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6 out of 7

In ARDS, not true is – (DNB June 2009)

A Decreased pulmonary compliance

B Normal pulmonary artery wedge pressure

C Increased left atrial pressure

D Severe hypoxemia

Ans. C

Increased left atrial pressure

Clinical physiology of ARDS

Clinical history of insult known to cause ARDS there is sudden onset of dyspnea

This is clue to Accumulation of proteinaceous pulmonary edema in the interstitium.

The diagnosis of ARDS is made on clinical grounds on the following criteria.

1). Acute onset

2). Bilateral pulmonary infiltrate on chest X-ray.

3). Pulmonary artery wedge pressure < 18mm/Hg (i.e. LA pressure is normal).

4). A partial pressure of arterial oxygen to fractional inspired oxygen concentration (PaO2/FiO2) <200mmHg.

Extra Edge : Left atrial (LA) pressure is normal in ARDS, (LA pressure is high in cardiac pulmonary edema) (MCQ)

Pathophysiology of ARDS

1). Diffuse alveolar damage.

2). Reduced surfactant (Injury to type II pneumocytes)

3). Increased permeability of alveolar capillary membrane.

4). Due to damage to the normally tight alveolar barrier, edema fluid rich in protein, accumulates, in the interstitial and alveolar spaces (non cardiogenic pulmonary edema).

5). The proteins then aggregate in air spaces with cellular debris and dysfunctional pulmonary surfactant to form hyaline membrane whorls.

6). It leads to O2 diffusion from alveolus to capillary but CO2 being a water soluble gas can diffuse from capillary to alveolus. It leads to acute type I respiratory failure.

7). Pulmonary edema + Hyaline membrane (fibrin + coagulated cell debris)

8). Reduced compliance (stiff lung)

9). Diminished tidal volume

10). Hypoxemia (Worsened gas exchange d/t pulmonary shunting)

11). Hypocapnia (CO2 washout)

Extra Edge: Pulmonary Shunting:

1). Significant shunt occurs in ARDS where pulmonary arterial blood shunts past flooded and collapsed Alveoli into pulmonary veins.

2). Shunting of the poorly oxygenated venous blood into arterial circulation significantly contributes to arterial hypoxemia.

3). Reduced compliance: Reduced lung compliance and stiff lung are characteristic features of ARDS

4). Hypoxemia: Arterial hypoxemia is a characteristic feature of ARDS and may be caused by reduced lung compliance and pulmonary shunting.

HAPE: (High Altitude pulmonary edema)

1). Pulmonary vasoconstriction is as the prime mechanism

2). May occur in both acclimatized and unacclamatized individuals

3). HAPE is exacerbated by exercise and is associated with high cardiac output.