Treating fluid collections and abscesses

Selecting collections to drain

The ubiquitous nature of computed tomography (CT) chest/abdo/pelvis scanning in most hospitals means that fluid collections are frequently discovered, particularly in postoperative patients. There are a number of reasons that a fluid collection may be present; haematoma, lymphocele or simple serous fluid for example. Drainage is indicated if there is infection or significant mass effect, e.g. large pleural effusion.

Evidence of an infected collection

• A defined fluid collection with a wall

• Enhancement of the wall

• Gas within the fluid (check it is not a bowel loop!)

• Hounsfield unit (HU) >5 – collections with very low HU are likely to be simple fluid.

Review the CT and look for evidence of infection elsewhere, it is not rare to be asked to drain a small pelvic collection in a patient with a raging pneumonia. A direct discussion with the referring clinician is essential. Draining every collection referred will do patients a disservice as you will almost certainly infect some sterile fluid collections and will certainly have exposed the patients to the risk of drainage without the benefits.

Preparation

Check coagulation and correct coagulopathy. Ensure the patient has received appropriate antibiotic cover, which depends on the site/source of infection – you are likely to make them bacteraemic during the drainage. Abscess drainage can be painful, so get effective local anaesthesia and use parenteral analgesia as required.

Selecting image guidance

Usually, the abscess will have been demonstrated on CT. The CT is invaluable in defining the relationship of the abscess to other structures. Try to plan a route that will allow you to use ultrasound guidance for the actual drainage. That means a route away from bowel, etc. Ultrasound is quicker, allows real-time visualization and does not get you irradiated. If bowel or other tricky obstacles are in the way, then use CT – ideally with CT fluoroscopy.

Diagnostic aspiration

Diagnostic aspiration tends to be used when the nature of the collection is unclear or when there is a small collection that is very difficult to drain. A diagnostic aspirate gives bacterial sensitivity and allegedly reduces the abscess wall tension, which helps antibiotic penetration. Always undertake aspiration under sterile conditions and avoid traversing bowel as there is a real opportunity to inoculate and infect a collection.

In theory, we should tell you to always start with a 20G needle but, in reality, unless the collection is in tiger country, most operators will use an 18G needle – it is easier to steer and you can aspirate thick pus through it. If you use a 20G needle and cannot aspirate then a further puncture is usually needed. If possible, plan the aspiration so that the route is suitable for drainage as if you aspirate pus that is usually the next step.

Tip

The aspirate is not a fine wine; there is nothing to be gained from sniffing it, gagging and suggesting ‘anaerobes’!

Drainage

Drainage is all about getting gravity on your side, so plan the drainage route so that the shortest access is achieved that will result in a drain in a dependent position and preferably with the drain hub below the level of the collection.

Think about the content of the collection. Ultrasound offers the most useful assessment of the viscosity of the collection and the presence of loculi and septa. If the collection is loculated, then target the largest locule first. Look for the following features, which will influence the type and number of drains required:

• Anechoic collection – probably clear fluid, e.g. urinoma

• Few scattered echoes – turbid fluid, e.g. thin pus

• Extensive or swirling echoes – thick fluid, e.g. viscous pus

• Diffuse echoes with gas – organized abscess or phlegmon.

Tip

The ultrasound appearances are not an infallible guide and occasionally will be misleading – always try a diagnostic aspiration.