperfusion after reversible regional ischemia in anesthetized dogs

Summary Earlier studies have demonstrated an improvement in the recovery of the regional myocardial function after reversible myocardial ischemia when dogs were treated with superoxide dismutase (SOD) + catalase (CAT). In all these studies, drug administration was started prior to the ischemic period. The aim of this study was to investigate the effects of SOD and CAT on the recovery of the regional contractile function in anesthetized beagle dogs when the drugs were administered at the time of reperfusion. The animals were subjected to 20 min of left coronary artery occlusion followed by 3 h reperfusion. The regional myocardial contractile function, measured as subendocardial segment shortening (SS, sonomicrometry) decreased to below zero and the regional blood flow in the ischemic subendocardium was reduced to about 5 % of pre-ischemic values during the coronary artery occlusion period. The size of the occluded bed was similar in the two groups. Saline (n = 8) or SOD (10 mg/kg) + CAT (3.4 mg/kg) (n = 8) were infused into the left atrium from 2.5 min prior to until 20 min after the start of reperfusion. The peak plasma level of SOD was 102 ± 15 mg/1 at 20 min reperfusion. There were no significant differences in the arterial blood pressure, cardiac contractile function and regional blood flow between the two groups at any time during the experiment. During reperfusion in the dogs given vehicle, SS recovered to 48 ± 7 % (mean ± SEM) after the first hour of reperfusion, and to 51 ± 6 % of pre-ischemic values after 3 h of reperfusion. The corresponding values in SOD + CAT treated dogs were 50 ± 5 % (1 h) and 53 ± 8 % (3 h), respectively. It is concluded that SOD + CAT, when given at the time of reperfusion, did not improve the regional contractile function after reversible ischemia in the anesthetized beagle dog.     

The transmural progression of the no-reflow phenomenon in globally ischemic hearts

Summary A no-reflow phenomenon (NRP) develops in hearts subjected to global ischemia and prevents reperfusion of the subendocardial myocardium upon restoration of arterial supply. In the present study the transmural progression of the NRP across the left ventricular wall in globally ischemic rat hearts was quantitatively defined by using autoradiographic nuclear track emulsion (NTE) as an indicator of microvascular competence.Rat hearts were isolated and perfused for 10 min with oxygenated Krebs-Henseleit buffer, then were made completely globally ischemic for from 0 to 60 min and were maintained at 37°C. They were then fixed by perfusion with glutaraldehyde after which NTE was injected into the coronary arteries. Transverse sections through the left ventricles were examined by scanning electron microscopy using back-scattered electron imaging and the vessels in a standard transmural contiguous series of photomicrographs were classified according to whether they did or did not permit the flow of NTE.Non-ischemic control myocardium showed a mean proportion of filled vessels of 99.4±0.5% SD, and those subjected to 15 min of ischemia showed only a slight overall reduction. After 30 min of ischemia 96±3% of vessels in the subepicardial third could be reperfused, but the proportion progressively diminished across the myocardium to total no-reflow near the endocardium. From 45–60 min of ischemia the totally non-reperfusible region remained confined to the subendocardial third but there was a significant reduction in the proportion of reperfusible vessels in the subepicardial third to 40%±27%.Ischemia thus progressively reduces the capacity of myocardium to be reperfused. This reduction occurs across the entire width of the left ventricular free wall but develops more slowly and is less severe in the middle and outer thirds than in the subendocardium where the capacity for microvascular reperfusion is totally lost.     

Regional oxygen supply and consumption balance in experimental left ventricular hypertrophy

