Respiratory Distress Syndrome Disease

DESCRIPTION

A patient with previously normal lungs who suffers a catastrophic pulmonary or nonpulmonary event followed by a cascade of events that occur at the alveolar-arteriolar capillary membrane. Reduction in functional residual capacity and lung compliance are hallmarks.

Hypoxemia, decreased pulmonary compliance, and an increasing shunt forms as the proteinaceous material collects in the interstitium and alveoli. Hypoxemic respiratory failure secondary to non-cardiogenic pulmonary edema almost always requires mechanical ventilation
System(s) affected: Pulmonary
Genetics: N/A
Incidence/Prevalence in USA: 150,000/year
Predominant age: All ages
Predominant sex: Male = Female

SIGNS AND SYMPTOMS

Most patients demonstrate similar clinical and pathologic features regardless of the cause of the acute lung injury.

There are 4 phases:
- Phase 1: Acute injury - normal physical exam, normal chest x-ray, tachypnea, tachycardia, and respiratory alkalosis
- Phase 2: Latent phase - 6-48 hours after injury: Hyperventilation, hypocapnia, increase in work of breathing, widening alveolar-arterial oxygen gradient
- Phase 3: Acute respiratory failure - tachypnea and dyspnea, decreased lung compliance, diffuse infiltrates on chest roentgenogram, high-pitched diffusely scattered crackles
- Phase 4: Severe abnormalities - severe hypoxemia unresponsive to therapy, increased intrapulmonary shunting, metabolic and respiratory acidosis
Signs and symptoms common to all 4 phases:
- Tachypnea and tachycardia during the first 12 to 24 hours
- Moist and cyanotic skin
- Breathing difficulty with intercostal and accessory respiratory muscles
- Dramatic increase in work of breathing
- High-pitched end-expiratory crackles are heard throughout all lung fields
- Increased agitation
- Lethargy, then obtundation
- Hypoxemia may be present long before clinical signs

CAUSES

Recent studies have revealed a number of mediators are involved in the initiation and perpetuation of ARDS:
- Cytokines (tumor necrosis factor, interleukin 1, interleukin 6)
- Complement activation
- Coagulation activation
- Platelet-activating factor
- Oxygen radicals
- Lipoxygenase pathways (Leukotrienes C4, D4 and E4)
- Neutrophil proteases
- Nitric oxide - may be deleterious or advantageous
- Endotoxin
- Cyclooxygenase pathway products (thromboxane A2, prostacyclin)
All of the following causes can initiate a systemic inflammatory response with activation of the previous mediators:
- Aspiration
- Pulmonary and systemic infections (bacterial, fungal, viral, and protozoan)
- Sepsis (gram negative, gram positive, fungi, tuberculous, Pneumocystis pneumonia)
- Pulmonary contusion
- Near-drowning
- Multiple fractures, especially long bones (fat embolism)
- Multiple transfusions
- Severe pancreatitis
- Head injury
- Inhalation of toxic gases (oxygen, smoke, NH3, chlorine, plastics, phosgene, cadmium)
- Burns
- Shock (hemorrhage, cardiogenic, septic, anaphylactic)
- Eclampsia
- Carcinomatosis
- Leukoagglutinin reaction
- Air or amniotic fluid emboli

RISK FACTORS

Systemic sepsis
Pulmonary contusion
Aspiration
Inhalation of toxic substances
Diffuse pneumonia
Multiple emergency blood transfusions

LABORATORY

PaO2 < 50 mm Hg with FiO2 > 0.6
Overall compliance < 50 mL/cm water (usually 20-30 mL/cm water)
Increased shunt fraction Q's/Q't and dead space ventilation VD/VT

Drugs that may alter lab results: N/A
Disorders that may alter lab results:

Multiple pulmonary embolism
Cardiogenic pulmonary edema
Severe chronic obstructive pulmonary disease
Severe pneumonia

