Pathogenesis of lung injury
Fat embolism is underpinned by a post-traumatic increase in blood lipase levels. This triggers a cascade of reactions leading to alveolar-interstitial edema. Damaging factors attack the pulmonary vascular endothelium and pneumocytes, and fibrin degradation products accumulate.
The consequence of this is impaired surfactant production, development of interstitial edema, and bronchospasm. Pressure in the lesser circulation rises (pulmonary hypertension), and the lung parenchyma itself loses its elastic properties. The work of breathing increases, along with the volume of dead space.
On days 1–2, the alveoli fill with exudative fluid containing protein, fibrin, erythrocytes, and leukocytes. Moist rales appear, and persistent hypoxia develops due to impaired alveolar gas diffusion.
Clinical markers (triad)
The classical triad of signs occurring after fractures (especially of the tibia and pelvis) allows for a high probability of suspecting the development of fat embolism:
- Development of severe acute respiratory failure (ARF).
- Persistent arterial hypoxemia ($pO_2$ < 65 mmHg) resistant to oxygen therapy.
- Mental disturbances up to a complete loss of consciousness.
Mechanisms of ARF compensation
The body attempts to cope with increasing hypoxia through two main pathways:
- Stimulation of external respiration. Dyspnea appears initially — energetically, it is more favorable for the body to first deepen inspiration and only then increase the respiratory rate (tachypnea ensues). However, it should be remembered: with excessive tachypnea, vital capacity (VC) and tidal volume plummet rapidly.
- Increased tissue $O_2$ utilization. Cells begin to absorb oxygen more actively due to an increase in its utilization coefficient and biochemical shifts that facilitate oxyhemoglobin dissociation directly in the tissues.
Stages of acute respiratory failure
Compensated stage:
- Skin is pale.
- Respiratory rate does not exceed 35 breaths per minute, accessory muscles participate moderately in the act of breathing.
- Auscultation reveals scattered dry rales.
- Hemodynamics: tachycardia up to 100 bpm.
Decompensated stage (severe condition):
- Sharp pallor gives way to pronounced cyanosis of the skin and nail beds.
- Respiratory rate reaches 40 or more per minute. The nasal alae and accessory muscles work actively.
- Auscultation reveals coarse dry and moist rales, noticeable even from a distance.
- Hemodynamics is unstable: pronounced tachycardia, frequent arrhythmias, and hypotension.
Triad of clinical markers: Fracture (tibia/pelvis) ➔ Drop in oxygen (<65) ➔ Mental clouding (psychics).
Class and exam questions
Why might ARF proceed without cyanosis in the presence of concomitant massive blood loss?
For clinically visible cyanosis to appear, a reduced hemoglobin concentration of at least 5% is required. In massive acute blood loss, this is often unachievable due to the absolute loss of hemoglobin along with the lost blood.
Which diagnostic method helps detect ARF at an early stage?
Chest X-ray examination. It allows for the detection of early signs such as enhanced vascular markings and phenomena of venous hypertension.
Is hypercapnia observed in ARF against the background of trauma?
No, in trauma, hypercapnia (excess carbon dioxide in the blood) is observed quite rarely, unlike pronounced arterial hypoxemia.
What else to study on this topic
- Principles of infusion-transfusion therapy and restoration of circulating blood volume in shock
- Timing of surgical interventions in traumatic disease
- Features of acid-base balance monitoring (why alkalosis develops)
- Pathogenetic treatment of obstructive and redistribution shock
- Prevention and proactive treatment of complications (wound infection, pneumonia)
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