Coronary steal: its role in detrimental effect of isoproterenol after acute coronary occlusion in dogs

Adenosine-Induced Coronary Steal Is Observed in Patients Presenting With ST-Segment-Elevation Myocardial Infarction

Background Adenosine is used to treat no-reflow in the infarct-related artery (IRA) during ST-segment-elevation myocardial infarction intervention. However, the physiological effect of adenosine in the IRA is variable. Coronary steal-a reduction of blood flow to the distal coronary bed-can occur in response to adenosine and this is facilitated by collaterals. We investigated the effects of adenosine on coronary flow reserve (CFR) in patients presenting with ST-segment-elevation myocardial infarction to better understand the physiological mechanism underpinning the variable response to adenosine. Methods and Results Pressure-wire assessment of the IRA after percutaneous coronary intervention was performed in 93 patients presenting with ST-segment-elevation myocardial infarction to calculate index of microvascular resistance, CFR, and collateral flow index by pressure. Modified collateral Rentrop grade to the IRA was recorded, as was microvascular obstruction by cardiac magnetic resonance imaging. Coronary steal (CFR <0.9), no change in flow (CFR=0.9-1.1), and hyperemic flow (CFR >1.1) after adenosine occurred in 19 (20%), 15 (16%), and 59 (63%) patients, respectively. Patients with coronary steal had higher modified Rentrop score to the IRA (1 [0, 1.75] versus 0 [0, 1], P<0.001) and a higher collateral flow index by pressure (0.25±0.10 versus 0.15±0.10, P=0.004) than the hyperemic group. The coronary steal group also had significantly higher index of microvascular resistance (61.68 [28.13, 87.04] versus 23.93 [14.67, 37.00], P=0.006) and had more disease (stenosis >50%) in the donor arteries (52.63% versus 22.03%, P=0.02) than the hyperemic group. Conclusions Adenosine-induced coronary steal may be responsible for a reduction in coronary flow reserve in a proportion of patients presenting with ST-segment-elevation myocardial infarction. Registration URL: https://www.clinicaltrials.gov; Unique identifier: NCT03145194. URL: https://www.isrctn.com; Unique identifier: ISRCTN3176727.

Carbon Dioxide and the Heart: Physiology and Clinical Implications

Carbon dioxide (CO2) is an end product of aerobic cellular respiration. In healthy persons, PaCO2 is maintained by physiologic mechanisms within a narrow range (35-45 mm Hg). Both hypercapnia and hypocapnia are encountered in myriad clinical situations. In recent years, the number of hypercapnic patients has increased by the use of smaller tidal volumes to limit lung stretch and injury during mechanical ventilation, so-called permissive hypercapnia. A knowledge and appreciation of the effects of CO2 in the heart are necessary for optimal clinical management in the perioperative and critical care settings. This article reviews, from a historical perspective: (1) the effects of CO2 on coronary blood flow and the mechanisms underlying these effects; (2) the role of endogenously produced CO2 in metabolic control of coronary blood flow and the matching of myocardial oxygen supply to demand; and (3) the direct and reflexogenic actions of CO2 on myocardial contractile function. Clinically relevant issues are addressed, including the role of increased myocardial tissue PCO2 (PmCO2) in the decline in myocardial contractility during coronary hypoperfusion and the increased vulnerability to CO2-induced cardiac depression in patients receiving a β-adrenergic receptor antagonist or with otherwise compromised inotropic reserve. The potential use of real-time measurements of PmO2 to monitor the adequacy of myocardial perfusion in the perioperative period is discussed.

Pathophysiology of coronary collaterals

While the existence of structural adaptation of coronary anastomoses is undisputed, the potential of coronary collaterals to be capable of functional adaptation has been questioned. For many years, collateral vessels were thought to be rigid tubes allowing only limited blood flow governed by the pressure gradient across them. This concept was consistent with the notion that although collaterals could provide adequate blood flow to maintain resting levels, they would be unable to increase blood flow sufficiently in situations of increased myocardial oxygen demand.

However, more recent studies have demonstrated the capability of the collateral circulation to deliver sufficient blood flow even during exertion or pharmacologic stress. Moreover, it has been shown that increases in collateral flow could be attributed directly to collateral vasomotion.

This review summarizes the pathophysiology of the coronary collateral circulation, ie the functional adapation of coronary collaterals to acute alterations in the coronary circulation.

Detection of moderate regional ischemia in pig hearts in vivo by near-infrared and thermal imaging: effects of dipyridamole

Effects of coronary vasodilator, dipyridamole, on epicardial oxygenation and flow were investigated under conditions of moderate coronary occlusion using near-infrared spectroscopic (NIRS) and thermal imaging. In anesthetized open chest pigs an inflatable occluder and flow probe were placed around the left anterior descending artery (LAD). In the ischemic group (n = 11) LAD occlusion (50% flow, 80 min) was followed by complete occlusion (10 min, n = 4), and reflow. Dipyridamole was infused (0.14 mg/min/kg/4 min) intravenously during 50% occlusion. In the control group (n = 6) LAD flow was temporarily increased (hyperemic response) by two 2-min periods of complete LAD occlusion applied 120 min apart, with a 4-min period of dipyridamole infusion between the two occlusions. NIRS and thermal images were acquired throughout the protocol. Maps of subepicardial oxygen saturation parameter (OSP), and epicardial temperature (T) were obtained. Partial occlusion reduced OSP and the temperature by 0.23 +/- 0.08 and 0.88 +/- 0.39 degrees C versus remote region, respectively. Dipyridamole decreased systolic blood pressure by 36%, which caused further decline in the LAD flow to 18% and OSP and T by 0.37 +/- 0.01 and 2.46 +/- 0.32 degrees C, respectively. Reflow restored OSP and T to their baseline levels. In control group dipyridamole and hyperemia increased LAD flow 2-4-fold associated with moderate increase in OSP and T. OSP and T showed linear dependence on the flow below 100%, which is leveled-off at flows above normal. Dipyridamole increases differences in the epicardial oxygenation and T between normal and moderately ischemic areas due to enhancement of disparity in perfusion of these areas.

