Halothane depression of myocardial slow action potentials

Effects of halothane on membrane currents associated with contraction in single myocytes isolated from guinea-pig ventricle

1. The effects of halothane on electrical activity and contraction were investigated in single myocytes isolated from guinea-pig ventricle. 2. Halothane depressed the plateau and shortened the duration of action potentials. 3. Halothane also reduced the amplitude of inward calcium currents and of additional inward current activated by cytosolic calcium under voltage-clamp conditions. 4. Contractions (measured by an optical technique) accompanying either action potentials or calcium currents were reduced by halothane. However, the extent of attenuation of contraction was greater than when a similar level of calcium channel blockade was induced by application of verapamil. 5. Actions of halothane on calcium-activated tail currents in double-pulse experiments were consistent with reduction by halothane of the cytosolic calcium transient, perhaps as a consequence of reduced uptake of calcium into sarcoplasmic reticulum stores. 6. It is concluded that the actions of halothane on inward currents contribute to its effects on action potentials. The reduction in contraction caused by halothane may result partly from a reduced influx of calcium to trigger contraction, and partly by a reduced release of calcium from sarcoplasmic reticulum stores.

Effects of halothane on membrane potentials and membrane ionic currents in single bullfrog atrial cells

Halothane exerts negative inotropic and negative chronotropic actions on the isolated heart in experimental animals. In order to assess directly the actions of halothane in myocardium, we studied the effects of halothane on membrane potentials and transmembrane ionic currents in single isolated frog atrial cells obtained by the enzymatic dissociation method. The results show: (a) that the action potential is prolonged and its plateau phase and overshoot are depressed, but the resting potential remains unchanged; (b) that there is a significant inhibition of a time- and voltage-dependent outward K+ current and a slow inward Ca2+ current, with a slight decrease of a fast inward Na+ current following halothane (1.0-4.0%) application; and that halothane has no effect on another K+ current, time-independent current.

Interaction between diltiazem and halothane or enflurane in the canine blood-perfused papillary muscle and sinoatrial node preparations cross-circulated by chronically instrumented conscious donor dog

Interaction of cardiovascular effects of diltiazem with those of halothane or enflurane was estimated in the canine isolated papillary muscle and sinoatrial node preparations perfused by arterial blood of the chronically instrumented conscious and halothane- or enflurane-anesthetized donor dog, into which diltiazem was infused i.v. at a rate of 20 micro g/kg/min for 60 min. One hour after diltiazem infusion, in the conscious donor dog, mean arterial pressure (MAP) and heart rate (DHR) were decreased to 84 +/- 3 and 84 +/- 2% and PQ interval (PQ) was prolonged to 148 +/- 5%, while in the isolated preparations, developed tension (DT) of the papillary muscle and sinoatrial rate (SAR) were decreased to 68 +/- 3 and 74 +/- 3% and blood flow (BF) was increased to 155 +/- 5% (n = 10). On the other hand, halothane (0.8%) anesthesia per se decreased MAP, DHR, DT and SAR to 89 +/- 8, 84 +/- 3, 79 +/- 3 and 89 +/- 5% (n = 7) of each basal value in conscious state 20 min after the inhalation. During halothane anesthesia, the same dose of diltiazem infused decreased MAP to 74 +/- 4 (n = 7), DHR to 66 +/- 4 (n = 6), DT to 62 +/- 7 (n = 7) and SAR to 69 +/- 1% (n = 3) of each value suppressed by halothane itself. Meanwhile, enflurane (1.7%) anesthesia itself decreased MAP, DHR, DT and SAR to 81 +/- 3, 85 +/- 2, 81 +/- 2 and 88 +/- 2% (n = 10) of each basal value in conscious state 30 min after enflurane inhalation. During enflurane anesthesia diltiazem decreased MAP to 74 +/- 3 (n = 10), DHR to 67 +/- 3 (n = 8), DT to 45 +/- 5 (n = 10) and SAR to 74 +/- 6% (n = 3) of each value under enflurane anesthesia alone. PQ interval of the donor dog heart was prolonged by halothane alone to 111 +/- 5% (n = 7) and by enflurane alone to 110 +/- 2% (n = 10) of the value before each anesthesia, and then diltiazem prolonged PQ interval to 160 +/- 8% (n = 6) and 174 +/- 10% (n = 8) of each value suppressed by the anesthetic itself during halothane- or enflurane-anesthesia, respectively. The second degree AV conduction block was induced in 1 of 7 halothane- and in 2 of 10 enflurane-anesthetized donor dogs, respectively. The sinus arrest was induced by diltiazem in 4 of 7 sinoatrial node preparations under halothane and in 7 of 10 ones during enflurane anesthesia. Moreover, plasma concentration of diltiazem 60 min after the start of infusion was 556 +/- 121 ng/ml in conscious dogs and tended to increase to 752 +/- 101 ng/ml in enflurane anesthetized donor dogs (n = 4), but there was no significant difference between two values (0.05 < P < 0.1). These results indicate that effects of diltiazem could be potentiated during halothane or enflurane anesthesia by elimination of compensatory reflex noted in conscious state, and that the negative inotropic effect of diltiazem was enhanced by enflurane anesthesia due to unknown mechanisms which probably include a slight but insignificant increase in plasma concentration.

