For some reason, this complication of cardiothoracic surgery has merited a whole 10-mark SAQ- specifically, Question 8 from the first paper of 2002. It has then been forgotten for about 13-14 years.
Course of the phrenic nerve
This image from Wikipedia illustrates it nicely.
According to a cadaver study, in about 80% of the population the left phrenic nerve passes over the obtuse cardiac margin and the left obtuse marginal vein and artery, where it is well out of the way. In the remaining 20%, it passes anteriorly over the left main coronary artery, where it becomes very inconvenient.
Causes of perioperative phrenic nerve injury
There are a few ways to damage the phrenic nerve during cardiac surgery:
- Sever it completely (hence difficult dissection makes it more likely)
- Put pressure on it accidentally (hence excessive retraction is to blame)
- Topical cardiac cooling (the icy slush causes a neuropraxia)
- Internal jugular vein haematoma following cannulation
- Unequal chest expansion
- Atelectasis on the affected side
- An elevated hemidiaphragm on CXR
- Difficult ventilation/weaning
- Poor inspiratory effort and poor cough
- With inspiration, the paralysed diaphragm paradoxically retracts upwards.
Management is supportive. One has no choice but to continue ventilator support until it resolves.
In the long term, unresolved phrenic nerve palsy may require diaphragmatic plication, which is a very old-school procedure. Generally speaking, this is not a procedure which is likely to result in a more rapid wean from the ventilator - rather, it is reserved for patients who are symptomatic in the long term.