In this example, volume expansion is indeed appropriate using a solution that is isotonic to the patient. Suppose one wishes to add KCL to the initial bolus fluid and provide 1L of isontoic fluid. If one wishes to add 40 meq to the first 1L bolus, then we would look to column 4 (Tonicity with 40 meq added to 0.5NS (mmol/L), and select a value that closely matches the patient's sodium of 120 mmol/L. In this case, 121 mmol/L would suffice and according to the table, one would proportion the bolus 10% NS and 90% 1/2NS plus 40 meq/L KCL, running at a total rate that you wish to administer. The order might appear as:
Infuse 100CC of NS and 900CC of 1/2 NS plus 40 meq/L over 1 hour
Note, you can run the two IV solutions simultaneously through 2 IVs or using a y-connector, an IV pump and a single IV site. Assuming the potassium normalizes, sodium remains 120 mmol/L but she requires ongoing IV boluses or maintenance, then one can simply choose a solution that matches the patients tonicity without containing potassium. For example, a 60:40 split of NS:1/2NS would provide a tonicity (123 mmol/L) closely matching that of the patient.