ISSUES IN PERIPHERAL NERVE BLOCKS

1. ISSUES IN PERIPHERAL NERVE BLOCKS BY KARIM YOUSSEF KAMAL, MD Lecturer of Anesthesia, Intensive Care and Pain Management AIN SHAMS UNIVERSITY

2. Peripheral Nerve Block Definition: A peripheral nerve block (PNB) is the injection of a local anesthetic around a nerve or group of nerves with blockade of nerve impulse conduction, causing temporary analgesia and loss of sensory and motor function.

3. Peripheral nerve blocks • More hemodynamic stability compared with neuraxial blocks. • Anticoagulation less of an issue. • Increasing popularity due to advances in ultrasound technology. • Introduction of perineural catheters prolongs post-operative pain control benefits.

4. Benefits • Avoidance of general anesthesia • Primary regional anesthetic vs. combination with “light” general anesthetic. • Most patients request intra-operative sedation. • Decreased PONV, sore throat, delirium, airway obstruction and respiratory depression. • Decreased time to discharge from PACU. • Increased patient satisfaction.

5. Benefits • Post-operative pain control • Decreased narcotic requirements and associated adverse side effects (e.g. nausea, pruritis, sedation, confusion, respiratory depression). • Earlier recovery of bowel function. • Improved tolerance of physical therapy. • Improved pain scores in PACU. • Increased patient satisfaction.

6. Disadvantages • Infrastructure requirements and potential for surgeon delays. • Failed Blocks. • Intraoperative awareness and non-operative discomfort (e.g. positioning). • Variable duration (approx. 4-40 hours). • Rare serious complications (e.g. local anesthetic toxicity, nerve injury).

7. PHARMACOLOGY OF LOCAL ANESTHETICS

8. Chemical structure of local anesthetics • Ester type: unstable, broken down by pseudocholine esterase. • Amide type: stable, broken down be liver enzymes

9. Types of local anesthetics Ester types • Lidocaine • Mepivacaine • Prilocaine • Etidocaine • Bupivacaine • Levobupivacaine • Ropivacaine Amide types Ester types Cocaine Procaine Chloroprocaine Tetracaine benzocaine

10. Mechanism of action Deposition of solution near nerves. Permeation of nerve axons. Binding to sites in voltage gated Na channels. Affection of nerve conduction.

11. Factors affecting distribution and elimination Protein binding: determines amount of free active part. Clearance: ester type depend on cholinesterase while amide type on hepatic blood flow. Cardiac disease: ↓ COP→ ↓distribution. Liver disease: ↑ blood level of amide type. Renal disease: minimal effect. Newborn: immature liver. Old age: ↓ dose

12. Additives to local anesthetics • Epinephrine: ↑duration, ↑ intensity, ↓systemic absorption. Also allowes early detection of I.V injection. • Steroids: antiinflammatory & analgesic effect. • Ketamine:NMDA anatagonist→analgesia. • Hyaluronidase:breaks collagen→ greater spread. • Clonidine:direct inhibition of nerve conduction plus indirect analgesia on spinal and supraspinal levels.

13. Complications of any PNB • Local anesthetic toxicity • Bleeding/hematoma • Infection • Nerve injury • Transient paresthesias 1-3% • Permanent nerve injury ~1/10,000 • Failed block

14. Local anesthetic toxicity • Intravascular injection (immediate onset) • Systemic absorption (delayed onset) • Central nervous system signs • 1st excitation: perioral tingling, tinnitus, agitation • 2nd depression: blurred vision, slurred speech, loss of consciousness • Seizure • Cardiovascular toxicity • Cardiac arrhythmia and/or circulatory collapse • Requires ~ 3x blood concentration that causes seizures

15. Lipid emulsion • For use in the treatment of life threatening local anesthetic toxicity. • Novel therapeutic indication for an old medication (component of TPN). • First case reported in 2006, now with over a dozen reported cases. • Mechanism of action unknown (“lipid sink?”).

