A species comparison of the effects of changing perfusion pressure on blood flow and metabolic heat production in the myocardium

Reduction of ST elevation in repeated coronary occlusion model depends on both altered metabolic response and conduction property

The aim of this study was to elucidate the mechanisms of altered electrical response to ischemia in repeated coronary occlusion model. To test its dependence on metabolic response, extracellular K+ concentration (eKC), myocardial pH and PCO2 were simultaneously measured with epicardial ECG during three consecutive 4 min of left anterior descending coronary artery (LAD) occlusion separated by 15 min of reperfusion in canine hearts. ECG changes induced by infusion of high K+-buffer (10 mM) into the coronary arterial bed via carotid artery-LAD bypass (referred to as high K+-challenges: HKC) were also tested prior to (the first HKC), and during each reperfusion period (the second to the fourth HKC). ST elevation was significantly reduced in subsequent occlusions (3.14 +/- 0.48 and 2.98 +/- 0.47 mV in the second and third occlusion, both P<0.05, compared to 4.91 +/- 0.78 mV in the first). This was accompanied by significant attenuation of the changes in eKC, tissue pH and PCO2. ST elevation induced by HKC also significantly reduced after repeated occlusion (4.09 +/- 0.79 mV in the fourth HKC vs. 5.64 +/- 0.68 mV in the first, P<0.05) in spite of the identical changes in eKC during HKC. This progressive decrease in ST changes by HKC was rather consistent with augmented conduction delay (86.4 +/- 7.1% increase in activation time in the fourth vs. 54.3 +/- 3.4% in the first, P<0.01). These findings indicate that repeated ischemia induces altered electrical response to subsequent ischemia based on both attenuated metabolic response and altered conduction property.

Relationship between epicardial and intramyocardial ST-segment voltage and myocardial blood flow during graded coronary constriction in the dog

This study was undertaken to examine the relationship between electrographic ST-elevation and regional myocardial blood flow during graded coronary constriction. Electrograms from the epicardial surface, outer and inner layers of the myocardium were recorded. Regional blood flow to the outer and inner layers of the myocardium was measured by means of a heat-clearance method. With the application of a coronary constriction, myocardial blood flow to the inner layer began to decrease with a 75% coronary constriction, while flow to the outer layer was maintained at near normal up to least an 80% coronary constriction. Non-linear squares curve of between ST-elevation and the decrease in myocardial flow resulted in y = 18.80 exp-0.06x - 0.02 in the outer layer and y = 9.22 exp-0.02x - 1.54 in the inner layer. Also the non-linear squares curve of between epicardial ST-elevation and the decrease in myocardial flow resulted in y = 50.91 exp-0.11 x + 0.04 in the inner layer and y = 10.29 exp-0.07 x + 0.03 in the outer layer. The standard deviations of the coefficients of the latter two equations were higher than those of the former two equations. The higher values indicated larger variations of the parameter coefficients and data points. These results clearly demonstrated that a regional intramyocardial electrogram more closely reflects local ischemia than does an epicardial electrogram.

The effects of distension of the small intestine on myocardial blood flow in anaesthetised cats: possible relevance to coronary vasospasm

The haemodynamic effects of distending the small intestine (with a balloon in the lumen) were examined in cats anaesthetised with chloralose. Particular attention was paid to blood flow changes in localised areas of the left ventricular wall (as assessed using the heated thermocouple technique). Intestinal distension led to an increase in systemic blood pressure but usually to a reduction in myocardial blood flow; no cardiac dysrhythmias were observed. When the effect of increased systemic (perfusion) pressure on blood flow was eliminated (using partial correlation coefficients) flow then bore a negative relationship to intestinal pressure, probably indicating constriction of the myocardial blood vessles. This may indicate that distension of hollow organs can lead to a visceral-cardiac reflex. The resulting coronary vasospasm might be one cause of pain in certain patients with angina pectoris.

