Basics of peripheral nerve stimulator and ultrasound

pns and usg basic

1. BASICS OF PERIPHERAL NERVE STIMULATOR AND ULTRASOUND Presented by- Dr.Hrishikesh Bharali,PGT Moderated by- Dr.Priyam Saikia , Asst. Proff Deptt . of Anaesthesiology and Critical Care, GMCH

2. INTRODUCTION “ Regional anaesthesia always works, provided you put the right dose of the right drug in the right place ”~Denny Harrop Historically, nerve blocks were performed using anatomical landmarks as a guide as to where to insert the needle and then eliciting paraesthesia . Multiple attempts to elicit paraesthesia which was percieved as painful, failure rate of nearly 20 %, and fear of neurological sequelae prompted search for safer guidance. The advent of Peripheral Nerve Stimulation (PNS) and later Ultrasound guidance have sought to add an objective end point to aid nerve location.

3. PERIPHERAL NERVE STIMULATION PNS in regional anesthesia is a method of using a low-intensity and short-duration electrical stimulus to obtain a defined response (muscle twitch or sensation) to locate a peripheral nerve or nerve plexus with an needle and deposit local anaesthetic around the nerve or in that compartment to provide a sensory and motor block for surgery and/or eventually analgesia for pain management..

4. HISTORY 1780 : GALVANI described the effect of Neuromuscular stimulation 1912 : PERTHES developed and described Electrical nerve stimulator 1955 : PEARSON – concept of Insulated needles for nerve location 1962 : GREENBLATT & DENSON – Portable variable current output nerve stimulator 1984 : FORD – Lack of accuracy with noninsulated needles , Suggested the use of Constant current nerve stimulator

5. NERVE STIMULATOR

6. COMPONENTS OF PNS Synchronize various functions of the device Frequency,du-ration,current intensity on display panel Display of adjustalble variables Delivers same current in the face of altering impedance

7. ELECTROPHYSIOLOGY Nerve cells have a resting membrane potential of -90 mV. When a neuron is “stimulated” a transient change in the ion permeability of the membrane (an increase in the conductance of the sodium channels) occurs. If the stimulus is strong enough it depolarises the membrane sufficiently to set off an action potential which then propagates along the nerve to stimulate the muscle and causes a contraction.

8. ELECTRICAL STIMULUS A certain minimum current intensity is necessary at a given pulse duration to reach the threshold level of excitation. Rheobase : The lowest threshold current required to initiate an action potential in the nerve is called the Rheobase . Chronaxie : Chronaxie is the length of time the current must be applied to the nerve to initiate an impulse when the current level is twice the rheobase . Chronaxie values provides an indicator of the relative excitability of a nerve.

9. Chronaxie of different nerves NERVE FEATURE CHRONAXIE-ms C Unmyelinated 0.40 A δ myelinated 0.17 A α myelinated 0.05 - 0.10 10

10. WHY MOTOR FIBERS HAVE A SHORTER CHRONAXY? Because of the relatively low capacitance of their myelinated membrane ( uninsulated nodes of Ranvier are the only places along the axon where ions are exchanged : Saltatory conduction). Possible to stimulate a motor nerve but not the sensory nerve by using a current of smaller chronaxie (shorter time) . This means a motor response can be seen without producing pain-----however patient still feels TINGLING .

11. Current INTENSITY Current intensity (I) is a measure of stimulus strength and is the flow of electrical charges used to depolarize the nerve and subsequently produce a motor response, or ‘‘twitch. The delivered current is described by Ohm’s law : V = I x R (or) I = V / R where V is voltage( kV) ; I, current (mA) ; R, resistance(k Ώ ) Resistance (R) is primarily independent of the stimulator and is largely a function of tissue impedance encountered by the needle, poor connection of the return electrode., Connecting wires. Modern nerve stimulators maintain a constant current by raising or lowering the voltage ( V) in response to changes in resistance.

12. FREQUENCY Frequency refers to the number of repeating events per unit time. The ideal electric parameters for comfortable stimulation is 1 to 2 Hz. A higher frequency will give more frequent feedback to the operator, but often causes greater discomfort to the patient. Cycles/second

13. NEEDLE TO NERVE DISTANCE COULOMB’S LAW : E= K(Q/r2) where, E is the stimulus intensity K is a constant Q is the minimum current from the needle tip r is the distance of the stimulus source from the nerve. Rearranging the equation, Q ∝ r2 Hence, at a low current intensity, the nerve will only be stimulated when the needle is very close to it and therefore the ability to stimulate the nerve at a very low current is an indication of proximity to the nerve.

14. POLARITY “negative to needle,positive to patient” When negative current is applied to the surface of a nerve , there is resultant depolarisation leading to generation of an action potential. It is better to have the needle as the cathode because if the needle is positive (the anode) then the nerve will get hyperpolarised and a larger current will be needed to depolarise the nerve and obtain a response.

