White Lung on CXR
We didn’t. There was slight tracheal deviation towards the patient’s right, towards the side of the white lung, unusual for a massive pleural effusion. Instead, we sound the patient’s right upper quadrant, and saw this:
If we had attempted a pleural tap earlier, we would technically ended up doing a lung biopsy. The right lung is completely non-aerated in the clip. Instead of air, a large solid hyperechoic mass now sits on top of the diaphragm.
Being tissue in nature, like the liver, the mass allows transmission of ultrasound waves. Which is why the vertebral column (seen as a thick, bright, discontinuous horizontal line at the 12-13cm mark) is now clearly visible above the diaphragm.
Indeed, if the lung above the diaphragm is aerated, the normally present vertebral column above the diaphragm will be normally absent on ultrasound, since it is entirely erased by the mirror image of the liver.
The diaphragm is usually described as a “bright” reflector. That’s not exactly correct. It is the diaphragm-air interface, where the huge acoustic impedance mismatch is, that is bright. The diaphragm is a piece of muscle, and should look like one – usually relatively dark. In this video clip, you can see the “muscle” diaphragm sandwiched between two brighter lines: pleural and peritoneum.
So much for anatomy and pathology. There is also some physiology observable here.
Notice the diaphragm in action? The muscle thickens to contract.
In fact, there is interest in looking at the diaphragm thickening as a predictive index of weaning from mechanical ventilation. This is how it's done:
Position: between anterior and mid-axillary line, 0.5-2cm below the costophrenic sinus (usually around 8-9th intercostal space)
At this position, i.e. zone of apposition of the muscle, you can see both sides of the diaphragm. The patient then inhale and exhale maximally, to visualize the maximal diphragmatic excursion and therefore thickening. One small study found that using diaphragm thickening fraction (DTF) cut off at 36%, it predicts a successful spontaneously breathing trial with a sensitivity of 0.82 and specificity of 0.88, with an AUC of 0.948 (95% CI 0.89-1.00). Awaiting validation, it holds promise as a new tool to help us get our patients off the vent safely.
It may not be easy to elucidate the causes of the white-out lung on CXR
Use ultrasound to rule-in pleural effusion, especially before intervention.
Seeing the vertebral column above the diaphragm on ultrasound is always abnormal.
The diaphragm is not bright; the diaphragm-pleural and diaphragm-peritoneal interfaces are.
Ferrari G, De Filippi G, Elia F, Panero F, Volpicelli G, Aprà F. Diaphragm ultrasound as a new index of discontinuation from mechanical ventilation. Crit Ultrasound J. 2014 Jun 7;6(1):8.
Dr. Toh Hong Chuen
Acute and Emergency Care Centre, Khoo Teck Puat Hospital. Singapore