Myocardial infarction in the monkey: studies on the collateral circulation after acute coronary occlusion

An Improved Monkey Model of Myocardial Ischemic Infarction for Cardiovascular Drug Development

Non-human primate monkey model of myocardial ischemic infarction is precious for translational medicine research. Ligation of the left anterior descending (LAD) artery is a common procedure to induce myocardial ischemic infarction. However, the consistency of the myocardial infarction thus generated remains problematic. The present study was undertaken to critically evaluate the monkey model of myocardial ischemic infarction to develop a procedure for a consistent cross-study comparison. Forty male Rhesus monkeys were divided into 4 groups and subjected to LAD artery ligation at different levels along the artery. In addition, the major diagonal branch was selectively ligated parallel to the ligation site of the LAD artery according to the diagonal branch distribution. Analyses of MRI, echocardiography, cardiac hemodynamics, electrocardiography, histopathology, and cardiac injury biomarkers were undertaken to characterize the monkeys with myocardial infarction. Ligation at 40% of the total length of the artery, measured from the apex end, produced variable infarct areas with inconsistent functional alterations. Ligation at 60% or above coupled with selective ligation of diagonal branches produced a consistent myocardial infarction with uniform dysfunction. However, ligation at 70% caused a lethal threat. After a thorough analysis, it is concluded that ligation at 60% of the total length coupled with selective ligation of diagonal branches, enables standardization of the location of occlusion and the subsequent ischemic area, as well as avoids the influence of the diagonal branches, are ideal to produce a consistent monkey model of myocardial ischemic infarction.

Coronary collateral development in the rhesus monkey (Macaca mulatta)

The ability of the rhesus monkey to form coronary collaterals was tested in ten animals. Ameroid constrictors were implanted on the left circumflex coronary artery and allowed to remain for 12 weeks. One animal died of an acute myocardial infarction nine days after surgery; the remaining animals survived without clinical signs referable to myocardial ischemia. The hearts were excised at 12 weeks postsurgery for perfusion fixation and coronary vascular injection with barium-gelatin. All hearts exhibited infarction scars in the circumflex-perfused regions, with infarcts varying from a transmural scar to cases with a thin margin of surviving myocardium at the epicardial surface. Coronary collaterals were infrequent and small in size, and particularly evident in the atria. We conclude that the rhesus monkey is unable to develop sufficient coronary collateral blood flow to prevent myocardial infarction after gradual, total coronary occlusion with ameroid constrictors.

Acute coronary occlusion in the pig: effect of nitroglycerin on regional myocardial blood flow

Myocardial blood flow was studied in 10 closed chest, anesthetized pigs after an acute balloon catheter occlusion of the left anterior descending coronary artery. With use of radioactive microspheres (15 mu), myocardial blood flow was measured before and during an intravenous nitroglycerin infusion and during a combined nitroglycerin-phenylephrine infusion. A significant zone of ischemis (myocardial blood flow less than 50 percent of normal zone flow) was produced by the occlusion and involved 15 percent of the combined left ventricular and interventricular septal mass. More than 50 percent of this ischemic zone was intensely ischemic (myocardial blood flow 0 to 3 percent of normal). Nitroglycerin resulted in a 20 to 30 mm Hg decrease in systolic blood pressure. Myocardial blood flow was unchanged in intensely ischemic areas but varied directly with the product of heart rate and systolic blood pressure in the moderately ischemic area (myocardial blood flow 26 to 50 percent of normal). S-T segment elevation was significantly increased during nitroglycerin infusion and returned to control level with the added infusion of phenylephrine sufficient to restore the systemic blood pressure to prenitroglycerin values. No improvement in ischemic zone perfusion could be demonstrated during the infusion of nitroglycerin alone or with phenylephrine. The endocardial-epicardial flow ratio in moderately ischemic areas was slightly lower than the normal zone flow ratio and decreased slightly during infusion of nitroglycerin. With the addition of phenylephrine, the ratios rose slightly and no longer differed from prenitroglycerin values. Blood flow distribution in acutely ischemic pig myocardium differs considerably from that observed in the dog. Nitroglycerin was not shown to have any beneficial effects with or without its relative hypotensive effect. More extensive study in animal models other than the dog is needed.

