Pulse oximetry is an essential, non-invasive tool used to monitor a patient’s oxygen saturation (SpO2) and heart rate, providing critical information about respiratory and cardiovascular function. However, obtaining accurate pulse oximetry readings can be challenging in certain situations, such as during peripheral vasoconstriction, on cold extremities, or in patients with significant edema or nail polish. In these cases, a digital nerve block can be a valuable method to improve the accuracy of pulse oximetry by increasing blood flow to the digit and ensuring reliable readings.
Pulse oximeters emit light through a pulsating capillary bed, usually at the fingertip or earlobe, and measure the amount of light absorbed by oxygenated and deoxygenated hemoglobin. Factors like poor peripheral circulation, vasoconstriction, or interference from external substances (like nail polish or dirt) can impair the accuracy of these readings. Indeed, cold environments, hypovolemia, or shock can lead to vasoconstriction, reducing blood flow to the extremities and leading to unreliable pulse oximetry readings. In addition, swelling or poor perfusion in the fingers can further complicate obtaining an accurate SpO2 reading. Finally, dark nail polish or pigmented skin can interfere with the pulse oximeter’s ability to transmit light through the digit, leading to inaccurate measurements 1–4.
A digital nerve block is a regional anesthesia technique commonly used to numb the fingers or toes for minor surgical procedures. This block targets the digital nerves, which run along the sides of each digit, providing sensation to the area. Administering a digital nerve block not only anesthetizes the finger but also causes local vasodilation by blocking sympathetic nerve fibers that control vasoconstriction. This vasodilation increases blood flow to the digit, which can enhance the accuracy of pulse oximetry readings.
A digital nerve block in pulse oximetry should be used when pulse oximetry readings are consistently unreliable due to poor peripheral perfusion, in patients with peripheral vascular disease, Raynaud’s phenomenon, or other conditions that result in vasoconstriction, or during procedures where it is critical to maintain accurate SpO2 monitoring, such as in patients with compromised respiratory function or during anesthesia 5–7.
The digital nerve block is a straightforward procedure that can be performed quickly at the bedside. First, the patient’s finger is cleaned with an antiseptic solution. A small gauge needle is typically used to minimize discomfort. Next, a local anesthetic is injected on the lateral and medial aspects of the base of the finger. Care is taken to avoid intravascular injection. Approximately 1-2 ml of anesthetic is injected on each side of the digit. The anesthetic is deposited just distal to the web space, ensuring coverage of the digital nerves. The block takes effect within a few minutes. The patient’s finger should be observed for signs of vasodilation, such as increased warmth and color. Pulse oximetry readings can then be reassessed 7.
When considering a digital nerve block in pulse oximetry, there are risks to keep in mind. The procedure, while generally safe, carries risks such as infection, bleeding, or local anesthetic toxicity. However, these remain rare with proper technique 7.
References
1. Barker, S. J. & Tremper, K. K. Pulse oximetry: applications and limitations. Int. Anesthesiol. Clin. 25, 155–175 (1987). doi: 10.1097/00004311-198702530-00010.
2. DeMeulenaere, S. Pulse Oximetry: Uses and Limitations. J. Nurse Pract. 3, 312–317 (2007).doi: 10.1016/j.nurpra.2007.02.021
3. Mardirossian, G. & Schneider, R. E. Limitations of Pulse Ox netry. Anesth Prog 39, 194–196 (1992).
4. Why it’s a problem that pulse oximeters don’t work as well on patients of color | MIT News | Massachusetts Institute of Technology. Available at: https://news.mit.edu/2022/pulse-oximeters-dont-work-as-well-patients-of-color-0802. (Accessed: 18th August 2024)
5. Cordoví de Armas, L., Espinaco Valdés, J., Jiménez Paneque, R. E., Costa Hidalgo, T. & Vallongo Menéndez, M. B. [Improved detection of the pulse oximeter signal with a digital nerve block in patients in poor health status]. Rev. Esp. Anestesiol. Reanim. 55, 481–486 (2008). doi: 10.1016/s0034-9356(08)70631-5.
6. Erasmus, P. D. Digital block improves pulse oximetry signal in vasoconstricted patients. Anaesthesia (2003). doi:10.1046/j.1365-2044.2003.03415_14.x
7. Napier, A., Howell, D. M. & Taylor, A. Digital Nerve Block. Adult-Gerontology Acute Care Pract. Guidel. Second Ed. 567–568 (2024).