3.1 Osmotic Agents

Hyperosmolar therapy is the pharmacologic rescue you reach for when intracranial pressure (ICP) threatens cerebral perfusion or herniation. This independent OpenExamPrep chapter covers how mannitol and hypertonic saline (HTS) work, how to dose them, what to monitor, and how to pick one agent over the other in a real neuro ICU. Both drugs pull water from brain tissue across an intact blood–brain barrier (BBB). Neither reverses the primary injury. Neither replaces airway control, head-of-bed elevation, fever treatment, seizure control, cerebrospinal fluid (CSF) drainage, or surgery.

Why osmotic agents matter on this exam

Items rarely ask you to name a brand. They embed a physiologic trap. Mannitol in a dry, hypotensive trauma patient drops cerebral perfusion pressure (CPP) because CPP equals mean arterial pressure minus ICP. A 23.4% saline push through a tenuous hand intravenous line risks catastrophic extravasation. A patient whose sodium has been 118 mEq/L for days can develop osmotic demyelination syndrome (ODS) if you jump sodium into the 140s in one hour. Crystals in a refrigerated mannitol bag delay a herniation bolus if nobody inspects the solution.

Brain Trauma Foundation (BTF) 4th Edition material treats hyperosmolar therapy as a method to lower ICP. A commonly cited treatment threshold is 22 mmHg. BTF does not establish mannitol or HTS as a silver bullet that independently improves survival. Use osmotherapy to protect the brain and buy minutes while you treat the cause.

Mannitol: dose, filter, and crystals

Mannitol is a sugar alcohol, usually as 20% (200 mg/mL) or 25% solution. The usual neurocritical care bolus is 0.25–1 g/kg IV. Many teams use 0.5–1 g/kg for frank herniation and 0.25–0.5 g/kg for milder ICP spikes. Infuse over about 15–30 minutes. A faster push increases the chance of abrupt plasma expansion followed by hypotension as diuresis begins.

Mannitol raises plasma osmolality, draws water from parenchyma, and reduces blood viscosity. The viscosity change can trigger reflex cerebral vasoconstriction and lower cerebral blood volume. ICP typically begins to fall within 15–30 minutes, peaks around 30–60 minutes, and lasts about 2–6 hours. The kidney filters mannitol and does not reabsorb it, so you get an osmotic diuresis. Where BBB is open, mannitol can enter tissue. When plasma osmolality later falls, water can follow into that tissue and produce rebound intracranial hypertension.

Twenty percent mannitol crystallizes when cold. Inspect the bag under light. If crystals are present, warm the bag until they dissolve; do not inject a slurry. Give the dose through an in-line filter (commonly 5 micron). A reliable peripheral intravenous line can carry a 20% bolus in many units, but extravasation still injures tissue. Intraosseous access is an emergency option when no vein is available.

Osmolar gap and volume

Calculated osmolarity (mg/dL glucose and BUN) is approximately 2 × Na + glucose/18 + BUN/2.8. The osmolar gap is measured osmolality minus that calculated value. Mannitol widens the gap, so the gap tracks remaining drug better than a lone osmolality number. Classic teaching is to hold further mannitol if the gap exceeds about 20 mOsm/kg or if measured osmolality is driven above roughly 320–340 mOsm/kg, because extra doses add renal and hypovolemic harm with diminishing ICP return. Those cutoffs are heuristics. A herniating patient with a gap of 18 is not a reason to withhold a needed bolus if surgery is 20 minutes away; a euvolemic patient with a gap of 25 and ICP 18 is not a reason to give another 1 g/kg out of habit.

Replace urine milliliter for milliliter with isotonic fluid unless the patient is already volume-overloaded. Hypovolemia after mannitol lowers MAP and therefore CPP. Mannitol is a poor choice in hemorrhagic shock, uncontrolled bleeding, or anuria that cannot clear the osmotic load. Acute kidney injury clusters with high cumulative dose, preexisting chronic kidney disease (CKD), and hypovolemia.

Hypertonic saline: 3% and 23.4%

HTS is sodium chloride above 0.9%, sometimes with acetate or lactate buffers. Neuro ICUs most often use 3% saline for titratable therapy and 23.4% saline as a small-volume herniation rescue.