TQ-ST segment mapping: critical review and analysis of current concepts

Controversy and confusion surround many aspects of TQ-ST segment mapping today. Technical standards pertaining to the recording and measurement of the TQ-ST deflection have not been uniformly established nor has the correlative value of the deflection as an indicator of myocardial injury been clearly ascertained. The TQ-ST deflection is believed to originate primarily although not exclusively as a result of extracellular potassium accumulation in the ischemic region and subsequent establishment of a transmembrane potential gradient during diastole and systole at the ischemic boundary. Nonspatial factors (including electrolytes, antiarrhythmic agents, heart rate) influence the TQ-ST deflection by altering this gradient. Spatial factors (including ischemic area and shape, electrode location) alter the relative position of the ischemic boundary to the electrode site and as such can be analyzed with the solid angle theorem. Further study of the complex behavior of the TQ-ST segment deflection, particularly in the presence of pharmacologic intervention, is necessary before mapping techniques can be used reliably in clinical studies designed to quantitate and modify ischemic damage.

Spatial and nonspatial influences on the TG-ST segment deflection of ischemia. Theoretical and experimental analysis in the pig

Spatial and nonspatial aspects of TQ-ST segment mapping were studied with the solid angle theorem and randomly coded data from 15,000 electrograms of 160 anterior descending artery occlusions each of 100-s duration performed in 18 pigs. Factors analyzed included electrode location, ischemic area and shape, wall thickness, and increases in plasma potassium (K(+)). Change from control in the TQ-ST recorded at 60 s (DeltaTQ-ST) was measured at 22 ischemic (IS) and nonischemic (NIS) epicardial sites overlying right (RV) and left (LV) ventricles. In IS regions, DeltaTQ-ST decreased according to LV > septum > RV and LV base > LV apex. In NIS regions, LV sites had negative (Neg) DeltaTQ-ST which increased as LV IS border was approached. However, RV NIS had positive (Pos) DeltaTQ-ST which again increased as RV IS border was approached. With large artery occlusion IS area increased 123+/-18%, DeltaTQ-ST at IS sites decreased (-38.1+/-3.6%), and sum of DeltaTQ-ST at IS sites increased by only 67.3+/-10.3%. In RV NIS Pos DeltaTQ-ST became Neg. With increased K(+), DeltaTQ-ST decreased proportionately to log K(+) (r = 0.97+/-0.01) at IS and NIS sites on the epicardium and precordium. TQ-ST at 60 s was obliterated when K(+) = 8.7+/-0.2 mM. All findings were significant (P < 0.005) and agreed with the solid angle theorem. Thus, a transmembrane potential difference and current flow at the IS boundary alone are responsible for the TQ-ST. Nonspatial factors affect the magnitude of transmembrane potential difference, while spatial factors alter the position of the boundary to the electrode site.

Comparison of the coronary collateral circulation in dogs and baboons after coronary occlusion

The relevance to man of experimental observations on coronary collateral blood flow (CCBF) in dogs has been questioned. The effect of 2 to 3 hour coronary occlusions in the anesthetized dog and a primate, the baboon, were therefore compared, with CCBF measured by injections of 85Kr distal to occlusion with precordial counting. Before killing, additional isotope was infused to compare inner/outer wall flow distribution and myocardial tissue samples were analyzed for electrolyte content. Effects of nitrates on hemodynamics and metabolism were also compared in dog and baboon. Similar values for CCBF and resistance following occlusions were found in dog and baboon (flow approximately 25 per cent control, calculated resistance increase four- to sevenfold). Greater subendocardial ischemia in both species was indicated by isotope distribution less to the inner wall, but electrolyte changes (k+ less and Na+ greater in the ischemic area compared to nonischemic) were similar transmurally in both species. Hemodynamic responses to nitrate infusion (isosorbide dinitrate) were similar, with increase in CCBF and decrease in resistance. In neither group were inner/outer wall isotope distribution or electrolyte changes influenced by nitrate. The coronary collateral response to occlusion is similar in dog and baboon in terms of both hemodynamics and metabolic changes. After 2 to 3 hours of coronary occlusion some hemodynamic benefit may be demonstrated with nitrates but no metabolic advantage, at least in the central area of ischemia.

