PHRENIC NERVE · AFTER A BLOCK
Diaphragm ultrasound
When to use it
- Check the phrenic nerve before and after interscalene, supraclavicular, superior trunk and cervical plexus blocks, especially in patients with poor lung function.
- Explain breathlessness after a block, and see weakness or paralysis of one hemidiaphragm after cardiac or neck surgery.
- Assess diaphragm function before weaning from ventilation.
- Probe
- Curved, low frequency
- Position
- Supine or semi-sitting, same position before and after
- Depth
- 12–16 cm
How to scan
- Right side: probe below the costal margin in the mid-clavicular or anterior axillary line, aimed up and back towards the dome. Use the liver as a window. Left side: use the spleen, more laterally.
- Find the diaphragm: a bright curved line deep to the liver. It moves towards the probe as the patient breathes in.
- Switch on M-mode and put the line across the diaphragm where it moves most. Measure the height of the wave during quiet breathing, a sniff and a deep breath.
- Compare with the other side, or with the same side before the block.
Right subcostal view through the liver. The M-mode line crosses the diaphragm; the inset shows its movement with breathing.
- Liver
- the liver is the acoustic window on the right; use the spleen on the left
- Diaphragm
- bright curved line deep to the liver; it moves towards the probe as the patient breathes in
- HV
- hepatic vein
- M-mode
- put the M-mode line across the diaphragm where it moves most; measure the height of each wave (excursion, cm)
Original schematic illustration © Nerve Blocks. Not to scale.
What you see and what it means
| On the screen | What it means |
|---|---|
| Quiet breathing: about 1 cm or more | Normal lower limit (about 0.9 cm in women, 1 cm in men) |
| Sniff: about 1.8 cm or more | Normal lower limit (about 1.6 cm in women, 1.8 cm in men) |
| Deep breath: 4–5 cm or more | Normal lower limit (3.7 cm in women, 4.7 cm in men) |
| Excursion falls by 25–75% after a block | Partial hemidiaphragmatic paresis, as defined in many block studies |
| Falls by over 75%, none, or moves the wrong way on sniffing | Complete paresis: paradoxical movement means paralysis |
Pearls
- Measure the same way, in the same position, before and after the block. Changes matter more than single numbers.
- The sniff test shows paralysis best: a paralysed diaphragm is sucked up (away from the probe) when the patient sniffs.
- Phrenic paresis is common after interscalene blocks, even with low volumes. Ultrasound-guided low-volume blocks reduce it.
References
- Boussuges A, Gole Y, Blanc P. Diaphragmatic motion studied by M-mode ultrasonography: methods, reproducibility, and normal values. Chest 2009;135:391–400. PubMed
- Renes SH, Rettig HC, Gielen MJ, et al. Ultrasound-guided low-dose interscalene brachial plexus block reduces the incidence of hemidiaphragmatic paresis. Reg Anesth Pain Med 2009;34:498–502. PubMed