[Calcium inhibitors and anesthesia]

Calcium blockers (CB) are routinely used. This could lead to possible interference with anaesthetic drugs. CB prevent calcium from entering the cell by inhibiting the slow voltage-dependent calcium channels. They act mostly on heart and smooth muscle. Of all the possible indications, the three that are confirmed are coronary heart disease, arterial hypertension and supraventricular rhythm disturbances. Most of the work published and the cases reported concerns interactions between CB and halogenated anaesthetic agents; the latter's actions on the heart depend on cellular calcium exchange. Also, the cardiovascular effects of these anaesthetics are similar to that of CB. Experimentally, halothane and enflurane have direct cardiac inhibitory effects similar to verapamil and diltiazem, whereas isoflurane's properties seem closer to the dihydropyridines (nifedipine and nicardipine). Giving verapamil or diltiazem increases the number of sino-atrial and atrio-ventricular blocks when using a halogenated agent. Clinically, interpreting the effects of CB during anaesthetic induction is difficult because of the pathology (coronary heart disease, cardiac failure), the other drugs (beta-blockers and nitrates) and the type of anaesthesia (emergency or elective). Interactions can give rise to anything from a severe cardiovascular collapse, requiring catecholamines, to a mild fall in blood pressure which responds well to plasma expansion, or even no effect on blood pressure. Rebound is seen on stopping CB in patients with coronary heart disease or arterial hypertension; stopping them before surgery does not therefore seem justified. However, extreme care must be taken when using halogenated agents for patients under treatment with CB and/or beta-blockers. A wary anaesthetist will be able to adapt the technique to the patient. It has been suggested that CB could be used to treat preoperatively myocardial ischaemia (diltiazem), hypertensive crises (nifedipine, nicardipine) and ventricular rhythm disturbances (verapamil); this must be done with caution, the patient being closely monitored (haemodynamic and electrocardiographic monitoring). Postoperatively, intranasal nifedipine, continuous intravenous nicardipine or diltiazem have been used to treat increases in arterial blood pressure during recovery and to adapt the cardiovascular system to the increased metabolic needs. Here again, close patient monitoring is essential. In any case, treatment with CB which has been stopped should be started up again as soon as possible.

[Prevention of per- and postoperative myocardial ischemia in non-cardiac surgery by intravenous diltiazem]