Visualization and grading of regional ischemia in pigs in vivo using near-infrared and thermal imagingThis paper is one of a selection of papers published in this Special Issue, entitled The Cellular and Molecular Basis of Cardiovascular Dysfunction, Dhalla 70th Birthday Tribute

Reductions in regional coronary flow result in tissue deoxygenation and decrease in surface temperature, changes detectable by near-infrared spectroscopic (NIRS) and thermal imaging, respectively. In anesthetized open-chest pigs, an inflatable occluder and flow probe were placed around the left anterior descending artery. Gated NIRS and nongated thermal images were acquired at baseline, partial (17% and 50%), and complete occlusion and reflow. At each step, dobutamine was infused (10 μg·min–1·kg–1) for 7–9 min to increase blood pressure and flow. Changes in the oxygen saturation parameter, rate of indocyanine green flow tracer passage, and the surface temperature were correlated with the measured left anterior descending artery flow. Location and sizes of the areas of reduced oxygenation, indocyanine green uptake, and temperature were similar. Decrease in the coronary flow to 50% and 17% of baseline resulted in progressive decrease in the above parameters, whereas increase in flow from 75% to ~250% achieved by dobutamine and reactive hyperemia did not significantly change them. Dobutamine increased total and epicardial flow in ischemic areas and increased subepicardial oxygenation. NIRS and thermal imaging provide epicardial maps of oxygen saturation and perfusion that reveal ischemic areas. Combination of these techniques may be useful in the coronary artery bypass graft (CABG) surgery setting.

Determinants of coronary steal in chronic total coronary occlusions donor artery, collateral, and microvascular resistance

OBJECTIVES

We aimed to assess the mechanisms of coronary steal by direct hemodynamic measurements of the collateral circulation in chronic total coronary occlusions (CTO).

BACKGROUND

Coronary steal may cause ischemia despite well-developed collaterals in coronary artery disease.

METHODS

Fifty-six patients were studied during recanalization of a CTO. Before recanalization, the fractional flow reserve in the donor artery (FFR(D)) at the takeoff of the collaterals and the coronary flow reserve were recorded. After crossing the occlusion, the distal coronary flow velocity was measured by a Doppler wire (APV(Occl)), and distal pressure by a pressure wire. Changes of these parameters were assessed during intravenous adenosine (140 microg/kg/min). Resistance indexes for the donor artery (R(D)), collaterals (R(C)), and microcirculation (R(P)) were calculated.

RESULTS

Adenosine caused a decrease of APV(Occl) (i.e., coronary steal, in 26 patients [group S], an increase in 19 patients [group R], and no change in 11 patients). The FFR(D) was lower in group S. R(D) and R(C) increased in group S, while R(D) did not change significantly and R(C) decreased in group R. Patients with steal had more severe regional dysfunction. Patients with steal but without an FFR(D) <0.8 tended to have an impaired microvascular function.

CONCLUSIONS

We could demonstrate that coronary steal in man is mainly due to a hemodynamically significant donor artery lesion, but can also occur due to an impaired vasodilatory reserve of the microcirculation in the absence of a donor artery lesion. Coronary steal may have an adverse influence on the preservation of myocardial function by collaterals.

Effects of minoxidil on ischemia-induced mechanical and metabolic dysfunction in dog myocardium

Effects of minoxidil on ischemia-induced myocardial mechanical and metabolic dysfunction were examined in anesthetized open-chest dogs. A regional portion of the left ventricle was made ischemic for 20 min by ligating the left anterior descending coronary artery, and then reperfused for 120 min. Dimethylsulfoxide or minoxidil (0.3, or 1.0 mg/kg) was injected intravenously 10 min before ligation. Ischemia decreased regional myocardial contraction, and reperfusion recovered it but incompletely. Myocardial metabolic derangement was observed during ischemia, such as decreases in the myocardial levels of ATP and creatine phosphate. These metabolic changes caused by ischemia were restored by reperfusion. Minoxidil injection at 0.3 and 1.0 mg/kg significantly decreased blood pressures but increased coronary flow. Pretreatment with minoxidil significantly enhanced the recovery of myocardial contraction during reperfusion after ischemia. The levels of ATP and creatine phosphate in the ischemic myocardium were significantly preserved by minoxidil at 0.3 mg/kg. No significant effect of minoxidil on the metabolism was observed in the 120 min reperfused myocardium. In conclusion, minoxidil improved the mechanical dysfunction in the reperfused heart and the drug at low dose preserved high-energy phosphates during ischemia.

Coronary sinus occlusion enhances coronary collateral flow and reduces subendocardial ischemia

On the hypothesis that coronary sinus occlusion (CSO) may reduce myocardial ischemia, we examined the effects of CSO on coronary collateral blood flow and on the distribution of regional myocardial blood flow (RMBF) in dogs. Thirty-eight anesthetized dogs underwent occlusion of the left anterior descending coronary artery with or without CSO and intact vasomotor tone. We measured RMBF and intramyocardial pressure (IMP) in the subendocardium (Endo) and subepicardium (Epi) separately. With intact vasomotor tone, CSO during ischemia significantly increased RMBF in the ischemic region (IR), particularly in Endo from 0.17 +/- 0.03 to 0.33 +/- 0.05 ml x min(-1) x g(-1) (P < 0.05), and increased the Endo/Epi from 0.59 +/- 0.10 to 1.15 +/- 0.15 (P < 0.01). These effects of CSO were partially abolished by adenosine. However, the Endo/Epi was still increased from 0.90 +/- 0.13 to 2.09 +/- 0.30 (P < 0.01). The changes in RMBF in IR were significantly correlated with the peak CS pressure during CSO. The Endo/Epi of IMP in IR was significantly decreased during CSO. In conclusion, CSO potentially enhances coronary collateral flow, and preserves the ischemic myocardium, especially in Endo.

Pharmacological modulation of the human collateral vascular resistance in acute and chronic coronary occlusion assessed by intracoronary blood flow velocity analysis in an angioplasty model

BACKGROUND

The pharmacological responsiveness of the coronary collateral circulation in humans has been studied only by indirect means.

