The heart provides blood to the lungs to collect oxygen to expel carbon dioxide, then pumps the blood out to the organs to supply them with oxygen and other nutrients. The organs need a continuous but variable supply of blood. The local blood vessels in and around an organ are able to compensate to some degree to optimize the flow of blood through the organ, for example by increasing or decreasing the diameter of arteries and thereby increasing or decreasing the flow. However, if the heart is beating outside a range of acceptable values--too high, too low or too uncoordinated--then the organs will not be able to efficiently compensate.
Bradycardia
According the Mayo Clinic, bradycardia means a low heart rate. It can be caused by problems such as heart blocks. The heart contracts by signaling from an electrical system; a heart block obstructs or slows the electrical signaling. Other causes include damage to the heart from aging or heart attacks, infections, heart surgery, low thyroid hormone levels, imbalances of electrolytes such as potassium in the blood and certain medications. Symptoms may include shortness of breath, chest pain, dizziness or fainting and fatigue. Treatments may involve treating the underlying condition such as changing medications or insertion of a pacemaker, an electrical device that tells the heart when to contract.
Tachycardia
Tachycardia means a high heart rate, according the Merck Manuals. If the rate is too high, each beat will not pump out much blood, and the heart may be working dangerously hard. Causes can stem from problems with the heart's natural pacemaker, damage to the heart muscle from infection or heart attacks, high thyroid levels, stimulants such as caffeine or other drugs, electrolyte imbalances or inherited disorders. Symptoms may be similar to those of bradycardia. Palpitations may occur. Shortness of breath and chest pain may also be felt. Treatment may be with drugs such as beta blockers or calcium channel blockers, or even defibrillation if the heart rate is severely high. Defibrillation is the application of an electrical impulse, or shocking the heart, to get it to reset to a more normal rhythm.
Other Arrhythmias
According the National Institutes of Health, sick sinus syndrome, or tachy brady syndrome, is a problem with the heart's primary intrinsic pacemaker, called the sinoatrial node, or SA node, that causes the heart to intermittently beat too quickly and too slowly. It is relatively uncommon. Symptoms are usually not noticed, but if present are similar to those of bradycardia and tachycardia. Causes include those for tachy and bradycardias as well. Treatment may be with radiofrequency ablation, which is when a physician specializing in cardiac electrophysiology performs a procedure to eliminate electrical centers in the heart that are not conducting properly. Alternatively or in addition, a pacemaker may need to be placed.
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