Failed Spinal Anesthesia: Possible Causes in This Case
The most likely cause of this failed spinal is inadequate dosing—2.2 mL of 0.5% hyperbaric bupivacaine provides only 11 mg, which is at the lower end of the recommended range for cesarean section, and the T10 sensory level with preserved motor function confirms insufficient spread of local anesthetic. 1
Primary Dosing Issue
The FDA-approved dosing for cesarean section is 7.5-10.5 mg (1.5-2.1 mL) of 0.5% hyperbaric bupivacaine as a minimum, with standard practice using 10-15 mg (2-3 mL) to achieve a T4 sensory level. 2, 1
This patient received 11 mg (2.2 mL), which falls within the lower therapeutic range but is clearly insufficient given the T10 block achieved. 1
The ED50 for achieving T5 block with hyperbaric bupivacaine alone is approximately 9.95 mg, with an ED95 of 13.55 mg, suggesting this patient's dose was marginal at best. 3
Technical Factors Contributing to Failure
Incorrect identification of the subarachnoid space:
The spinal needle may have been placed in the epidural space rather than the subarachnoid space, resulting in an epidural block with the intended spinal dose—this would explain the inadequate sensory level and preserved motor function. 4, 5
Partial subarachnoid placement can occur where only a portion of the injectate enters the CSF, with the remainder deposited epidurally. 5
Catheter or needle malposition:
The CSE technique involves locating two anatomical spaces, and the epidural needle may have been misplaced, affecting the trajectory of the spinal needle. 6
If the spinal needle did not fully penetrate the dura, the local anesthetic may have been deposited subdurally rather than subarachnoidally. 5
Drug Distribution Problems
Inadequate spread of local anesthetic:
Despite using hyperbaric bupivacaine (which should spread predictably with gravity), the T10 level suggests the drug did not spread cephalad sufficiently. 2, 5
Left lateral tilt positioning may have contributed to unilateral or asymmetric spread, though the patient had bilateral T10 block. 5
The addition of 300 mcg diamorphine (which should be 300 micrograms, not milligrams—this appears to be a transcription error in the case) does not influence block height, so this is not a contributing factor. 3
Patient-Related Factors
Anatomical variations:
Increased CSF volume in pregnancy can dilute local anesthetic, though this typically requires higher doses rather than causing complete failure. 5
Abnormal spinal anatomy or previous spinal surgery could create barriers to drug spread, though this was not mentioned in the history. 5
Timing of assessment:
- Five minutes may be insufficient time for full block development with hyperbaric bupivacaine, which typically reaches maximum height at 15-20 minutes. 5, 3
Critical Pitfalls to Avoid
Do not administer a repeat full spinal dose without careful consideration—this carries a 1 in 4367 risk of high or total spinal block requiring cardiovascular and respiratory support. 4, 7
Monitor block height every 5 minutes until no further extension is observed before declaring failure. 4, 2
Watch for signs of high spinal: increasing agitation, significant hypotension, bradycardia, upper limb weakness, dyspnea, or difficulty speaking. 4, 7
Immediate Management Options
If block remains inadequate after 15-20 minutes:
Consider incremental epidural dosing through the CSE catheter if properly positioned and tested. 6
Provide supplemental analgesia with intravenous opioids and anxiolytics while preparing for alternative anesthesia. 8
Offer conversion to general anesthesia if the block cannot be extended adequately—this should have been discussed during preoperative consent. 8
If attempting repeat spinal, use extreme caution with dosing: start with 1.25 mg increments of hyperbaric bupivacaine every 3 minutes, monitoring closely for high block. 2
Alternative explanation requiring urgent action: