Heart rhythms

Lethal Arrhythmias. Arrhythmias are a second significant source of cardiovascular problems. An arrhythmia is an abnormal or irregular heart rhythm. Bradyarrhythmias result in heart rates that are too slow tachyarrhythmias cause abnormally fast rates. A bradyarrhythmia can be debiUtating, causing a person to be short of breath, unable to climb stairs, black out, or even to go into cardiac arrest. Tachyarrhythmias can be un settling and painful at best, life-threatening at worst. 

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. 

Cardiac-changes in pulse rate or rhythm electrocardiographic changes, such as bradycardia, tachycardia, premature ventricular contractions, bigeminal (two beats followed by a pause), or trigeminal (three beats followed by a pause) pulse. Other arrhythmias (abnormal heart rhythms)also may be seen. 

The major drawback of these models, however, is their lack of a clear reference between model components and constituent parts of the biological system (e.g. structures like ion channels, transporter proteins, receptors, etc.). These models, therefore, do not permit the simulation of patho-physiological detail, such as the series of events that follows a reduction in oxygen supply to the cardiac muscle and, ultimately, causes serious disturbances in heart rhythm. 

Clearly, cardiac function may not be addressed exclusively on the basis of describing the working mechanisms of single cells. Both normal and disturbed heart rhythms are based on a spreading wave of electrical excitation, the meaningful investigation of which requires conduction pathways of at least hundreds if not thousands of cells in length. 

The same applies to pathologically-disturbed function. A simulated reduction in coronary blood flow (heart attack) would lead to reduced oxygen supply to the cells in the virtual heart, which would reduce efficiency of cardiac contraction and possibly give rise to heart rhythm disturbances. Ventricular pressure development would be compromised, as would the blood supply to all organs of the body, including the heart. All these implications can be studied in a virtual heart. 

Blood chemistry tests are performed with particular attention given to potassium and magnesium, which may affect heart rhythm. 

Skeletal muscle is neurogenic and requires stimulation from the somatic nervous system to initiate contraction. Because no electrical communication takes place between these cells, each muscle fiber is innervated by a branch of an alpha motor neuron. Cardiac muscle, however, is myogenic, or self-excitatory this muscle spontaneously depolarizes to threshold and generates action potentials without external stimulation. The region of the heart with the fastest rate of inherent depolarization initiates the heart beat and determines the heart rhythm. In normal hearts, this "pacemaker region is the sinoatrial node. 

Judd-Hunter color difference scale, 7 321 Juglone, in skin coloring products, 7 847 Juglone derivatives, 21 264-265 Juice softening, 23 463 Junctional heart rhythm, 5 107 Junction capacitance, 22 244 Junction devices, 22 180-181 Junction FETs (JFETs), 22 163, 164. See also Field effect transistors (FETs) physics of, 22 241-245, 249 Junction potentials, 9 582 Junctions, stacking, 23 38-39. See also Josephson junctions p-n junction Just-in-Time technique, 21 172 Jute, 11 287, 288, 292, 293. See also China jute... 

Zipes, D.P. (2007) Heart rate-corrected QT interval in men increases during winter months. Heart Rhythm, 4, 277-281. 

Shah, R.R. and Hondeghem, L.M. (2005) Refining detection of drug-induced proarrhythmia QT interval and TRIaD. Heart Rhythm, 2, 758-772. 

Cisapride monohydrate Propulsid Janssen Pharmaceuticals Heartburn 7/29/1993 7/14/2000 7 years Heart rhythm abnormalities US 2.5 billion... 

Asternizole Hismanal Janssen Pharmaceuticals Antihistamine 12/19/1988 5/18/1999 6 months Heart rhythm abnormalities 23 million... 

Grepafloxacin Raxar Glaxo Wellcome Antibiotic 11/6/1997 11/1/1999 2 years Heart rhythm abnormalities 23.5 million... 

Terfenadine Seldane, Seldane-D Hoechst Marion Roussel Antihistamine 5/8/1985 2/27/1998 2.75 years Heart rhythm abnormalities Not available... 

TCAs can be lethal in overdose and can cause dangerous heart rhythm abnormalities in susceptible patients. Please refer to Chapter 3 for more complete discussion of the TCAs. 

After checking a baseline EKG to rule out undetected heart rhythm abnormalities, many clinicians use a low dose of imipramine or protriptyline to treat the auxiliary symptoms of narcolepsy. Either of these can be started at 10 mg taken once a day and then slowly increased over several weeks as needed until the symptoms... 

The electrical impulse for contraction (propagated action potential p. 136) originates in pacemaker cells of the sinoatrial node and spreads through the atria, atrioventricular (AV) node, and adjoining parts of the His-Purkinje fiber system to the ventricles (A). Irregularities of heart rhythm can interfere dangerously with cardiac pumping func-tioa... 

Aluminum Hydroxide + Magnesium Hydroxide (Maalox) [OTC] [Antacid/Aluminum Magnesium Salts] Uses Hyper-acidity (peptic ulcer, hiatal hernia, etc) Action Neutralizes gastric acid Dose Adults. 10-20 mL or 2-4 tabs PO qid or PRN Feds. 5-15 mL PO qid or PRN Caution [C, ] Disp Tabs, susp SE May cause t Mg in renal insuff, constipation, D Interactions In addition to AlOH, X effects OF digoxin, quinolones, phenytoin, Fe supl, ketoconazole EMS None OD May cause constipation, loss of appetite, painful urination, heart rhythm changes, muscle weakness, and peripheral edema symptomatic and supportive... 

As a result of the weight of evidence for ICD therapy in prevention of SCD in patients with ischemic cardiomyopathy, the American College of Cardiology, American Heart Association, and the North American Society for Pacing and Electrophysiology (now Heart Rhythm Society) revised guidelines for ICD implantation in October 2002... 

Gold MR, Auricchio A, Hummel JD, et al. Comparison of stimulation sites within left ventricular veins on the acute hemodynamic effects of cardiac resynchronization therapy. Heart Rhythm 2005 2 376-81. 

Metabolic acidosis, a potentially fatal condition, is characterized by nausea, vomiting, abdominal pain, lethargy, elevated heart rate, and irregular heart rhythm. 

A patient with heart failure developed a serious abnormal heart rhythm, ventricular tachycardia, and it was decided to treat this with lig-nocaine (also known as lidocaine) by the intravenous route. He was given a loading dose designed to raise the plasma concentration to an effective 2 mg/1, and then given a constant-rate intravenous infusion aimed at maintaining that concentration. In about an hour he was observed to be tremulous and then had a brief generalized convulsion (a fit). The plasma lig-nocaine concentration was found to be 8 mg/1 (desired therapeutic range 1 - no more than 5 mg/1). 

A) Abnormal heart rhythms alcohol induces cytochrome P450 isozymes that convert ibuprofen to a cardiotoxic free radical metabolite... 

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