Ultrasound better than Chest X ray for post- central venous catheterization screening

central vein usg

Central venous catheterization is a common procedure done in the critical care units. The common indications include venous access, drug delivery, CVP measurement, total parenteral nutrition, shock management etc. As with any medical procedures, complication may occur after central venous cannulation.  Catheter tip misplacement, pneumothorax, arterial puncture, neural injury, hematoma are some of the reported complications. Catheter tip misplacement can cause thrombosis, arrhythmias, venous and right atrial perforations. Hence catheter tip should not be placed in or allowed to migrate into the heart. Early screening with chest X ray is done routinely to rule out pneumothorax and catheter tip misplacement. 

But it has been well documented that supine chest X rays have poor sensitivity in detecting small pneumothoraces. Since most of the chest X rays are done within one hour of the procedure, the pneumothorax is likely to be small at that time and the pleural air may preferentially move to the non-dependent anterior and medial part, which are difficult to detect in supine chest X ray. Futhermore the anatomical landmarks available in the chest X ray cannot accurately predict the catheter tip position in significant number of cases. 

he chest X rays have other disadvantages including 

Radiation exposure to the patient and healthcare personnel in ICU

Time delay in obtaining chest X ray compared to the easier bedside ultrasound

Studies have documented the overall cost of obtaining a chest X ray to be higher than bedside ultrasound. 

The absence of radiation, bedside availability, high sensitivity and specificity, real time images and possibility of repeat examination makes ultrasound a better radiological tool than chest X ray in intensive care units. Studies have shown close concordance between results obtained from ultrasound and chest radiography in detecting pneumothorax and catheter tip placement. Hence ultrasound can be used as an initial screening tool post central vein catheterization and chest X ray can be done in selected cases where ultrasound is deemed inadequate. 

Detecting Pneumothorax

The linear ultrasound probe is used to look for the lung sliding in the anterior and lateral intercostal spaces. For effective assessment the lung sliding on the particular hemithorax can be noted even prior to the central venous cannula insertion and then compared post-procedure. The presence of lung sliding and comet tail artefacts rules out pneumothorax. The sensitivity of ultrasound in detecting even small pneumothorax is very high. The absence of both lung sliding and comet tail artefacts suggests pneumothorax in greater than 95% of cases. 

Detecting Catheter tip position

The ultrasound probe is placed in the sub-costal/epigastric acoustic window along the short axis of the heart. The right atrium, SVC and IVC are easily visualized in this window.  The presence of the catheter tip in either of these vascular structures is explored in B mode initially. The catheter is identified as a line of hyperechoic dots in B mode. Color doppler can be done to enhance identification while injecting 3-5 ml of saline solution. In addition contrast enhanced ultrasound has been shown to increase detection rate.  To obtain contrast, 5 ml of agitated saline-air mixture (4.5 ml of saline and 0.5 ml of air) is injected through the catheter as a bolus solution. The characteristics and the time delay of the bubbles are noted in the ultrasound image. Numerous bubbles with a linear flow coming from the SVC within 2 seconds indicate correct catheter tip positioning. If the bubbles originate in the right atrium/IVC/right ventricle or if there is a significant delay before the bubbles appear or if only few bubbles can be visualized, then it implies that the catheter tip position is inappropriate. 

Limitations

The acoustic window and visualization may be difficult in certain patients. Artefacts may interfere with correct identification of the catheter tip. 

Date: 
Thursday, November 28, 2013