Summary The aim of the present study was to determine if the relationship between myocardial O₂ supply and O₂ consumption was preserved after prolonged pressure overload due to aortic valve stenosis. This was examined in anesthetized open-chest dogs in which the aortic valve was plicated 6 months previously. We measured coronary blood flow with radioactive microspheres and regional small vessel O₂ saturation with microspectrophotometry, to obtain O₂ supply, and O₂ consumption. Regional O₂ consumption was calculated as the product of flow and O₂ extraction. The left ventricular weight/body weight ratio was 81% greater in the dogs with aortic valve stenosis. There were no hemodynamic differences between the groups except that left ventricular systolic pressure was 38±22 mm Hg greater than aortic in the hypertrophied group. Coronary blood flow did not differ between the control and hypertrophied groups nor were there subepicardial vs subendocardial differences. When maximal coronary flow was determined with chromonar (10 mg/kg), the flow increase was significantly attenuated in the hypertrophied subendocardium (242.1±82.3 (hypertrophy) vs 512.4±204.1 ml·min^–1·100 g^–1 (control)). There were no significant differences in O₂ extraction or O₂ consumption/g between control and hypertrophied animals. There was a significantly lower O₂ supply/consumption ratio in the subendocardium compared to the subepicardium of both groups. However, the O₂ supply/consumption ratio was not decreased by hypertrophy. Thus, despite significant hypertrophy, a loss of flow reserve and a high left ventricular pressure, O₂ supply/consumption balance is preserved in valvular aortic stenosis at rest.     

Regional myocadial blood flow and cardiac mechanics in dog hearts with CO2 laser-induced intramyocardial revascularization

Summary Laser-induced intramyocardial revascularization (LIR) has been used to promote direct communications between blood within the ventricular cavity and that of the existing myocardial vasculature in an attempt to increase perfusion in patients with ischemic heart discase. This study was conducted to measure the effects of LIR channels on regional myocardial flood flow (microspheres), cardiac mechanics (sonomicrometers), and myocardial tissue pressures in 18 dogs. Under baseline hemodynamic conditions (mean HR=165.2±11.4 bpm, LVP=123.6±22.9/4.0±1.8 mmHg, AoP=112.8±27.1/77.0±22.5 mmHg), myocardial blood flow in laser-treated tissue (mean =1.11±.10 cc/min/gm before laser; .71±.19 cc/min/gm after laser) was reduced as compared to blood flow in control tissue (mean=1.12±.15 cc/min/gm before laser; 1.25±.22 cc/min/gm after laser). Regional myocardial systolic shortening (11.32%±3.82% before laser; 7.49%±2.86% after laser) was decreased by 33%. During simultaneous reversible ligation of the LAD and LCCA for 2 min, when intramyocardial channels represented the only tissue access for the injected microspheres, blood flow in laser-treated tissue was not increased above that of the control non-lasered tissue. However, regional blood flow was greater in laser-treated ischemic tissue (mean=.61±.12 cc/min/gm) than in untreated ischemic areas (mean=.04±.03 cc/min/gm) when left ventricular pressure (LVP) was acutely elevated (mean SLVP=207.0±16.1 mmHg). Using these measurements, a model is proposed to predict regional systolic pressure gradients between the left ventricular cavity and coronary intramyocardial vasculature required to permit restoration of blood flow to ischemic myocardium. We conclude that improved perfusion via laser-induced intramyocardial channels does not occur in otherwise normal myocardium exposed to acute coronary ligation and only small improvements in perfusion are noted when LVP is significantly elevated. Consideration of further clinical application of this approach is seriously cautioned awaiting additional experimental studies.This study was supported by U.S. Public Health Service Grant R01 HL32897-01 from the National Heart, Lung, and Blood Institute, by grants in aid from the American Heart Association, and by the Fulbright-Hays Scholarship Grant.     