Concentration of digoxin, methyldigoxin, digitoxin and ouabain in the myocardium of the dog following coronary occulsion

26 mongrel dogs were given a single dose of 0.03mg/kg tritium-labelled digoxin, beta-methyldigoxin, digitoxin or ouabain 2 hrs or 95 hrs following experimental coronary occlusion. Examination of the epicardial ECG was performed by moving from intact to ischemic or necrotic zones. 60 min after glycoside administration the animals were sacrificed and tissue samples from the marked heart muscles areas and from the skeletal muscle were analysed for glycoside content. The early glycoside uptake in acute ischemic or necrotic myocardium was diminished independently of the physicochemical properties of the glycoside. Significantly higher glycoside concentrations (ng/g wet weight) were measured in the injured myocardium 3 hrs after coronary occlusion than 96 hrs afterward (p less than 0.005). The values in acute ischemic myocardium varied considerably. This nonhomogeneity of glycoside uptake in the acute ischemic heart muscle may partly explain the increased sensitivity to glycosides in myocardial infarction. The decline of glycoside concentration correlates with the alterations in the epicardial ECG. The cardiac effects of cardenolides 60 min after intravenous administration was caused by the unchanged glycoside. In contrast to the myocardium, glycoside accumulation could not be found in the skeletal muscle. The concentrations of digoxin, beta-methyldigoxin and digitoxin in the skeletal muscle were significantly higher than the concentration of ouabain, which was rapidly eliminated via the urine.

Quantitative measurement of local blood flow with heat clearance

Measurements of the local blood flow through organs by means of heated thermistor-probes enable one to perform continuous recordings of flow. The principle is based on the measurement between a heated and an unheated probe, both implanted in the tissue or placed on the superficial layer of the organ. The flow-measure for this type of blood flow recording is the heat transported from the heated site of the tissue. It is measured in cal-cm(-1)-sec(-1)-degree C(-1). This is the dimension of the heat conductance. Quantification of continuous blood flow measurements in ml flow/g tissue-min has been carried out with combinations of gas clearance and heat transport. It has now become possible to measure blood flow in ml blood-ml(-1) tissue-min(-1) solely with the aid implanted thermistors. Two types of local blood flow measurements are described. 1. Measurements with "slug heating" by which a thermistor is heated for a very short time and the temperature change is subsequently recorded. 2. The measurement of flow by sudden introduction of heat, if a heating coil around a micro-thermistor is switched on or off and the course of the tissue temperature is subsequently followed. It is proven, that the temperature of a point-shaped micro-thermistor in nonperfused tissue and in homogeneous perfused tissue differs only by an exponential factor (see article) where uslug is the temperature field after a slug injection of heat dependent on location, time, blood flow (phi) and partition coefficient for heat (lambda) and where index 0 = unperfused tissue. If the heater is not heated in the form of a delta function but is switched on and the temperature is subsequently recorded or if the heater, after a period of constant temperature, is switched off, the derivatives in respect to time differ only by an exponential factor and can be used as a measure of flow. The evaluation of blood flow is described in detail. The measurements show the following advantages, compared with former techniques: 1. Heat clearance and continuous blood flow measurements by means of heat transport measurements are both possible at the same site. 2. No other operation and procedures are necessary except implantation of the thermistor. 3. Turning on the heater does not affect the tissue and does not cause emotional reactions. As examples the measurements of local blood flow in brain tissue and in the myocardium of conscious animals are described.

Interruption of the arterial inferior alveolar flow and its effects on mandibular collateral circulation and dental tissues

The interruption of circulation through the inferior alveolar artery was followed by the establishment of a fast retrograde blood flow through the vessel. The mental artery and the mandibular branch of the sublingual artery were the main vessels to contribute to that flow. No histopathologic changes were found in the experimental hemimandibles; however, temporary regressive changes were found in the dental pulps of molars.

Early changes in the electrocardiogram and vectorcardiogram and myocardial histology following coronary artery ligation in monkeys

The ramus interventricularis anterior was ligated at different levels (upper, middle, low third) in 12 anesthetized monkeys (Macaca mulatta). Morphological changes and their localization were compared with the electrocardiogram (ECG) in 12 leads and vectorcardiogram (VCG) according to Akulinichev's 5-plane system). The observations were performed for 6-7 hours after ligation. It has been established that upper third ligation of the ramnus interventricularis anterior caused ECG changes chiefly limited to the chest leads. Normalization of the ECG and VCG may occur within 4-6 hours after middle third ligation. In this case, monkeys develop morphological changes in form of insignificant dystrophic changes in the myocardium. This localization of ligature does not seem to disturb collateral blood circulation ensuring myocardial blood supply. Low third ligation was never followed by any normalization. Pathological wave Q on the ECG and its equivalent on the VCG may be observed in case of myocardial damage without large necrotic foci. In such cases, wave Q may decrease after blood supply improvement in the damaged focus.