3% NaCl has an osmolarity of about 1027 mOsm/L. Typical adult boluses are 2–5 mL/kg (often 250 mL) over 10–20 minutes. Continuous infusions often start near 0.5–1 mL/kg/h and are titrated to sodium and ICP. Many hospitals allow peripheral 3% boluses through a large-bore, well-functioning catheter because the volume is limited. Continuous peripheral 3% still risks phlebitis and extravasation, so central access is preferred for a prolonged drip. Recheck the insertion site every time you walk in the room.

23.4% NaCl is about 8 mOsm/mL. The usual adult rescue is 30 mL over 10–20 minutes (some herniation protocols use a shorter push). This concentration is caustic. Give it through a central venous catheter whenever possible. Intraosseous access is an emergency alternative. Peripheral 23.4% risks severe tissue necrosis if it leaks. ICP can fall within minutes. Repeat dosing is limited by the sodium and chloride you just added.

HTS expands intravascular volume rather than causing a brisk diuresis. That is why it is attractive in hypovolemic brain-injured patients who still need preload. Teams often hold serum sodium in the mid-140s to low-150s mEq/L while ICP is labile and avoid driving sodium much above 160 mEq/L without a compelling reason. There is no single universal ICP sodium setpoint. Check sodium frequently during active titration—every 4–6 hours on an infusion, and sooner after 23.4% boluses. Do not chase ever-higher sodium when ICP is already controlled.

Rebound occurs if a high sodium is dropped too quickly: water returns to the brain and ICP climbs. Wean infusions and liberalize free water gradually. Chloride-rich HTS commonly produces hyperchloremic metabolic acidosis, a non-anion-gap acidosis that can affect renal perfusion and hemodynamics. Balanced hypertonic mixtures exist in some centers; 3% and 23.4% NaCl remain chloride loads.

Osmotic demyelination

ODS, including central pontine myelinolysis, follows over-rapid sodium correction. Risk is highest with chronic hyponatremia, alcoholism, malnutrition, and liver disease. In a patient whose sodium has been 118 mEq/L for days, a 23.4% bolus that jumps sodium by 12 mEq/L in an hour can devastate white matter even if the ICP number looks prettier. Acute sodium falls from cerebral salt wasting or mannitol diuresis are not the same physiology as a sodium that has been low all week, but you still avoid cavalier swings. For chronic hyponatremia, keep correction within commonly cited limits of about 8–10 mEq/L in 24 hours unless a herniation crisis forces a more rapid rescue—and even then, stop the rise once the emergency is controlled.

When to prefer saline versus mannitol

Choose from physiology, not from which bag is closer to the door.

Heart-failure physiology is a frequent discriminator. Both agents initially expand plasma volume. Mannitol then diureses if the kidneys work. A patient with pulmonary edema, adequate MAP, and urine output may tolerate mannitol better than a sustained HTS infusion. A septic or polytrauma patient who is behind on volume will often look worse after mannitol. CKD and oliguria tilt strongly toward HTS because mannitol is filtered and is nephrotoxic when it stagnates.

Doses, onset, and adverse effects

Adjuncts in the same ICP algorithm are not osmotic drugs: head-of-bed elevation, neutral neck position to preserve venous outflow, brief hyperventilation as a bridge (avoid PaCO2 below 25 mmHg), adequate sedation, neuromuscular blockade only when the patient is fighting the ventilator, CSF drainage, and decompressive surgery. Do not stack osmotic boluses as a way to postpone a needed hemicraniectomy.

Exam traps

Using mannitol as a default in a hypotensive TBI patient. Omitting the filter or injecting crystallized mannitol. Giving 23.4% through a 22-gauge hand IV. Treating a sodium of 158 mEq/L with another 23.4% bolus because ICP was 25 once an hour ago. Correcting chronic hyponatremia to 140 mEq/L in a single shift. Claiming that osmotherapy has randomized evidence of a large mortality benefit independent of ICP control. BTF-style care uses these drugs to treat intracranial hypertension, not as a survival panacea.