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Empyema and Complicated Lung Infections - 9/13/2012

posted Sep 13, 2012, 2:52 PM by Rohit Das   [ updated Dec 27, 2012, 6:54 AM by Purnema Madahar ]

I wanted to go back in time and talk a bit about our topic for CRS last Friday – Empyema.

There are a number of reasons why pneumonias can progress and become complicated. Usually, this is related to misdiagnosis of the pathogen, a delayed presentation of an indolent pathogen, a resistant pathogen, and/or patient comorbidities. Though I don’t really want to go into this too much, noninfectious etiologies (BOOP, Wegener’s, certain neoplasms) can also mimic “nonresolving” pneumonias. 

Empyema is the far end of the spectrum of what we think of as complications from an initial bacterial pneumonia. Uncomplicated parapneumonic effusions occur in up to 40% of bacterial pneumonias, but progression past that is rare, as the large majority will resolve with appropriate antibiotic therapy. As a complication of community acquired pneumonia, empyema is exceedingly rare, occurring about 1% of the time. Let’s go through the spectrum of complications, which also correlate with evolving pathophysiology:

  • Uncomplicated effusion – Very common, due to increased interstitial lung fluid and movement across the visceral pleura. They are characterized by exudative findings (i.e., positive Light’s criteria), and resolve with resolution of the pneumonia invariably.
  • Complicated effusion/Empyema – These happen when bacteria invade the pleural space. Though they also have exudative characteristics, the presence of bacteria will lead to increased neutrophils in the fluid, as well as pleural acidosis. Empyema occurs when bacterial burden in the pleural space becomes increasingly evident – pus can be aspirated, and bacteria can be seen on gram stain.
On that note, quick review of pleural fluid analysis:
  • Transudate versus exudates is the main differentiation to make; this is where Light’s Criteria  comes into play (one of the two is good enough) – fluid protein/serum protein > 0.5 OR fluid LDH/serum LDH > 0.6.
  • Of note, though pleural protein is usually <3 in transudates, diuresis can elevate protein levels, mimicking an exudative process.
  • Very high triglycerides levels (>110) are very supportive of a chylothorax.
  • Glucose is also useful, as a low level, <60, is only seen in a few exudative processes – empyema, pleural TB, SLE/RA with pleuritis, and malignancy to name the most common. Adenosine deaminase levels are also high in TB pleurisy, and can help with that diagnosis.

What are the causative agents behind most complicated parapneumonic effusions and empyemas? In one retrospective case series that I found, 40% of cases were due to Strep. Pneumoniae and Staph – which make sense since these are common causes of bacterial PNA. However, 20-30% of cases were due to solely anaerobic organisms, usually in the context of some risk factor for aspiration. This reinforces a key point – the pneumonias that progress to empyema are often due to indolent pathogens (like anaerobes) that more often present with pneumonia-related complications. The majority of cases due to common community-acquired organisms present much earlier, before they get a chance to develop into something more complicated.

Regarding management – antibiotics are an obvious important aspect to treatment, and should be directed against the documented or suspected pathogen. More importantly, when do you need to more than that?

  • For the most part, uncomplicated pleural effusions that are small, flee flowing on imaging, and have a pH of 7.2 or greater can be observed and managed with just antibiotics.
  • Large effusions, loculated effusions (not free-flowing on imaging) and effusions with a pH of <7.2 are unlikely to get better with just antibiotics and require drainage.
  • Empyemas will require drainage invariably, and more often require thoracic surgery with VATS or open debridement/drainage.

Attached are two good review articles on this subject. The daily shall return Monday…have a good weekend!

Parapneumonic Effusion and Empyema

Hamm et. al., Eur Respir J 1997, Volume 10: 1150-56

Bartlett, Anaerobe 2012, Volume 18: 235-239