Ultrasound is the transmission and reception of high frequency sound waves by a crystal stimulated by electricity. Probes used in anesthesia may have many crystals, or a single one as in the case of U-Blok, but the principle is always the same: The sound waves leaving the crystal or crystals then propagate through tissues at different speeds and then return to the crystal to generate an electrical current which is then translated into the image that we see.
As sound travels through tissue they are either absorbed, reflected or allowed to pass through, depending on the tissue's echo density All ultrasound dissipates in tissue producing heat. Time between sending and receiving equals distance.
The frequency of the ultrasound waves is measured in Megahertz (MHz - 1 MHz equals one million cycles per second) in ultrasound. Most devices used in anesthesia will use frequencies in the range of 3-24 MHz. Wavelength is inversely proportional to frequency, so the higher the frequency you select, the smaller the wavelength, and the better resolution you will have at shallow depths. (Think of the wave peaks closer to each other, thus giving you a higher resolution.) The opposite is true - if you are planning on performing a sciatic nerve block, a low frequency probe (3-5 MHz) will allow higher wavelengths to penetrate deeper (but with a lower resolution). Most ultrasound devices will remind you of this Frequency-Depth couplet to help you properly select a frequency when starting a given block.
I-Streme Echo connected to MS Surface 2 tablet USB port
Once your tablet is displaying an image, a few basics need to be understood before you can begin to perform blocks. Before working with needles, become very familiar with the basics of ultrasound, the anatomy you are viewing (spend time in the Neuraxiom web page, where you will see the same flash files you will be able to see in your I-Strene Echo system while you are performing your blocks), and how this anatomy is displayed with your device.
When viewing a vessel or nerve, your probe can be placed in either of two directions: 1) short axis in which you hold the probe at a right angle to the direction of the structure, and 2) long axis in which the probe and the structure viewed is held parallel or aligned to it. This is very important concept and one that you should ask yourself at the beginning of each procedure.
As an example, if you use the probe to view the radial artery in your wrist, a short axis view will display a small circle which is dilating and contracting. If you now rotate the probe 90 degrees to a long axis view, the artery will be now seen as a cylinder which is pumping. If you want to cannulate it, this would probably be the ideal view to use.
If you bought an ultrasound, this is what you really want to use it for: placing your needle tip at an exact location to best obtain a block. There are two ways to best view a needle: 1) in-plane, in which the needle is placed in the same plane as the ultrasound beam, and 2) out-of-plane in which the needle is placed perpendicular to the beam.
If you want to see the entire needle during a block, the in-plane approach is the one to use. Use this method to also view the needle tip. In the out-of-plane approach you will only see the needle as a bright spot which represents some part of the needle.
Visibility of needles may be tricky, but a few basic tips will help you in seeing them better: 1) Remember that a typical ultrasound beam is only 2mm wide, so proper alignment of your probe is important. 2) Inserting your needle at a flatter angle (30 degrees) is ideal for an in-plane approach. If you are using an out-of-plane approach, steeper angles (> 60 degrees) allow you to see the needle better.
Learn how to differentiate tissues by how their echoes behave under ultrasound.
1. Arteries are pulsatile and hypoechoic - meaning they pulse and are black inside (fluid passes through sound waves, which is why bladder and blood vessels appear black. Structures behind them can show an artifact of increased intensity.)
Tissue densities as seen with our former model (U-Blok SP probe) in the axilla - note grape pattern of nerves
2. Veins are non-pulsatile, hypoechoic, compressible and get larger with a Valsalva.
3. Muscle is hypoechoic (dark) with striated lines.
4. Tendons show up as hyperechoic and with bright striated lines. Movement is very easy to see.
5. Nerves can be hyper or hypoechoic. The more peripheral nerves show up as straws bundled up. The more proximal ones are more hypoechoic.
6. Bone is very hyperchoic.
Learning how to ultrasound is similar to learning how to ride a bicycle. At first you will feel awkward and have difficulties holding the probe, orienting it, or knowing when to move it or when to keep still. Be patient - these skills will come naturally to you with a little bit of practice.
Don't be skimpy with this and try to make sure you have enough to have a good spread between your entire probe end and the skin. You will also learn quickly when you do not have enough. Make sure you use sterile gel for all your blocks.
- Probe movement
Very often, the best images are obtained by gently moving your probe around in different directions - kind of like when you slow down in a bicycle and you make many fine movements so as not to fall.
- Probe orientation
Begin your blocks by paying attention to whether you are to be in-plane or out-of-plane and whether your button probe is aligned with the blue beacon on the top your screen. If it is aligned, and you are planning an in-plane block, you will begin to see your needle coming from that side.
- Needle direction
The block that will confuse you at the beginning is the popliteal block if you are using a lateral approach. In most blocks, moving the needle deeper will show the needle moving away from the probe. But in the case of a lateral popliteal block, moving deeper into the popliteal region will mean moving the needle towards the probe.
For every block you practice, always have the same methodology. As an example, if you are going to do a supraclavicular block here is a good checklist to follow:
1. Position patient properly with head looking away from shoulder to blocked. Connect monitors. O2.
2. Prep the area.
3. Sterile gel on area.
4. Stand behind the patient as if you were going to intubate - consider back up 30 degrees for your comfort.
5. Pick up probe with non-dominant hand and have probe button facing laterally.
6. Identify ext. jugular vein and place probe in that area and just above the clavicle.
7. Look for the pulsating subclavian artery and next to it the brachial plexus. Look for other landmarks such as the first rib, pleura (comet-tail sign)
8. Introduce your needle about 1cm lateral to probe, and begin to look for it on the right part of the screen if you aligned your probe button properly with the blue beacon on the screen. If you cannot see the needle, you will see tissue movement. Gently move the probe sideways so as to align the probe beam with the needle. Also rotating it gently back and forth (clockwise, counterclockwise) will sometimes help you find the needle.
9. Advance your needle slowly and try to understand what you are going through. If you are using a nerve stimulator, look for a twitch to confirm that your needle tip is near the plexus.
10 Inject 1 ml initially to visually confirm that you are where you think you are and that you are not intra-neural (feel for resistance or ask the patient if he/she is feeling pain - both signs of intra-neural injection.)
11 Inject the rest of the anesthetic slowly. Consider moving your needle around to get an even spread and coverage of the three branches. Watch for intravascular injection by aspirating every 5 mls.
12.Remember that because ultrasound is far more accurate in needle placement, you will need much lower volumes. We rarely use 20 mls of local anesthetic for any block that we perform.
Avoiding failures or mishaps
Lack of attention when performing a block under ultrasound can also lead to intravascular injections. If you are particularly close to a vessel, aspirate every 5 mls when injecting and look for bubbles inside the vessel.
Intraneural injections will not always be evident under ultrasound, so pay attention to resistance to injection and/or the patient complaining of pain.
Failed blocks are most commonly due to the local anesthetic going to a nearby compartment (local anesthetic in scalene muscles vs. plexus). So if in doubt, do not inject your entire volume at a single location. (You were taught to do this before, but with ultrasound this is not the way to do things.) As an example, in an axillary block we typically inject local anesthetic at four different locations to obtain a solid block.`
Just like when you were able to ride a bicycle, you really were not perfectably comfortable for months especially if you wanted to take it into rough terrain. Becoming proficient in ultrasound will take some time too. But one day, and this will be when you least expect it, you will realize that you have mastered its art and that you are ready to leave the Shaolin temple of ultrasound learning. At that time you will be able to innovate and perfect your own techniques. And because U-Blok is so easy to carry around, set up, and use - you should reach this milestone rather quickly.