Test dosing in regional anesthesia is designed to enhance patient safety by identifying unintended intravascular or intrathecal placement of needles or catheters. Administering a small, specific test dose of local anesthetic, often combined with epinephrine, allows clinicians to detect inaccurate needle placement during regional anesthesia before delivering the full anesthetic dose, thereby minimizing potential complications.

The primary objective of a test dose during regional anesthesia is to confirm the correct placement of the needle or catheter within the intended epidural or peripheral nerve space. Accidental intravascular injection can lead to local anesthetic systemic toxicity (LAST), characterized by symptoms such as seizures, cardiac arrhythmias, and, in severe cases, cardiovascular collapse. Inadvertent intrathecal injection may result in high or total spinal anesthesia, presenting as paralysis, loss of consciousness, respiratory arrest, hypotension, bradycardia, and possible cardiac arrest. By administering a test dose, healthcare providers can promptly identify and rectify such misplacements, thereby enhancing patient safety.¹

A commonly used test dose comprises 3 mL of 1.5% lidocaine (45 mg) with epinephrine 1:200,000 (15 µg). This combination serves dual purposes: lidocaine acts as a local anesthetic, while epinephrine, a vasoconstrictor, helps in detecting intravascular placement by eliciting a measurable physiological response. The inclusion of epinephrine is particularly useful, as an intravascular injection can lead to a noticeable increase in heart rate, alerting the clinician to the misplacement.²

The test dose should be administered rapidly, as slow injection may result in the redistribution and metabolism of the drugs before they can produce detectable effects. During and after administration, vigilant monitoring of the patient’s physiological responses is essential. Key parameters include heart rate, blood pressure, electrocardiogram (ECG) changes, and any alterations in consciousness. An increase in heart rate by 20 beats per minute or more within one minute of injection suggests intravascular placement, while signs such as sudden numbness or motor blockade may indicate intrathecal injection.²

In obstetric anesthesia, the use of epinephrine in test doses has been a topic of debate. Epinephrine can reduce uterine blood flow, potentially affecting fetal well-being. Therefore, some practitioners opt for alternative agents or adjust the epinephrine concentration when administering test doses to pregnant patients. Additionally, physiological changes during pregnancy, such as increased cardiac output and blood volume, may alter the patient’s response to the test dose, necessitating careful interpretation of hemodynamic changes.¹

While the traditional test dose is a valuable tool for regional anesthesia, it is not infallible. A negative test dose does not guarantee correct placement, as false negatives can occur. Factors such as the use of beta-blockers, which blunt the heart rate response, or anatomical variations can affect the reliability of the test dose. Consequently, there is ongoing discussion about the need to reassess and potentially update test dose protocols to enhance their sensitivity and specificity.³

Test dosing remains a fundamental practice in regional anesthesia, serving as a safeguard against the complications associated with inadvertent intravascular or intrathecal injections. By understanding its purpose, proper administration, and limitations, healthcare professionals can effectively utilize test dosing to enhance patient safety during regional anesthesia procedures.

References

1. Sween, Lindsay K. “Test Dose.” Obstetric Anesthesia: Quick References & Practical Guides Eds. Philip E. Hess, et al. McGraw Hill, 2023, https://accessanesthesiology.mhmedical.com/content.aspx?bookid=3328&sectionid=277013527.
2. Freeman, Brian S. “Epidural Test Dose.” Anesthesiology Core Review: Part One Basic Exam Eds. Brian S. Freeman, and Jeffrey S. Berger. McGraw-Hill Education, 2014, https://accessanesthesiology.mhmedical.com/content.aspx?bookid=974&sectionid=61588399.
3. La Colla, L., Starnari, R., & Stafford-Smith, M. (2021, October). The epidural “Test dose” — time to Rethink Our Thoracic Practice? Journal of Cardiothoracic and Vascular Anesthesia. https://www.jcvaonline.com/article/S1053-0770(21)00200-7/fulltext.