From the Guidelines
Isotonic fluids are the most appropriate choice for maintaining hydration in most patients, as they have the same concentration as body fluids and cause no net movement of water across cell membranes, thereby minimizing the risk of cellular dehydration or swelling, as supported by the most recent and highest quality study 1.
Key Points
- Isotonic fluids have the same concentration as body fluids (0.9% sodium chloride or normal saline) and are ideal for maintaining hydration without changing cellular volume.
- Hypertonic fluids have higher solute concentration than body cells (like 3% saline or D50W) and can reduce cellular edema but may cause cellular dehydration if used excessively.
- Hypotonic fluids have lower solute concentration than body cells (like 0.45% saline or D5W after glucose metabolism) and can help rehydrate cells but can lead to cellular swelling and potentially dangerous cerebral edema if administered too quickly.
- The choice of fluid depends on the patient's condition, with isotonic fluids used for general volume replacement, hypertonic fluids for treating cerebral edema or severe hyponatremia, and hypotonic fluids for cellular dehydration.
Clinical Considerations
- According to the guidelines for the choice of intravenous fluids for vascular filling in critically ill patients, 2021, isotonic fluids are the most appropriate maintenance IVF and are the least likely to result in a disorder in serum sodium 1.
- The use of hypertonic saline in critical care has been reviewed, and it is recommended that intravenous hypertonic saline be used in the treatment algorithm for raised intracranial pressure, but it does not improve neurological outcomes or survival in states of raised intracranial pressure 1.
- The most recent study on guidelines for the choice of intravenous fluids for vascular filling in critically ill patients, 2021, provides the most up-to-date and highest quality evidence for the use of isotonic fluids in maintaining hydration 1.
From the Research
Effects of Isotonic, Hypertonic, and Hypotonic Fluids on the Body
- Isotonic fluids have the same concentration of solutes as blood and do not cause a significant shift in fluid balance between the intravascular and interstitial spaces 2, 3.
- Hypertonic fluids have a higher concentration of solutes than blood and can cause fluid to shift from the interstitial space into the intravascular space, increasing blood volume and pressure 4, 5.
- Hypotonic fluids have a lower concentration of solutes than blood and can cause fluid to shift from the intravascular space into the interstitial space, decreasing blood volume and pressure 5, 6.
Clinical Implications
- The use of isotonic saline rather than albumin is conditionally recommended in patients with traumatic brain injury, as it may help to maintain optimal fluid balance and prevent cerebral edema 2.
- Hypertonic saline may be beneficial in the treatment of severe head injury, as it can help to reduce intracranial pressure and improve cerebral perfusion pressure 5.
- The use of hypertonic saline-dextran solution has been shown to be effective in resuscitating severely dehydrated animals, and may have implications for the treatment of human patients with severe dehydration 6.
Key Findings
- A study comparing lactated Ringer's solution and hypertonic saline in severely head-injured children found that hypertonic saline was associated with fewer interventions, fewer complications, and shorter ICU stay times 5.
- A study comparing hypertonic saline-dextran solution and lactated Ringer's solution in severely dehydrated calves found that both treatments were effective, but hypertonic saline-dextran solution was associated with a more rapid and sustained response 6.
- The European Society of Intensive Care Medicine recommends the use of crystalloids rather than albumin for volume expansion in critically ill patients, and provides conditional recommendations for the use of balanced crystalloids rather than isotonic saline in certain patient populations 2.