Myocardial and circulatory effects of E. coli endotoxin; modification of responses to catecholamines

1. The predominant acute effect of E. coli endotoxin in anaesthetized, ventilated cats was pulmonary hypertension resulting from a 8-12 fold increase in pulmonary vascular resistance. This was followed by decreases in left ventricular (LV) and systemic arterial pressures and in LV dP/dt max. Recovery occurred within 2-4 min and was dependent upon increased sympathetic drive; recovery did not occur in cats treated with the beta-adrenoceptor blocking drug alprenolol.2. The pulmonary vasoconstriction was reduced in cats given compound 48/80 and evidence is presented that it results primarily from histamine release.3. Over the 2-3 h period following endotoxin injection, systemic arterial pressure tended to decrease and heart rate and myocardial metabolic heat production to increase. Myocardial blood flow and LV dP/dt remained fairly stable until the terminal stages of shock.4. The predominant delayed effect of E. coli endotoxin in cats were a markedly reduced stroke volume, an increase in peripheral vascular resistance and a severe metabolic acidosis (arterial base excess-20 mEq/litre). Arterial pO(2) and pCO(2) were not significantly affected. It is concluded that myocardial contractility is maintained at this time through the release of catecholamines and that endotoxin itself depresses contractility.5. The effects of adrenaline and noradrenaline infusions on systolic and diastolic blood pressures, heart rate, cardiac output, myocardial blood flow and LV dP/dt max were markedly reduced in the period 2-3 h after endotoxin. In a few animals some recovery of the response to noradrenaline occurred and was associated with a general circulatory improvement and a reduced metabolic acidosis.

Haemodynamic and coronary effects of quazodine in cats with developing myocardial infarcts

Acute ligation of the descending branch of the left coronary artery in anaesthetized cats resulted, within 1–2 h, in a 30% decrease in local blood flow in the region mainly supplied by the ligated vessel, a fall in systemic blood pressure, in cardiac output, and in left ventricular dP/dt max (LVdP/dt). There was electrocardiographic evidence of myocardial ischaemia (pronounced ST elevation). In these animals with developing myocardial infarcts, intravenous infusions of quazodine (MJ1988; 6,7-dimethoxy-4-ethyl-quinazoline) markedly increased myocardial contractility and local myocardial blood flow in the developing infarct, and decreased systemic arterial pressure, peripheral vascular resistance and left ventricular end-diastolic pressure, effects similar to those observed in normal cats. The increase in cardiac contractility (cardiac output and LVdP/dt) occurred without a concomitant increase in myocardial metabolic heat production. This ‘oxygen sparing effect’ probably results from a decrease in left ventricular wall tension. It is suggested that quazodine warrants further investigation as a cardiac stimulant in power failure following myocardial infarction in man.

Cardiovascular pharmacology of quazodine (MJ-1988), with particular reference to effects of myocardial blood flow and metabolic heat production

1. The effects of intravenous infusions of quazodine (6,7-dimethoxy-4-ethylquinazoline; MJ-1988) on myocardial blood flow, myocardial metabolic heat production and on general haemodynamics have been studied in cats anaesthetized with sodium pentobarbitone.2. Quazodine (0.25 and 0.5 (mg/kg)/min for 10 min) decreased diastolic blood pressure, peripheral vascular resistance, systolic ejection time and left ventricular end-diastolic pressure. Heart rate, cardiac effort, output and external work and left ventricular dP/dt were markedly increased. These changes are indicative of increased myocardial contractility and peripheral vasodilatation.3. In a dose of (1.0 mg/kg)/min, quazodine had a more marked hypotensive effect, systolic pressure being significantly reduced, and had less effect on left ventricular dP/dt and cardiac effort. Calculated external cardiac work was slightly reduced and there were very occasional nodal arrhythmias.4. Changes in heart rate, aortic dP/dt and diastolic blood pressure induced by quazodine were unaffected by the previous administration of the beta-adrenoceptor blocking agent alprenolol in a dose (1.0 mg/kg) which abolished the effects of isoprenaline.5. In all doses, quazodine markedly increased local blood flow (by 70-540%) around an implanted myocardial heated thermocouple recorder. ;Corrected temperature', an index of local myocardial metabolic heat production, was almost unchanged and it is suggested that increased myocardial contractility, occurring with unchanged metabolic heat production and oxygen consumption, probably results from a concomitant decrease in intramural wall tension.

