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# Fat, air and thromboembolism after injury

- Finding

After an injury, fat, air or blood clots can travel to the lungs or brain and kill; the autopsy must look for them on purpose.

Updated Nov 13, 2026

## Key points

- Fat embolism: after long-bone or pelvic fractures; syndrome after about one to three days; confirmed with fat stains on lung and brain.
- Some fat in lung vessels is common after any fracture or resuscitation; only a heavy load or brain involvement explains death.
- Air embolism: neck vein wounds, chest injury, procedures; show the air before opening the heart (radiograph or CT, or opening the heart under water).
- Pulmonary thromboembolism: days to weeks after injury and immobility; look for the source in the leg and pelvic veins.
- Bone marrow emboli in the lungs indicate fractures in life (including resuscitation fractures).

On this page

- The basics Key points
- What it is
- How to tell it apart
- How to recognise it
- How it forms
- Step by step
- Next step: what to do and document
- Common mistakes
- Elsewhere in DurgiPedia

- Study Test yourself

- Connections Connections

## What it is

Three kinds of embolism link an injury to a later death. Fat embolism follows fractures of long bones and pelvis, soft-tissue crushing and some procedures; fat droplets block small vessels of the lungs and brain, and the fat embolism syndrome appears about one to three days later. It is confirmed at autopsy with fat stains on lung and brain tissue. Air embolism follows wounds of the neck veins, chest injuries, and procedures such as central lines; air enters the veins and blocks the right heart, and it must be shown before the heart is opened. Pulmonary thromboembolism follows immobility after injury, usually days to weeks later, from clots in the leg or pelvic veins. Each links the death to the injury, but each also occurs without trauma, so the setting matters.

## How to tell it apart

- Fat must be stained on frozen sections; routine processing dissolves it.
- Grading systems quantify pulmonary fat; resuscitation can add some fat but rarely enough to explain death.
- Postmortem gas from decomposition can mimic air embolism; the test is valid only in fresh bodies.
- A thromboembolism after an injury and immobility is a complication of the injury; without injury it may be natural.

## How to recognise it

- Fat: optically empty droplets in lung capillaries on routine stains; orange or red with fat stains on frozen sections; brain petechiae in white matter.
- Air: frothy blood in the right heart, air in the neck veins, visible on postmortem imaging.
- Thrombus: a coiled clot in the pulmonary arteries; the source clot in the calf, thigh or pelvic veins.
- Bone marrow: fragments of marrow with fat cells in pulmonary vessels.

## How it forms

- Fat: droplets from broken marrow or crushed fat enter torn veins; free fatty acids also injure the lung vessels.
- Air: negative pressure in veins near the heart sucks air in; a large volume blocks the right ventricle.
- Thrombus: stasis, vessel injury and a clotting tendency after trauma (the Virchow triad).
- Through a patent foramen ovale, fat or air can reach the brain and other organs (paradoxical embolism).

## Step by step

- In suspected air embolism, obtain imaging first or open the chest and the heart under water to see escaping gas.
- Take unfixed lung and brain tissue for fat stains on frozen sections.
- Dissect the leg and pelvic veins to find the source of a pulmonary embolus.
- Record the date of injury, the immobilisation and any prevention of clots from the medical record.

## Next step: what to do and document

- Record fractures, their fixation and the date of first respiratory or neurological signs in an injured patient.
- Document central lines and other procedures that can let air in, with date and operator.
- Record immobilisation and clot prevention measures; they matter for causation if the patient dies of an embolus.
- On the death certificate, link the embolism to the original injury and report the death as violent.
- Tell the autopsy team when air embolism is suspected, so the heart is not opened in the usual way.

## Common mistakes

- Processing lung tissue routinely and missing the fat.
- Opening the heart before testing for air.
- Calling decomposition gas an air embolism.
- Certifying a post-injury pulmonary embolism as natural.

## Test yourself

Think of the answer, then tap the question to see it.