The oxygen consumption paradox of “stunned myocardium” in dogs

Summary The contractile state of the heart is a major determinant of myocardial oxygen consumption. Since regional myocardial contractility can be severely impaired following a transient coronary occlusion, post-ischemic myocardium is frequently assumed to consume less oxygen. To test this assumption, regional myocardial function and oxygen consumption were studied in ancsthetized dogs during 2 h of myocardial reperfusion following either a 15-min (Group I) or 4-h (Group II) left anterior descending coronary artery occlusion. Both groups developed similar post-ischemic regional dysfunction characterized by paradoxical motion (negative shortening). Measured as a percent of baseline segment shortening, anterior wall function in Group I (n=8) and Group II (n=5) at 30 min of reperfusion was –33±11% and –34±16% (p=NS) and at 120 min was –23±9% and –40±16% (p=NS). However, the two groups showed a marked difference in regional myocardial oxygen consumption during reperfusion. Despite the abnormal wall motion, regional oxygen consumption in Group I at 30 and 120 min of reperfusion was unchanged from pre-ischemic levels as measured as a percent of bascline: 104±20% (p=NS) and 111±21% (p=NS). In contrast, regional oxygen consumption in Group II was markedly depressed from bascline at 30 and 120 min of reperfusion: 42±7% (p<.01) and 40±8% (p<.01). To determine whether the dissociation between regional myocardial oxygen consumption and function in Group I was related to mitochondrial uncoupling, six additional dogs were studied. Tissue samples were obtained from post-ischemic myocardium after 120 min of reperfusion following a 15-min coronary artery occlusion, and compared to non-ischemic myocardium. There were no differences in the in vitro mitochondrial respiratory rates or oxidative phosphorylation capacity between the post-ischemic and non-ischemic myocardium. Therefore, in the post-ischemic myocardium, significant depressions in regional contractility may not be associated with falls in oxygen consumption. Following a 15-min coronary artery occlusion, the injured myocardium maintains a paradoxically high oxygen consumption with normal mitochondrial function despite decreased contractility and abnormal wall motion.Grant Support: Dr. Dean was a Fellow of the American Heart Association. Dr. Nicklas is supported by the NIH Clinical Investigator Award, HL 011170.     

Effects of dipyridamole on collateral flow and regional myocardial function in conscious dogs with newly developed collaterals

Summary Studies were conducted on six conscious dogs instrumented for measurement of subendocardial segment lengths in the area perfused by the left anterior descending coronary artery (LAD) and left circumflex coronary artery (LCCA), LCCA flow, and left ventricular pressure. Externally inflatable occluders were placed around the proximal LAD and LCCA. Collateral channels sufficient for the resting metabolic demands in the occluded LCCA perfusion territory were induced by repeated, brief LCCA occlusions. Dogs were then subjected to two consecutive brief periods of LAD occlusion. Dipyridamole (0.25 mg/kg) was injected intravenously 3 min prior to the second LAD occlusion. The collateral blood flow from the LCCA to the occluded LAD area was measured as the stepwise decrease in LCCA flow upon release of the LAD occlusion. During LAD occlusion after dipyridamole treatment collateral blood flow velocity decreased to 3.8±1.1 cm/s (±standard error) compared with a value of 4.9±0.9 cm/s measured during LAD occlusion without dipyridamole treatment. Percentage systolic segment shortening in the collateral dependent zone significantly deteriorated from 14.3±5.2 to 9.7±5.0% (p<0.05). Electrocardiograms taken simultaneously from endocardial ultrasonic transducers in the ischemic segment revealed significant increases in ST-segment level from 4.2±0.6 to 5.4±0.6 mV. These findings indicate that dipyridamolc adverscly affects the extent of myocardial ischemia in the collateral-dependent zone.Supported by grant HL 32800 from the NHLBI     

Factors affecting penetration of retrograde coronary venous injections into normal and ischemic canine myocardium: assessment by contrast echocardiography and digital angiography

Summary This experimental study described myocardial echo contrast enhancement through coronary venous injections. Retrograde administration of renografin was performed in 15 closed-chest dogs. Two-dimensional echocardiography was used to study myocardial echo contrast enhancement before and after coronary artery occlusion. Digital subtraction venography was used to assess delivery, drainage and shunting of the retrograde injectate. Systolic/diastolic blood pressure in the great cardiac vein measured 7±3/1±0.6 mm Hg and increased to 29±11/5±3 after coronary sinus occlusion and to 55±2.3/15±12 mm Hg during coronary sinus contrast injection. Myocardial contrast echo appearance in a midpapillary left ventricular short axis cross-section was limited to the anteroseptal region, extending to 28.4±11.3% of the section circumference after great cardiac vein injections and 35.3±17% after coronary sinus injections (difference NS). After occlusion of the left anterior descending coronary artery, great cardiac vein contrast injections resulted in opacification of 36.6±9.7% of the section circumference (N.S. vs preocclusion control) and opacified most, but not all asynergic segments. After occlusion of the circumflex coronary artery, myocardial echo contrast uptake was restricted to the septum and the anterior wall. The ischemic and asynergic posterolateral myocardial segments were not opacified. Digital subtraction coronary venography revealed rapid drainage of retrogradely injected contrast to the right atrium, in spite of coronary sinus balloon occlusion via venovenous anastomoses.Retrograde coronary venous contrast injections may help define myocardial regions which are accessible with retrograde coronary venous interventions.Dr. Punzengruber was supported by a Grant from the Max Kade Foundation, New York     