15. STIMULATING NEEDLES Insulated needles are coated with a layer of non conducting material – Teflon or silicon Upon stimulation , the current density focusses on needle tip Hence, a low threshold current is sufficient to stimulate the target nerve

16. STIMULATION AND INJECTION TECHNIQUE 17 Desired initial: Current:1-2mA Pulse duration:0.1 ms Frequency:1-2Hz Current gradually reduced & needle advanced slowly once sought after muscle response obtained Threshold: 0.2-0.5 mA at 0.1ms Aspirate~test dose of 1-2 ml LA injected which abolishes muscle twitch Failure of the twitch to disappear, pain on injection of solution or high injection pressures suggests intraneural placement of the needle tip and warrants small withdrawal of the needle tip (0.5 -1mm) Increase the current to initial level No stimulatory response-inject the remaining drug

17. IDEAL ELECTRICAL CHARACTERISTICS OF A PNS 18

18. BASICS OF ULTRASOUND

19. ULTRASOUND FOR ANAESTHESIOLOGISTS Current and potential future applications of US in anesthesiology are summarized as follows:

20. ULTRASOUND principle Sound waves at higher frequencies than can be detected by human ear (>20,000 Hz). Medical ultrasound : very high frequency (1-20 MHz). US machines utilize the pulse echo principle US waves are created by a vibrating crystal within a ceramic probe-> transmitted into the patient --> echoes return from various tissue interfaces --> detected by probe --> processed by computer --> visualised as an image on screen

21. ULTRASOUND MACHINE PARTS Transducer probe Central Processing Unit(CPU)- Computer that does all of the calculations Transducer pulse controls- changes the amplitude,frequency and duration of the pulses emitted from the probe Display - displays the image from the data processed by the CPU Keyboard Disk storage devices ( hard,floppy,CD ) Printer

22. Signal Intensity Determined by degree of reflected waves returning to the transducer Larger intensities = Strongly reflected = Hyperechoic image (Whiter) Weaker intensities = Weakly Reflected = Hypoechoic (Darker) 23

23. Image: (1) Sites et al Ultrasound Wave Interaction with Tissues Reflection SPECULAR (large smooth objects like a needle) (d) SCATTERING (most neural images) (a) Refraction (c) Transmission (b)

24. Reflection of Ultrasound Waves Proportional to the difference in acoustic impedance between adjacent tissues Greater difference = better distinction = better resolution Explains the varying appearances of nervous tissue on U/S imaging Interscalene vs Popliteal

25. Refraction Occurs at tissue interfaces ( unreflected ) Refraction can diminish image quality Increases with angle of incidence Optimal angle of incidence is 90° Perpendicular probe minimizes effect

26. Attenuation Progressive loss of energy with signal propagation Results in progressive decrease in returning signal Major source is conversion of acoustic energy to heat Loss of signal is directly related to depth High frequency results in greater attenuation Represented by the Attenuation Co-efficient Bone>>Air>>>Muscle>Brain>Fat>Blood>>> Water.

27. Overcoming Loss of Signal from Attenuation Artificial Enhancement (Adjusting Gain ): the intensity of the reflected sound wave is amplified Time Gain Compensation Adjusts gain independently at specified depth intervals Most modern U/S machines do this automatically ( autogain ) Choosing lower frequencies for deeper tissues (posterior sciatic)

28. RESOLUTION Resolution refers to the ability to distinguish one object from another. Types: Spatial -smallest distance that two target can be separated for the system to distinguish between them. Two components- Axial Lateral Temporal

29. Axial resolution The minimum separation between the structures the ultrasound beam can distinguish parallel to its path. Determinant : Size of wavelength-high frequency wavelength better .

30. Lateral resolution Minimum separation between structures the ultrasound beam can distinguish in a plane perpendicular to its path. Determinants: Beam width-smaller the better Temporal resolution Ability of system to accurately track moving targets over time. Determined by Frame rate

31. Frequency Summary High frequency Improved resolution Depth of penetration less For superficial structures Low frequency Poorer resolution Depth of penetration more For general abdominopelvic uses

32. PROBE (TRANSDUCER)

33. Every probe has an orientation marker that correlates with another marker displayed on the ultrasound screen

34. ULTRASOUND GEL Ultrasound gel is a type of conductive medium that enables a tight bond (acoustic seal) and removes air between the skin and the probe or transducer, letting the waves transmit directly to the tissues beneath and to the parts that need to be imaged.

35. MODES of ULTRASOUND A-mode (A=amplitude): Oldest mode.Amplitude of reflected ultrasound is displayed as one dimensional image.Currently used only in opthalmology . B-mode(B=brightness): a two dimensional mode and provide a cross sectional image through the area of interest and is the primary mode currenly used in regional anaesthesia . M-mode (M=motion): one dimensional mode against time mostly used for cardiac imaging.

36. D-mode(D= doppler ):measures the shift in the frequency between the incident and the reflected wave after hitting a moving object. Color Doppler Continuous Wave Doppler Pulse wave Doppler Dopler Duplex

37. Frequency Summary High frequency Improved resolution Depth of penetration less For superficial structures Low frequency Poorer resolution Depth of penetration more For general abdominopelvic uses

38. All Ultrasound Guided Blocks Involve Three Steps: Choosing One of Two Imaging Views Short Axis View Long Axis View Scanning the Nerve Track for Image Optimization Choosing a Needle Approach Technique In Plane Out of Plane

39. Imaging Plane Options Long Axis View or Longitudinal View Rarely Used Short Axis View or Transverse View Most Commonly Used Relatively Easy Better Resolution of Fascial Barriers that Surrond Nerves Dynamic Assessment of Circumferential Local Anesthetic Spread Workable Image Even with Slight Movement of Transducer Probe

40. Where is The Needle Coming From? Out of Plane Technique Inserting the needle so that it crosses the plane of imaging near the target. In Plane Technique Inserted within the plane of imaging to visualize the entire shaft and tip.

41. Out of Plane Approach

42. In Plane Approach

43. Out of plane In plane Advantage: Shorter Needle Insertion Paths Less Patient Discomfort Easier to Perform Disadvantage: Unable to accurately track needle tip Advantage: Ability to Track the Needle Tip Theoretically Safer Disadvantage: More Time Consuming More difficult to perform Can be more painful secondary to longer insertion paths