17. Contraindications • Needle phobia or otherwise uncooperative. • Excessive sedation (adults). • Infection (local and untreated systemic). • Anticoagulation? • Pre-existing nerve injury. • Surgery specific (e.g. motor block and post-op neurological examination). • Block specific (e.g. pulmonary disease and interscalene block).

18. Placement safety precautions • Monitoring. • Availability of resuscitation equipment (suction, airway management). • Availability of resuscitation drugs (induction agents, ACLS drugs, lipid emulsion). • Pre-procedure confirmation (“timeout”). • Aspiration before injection. • Incremental injection. • Do not inject when paresthesia present.

19. PNB placement techniques • Anatomy. • Loss of resistance and tactile feedback • Evoked paresthesia. • Nerve stimulator (goal 0.3-0.5 mA). • Ultrasound guided.

20. Does Peripheral Nerve STIMULATOR Give up to Ultrasound?

21. PNS Ultrasound

22. Indications of nerve stimulators • Children and adults who are already anesthetized when a decision is made that regional block is an appropriate technique. • Individuals who are unable to report paresthesias accurately. • In performing local anesthetic administration on specific nerve. • In placement of stimulating catheters for anesthesia or postoperative analgesia. • Patients with chronic pain, in whom accurate needle placement and reproduction of pain with electrical stimulation or elimination of pain with accurate administration of small volumes of local anesthetics may improve diagnosis and treatment.

23. PERIPHERAL NERVE STIMULATORPHYSIOLOGICAL CONSEDERATION • Motor fibers (Aα)require less current than sensory element (Aδ , C fibers). • Cathode(negative)electrode should be attached to the stimulating needle, not anode(positive)electrode. • Current less than 0.5 mA is essential to locate the nerve successfully.

24. PERIPHERAL NERVE STIMULATORApparatus • Constant current output. • The stimulating pulse width should be shorter(50-100sec). • A battery indicator is essential. • The polarity of the leads should be clearly marked.

25. The general procedure for nerve stimulation • Stimulating needle is connected to the cathode. • After the skin is disinfected, the needle is advanced towards the nerve with the stimulator at a relatively high current intensity (1–2 mA) and with a pulse width of 100 to 200 μs. • This higher current amplitude is necessary to stimulate the nerve at some distance from the needle.

26. The general procedure for nerve stimulation • The current intensity decreased as the needle approaches the nerve, threshold current usually at 0.4–0.5 mA. • Attempting to observe a twitch at lower intensities (<0.3–0.4 mA) may result in inadvertent intraneural injection.

27. The general procedure for nerve stimulation Once the acceptable threshold current is reached, aspiration for potential intravascular placement is performed. With a negative aspiration a test injection of local anesthetic or normal saline (1–2 mL) is performed. The muscle twitch should diminish following the test injection (Raj test).

28. The general procedure for nerve stimulation • The mechanism of the Raj test was previously thought to be due to displacement of the nerve by the injectate. • A recent interpretation of the Raj test suggests that ionic solutions influence the response to nerve stimulation.

29. The general procedure for nerve stimulation With pure sensory nerves, the response will be a radiation of paresthesia with each pulse along the distribution of the nerve. Additionally, the pulse width used for nerve localization should be somewhat higher (300 μs–1 ms).

30. Insulated needles

31. Continuous plexus kit

32. Surface electrodes for percutaneous electrode guidance.

33. Principles of ultrasound technique

34. Ultrasound machine • Sound is mechanical vibration of particles in a physical medium (air, blood, fat, etc). • Sound comes in three "flavors": infrasound, audible sound, and ultrasound, based on its frequency • There are 5 major steps that ultrasound takes before an image can be made: sound generation, transmission, reception or reflection, reflection to transmission, reception to processing image production • 8-12 MHz (multifrequency) linear probe (this probe alone will suffice for 90% of practical nerve blocks)

35. Propagation through dense objects e.g. bone is poor with nearly the entire ultrasound beam reflected (hyper echoic (bright) image ). Fat and tendon have low reflectivity thus they form hypo echoic (dark) images. The outline of an object is generally best delineated when the ultrasound beam is at 90 degrees. Generally speaking, nerves appear in the transverse (cross sectional) view as round to oval shaped structures that are nodular and they can be hypo echoic or hyper echoic depending on location. On the transverse view, peripheral nerves in cross section often appear to have an internal honey comb texture.

