Resource Respiratory

ARDS – Definition and Pathophysiology

 
Dr John Griffiths DICM MRCP FRCA MA
CriticalCareUK Editor

Focus on definition and associations of ALI and ARDS

Acute lung injury (ALI) and adult respiratory distress syndrome (ARDS) develop in response to lung injury. A wide variety of precipitating causes are recognised (Tables 1a and 1b). Severe sepsis is the leading cause, followed by pneumonia, aspiration of gastric contents, massive blood transfusion, multiple trauma and pregnancy-related ARDS. ALI and ARDS develop very soon after the precipitating event, usually within 12-72 hours and often within 6 hours. In 1994, The American-European Consensus Conference Committee proposed definitions for ALI and ARDS (Table 2). It can be seen that ALI is a continuum of injury. When the oxygenation abnormality is more severe, the condition is termed ARDS. There are estimated to be about 3/100,000 cases of ARDS each year. Recent improvements in critical care have seen a reduction in the mortality of ARDS but it still remains at about 50% for the general population and 25% if it is pregnancy related. Fewer than 20% of deaths are due to refractory respiratory failure. This emphasises the importance of identifying and treating the precipitating cause.

Table 1b. Causes of ARDS classified into ‘direct’ and ‘indirect’ lung injury

Adapted from Ware LB et al.

Adapted from Bernard GR, Artigas A, Brigham KL et al.

Focus on pathophysiology of ARDS

ALI and ARDS develop when inflammatory cytokines or exogenous agents injure both the epithelium and endothelium of the lung. These inflammatory insults can occur locally in the lung (“primary” ARDS) or be part of a systemic inflammatory syndrome (“secondary” ARDS). Irrespective of the precipitating cause, the pathophysiological mechanisms driving the process are identical and progress through recognised phases (Table 3). ARDS classically affects the lung in a non-homogenous manner. Supine CT scans of ARDS lung taken during the initial phases show striking asymmetry of lung involvement. Dependent posterior regions are preferentially infiltrated, consolidated or collapsed. Anterior areas are often normally or even excessively aerated during mechanical ventilation. It is this heterogeneity of lung involvement that makes effective and safe ventilation difficult to achieve.

Focus on clinical manifestations and outcome from ARDS

At the bedside, the patient is clearly in respiratory distress. Although orthopneoa may be present, the other features of congestive heart failure are seldom present. Chest X-ray reveals diffuse, bilateral infiltrates that are often patchy and asymmetric (Table 4). Arterial blood gases reveal hypoxaemia that is often refractory to oxygen therapy. An initial respiratory alkalosis invariably leads to hypercapnia as dead space ventilation increases and muscle fatigue sets in. As ARDS evolves, the cardiovascular system is commonly affected and multi-organ failure invariably ensues. Over half the patients will develop associated renal failure. Immediate prognosis is related to the number of organ systems involved. In the general population, patients with only lung involvement have 15-30% mortality. If three or more organs are involved, this becomes greater than 80%. If the multi-organ failure persists beyond 4 days, mortality is 100%.

Key references

Bernard GR, Artigas A, Brigham KL, et al.
The American-European consensus Conference on ARDS: definitions, mechanisms, relevant outcomes, and clinical trial coordination.
Am J Respir Crit Care Med 1994; 149: 818-824.