## How long after a long-bone fracture does fat embolism syndrome usually appear?

Usually one to three days after the injury.

## Why must fat stains be done on frozen sections?

Because routine tissue processing dissolves the fat and leaves only empty spaces.

## How is air embolism shown at autopsy?

By imaging before the autopsy, or by opening the chest and the heart under water to see gas escape, before the usual dissection.

## A trauma patient dies of a pulmonary embolus two weeks after a femoral fracture. Is the death natural?

No. The embolus is a complication of the injury and immobility, so the injury remains the underlying cause.

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## Elsewhere in DurgiPedia

- [Fat, air and amniotic fluid embolism](https://durgical.com/en/general-pathology/fat-air-and-amniotic-fluid-embolism/)
- [Pulmonary thromboembolism: where it comes from and what it does](https://durgical.com/en/general-pathology/pulmonary-thromboembolism-where-it-comes-from-and-what-it-does/)
- [Thrombosis and Virchow's triad](https://durgical.com/en/general-pathology/thrombosis-and-virchow-s-triad/)

## Connections

## Related 5

- [Complications of injury and the chain of causation](https://durgical.com/en/forensic-medicine/complications-of-injury-and-the-chain-of-causation/)
- [Chest injuries, aortic rupture and resuscitation artefacts](https://durgical.com/en/forensic-medicine/chest-injuries-aortic-rupture-and-resuscitation-artefacts/)
- [Cause, mechanism and manner of death](https://durgical.com/en/forensic-medicine/cause-mechanism-and-manner-of-death/)
- [The medico-legal autopsy: when it is mandatory in Colombia](https://durgical.com/en/forensic-medicine/the-medico-legal-autopsy-when-it-is-mandatory-in-colombia/)
- [Decomposition: autolysis and putrefaction](https://durgical.com/en/forensic-medicine/decomposition-autolysis-and-putrefaction/)

## Related conditions 6

- [Fat embolism syndrome](https://durgical.com/en/conditions/fat-embolism-syndrome/)
- [Pulmonary embolism](https://durgical.com/en/conditions/pulmonary-embolism/)
- [Deep vein thrombosis](https://durgical.com/en/conditions/deep-vein-thrombosis/)
- [Femoral shaft fracture](https://durgical.com/en/conditions/femoral-shaft-fracture/)
- [Pelvic fracture](https://durgical.com/en/conditions/pelvic-fracture/)
- [Acute respiratory distress syndrome (ARDS)](https://durgical.com/en/conditions/acute-respiratory-distress-syndrome/)

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Educational content on forensic and legal medicine; not legal advice and no substitute for the technical guides of Colombia's National Institute of Legal Medicine and Forensic Sciences. Laws change: each page links the official text and says when it was last checked. For a real case, follow the guide in force and ask the competent authority.

Sources: [Fat Embolism and Fat Embolism Syndrome](https://www.ncbi.nlm.nih.gov/books/NBK499885/) (StatPearls Publishing (NCBI Bookshelf), 2026); [Fat embolism: a systematic review to facilitate the development of standardised procedures in pathology](https://pmc.ncbi.nlm.nih.gov/articles/PMC11964584/) (Histopathology (PMC), 2025); [Forensic evidence from a 30-year study of non-thrombotic pulmonary embolism](https://pmc.ncbi.nlm.nih.gov/articles/PMC13658339/) (International Journal of Legal Medicine (PMC), 2026); [Fatal Consequences of Head Trauma: A Case Report of Venous Air Embolism Complicated by Substance Abuse and Liver Disease](https://pmc.ncbi.nlm.nih.gov/articles/PMC11570429/) (Cureus (PMC), 2024); [Deep Venous Thrombosis](https://www.ncbi.nlm.nih.gov/books/NBK507708/) (StatPearls Publishing (NCBI Bookshelf), 2023).

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/forensic-medicine/fat-air-and-thromboembolism-after-injury/