METHODS AND RESULTS

Patients with one-vessel disease and recruitable (n = 14) or spontaneously visible (n = 24) collateral vessels were studied during coronary angioplasty. Collateral flow in the recipient coronary artery was determined with a 0.014-in Doppler guide wire during balloon coronary occlusion and expressed as the diastolic blood flow velocity integral (dVi). Collateral blood flow velocity, mean aortic pressure (Pao), and coronary wedge pressure (Pw) were used to calculate the collateral vascular resistance index: Rcoll = (Pao-Pw)/ dVi (mm Hg/cm) and the peripheral vascular resistance index of the recipient coronary artery: R4 = Pw/dVi (mm Hg/cm). Adenosine (12 to 18 micrograms) and nitroglycerin (0.2 mg) were injected as a bolus in the donor coronary artery during subsequent balloon inflations to assess their effect on these hemodynamic variables. The administration of adenosine or nitroglycerin in patients with recruitable collateral vessels did not induce a change in dVi and Pw/Pao ratio. In patients with spontaneously visible collateral vessels, dVi increased from 8.0 +/- 4.5 to 10.8 +/- 8.0 cm (P = .01) after adenosine and from 7.4 +/- 4.5 to 10.3 +/- 6.9 cm (P = .003) after nitroglycerin. The Pw/Pao ratio remained unchanged after adenosine and nitroglycerin. Rcoll decreased from 10.3 +/- 9.5 to 8.6 +/- 8.5 mm Hg/cm (P = .01) after adenosine and from 11.6 +/- 10.4 to 8.3 +/- 8.9 mm Hg/cm (P < .001) after nitroglycerin. R4 decreased from 7.7 +/- 5.5 to 5.9 +/- 5.1 mm Hg/cm (P < .001) after adenosine and from 8.4 +/- 6.6 to 7.1 +/- 7.2 mm Hg/cm (P = .01) after nitroglycerin.

CONCLUSIONS

Coronary collateral blood flow can be increased with adenosine and nitroglycerin in patients with one-vessel disease and spontaneously visible collateral vessels, which is in contrast to patients with recruitable collateral vessels. This effect is the result of a reduction in the collateral vascular resistance and peripheral vascular resistance of the recipient coronary artery.

Impairment of the myocardial vasomotor response to cold pressor stress in collateral dependent myocardium

OBJECTIVE

To study the vasomotor response (cold pressor/basal flow) in myocardium perfused entirely by collaterals, using the reflex sympathetic stimulation of cold pressor stress.

DESIGN

Regional myocardial blood flow was measured in collateral dependent and in remote myocardium using positron emission tomography with 15O water at basal and at cold pressor stress. Regional ischaemia was measured with 18F-fluorodeoxyglucose (FDG).

PATIENTS

Nine patients (mean (SD) age 53 (6) years) with an occluded coronary artery supplied entirely by collaterals from other angiographically normal arteries.

RESULTS

In remote myocardium, basal and cold pressor flow were 0.99 (0.26) and 1.46 (0.60) ml/min/g (P < 0.05), respectively, a myocardial vasomotor response of 1.46 (0.45). In collateral dependent myocardium, basal and cold pressor flow were 0.91 (0.20) and 0.87 (0.35) ml/min/g, respectively (the latter value, P < 0.05 v remote region), a myocardial vasomotor response of 0.97 (0.43) (P < 0.05 v remote region). The myocardial vascular resistance (mean arterial pressure/flow) during cold pressor was higher in the collateral dependent than in remote myocardium, at 147.0 (61.1) and 85.6 (32.3) mm Hg.min.g/ml (P < 0.05), respectively, but with no relative increase in FDG uptake.

CONCLUSIONS

In contrast to the decrease in myocardial resistance in remote myocardium with cold pressor, an increase was observed in collateral dependent myocardium suggesting a vasoconstrictor response in resistive vessels, without demonstrable myocardial ischaemia.

Preferential dilation of large coronary microvessels by the mononitrates SPM-4744 and SPM-5185

A novel aspect of the pharmacodynamic action of nitroglycerin is that it is a potent dilator of larger coronary arteries, yet it dilates smaller coronary microvessels submaximally and only in high concentrations. We sought to determine whether this property was shared by other organic nitrates. The effects of two mononitrates. SPM-4744 and SPM-5185 (the latter of which possesses a thioester in its structure), on coronary microvessels of different sizes were studied. Large (200-microns diameter) and small ( < 100-microns diameter) porcine coronary microvessels were studied in vitro while pressurized in a no-flow state. After constriction with the thromboxane analogue U46619, maximal dilations (as a percent of preconstricted tone at the highest applied concentration, 10 microM) of small coronary microvessels were 18 +/- 3 and 16 = 2% in response to SPM-4744 and SPM-5185, respectively. The dilations of larger coronary microvessels to SPM-4744 and SPM-5185 were 55 +/- 5 and 43 +/- 6%, respectively (both p < 0.001 vs. the small vessel responses). This pattern of differential vasodilatation of large and small coronary microvessels was similar to that produced by nitroglycerin. In contrast, sodium nitroprusside produced equivalent degrees of vasodilation of small and large coronary microvessels. Additional experiments demonstrated that both SPM compounds produced dilation of the coronary microcirculation in isolated rat heart and relaxed isolated segments of rat aortic rings only in high ( > or = 1 microM) concentrations. These data demonstrate that the organic mononitrates are similar to nitroglycerin in their selectivity for larger coronary microvessels and produce only minimal dilation of coronary microvessels < 100 microM in diameter.

Pharmacological properties of KRN2391, a novel vasodilator of the nitrate-potassium channel opener hybrid type

1. The pharmacological properties of KRN2391 in animal experiments are reviewed. 2. The vasodilating mechanism of KRN2391 is based on both a nitrate action and a K channel opening action, and whether KRN2391 acts as a nitrate and/or a K channel opener depends on the type and the segment of blood vessels. 3. KRN2391 causes a preferential increase in coronary blood flow in anesthetized dogs. 4. KRN2391 produces an increase in oxygen supply to the heart and a decrease in myocardial oxygen consumption in anesthetized dogs. 5. KRN2391 shows antiangial effects in various anginal models of rats and cardioprotective effects in perfused rat hearts. 6. KRN2391 does not develop self-tolerance or cross-tolerance between KRN2391 and other nitrates in coronary dilating and vasodepressor effects. 7. The pharmacological properties of KRN2391 are thought to be beneficial for the treatment of patients with ischemic heart disease.