A controlled double-blind trial was carried out to assess the efficacy and safety of a continuous intravenous infusion of diltiazem in preventing perioperative myocardial ischaemia in patients with coronary artery disease. Sixty-six patients undergoing non cardiac surgical procedures (vascular surgery, n = 37; other, n = 29) were randomly chosen to receive either diltiazem (group D, n = 32); or placebo (group P, n = 34); there was no difference between these groups in the number of patients in each NYHA class (I: 13/16; II: 14/14; III: 5/4) or having had a previous myocardial infarct (20/22). ECG leads CM5 and CL5 were recorded continuously with an ICR 7200 Holter monitor. After starting recording, either placebo or a loading dose (0.5 mg.kg-1) of diltiazem was given, followed by an infusion of 5 micrograms.kg-1.min-1. Anaesthesia was induced by thiopentone and suxamethonium, and maintained with nitrous oxide (50%), fentanyl and either halothane or droperidol. The number of myocardial ischaemic episodes was significantly (p less than 0.05) lower in group D (2 ST depressions in two patients) than in group P (8 ST depressions in six patients, 2 myocardial infarcts and 1 pulmonary oedema). No conduction disturbance was observed; the lowest cardiac frequency was found in group P (32 b.min-1). Systolic and diastolic arterial blood pressures were lower in group D than in group P, but no difference was found in heart rate and rate-pressure product.(ABSTRACT TRUNCATED AT 250 WORDS)

Volatile anesthetics depress the depolarization-induced cytoplasmic calcium rise in PC 12 cells

In the rat pheochromocytoma cell line PC 12, the effects of four volatile anesthetics (halothane, isoflurane, enflurane, methoxyflurane) on the K+-evoked intracellular calcium [( Ca2+]i) rise were investigated using the Ca2+-sensitive fluorescence dye fura-2. The [Ca2+]i rise was depressed, at clinical concentrations, by all anesthetics with almost identical aqueous IC50 values. The study extends to neuronal cells the observation made previously in cardiac tissue that volatile anesthetics may interfere with Ca2+ fluxes through voltage-gated channels.

Calcium reverses global and regional myocardial dysfunction caused by the combination of verapamil and halothane

In order to evaluate the effects of the combination of halothane and verapamil on left ventricular function and coronary blood flow (CBF), six sheep were anaesthetized with halothane (1.2% inspired) and given increasing cumulative doses of intravenous verapamil. Regional myocardial function was assessed by sonomicrometry in the areas supplied by the left anterior descending coronary artery (LAD) and the left circumflex coronary artery (LC). Changes in global haemodynamics, atrioventricular conduction, LV relaxation and systolic shortening after 0.32 mg X kg-1 intravenous verapamil indicated impaired left ventricular function. Significant myocardial dysfunction (post-systolic shortening) occurred in the LAD territory, accompanied by a 64% decrease (42 +/- 6 to 15 +/- 3, P less than 0.01) in coronary perfusion pressure (CPP). Coronary blood flow in the LC segment decreased 83% (102 +/- 15 to 17 +/- 13, P less than 0.01) as coronary reserve was exhausted with the decrease in CPP. Calcium chloride reversed the impairment of global and regional myocardial function observed with verapamil, improved the impaired left ventricular relaxation, but did not significantly alter atrioventricular conduction. Thus the combination halothane-verapamil can cause significant left ventricular depression and myocardial dysfunction, possibly by inducing subendocardial ischaemia or by direct pharmacologic effect. Calcium chloride reverses this regional myocardial dysfunction as well as the deleterious global haemodynamic changes caused by halothane-verapamil; however, the changes in atrioventricular conduction are not corrected by calcium.

Differential inotropic effects of halothane and isoflurane in dog ventricular tissues

The differential effects of halothane (0.25-0.75%) and isoflurane (0.5-1.25%) on the electromechanical activity of canine ventricular tissues were compared in vitro. In Purkinje fibres, halothane but not isoflurane could induce an initial increase of contractile force which was not blocked by diltiazem or propranolol. In ventricular muscles, halothane decreased the resting state contraction more markedly than isoflurane. The results suggest that halothane induces a greater negative inotropy than isoflurane through a differential alteration of intracellular Ca2+ stores.

Mechanisms of halothane action on synaptic transmission in motoneurons of the newborn rat spinal cord in vitro

Action of halothane on synaptic transmission was studied on the isolated newborn rat spinal cord. Clinical doses of halothane (less than or equal to 3%) suppressed mono- and polysynaptic reflexes, dorsal root reflexes, excitatory and inhibitory postsynaptic potentials as well as the spontaneous synaptic potentials caused by impulse bombardment. However, the spontaneous miniature inhibitory postsynaptic potentials observed after blocking impulse activities by tetrodotoxin were not all suppressed by halothane. During halothane administration, the membrane potential of motoneurons was hyperpolarized by several millivolts, associated with an increase in input conductance. However, the threshold potential level for spike generation was virtually unaffected. Depression of synaptic transmission in spinal motoneurons by halothane is suggested to be due to two factors: a reduction in the amount of transmitter release secondary to interference with Ca2+ entry into nerve terminals, either by partial blockade of impulse invasion or voltage-dependent Ca2+ channels; and an increase in the depolarizing current necessary for excitation of motoneurons owing to hyperpolarization and decreased input resistance.

