Tasty Morsels of Critical Care 038 | Lung Transplantation

26 Apr

Welcome back to the tasty morsels of critical care podcast. A meandering monologue through critical care fellowship exam preparation.

Oh’s Manual chapter 104 has a decent chapter on the intensive care aspects of lung transplant. A lot of this stuff will end up fairly centre specific but there is a good general overview to be had here.

Lung transplants do reasonably well overall (most likely due to careful selection) with a median survival of 7 years and 80% alive at one year. Donation after cardiac death is becoming an increasing source of donor organs and the lungs seem relatively tolerant of the short warm ischaemic time (though kidneys are still the best overall).

Who might be a candidate for lung transplantation? The generic phrase is “advanced, non malignant lung disease, for which there is no alternative therapy” However a list of common potential indications might go as follows

  • advanced non malignant lung disease without alternative
  • COPD/Alpha1 antitrypsin deficiency
  • Pulm fibrosis (single lung Tx still done for this sometimes)
  • CF
  • Bronchiectasis
  • Pulmonary vascular disease

The other major component of patient selection is how well they’re going to tolerate the overall burden of surgery, intensive care and the long term prospects of immunosuppression. As a result candidates are typically fairly stable with severe disease and generally admitted from home unlike the liver transplant patient who may well get their transplant in the midst of multi-organ failure. There are exceptions such as using VV-ECMO as a bridge to transplant akin to an LVAD as a bridge to transplant but these should be considered as truly exceptional.

The potential donor is typically split into one of two categories

  • standard donor
  • high risk (marginal) donor

These are distinguished by obvious factors such as age and smoking history and the state of the donor lung at the time of procurement.

As someone without an anaesthesia background, I have little insight or involvement with the procurement process but I do find the recent development of ex-vivo perfusion devices to be fascinating. These are now recently developed for the lungs which allows donor lungs to be perfused and even optimised prior to implantation.

The surgical technique used is typically a sequential single lung transplant, one at a time with individual bronchial anastamoses rather than a complete “en-bloc” technique with an anastamosis at the trachea. If the native lungs are good enough to tolerate single lung ventilation then it can be done without bypass but the second lung is obviously dependant on immediately effective perfusion and gas exchange in the first implanted lung to allow the second lung to be implanted. This is certainly outside the realm of intensive care but remains a somewhat magical act that my surgical and anaesthetistic colleagues perform in the operating room.

Post op is where intensive care is subbed onto the field to take over the physiology of the fairly roughed up transplant patient. There are a range of potential issues to cover that are probably best split up by system.

From a respiratory perspective the grafted lungs are denervated and therefore there isn’t the usual cough and mucociliary clearance at least in the early phases. The general idea is to wake and wean and extubate but ~15% need prolonged ventilation.

Single lung transplant can cause a lot more issues as the two lungs can often have markedly different respiratory dynamics and will experience the pressures from the ventilator very differently. The same discrepancy between lungs is also experienced by the right heart which continues to pump expecting the long standing increased PVR it’s used to and now once blood gets into the pulmonary trunk it is faced with either an obstructive, high resistance PA on one side and nice low resistance system in the other PA. This can end up with the new lung receiving a disproportionate amount of the cardiac output that can flood the lung. Indeed the right heart is one of the major causes for concern overall here.

Immunosuppression typically comes with some early basiliximab and ultimately an antimetabolite and a calcineurin inhibitor.

In terms of infection, prophylaxis plays a big role as the lungs are more exposed to the manky, dirty outside world than most of the other transplanted organs. You will typically have some sputum cultures from the donor already growing before the transplant so that can help guide your antimicrobials. PJP prophylaxis will be needed but there’s no great rush. CMV is needed for all but as usual mismatched donors/recipients have the highest risk.

Primary graft dysfunction typically presents as worsening gas exchange, decreased compliance and alveolar infiltrates. Ischaemia reperfusion injury is a common cause. It is non immune mediated and an important cause of early mortality and causes long term morbidity. It’s worth noting that it’s a recognised indication for VV-ECMO.

Other things to look for in the early stages are anastamotic ischaemia (the bronchial arteries are typically not anastamosed so perfusion is not as good as native). This occurs in ~2%. Phrenic nerve palsy is an understandable complication in ~5%. A fairly niche one is pulmonary venous kinking where one of the newly anastamosed pulmonary veins has got kinked in the process. TOE or a CTA can see this.

Acute rejection on the other hand is a delayed phenomenon with lots of changes that can look like infection and indeed both can happen at the same time. Ultimately you need a biopsy and once you find it, it’s big doses of pulsed steroids a treatment.

Post transplant lymphproliferative disorder (PTLD) can occur in 5-10% of lung transplants (much higher than other solid organ transplants). Usually this is with EBV infections with B cell proliferation which is unopposed as the T cells are suppressed by the immunosuppression.

References:

Oh’s Manual chapter 104

LITFL

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