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Letter to the editor
Chart to estimate the depth of the target nerve/vessel during nerve block and vascular cannulation
  1. Anthony M-H Ho and
  2. Glenio Bitencourt Mizubuti
  1. Department of Anesthesiology and Perioperative Medicine, Queen’s University, Kingston, Ontario, Canada
  1. Correspondence to Professor Glenio Bitencourt Mizubuti, Department of Anesthesiology and Perioperative Medicine, Queen’s University, Kingston, ON K7L 2V7, Canada; gleniomizubuti{at}

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To the Editor,

When performing nerve blocks or vascular cannulation under ultrasound (US) guidance, the depth of the target nerve/vessel and/or the defining landmark (d US cm; figure 1) is seen on the US screen and noted by the practitioner. This depth serves as a guide for determining the depth at which the needle needs to be inserted (d N) to reach the target. For obvious reasons, d N is always larger than d US. For steep needle entry (small α), the difference between d N and d US is relatively small. For less steep needle entry, the difference is relatively large. This same principle also applies to nerve blocks when US is not used. For example, in paravertebral block using the landmark approach, once the block needle contacts the transverse process, the practitioner may tilt the needle in a cephalad or caudad direction to ‘walk off’ the transverse process. Here the depth at which the needle first touches the transverse process is used as a guide. Depending on how much the needle has to be tilted (and this is quite variable) to just clear the transverse process, the depth at which the needle actually clears the transverse process could be very close to the depth of the transverse process for small tilt angles or could be substantially larger for larger tilt angles.α

Figure 1

The block or vascular needle sustains a certain angle (α) with the ultrasound probe.

The needle tip is not always easily seen on US in an in-plane approach, especially if α is small (steep needle entry), and is not seen in an out-of-plane approach. Having a reasonable estimate of d N may reduce the chance that the needle tip is way off (not close enough or too deep), and may reduce the chance of complications (hitting the pleura in supraclavicular block or hitting a posterior lying carotid artery in internal jugular cannulation using short-axis out-of-plane approach). Likewise, in landmark (without US) paravertebral block as discussed above, knowing when the needle tip clears the transverse process is also important as it is beyond this point that loss of resistance is sought or the needle is advanced by ~1 cm to reach the paravertebral space.1

As we cannot expect practitioners to determine in their head or use a calculator in the middle of a procedure, we have devised a simple chart (table 1) as a guide for estimating d N. It is strategically placed next to the US screen or on the block cart. This is a sample chart and details can be added or subtracted. Next to the table is a large diagram of a protractor to help practitioners estimate the angle of the needle (figure 2). This technique is a variation of a trigonometric method published earlier.2

Table 1

Depth (cm) at which the block or vascular needle needs to be inserted (d N) according to the depth of the target/landmark on ultrasound (US) and the angle between the US probe and the needle (also see figure 1)

Figure 2

An aid to help practitioners estimate the angle (α) of the needle.



  • Contributors All authors meet all of the following criteria: (1) substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data for the work; (2) drafting the work or revising it critically for important intellectual content; (3) final approval of the version to be published; and (4) agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

  • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

  • Competing interests None declared.

  • Patient consent for publication Not required.

  • Provenance and peer review Not commissioned; externally peer reviewed.