Vasorelaxant mechanism of KRN2391 and nicorandil in porcine coronary arteries of different sizes

1. The relaxant mechanisms of action of KRN2391, a novel vasodilator, and nicorandil on epimyocardial coronary artery (2.5- 3.0 mm outer diameter) and mid-myocardial coronary artery (0.8-1.0 mm outer diameter) were investigated in porcine isolated coronary arteries. In addition, the vasorelaxant responses of KRN2391 and nicorandil were compared with those of nitroglycerin and cromakalim, a K+ channel opener, in epi- and mid-myocardial coronary arteries. 2. Nitroglycerin showed a more potent relaxant effect on epi-myocardial coronary arteries than on mid-myocardial coronary arteries, whereas cromakalim produced greater relaxation responses in mid-myocardial coronary arteries. There was no difference between epi- and mid-myocardial coronary arteries in terms of the relaxant effect of KRN2391 and nicorandil. 3. Relaxation induced by KRN2391 in epi- and mid-myocardial coronary arteries was inhibited by oxyhaemoglobin, a pharmacological antagonist of nitrovasodilators, and glibenclamide, a pharmacological antagonist of K+ channel opening drugs. However, the inhibitory effect of glibenclamide on KRN2391-induced relaxation was greater in mid-myocardial coronary artery than in epi-myocardial coronary artery. 4. Relaxation induced by nicorandil was inhibited by oxyhaemoglobin alone in epi-myocardial coronary arteries and by both oxyhaemoglobin and glibenclamide in mid-myocardial coronary arteries. 5. In epi- and mid-myocardial coronary arteries, relaxation induced by cromakalim was inhibited by glibenclamide but not by oxyhaemoglobin, whereas relaxation induced by nitroglycerin was inhibited by oxyhaemoglobin but not by glibenclamide. 6. These results suggest that KRN2391 and nicorandil exhibit a dual mechanism of action acting partly as a nitrate and partly as a K+ channel opener. The mechanism of action of these drugs depend on the segment of coronary artery studied. Furthermore, the dual mechanism of action of KRN2391 and nicorandil seems to contribute to the equipotent relaxant effect between epi- and mid-myocardial coronary arteries.

Effects of steal-prone anatomy on intraoperative myocardial ischemia. The SPI Research Group

OBJECTIVES

Our study objective was to determine whether the presence of steal-prone anatomy conferred an increased risk in the development of intraoperative myocardial ischemia.

BACKGROUND

Coronary artery steal of collateral blood flow has been demonstrated for many vasodilators, including isoflurane, the most commonly used inhalational anesthetic agent in the United States. It has been postulated that patients with steal-prone anatomy (total occlusion of one coronary artery that is supplied distally by collateral flow from another coronary artery with a > or = 50% stenosis) may be particularly at risk for the development of intraoperative myocardial ischemia when an anesthetic with a vasodilator property is being administered.

METHODS

We evaluated the risk of myocardial ischemia under isoflurane anesthesia (vs. a high dose narcotic technique using sufentanil) using continuous intraoperative electrocardiography and transesophageal echocardiography in patients with and without steal-prone anatomy undergoing coronary artery bypass graft surgery.

RESULTS

Sixty-two (33%) of the 186 patients had steal-prone anatomy: in 5 (8%) the collateral-supplying vessel was > or = 50% to 69% stenosed, in 24 (39%) it was > or = 70% to 89% stenosed and in 33 (53%) it was > or = 90% stenosed. The incidence of ischemia (transesophageal echocardiography or intraoperative electrocardiography, or both) was similar in patients with and without steal-prone coronary anatomy (18 [29%] of 62 patients vs. 39 [31%] of 124 patients, p = 0.87, 95% confidence interval = -0.13 to 0.17). The incidence of intraoperative ischemia was similar in patients who received isoflurane or sufentanil anesthesia (20 [32%] of 62 patients vs. 37 [30%] of 124 patients, p = 0.87). The incidence of tachycardia and hypotension was low (increases in heart rate = 9.8%, and decreases in systolic blood pressure = 10.8% of total monitoring time during the prebypass period compared with preoperative baseline values). The incidence of adverse cardiac outcome was similar in patients with and without preoperative steal-prone coronary anatomy (4 [7%] of 62 patients vs. 14 [11%] of 124 patients, p = 0.53).

CONCLUSIONS

These findings demonstrate that under strict hemodynamic control the presence of steal-prone anatomy does not confer an increased risk in the development of intraoperative myocardial ischemia.

Neurohumoral regulation of coronary collateral vasomotor tone

As a result of gradual coronary occlusion, coronary collaterals are stimulated to develop. This maturation process involves not only dilatation of the vessel, but the development of new vascular smooth muscle. Experiments have been performed to examine vasomotor characteristics of mature coronary collaterals from dogs 3 to 6 months following ameroid constrictor placement. Studies in Langendorff blood-perfused hearts have shown that transcollateral resistance does not change during either the administration of alpha 1- or alpha 2-adrenergic agonists. Isolated collateral vessels studied as rings in organ chambers do not constrict to either alpha 1- or alpha 2-adrenergic agonists. These studies show that mature collateral vessels are not likely to possess functioning alpha-adrenergic receptors. Subsequent experiments using a cover slip autoradiographic ligand-binding approach have demonstrated a population of beta-adrenergic receptors on mature coronary collaterals. Studies of isolated collaterals have demonstrated beta-adrenoceptor-mediated relaxation that appears due to a population of mixed beta 1- and beta 2-adrenergic receptors. Subsequent studies have demonstrated that mature collateral vessels are hyperresponsive to the vasoconstrictor effects of vasopressin and that concentrations of vasopressin which may be encountered in pathophysiologic conditions can markedly attenuate coronary collateral perfusion. Finally, the microcirculation of the collateral-dependent myocardium develops endothelial cell dysfunction. This results in impaired endothelium-dependent relaxations to adenosine diphosphate and acetylcholine and enhanced vasoconstriction to vasopressin. These alterations of the coronary circulation may have important implications regarding neurohumoral regulation of myocardial perfusion in collateral-dependent myocardium.