36. Piezoelectric CrystalsTransform energy in both directions • Ultrasound is generated when multiple piezoelectric crystals inside a transducer rapidly vibrate in response to an alternating current. • To generate a clinically useful image, ultrasound waves must reflect off tissues and return to the transducer. The transducer, after emitting the wave, switches to a receive mode. • When ultrasound waves return to the transducer, the piezoelectric crystals will vibrate once again, this time transforming the sound energy back into electrical energy.

37. High frequency beam has a narrower beam width • Resolution refers to the ability to clearly distinguish • two structures lyingbeside one another.

38. High frequency = high resolution but limited depth of penetration Low frequency = greater depth of penetration but lower resolution

39. Anisotropy

40. Ultrasound Terminology • Acoustic impedance = resistance of a medium to the sound transmission • Attenuation = loss of wave energy traveling through a medium of different acoustic impedance • Cycle = the combination of one rarefaction and one compression equals one cycle • Doppler effect = sound + movement -> modified frequency • Echogenicity = capacity of a structure in the path of an ultrasound beam to reflect back sound waves • Frequency = the number of cycles per unit time. frequency is the reverse of wavelength. The higher the frequency, the smaller the wavelength.

41. Ultrasound Terminology • Hyperechoic = an ultrasound image with echoes stronger than normal or than surrounding structures; images are bright on monitor; examples are bone and pleura. • Hypoechoic = an ultrasound image with weaker echoes than the surrounding structures; images are dark on monitor; examples are vessels and cyst. • Interface = boundary between two substances that transmit sound at different velocities. • Velocity = speed at which sound waves travel through a particular medium; velocity = frequency x wavelength; ultrasound velocity through human soft tissue = 1540 m/sec. • Wavelength = the distance between the onset of peak compression or cycle to the next.

42. Several Types of Probes • Linear vs curved. • High frequency vs low frequency.

43. BiosoundEsaoteMyLab®30CV

44. Needling Technique • “In-Plane” • Safer • Always see tip of needle • Out-of-Plane” • May be easier. • Risk losing sight of needle tip.

45. Adjusting the Depth • Superficial nerves need shallow settings. • Deep nerves need deep settings.

46. Adjusting the Picture • Resolution • Set by the transducer frequency. • Gain • Near • Far • Color doppler • Shows blood flow.

47. Freeze and Save • Can save to hard drive or disk. • Able to print out hard copy. • Able to save video. • New reimbursement requirements Must have proof that block was done with ultrasound if you billed for it.

48. Regardless of the machine or transducer selected, there are four basic transducer manipulation techniques, which can be described as the “PART”of scanning: • Pressure (P): Various degrees of pressure are applied to the transducer that are translated onto the skin. • Alignment(A):Sliding the transducer defines the lengthwise course of the nerve and reference structures. • Rotation(R):The transducer is turned in either a clockwise or counterclockwise direction to optimize the image (either long or short-axis) of the nerve and needle. • Tilting(T): The transducer is tilted in both directions to maximize the angle of incidence of the ultrasound beam to the target nerve, thereby maximizing reflection and optimizing image quality.

49. Advantages of Ultrasound • Safety - no radiation. • Visualize nerves. • Visualize surrounding structures. • Avoid complications. • Visualize spread of local anesthetic. • May be more reliable and quicker onset of action. • Visualize the internal anatomy. • Advance the needle under real time guidance (dynamic images). • Observe the local injection (accuracy).

50. Summary • Any block technique works if you are good • All current RCTs are small • Ultrasound decreases:: • Onset time • Number of needle passes • Discomfort if stimulation is avoided • Decreased LA volumes • Increased successful rate