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US Guided Thoracentesis | Paracentesis

In this chapter we will take a look at the use of ultrasound guided thoracentesis and paracentesis. This chapter assumes you know how to use ultrasound of the lung for diagnostic purposes so head out here in case you may have missed it. Some of the equipment and steps are similar for both procedures but we will deal with them separately in this section.


US Guided Thoracenthesis

Thoracentesis is the removal of pleural fluid from the pleural space in the thoracic cavity and can be done for either diagnostic or therapeutic reasons. Ultrasound has several clinical implications when used to guide the procedure including:

1. Ultrasound (US) can be used to identify and guide the percutaneous removal of pleural fluid. The technology allows identification of small amounts fluid (<30ml) that cannot be identified with the use of radiography which typically detects volumes in excess of 50ml of fluid. Ultrasound can also identify loculations more readily than with radiography. 

2. US should be used to guide thoracentesis to reduce the risk of complications, the most common being pneumothorax. It does this by identifying patients in whom thoracentesis cannot be safely performed, allowing selection of the safest needle insertion site and revealing the optimal depth of needle insertion. 

3. US should be used to increase the success rate of thoracentesis. It's use has lower rates of failed attempts. 

Contraindications to thoracentesis include severe bleeding diathesis, skin infection or wound over the needle insertion site. 


Anatomic considerations Thoracentesis

The space between the parietal and visceral pleura is the pleural space and the location of pleural effusions. It is this space that we need to access with thoracentesis. The pleural space extends inferiorly to approximately the 10th intercostal space. The intercostal  neurovascular bundle runs along the inferior border of the rib (as shown in the diagram to the right) and should be avoided when performing the thoracentesis.


Internal  intercostal  muscle 

Parietal  Pleura 

Visceral  Pleura 

Diagram showing a section of an intercostal space and relevant structures.

Safe limits for thoracentesis 

Woman with Bare Back

A safe region for thoracentesis is the area superior to the 9th rib (due to diaphragmatic excursion), 5cm lateral to the spine and posterior to the mid axillary line (area shown on the picture to the left).

Safe region for thoracentesis. See text


Equipment and Technique

To locate the entry point of the needle for thoracentesis a low frequency (1-5 Mhz) probe work best by allowing us to look at deeper structures. A curved linear and a cardiac (phased array) probe would work best for this. The advantage of the curved linear probe would be that the lung sliding would be easier to visualize than with a cardiac probe. However, it has a larger probe footprint and thus real time guidance needle guidance may be challenging to perform with this probe.

A linear probe is useful if the procedure is intended to be done using real time ultasound. 

Example of low frequency probes that can be used for thoracentesis. Once these probes have been initially used to confirm potential needle locations for drainage, a linear probe can then be used if real time ultrasound is warranted.

Probe selection

Thoracentesis equipment

The provider should have the necessary equipment before attempting thoracentesis. Several commercially available kits are available that contain items needed for the thoracentesis (see picture below). Most kits contain a 8-Fr over-the-needle catheter (for therapeutic purposes), an 18-gauge needle (for diagnostic purposes), a stopcock and a 35 to 60ml syringe with a thoracentesis drainage system. You may need an ultrasound probe cover with sterile gel since it is typically not included in the kit.


8 F over an 18G Needle

18 G Needle

An example of a thoracentesis kit. This is the Arrow-Clarke Pleural Seal Kit.

Thoracentesis Steps

Thoracentesis steps summary.

The following is a summary of the steps to guide you in performing thoracentesis after positioning the patient sitting upright on the edge of the bed with arms extended. :

 2Sterile prep and drape. Follow full barrier precautions and prepare the surgical area is prepped with chlorhexidine (if not allergic) for 60 seconds. The ultrasound probe can also be used to guide the needle as it goes into the thorax. If this is the case then the probe should also be covered with a sterile cover. 

 3.  Apply local anesthesia. The proposed entry site from step 1 should have resulted in an indentation of the skin by firmly pressing the skin. Local anesthesia should be given at the level of the skin by creating a skin wheel at this site.

 1 Pre procedural scanning. We start by evaluating and mark the location of the site of insertion before an actual attempt is made with the use of ultrasound. This critical step ensures the correct side and amount of fluid that is meant to be sampled or collected. It also ensures that other organs have been accounted for and out of the way of the needle pathway. Remember the angle of approach, which is the angle between the probe and the skin. The thoracentesis site should be in the mid scapular or posterior axillary line and one or two intercostal levels below the highest level of effusion. We will be exploring this step further in the section on sonoanatomy.


Woman with Bare Back_edited.jpg

Pre Procedural scanning. 

 4 Needle insertion. In order to minimize complications and to gain access to the thoracic cavity a special consideration must be made regarding the location of the entry site. The provider needs to avoid the neurovascular bundle that runs on the underside (below the rib) of the interspace being punctured (seen on the image). The needle is advanced while performing negative aspiration on the plunger and is advanced until there is fluid on the syringe or loss of resistance. The recommendation here is to use the same angle of approach as that used with the probe during the pre procedural phase. The providers will have two options here:


 4iIf the effusion is deemed large enough the provider may continue with the procedure without the additional use of ultrasound.

 4iiIf the effusion is small or confirmation of the target space is needed then real time ultrasound is use to guide the needle into the thoracic cavity. 

Diagram showing a section of an intercostal space and a pleural effusion. Notice the insertion site of the needle at the intercostal space.

 5 Advance catheter. Negative aspiration has produced pleural fluid coming into the syringe as a results of the steps above. We can now collect fluid for sampling. If using a catheter over the needle then it is advanced and secured and then have the collection bag placed. 

 6 Confirm normal lung sliding with Ultrasound. Confirmation of normal lung sliding can be used to rule out pneumothorax pre and postprocedure. Post procedure chest radiography may be considered in those patients who are mechanically ventilated especially if they present with high airway pressures. 


Sonographic anatomy for thoracentesis

US should be used to identify the chest wall, pleura, diaphragm, lung and sub diaphragmatic organ before selecting a needle insertion site. Lets explore these structures in detail.

Thoracentesis US

Right diaphragm and intra-abdominal organs.


In these clips we see a side by side comparison of the windows we obtain by moving the ultrasound probe from the right lower back moving upwards in order to identify the diaphragm. On the right side we should see the liver and the right kidney and as we move the probe more cephalad, the diaphragm appears. We can also appreciate the acoustic shadows cast by the ribs that obscure the visualization of deeper structures.

Woman with Bare Back_edited.jpg










Right lower chest. Movement of the probe cephalad (from position 1 to position 2 which lies more posterior and higher up) and corresponding windows on the right. LS, lung sliding; L, liver; D, diaphragm; K, kidney. Notice the curtain effect which is an artifact seen when the diaphragm contracts which then lowers the lung into the US window and since there is normal lung sliding, this will in turn obscure deeper structures including the diaphragm.  

Right diaphragm and intraabdominal organs.

In these clips we see a side by side comparison of the windows we obtain by moving the ultrasound probe from the left lower back moving upwards in order to identify the diaphragm. On the left side we should see the spleen and the left kidney and as we move the probe more cephalad, the diaphragm appears. We can also appreciate the acoustic shadows cast by the ribs that obscure the visualization of deeper structures. Notice that we see a curtain effect on this side and we cannot visualize the diaphragm directly. This is because at ultrasound beam has encountered lung sliding more proximal to the probe and thus sound waves are not able to penetrate deeper into the tissue. 

Woman with Bare Back