Neurogenic Pulmonary Edema

Neurogenic Pulmonary Edema(NPE)

Introduction

Neurogenic pulmonary edema (NPE) is an acute pulmonary edema syndrome occurring after a severe acute CNS insult, particularly when the neurological event causes an abrupt rise in intracranial pressure and/or massive sympathetic activation.

• It can be confused with:
• aspiration pneumonitis
• cardiogenic pulmonary edema
• ARDS
• pneumonia
• fluid overload
• pulmonary embolism
• NPE may coexist with neurogenic stunned myocardium/Takotsubo syndrome, making the distinction between cardiogenic and non-cardiogenic mechanisms difficult. NPE can also evolve into an ARDS-like phenotype, so these categories are not always clinically separable.Aspiration and NPE can coexist so Do not force a single diagnosis.

Approximately 20–30% of patients with acute severe brain injury may develop some form of NPE, although reported incidence varies substantially depending on the definition and population studied. 

Etiology

Almost any sufficiently severe acute CNS insult can potentially trigger NPE. Example -Sudden severe neurological insult(aneurysmal SAH)+ sudden hypoxemia + bilateral pulmonary edema + no convincing primary cardiac/pulmonary explanation.

Pathophysiology 

The pathophysiology is multifactorial and incompletely understood.

Several mechanisms probably operate simultaneously.

The major theories are:

1. Sympathetic/catecholamine surge
2. Neurohemodynamic or “blast injury” theory(the pulmonary capillary bed is exposed to an acute pressure/volume “blast.”This can produce pulmonary edema even without primary LV failure.)
3. Increased pulmonary capillary permeability
4. Neurocardiac injury / stunned myocardium
5. Adrenergic hypersensitivity of pulmonary vasculature
6. Inflammatory mechanisms
7. Altered autonomic control of pulmonary vascular tone

These mechanisms should not be viewed as mutually exclusive. 

The central mechanism: sympathetic storm(most important concept.)

CNS insult

↓

Sudden increase in ICP / brainstem-hypothalamic activation

↓

Massive sympathetic discharge

↓

Huge catecholamine release

• norepinephrine
• epinephrine
• dopamine

↓

Systemic vasoconstriction + pulmonary vasoconstriction

↓

Increased systemic vascular resistance and Increased pulmonary vascular resistance

↓

Blood shifts from systemic circulation → pulmonary circulation

↓

Abrupt increase in pulmonary vascular pressure

↓

Pulmonary edema

This is sometimes called the “blast injury” or neurohemodynamic theory. 

Anatomical basis of sympathetic activation

The hypothalamus and brainstem regulate sympathetic outflow.

Severe injury or abrupt ICP elevation can activate these pathways.

↓

sympathetic preganglionic neurons

↓

sympathetic nervous system

↓

catecholamine surge.

The insula is particularly important in autonomic cardiovascular regulation.

Therefore, two patients with NPE can look completely different

Patient A — predominantly neurohemodynamic NPE

• severe hypertension
• tachycardia
• normal LV function
• normal/near-normal filling pressures
• pulmonary edema

Patient B — NPE + neurogenic stunned myocardium

• pulmonary edema
• hypotension
• elevated troponin
• ECG abnormalities
• LV dysfunction
• regional wall-motion abnormalities
• possibly Takotsubo pattern

Both may be triggered by the same CNS event.

Two clinical forms of NPE

Clinical manifestations

LUS is particularly useful for serial assessment.

For example:

Before treatment

Diffuse B-lines

↓

After stabilization

B-lines decrease

↓

improving extravascular lung water.

Lung ultrasound has been proposed as a useful bedside method for monitoring pulmonary fluid while simultaneously managing the competing cerebral fluid requirements. 

Differential Diagnosis

• Aspiration pneumonitis.
• Pneumonia (including aspiration pneumonia).
• Fat embolism syndrome.
• Pulmonary contusion.
• Transfusion-related acute lung injury (TRALI).
• Negative pressure pulmonary edema 
• SCAPE

Diagnostic criteria

There is no universally accepted gold-standard diagnostic test.

NPE is essentially a:clinical diagnosis of exclusion

There is no specific biomarker for NPE.

Potential findings:

• elevated catecholamines
• elevated BNP/NT-proBNP
• elevated troponin
• leukocytosis
• metabolic abnormalities related to the CNS insult

However:

Troponin elevation does NOT automatically mean ACS.

In SAH, catecholamine-mediated neurocardiac injury can produce troponin elevation without obstructive coronary disease. 

Echocardiography-Differentiate between  cardiogenic or neurogenic/non-cardiogenic Pulmonary edema?