The effects of nitroglycerin on coronary collaterals and myocardial contractility

Nitroglycerin (TNG) causes a prolonged dilatation of coronary collaterals. To demonstrate a functional significance of this dilatation we measured the effect of TNG on myocardial contractile force in dogs 2(1/2)-4 wk after the left anterior descending coronary artery (LAD) had been embolized in closed-chest animals. Development of collaterals was documented by angiography. Via a left thoracotomy the main left coronary artery (LCA) and LAD distal to the embolized plug were cannulated. Coronary flow and perfusion pressure were recorded. Contractile force was measured with gauges sutured to epicardial areas supplied by the left circumflex coronary artery (LCf) and occluded LAD. Coronary perfusion pressure in the LCA was gradually decreased until the contractile force recorded by the LAD gauge diminished while the LCf gauge was unaffected. Under these conditions, with coronary perfusion pressure held constant with the aid of a Starling resistance, TNG (18 mug) injected into the LCA increased peripheral LAD pressure by 3-12 mm Hg and contractile force in the LAD region by 36% (range 20-90%), returning it to near-normal levels, while having minimal effect in the LCf area. These changes persisted for 5 min. When LCf and LAD areas were both ischemic, intracoronary TNG had minimal effect on peripheral LAD pressure and contractile force. Thus, TNG causes prolonged dilatation of coronary collaterals and presumed increased collateral flow with subsequent enhancement of myocardial contractile force in ischemic areas. This effect is seen only when ischemia is limited to an area supplied by the collaterals. When the whole heart is ischemic, collaterals are unresponsive to TNG, suggesting that these collaterals dilate fully when the regions from which they originate become ischemic.

Acute metabolic response in myocardial infarction

Acute myocardial infarction is viewed as a severe trauma causing a generalized metabolic reaction; an acute emotional stress with further metabolic implications; and a localized wound in which there is an acute increase in carbohydrate metabolism, followed by protein synthetic reactions leading to scar formation. The metabolic response is vital to the patient's successful adaptation to his myocardial infarction.

Observations on coronary collateral communications and the control of flow in the coronary circulation of the dog

1. Pressure was measured in the small arterial anastomosing branches of the coronary vascular network. The mean value was 30 mm Hg not significantly different from the mean value of 33 mm Hg for peripheral coronary pressure measured distal to a ligature on the anterior descending branch of the left coronary artery. Evidence was adduced to show that either the anterior descending or the circumflex artery had the capacity to maintain network pressure at levels adequate for tissue perfusion.2. The network has both capacity and compliance. Filling of the network compliance during systole probably accounts for the systolic phase of coronary flow. Flow through the microcirculation is probably entirely diastolic, the combined compliance of the aorta and large vessels together with the network provides the necessary reservoir, the potential energy indicated by diastolic pressure provides the perfusion pressure head.3. Resistance of vessels between the aorta and network cannula (pre-net) was approximately double that of the microcirculation (post-net). The smaller pre-network vessels are of the order 70 mum in diameter. Both pre- and post-network vessels are vaso-active and respond similarly to adrenaline and haemorrhage.

Experimental myocardial infarction in the dog. Comparison of myocardial blood flow within, near, and distant from the infarct

Control measurements of myocardial blood flow were made in intact dogs by a method of 133 Xe clearance, using a catheter positioned radiographically in the circumflex artery. Acute myocardial infarction was then caused by tying the anterior descending artery. A small catheter was inserted distally beyond the obstructed artery into the infarct, and another proximally toward the anterior ventricular artery supplying adjacent left ventricular muscle. Their ends were brought out as the chest was closed. 133 Xe in solution was injected serially over 4 hours to determine myocardial blood flow within the infarct, in adjacent muscle, and in more distant muscle supplied by the circumflex artery. Myocardial blood flow in the circumflex artery usually fell as a result of infarction. A constant finding was that adjacent flow exceeded the more distant flow by 6% up to 43% (mean 20%) regardless of either spontaneous or induced changes in blood pressure. The difference was not related to the collateral flow rate and was abolished by dipyridamole. Perfusion of muscle adjacent to the infarct was less homogenous than that of more distant muscle.

In the normal dog rates of myocardial blood flow in the distribution of the circumflex and anterior descending branches of the left coronary artery were the same, contrasting with lower rates for the right.