ARDS, Acute Lung Injury

Question 1

how is acute lung injury characterized?

Answer 1

abrupt onset of hypoxemia and bilateral pulmonary edema in the absence of cardiac failure (non-cardiogenic pulmonary edema)

acute respiratory distress syndrome (ARDS) is a manifestation of severe ALI

Question 2

what is the histological manifestation of acute lung injury (ALI) and acute respiratory distress syndrome (ARDS)?

Answer 2

diffuse alveolar damage

*appears as diffuse bilateral infiltrates on radiographic examination

Question 3

describe the development of acute lung injury (ALI) / acute respiratory distress syndrome (ARDS)

{how does lung injury come about}

Answer 3

initiated by injury of pneumocytes and pulmonary endothelium

—> endothelial activation, secondary to pneumocyte injury or circulating inflammatory mediators
—> neutrophil migration and degranulation/release of proteases, ROS, cytokines
—> accumulation of intra-alveolar fluid and formation of hyaline membranes (protein-rich edema + dead alveolar epithelial cells)
—> resolution/type II pneumocyte proliferation (stem cells)

Question 4

why do the lungs become stiff in ALI/ARDS?

Answer 4

Damage to pneumocytes and pulmonary endothelium leads to loss of functional surfactant —> lungs become stiff (reduced compliance), leading to need for intubation and high ventilatory pressures to maintain adequate gas exchange

Question 5

How does chest radiograph imaging change when acute lung injury develops into acute respiratory distress syndrome?

Answer 5

ALI: diffuse bilateral infiltrates

ARDS: bilateral alveolar opacities

Question 6

how is the severity of hypoxemia in ARDS measured in clinical practice?

Answer 6

can be assessed with alveolar-arterial PO2 difference, but in clinical practice more commonly done by calculating the ratio of arterial PO2 and the inspired O2 fraction (P/F ratio)

diagnosis of ARDS requires P/F ratio below 300 (lower value = worse V/Q inequality)

Question 7

What pathological change is behind the marked fall in FRC in patients with ARDS?

Answer 7

increased recoil caused by alveolar edema and exudate that increases surface tension forces, plus decreased surfactant production —> stiff lungs, reduced compliance, fall in FRC (functional residual capacity)

Question 8

can high inspired oxygen concentrations help a patient with ARDS?

Answer 8

YES - hypoxemia is caused by ventilation perfusion inequality, and shunt

(lungs are very stiff, fall in FRC, increased recoil/decreased compliance)

Question 9

What is the pathophysiology behind hypercapnia developing in early ARDS?

Answer 9

Microvascular occlusion reduces pulmonary arterial blood flow to ventilated portions of the lung —> increased dead space —> increase PCO2