Synchronized cycles of contraction and relaxation allow the heart to efficiently pump blood and deliver oxygen throughout the body. These cycles are governed by the fluctuation of positively charged molecules in and out of each cell. Potassium is a major contributor to the regulation of the cardiac cycle. Disruptions in the availability of potassium can have detrimental effects on the heart's ability to function rhythmically.
Cardiac Cycle
In the text "Cardiovascular Physiology," Drs. Mohrman and Heller describe a single cardiac cycle as a period of muscular contraction followed by a period of relaxation. This sequence is repeated approximately 60 to 80 times per minute in a healthy, resting adult, and accounts for his resting heart rate. More than 100, or fewer than 60, cardiac cycles occurring in one minute in a resting adult is considered an arrhythmia--an abnormal rhythm. Disruption in the quantity of molecules that initiate the periods of contraction and relaxation contributes significantly to the presence of arrhythmias.
Potassium
The cardiac cycle is regulated primarily by fluctuations of potassium, sodium and calcium into and out of each cardiac cell. These molecules are electrically charged particles that create an imbalance between the electrical charge inside and outside of each cell. At rest, the outside of the cardiac cell is more positively charged than the inside. The exchange of sodium and potassium, via the sodium-potassium pump, allows the cell to maintain this resting state. For a contraction to occur, more sodium and potassium will infiltrate the cell, causing an equilibrium in charges between the inside and the outside of the cell. This process is called depolarization and is considered the start of a cardiac muscle contraction, or heartbeat. When the contraction has ended, the sodium-potassium pump will expel sodium and potassium back out of the cell to re-establish the relaxed, resting state.
High Potassium Levels
Hyperkalemia is a condition in which the blood contains high levels of potassium. According to the Mayo Clinic, too much potassium in the blood can be dangerous. Hyperkalemia interrupts the normal function of the heart by creating an abnormal electrical equilibrium between the inside and outside of the heart's cells. This causes the heart to depolarize, or fire, when it should be resting, and prevents a return to the resting state. Ultimately this could lead to sudden cardiac arrest by preventing the heart from contracting and relaxing normally. Kidney failure is often the cause of hyperkalemia; however, certain medications for the treatment of heart disease and Type I diabetes have also been linked to hyperkalemia.
Low Potassium Levels
Similar to kyperkalemia, a very low blood level of potassium is also a medical emergency. A low level of potassium in the blood is referred to as hypokalemia, and is generally the result of excessive potassium loss through urine. The resulting imbalance between sodium and potassium alters the ability of the cardiac cells to create the necessary fluctuations for each phase of the cardiac cycle. "Heart Physiology: From Cell to Circulation" explains that low potassium depresses the heart's ability to contract at regular intervals and slows the cardiac cycle to life-threateningly low rates. Like hyperkalemia, hypokalemia can be caused by kidney failure and other instigators such as severe diarrhea, vomiting and diuretic medications.
Treatment
Treatment for hypo- and hyperkalemia involves re-establishing the normal electrical imbalance between the inside and outside of the cell. This is achieved either by administering high levels of potassium or by providing intravenous medications to enhance the cellular uptake of potassium from the blood, respectively. Because cardiac arrhythmias are caused by both high and low potassium levels, cardiac monitoring should always accompany treatment of these conditions.
References
- "Cardiovascular Physiology"; David E. Morhman, Ph.D.; 2003
- Mayo Clinic: Hyperkalemia
- Mayo Clinic: Hypokalemia
- "The Heart: From Cell to Circulation"; Lionel H. Opie, M.D.; 1998
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