Efficacy of nitroglycerin in stress-induced regional myocardial dysfunction is dependent on the chosen model: investigations in anesthetized and conscious dogs

The antianginal efficacy of nitroglycerin (1 microgram/kg per min) was investigated in two different experimental models, one using chloralose-anesthetized open-chest dogs, the other using conscious, chronically instrumented dogs. Heart rate, arterial pressure, left ventricular dp/dtmax, and left ventricular end-diastolic pressure were registered. Left ventricular regional contractile function in the area supplied by the left circumflex coronary artery (LCX) and the left anterior descending artery (LAD) were assessed using sonomicrometry. In both models, the coronary flow reserve was limited by a hydraulic occluder around the LCX. Cardiac stimulation was achieved by a bolus injection of isoproterenol (ISO 0.5 microgram/kg) in the anesthetized animals and by graded treadmill exercise in the conscious animals. In both cases, transient contractile dysfunction occurred in the area supplied by the stenosed vessel. This contractile dysfunction was completely abolished by nitroglycerin in the conscious animals, while nitroglycerin failed to show any antianginal effect in the anesthetized dogs. Although hemodynamic differences in open and closed chest should be considered, remarkable differences in mechanisms of blood-pressure regulation according to the mode of stimulation were observed: in contrast to the situation during treadmill exercise, the ISO-induced decrease in arterial blood pressure does not correspond to the clinical picture of an anginal attack. These results show that it is most important to mimic the complex pathophysiological reactions of angina pectoris in man as closely as possible in the experimental model.

Can new inodilators displace digitalis in the therapy of congestive heart failure?

New inodilators that possess both positive inotropic and vasodilator actions have many favorable effects in patients with congestive heart failure, even in those with refractory heart failure. These effects are expected to prevent myocardial injury, improve peripheral circulation, depress the excessive endogenous neurohumoral activation, and, finally, improve the quality of life, and increase lifespan. However, experience with new inodilators has only begun. Several questions remain to be answered before these drugs can be widely used with safety, including whether life-threatening adverse effects appear, mortality rate is lessened, and drug tolerance occurs. The therapeutic level of the dose and the relation between the effectiveness of the drug and the degree of the severity of heart failure should also be established. Therefore, long-term, randomized, double-blind, placebo-controlled clinical trials will be necessary before the new inodilators can take the place of digitalis and thus become the mainstay of the therapy of congestive heart failure.

The effects of nitroglycerin on regional myocardial contractile dysfunction produced by treadmill exercise or isoprenaline stimulation in dogs

1. To compare different methods of cardiac stress testing that are clinically applied in the management of coronary heart disease, 2 groups of dogs each were chronically instrumented and subjected to treadmill exercise or isoprenaline infusion in the presence of coronary stenosis. 2. It was of interest to determine differences in haemodynamic and regional myocardial contractile parameters, the response to antianginal therapy (nitroglycerin 15 micrograms kg-1 15 min-1, i.v.), and, in particular, whether this response differed according to the mode of cardiac stimulation, i.e. treadmill exercise or isoprenaline infusion. 3. After stenosis of the circumflex branch of the left coronary artery which affected resting myocardial function only minimally, treadmill exercise or isoprenaline infusion induced transient regional contractile dysfunction. Heart rate, arterial blood pressure, left ventricular end-diastolic pressure and left ventricular dp/dtmax were registered and myocardial oxygen demand was calculated. Regional contractile performance was assessed by ultrasonic distance measurement in the underperfused and in a normally perfused area. 4. Treadmill exercise led to an increase in systolic arterial and left ventricular end-diastolic pressure. In contrast, isoprenaline-induced stimulation led to a decrease in diastolic arterial and left ventricular end-diastolic pressure. Regional contractile function in the critically underperfused area showed a deterioration during both modes of stress. Nitroglycerin completely abolished stress-induced contractile dysfunction only in the group where treadmill exercise was employed for stimulation. 5. The inability of nitroglycerin to prevent myocardial dysfunction in the isoprenaline group may be due to exhaustion of the arterial and/or venous vasodilator potency of nitroglycerin in the presence of adrenoceptor vasodilatation induced by isoprenaline. 6. These findings indicate that clinical antianginal drug testing and the evaluation of the course of disease in patients with coronary heart disease may be highly dependent on the test method chosen.

Effects of calcium channel blocker on responses of blood flow, function, arrhythmias, and extent of infarction following reperfusion in conscious baboons

Two groups of chronically instrumented, conscious baboons were studied. The effects of coronary artery occlusion for 3 hours and reperfusion for 1 week were examined on measurements of left ventricular function, ischemic-zone wall thickness, regional myocardial blood flow, arrhythmias, and extent of necrosis. The experimental group of animals (n = 7) was treated with the calcium channel blocker nisoldipine (0.1 microgram/kg/min) from 1 hour after coronary occlusion to 3 hours after coronary reperfusion. The control group (n = 6) received the vehicle (n = 4) or saline (n = 2). The effects of coronary artery occlusion and reperfusion on arterial pressure, left ventricular systolic pressure, heart rate, and left ventricular dP/dt were similar in both groups. Systolic wall thickening was reversed to paradoxical wall thinning during occlusion in both groups, and there was no recovery to systolic wall thickening over the 1-week period in either group. There were differences in regional blood flow; during coronary artery occlusion, nisoldipine increased blood flow significantly in the endocardium and epicardium of nonischemic and ischemic zones. There was a major difference in the number of arrhythmic beats per minute on reperfusion; during reperfusion, the number of arrhythmias rose markedly in the vehicle-treated group but actually fell in the nisoldipine-treated group. The size of areas at risk, infarcts, infarcts related to the area at risk, and amount of total creatine kinase (CK) and MB-CK appearing in blood were not significantly different in the two groups. Thus, in the conscious baboon, nisoldipine administered 1 hour after coronary artery occlusion exerted a marked effect in diminishing reperfusion-induced arrhythmias and improved blood flow to the ischemic zone during occlusion but did not salvage ischemic tissue.

Myocardial energy production and consumption remain balanced during positive inotropic stimulation when coronary flow is restricted to basal rates in rabbit heart

The effect on myocardial energy balance of increasing oxygen demand without altering basal myocardial perfusion rate was assessed in isolated, isovolumic, retrograde blood perfused rabbit hearts. Myocardial energy requirements were increased with paired stimulation. The capacity of rapid paired stimulation to increase mechanical energy consumption was demonstrated in the presence of increased perfusion with the rate X pressure product and oxygen consumption increasing 86 and 148%, respectively, compared with control values. In contrast, rapid paired stimulation under constant, basal flow conditions did not alter the rate X pressure product, while oxygen extraction and consumption increased only 40% relative to control. Myocardial ATP, creatine-phosphate, and lactate content were identical under control and constant flow-paired stimulation conditions. The results of this study indicate that no detectable energy imbalance was produced by rapid paired stimulation with flow held constant at basal rates. These results suggest that the myocardium does not increase mechanical energy expenditure in response to inotropic or rate stimulation in the presence of restricted flow reserve and are inconsistent with the concept of "demand-induced" or "relative" myocardial ischemia.