Myocardial and haemodynamic effects of phentolamine

1. In cats anaesthetized with pentobarbitone, intravenous infusions of phentolamine ((10-50 mug/kg)/min for 5 min) increased heart rate, left ventricular dp/dt max (without increasing end-diastolic pressure), aortic dp/dt, cardiac output, myocardial blood flow and metabolic heat production.2. Phentolamine-induced increases in myocardial contractility occurred irrespective of the direction or magnitude of the blood pressure change and were maintained well beyond the actual infusion period.3. In cats treated with alprenolol, bretylium or reserpine there was no evidence of increased cardiac contractility following phentolamine administration.4. It is concluded that phentolamine, in doses less than those required to produce significant alpha-adrenoceptor blockade, increased myocardial contractility through an effect on the sympathetic nervous system.

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.

The effect of "selective" beta-adrenoceptor blocking drugs on the myocardial circulation

1. A comparison has been made of the effects of a relatively specific beta(1)-adrenoceptor blocking drug (practolol) and a relatively specific beta(2)-adrenoceptor blocking drug (butoxamine) on myocardial and general haemodynamics in anaesthetized cats.2. Practolol, in a dose (10 mg/kg, intravenously) which had little effect on arterial pressure, heart rate, myocardial blood flow or myocardial vascular resistance, markedly reduced the effects of isoprenaline infusions on heart rate, aortic dp/dt, myocardial blood flow, vascular resistance and metabolic heat production, and the cardiac effort index. Isoprenaline induced vasodepression was unaffected.3. Butoxamine (5 mg/kg, intravenously) decreased heart rate, aortic dp/dt, the cardiac effort index and myocardial blood flow and increased myocardial vascular resistance. This is taken as further evidence for the existence of beta(2)-adrenoceptors in the myocardial microcirculation.4. After butoxamine, the effects of isoprenaline on myocardial blood flow, myocardial vascular resistance and heart rate were unaffected but the peripheral vasodilator effect was abolished. The effects on aortic dp/dt and the cardiac effort index were potentiated.5. It is concluded that the effect of isoprenaline in increasing myocardial blood flow is due predominantly to increased cardiac work and oxygen consumption and that practolol, since it has little direct effect on myocardial blood flow yet abolishes the cardiac stimulant and oxygen wasting effects of released catecholamines, has properties which indicate that it should be an effective and safe anti-anginal drug.

The effect of catecholamine infusions on myocardial blood flow, metabolic heat production and on general haemodynamics, before and after alprenolol (H56-28), in anaesthetized cats