ECG—rule out Acute coronary syndrome

CT Chest—helps to differentiate other differential diagnosis 

Management

• Treat the neurological catastrophe.
• Support oxygenation/ventilation while preventing secondary brain injury.
• It is generally self-limiting, with spontaneous resolution over a few days

Mechanical ventilation

NIV—Use nasal interface as patients usually have altered mental status, with an increased risk of aspiration.

If intubated, use lung-protective ventilation.

PEEP can be very useful in NPE.

It:

• recruits alveoli
• improves oxygenation
• reduces intrapulmonary shunt
• decreases work of breathing.

But in severe brain injury:

PEEP must be balanced against intracranial physiology.

Potential effects include:

• increased intrathoracic pressure
• reduced venous return
• altered cerebral venous drainage
• potential increase in ICP
• reduction in CPP if systemic pressure falls.

Therefore:PEEP is not contraindicated in brain injury.

Rather:Use the lowest PEEP that provides adequate recruitment/oxygenation while maintaining acceptable ICP and CPP.

The effect is highly dependent on lung compliance, intrathoracic pressure transmission, volume status and intracranial compliance.

Fluid management

Pulmonary edema suggests:“Restrict fluids.”

But many CNS conditions, particularly SAH, may require:

adequate euvolemia and maintenance of cerebral perfusion.

Therefore:Avoid two extremes

❌ indiscriminate fluid loading and ❌ aggressive diuresis causing hypovolemia.

The goal is:

euvolemia + adequate cerebral perfusion + avoidance of pulmonary fluid overload.

Fluid management should be individualized using:

• clinical examination
• arterial pressure
• urine output
• lactate
• echo
• lung ultrasound
• dynamic indices where appropriate
• venous congestion assessment
• serial weight/fluid balance.

Lung ultrasound may be particularly useful for monitoring extravascular lung water while managing cerebral fluid requirements. 

Furosemide?

NPE is not automatically a condition of systemic volume overload.

Do not reflexively give diuretics solely because bilateral pulmonary edema is present.

Diuretics are reasonable when there is evidence of:

• volume overload
• elevated filling pressures
• cardiogenic pulmonary edema
• significant systemic congestion.

Treat increased ICP But Hyperventilation?

Routine aggressive hyperventilation is not recommended.

Hypocapnia causes:cerebral vasoconstriction which reduced cerebral blood flow.Therefore, in most neurocritical patients:

target normocapnia

unless brief controlled hyperventilation is being used as an emergency temporizing maneuver for impending herniation.

Sympathetic blockade

Because catecholamine excess is central to NPE pathophysiology, sympathetic blockade has theoretical appeal.

Potential approaches studied include:

• α-blockade
• β-blockade
• combined adrenergic modulation.

However:

There is insufficient high-quality evidence to recommend routine pharmacological sympathetic blockade specifically for NPE.

Prone positioning?

If NPE produces severe refractory ARDS physiology, prone positioning may theoretically be considered according to standard severe ARDS principles.

But in acute brain injury:

• ICP must be considered
• cerebral venous drainage must be maintained
• head/neck positioning must be carefully controlled
• invasive devices must be protected.

Therefore:

Severe brain injury is not an automatic absolute contraindication to prone positioning, but it requires neurocritical expertise and careful ICP/CPP monitoring.

Resolution

• One of the interesting characteristics of NPE is its potentially rapid reversibility.
• Many patients improve substantially over:24–72 hours once the neurological insult and sympathetic surge stabilize and appropriate respiratory support is provided. 

What NOT to do

❌ Don’t assume every pulmonary edema in SAH is cardiogenic.

❌ Don’t assume every bilateral infiltrate is pneumonia.

❌ Don’t reflexively give large-volume fluids.

❌ Don’t reflexively diurese.

❌ Don’t routinely use steroids.

❌ Don’t routinely use antibiotics without evidence of infection.

❌ Don’t aggressively reduce BP without considering CPP.

❌ Don’t avoid PEEP simply because the patient has brain injury.

❌ Don’t hyperventilate routinely.

❌ Don’t forget echocardiography.

❌ Don’t focus only on the lungs—treat the brain.

References

1. Davison DL, Terek M, Chawla LS. Neurogenic pulmonary edema. Crit Care. 2012;16(2):212. 
2. Davison DL, et al. Neurogenic pulmonary edema. Crit Care Med. 2015;43(7):1645-51. doi:10.1097/CCM.0000000000001101. 
3. Busl, K., & Bleck, T. (2015). Neurogenic Pulmonary Edema. Critical Care Medicine, 43(8), 1710–1715. https://doi.org/10.1097/CCM.0000000000001101