Alanine,glutamate, and ammonia exchanges in acutely ischemic swine myocardium

Summary Coronary artery disease causes an increase in glutamate uptake and alanine output by the heart. We assessed the effects of acute myocardial ischemia on alanine and glutamate exchange and ammonia production in 10 anesthetized open-chest domestic swine (46.9±0.7 kg). Coronary blood flow was controlled through an extracorporal perfusion circuit. After a nonischemic control period (aerobic) the blood flow in the left anterior descending coronary artery was reduced by 60%. Arterial and anterior interventricular venous samples where drawn before and during 35 min of ischemia. Subendocardial blood flow, measured using radiolabeled microspheres, decreased from 1.27±0.16 to 0.25±0.09 (ml/g)/min, and left-ventricular wall-thickening fell to 47% of aerobic values. Ischemia resulted in a significant increase in the rate of glucose uptake (p<0.05) and a switch to net lactate production (p<0.01). Ischemia did not affect the rates of alanine output (–0.9±1.0 vs. –0.3±0.3 mol/min) or glutamate uptake (–0.4±1.1 vs. 0.3±0.6 mol/min), but did increase the venous-arterial difference for ammonia (–4.1±4.1 to 52.7±5.5 M, p<0.0001) and the ammonia output (–0.33±0.24 to 1.34±0.14 mol/min, p<0.0001). In conclusion, acute ischemia did not stimulate greater alanine output or glutamate uptake. However, acute ischemia did cause an increase in anaerobic glycolysis rate and ammonia output, which reflects a profound disruption in myocardial energy metabolism.     

Influence of repetitive coronary occlusions on myocardial adenine nucleosides,high energy phosphates and ultrastructure

Summary We compared the effects of repeated short periods of myocardial ischemia with those of permanent occlusion (canine open-chest) with regard to tissue content of adenine nucleotides, nucleosides, creatine phosphate, and ultrastructure. Coronary occlusion for 3 min followed by a reperfusion period of 7 min was repeated up to a cumulative occlusion time of either 45 or 90 min. After cumulative occlusions of 15, 30, 45, and 90 min, transmural needle biopsies were taken from the ischemic area to be analyzed for adenine nucleotides, nucleosides, creatine phosphate, and ultrastructural changes. At the end of each experiment, tetrazolium salt staining was used for macroscopic detection of myocardial necrosis. These data were obtained with those obtained from dogs with a permanent coronary occlusion of 45 and 90 min, respectively. After repeated coronary occlusions at a cumulative occlusion time of 45 min, macroscopic detection of necrosis was negative, and after 90 min of cumulative coronary occlusion, patchy subendocardial tissue necrosis was found in only one out of 13 dogs, whereas in the group with permanent coronary occlusion, small patchy subendocardial necrosis was found in 95% after 45 min, and after 90 min permanent coronary occlusion, large subendocardial necrotic areas spreading towards the epicardial layers were found in 90% of the hearts.Ultrastructural investigations showed only slight to moderate ischemic injury after 45 and 90 min intermittent coronary occlusion, whereas permanent coronary occlusion produced moderate to severe ischemic injury after 45 min; and 90 min permanent coronary occlusion produced irreversible ischemic injury in all subendocardial tissue samples and in 80% of the subepicardial tissue samples.ATP content was decreased significantly less during intermittent coronary occlusion compared with that during permanent coronary occlusion.AMP and nucleosides did accumulate during permanent occlusion but not with repetitive brief coronary occlusions. Our results show that intermittent reperfusion significantly delays ischemic injury in comparison with permanent coronary occlusion.     

