Pathophysiology during acute/exudate phase of ARDS/ALI

Phases of injury

  1. Exudative – Characterized by acute development of decreased pulm compliance and arterial hypoxemia
  2. Fibroproliferative – Characterized by increased alveolar dead space fraction, chronic inflammation, and scarring of alveolar-capillary unit
  3. Recovery – Characterized by restoration of the alveolar epithelial barrier, gradual improvement in pulm compliance, resolution of arterial hypoxemia, and eventual return of pulm function

Clinical presentation

First signs are tachypnea, dyspnea, agitation and hypoxemia. May occur over hrs to 1-5 days. As lung becomes edematous and consolidated, tachypnea and hypoxemia are caused by progressive restrictive lung dz and muscle fatigue.

CXR w/ diffuse opacities +/- small effusion.
CT w/ dense regions in dependent areas reflecting collapse of edematous lung/ w secondary atelectasis. Aerated regions prevail in non-dependent areas. PFTs show decreased FRC. Total lung compliance is ↓’d but some regions may be normal.

Hypoxemia results from intrapulmonary shunting and V/Q mismatch. In fibroproliferative phase, lung compliance is reduced by progressive lung fibrosis. PEEP effects on oxygenation are less impressive. CO2 retention is common.
Significant bronchoreactivity can be seen post-recovery. Muscle wasting and weakness are most prominent extrapulmonary complications.

Figures

Oxygenation/ventilation strategies

Lung protective strategies

PEEP – High PEEP, used to maximize alveolar recruitment, improve oxygenation insufficiency, and minimize need for O2. Can cause overdistension and hemodynamic compromise if too high.

TIDAL VOLUME – Low TV, 6-8cc/kg.

PERMISSIVE HYPERCAPNIA – Well tolerated at pCO2 levels of 65-85mmHg, pH < 7.25.

PRONE POSITIONING – Recruits atelectatic dependent zones.

Adjunct therapies

HFOV – Extreme form of low tidal volume.

iNO – Relaxes pulmonary vascular smooth muscles.

Surfactant replacement – May improve compliance and reduce intrapulmonary shunting

Corticosteroids – Used in most severe forms of ARDS.

ECMO – Will provide gas exchange and circulatory support in life-threatening cases, rescue until ARDS resolves.