Prolongation of the action potential and reduction of the delayed outward K+ current by halothane in single frog atrial cells

Halothane has been shown to produce significant changes in heart rate and strength of contraction. We have studied the mechanism(s) of these effects by recording action potentials and transmembrane ionic currents in single cardiac cells from bullfrog atrium. Our results show that the action potential was prolonged and its plateau was depressed; and that there was a significant inhibition of a potassium current, Ik, and a Ca2+ current, Isi, following halothane (2%) application.

[Prevention of hypertensive attacks after carotid surgery. The value of nifedipine and diltiazem]

A randomized study was carried out to determine whether postoperative administration of either intranasal nifedipine or intravenous diltiazem was effective in preventing hypertensive episodes after carotid endarterectomy. Sixty-three consecutive patients undergoing this surgery were randomly assigned to three groups (n = 21): control (C), nifedipine (N) and diltiazem (D). Anaesthesia was induced with flunitrazepam (0.02 mg X kg-1), fentanyl 6 micrograms X kg-1 and pancuronium (0.1 mg X kg-1), and maintained by N2O/O2 (50%), additional fentanyl and halothane or enflurane when warranted. Postoperatively, patients were warned whilst under mechanical ventilation in a recovery room. Upon arrival in the recovery room, intranasal nifedipine (10 mg) was given to patients included in group N, while patients from group D received an intravenous dose of 0.3 mg X kg-1 of diltiazem, followed by a continuous infusion of 3 micrograms X kg-1 X min-1 until 15 min after extubation. No preventive treatment was given to patients in group C. If postoperative systolic blood pressure rose to more than 180 mmHg, 10 mg of nifedipine were administered intranasally. During the postoperative period, 13 patients from group C, 5 from group N and 4 from group D exhibited hypertensive episodes (systolic blood pressure greater than 180 mmHg or diastolic blood pressure greater than 100 mmHg). Intranasal administration of 10 mg nifedipine led to the normalization of blood pressure in 20 out of these 22 patients. This study confirmed the high incidence of hypertensive attacks after carotid endarterectomy, and showed that prophylactic administration of nifedipine or intravenous diltiazem was highly effective in preventing such hypertensive episodes.

Lack of effects of d-tubocurarine and pancuronium on the slow action potential of the guinea pig papillary muscle

Inotropic effects of non-depolarizing muscle relaxants were examined with guinea pig ventricular papillary muscle depolarized to -47 mV in high K Ba-Tyrode solution. Field stimulation of 0.1 Hz elicited the slow action potential, a measure of the calcium current. The amplitude, the duration at 0 mV level and dV/dt of the action potential were monitored together with the contractile tension. Amelizol (3 mg X ml-1 d-tubocurarine (d-tc) and 5 mg X ml-1 chlorobutanol) depressed the four functions in a dose-dependent manner, while crystalline d-tc did not. Chlorobutanol (the antimicrobial preservative) had the same effects as Amelizol. Neither Mioblock (2 mg X ml-1 pancuronium and unpublished preservative) nor crystalline pancuronium altered the functions. These findings suggest that the negative inotropic effect of Amelizol is not due to d-tc but to chlorobutanol, which may exert its effect by depressing the calcium current. The lack of change in the slow action potential seen with pancuronium may indicate no direct effect on the calcium current, thereby further suggesting absent direct beta-adrenomimetic action of this agent.

