EPIDURAL TEST DOSE: WHAT and WHY

INTRODUCTION

Central Neuraxial block (CNB) is the most popular and most widely practiced technique amongst all anaesthetic techniques.  Report and findings of the 3rd National Audit Project of the Royal College of Anaesthetists showed in a conservative estimate, that CNB is used for at least 8–10% of all operations in the UK.

Central Neuraxial block (CNB) techniques can produce highly effective pain relief for a wide variety of indications and may decrease patient morbidity after major surgery. Individual studies and meta-analyses have examined this effect and suggested benefit, [1–2] with even cautious commentators accepting that there is merit in the suggestion.[3] Yeager’s very small study reporting that epidural analgesia markedly reduced complication rates (overall complications, cardiovascular failure, major infections, cortisol rise) in high-risk patients was one of the earlier studies to suggest benefits out with improved analgesia. [4] Rodgers metanalysis [5] reported a reduction in deep vein thrombosis (DVT) by 44%, pulmonary embolism (PE) by 55%, transfusion requirements by 50%, pneumonia by 39%, and respiratory depression by 59%, all statistically significant effects. There were also non-significant reductions in myocardial infarction and renal failure. 

The wider use of CNB for operative delivery has almost certainly been the major factor in reducing the incidence of maternal death due solely to anaesthesia.[6] In surgery, many new procedures (orthopaedic joint replacement, transurethral urology, vascular surgery) have been introduced to the benefit of an increasingly elderly population, but such patients suffer from much intercurrent disease and receive complex drug therapy, both of which complicate general anaesthesia. Regional techniques were (and still are) seen as providing clear benefits in these very diverse clinical situations.

 Different Local Anaesthetic Agents and its maximum dose with Duration of Action:

ADVERSE EFFECTS AND COMPLICATIONS

Serious and permanent complications of neuraxial anesthesia are very rare. In a prospective audit report from the UK, the incidence of permanent injury from neuraxial anesthesia was 4.2 per 100,000 [7]. This report also highlighted that most injuries resolved within six months.

Among all the complications and adverse effects of regional anaesthesia, the reported incidence of major LAST events (i.e., seizures or cardiac arrest) is very low. Major LAST events have decreased markedly since the early 1980s, likely related to increased awareness and routine incorporation of preventive measures into clinical practice. [8,9]

Local anesthetic systemic toxicity (LAST) is a potentially devastating but rare complication of regional anesthesia. Estimating the frequency of LAST events in the context of peripheral nerve blocks (PNBs) is challenging due to their infrequent occurrence as well as the range of their attendant clinical signs and symptoms of LAST. Local anesthetic systemic toxicity (LAST) can occur with any route of administration of local anesthetics but is much less likely with spinal than with epidural anesthesia, because such a small dose of local anesthetic (LA) is injected for spinal anesthesia. LAST is a rare but potentially lethal event, that may consist of central nervous system and/or cardiovascular effects ranging from minor manifestations (eg, perioral numbness, tinnitus, twitching), to major events, including seizures, coma, severe hypotension, arrhythmias, and asystole. LAST occurs most commonly with inadvertent intravascular injection of LA with almost immediate onset of signs and symptoms, but delayed onset may also occur with epidural infusion after systemic absorption of LA or migration of the catheter into a blood vessel.

Association between Plasma concentration of Lidocaine and Side effects:

Plasma Concentration	Possible side effects
4-6 µg/mL	Lightheadedness, numb tongue, metallic taste, increased blood pressure, dizziness.
8 µg/mL	Visual and auditory disturbances, dissociation, muscle twitching, decreased blood pressure.
12 µg/mL	Convulsions (Benzodiazepines sensitive)
16 µg/mL	Come
20 µg/mL	Respiratory arrest
24 µg/mL	Cardiovascular system depression/Collapse

Ref: Toxicity Ferrini, 2000

Recent estimates demonstrate a low incidence and suggest a decreasing rate trend; however, most of these studies have been determined from regional anesthesia registries and single institution quality improvement or billing queries, thus lacking generalizability to national practice. [10-13] A recent study using a United States database found the incidence of LAST was 1.8/1,000 PNBs, noticeably higher than previous estimates.[14] However, this study did not directly identify a diagnosis of LAST but used the presence of surrogate outcomes, such as a diagnosis of seizure, or cardiac arrest and administration of Intralipid, to identify the incidence of LAST.[14] The results lead to a question of whether improvements in technique, utilization of ultrasound guidance and awareness of LAST have led to decreases in the rate of LAST in the United States.

Smaller quantities of local anaesthesia can produce blood levels above the seizure threshold if injected intravenously or intra arterially in the cerebral circulation (A seizure threshold is a person's likelihood to have a seizure. The higher the threshold the less likely it is that a seizure will happen).