Superoxide dismutase decreases early reperfusion release of conjugated dienes following regional canine ischemia

Summary Oxygen radical-induced myocardial lipid peroxidation may cause injury during regional ischemia and reperfusion. However, in vivo detection of lipid peroxidation is difficult. Since conjugated dienes are lipid peroxidation products of unsaturated fatty acids, we evaluated the potential value of detection of these double-bonded fatty acids as a marker of oxygen radical injury. In seven untreated and five superoxide dismutase-treated anesthetized dogs exposed to 90 min of coronary occlusion and subsequent reperfusion, coronary sinus plasma draining the ischemic and reperfused region was assayed for dienes. Lipids were extracted and diene optical density measured at 233 nm wavelength. Superoxide dismutase (5 mg/kg, total dose) was infused into the left atrium during ischemia and the first 30 min of reperfusion. Coronary sinus diene optical density increased in untreated animals at 5 and 10 min of reperfusion (reperfusion optical density (x±SEM): 5 min=1.49±0.20 absorbance units, 10 min=1.36±0.06; both p<0.05 vs preocclusion optical density=1.10±0.05 and 25 min reperfusion=1.20±0.07). No increase in diene optical density occurred in superoxide dismutase-treated dogs. Myocardial lipid peroxidation products, as conjugated dienes, increased in coronary sinus plasma during early reperfusion and this increase was prevented by superoxide dismutase infusion.     

Tolerance of myocardium of aged animals to repeated oxygen deficiency

Summary Hemodynamic and metabolic effects of three times 4 min of oxygen deficiency were investigated in 18-month-old rats in comparison to 4-month-old Wistar rats. Left-ventricular isovolumicpressure-generating capacity and dp/dt_max during isovolumic conditions and hemodynamic indices during intact circulation were determined in open-chest rats. Additionally, high-energy phosphates were measured at the end of the experiments after 20 min of postasphyxial recovery.Older rats had a significantly reduced isovolumic left-ventricular pressure generating capacity (236±9 vs 269±5 mm Hg; p<0.05) and a low cardiac index (55±9 vs 117±8 ml×min^–1×kg^–1). The effects of the oxygen deficiency were comparable in both groups. The isovolumic pressure generating capacity was reduced for 11% vs 14%, and dp/dt_max for 13% vs 13%. The myocardial ATP-content was also decreased for the same extent in both groups (0.6 vs 1.0 mol/gww). Both hemodynamic and biochemical results indicate that aged myocardium does not have a reduced tolerance to repeated periods of oxygen deficiency.Supported in part by the Deutsche Forschungsgemeinschaft, Bonn, FRG (HO 1003/1-2)Parts of the results were presented at the 10th meeting of the Internat. Soc. for Heart Research (Europ. Section), Rotterdam, 1989     

Alterations in the myocardial capillary vasculature accompany tachycardia-induced cardiomyopathy

Summary Changes in capillary structure and distribution within the left ventricle (LV) occur with pressure and volume overload hypertrophy. These changes may cause an impairment in myocardial blood flow (MBF) and oxygen delivery resulting in myocyte injury and LV dysfunction. However, it is unknown whether changes in the capillary vasculature accompany the development of LV dysfunction in dilated cardiomyopathies. Accordingly, this study examined the relation between LV function, MBF and capillary architecture after the development of dilated cardiomyopathy in pigs produced by chronic supraventricular tachycardia (SVT). LV function was examined by echo-catheterization, and capillary morphometrics were computed using lectin histochemistry in two groups of pigs: sham controls (n=8); and after 3 weeks of pacing-induced SVT (n=8). Chronic SVT resulted in significantly incresed LV end-diastolic dimension and pressure with a 50% reduction in LV fractional shortening compared to CON (p<0.05). Although no significant change in capillary density (2180±164 vs 2402±147/mm², p=0.25) occurred in the SVT group compared to CON, a significant reduction in the volume fraction of capillaries (12.2±0.5 vs 9.9±0.7%, p<0.05) and increased capillary diffusion distance (8.4±0.5 vs 7.5±0.3 m, p<0.05) was observed. Frequency distribution analysis revealed a higher percentage of smaller diameter capillaries with chronic SVT vs CON (p<0.05). Ultrastructural examination revealed an increased capillary-myocyte distance with chronic SVT (0.95±0.08 vs 1.95±0.12 m, p<0.05). These changes were accompanied by ultrastructural evidence of significant subendocardial injury (p<0.05). MBF was measured using microspheres in five additional conscious pigs in each group. MBF was significantly reduced and coronary vascular resistance increased in the SVT group compared to CON (p<0.05). Chronic SVT caused significant remodeling of the capillary vasculature; these changes were associated with reduced MBF, myocyte injury, and LV dysfunction.     

