Final answer:
Hypercalcemia is the electrolyte imbalance that causes a shortened PR interval. It impacts HR and contractility, and high calcium levels are associated with various ECG changes, including potentially a short QT interval and a PR interval. Electrolyte balance is essential for maintaining normal heart rhythm.
Step-by-step explanation:
The electrolyte imbalance causing a shortened PR interval includes hypercalcemia. High levels of calcium ions, or hypercalcemia, have significant effects on the heart rate (HR) and contractility. An increase in calcium ion levels is associated with an increase in HR and contractility. Specifically, hypercalcemia can lead to a short QT interval on an electrocardiogram (ECG) and, though not commonly discussed, can also cause a shortened PR interval. Conversely, other electrolyte imbalances such as hypokalemia can result in heart arrhythmias, while hyperkalemia leads to a weakened and flaccid heart which may ultimately fail.
Maintaining proper electrolyte balance is crucial for the normal electrical activity of the heart, including the sequence of depolarization and repolarization that produces the standard rhythm. Drugs that influence calcium channels, such as calcium channel blockers, can slow down the HR by interfering with the movement of calcium ions, affecting the heart's normal functioning and showing changes in ECG readings.