12a ULTRASOUND AND BLOCKS (24)

Question 1

WHAT IS ULTRASOUND?

Answer 1

• the normal range of human hearing is 20 Hz to 20 kHz
• sound of frequencies higher than 20 kHz = ULTRASOUND, although Diagnostic Ultrasound generally ranges from 1-20 MHz

Question 2

3 (+2) ANATOMICAL PLANES FOR ULTRASOUND

Answer 2

there are 3 main anatomical planes used to describe ultrasound:

1. the AXIAL PLANE slices across the long axis of the body, like slicing a salami
2. the CORONAL PLANE divides the anterior and posterior surfaces (like a CROWN or Tiara on the head)
3. the SAGITTAL plane divides the Left and Right sides (like the Sagittal sinus)

in addition, when scanning a long tubular structure like a vein, the words SHORT and LONG AXIS may be used to describe views perpendicular and parallel to it

Question 3

ULTRASOUND PROBE ORIENTATION

Answer 3

• the probe is normally oriented with the marker to the patients RHS or Head unless standing above (eg IJV), when its directed L, to maintain screen orientation

Question 4

WHICH HAND HOLDS THE US PROBE?

Answer 4

• for DIAGNOSTIC IMAGING, the DOMINANT hand holds the probe
• for PROCEDURAL IMAGING, the NON-DOMINANT hand does

Question 5

ULTRASOUND PROBE COVERS AND CLEANING

Answer 5

• for DIAGNOSTIC IMAGING on clean unbroken skin, no cover is required, and unsterile gel suffices
• for _SINGLE SHOT BLOCKS AND PERIPERAL IV_s, sterile gloves, sterile probe sleeve and sterile gel inside and outside* are required, .
• for CENTRAL VASCULAR ACCESS AND NERVE CATHETERS: use a full sterile field as well.
• To clean: unsheath, wipe clean with TUFFY WIPES, including the cable, & air dry
• * in theory, unsterile gel inside is ok, provided the sleeve has no microperforations*

Question 6

ULTRASOUND MODES

Answer 6

there are 4 main modes

1. B MODE = BRIGHTNESS MODE, this is the basic 2D realtime image.
2. M MODE = MOTION MODE, which displays motion over time, eg when looking for PT
3. COLOUR DOPPLER MODE uses colour to display direction of flow
4. POWER DOPPLER, a more sensitive Doppler mode that displays PRESENCE of flow without indicating its direction

Question 7

ULTRASOUND FREQUENCY SETTINGS

Answer 7

• Low frequency = 1-5 mHz
• Intermediate = 5-10 &
• High frequency = 10-15 mHz
• Higher frequency = better resolution but less penetration, you chose the highest frequency consistent with the target tissue depth

Question 8

2 COMMONLY USED ANAESTHESIA/ED ULTRASOUND PROBES

Answer 8

1. The HIGH FREQUENCY LINEAR PROBE: this is a large probe with a flat face, used for high res imaging of small shallow structures, eg nerves and vessels
2. The LOW FREQUENCY PHASED ARRAY PROBE: this is a small* probe used for imaging deeper structures with restricted access windows: eg between the ribs in FAST

* the probe size is kept small by electronically scanning the beam from side to side, instead of arranging the crystals in a straight line: this produces a wedge shaped screenview

Question 9

ULTRASOUND KNOBOLOGY

Answer 9

There are 4 important control knobs on an US machine:

1. DEPTH: adjusts the depth of tissue shown on the screen: set to place the target tissue in the middle of the screen
2. GAIN: amplifies the returning signal to give satisfactory screen brightness, and may include NEAR, FAR & OVERALL GAIN: more gain is generally less accurate due to amplification of artefacts.
3. FOCUS: the beam first converges as it leaves the probe, then diverges, with best resolution at the focal point between the 2, so focal depth should be set to match target tissue depth, although many machines now automatically optimise focus for display depth
4. DYNAMIC RANGE: is the number of shades of grey used in the image: reducing range increases contrast, which is helpful for vascular access.

