Electrical activity of the heart

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

Ca++ channels start to become activated allowing Ca++ ion influx, which continues to depolarize the membrane toward threshold. 

Phase 0 begins when the membrane potential reaches threshold (-40 mV). Recall that the upstroke of the action potential in neurons is due to increased permeability of fast Na+ channels, resulting in a steep, rapid depolarization. 

However, in the SAnode, the action potential develops more slowly because the fast Na+ channels do not play a role. Whenever the membrane potential is less negative than -60 mV for more than a few milliseconds, these channels become inactivated. With a resting membrane potential of -55 mV, this is clearly the case in the SA node. Instead, when the membrane potential reaches threshold in this tissue, many slow Ca++ channels open, resulting in the depolarization phase of the action potential. The slope of this depolarization is less steep than that of neurons. 

Phase 3 begins at the peak of the action potential. At this point, the Ca++ channels close and K+ channels open. The resulting efflux of K+ ions causes the repolarization phase of the action potential. 

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One of the more intriguing cardiovascular developments is cardiomyoplasty where implantable technologies are blended with another part of the body to take over for a diseased heart. One company, Medtronic, in close collaboration with surgeons, has developed a cardiomyoplasty system to accompany a technique of wrapping back muscle around a diseased heart which can no longer adequately pump. A combination pacemaker and neurological device senses the electrical activity of the heart and correspondingly trains and stimulates the dorsal muscle to cause the defective heart to contract and pump blood. Over 50 implants have been performed to date. 

An arrhythmia may occur as a result of heart disease or from a disorder that affects cardiovascular function. Conditions such as emotional stress, hypoxia, and electrolyte imbalance also may trigger an arrhythmia An electrocardiogram (ECG) provides a record of the electrical activity of the heart. Careful interpretation of the ECG along with a thorough physical assessment is necessary to determine the cause and type of arrhythmia The goal of antiarrhythmic drug therapy is to restore normal cardiac function and to prevent life-threatening arrhythmias. 

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... 

The electrocardiogram (ECG) is a non-invasive means of measuring the electrical activity of the heart. The relationship... 

Electrocardiogram A noninvasive method of recording the electrical activity of the heart. 

Figure 13.4 Electrocardiogram. The electrocardiogram (ECG) is a measure of the overall electrical activity of the heart. The P wave is caused by atrial depolarization, the QRS complex is caused by ventricular depolarization, and the T wave is caused by ventricular repolarization.

Intraventricular conduction delay often leads to late activation of the left ventricular free wall with significant mechanical consequences. The mechanical consequences of abnormal electrical activation of the heart have long been recognized [58, 60, 86]. These include dyssynchrony between the atria. 

SeifterJ, Ratner A, Sloane D. The electrical activity of the heart. In Concepts in Medical Physiology. Philadelphia Lippincott, Williams and Wilkins 2005... 

The sinoatrial node (SA), consisting of spindle-shaped cells, initiates the electrical activity of the heart. From its location in the right atrium in proximity to the superior vena cava, the electrical activity spreads to the atria whose cells are larger than those of the SA. The pulse from the atria spreads to the atrioventricular node (AV), the gateway to the ventricles. The atria and the ventricles are electrically isolated. The AV node also slows down the electrical activity giving the atria time to fill. The bundle of His is the upper end of the electrical path, which through the Purkinje fibers allows the electrical signal to activate the ventricles and thus to pump the blood. 

Electrocardiographic Signature of OPs Disturbances in the electrical activity of the heart caused by xenobiotics are readily discernible in a surface electrocardiogram (Yurumez et al, 2008 Dalvi et al, 1986 Chuang et al, 1996 Chhabra et al, 1970). 

The electrical activity of the heart is modulated by hormones and neurotransmitters. Xenobiotics disturb their balance. The parasympathetic system releases ACh and the sympathetic system releases catecholamines (norepinephrine and epinephrine). These bind to a and p types of receptors. ai-receptors are present on the post-synaptic member of the organ and mediate vasoconstriction and stimulation of Na" /K -ATPase, the Na" /Ca exchanger, and the Na /H exchanger. This affects the Ikatp and inhibits the Ij,ia+ and Ito. The a-receptor stimulation thus effectuates depolarization, and the a2-receptor inhibits norepinephrine release. 

Before discussing the various arrhythmias and the drugs used to treat them, it is advisable briefly to review a normal sinus rhythm and some of the electrical activities of the heart. 

The electrocardiogram is a skin surface measurement of the electrical activity of the heart muscle (known as myocardium). With each heart beat ions move through the heart muscle and gradients of charge are formed. Taking a differential measurement of the electrical potential on the body surface at different locations generates different ECG vectors. These vectors are known as leads. The ECG leads are formed from three electrodes placed on the body, with an optional fourth electrode used as a reference (MIT, 2013). 

ECG records the electrical activity of the heart from the skin s surface. With each heartbeat, an electrical signal spreads from the top to the bottom of the heart. As it travels, the signal causes the heart to contract and pump blood. Analysis of the timing and morphology of the electrical signal can help to diagnose heart problems. ECG is... 

The practical interest in this problem lies primarily in the fact that the impulse behavior in a closed circuit is very similar to excitation circulation along the myocardium in the case of abnormal electric activity of the heart. 

There are also six precordial leads, V, to V , that track along the rib cage over the lower heart region that are used to give more localized ECG spatial information. This total of 12 leads are used clinically to allow the cardiologist to view the electric activity of the heart from a variety of electrical perspectives and to enable a diagnosis based on a more comprehensive data set. 

The difference amplifier shown in Figure 27.8 is a basic amplifier block that can be extended to measure very small signals differentially in the body. Such measurements are the basis behind common connected health signal-acquisition targets such as the electrical activity of the heart (EKG/ECG), those of the muscles (EMG), and that... 

Electrocardiogram (ECG) The visual display of the electrical activity of the heart as recorded from electrodes placed on the skin surface. 

The electrical activity of the heart is a complex process which includes cellular... 

As a result of the electrical activity of the heart, electrical potentials appear at all points of the body surface. These potentials reflect both the electrical sources within the heart, and the electrical properties of the torso volume conductor. The... 

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