Long-term nitroglycerin treatment: effect on direct and endothelium-mediated large coronary artery dilation in conscious dogs

We examined the effect of nitroglycerin (GTN) tolerance on an important determinant of nitrate-antianginal action, large coronary artery dilation, in 11 chronically instrumented conscious dogs. In addition, endothelium-mediated coronary artery dilation was studied because this shares a common dilator pathway with the nitrates, i.e., activation of soluble guanylate cyclase. With long-term GTN (1.5 micrograms/kg/min iv for 5 days) the diameters of the left circumflex and anterior descending coronary arteries showed an initial increase of 8.2 +/- 0.3% and 10.8 +/- 0.9%, respectively, returning to control levels by the second to third day of treatment. On days 4 and 5, the dose-response relations for GTN-induced epicardial artery dilation were shifted (p less than .01) to 17- to 20-fold higher doses. However, there was no attenuation of epicardial artery dilation induced by SIN-1 (n = 7), another activator of guanylate cyclase, or of endothelium-mediated dilation assessed both as flow-dependent dilation (n = 7) and as direct intra-arterial acetylcholine-induced dilation (n = 4). In addition, there was no clear tolerance to the peripheral vascular actions of GTN responsible for reflex tachycardia and increased coronary flow. We conclude that a moderate degree of nitrate tolerance to epicardial artery dilation does not affect the responsiveness to other exogenous or endogenous activators of guanylate cyclase. However, this tolerance to epicardial artery dilation, together with the maintenance of peripheral vascular actions that can induce reflex tachycardia, result in a potentially unfavorable balance of GTN effects.

Prostacyclin reduction of regional ischemic injury in the canine myocardium

The effect of prostacyclin (PGI2) on the myocardium of the awake dog subjected to coronary artery occlusion was examined. Animals were randomly administered PGI2 200 ng/kg/min (n = 6), PGI2 100 ng/kg/min (n = 6), or the vehicle control (n = 6), beginning 30 min prior to coronary artery occlusion. Radiolabeled microspheres (15 microns) were used to measure myocardial blood flow. The myocardial region at risk was determined by fluorescein injection, and infarct size was assessed by triphenyl tetrazolium staining. Segmental myocardial function was evaluated from the systolic ejection shortening (SES) by subendocardial ultrasonic dimension crystals in normal, ischemic, and border zones. PGI2 200 ng/kg/min produced significant decreases in aortic pressure and systemic vascular resistance. PGI2 100 ng/kg/min, which achieves 95% platelet inhibition, had no significant hemodynamic effects. Animals receiving PGI2 200 ng/kg/min had significantly higher blood flow to the ischemic region, better border zones SES, and a smaller infarct. PGI2 ameliorates myocardial injury and reduces functional impairment produced by ischemia in doses that elicit vasodilation. This beneficial effect of PGI2 does not appear to be mediated solely by an antiplatelet mechanism.

The regional difference of relaxations induced by various vasodilators in isolated dog coronary and mesenteric arteries

Relaxant responses to vasodilators, including nitroglycerin, sodium nitroprusside, prostaglandin I2 sodium salt (PGI2), prostaglandin E1 (PGE1), diltiazem hydrochloride and adenosine, were compared in helical strips of dog coronary arteries of different sizes and in coronary and mesenteric arterial strips. The relaxant responses to nitroglycerin, sodium nitroprusside, diltiazem and adenosine were significantly greater in coronary arteries than in mesenteric arteries, whereas the responses to PGI2 and PGE1 in these arteries did not significantly differ. In coronary arteries of different sizes, the relaxation induced by nitroglycerin was in the order of large greater than medium greater than small-size, while in contrast, the relaxations by adenosine, PGI2 and PGE1 were greatest in the small-size arteries and least in the large-size arteries. The relaxant responses to sodium nitroprusside and diltiazem did not differ in the coronary arteries of different sizes. Nitroglycerin, sodium nitroprusside and diltiazem appear to dilate coronary arteries more predominantly than mesenteric arteries. The preferential dilator action of PGI2 and PGE1 on distal coronary arteries, like that of adenosine, may lead more blood to re-distribute to the non-ischemic region of the heart in anginal patients.

Myocardial blood flow during cardiac tamponade in dogs with coronary occlusion: effects of isoproterenol

To determine the effects of cardiac tamponade in dogs with ligation of the left anterior descending coronary artery, fluid was introduced into the pericardial space to raise right and left atrial and pericardial pressures, first to 7 to 9 mm Hg and then to 11 to 12 mm Hg. Normal and ischemic myocardial blood flow fell approximately 20% to 25% during mild tamponade (1.27 +/- 0.16 to 1.00 +/- 0.06 ml/min/gm and 0.52 +/- 0.12 to 0.39 +/- 0.08 ml/min/gm, respectively) and by 50% during moderate tamponade (0.66 +/- 0.08 and 0.23 +/- 0.05 ml/min/gm, respectively). The inner/outer left ventricular wall blood flow ratio decreased modestly from 1.16 to 1.08 (p less than 0.025) in normal areas but increased from 0.53 to 0.61 (p less than 0.05) in the ischemic regions, suggesting possible epicardial vessel compression. Isoproterenol resulted in prompt decreases in pericardial and filling pressures, 16% increase in aortic pressure, and 200% rise in cardiac output. Normal myocardial blood flow more than doubled (1.55 +/- 0.12 ml/min/gm, p less than 0.001). Although average ischemic blood flow rose slightly to 0.042 +/- 0.10 ml/min/gm, the increase was not significant. Furthermore, changes in ischemic blood flow were heterogeneous with frank decrease in one dog. Therefore, although isoproterenol has salutary hemodynamic effects, its unpredictable action on myocardial blood flow should cause one to use it cautiously in those with tamponade who are believed to have coronary occlusive disease.