Question 10

5 THINGS TO SELECT BEFORE SCANNING

Answer 10

1. Select the PROBE
2. Select the MODE (B, M, Doppler etc)
3. Select the PROGRAM (nerve, vasc etc)
4. Select the FREQUENCY (Res, Pen,Gen)
5. Select the DEPTH

Question 11

DISTINGUISHING NERVES, VEINS & ARTERIES ON ULTRASOUND

Answer 11

• NERVES have a speckled appearance in cross section, and linear streaks in longitudinal
• VEINS are easy to compress, distend with valsalva (humming) and collapse with a sniff
• ARTERIES are difficult to compress, and pulsate
• DOPPLER shows direction of flow

Question 12

ACRONYM FOR DOPPLER COLOUR SHIFT?

Answer 12

BART:

• Blue = Away
• Red = Towards

Question 13

HOW TO DO AN AXILLARY BLOCK (by landmarks)

Answer 13

blocks the arm below the shoulder, but best below the elbow, and tends to miss the radial forearm (MCN)

METHOD

• arm and shoulder at 90 degrees
• insert needle under ant axill fold, give 40 mls 0.5% Ropivacaine
• top up MCN with 5 mls lateral to the biceps tendon in the cubital fossa

Question 14

WHAT ARE THE 3 ANATOMICAL TRIANGLES IN THE NECK?

Answer 14

the 3 anatomical triangles in the neck comprise:

1. The ANTERIOR TRIANGLE, bounded by the SCM, midline & mandible. It has little anaesthetic relevance
2. The POSTERIOR TRIANGLE, bounded by SCM, trapezius & clavicle, and important for the INTERSCALENE GROOVE between anterior & middle scalenes in its floor, containing the brachial plexus nerves.
3. The MIDDLE TRIANGLE, a narrow sliver bounded by the STERNAL & CLAVICULAR heads of the SCM, and the clavicle, and important because its apex (between the 2 heads) marks the needle entry point for anatomically based approaches to IJV puncture.

Question 15

HOW TO DO A SCALENE BLOCK (landmark & USG)

Answer 15

covers the shoulder and arm, but may miss the ulnar hand, beware phrenic block

LANDMARK METHOD

• supine with head turned away
• at C6 (CRICOID) level, feel for the interscalene groove
• give 40 mls 0.5% Ropivacaine

USG

• pt semi sitting with head on a pillow and turned away
• place a linear high freq probe in the supraclavicular fossa parallel to the clavicle to identify the brachial nerves immediately lateral to the Subclavian artery
• slide the probe cranially* until the 3 dark circles of the roots are seen stacked vertically in the interscalene groove at ~C6 level
• advance the needle from posteriorly, through the middle scalene, and give 30mls 0.5% ROPIV

*noting, as you do, that the probe rotates so lateral becomes posterior

Question 16

SCIATIC NERVE COVERAGE & HOW TO DO A POPLITEAL SCIATIC BLOCK

Answer 16

the SCIATIC NERVE supplies the post thigh, lateral leg, and dorsum/lateral/sole of the foot. It carries most of the sensory supply below the knee and can be blocked at the knee with a POPLITEAL SCIATIC BLOCK:

METHOD: LANDMARK

• pt supine - insert needle approx 1 handbreadth superior to patella, just anterior to the hamstring tendon
• go in from laterally ~ 5cm in the plane of the floor
• give 30 mls 0.5% Ropivacaine

METHOD: USG

• position either lateral with pillow between knees (if awake) or supine with calf on bolster, & hip & knee at 90 degrees
• place a linear high freq probe axially over the middle of the popliteal fossa and identify the TIBIAL component of the nerve in the midline, immediately superficial to the vein and artery*.
• Slide the probe a few cm proximally until you see the PERONEAL branch of the nerve joining**.
• Advance the needle from directly lateral, aiming at the junction, and give 20mls 0.5% Ropiv (note, onset can take >30m)
• * the ‘snowman in the back of the knee’*
• **otherwise your block will miss this component*

Question 17

FEMORAL NERVE COVERAGE & HOW TO DO A FEMORAL BLOCK

Answer 17

the FEMORAL NERVE supplies sensation to the hip joint, femur & knee joint, ant & medial thigh, and medial leg & foot, but misses the medial knee (Obturator n) & lateral thigh (LCN of thigh)

it runs deep to the midpoint of the GROIN crease, MEDIAL to the ILIACUS muscle and LATERAL to the Artery (I-NAVY).

