Cardiac conduction system

Mechanisms of Cardiotoxicity Chemical compounds often affect the cardiac conducting system and thereby change cardiac rhythm and force of contraction. These effects are seen as alterations in the heart rate, conduction velocity of impulses within the heart, and contractivity. For example, alterations of pH and changes in ionic balance affect these cardiac functions. In principle, cardiac toxicity can be expressed in three different ways (1) pharmacological actions become amplified in an nonphysiological way (2) reactive metabolites of chemical compounds react covalently with vital macromolecules... 

Halogenated hydrocarbons depress cardiac contractility, decrease heart rate, and inhibit conductivity in the cardiac conducting system. The cardiac-toxicity of these compounds is related to the number of halogen atoms it increases first as the number of halogen atoms increases, but decreases after achieving the maximum toxicity when four halogen atoms are present. Some of these compounds, e.g., chloroform, carbon tetrachloride, and trichloroethylene, sensitize the heart to catecholamines (adrenaline and noradrenaline) and thus increase the risk of cardiac arrhythmia. 

Although blood pressure control follows Ohm s law and seems to be simple, it underlies a complex circuit of interrelated systems. Hence, numerous physiologic systems that have pleiotropic effects and interact in complex fashion have been found to modulate blood pressure. Because of their number and complexity it is beyond the scope of the current account to cover all mechanisms and feedback circuits involved in blood pressure control. Rather, an overview of the clinically most relevant ones is presented. These systems include the heart, the blood vessels, the extracellular volume, the kidneys, the nervous system, a variety of humoral factors, and molecular events at the cellular level. They are intertwined to maintain adequate tissue perfusion and nutrition. Normal blood pressure control can be related to cardiac output and the total peripheral resistance. The stroke volume and the heart rate determine cardiac output. Each cycle of cardiac contraction propels a bolus of about 70 ml blood into the systemic arterial system. As one example of the interaction of these multiple systems, the stroke volume is dependent in part on intravascular volume regulated by the kidneys as well as on myocardial contractility. The latter is, in turn, a complex function involving sympathetic and parasympathetic control of heart rate intrinsic activity of the cardiac conduction system complex membrane transport and cellular events requiring influx of calcium, which lead to myocardial fibre shortening and relaxation and affects the humoral substances (e.g., catecholamines) in stimulation heart rate and myocardial fibre tension. 

Sites of endothelin-receptor expression. ETA receptors are expressed in the smooth muscle cells of the vascular medial layer and the airways, in cardiac myocytes, lung parenchyma, bronchiolar epithelial cells and prostate epithelial cells. ETB receptors are expressed in endothelial cells, in bronchiolar smooth muscle cells, vascular smooth muscle cells of certain vessels (e.g. saphenous vein, internal mammary artety), in the renal proximal and distal tubule, the renal collecting duct and in the cells of the atrioventricular conducting system. 

FIGURE 6-1. The cardiac conduction system. AV, atrioventricular. (Reprinted with permission from Cummins RO, (ed.) ACLS Provider Manual. Dallas American Heart Association 2003 253.)... 

General definitions relating to action potentials are given in Section 9. This section deals specifically with action potentials within the cardiac pacemaker cells and conducting system. 

Parenteral therapy The dangers of parenteral use of quinidine are increased in the presence of AV block or absence of atrial activity. Administration is more hazardous in patients with extensive myocardial damage. Use of quinidine in digitalis-induced cardiac arrhythmia is extremely dangerous because the cardiac glycoside may already have caused serious impairment of intracardiac conduction system. Too rapid IV administration of as little as 200 mg may precipitate a fall of 40 to 50 mm Hg in arterial pressure. 

Conduction system abnormalities are common in chronic heart failure, occurring in 15-30% of the population with low left ventricular ejection fraction (LVEF) [1-3]. The prevalence in ischemic heart disease is roughly similar to that seen in other forms of dilated cardiomyopathy. Conduction system disease can occur both at the time of an acute myocardial infarction as well as slowly progressing in chronic ischemic heart disease. Intraventricular conduction delays are associated with a poor prognosis in heart failure, with up to a 70% increase in the risk of death, and are also more prevalent in patients with advanced symptoms [2,4]. In ischemic heart disease, all components of the conduction system are at risk of ischemic injury, from the sinoatrial node to the His-Pukinje system. These conduction system abnormalities have the potential to impair cardiac function by a number of mechanisms. Since conduction abnormalities impair cardiac function, it is logical that pacing therapies to correct or improve these conduction abnormalities may improve cardiac function. 

These are the agents which block the action of sympathetic nerve stimulation and circulating sympathomimetic amines on the beta adrenergic receptors. At the cellular level, they inhibit the activity of the membrane cAMP. The main effect is to reduce cardiac activity by diminishing (3 receptor stimulation in the heart. This decreases the rate and force of myocardial contraction of the heart, and decreases the rate of conduction of impulses through the conduction system. They are classified as in table 3.3.2. 

The term, arrhythmogenic substrate, became a matter of interest to many researchers. The arrhythmogenic substrate means the pathologic and anatomic preconditions for the initiation of tachyarrhythmias such as myocardial fibrosis, aneurysm, the border zone between normal and ischemic or infarcted tissue, scars, diffuse myocardial injury in cardiomyopathy or the chronic alterations induced by myocarditis, and furthermore, accessory pathways or variations in the specific cardiac conduction system. These anatomic or pathologic altera-... 

Exerts direct action on cardiac muscle and the specialized conduction system... 

Octreotide increases systemic vascular resistance, and bradycardia may be a baroreceptor-induced response. Octreotide also has direct effects on the heart, the main effects being reduced heart rate, reduced myocardial contractility, and slowing of the propagation velocity along the cardiac conduction system. 

Although cardiac arrhythmias involve the electrical conduction system of the heart including the sinus node, atrioventricular... 

Figure 4.11. Generation of spontaneous action potentials in the cardiac conduction system. Depolarization starts as a slowly ascending prepotential that is due to the Caj. channel. Once the corresponding threshold is reached, the Ca channel opens, and the action potential is triggered. It is terminated by inactivation of the Ca chaimels, and by the opening of Ky chaimels (which have the same role here as in the skeletal muscle and nerves).

Special caution is recommended when acetylcholinesterase inhibitors are given to patients with inflammatory, infiltrative, or degenerative disease of the cardiac conducting system, patients taking digitalis, calcium channel blockers, or beta-blockers, and patients with myocardial ischemia. Appropriate resuscitative equipment should be readily available. 

Carbamazepine can depress the cardiac conducting system (SEDA-16, 71). There have been a few reports of reversible atrioventricular block (SED-12,130) (2-4), and asystole has been described in a patient with GuiUam-Barre syndrome (SEDA-18, 62). 

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