Effects of isoproterenol on adenine nucleotide catabolism in cat hearts with acute regional ischaemia

Previous studies have demonstrated that beta-adrenergic agonists cause extension of ischaemic lesions in the myocardium. In the present study the effects of isoproterenol on adenine nucleotide metabolism were investigated in cat hearts with regional ischaemia following permanent coronary occlusion for 45 min. Adenine nucleotides and their metabolites were measured by high performance liquid chromatography and regional myocardial blood flow was measured by 15-microns radiolabelled microspheres in a total of 255 paired tissue samples. Compared with untreated control cats, ATP and the energy charge were more reduced in ischaemic myocardium of isoproterenol-treated cats. Increased amounts of the degradation products inosine and hypoxanthine/xanthine were also found in these regions. These results could be ascribed to increased cardiac performance caused by isoproterenol, which augments the imbalance between energy production and oxygen supply in ischaemic myocardium.

A canine model of transient myocardial dysfunction regional shortening in the presence of critical stenosis and cardiac stimulation

An experimental model of exercise-induced angina pectoris was developed in anesthetized dogs. Two i.v. bolus injections of isoproterenol (ISO, 0.25 microgram/kg; time interval: 10 min) were administered, followed by constriction of the circumflex coronary artery (LCX; critical stenosis). Further i.v. bolus injections of ISO were administered before and after i.v. infusion of 0.9% NaCl (control infusion). Stenosis of the LCX did not modify regional function at rest as assessed with ultrasonic crystals implanted subendocardially both in the distribution areas of the circumflex coronary artery and of the left anterior descending coronary artery (LAD). In the presence of critical stenosis, ISO induced a reproducible, transient decrease in systolic shortening and percent systolic shortening in the area supplied by the LCX and a marked increase in systolic shortening and percent systolic shortening in the area supplied by the LAD. During a 60-min observation period following control infusion, no further substantial changes in regional dimensions were observed. The ISO-induced hemodynamic changes were reproducible during this period. The experimental model of exercise-induced angina pectoris presented in this study can be assumed to provide a reliable basis for evaluating the efficacy of various therapeutical interventions.

Effect of nitroglycerin on regional myocardial blood flow following an experimental coronary spasm

This study was designed to evaluate the effect of nitroglycerin (30 micrograms given as an i.v. bolus) on regional distribution of myocardial blood flow in conscious dogs, following an acute coronary occlusion similar to a coronary spasm. The left anterior descending (LAD) coronary artery was acutely occluded with a balloon cuff occluder. The distribution of blood flow between the endocardium and the epicardium of both the normal and ischemic area of the left ventricle was determined by means of the radioactive microsphere technique. Acute occlusion of the left anterior descending coronary artery produced a significant decrease of blood flow reaching the area irrigated by this artery; this decrease was of a lesser magnitude after administration of nitroglycerin. In addition, ischemia produced a disproportionate decrease in endocardial blood flow. This decrement was also of a lesser magnitude following administration of nitroglycerin. Blood perfusion to the non-ischemic myocardium was not altered. These results indicate that an intravenous bolus of nitroglycerin, given after a brief coronary occlusion simulating a coronary spasm, increases blood flow to the ischemic myocardium, induces a favorable redistribution of blood flow toward the ischemic endocardium and does not produce any decrement of blood perfusion to the non-ischemic myocardium.

Management of acute myocardial infarction with natural physiological agents

A number of natural physiological agents deserve evaluation in the treatment of acute myocardial infarction. Prostacyclin and magnesium dilate large coronary arteries and could promote collateral circulation to ischemic regions, especially if used in conjunction with alpha-agonists to prevent a drop in coronary perfusion pressure. In addition, prostacyclin has anti-aggregatory and de-aggregatory effects on platelets and a stabilizing action on hypoxic tissue, while magnesium has anti-arrhythmic, potassium-retaining, and fibrinolytic effects, all of which could improve the outcome in acute MI. Adenosine or ribose infusion could be used to promote rapid repletion of adenine nucleotides in reperfused tissue, but unfortunately arteriolar vasodilation by adenosine might reduce collateral perfusion by "coronary steal". High-dose insulin has positive-inotropic (at minimal oxygen cost) and potent anti-arrhythmic actions that have not been adequately tested in previous clinical trials of "polarizing solutions". Carnitine infusion could improve the bioenergetics of ischemic myocardium by relieving inhibition of mitochondrial adenine nucleotide translocase.

The effects of intracoronary Renografin injection on coronary flow and distribution in dogs following coronary occlusion

To study the effects of a contrast agent commonly used in coronary angiography (Renografin) on myocardial flow distribution, the main left coronary arteries of 9 dogs were cannulated and perfused from the carotid arteries. The left anterior descending coronary artery (LAD) was ligated and cannulated to monitor peripheral coronary pressure (PCP). Myocardial blood flow and distribution were determined with radioactive microspheres before Renografin administration and after resolution of the brief period of hypotension following injection of 2-3.5 cm3 of the contrast agent directly into the coronary perfusion tubing. Renografin significantly increased blood flow to normal myocardial regions (0.90 to 1.51 cm3 . g-1 . min-1, p less than 0.001). Despite this flow augmentation and normal aortic and coronary perfusion pressures. PCP decreased from 28.0 +/- 3.4 to 24.4 +/- 2.8 mm Hg (p less than 0.001), and myocardial blood flow to the central ischemic LAD area halved from 0.17 +/- 0.04 to 0.09 +/- 0.03 cm3 . g-1 . min-1 (p less than 0.025). The inner/outer left ventricular wall blood flow ratio in the central ischemic region increased from 0.28 to 0.39 (p less than 0.05). Thus the contrast agent causes a coronary steal or redistribution of coronary flow away from already ischemic regions, and the endocardium appears to be less affected than the epicardium.