Reduction of the slow inward current of isolated rat ventricular cells by thiamylal and halothane

The barbiturates and halothane exert a negative inotropic effect on the myocardium. A reduction in the slow inward current, carried mainly by calcium ions, is an important factor for the underlying mechanism because the calcium current during the action potential provides the calcium ions for accompanying contraction, supplies Ca ions to the sarcoplasmic reticulum for subsequent contractions, and induces Ca release from the store site. It has been suggested that reduction in the slow inward current caused by anesthetics is indicated by depression of the slow action potential of the partially depolarized myocardium. In order to assess directly the effect of anesthetics on the slow inward current, we carried out voltage clamp experiments with single isolated rat ventricular cells obtained by an enzymatic dissociation method. Thiamylal (10(-4) mol . l-1) and halothane (1%) decreased the slow inward current to 60 +/- 5% (mean +/- s.d., n = 8) and to 65 +/- 10% (mean +/- s.d., n = 8) of the control value, respectively, without changing the configuration of the current-voltage curve. The results provide further evidence for anesthetic reduction of the slow inward current of the myocardium, and suggest that the negative inotropic effect is at least partly due to the reduction in that current.

Calcium antagonists--drug interactions

Evaluations of drug interactions should be done with caution. One needs to be aware of the reported interactions and apply the information on an individual basis. This review may therefore serve as a guide to the more common drug interactions and when drug therapy should be monitored closely in clinical practice. Major drug interactions with calcium antagonists are summarized in Table 2.

[Hemodynamic interferences between diltiazem and thiopental--experimental study in the pig]

The clinical applications of calcium channel blockers are rapidly growing; as barbiturates interfere with intracellular calcium movements, the possibility of drug interaction at this level must be considered. We have studied the haemodynamic interaction of Thiopentone and Diltiazem in pigs. A loading dose of Diltiazem (0.15 mg X kg-1) was injected followed by a continuous administration of 0.07 mg X kg-1 X hour. From the fifth to the tenth minute, the reduced afterload was associated with a moderate depression of the contractility of the left ventricle. These values returned to the initial level within 30 minutes. Heart rate and cardiac output were not modified. The stability of these two parameters is thought to result from the complex interaction of both drugs on the sympathetic nervous system and the ventricular function.

Effects of prostaglandin E2, propranolol and nitroglycerine with halothane, pethidine or pentobarbitone anaesthesia on arrhythmias and other responses to ligation of a coronary artery in rats

The effects of various cardiovascular drugs (prostaglandin E2 (PGE2), propranolol and nitroglycerine) and anaesthetic regimens (halothane, pethidine and pentobarbitone), upon the outcome of coronary artery ligation in acutely prepared rats were determined. Effects upon arrhythmias, blood pressure, heart rate, mortality, ECG and the size of the occluded zone were determined for each drug in the presence of each anaesthetic. PGE2 and nitroglycerine had no statistically significant effects on the outcome of ligation whatever the anaesthetic. Propranolol had limited antiarrhythmic actions. The anaesthetic used had major effects upon the outcome of ligation, regardless of the cardiovascular drugs administered. Pentobarbitone anaesthesia resulted in the highest mortality, and most arrhythmias. Pethidine-N2O anaesthesia was associated with fewer arrhythmias. Halothane-N2O anaesthesia markedly decreased the incidence and severity of arrhythmias, independent of the cardiovascular drug. It was concluded that the anaesthetic used can have a major influence on ligation-induced arrhythmias in acutely prepared anaesthetized rats.

The calcium channel blockers: pharmacology and clinical applications

The calcium channel blockers provide an exciting and effective new therapeutic tool in the management of ischaemic cardiac syndromes and may prove popular and effective in the treatment of a variety of other disorders. They have provided a new approach to treatment and have added new insights into the pathogenesis of ischaemic cardiac syndromes. Their introduction into clinical practice has been swift and many of our concepts regarding their pharmacologic activities in man remain based on theoretic considerations. Their expanding clinical use and further comparative studies will undoubtedly provide further information in regard to indications, adverse effects, drug interaction and long-term safety. Particular caution is advised when they are combined with certain antiarrhythmic agents, digitalis and particularly beta adrenergic blocking agents. Little is known about their interaction with various general anaesthetic agents and for this reason particular vigilance is required as more patients receiving these agents are admitted for surgical procedures.