1. Marking and measuring with US for TPVB Barys Ihnatsenka, MD

2. The idea Live US guided paravertebral blocks could be challenging One can use US to mark skin projection of important deep non palpable landmarks and measure important distances and then put transducer down and perform block without US by using other end points (distance, LOR) This approach could be easier but still require some basic training

3. Mark the middle of the probe to outline skin projection of the TP in sagittal plane Position a structure of the interest (here TP in sagittal plane) in the middle of the screen Marking and measuring with US (basic principles) Note the depth of TP By looking at the depth marks on the right side of the screen (in this case a bit more than 1.5 cm) One can also use a US caliper tool for more precision 123

4. If ultrasound probe is not perpendicular to all planes (tilted or rocked) skin marking and depth measurements will be inaccurate if needle is advanced via the marked point perpendicular to all planes. Marking only works well when both US probe and needle are perpendicular to the surface in all planes or needle angulation is the same as was US plane angulation Pitfalls of marking and measuring with US (probe tilting/rocking as potential problems) TP Despite the intention to be perpendicular to all plane, one may tilt transducer up during image acquisition rocking Needle that is advanced at right angle via the mark will end up caudad to the target Needle will end up medial to the intended spot

5. Avoid tilting the probe in axial(above) or sagittal plane (below) unless you going to reproduce the same tilt with the needle correct US probe TILTING examples

6. Avoid rocking the probe in sagittal (above) or axial plane US probe in Sagittal plane US probe in axial plane correct US probe rocking examples

7. The rule of 2 orthogonal planes in correct order (axial then sagittal) It is easy to inadvertently tilt or rock the probe even when we pay attention to it By visualizing and marking the same structure in two orthogonal planes in correct order: axial scan first and then sagittal, we will improve the precision of marking y X y X

8. Another potential pitfalls of marking with US: confusing tip and root of TP on sagittal US image When marking middle of the TP in sagittal view make sure that US plane is “cutting” via its tip and not the root. Tip of TP is a bit more cephalad and It is more shallow (see more on next slide) Quick slight tilt of the US probe laterally will help to confirm its correct position (if you are on the tip of TP US plane will slide of TP into the ribs) Again if we do axial scan first and outline tip of TP on it (very distinct appearance) and only then place probe sagittal over the mark we got in axial plane, we would avoid the problem

9. Sagittal scan over tip of TP on the simulator (upper left) Note that image below on the right demonstrates the view that is acquired not over tip of TP but rather over root of TP. (TPs are deeper, they are wider with cephalad edge of TP being more superficial than caudad edge, the distance between TP is slightly less, more rib sticks out cephalad below TP, lung is almost not seen due to unfavorable angle of incidence) To differentiate one view from another one also may tilt the probe slightly laterally for a moment. In first case (view over TP tip) ultrasound plane will slide to the rib while in the second only to TP tip

10. Avoid sloppy skin marking despite correct image and probe position Our observations showed that not uncommon despite correct position of US probe, novice marks middle of the probe up to 5 mm above or below it If a US transducer does not have a middle mark we recommend to mark it in advance instead of relying on your ability to estimate the middle of the transducer with your eye Make sure that you move your head around (to avoid parallax) when marking the skin at the middle of the probe

11. The summary of secrets of the precise marking Visualize and mark any structure in 2 orthogonal planes in correct order and then connect those 2 marks When acquire the image try your best to keep probe perpendicular to all plane (no tilting or rocking) Identify structure correctly (good knowledge of sonoanatomy) Move your head around while marking middle of the probe to avoid parallax error y X y X

12. Last few pieces of the puzzle for US assisted blocks Marking and measuring with following needling without US will only work well when both US probe and needle are perpendicular to the surface in all planes or needle angulation is the same as was US plane angulation Learn not only keep the probe perpendicular to all planes but reproduce the same perpendicularity with the needle TIP 1: eye level is extremely important to avoid errors

13. “Eye level” and needle or transducer perpendicularity to the surface When axis of your eye sight is coaxial with the needle, hub will be positioned over the tip Marking the “cross-fire” over the needle entry may help one to align the eye axis and the needle axis perpendicular to the surface Here needle’s hub must be slightly elevated and moved to the right so hub of the needle completely overlap the tip And is in the middle of cross-fire

14. Another tip (look from the side) Alternatively to “cross- fire” trick, one can move his/her head around to confirm perpendicularity of the needle trajectory or ask for assistance from the side

15. Another tip (know the distance, plan the correction trajectory) If one know the distance to the target then it is clear when target is missed and correction is in order Correction is quick and easy when your first pass is planned with correction trajectory in mind By starting slightly below the target with only slight cephalad angulation we increase our chance of hitting it on our second pass by angling more up if we missed it on our first pass (this approach is handy when we plan to walk off caudad from TP) When first needle trajectory is slightly incorrect (green dash line), one will miss TP on the first pass and realize it as soon as fingers that serve as stoppers will touch the skin Pulling needle back and redirecting it up will put the needle on the TP (red dash line). Note that needle entry was deliberately chosen below TPs caudad edge skin projection Do not exaggerate cephalad angle of your first pass (take baby steps-10 degree angle): TP is only 1 cm in craniocaudal dimension and if it is deep small angulation may translate in large steps at the deep layers TP Long fingers at TP landing distance Serve as a stopper to prevent accidental advancement past safe distance

16. Use of ultrasound for distance assessment Ultrasound can be used to assess important distances, but the following must be kept in mind (the “The US distance may differ from needle distance”) 1.When we measure any distance for future block the ultrasound plane during depth measurement should coincide with the future needle pass 2.Excessive soft tissue compression will lead to under estimating the depth 3.Elastic structures such as membranes, fascial planes etc. will stretch before blunt needle perforation (so this will increase “needle distance to end point such as LOR, pop or twitch compare to static US distance”

17. Avoid excessive soft tissue compression with the probe when measuring the distances with US On the left depth from skin to TP is about 5 mm less than on the right due to excessive tissue squish Tissue compression will lead to underestimation of the depth (from skin to TP for example). We recommend to add 5 mm to ultrasound measured distance even with minimal transducer pressure The compression of soft tissues with transducer is less relevant when we measure distance between deep stable structures : TP and pleura