Isotonic, Hypotonic, and Hypertonic IV Fluid Solution 2020
NCLEX™ Review

Fluid management is a core responsibility of nurses and physicians at every hospital throughout the United States. Patients may present with a variety of conditions that can alter the balance of free water and electrolytes in the body. For the body to function properly it is imperative for a specific volume in the fluid and in the cells to be maintained in addition to the concentration of solutes and electrolytes.

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The body manages these levels primarily through the kidneys which is charged with the task of excreting or retaining fluids and electrolytes. When the kidneys sense an imbalance, they can adjust for it and ideally correct for it to restore normal body functions. The kidneys also play a key role in the body by filtering the blood of solutes, electrolytes, and other molecules (e.g. glucose) that can be excreted or retained as needed.

It is important to understand some terminology when evaluating patients with a fluid imbalance. Conceptually it is important to understand that the balance of fluids and electrolytes mostly refers to the volume and concentration of solutes inside the cell referred to as intracellular fluid (ICF) and the volume and concentration outside the cell called extracellular fluid (ECF). Patients may have too much or too little water in either of these parts of the body. When practitioners are diagnosing the imbalance, they generally are looking at the levels of the ECF where their labs are assessed. If fluid levels in the ECF are high it is called hypervolemic and if the levels are low, it is called hypovolemic. If fluid levels are balanced it is called isovolumic.

The key electrolyte that lives within the ICS is potassium which is regulated by the sodium potassium pump. The sodium potassium pump is an important component of cell membrane which acts to drive sodium out of the cell and potassium in using energy in the form of ATP (active transport) to do so. The ECF describes fluids that exists in blood vessels, the skin, spinal cord fluid (SCF), synovial fluid, pleural fluid, and digestive secretions. The ECF contains 6 liters of water in the blood vessels, 11-12 liters in the interstitial fluids, and 1 liter in the transcellular spaces (between the cells).

The body may also lose water via sweating and breathing. Patients with diaphoresis or excessive sweating or those in heat stroke or with a fever may become dehydrated. Patients who are hyperventilating may also expire extra moisture from their lungs and outside of the body. Other routes where fluid levels may become imbalanced are via vomiting and diarrhea where extra water will get excreted from the body.

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Other key terms to be aware of are osmosis which describes the movement of fluid from lower solute concentrations to higher concentration. Osmolality describes the number of osmotically active particles per Kg of water in the body. The concentration of a solution in the body is measured as osmotic pressure as milliosmoles (mOsm). Patients who are hypertonic have a higher concentration of osmotically active particles (> 300 mOsm/L) – hypotonic is a low concentration of these particles (< 270 mOsm/L).

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Lastly, an important concept to understand is how much fluid the various tissues of the body can hold. Muscle contains the most water out of all tissues in the body while fatty tissue contains much less. As a result, patients with larger muscle mass will require extra fluid intake to maintain their fluid balance. Patients who are obese will contain extra water but not as much per kg of muscle. Additionally, this implies that men tend to retain more water than women since men generally have a higher muscle mass than their female counterparts.

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Causes of Fluid Imbalance

Dehydration:

Dehydration can be caused by several factors ultimately described as a greater loss of free water compared to intake or retention. Patients who are dehydrated may also have hypovolemia which more specifically refers to a decreased volume of circulating blood in the body. Dehydration can be caused by over excretion of fluids which can be due to diuretic use as either medication (e.g. loop diuretics, thiazides) or substances (e.g. alcohol, caffeine). Patients with diabetes are at an increased risk for dehydration in the event of high blood sugar which draws extra water into the blood – from the cells – to be excreted via urine. Other causes of dehydration include excessive sweating (diaphoresis) and severe burns as the skin will draw water to itself as part of the healing process.

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Overhydration:

Volume overload or fluid retention can be caused by a variety of conditions many of which can be very severe. Patients with kidney failure or heart failure may experience increased retention of water as a downstream consequence of their condition. Other potential causes may be increased electrolyte consumption via diet or from IV fluids. Other causes can be hormonal signals via the renin angiotensin aldosterone system which generally act to increase the retention of water to adjust for low blood pressure. This system can become dysfunctional in patients with cardiovascular disease which ultimately have serious consequences down the line.

Signs, Symptoms and Clinical Manifestations of Fluid Imbalance

Dehydration:

Signs and Symptoms

  • Flat neck and veins
  • Dry/flat skin
  • Increased skin turgor (slow, sluggish skin after getting pinched)
  • Weight loss
  • Difficulty breathing (dyspnea)
  • Orthostatic hypotension (sudden drop of blood pressure after changing position)

Clinical Manifestations

  • Low blood pressure
  • Increased heart rate (tachycardia)
  • Decreased central venous pressure
  • ECG abnormalities (due to potassium fluctuations)
  • Increased respiratory rate (tachypnea)
  • High lab values
    • High hemoglobin and hematocrit
    • High electrolyte panels
    • High serum osmolality
    • High BUN
    • Increased urine specific gravity

Overhydration:

Signs and Symptoms

  • Peripheral edema (especially in the limbs)
  • Pulmonary edema (fluid in lungs)
  • Crackling sounds in the lungs
  • Bounding pulses
  • Rapid weight gain

Clinical Manifestations

  • High blood pressure
  • ECG abnormalities
  • Jugular venous distension (JVD)
  • Increased central venous pressure
  • Heart damage
  • Lab values
    • Elevated BNP
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Treatment

Pharmacology and Nursing Interventions

Pharmacology:

Fluid replacement is the main intervention that can be implemented for patients with electrolyte imbalances. There are many different types of solutions that can be used depending on the concentration of electrolytes in the ECF. Understanding these measurements is critical to improve the patient’s condition while also preventing side effects. It is important to understand that all IV fluid replacements should be administered slowly to minimize the risk of fluid overload or exacerbating electrolyte abnormalities.

