How does a serratus anterior block work?

How Does a Serratus Anterior Plane Block Work?

The serratus anterior plane block works by injecting local anesthetic either superficially (between the serratus anterior muscle and latissimus dorsi muscle) or deeply (between the serratus anterior muscle and intercostal muscles) to block the lateral cutaneous branches of the intercostal nerves (typically T3-T6), along with the long thoracic and thoracodorsal nerves, providing analgesia to the anterolateral chest wall. 1, 2

Anatomical Mechanism

The block targets specific tissue planes around the serratus anterior muscle at the level of the midaxillary line, typically at the fifth rib 2:

• Superficial approach: Local anesthetic is deposited above the serratus anterior muscle, between it and the latissimus dorsi muscle 1
• Deep approach: Local anesthetic is placed below the serratus anterior muscle, between it and the intercostal muscles 1

The injected local anesthetic spreads within these fascial planes to block the lateral cutaneous branches of the intercostal nerves, which provide sensory innervation to the anterolateral chest wall 2, 3. This anatomical coverage makes it particularly effective for procedures involving the lateral thoracic wall, including VATS incisions and chest tube sites 3.

Clinical Efficacy Evidence

Pain Control Outcomes

Multiple studies demonstrate significant analgesic benefits compared to no block 1:

• Deep serratus anterior plane blocks consistently show lower pain scores and reduced tramadol consumption as rescue medication 1
• Patients receiving the block (either deep or superficial) demonstrate lower pain scores in the first 8 hours and significantly lower opioid consumption 1
• Postoperative nausea and vomiting rates are lower in patients receiving serratus anterior plane blocks 1

Comparison with Other Techniques

The evidence shows mixed results when comparing serratus anterior plane block to other regional techniques 1:

• Versus intercostal nerve blocks: No significant difference in opioid consumption or pain scores 1
• Versus erector spinae plane (ESP) block: Some studies favor ESP block with lower pain scores and opioid consumption 1, while others show no difference between deep serratus anterior plane block and ESP block 1
• Versus local anesthetic infiltration: Serratus anterior plane block provides superior pain control at 2 and 8 hours postoperatively, with longer time to first significant pain (VAS ≥4) 1

Practical Implementation

Drug Selection and Dosing

Common local anesthetic regimens include 1, 3:

• Ropivacaine 0.125-0.25% in volumes of 20-25 mL is widely used for thoracic surgery 3
• Levobupivacaine 0.25% provides effective analgesia while avoiding adverse reactions 3

Adjuvants for Enhanced Effect

Dexmedetomidine at 1 mcg/kg as an adjuvant to ropivacaine produces significantly lower pain scores and reduced opioid consumption compared to lower doses (0.5 mcg/kg) or plain local anesthetic 1. This represents the highest quality evidence for adjuvant selection, though clinicians should monitor for bradycardia and hypotension 4.

Advantages Over Alternative Techniques

The serratus anterior plane block offers several practical benefits 3, 5:

• Simpler operation compared to thoracic epidural analgesia and thoracic paravertebral block 3
• Increased safety profile with fewer complications 3
• Hemodynamic stability without the hypotension risk associated with epidural techniques 3
• Good ultrasonic anatomical basis allowing for visualization and higher success rates 2
• Can be performed as continuous catheter technique for prolonged analgesia 5

Clinical Applications

The block is effective for 6, 2, 7:

• Video-assisted thoracoscopic surgery (VATS) 1, 3
• Thoracotomy pain management 5
• Rib fractures 2
• Breast surgery 2
• Pectus excavatum repair (showing superior results to cryoanalgesia with VAS <4 at 3 hours postoperatively) 7
• Posttraumatic chronic intercostal neuralgia 6

Important Caveats

Always use ultrasound guidance to increase accuracy and safety of the block 8. The block should be combined with multimodal analgesia including paracetamol and NSAIDs as baseline therapy, as this combination consistently produces the best outcomes in the evidence 1. The choice between superficial and deep approaches may depend on operator preference, as both show efficacy, though deep blocks may provide slightly better pain control in some studies 1.