Effect Of Electrolytes On Cardiac Rhythm Notes

Effect Of Changes In Electrolyte Concentration On Heart Introduction

The distribution of electrolytes in extracellular fluid and intracellular fluid is responsible for the electrical activity of the tissues including myocardium. Thus, any change in the concentration of any of the electrolytes will definitely alter the electrical activity of the cardiac muscle.

Effect Of Changes In Sodium Ion Concentration

• Normal sodium concentration in blood is 135-145 mEq/ L. The change in the concentration of sodium does not alter the electrical activity of the heart severely.
• Only the low level of sodium in the body fluids reduces the electrical activity of cardiac muscle and the ECG shows low voltage waves.
• The changes in the concentration of potassium and calcium ions have significant effects on the heart.

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Effect Of Changes In Potassium Ion Concentration

The normal potassium concentration in the blood is about 3.5-5 mEq/L. The changes in ECG are developed when the potassium level increases to 6 mEq/L or when it decreases to 2 mEq/L.

Effect Of Hyperkalemia: Hyperkalemia is the increase in potassium ion concentration in the blood.

Hyperkalemia decreases:

1. Resting membrane potential leads to hyperpolarization
2. Excitability of the muscle.

The effects of hyperkalemia on the excitability of cardiac muscle depend upon the severity of hyperkalemia:

• When the potassium ion concentration increases slightly to 6 or 7 mEq/L., in ECG, the T wave is tall and tented. P-R interval and QRS complex are normal
• With a further increase in potassium level to 8 mEq/L, the P-R interval and the duration of the QRS complex are prolonged.
• It is because hyperkalemia decreases the rate of conduction. P wave may be small.
• Severe hyperkalemia beyond 9 mEq/L makes the atrial muscle unexcitable. So, the P wave is absent in ECG.
• The QRS complex merges with the T wave. This condition is fatal because it leads to ventricular fibrillation or stoppage of the heart in diastole due to the lack of excitability.

Effect Of Hypokalemia

Hypokalemia is the decrease in potassium ion concentration in the blood. It decreases the sensitivity of the heart muscle.

When the potassium level falls to 2 mEq/L, changes in ECG are:

1. The S-T segment is depressed
2. T wave is small, flat, or inverted
3. U wave appears. Sometimes, the U wave merges with T wave. Because of this, the Q-T interval is mistaken for being prolonged.

Further decrease in potassium level, i.e. below 2 mEq/L, causes:

1. Depression of the S-T segment below the isoelectric baseline.
2. Inversion of the T wave
3. The appearance of prominent U wave
4. Prolongation of P-R.

Effect Of Changes In Calcium Ion Concentration

• The normal concentration of calcium in the blood is 9-11 mg/dL (4.5-5.5 mEq/L).
• Mostly, hypocalcemia affects the heart rather than hypercalcemia.

Effects Of Hypercalcemia

Hypercalcemia is an elevated calcium level. It increases the excitability and contractility of the heart muscle. In clinical conditions, the effect of hypercalcemia is very rare.

If it occurs, it causes changes in ECG:

1. Shortening the duration of the S-T segment
2. Shortening of QT interval
3. The appearance of U wave appears

Calcium Rigor

• In animals, under experimental conditions, the heart stops in systole when a large quantity of calcium ions is infused.
• The stoppage of the heart in systole is called the calcium rigor. The calcium rigor is a reversible phenomenon and the heart starts functioning normally when the calcium ions are washed.

Effects Of Hypocalcemia

Hypocalcemia is the reduction in blood calcium levels. It reduces the excitability of the cardiac muscle. So, the changes in ECG are:

1. Prolongation of S-T segment
2. Prolongation of Q-T interval
3. The appearance of a prominent U wave.

Experimental Evidence

The effects of ions on the heart are studied by perfusing the brain of animals such as frogs and rabbits.