The concept of the fluid compartment model has existed for more than a century. However, while this model is routinely taught to physicians and advanced practice providers, many nurses remain unfamiliar with the critical concepts.

Why? Historically, nursing education has focused on understanding visible clinical findings and immediate interventions. Nursing education has not prioritized an advanced understanding of physiologic mechanisms during fluid loss.

As trauma nursing evolves — especially in acute care settings — understanding fluid physiology is becoming increasingly valuable because it can empower nurses to provide even better patient care.

Nurses who understand the fluid compartment model can use its clinical implications to enhance clinical reasoning, improve assessment accuracy and foster clear communication within interdisciplinary teams. This can help optimize patient outcomes and prevent complications associated with trauma resuscitation.

What is the fluid compartment model?

The human body maintains fluid within two primary compartments — the intracellular compartment (approximately 60% of body fluid) and the extracellular compartment (approximately 40% of body fluid).

Extracellular fluid further divides into interstitial fluid, intravascular fluid (plasma) and specialized fluids like cerebrospinal fluid, lymph and pleural fluid.

The following illustration shows the breakdown of the three main fluid compartments, including the two membrane systems that separate those compartments:

How this model explains the progression of hemorrhage to shock

This fluid compartment model becomes very useful in trauma when we understand that traumatic bleeding initiates rapid fluid shifts across the three main compartments. These shifts occur in three phases:

Phase 1: Initial fluid loss. Immediately following injury, intravascular fluid loss causes rapid depletion of plasma volume. This leads to decreased cardiac output and, ultimately, poor tissue perfusion.

Phase 2: Compensation (also known as “autotransfusion”). In response to decreased intravascular volume, fluid shifts from interstitial to intravascular spaces. This is the body’s attempt to restore circulating volume. This process may create the illusion that the patient is stable or stabilizing, but in reality any signs of hemodynamic stability will be temporary.

Phase 3: Cellular and capillary leak. If compensatory shock is not recognized and corrected, the inflammatory cascade triggered by trauma will increase capillary permeability, result in fluid shifting from intracellular to interstitial spaces. This exacerbates edema, reduces tissue perfusion and creates profound systemic complications. As injury progresses, cellular fluid depletion accompanied by anaerobic metabolism leads to acidosis and electrolyte loss. During this phase, patients will require aggressive fluid resuscitation, blood product transfusion and close monitoring of physiologic response. Patient progression is often irreversible and leads to multi-organ failure.

How trauma nurses can use the fluid compartment model to guide practice

As noted above, the autotransfusion phase is a “danger zone” because it can give a false sense of patient stability when in fact the patient is about to go over the hemodynamic cliff.

During this phase, nurses should be alert for subtle signs of compensatory shock, such as:

• Mild tachycardia
• Narrowing pulse pressure
• Clammy, cool skin
• Altered mentation or anxiety

Recognition of these symptoms is often missed and, as a result, patients can quickly spiral into decompensate shock and the lethal trauma diamond — coagulopathy, acidosis, hypothermia and hypocalcemia.

What to watch out for and what to do

Prompt recognition and prevention are key.

1. If early coagulopathies are noted in the patient’s initial labs or if you are preparing for blood transfusion — administer calcium according to your local protocols. Remember, banked blood contains citrate preservative. Citrate binds to calcium, which will lead to worsening coagulopathies.

2. If blood pressure is stable but the patient’s heart rate or diastolic blood pressure is rising — reassess for other signs of shock. Remember, tachycardia is often the first sign of fluid loss. Do not assume all tachycardia in trauma is related to pain.

3. If the patient’s skin is cool, pale or clammy — assume peripheral vasoconstriction caused by the shunting of blood to vital organs during autotransfusion. Along with reassessing fluid needs, keep your patient warm.

4. If assessment reveals a patient with anxiety, restlessness or confusion — do not assume illicit substances, dementia or situational anxiety. Until proven otherwise, consider that this patient is showing signs of inadequate cerebral perfusion.

Crucial role in early recognition of shock

Nurses play a crucial role in trauma resuscitation and are uniquely positioned to impact patient outcomes through initial assessments and ongoing care.

A strong understanding of fluid dynamics and evidence-based practices equips nurses to recognize early signs of shock, intervene effectively and collaborate seamlessly within the healthcare team, ultimately enhancing patient care and improving survival rates.

Jacki Roland, MSN, RN, CEN, TCRN is Emergency Department Clinical Nurse Educator at UCHealth University of Colorado Hospital, a Level I trauma center in Aurora, Colorado, and UCHealth Metro Denver Freestanding Emergency Departments.