The effect of thyroid state on monophasic action potential in human heart

Right atrial monophasic action potential in healthy males. Studies during spontaneous sinus rhythm and atrial pacing

Right atrial monophasic action potentials (MAP) have been recorded in 40 healthy males. At least one MAP recording was performed in each individual during spontaneous sinus rhythm. Right atrial pacing and simultaneous MAP recording were performed in 25 of the 40 individuals. The paced cycle length was successively decreased in steps of 100 msec between 800 and 400 msec. The durations of the MAP at 50 and 90% repolarization and the relative repolarization rate during phase 3 (RRR ph 3) were calculated and normal values for the variables are given. A correlation was demonstrated between the duration of the MAP and the cycle length-the longer the cycle length, the longer the duration of the MAP-both during sinus rhythm and during the atrial pacing. A significant correlation was also shown between the RRR ph 3 and the duration of the MAP at 90% repolarization-the shorter the MAP, the faster the RRR ph 3. One age group (45-54 years) deviated significantly from the rest in some respects, but in general there was no age trend in the whole material.

Nature of cardiac arrhythmias and electrolyte disturbances. Role of potassium in atrial fibrillation

Atrial fibrillation is highly favoured by an accelerated atrial muscle repolarization. In hyperthyroidism, this occurs without changes in resting membrane potential level of the atrial muscle cells and without changes in the ionic composition of the cell. In patients prone to atrial fibrillation but without signs of hyperthyroidism, accelerated atrial repolarization can also be demonstrated. In such patients, a decreased atrial muscle potassium content seems responsible for the electrophysiological changes that highly favours the occurrence of atrial fibrillation.

Effects of hyperthyroidism on delayed rectifier K+ currents in left and right murine atria

Hyperthyroidism has been associated with atrial fibrillation (AF); however, hyperthyroidism-induced ion channel changes that may predispose to AF have not been fully elucidated. To understand the electrophysiological changes that occur in left and right atria with hyperthyroidism, the patch-clamp technique was used to compare action potential duration (APD) and whole cell currents in myocytes from left and right atria from both control and hyperthyroid mice. Additionally, RNase protection assays and immunoblotting were performed to evaluate the mRNA and protein expression levels of K(+) channel alpha-subunits in left and right atria. The results showed that 1) in control mice, the APD was shorter and the ultra-rapid delayed rectifier K(+) conductance (I(Kur)) and the sustained delayed rectifier K(+) conductance (I(ss)) were larger in the left than in the right atrium; also, mRNA and protein expression levels of Kv1.5 and Kv2.1 were higher in the left atrium; 2) in hyperthyroid mice, the APD was shortened and I(Kur) and I(ss) were increased in both left and right atrial myocytes, and the protein expression levels of Kv1.5 and Kv2.1 were increased significantly in both atria; and 3) the influence of hyperthyroidism on APD and delayed rectifier K(+) currents was more prominent in right than in left atrium, which minimized the interatrial APD difference. In conclusion, hyperthyroidism resulted in more significant APD shortening and greater delayed rectifier K(+) current increases in the right vs. the left atrium, which can contribute to the propensity for atrial arrhythmia in hyperthyroid heart.

Amiodarone and thyroid function

Amiodarone blocks the action of thyroid hormone by the inhibition of 5'-deiodinase which reduces production of T3 in peripheral tissues and possibly by blocking nuclear binding of T3. Since the drug inhibits peripheral conversion of T4 to T3, many patients taking amiodarone have abnormal thyroid function studies (increased T4 and rT3; decreased T3) despite being euthyroid. Treatment of patients with amiodarone generates an iodine excess, which contributes greatly to the significant incidence of altered thyroid status in this population. The diagnosis of hyperthyroidism and hypothyroidism can be difficult. However, using the overall clinical picture and the tolerance limits of hormone levels determined for patients remaining euthyroid on amiodarone therapy, the accurate diagnosis of clinically significant thyroid dysfunction can almost always be made. To screen for thyroid disease, thyroid function should be assessed before initiating therapy, semiannually during therapy or whenever clinical features of thyroid dysfunction occur. Subclinical hypothyroidism as denoted by modest increases in TSH levels do not require treatment or the discontinuation of amiodarone therapy. An appreciation of the mechanism of the interaction between amiodarone and thyroid hormone metabolism allows the clinician to recognize thyroid dysfunction at an early stage and initiate appropriate therapy, thereby minimizing the morbidity associated with forms of amiodarone toxicity.

