Hospital staff have stated that grandmother has low potassium levels. what would the effect be on someone in their 80's?

Arrhythmias: Hypokalemia=low potassium. It depends how low. Mild: usually nothing, rarely mild arrhythmias. Moderate(potassium 2.5-3 meq/l (normal 3.5-5.0)): arrhythmias; muscle weakness, pain, and cramps; constipation. With more severe hypokalemia: paralysis, reduced reflexes.

Depends on how low. : By the time your blood potassium is low the total body potassium is really depleted. Slightly low potassium on it's own should not cause any arrhythmias. In combination with certain medications heart rhythm abnormalities may occur. Very low levels of potassium can cause serious arrhythmias. You should have your pcp follow this closely.