PATHOLOGICAL FINDINGS

Lungs show exudative phase, early proliferative phase or late proliferative phase
Interstitial and alveolar edema
Inflammatory cells and erythrocytes spill into interstitium and the alveolus
Type I cells are destroyed, leaving a denuded basement membrane
Protein-rich fluid fills the alveoli
Type II alveolar cells appear unaltered initially
Type II cells begin to proliferate within 72 hours of initial insult
The type II cells cover the denuded basement membrane
Aggregates of plasma proteins, cellular debris, fibrin and surfactant remnants form hyaline membranes
Over next 3-10 days, alveolar septum thickens by proliferating fibroblasts, leukocytes and plasma cells
Capillary injury begins to occur
Hyaline membranes begin to reorganize
Fibrosis becomes apparent in respiratory ducts and bronchioles

SPECIAL TESTS

Pulmonary artery catheter
Measure pulmonary edema fluid content

IMAGING

Chest roentgenogram

DIAGNOSTIC PROCEDURES

Pulmonary artery catheterization to demonstrate:
- Normal pulmonary arterial occlusion pressure (PAOP)
- Note: The main point of this study is to determine if the PAOP is inconsistent with cardiogenic pulmonary edema. A low PAOP with low serum albumin may lead to cardiogenic etiology. The patient with chronic congestive heart failure may have high wedge, but still develop adult respiratory distress syndrome.

APPROPRIATE HEALTH CARE

Intensive care unit

GENERAL MEASURES

Treat underlying etiology as appropriate
Corticosteroids have theoretical, but not proven, benefit. This is still controversial. Do not use if occult infection exists.
Support ventilation mechanically as necessary
Support oxygenation with positive end-expiratory pressure (PEEP) with an amount that allows adequate hemoglobin saturation (90-95%) with an FiO2 of ≤ 60%. PaO2 is less important than oxygen delivery.
Maintain oxygen delivery:
- Increase O2 content with packed red blood cell transfusion as necessary
- Optimize cardiac output with fluid or inotropes. Measure CO with each PEEP change. If CO decreases, give fluids to regain adequate CO and allow additional PEEP adjustments if necessary.
Avoid over-ventilation
Avoid large tidal volumes (> 10 cc/kg) if peak airway pressures are high
A pulmonary artery catheter may be helpful in assessing left ventricular function, CO, oxygen delivery and consumption. It is necessary if high levels of PEEP are used.
Be aware of possible respiratory superinfection
Deep vein thrombosis prophylaxis is essential
Consider paralyzing agents (to improve compliance and decrease barotrauma) if patient is fighting ventilator. (Initiate anxiolytics when instituting treatment with paralytics.)
Ulcer prophylaxis
Extraordinary management:
- Extracorporeal membrane oxygenation (ECMO) is conceivable in children
- High-frequency ventilation - it achieves adequate gas exchange; however, there is no improvement on outcome
- Pressure controlled inverse ratio ventilation (PCIRV)
- Extracorporeal CO2 removal with low frequency ventilation (Used in Europe and Salt Lake City on NIH trial)

SURGICAL MEASURES

N/A

ACTIVITY

Bedrest

DIET

Nutritional support - avoid excess carbohydrates (could increase respiratory quotient)

PATIENT EDUCATION

N/A

PREVENTION/AVOIDANCE

N/A

POSSIBLE COMPLICATIONS

Multiple organ dysfunction syndrome (MODS)
Death
Permanent lung disease
Oxygen toxicity
Barotrauma
Superinfection

EXPECTED COURSE AND PROGNOSIS

50% mortality rate; the syndrome is heterogeneous and mortality runs from 15-90%, depending on multiple factors; mainly, underlying cause, age of patient, number of failed organs.
There is a decrease in diffusing capacity due to fibrosis and restrictive lung disease. Many people will have abnormal pulmonary function tests up to 6 months after illness. Most who survive will reveal abnormalities only with sensitive function testing, such as pulmonary exercise tests.

ASSOCIATED CONDITIONS

See Causes

AGE-RELATED FACTORS

N/A
Pediatric: N/A
Geriatric: Increasing mortality with increasing age
Others: N/A

PREGNANCY

N/A

OTHER NOTES

N/A

ABBREVIATIONS

PEEP = positive end-expiratory pressure

Clinical Investigations

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