Amiodarone, thyroid hormone indexes, and altered thyroid function: long-term serial effects in patients with cardiac arrhythmias

Amiodarone, a drug that has electrophysiologic actions resembling those of hypothyroidism, increases serum levels of T4 and reverse T3 (rT3) and decreases T3. The drug's long-term effects on thyroid function are poorly defined. Serial thyroid hormone indexes in 76 patients given amiodarone for 6 to 32 months (mean +/- standard deviation 16 +/- 7) for arrhythmias were determined serially. Over this period, 68 patients (89%) remained euthyroid; hypothyroidism developed in 6 (8%) and hyperthyroidism developed in 2 (3%). In patients who remained euthyroid, thyroid hormone alterations attained steady-state values at 3 months: T4 increased 42% (p less than 0.01), rT3 increased 172% (p less than 0.01) and T3 decreased 16% (p less than 0.05), without significant effect on thyroid stimulating hormone. For the euthyroid patients, the 90% tolerance limits (95% confidence) over the follow-up period for T4 was 5 to 19 micrograms/dl (normal 4 to 12), for T3 36 to 163 ng/dl (normal 60 to 160), for rT3 22 to 131 ng/dl (normal 15 to 50) and for thyroid stimulating hormone 0 to 14 microU/ml (normal 1 to 6). The changes in hormone indexes in hyperthyroid or hypothyroid patients were unrelated to the cumulative dose or duration of drug therapy. The most reliable diagnostic indexes for amiodarone-induced altered thyroid state were: thyroid stimulating hormone level over 20 microU for hypothyroidism and T4 over 20 ng/dl or high T3 over 200 ng/dl for hyperthyroidism. All levels were within the 90% tolerance limits derived for these hormones from patients remaining euthyroid on amiodarone long-term.(ABSTRACT TRUNCATED AT 250 WORDS)

A classification of antiarrhythmic actions reassessed after a decade of new drugs

The past decade has seen the introduction of many new class 1 drugs, restricting fast inward current. Confirmative evidence has been obtained that the antiarrthymic action of lidocaine and diphenylhydantoin is indeed due to their effect as class 1 agents depressing conduction. The original class 3 drug, amiodarone, is increasingly in use as an antiarrhythmic of first choice for WPW and for arrhythmias associated with hypertrophic myopathy, and as a reserve drug in resistant arrhythmias of other types. Other compounds delaying repolarization have proved to be clinically effective as antiarrhythmics. In addition to their class 2 antiarrhythymic action exhibited acutely, on long-term treatment beta blockers have a class 3 action, which might be, at least in part, responsible for the protection of postinfarction patients against sudden death. Recent research suggests that inhibition of slow inward current may lead, as a secondary consequence of lowered [Ca]i, to improved cell-to-cell conduction. Finally, all but one of the new antiarrhythmic drugs, none of which existed in 1972, have turned out to possess one or more of the four classes of action originally described. This can hardly be a coincidence. The single exception, alinidine, a selective bradycardic agent, may restrict anionic currents, which would constitute a fifth class of action, but this is far from proved.

Right atrial monophasic action potential in patients with paroxysmal supraventricular tachyarrhythmias

Thirteen patients with paroxysmal atrial tachyaarhthmias (six atrial supraventricular tachycardia, six atrial fibrillation, and one atrial flutter) have been investigated by recording of monophasic action potentials (MAP) during a period free from arrhythmias. None of the patients differed from healthy individuals with respect to any of the MAP variables. Occasional recordings showed values not found in healthy individuals, such as prolonged duration of the MAP or slow phase 4 depolarization. The atrial effective refractory period (AERP) was determined in two patients. One AERP was short compared with values found in healthy individuals. Thus some MAP recordings gave a possible explanation for the mechanism behind the arrhythmias, while other recordings showed no differences from results in healthy individuals.

Monophasic action potential of the right atrium in paroxysmal atrial flutter and fibrillation

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