Frequent aspiration, careful fixation of the needle, careful test doses and incremental injections are clearly indicated whenever injections are made near blood vessels, thereby early detection and prompt action is the key to prevent perioperative mortality associated with LAST.

                                  

Before initiating an epidural local anaesthetic infusion, a test dose may be administered. A test dose [15] is the injection of a quantity of drug sufficient to produce some reliable subtoxic clinical sign if injected intrathecally or intravascularly. The purpose of this test is to exclude the misplacement of the epidural catheter.

A small volume of lidocaine 1.5% with epinephrine is traditionally used for this purpose. If the catheter is in a blood vessel, the injection of a small volume of an epinephrine solution will result in a tachycardia. This is not without a risk in the patient with severe heart disease. If the catheter lies in the subarachnoid space, then the injection of a dose of local anaesthetic similar to that used in intrathecal anaesthesia should result in a sudden onset of loss of sensation and weakness in the legs in the awake patient. Hypotension may also develop swiftly. [15]

Till now it is the most reliable and practical test for early recognition of LAST.

The reliable indicators of intravascular injection are- [16]

• The tachycardia produced by 15 µgm epinephrine injected intravenously. 
• Increase in Heart rate by 30% (from 80-110 beats per minute) and Increase in systolic blood pressure (SBP) by 15 mmHg.

• Within 20 seconds following injection.
• The increase is transient.
• Changes return towards normal within 60 seconds.

   A mechanical beat to beat pulse counting device is essential rather than a manual count. 

Criteria for Epinephrine Test Doses:

Situation	Hemodynamic criteria
Normal patient	HR increase >20 bpm
	SBP increase >15 mmHg
Beta blockade	HR- no change
	SBP increase >15 mmHg
Advanced age	HR increase >9 bpm
	SBP increase >15 mmHg
General anaesthesia	HR increase >8 bpm
	SBP increase >13 mmHg

HR- Heart rate, SBP- Systolic blood pressure, BPM- Beats per minute

 There are several limitations, thus multiple safety steps are always important to the test- 

• Patients receiving beta blocking drugs will not respond with a tachycardia and changes in systolic blood pressure, so will need to be monitored or an alternative test used.
• Elderly patients may also have a reduced response to epinephrine.
• Pre medicated patients cannot be relied on to report subjective clinical signs.
• General anaesthesia will also reduce the response to epinephrine by 50%.

 Alternative tests:[16]

• Doses of 100mg chloroprocaine, 100mg lidocaine or 25mg bupivacaine injected intravenously will produce subjective signs and may be a reliable substitute in this situation.
• Another alternative is the injection of 1 ml of air while monitoring the heart sounds with a precordial doppler stethoscope. This has been used in obstetric anaesthesia, but it may not be practical in the operating room.

 References:

1. Ballantyne JC, Carr DB, deFerranti S, et al. The comparative effects of postoperative analgesic therapies on pulmonary outcome: cumulative meta-analysis of randomized controlled trials. Anesth Analg 1998;86:598–612. 

2 Rodgers A et al. Reduction of postoperative mortality and morbidity with epidural or spinal anaesthesia: Results of overview of randomized trials. Br Med J 2000;321:1493–1497. 

3 Buggy DJ, Smith G. Epidural anaesthesia and analgesia: better outcome after major surgery? Growing evidence suggests so. Br Med J 1999;319:530–531.

4.Yeager MP et al. Epidural anesthesia and analgesia in high-risk surgical patients. Anesthesiology 1987;66:729–736.

5. Rodgers A et al. Reduction of postoperative mortality and morbidity with epidural or spinal anaesthesia: results from overview of randomized trials. Br Med J 2000;321:1493.

6. Cooper GM, McClure JH. Anaesthesia. Chapter 9 in: Why Mothers Die: Sixth Report of the Confidential Enquiries into Maternal Deaths in the United Kingdom. RCOG Press, London 2004

7. Cook TM, Counsell D, Wildsmith JA, Royal College of Anaesthetists Third National Audit Project. Major complications of central neuraxial block: report on the Third National Audit Project of the Royal College of Anaesthetists. Br J Anaesth 2009; 102:179.

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10. Sites BD, Taenzer AH, Herrick MD, et al. Incidence of local anesthetic systemic toxicity and postoperative neurologic symptoms associated with 12,668 ultrasound-guided nerve blocks: an analysis from a prospective clinical registry. Reg Anesth Pain Med. 2012;37:478–482. [PubMed] [Google Scholar]

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16. Mulroy Regional Anaesthesia: An illustrated Procedure Guide. Complications of regional anaesthesia, 2002;3:33-35.