MD Notes

creation date: 2025-01-22 19:58
status: note
tags: Clinical & Communication Skills, Directed Independent Learning

POCUS

Introduction to POCUS

Intro

  • POCUS > RADUS for:
    • At bedside
    • Guide needle/removal of foreign body
    • Rapidly done
    • Real time interpretation
    • Easy to repeat
    • Answer targeted questions
      • Blood?
      • Is there AAA?
      • Is there cardiac tamponade?
      • Is there miscarriage?
  • POCUS does not replace RADUS, but acts as an excellent adjunct to the clinical findings

Physics

  • US waves 2-10 million Hz
  • At density change, reflection of waves return to transducer
    • Greater change in density at media border = greater reflection
    • Brighter US image between tissue changes
  • Impedance = loss of sound waves through a medium
    • Shade of grey in image reflected by impedance
    • Fluid has low impedance → low reflection → shows up black/darker
      • Anechoic = reflects little/none
      • Can use fluid filled organs to “see through them”; eg. full bladder before uterus ultrasound
    • Bone has high impedance → high reflection → shows up white/bright
      • Hyperechoic = reflects everything
        • Eg. bones like the spine
        • Area behind hyperechoic structure = acoustic shadow
      • Greying out behind a solid structure (that is not necessarily hyperechoic)
  • Scatter = scattered waves due to gas
    • Makes image hard to interpret

Probes

  • Curvilinear Probe
    • Low frequency
      • High depth of penetration
      • Low resolution
    • Good for most applications
    • Large footprint (size of probe space) makes it difficult to squeeze through spaces like the ribs
  • Phased Array Probe
    • Low frequency
      • High depth of penetration
      • Low resolution
    • Smaller footprint (good for thorax by going between ribs)
    • Sacrifices resolution for capturing motion
      • Good for visualizing chambers of heart
  • Linear Probe
    • High frequency
      • Low depth of penetration (<6 cm from surface)
      • High resolution
  • Endocavitary Probe
    • Transvaginal scans
  • Probes will have an indicator to correspond with the US screen
  • Convention for radiologists:
    • Marking is on left of screen
    • On probe, have marking pointed to patient’s right (for transverse plane) or head (for sagittal and coronal planes)
    • Note: this is different for cardiologists

Screen orientation

  • Near field: top of screen, closest to the probe
  • Far field: bottom of screen, furthest from the probe
  • Screen left: direction of probe indicator/marking
  • Screen right: opposite of probe indicator

Probe movement

  • Sliding: moving horizontal or vertical to find structure; note that the probe does not rotate
  • Rotating (or twisting): clockwise or counterclockwise
  • Fanning (or tilting or fanning): change angle of short axis of probe; to get multiple cross sections
  • Heel/Toe (or rocking): changing angle of long axis; toe = rock towards indicator, heel = rock away from indicator

Knobs

  • Depth
    • Higher depth means computer will “listen” for echos longer → can detect deeper structures
      • If the structure of interest is shallow, having a high depth will mean the structure will only be a small part at the near field
    • Lower depth means computer “listens” shorter time
      • Will magnify the near field
    • Balance of magnification and ensuring area of interest is within range
    • Start scans with maximum depth and decrease as area of interest is found
  • Gain
    • How sensitive probe is at returning waves
    • Low gain = darker
      • Generally better for structures like the heart
    • High gain = can increase noise
      • Better for echogenic structures like the spine

Artifacts

  • Objects on screens that aren’t actually there
  • Important artifacts to note:
    • Acoustic shadow
      • Shadow behind hyperechoic structures
      • Do not be fooled into thinking it is fluid
      • Note bright white boundary of hyperechoic structure
    • Refraction/edge artifact
      • Shadows directly behind the edges due to waves hitting a smooth, cystic like structure resulting in their refraction and not reflection
      • Not fluid!
    • Enhancement
      • When wave passes through fluid filled structure
      • Waves going through fluid is not attenuated (less impedance in fluid compared to tissue)
      • Structures behind the fluid filled structure are brighter and appears hyperechoic

Cardiac POCUS and IVC

Intro

  • Does not replace ECHO
  • But can be indicated if heart issues
  • Note: cardiology/IM convention puts probe indicator on RIGHT

Probe

  • Phased Array Robe (2-5 MHz)
  • Curvilinear Probe can work too but may not be as ideal
  • Use Cardiac preset

Cardiac Views

  • Parasternal Long Axis

    • Upper chest (around 3rd-4th intercostal space)
    • Just left of sternum (parasternal)
    • Direct indicator to right shoulder
    • Beam direct posterior
    • Slide probe cephalad/caudad
    • Example views:

    • Findings/investigations
      • Pericardial effusion (fluid between the heart and the descending thoracic aorta; bound by pericardium)
        • As opposed to left pleural effusion which will be posterior to descending thoracic aorta
      • Rules of 3rd’s for diameters of RVOT, aortic route, and left atrium
        • Should be roughly 1:1:1
      • LV systolic function
        • Estimate: grossly normal, moderately depressed, severely depressed, or hyperdynamic LV function
        • Based on LV wall thickening during systole (should thicken on contraction), change in LV cavity diameter (normal = collapse by 30-50% during systole), interior mitral valve leaflet “slap” (should come within 1cm of interventricular septum)
          • Hyperdynamic = collapse of LV

  • Parasternal Short Axis

    • 90 degrees clockwise turn from long axis
    • Direct indicator to left shoulder
    • Can slide for different cross sections; 3 levels of note
      • Mid-papillary level
        • Examples:

          • 90 degrees from long axis will give this view
          • LV should be circular, if oval: rotate probe
      • Mitral valve level (“Fish mouth view”)
        • Slide probe up towards sternum
        • Example views:

      • Aortic valve view (“Mercedes benz view”)
        • Slide valve superiorly from mitral view
        • Example views:

      • Findings/investigations
        • LV systolic function
          • Same concept as long axis (thickening wall, contraction of wall collapsing to centre point)
          • Entire wall collapse = hyperdynamic
        • Abnormal septal motion
          • Indicates increased RV pressure
            • Possible obstruction in pulmonary circulation, eg. PE
          • Causes IVS to flatten, giving LV “D” shape

  • Apical 4 chamber

    • Place probe near the apex of heart
      • 1-2 intercostal spaces below left nipple
    • Direct indicator to 2-3 o’clock position
    • Aim beam towards left shoulder to visualize all 4 chambers
    • Example views:

      • IVS should be vertical on screen; centre image on LV
    • 5 chamber view is possible (5th = LVOT)
    • Findings/Investigations
      • Chamber size and enlargement/dilatation
        • View must have mitral and tricuspid valves
        • Normal: RV is 2/3 LV
      • Qualitative RV and LV function
        • Look for sufficient changing cavity size
      • Pericardial effusion
        • Fluid can be seen next to chambers

  • Subxiphoid/subcostal

    • Place probe just below the xiphoid process
    • Direct indicator to 3 o’clock
    • Flatten probe against the abdomen to image under costal margin
      • Use over-the-probe grip
    • Example views:

      • Liver used as acoustic window
      • Similar to A4C view but different orientation
    • Findings/Investigations:
      • Inferior surface of heart and IVS comes together at the apex of heart to form the a 7; known as the “7 sign”
      • