Aspiration Technique: A Critical Step Before Anaesthesia Administration

The Basis of Aspiration

Syringe aspiration prior to injecting is advised not only in dental anaesthesia administration but also for most medication injections in an IM or interstitial space.¹ Its goal is to determine where the needle tip is situated, whether inside a blood vessel or not.² Signs of blood inside the cartidge during aspiration would indicate that the needle bevel is in a vein or artery and needs to be retracted to prevent intravascular injection.¹

A recent study found that dental anaesthetic injections had a reported rate of positive aspiration (i.e., suggesting the needle bevel was intravascular) between 3.2% and 8%, depending on the type of syringe used and the type of nerve block being performed.¹

Intravascular injection of local anaesthetic can result in a number of side effects (particularly if the anaesthetic has epinephrine in its formulation), such as:^3,4

• Palpations
• Inadequate anaesthesia
• Shooting pain
• Pallor, inadequate vascular perfusion
• Lip paresthesias
• Visual disturbances
• Headaches
• Vertigo

Although the doses used in dental anaesthesia are small and the symptoms should be transient and self-limited if they occur ³, they highlight the critical role of aspiration in minimizing patient risk and ensuring safe, effective delivery of dental anaesthesia.

To aspirate, the practitioner must create negative pressure within the dental cartridge by pulling the piston back gently (when using a manual aspiration syringe). The negative pressure then translates to the tip of the needle, drawing back whatever is lying in the soft tissues around it (e.g. blood, tissue, or air in case it is tested out of the mouth). The presence of blood inside the cartridge is an indication that the needle tip is inside a blood vessel, warning the practitioner not to inject the anaesthetic solution at this location.

Aspiration is a simple enough procedure, and it makes a difference in terms of safety during anaesthesia administration. However, the technique should be performed with care because it may increase the pain of injection, particularly in paediatric patients.^1,5 To avoid the discomfort, the practitioner can apply topical anaesthesia on the site of injection.

Aspiration Techniques

When using manual aspiration syringes, the practitioner should insert the needle into the injection site and, keeping it steady, retract the piston by 1 to 2 mm to generate negative pressure.²

Proper aspiration technique is generally described in the following steps:³

1. Choose the correct equipment for the particular nerve block: a dental syringe (self-aspirating is recommended) and a 25, 27, or 30 gauge dental needle (long or short).
2. Locate the site of injection using the appropriate anatomical landmarks.
3. Position the needle tip, advancing it until it hits bone or the target anatomical location.
4. Aspirate by slowly pulling back the plunger 1-2 mm.
5. If blood is aspirated, reposition the needle tip and re-aspirate.
6. If initial aspiration is negative for blood, aspirate in two planes by repositioning the needle and aspiring in another plane. This is because a false negative aspiration can result if the needle bevel is occluding a blood vessel.
7. If both aspirations are negative, slowly inject the drug while aspirating intermittently.

To make aspiration easier and more comfortable for practitioners during dental anaesthesia administration, self-aspirating syringes have been positively evaluated for their ergonomic design and effectiveness (Figure 1). Auto-aspirating syringes generate negative pressure automatically when the practitioner releases positive pressure, requiring only a gentle push and release to perform the aspiration test before injecting the anaesthetic.²

Figure 1. The Prestoject® FL self-aspirating syringe.

^{Source: Inibsa.com (This product complies with the medical device regulation.)}

Given the potential toxicity of anaesthetic agents, an understanding of vascular anatomy is all the more important for dental anaesthesia (see Figure 2).¹

Figure 2. The maxillary arteries and their branches.

Source: Study.com

Most of the complications reported in the literature are associated with the conventional inferior alveolar nerve (IAN) block.^3,4 The IAN block also has the highest risk of positive aspiration.⁴ The higher complication rate is because the procedure is deeper, has a relatively blind approach, and is anatomically close to the inferior alveolar blood vessels.³ As well, the traditional aspiration technique may produce a false negative because the inferior alveolar vein may be pinned against bone by the needle.⁴ This can be largely overcome by withdrawing the needle 1-2 mm before aspirating while performing an IAN block.⁴

The Bottom Line for Your Practice

Based on the current state of the research literature, dental professionals should aspirate their syringes prior to injecting local anaesthetics.³ This can be accomplished by using a self-aspirating syringe, or by gently pulling back on the syringe plunger 1-2 mm before injecting. If bloody aspirate is obtained, the needle should be repositioned and re-aspirated.

References

^{1. Sepah Y, Samad L, Altaf A, et al. Aspiration in injections: Should we continue or abandon the practice? F1000Research. 2017;3:157. doi:10.12688/f1000research.1113.3}

^{2. Malamed, S. F. (2020). Handbook of local anesthesia (7th ed.). Elsevier.}

^{3. Mathison M, Pepper T. Local anaesthesia techniques in dentistry and oral surgery. StatPearls. 2023. https://www.ncbi.nlm.nih.gov/books/NBK580480/}

^{4. Garcia-Blanco M, Scanlan S. Accidental intravascular local anaesthetic injection of the facial artery. Anesth Prog. 2021;68(1):29-32. doi:10.2344/anpr-67-03-03}

^{5. Herraiz-Adillo Á, Martínez-Vizcaíno V, Pozuelo-Carrascosa DP. Aspiration before intramuscular vaccines injection, should the debate continue? Enferm Clin. 2022;32(1):65-66. doi:10.1016/j.enfcle.2021.10.002}