1. In cats anaesthetized with pentobarbitone sodium, infusions of adrenaline, noradrenaline (0.5 mug/kg per min) and isoprenaline (0.25 mug/kg per min) increased myocardial blood flow, myocardial heat production, left ventricular systolic and end-diastolic pressures, left ventricular +ve and -ve dp/dt max, and calculated cardiac output, effort and oxygen consumption. These effects (apart from the effect of noradrenaline on left ventricular systolic pressure) were markedly reduced by previous administration of alprenolol (0.5 or 1.0 mg/kg).2. Infusions of adrenaline and noradrenaline increased arterial diastolic blood pressure and isoprenaline reduced it. After alprenolol the effects of adrenaline and noradrenaline were potentiated and that of isoprenaline abolished; in some experiments isoprenaline increased arterial diastolic pressure after alprenolol. Alprenolol did not influence the increases in arterial systolic pressure which followed the administration of adrenaline and noradrenaline.3. Isoprenaline-induced tachycardia was markedly reduced and adrenaline tachycardia was converted to bradycardia after alprenolol. The bradycardia which occurred during noradrenaline infusions was unaffected.4. After blockade by alprenolol, recovery of the effects of isoprenaline on left ventricular dp/dt and on heart rate occurred more quickly than recovery of the effects on arterial diastolic pressure. This suggests that alprenolol has a greater affinity for beta(2)- than for beta(1)-adrenoceptors.5. Intravenous administration of acetylcholine decreased arterial blood pressure, left ventricular pressure and +ve and -ve dp/dt max. During recovery from these effects there was a marked increase in +ve dp/dt max. which was absent after the administration of alprenolol (0.5 mg/kg). Because this dose of alprenolol is thus able to block the effects of reflex sympathetic cardiac nerve stimulation but does not completely antagonize the effects of exogenous adrenaline on dp/dt, it is suggested that alprenolol may have some adrenergic neurone blocking activity.6. Increases in liver and myocardial blood flow and heat production produced by noradrenaline, adrenaline and isoprenaline were reduced after alprenolol.7. Isoprenaline reduced air-way resistance and this effect was abolished by alprenolol; increases in air-way resistance produced by adrenaline and nor-adrenaline were augmented. All three amines inhibited intestinal smooth muscle contractions in vivo. Only the effect of isoprenaline was reduced by alprenolol.

Myocardial and haemodynamic effects of the beta-adrenoceptor blocking drug alprenolol (H56/28) in anaesthetized cats

1. The effects of the beta-adrenoceptor blocking drug alprenolol (H56/28) on myocardial and general haemodynamics were studied in anaesthetized cats.2. Alprenolol (0.5 mg/kg and 1.0 mg/kg) reduced femoral systolic and diastolic pressures, heart rate and left ventricular systolic pressure. The rate of rise of the left ventricular pressure pulse (dp/dt) was reduced despite a significant elevation of left ventricular end-diastolic pressure. This is evidence for decreased myocardial contractility. On some occasions there was a transient initial increase in +ve dp/dt max. possibly indicative of moderate beta-adrenoceptor stimulant activity.3. Myocardial and liver blood flows were measured using a heated thermocouple technique. Alprenolol slightly decreased both myocardial and liver blood flows (mean of 17% and 15% respectively with a dose of 1.0 mg/kg). Myocardial and liver vascular resistances were only very slightly increased.4. Alprenolol had no direct effect on calculated myocardial and liver metabolic heat production.5. In doses up to 1.0 mg/kg alprenolol had no effect on airway resistance but occasionally decreased (in vivo) intestinal muscle movement.6. Since alprenolol (although reducing calculated myocardial oxygen consumption and the myocardial and metabolic heat stimulant effects of catecholamines) has no significant effect on myocardial vascular resistance, it is suggested that it would be a useful adjunct in the therapy of angina pectoris.

Studies on the mode of action of hexobendine, a prospective anti-anginal drug

1. In cats anaesthetized with pentobarbitone sodium, hexobendine (0.25 mg/kg) markedly increased myocardial blood flow (measured using a heat clearance technique) and usually decreased myocardial metabolic heat production, without influencing cardiac contractility or systemic arterial blood pressure. These effects lasted for about 45 min.2. Larger doses (0.5 and 1.0 mg/kg) decreased systemic arterial blood pressure and the rate of rise of the left ventricular pressure pulse (dp/dt), although left ventricular end-diastolic pressure was usually increased. This is indicative of a decrease in myocardial contractility.3. In a concentration of 5 x 10(-6) g/ml., hexobendine protected isolated rabbit atria against the decrease in contractility that follows the removal of oxygen.4. Hexobendine did not antagonize the systemic and myocardial effects of infusions of adrenaline and noradrenaline, nor (except in concentrations of 10(-5) g/ml.) the increase in contractility induced by these catecholamines on isolated rabbit atria.