Heart conduction system

Heart Conduction system Slower rhythm and conduction (muscarinic) Faster rhythm and conduction (Pp largely due to circulating epinephrine)... 

Fig. 3. (a) Coronary arteries which form the heart s own blood supply (b) electrical conduction system which powers the human heart. 

Arrhythmias. The first solution to cardiovascular problems arising from arrhythmias came about as a result of a complication caused by open-heart surgery. During procedures to correct congenital defects in children s hearts, the electrical conduction system often became impaired, and until it healed, the heart could not contract sufficiently without outside electrical stimulation. A system that plugged into a wall outlet was considered adequate until an electrical storm knocked out power, lea ding to the development of the first battery-powered external pacemaker. 

The conduction of an impulse can be slowed or stopped at any point along the conduction system of the heart. Slowing of conduction is called first-degree block, block of some impulses is called second-degree block, and a block of all impulses is called third-degree block. 

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. 

The sinus rhythm is the heart rhythm in which the sinus node generates an electrical impulse that travels through specialized cells (that form a conduction system) and leads to a ventricular contraction. 

The cardiotonics affect the transmission of electrical impulses along the pathway of the conduction system of tiie heart. The conduction system of die heart is a group of specialized nerve fibers consisting of die SA node, die AV node, the bundle of His, and die branches of Purkinje (Fig. 39-2). Each heartbeat (or contraction of tiie ventricles) is tiie result of an electrical impulse tiiat normally starts in tiie SA node, is tiien received by die AV node, and travels down die bundle of His and through tiie Purkinje fibers (see Fig. 39-2). The heartbeat can be felt as a pulse at the wrist and otiier areas of die body where an artery is close to the surface or lies near a bone When the electrical impulse reaches the... 

Figure 39-2. The conducting system of the heart. Impulses originating in the node are transmitted through the atria to the AV node down the bundle of His and the bundle branches through the Flirkinje fi bers to the ventrides.

Cardiovascular Effects. Eesions in the heart and blood vessels have been reported in humans acutely intoxicated with methyl parathion (Wofatox) (Fazekas 1971) and are discussed in Section 3.2.2.2. However, many of these lesions may be secondary to the effects of methyl parathion on the conduction system of the heart, to other components ingested, or to therapeutic regimens that some of these patients received. 

Renal Effects. Acute nephrosis has been reported in humans after acute, lethal intoxication (Fazekas 1971) by methyl parathion (Wofatox). This may be a secondary effect of hypoxia related to the neurologic effects of methyl parathion on vascular smooth muscle and on the electrical conduction system of the heart. It could also be related to therapeutic efforts. 

The mechanical activity of the heart (contraction of the atria and ventricles) occurs as a result of the electrical activity of the heart. The heart possesses an intrinsic electrical conduction system (Fig. 6-1). Normal myocardial contraction cannot occur without proper and normal function of the heart s electrical conduction system. Electrical depolarization of the atria results in atrial contraction, and ventricular depolarization is... 

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.)... 

Describe the components of the specialized electrical conduction system of the heart... 

The specialized excitation and electrical conduction system in the heart consists of ... 

From the AV node, the electrical impulse spreads through the AV bundle or the bundle of His. This portion of the conduction system penetrates the fibrous tissue separating the atria from the ventricles and enters the interventricular septum where it divides into the left and right bundle branches. The bundle branches travel down the septum toward the apex of the heart and then reverse direction, traveling back toward the atria along the outer ventricle walls. This route of conduction of the impulse facilitates ejection of blood from the ventricles. If the impulse were to be conducted directly from the atria to the ventricles, the ventricular contraction would begin at the top of the chambers and proceed downward toward the apex. This would trap the blood at the bottom of the chambers. Instead, the wave of ventricular electrical stimulation and, therefore, contraction moves from the apex of the heart toward the top of the chambers where the semilunar valves are located and ejection takes place. 

Similar to the TCAs, there have been reports of sudden deaths reported in children with ADHD who were taking both clonidine and a psychostimulant. For this reason, precautionary measures should be taken before starting a patient (particularly a child) on clonidine. A baseline EKG should be obtained before starting the medication. If the EKG shows any problems in the heart s electrical conduction system, then clonidine should not be used. In addition, if the patient has any history of known heart problems, fainting spells, slow heart rate (i.e., less than 60 beats per minute), or low blood pressure, we would recommend avoiding clonidine. When clonidine is used, the patient s blood pressure and pulse rate should be measured at each office visit, and an EKG should be periodically repeated. 

The rhythm of heart contractions depends on many parameters condition of pacemaker cells and the conduction system, myocardial blood flow, and other factors consequently, arrhythmia can originate for different reasons that are caused by disruptions in electrical impulse generation or conduction. They can be caused by heart disease, myocardial ischemia, electrolytic and acid-base changes, heart innervation problems, intoxication of the organism, and so on. 

Drugs used for treating arrhythmia can have an effect on the electrical conduction system of the heart, its excitability, automatism, the size of the effective refractory period, and adrenergic and cholinergic heart innervation. Accordingly, compounds of various chemical classes can restore heart rate disturbances. 

Classifying antiarrhythmic drugs is based on different principles for example, the location of the drug action. They can be substances that act directly on the myocardium and the conduction system of the heart itself, or substances that have an effect on the efferent... 

Drugs of this group are calcium channel blockers that inhibit slow transmembrane calcium ion flow in the cell of the conductive system of the heart during depolarization, which causes a slowing of atrioventricular conductivity and increased effective refractive period of atrioventricular ganglia, which eventually leads to the relaxation of smooth muscle of heart musculature and restores normal sinus rhythm during supraventricular tachycardias. 

Conduction System Disease in Ischemic Heart Disease... 

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. 

Historically and romantically, the heartbeat is recognized as the quintessential hallmark of life. Normally, the heart beats at 60-100 beats per minute (bpm), with each beat yielding a ventricular contraction that ejects blood out to the body. Each heartbeat is an electrical event that originates from a collection of electrically excitable cells within the heart called the sinoatrial node (SA), anatomically located at the upper pole of the heart. The sinoatrial node is the primary pacemaker of the heart. The electrical impulse generated in the sinoatrial node spreads rapidly downward from the atria chambers of the heart and reaches the atrioventricular node (AV), a collection of electrically excitable cells that constitutes the electrical interface between the atria and ventricles of the heart. Erom the AV node, the impulse propagates throughout the ventricles via an electrical conduction system referred to as the His-Purkinje system. The electrical transmission... 

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. 

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