Acute effect of intravenous dipyridamole on regional coronary hemodynamics and metabolism

The acute coronary hemodynamic and metabolic effects of intravenous dipyridamole were studied in 13 patients. Total left ventricular (LV), anterior (supplied by the left anterior descending coronary artery) and inferior (supplied by circumflex and right coronary arteries) regional flows and metabolic responses were assessed from the coronary sinus and great cardiac vein. Perfusion to LV regions was classified as potentially "normal" or "abnormal," based on coronary angiographic findings. Before dipyridamole, coronary flow, LV oxygen delivery and lactate extraction in both the normal and abnormal regions were similar. Within 1 minute after injection of 20 mg of dipyridamole by i.v. bolus, total coronary flow increased 51% (p less than 0.05). Fifteen minutes after injection the flow increase persisted. Flow decreased to approximately control level by 20 minutes. The major component of this increased total coronary flow resulted from increased flow in normal regions (75% at 1 minute, p less than 0.05). Mean regional LV oxygen delivery and lactate extraction were not changed significantly in either normal or abnormal regions. However, lactate production occurred more often after dipyridamole in abnormal regions. These results suggest that during dipyridamole-induced hyperemia, regional coronary flow and metabolic responses depend upon the status of the arteries supplying the LV region. Regional differences in flow and metabolism occur independent of major changes in heart rate and aortic and LV pressures.

Dobutamine infusion in conscious dogs with and without acute myocardial infarction. Effects on systemic hemodynamics, myocardial blood flow, and infarct size

We infused dobutamine (20 microgram/kg per min) intravenously, once before and once after coronary artery occlusion, in 10 chronically instrumented dogs. Both infusions increased cardiac output and left ventricular dP/dt and dP/dt/P, but divergent effects on heart rate and aortic blood pressure were observed. Dobutamine decreased heart rate and increased mean aortic blood pressure before coronary artery occlusion, whereas after occlusion it increased heart rate while mean aortic blood pressure remained unchanged. A greater decrease in total peripheral vascular resistance occurred during dobutamine infusion after coronary artery occlusion than before. These differences may relate to withdrawal of enhanced sympathetic tone after coronary occlusion. Similar infusions of normal saline (n = 9) produced no systemic hemodynamic changes either before or after coronary artery occlusion. Myocardial blood flow increased to both non-ischemic and ischemic regions of the heart during dobutamine infusion, but the endocardial:epicardial blood flow ratio did not change significantly. In addition, infarct size, measured by nitroblue tetrazolium stain, was smaller in the dobutamine group (10 +/- 1 g) than in the normal saline group (15 +/- 2 g). Neither left ventricular weight nor risk zone differed between the two groups. These results indicate that dobutamine may be a useful inotropic agent during acute myocardial infarction.

Effect of intracoronary glyceryl trinitrate on perfusion distribution in the collateralised human myocardium

The effect on myocardial perfusion distribution of intracoronary glyceryl trinitrate in a dose (60 micrograms) insufficient to cause alterations in systemic blood pressure or heart rate was studied in eight patients with angiographically demonstrated collaterals from the left coronary system to the distal right coronary artery. Double isotope imaging using technetium-99m and iodine-131 labelled albumin macroaggregates allowed each patient to serve as his own control. The reproducibility of the imaging and data handling techniques was shown in 12 control patients. Glyceryl trinitrate caused a significant diminution in the collateral-mediated fractional perfusion while increasing that of the native coronary bed.

Coronary steal-induced increase in myocardial infarct size after pharmacologic coronary vasodilation

This study was performed to determine if maximal coronary arterial vasodilation of nonischemic areas would produce an increase in myocardial infarct size through a "steal" of collateral flow from an ischemic region. Myocardial infarction was produced by a 2 hour occlusion and reperfusion of the distal left anterior descending coronary artery in anesthetized dogs. Five minutes after occlusion, 7 dogs were given saline solution, and in 12 dogs the coronary vasodilator chromonar (8 mg/kg, intravenously) was administered. Chromonar produced a significant increase (p less than 0.05) in blood flow to nonischemic regions and a concomitant decrease in flow to ischemic areas. Associated with these changes in flow was an elevation in total release and peak plasma creatine kinase compared with values in saline-treated control dogs. Myocardial infarct size determined with nitroblue tetrazolium stanining was significantly increased (p less than 0.05). These demonstarte that maximal coronary vasodilation of nonischemic areas can result in an extension of myocardial infarction by a steal of collateral flow away from the ischemic region.

The dependence of coronary collateral blood flow on regional vascular resistances. Pharmacological studies with glyceryl trinitrate, adenosine and verapamil

A method was applied in anesthetized dogs enabling the measurement of regional resistances up to and behind the start of collaterals and the collateral resistance. The studies show that peripheral coronary pressure, i.e. perfusion pressure of the collaterals, can change when the ratio of pre- and post-collateral resistance alters. Drugs can influence collateral blood flow not only by directly effecting the collaterals but also by altering collateral perfusion pressure. Glyceryl trinitrate given in minor doses improved collateral blood flow by directly dilating the collaterals and also by increasing collateral perfusion pressure. Higher doses did not improve collateral flow due to a decrease of collateral perfusion pressure. A steal-phenomenon occurred in some cases. Adenosine and verapamil had no direct influence on the collateral resistance. Verapamil given in small doses increased perfusion pressure slightly but not enough to improve collateral blood flow. High doses of verapamil, like low doses of adenosine, had no significant influence on collateral perfusion pressure and collateral blood flow. Adenosine given in high dosage led to a diminution of collateral flow by decreasing collateral perfusion pressure, i.e. a steal-phenomenon.

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.

Pharmacological alterations of coronary collateral circulation; implication to the steal-phenomenon

Measurements of resistances were performed on different parts of coronary vessels, including spontaneous collaterals in anesthetized dogs, following ligation and embolization of the descending branch of the left coronary artery. The normal situation was compared with the state of maximal pharmacological dilatation. The pharmacological dilatation decreased the collateral resistance by 24% in spite of the fact that the collaterals supplied an infarcted area in this case; the precollateral resistance, which chiefly consists of large vessels was also diminished. The collateral perfusion pressure is not only dependent on the aortic pressure, but also on the flow rate in the corresponding large coronary vessel and thereby on the ratio of precollateral to postcollateal resistance. A decrease of the collateral perfusion pressure causes an insufficient perfusion of the area supplied by the collaterals only when the pressure fall is not compensated for by an adequate decrease in the collateral resistance and when the ratio of the collateral to the nutritive resistance increases. Thus, both, the extent of the pressure fall across the precollateral resistance and the dilating capacity of the collaterals determine whether is not a pharmacological dilatation of the coronary vascular bed results in an insufficient supply of an infarcted area, i.e. in a so called "Steal-phenomenon". The results show further, that the given physical model of the microcirculation satisfactorily approximates the observed behaviour of the vascular system.