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INDICATIONS, LIMITATIONS AND SAFETY
INDICATIONS
Lung US is indicated in any patient presenting with cardiopulmonary symptoms, including cardiac arrest.
LIMITATIONS
Subcutaneous emphysema and dressings may limit US evaluation of the lung.
Lung US involves pattern recognition and requires clinical judgement to interpret the findings alongside the patient's presentation. In some instances the lung US may only narrow the differential diagnosis and further imaging will be required.
SAFETY
In all tissues, ultrasound has mechanical and thermal effects (1). Mechanical effects occur as the US waves pass through the tissues causing oscillation of particles and cavitation. The thermal effect is due to the release of heat as the US wave loses energy to the tissues.
The lung, being an air filled structure is most vulnerable to the mechanical effects of US. In extreme cases (demonstrated in animal studies) the mechanical effects of US can lead to pulmonary haemorrhage (2).
The risk of mechanical effects increases with prolonged scanning and an increased mechanical index (MI). Human modelling studies have shown that haemorrhage does not occur with a US frequency >1.69 and a MI <1.9. Standard lung and abdominal setting always have a MI < 1.9.
However, with this in mind, use the ALARA (As Little As Reasonably Achievable) principle during lung scanning:
1. the shortest duration of scanning
2. the highest frequency
3. the least power: B mode
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REFERENCES
1. American Institute of Ultrasound in Medicine. Section 4--bioeffects in tissues with gas bodies. J Ultrasound Med. 2000a;19:97-108. 154-68.
2. Church CC, O'Brien WD., Jr Evaluation of the threshold for lung hemorrhage by diagnostic ultrasound and a proposed new safety index. Ultrasound Med Biol. 2007;33:810-818.