Lack of significant effects of superoxide dismutase and catalase on development of reperfusion arrhythmias

Summary It has been reported that agents having the ability to scavenge oxygen-derived free radicals reduce the severity of ventricular arrhythmias that occur after brief coronary occlusion and reperfusion. Superoxide dismutase plus catalase (SOD + CAT) or placebo was administered in a blinded randomized fashion prior to coronary occlusion in rats (n = 25 each group) undergoing a 5-min left coronary occlusion followed by 15 min of reperfusion. During reperfusion, ventricular tachycardia (VT) developed in 96 % of animals in both groups. Reperfusion ventricular fibrillation (VF) developed in 60 % of the placebo group vs 56 % in the SOD + CAT group (p =1.0). Irreversible VF occurred in 40 of the placebo group vs 20 % in the SOD + CAT group (p = 0.22). Atrioventricular block occurred in 12 % of placebo and 4 % of SOD + CAT animals (p = 0.61).There were no significant differences between groups in duration of VT (85 ± 15 s (mean ± SEM) placebo vs 81 ± 14 s SOD + CAT, p = 0.81), total duration of VT plus VF (391 ± 76 s placebo vs 256 ± 64 SOD + CAT, p = 0.45) or numbers of single ventricular ectopic beats (65 ± 15 placebo vs 97 ± 18 SOD + CAT, p = 0.18). Heart rate at reperfusion was slightly higher in control than SOD + CAT animals (340 ± 33 vs 319 ± 32, p = 0.02). Risk zone size, determined by Monastral blue injection, was equal in both groups (34 ± 2 % of ventricular mass). The occurrence of reperfusion VF in this model could not be predicted by heart rate at reperfusion (331 ± 33 VF animals vs 328 ± 36 no VF, p = 0.77), or by risk zone size (34 ± 2 %, VF and no VF groups).Treatment with SOD + CAT did not reduce the overall incidence or duration of reperfusion arrhythmias in this model, though such treatment was associated with a non-significant trend toward less irreversibility of VF.Supported in part by National Institutes of Health SCOR HL-26215, Bethesda, Maryland 660     

Beneficial actions of acidotic initial reperfusate in stunned myocardium of rat hearts

Summary The effects of metabolic acidosis and alkalosis in the initial reperfusate on post-ischemic stunned myocardium were investigated in isolated rat hearts. Metabolic acidosis and alkalosis were produced by altering the doses of artificial buffer (Tris) in place of sodium bicarbonate. All hearts were subjected to global ischemia for 15 min at 37°C. The initial reperfusate under study was given during the subsequent 10 min of reperfusion, just prior to release of the aortic clamp. After that, reperfusion using normal Krebs-Henseleit buffer solution was carried out for 40 min. The acidotic initial reperfusate (pH 6.8) resulted in better protection than the alkalotic initial reperfusate (pH 7.8), as demonstrated by 1) a higher recovery of aortic flow (80.6 % ± 3.8 % vs 32.7 % ± 4.8 %, p < 0.01), 2) a smaller leakage of creatine kinase during the initial reperfusion phase (6.0 ± 0.7 vs 14.6 ± 2.1 IU/10 min/g dry weight, p < 0.05) and during the post-ischemic Langendorff perfusion phase (8.8 ± 1.7 vs 37.3 ±5.2 IU/10 min/g dry weight, p < 0.05), and 3) a lower myocardial water content at the end of reperfusion (84.8 ± 0.2 % vs 85.7 % ± 0.3 %, p < 0.05). Not only Tris buffer system, but also HEPES buffer system indicated that acidotic initial reperfusate was effective to protect against myocardial injury. These results suggest that 1) the extracellular pH during initial reperfusion profoundly influences the reversible myocardial dysfunction (stunned myocardium), and 2) the acidotic initial reperfusate improves post-ischemic myocardial performance.     