  • Hypertonic solutions: Contains higher concentrations of solutes than what is observed in the body. These solutions are generally utilized when patients have low lab values (e.g. low sodium, low glucose, etc.) Hypertonic solutions can also draw water from the ICF to the ECF which can be useful for patients with edema. These solutions may also be used for patients with heat exhaustion (where too many electrolytes were wasted) or peritonitis (inflammation of the peritoneal cavity. Examples of hypertonic solutions includes...

    • 3% normal saline (3% NS)
    • 5% normal saline (5% NS)
    • 10% dextrose in water (D10W)
    • 5% dextrose in water with ½ normal saline (D5 ½ NS)
    • 5% dextrose with lactated ringers (D5LR)
    • 50% dextrose in water (D50W)
  • Hypotonic solutions: Contains lower concentrations of solutes relative to the water in the bag. Hypotonic solution are used when patients already have high values of electrolytes especially hypernatremia (serum sodium > 145 mEq/L). Patients with elevated sodium and are given an isotonic or hypertonic solution this can exacerbate their hypernatremia and cause serious side effects. The main function of hypotonic solutions are to treat cellular dehydration which results from hyperosmolar conditions (e.g. hypernatremia, hyperkalemia, hyperglycemia, etc.)

    Diabetic patients may require this therapy in the event of uncontrolled blood sugar as the body will increase the excretion of glucose and water as a result. Patients with HHNS or diabetic ketoacidosis are prime examples of situations where hypotonic solutions may be administered. of Examples of hypotonic solutions includes...

    • 0.45% normal saline (½ NS)
    • 0.33% normal saline (½ NS)
    • 0.25% normal saline (¼ NS)
    • 2.5% dextrose in water and 5% dextrose in water (D5W)
      • Isotonic in the bag but hypotonic in the body after the cells consume the glucose
      • Should be used with caution for patients with hyperglycemia (e.g. diabetics)
      • Should not be use in patients with high intracranial pressure (ICP)
  • Isotonic solutions: Contains approximately the same percentage of solute to solution as in the human body. These are used in situations where fluid levels need to be replenished without shifting the fluids in or out of the cell. Isotonic solutions are very commonly used as these are effective at rehydrating patients. Examples of isotonic solutions includes...

    • Normal saline (0.9% NS)
    • Lactated ringers
      • Use with caution in renal impairment
      • Contains potassium (can affect ECG)
      • Contraindicated in liver impairment (cirrhosis, hepatitis) as the liver is needed to metabolize lactate
    • D5W
      • Isotonic in the bag but hypotonic in the body
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Other considerations should include an evaluation of the patient’s medication regimen. Since many drugs can alter fluid or electrolyte levels it is imperative to determine if the medications that patient is on is exacerbating their situation. Examples of medications that can cause dehydration includes diuretics, SGLT2 inhibitors, ACE inhibitors and ARBs, and more. Patients in extreme conditions should be evaluated by the physician or pharmacy to determine if a medication may need to be discontinued.

Nursing Interventions:

It is imperative when administering IV fluids to never infuse the solutions too quickly as this can be harmful to the patient. Hypertonic solutions are at an increased risk of causing fluid overload as it can draw water from the cells into the blood. This can be dangerous when given to patients with heart failure or renal failure. Other side effects can include tachycardia or hypovolemia (increased risk in liver disease, trauma, or burns).

Patients who are being treated for fluid or electrolyte imbalances should have their weight monitored daily. Rapid weight gain or loss suggests problems with key organs in the body (e.g. the kidneys, digestive tract, lungs, skin). The ins and outs should also be monitored in these patients to track sensible water loss or gain. It is important to track urine output in this situation monitoring for excretions of 240 ml in 3 hours or 480 ml in 6 hours. Generally, urine output should be roughly a liter every 12 hours. If patients are not excreting at these levels may have a critical condition that requires immediate treatment. If BUN or serum creatinine are elevated this suggests serious kidney problems – if only BUN is elevated the patient may just be dehydrated.

One of the more important interventions to make for patients who frequently struggle with fluid management should involve a consultation on signs and symptoms of overhydration or dehydration. A key symptom of dehydration to be aware of is low blood pressure which can cause orthostatic hypotension – a condition where patients change positions too quickly and feint due to lack of blood supply to the brain. Older patients who are prone to dehydration should be advised to increase their fluid and electrolyte intake as this can quickly lead to dehydration in addition to excessive vomiting. Patients who are on diuretics should also be instructed to increase water intake to ensure proper hydration.

Conclusion

It is very important that patients with fluid and electrolytes receive proper treatment. Although administering IV fluids is generally considered harmless – patients who are prone to these imbalances may already have underlying conditions that can be dangerous if the wrong solution is used or if it is given too quickly. Practitioners play a key role at managing these important levels in the patient’s body which is why understanding these concepts fully is imperative when preparing for the NCLEX™ exam.