Comprehensive guide to ultrasound-assisted vascular cannulation techniques
Ultrasound aids placement of central lines as well as peripheral lines, both in terms of speed of access and reduction in complications. The aim is twofold:
Ultrasound guidance significantly improves success rates for vascular access while reducing complications such as arterial puncture, hematoma formation, and pneumothorax.
Ultrasound should be used at all times, unless time-critical intervention mandates otherwise (e.g., in cardiac arrest).
Ultrasound is a very useful adjunct for femoral vein cannulation.
Ultrasound may be used when conventional access fails (e.g., in an ill patient who is shut-down, or in an intravenous drug user whose veins are damaged).
The difference between veins and arteries can be determined by compressibility - veins compress, arteries do not.
Arteries tend to be circular in transverse view, whereas veins are often oval.
If color flow is available, it can be utilized to determine flow direction. With the probe angled so that the flow is marginally towards or away from the probe, the color flow convention is: blue = away and red = towards (B.A.R.T.).
A linear, high frequency probe should be selected. The machine should be set to a depth of around 5 cm for central access, 4 cm for femoral and 3 cm for basilic cannulation. The time gain compensation (TGC) should be flat. In all cases avoid zoom or the skin surface will not be seen.
This is a function of the physical characteristics of a linear probe. The transducer array will effectively only 'see' a needle if it sits within the tolerances of the lateral and slice thickness resolution. This means that the operator needs to keep the needle within the very narrow slice thickness of the probe, typically 5 mm or less.
If the angle of approach of the needle into the tissue is too steep, the reflected sound does not return to the probe, and consequently no image is seen. Angles less than 35 degrees to the skin are needed to see the needle, and the smaller the angle, the better the image.
Anatomic structures are identified and an insertion position identified with ultrasound and marked on the skin. The cannulation then proceeds as it would without ultrasound.
The ultrasound transducer is placed in a sterile covering and the procedure is performed with simultaneous ultrasound visualisation of the cannulation. In most cases, this is the recommended approach.
Allows recognition of the target vein, and differentiation from the artery by virtue of its compressibility. However, this view only allows the cross section of the needle to be visualised by the ultrasound beam and may lead to errors in depth perception. In addition the acoustic shadow can lead to confusion. Therefore, this view is not optimal for real-time guided cannulation.
Allows the operator to trace the entire path and angle of the needle from the entry site at the skin and into the vessel. Therefore, after obtaining the transverse view, the operator should rotate the probe to view the vein in longitudinal section (and check to see if it is still compressible) before starting cannulation.
For optimal results, begin with the short axis view to identify the target vessel and confirm its compressibility, then rotate to the long axis view to guide the needle insertion under direct visualization.
During training, initially the trainee should work with the trainer in the dual operator technique with the ultrasound carried out by the trainer and the cannulation by the trainee. This allows for skill development in both ultrasound manipulation and needle guidance.