Effects of physical training on the myocardium of streptozotocin-induced diabetic rats

Summary Effects of endurance swimming training on myocardial contractility and left ventricular myosin isoenzymes were examined in diabetic rats. A diabetic condition was induced in 15-weck-old male Wistar rats, by intravenous injection of streptozotocin (50 mg/kg). Swimming training was carried out for five to six weeks (90 min/day, 6 days/week). In order to estimate myocardial contractility, the isometric developed tension of the isolated left ventricular papillary muscle was measured. Myosin isoenzymes were obtained by pyrophosphate gel electrophoresis. Fasting blood glucose of the trained group was significantly lower than that of the sedentary group (sedentary vs. trained=409.6±25.9 vs. 266.3±20.5 mg/dl, p<0.001). There was no significant difference in isometric developed tension (T) between the two groups, and the dT/dt_max of the trained group showed a tendency to increase (sedentary vs. trained, T: 2.8±0.8 vs. 2.9±0.8 g/mm², dT/dt_max: 23.1±3.6 vs. 26.2±3.5 g/mm² · 2, p<0.1). Myocardial mechanical responses to isoproterenol and dibutyryl cAMP were increased in the trained group. Left ventricular myosin isoenzyme pattern was shifted towards VM-1 by endurance swimming (sedentary vs. trained, VM-1: 5.6±4.5 vs. 19.6±8.8%, p<0.001, VM-3: 75.1±10.0 vs. 54.9±14.7%, p<0.001). These results indicate that endurance swimming can improve disordered glucose metabolism and also influence myocardial contractility, myocardial catecholamine responsiveness, and energetics in myocardial contraction.     

Influence of collateral flow reduction on electrophysiologic properties of preexisting ischemic area and inducibility of ventricular arrhythmias in the dog

Electrophysiologic properties of the left ventricle, vulnerability to ventricular arrhythmias and regional myocardial blood flow (RMBF) of the left ventricle were examined during superposition of acute ischemia on a healed myocardial infarction. The left circumflex coronary artery (LCX) was ligated in 13 dogs with a 28-day-old anteroapical infarction. Six (46%) of 13 dogs had reproducible ventricular tachycardia in response to programmed ventricular stimulation before LCX ligation. Ventricular fibrillation could be induced in 2 of these 6 dogs. After LCX ligation, 11 (85%) of 13 dogs had ventricular arrhythmias induced by ventricular stimulation. Nine of 13 dogs had ventricular tachycardia and 7 of 13 dogs had ventricular fibrillation. The heterogeneity of the effective refractory period (delta ERP) and the local intraventricular conduction time (LIVCT) in the border and the infarct zones of the left ventricle increased significantly after LCX ligation. RMBF in the border and the infarct zones were markedly decreased by LCX ligation. The magnitude of reduction of RMBF was correlated significantly with the prolongation of LIVCT. In conclusion, acute critical reduction of the collateral blood supply causes a more heterogeneous refractory period and conduction delay in the preexisting ischemic area of the heart, increasing the vulnerability to lethal ventricular arrhythmias.     

Ibuprofen abolishes the increase in leucocyte chemiluminescence observed during ischemic myocardial failure,but fails to improve hemodynamic function

Summary The aim of the study was to evaluate 1) whether the ability of leucocytes to produce oxygen radicals was increased by ischemia and 2) if ibuprofen pretreatment could influence leucocyte oxygen radical production, hemodynamic function, and myocardial oxygen consumption during acute ischemic myocardial failure. We studied two groups of anesthetized dogs (control and ibuprofen-treated), both subjected to coronary embolization with polystyrene microspheres (diameter 50 m). The embolization procedure was ended when left-ventricular end-diastolic pressure in both groups exceeded 20 mm Hg. Before and after induction of ischemia leucocytes were isolated and stimulated with opsonized zymosan, and oxygen radical production was measured using the luminol-dependent chemiluminescence technique. Significant increase occurred in oxygen radical production (from 10.9±2.2 to 16.3±2.3·10⁵ counts·10⁶ cells^–1·60 min^–1) 90 min after failure in the control group, whereas in ibuprofen-pretreated dogs oxygen radical production was unchanged. Hemodynamic registrations and myocardial oxygen consumption 90 min after failure were, however, not significantly different in control dogs and dogs pretreated with ibuprofen. Thus, in the present study, within the first 90 min of acute ischemic failure, a decrease in the ability of leucocytes to produce oxygen radicals was not related to significant changes in myocardial function.