What causes a patchy block in anesthesia?

Causes of Patchy Block in Anesthesia

Patchy block in anesthesia occurs primarily due to inadequate spread of local anesthetic within the epidural or subarachnoid space, often resulting from technical factors such as catheter misplacement, incorrect identification of the epidural space, or anatomical barriers to local anesthetic distribution. 1

Technical Factors Contributing to Patchy Block

• Provider experience: Inexperienced providers have significantly higher rates of epidural failure, with low-volume specialists having 3.77 times greater odds of inadvertent dural puncture compared to high-volume specialists 1
• Timing of procedure: Blocks placed during night shifts (19:00-08:00) have a 6.33 times higher relative risk of complications, likely due to provider fatigue and less experienced staff 1
• Incorrect identification of the epidural space using loss of resistance technique can lead to catheter misplacement and patchy anesthesia 1
• Catheter dislodgement at the skin level occurs in approximately 0.53% of cases, leading to inadequate anesthetic delivery 1
• Multiple attempts at needle placement can cause tissue trauma and bleeding, potentially interfering with anesthetic spread 2

Pharmacological and Physiological Factors

• Inadequate spread of local anesthetic within the cerebrospinal fluid can occur with intrathecal catheters, especially with low flow rates 1
• Anatomical barriers such as septations within the epidural space can prevent uniform spread of anesthetic 1
• Uneven distribution of anesthetic due to patient positioning during or after administration 1
• Properties of local anesthetics: Different agents have varying onset times and distribution patterns - chloroprocaine, lidocaine, mepivacaine, prilocaine and etidocaine have a rapid onset, while procaine, tetracaine and bupivacaine are characterized by a longer latency period 3
• Insufficient volume or concentration of local anesthetic solution can lead to inadequate spread and patchy block 4

Clinical Manifestations of Patchy Block

• Need for frequent bolus doses during labor analgesia indicates inadequate spread of anesthetic 1
• Uneven sensory blockade with areas of preserved sensation within the expected field of anesthesia 1
• Asymmetric motor blockade when assessed using scales such as the Bromage scale 5

Prevention Strategies

• Combined spinal-epidural technique may be more reliable than epidural alone, as free flow of CSF through the spinal needle confirms correct midline placement 1
• Ultrasound guidance can improve the accuracy of needle placement and reduce the risk of patchy blocks when employed by clinicians experienced in its use 2
• Multi-orifice catheters are more likely to produce reliable anesthetic spread compared to single-orifice catheters 1
• Proper patient positioning during block placement can affect success rates, with some evidence suggesting lateral decubitus position may be preferable to sitting in some cases 1
• Use of appropriate anesthetic agents: The onset, duration and quality of regional anesthesia are enhanced by an increase in dose achieved by either an increase in concentration or in the volume of anesthetic solution 4

Management of Patchy Block

• Catheter manipulation may help redistribute the anesthetic solution 1
• Administration of supplemental anesthetic boluses targeted to areas with preserved sensation 1
• Consideration of alternative anesthesia techniques if patchy block persists despite interventions 1
• Clear labeling of catheters and good communication between healthcare professionals are essential to prevent dosing errors that could contribute to patchy blocks 5

Special Considerations

• Monitoring for high or total spinal block: Signs include increasing agitation, significant hypotension, bradycardia, upper limb weakness, dyspnea or difficulty speaking 5
• Management of high block complications: Provide circulatory support with vasopressors and fluids, supplemental oxygen, and possibly tracheal intubation and ventilation if a patchy block suddenly extends to a high block 5
• Assessment of block height should be performed at least once every 5 minutes until no further extension is observed when complications are suspected 5