METHOD USG

• place a linear high freq probe over the midpoint of the groin crease and insert needle under the lateral edge.
• advance just superficial to the ILIACUS muscle aiming toward the femoral artery
• inject 20 mls 0.5% Ropivacaine, watching for spread around the nerve but BELOW the artery (spread above indicates you are still outside the FASCIA ILIACA* and LA will not reach the nerve)

* which encases iliacus: the femoral N lies under this fascia

Question 18

LATERAL CUTANEOUS N OF THIGH BLOCK

Answer 18

• The LCN of the thigh supplies the skin over the lateral thigh and so can carry pain from a NOF or TKR incision which is missed by femoral nerve block
• it runs deep to the lateral end of the inguinal ligt and can be blocked by advancing a needle vertically at a point 2cm medial and 2cm inferior to the ASIS until the pop of the fascia lata is felt, then 5mls of 0.5% Ropivacaine is injected

Question 19

WHATS A ‘3 IN 1 BLOCK?

Answer 19

the ‘3 in 1’ block is a high volume (30mls) Femoral Nerve block, aiming to also block, by transverse spread under the fascia:

• the LCN thigh (which covers the skin of the lateral thigh, including NOF and TKR incisions)
• the OBTURATOR N, (which covers the skin of the medial knee)*

* obturator coverage is harder to get

Question 20

HOW TO DO A FOOT BLOCK

Answer 20

RECALL that the foot is supplied by 5 nerves:

• 2 deep nerves (Posterior tibial & Deep Peroneal: which run with the pulses)
• 3 superficial nerves

METHOD:

start with a numb foot, then take 20 mls 2% lignocaine and give:

• 4 mls to each deep n (over the dorsal midfoot and behind the medial malleolus)
• use the rest to ring block the ankle - may top up with a digital/ray block also

Question 21

HOW TO DO A TAP BLOCK

Answer 21

the TAP block aims to block sensation to the anterior and lateral abdominal wall from COSTAL MARGIN to GROIN CREASE (T7 - L1) by blocking the anterior rami of the segmental nerves with a pool of local deposited into the plane between the Internal Oblique and Transversus Abdominus muscles

METHOD USG

• place a linear high freq probe axially on the MAL over the flank between costal margin and iliac crest
• identify the 3 layers of the abdo wall (EOM, IOM, TAM) noting that TA is comparatively thin and bowel lies directly below!
• advance the needle posteriorly from the anterior edge of the probe, aiming for the space between IOM & TAM (a series of 3 pops may be felt as the needle traverses the fascia sup to the EOM, between EOM/IOM and deep to IOM)
• inject 20mls 0.5% Ropivacaine, watching for separation of the layers
• repeat on other side if needed

Question 22

HOW TO DO A BIERS BLOCK

Answer 22

needs a BIERS CUFF (monitored but not necessarily double)

METHOD

• IVT both sides
• lift and exanguinate limb, cuff up and check pulse loss
• give 40 mls 0.5% PLAIN LIGNOCAINE
• works in 15m, dont release before 20, release causes rapid loss of block

Question 23

USG IJV CANNULATION

Answer 23

• stand at the head of the patient with the machine on the R and the probe marker oriented to the patients L
• place a linear high frequency probe axially over the apex* of the MIDDLE TRIANGLE to identify the IJV which is usually larger, more superficial and more lateral than the CAROTID

* ie over the entry point for the anatomical technique

Question 24

USG SUBCLAVIAN LINE CANNULATION

Answer 24

• place a linear high freq probe parasagitally over the midpoint of the clavicle to identify the vein caudal to the clavicular shadow
• advance the needle from lateral, noting that lung lies deep to the vein, so posterior wall puncture should be avoided