arrhythmia – n. 心律不齐
automaticity – n. 自动节律性
conductivity – n. 传导性
action potential – 动作电位
conduction pathway – 传导通道
cardiac conduction – 心脏传导
impulse – n. 冲动
sinoatrial – a. 窦房的
sinoatrial (SA) node – 窦房结
depolarization – n. 去极化，除极
sodium gates – 钠离子通道，钠门
electrical potential – 电势，电位
repolarization – n. 再极化，复极
resting potential – 静息电位
rhythmicity – n. 节律
atrioventricular – a. 房室的
atrioventricular (AV) node – 房室结
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Arrhythmias 心律不齐
Arrhythmias involve changes to the automaticity or conductivity of the heart cells. These changes can result from several factors, including electrolyte imbalances that alter the action potential, decreased oxygen delivery to cells that changes their action potential, structural damage that changes the conduction pathway, or acidosis or waste product accumulation that alters the action potential. In some cases, changes to the heart’s automaticity or conductivity may result from drugs that alter the action potential or cardiac conduction.
With normal heart function, each cycle of cardiac contraction and relaxation is controlled by impulses arising spontaneously in the sinoatrial (SA) node and transmitted via a specialized conducting system to activate all parts of the heart muscle almost simultaneously. These continuous, rhythmic contractions are controlled by the heart itself. This property allows the heart to beat as long as it has enough nutrients and oxygen to survive, regardless of the status of the rest of the body.
All cardiac cells possess some degree of automaticity in which the cells undergo a spontaneous depolarization during diastole or rest because they decrease the flow of potassium ions out of the cell and probably leak sodium into the cell, causing an action potential.
所有的心脏细胞都具有一定程度的自动节律性，由于这一自动节律性，细胞可以在舒张或静息期间自动除极，因为他们可以减少细胞钾离子的流出，并可能使钠进入细胞，引起动作电位。
The action potential of the cardiac muscle cell consists of five phases: 心肌细胞动作电位分5期：
l Phase 0 occurs when the cell reaches a point of stimulation. The sodium gates open along the cell membrane, and sodium rushes into the cell; this positive flow of electrons into the cell results in an electrical potential. This is called depolarization as the membrane no longer has a charge difference between the inside and outside of the membrane.
l Phase 1 is a very short period during which the sodium ion concentration equalizes inside and outside the cell.
期：极短；期间，细胞内和细胞外钠离子浓度相等。
l Phase 2, or the plateau stage, occurs as the cell membrane becomes less permeable to sodium, calcium slowly enters the cell, and potassium begins to leave the cell. The cell membrane is trying to return to its resting state, a process called repolarization or returning of the charge differences to the membrane.
l Phase 3 is a time of rapid repolarization as the sodium gates are closed and potassium flows out of the cell.
期：快速复极时间，钠门关闭，钾流出细胞。
l Phase 4 occurs when the cell comes to rest; the sodium–potassium pump returns the membrane to its resting membrane potential, and spontaneous depolarization begins again.
期：细胞处于静息期；钠钾泵使膜恢复到静息膜电位，自动除极再次开始。
Each area of the heart has a slightly different-appearing action potential that reflects the complexity of the cells in that area. Because of these differences in the action potential, each area of the heart has a slightly different rate of rhythmicity. The SA node generates an impulse about 60 to 100 times per minute, the atrioventricular (AV) node about 40 to 50 times per minute, and the complex ventricular muscle cells about 10 to 20 times per minute.
1. True or False: Patients with left-sided diastolic dysfunction heart failure usually have a normal ejection fraction.
A. True B. False 2. Blurred vision or halos are signs of:
A. Subtherapeutic digoxin levels.
B. Digoxin toxicity.
C. Nothing related to digoxin.
D. Corneal side effects of digoxin.
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答案 Answer 1. A. True. Rationale: Patient with left-sided DIASTOLIC dysfunction heart failure normally have a normal ejection fraction. However, patients with left-sided SYSTOLIC dysfunction heart failure usually do not because the heart is unable to CONTRACT efficiently rather than fill properly as with diastolic dysfunction. 2. B. Digoxin toxicity. Rationale: Halos is a hallmark sign of digoxin toxicity. Digoxin exhibits its therapeutic and toxic effects by poisoning the sodium-potassium ATPase. The subsequent increase in intracellular sodium leads to increased intracellular calcium by decreasing calcium expulsion through the sodium-calcium, cation exchanger. A, C and D are incorrect because subtherapeutic digoxin levels have no such effects.
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