Murray Score for Acute Lung Injury - An Overview

Introduction:

Acute lung injury and its more severe form, acute respiratory distress, is a syndrome of acute respiratory failure known by bilateral lung infiltrates and reduced oxygen levels dissolved in the blood. It is a life-threatening inflammatory condition of the lungs. Globally, it affects three million patients annually, and almost 10 percent require intensive care unit (ICU) admissions.

What Is Acute Respiratory Distress Syndrome?

Acute respiratory distress syndrome or ARDS is an acute, diffuse type of pulmonary inflammatory response that is neither a primary disease nor a single entity. Rather, it expresses countless other diseases that can lead to diffuse inflammation in the lungs, often accompanied by inflammation in other organs. It is also the most common cause of acute respiratory failure.

It is characterized by the following:

• Neutrophil sequestration in pulmonary capillaries.

• Increased capillary permeability.

• Protein-rich pulmonary edema with hyaline membrane formation.

• Damage to type 2 alveolar cells or pneumocytes leads to surfactant depletion.

• Alveolar collapse.

• Reduction in lung compliance.

If the initial phase does not resolve with treatment of the underlying cause, a fibroproliferative phase ensues and causes progressive pulmonary fibrosis. The term ARDS is often limited to patients requiring ventilatory support in the ICU, but less severe forms are conventionally referred to as acute lung injury.

What Is the Pathophysiology of ARDS?

The pathophysiology of ARDS involves tissue damage, either pulmonary or extrapulmonary, with the release of inflammatory mediators such as interleukin-1 (IL-1), followed by migration of neutrophils into the alveoli and excessive release of neutrophilic mediators such as cytokines and reactive oxygen species and causing injury to the alveolar capillaries and endothelial cells, a pathology known as diffuse alveolar damage (DAD).

This will result in an exudative or early phase in which excessive interstitial fluid accumulates causing pulmonary edema. Neutrophils and protein-rich interstitial fluid in the alveolar space cause the formation of a hyaline alveolar membrane. In addition, damage results in impaired gaseous exchange and hypoxemia. Damaged alveolar cells cause a sharp decline in surfactant production, causing alveolar collapse. The late stage, is characterized by the proliferation of type 2 alveolar cells and infiltration of fibroblasts resulting in progressive interstitial fibrosis.

What Are the Conditions That Predispose to ARDS?

Conditions predisposing to ARDS:

Direct or Inhalation Conditions:

• Aspiration of gastric contents.

• Pneumonia.

• Blunt chest trauma.

• Toxic gases or burn injury.

• Near drowning situations.

Indirect or Blood-Borne Conditions:

• Sepsis.

• Multiple trauma.

• Pancreatitis.

• Cardiopulmonary bypass.

• Severe burns.

• Necrotic tissue.

• Fat emboli.

• Major transfusion reactions.

• Anaphylaxis.

• Carcinomatosis.

• Obstetric crisis.

What Are the Signs and Symptoms of ARDS?

The earliest clinical signs of ARDS are severe dyspnea, tachypnea, cyanosis progressive reduced oxygen saturation levels in the blood, usually refractory to oxygen, which causes diffuse pulmonary infiltrates in chest X-ray within 24 hours and may lead to respiratory failure, thus requiring mechanical ventilation within 48 hours of illness. In addition, if the injurious factor is not removed, the number of inflammatory mediators secreted by the lungs may result in systemic inflammatory response syndrome (SIRS) and multi-organ dysfunction syndrome (MODS). This adds to impaired oxygenation, the central problem of ARDS, and further impairs oxygen delivery.

What Are the Criteria Defining ARDS?

The criteria defining ARDS is a triad of the following:

• Hypoxemia, where the ratio of pulse oximetric saturation to the fraction of inspired air (PaO2/FiO2) is less than 200 mmHg for ARDS and less than 300 mmHg for ALI (acute lung injury).

• Chest radiographic features show diffuse bilateral opacities.

• Reduced lung compliance.

What Is Murray Score of Lung Injury?

The scoring system developed by Murray et al is intended for cases of severe respiratory failure. It might help decide whether the patient requires a conventional method of ventilation or may require extracorporeal membrane oxygenation (ECMO). The scoring system is based on different pulmonary variables:

• Hypoxemia.

• Positive end-expiratory pressure (PEEP).

• Lung Compliance.

• Radiographic findings.

Each variable is given a score ranging between zero and four based on the severity of the condition. A final score is calculated by dividing the collective score by the number of variables included. The lung injury score according to Murray's scoring pattern:

• 0 score - Indicates no lung injury.

• Less than 2.5 score - Indicates mild to moderate lung injury.

• More than 2.5 score - Indicates presence of severe ARDS.

The scoring method is a clinical predictor tool for early tracheostomy in patients with respiratory failure in intensive care units. It considers positive end-expiratory pressure and lung compliance, both sensitive indicators of lung injury. The disadvantage of the scoring system is that lung compliance is not measured routinely, especially in large clinical studies.

What Is the Other Scoring System Available for Lung Injury?

Another widely used scoring system is the oxygenation index (OI). It measures the degree of lung injury and hypoxemia in young children. The mean alveolar pressure is the most important determining factor of oxygenation. It is the product of mean airway pressure, the fraction of inhaled oxygen divided by the partial pressure of oxygen. It determines the mortality of the infant and is a more sensitive predictor of the severity of pulmonary diseases. The oxygenation index reveals:

• 2-7 - Normal or pulmonary dysfunction.

• 8-9 - Moderate pulmonary dysfunction.

• More than 10 - Severe pulmonary dysfunction.

• More than 30 - Need for extracorporeal membrane oxygenation (ECMO). It is an important treatment modality in pediatric patients with cardiopulmonary failure.

How to Manage ARDS?

Patients with ARDS need to be hospitalized and require treatment in an intensive care unit. There is no specific management for ARDS; only symptomatic relief can be achieved by administering supplemental oxygen and mechanical ventilation. On changing the position from supine to prone, oxygenation of the patient improves exponentially as the perfusion redistribution and functional residual capacity also increase. Also, lateral rotation therapy is highly beneficial in stimulating postural drainage and helping mobilize the secretions. The lateral movement of the bed is done slowly for 18 to 24 hours. The primary goal is to improve oxygenation to correct the hypoxemia. Drugs that may improve the conditions are antibiotics, bronchodilators, and corticosteroids.

Conclusion:

Acute lung injury and respiratory distress syndrome are severe lung inflammatory conditions requiring immediate intervention. It is a life-threatening situation; therefore, Murray's score for lung injury plays an important clinical predictor tool for the need for early tracheostomy in patients with acute respiratory failure.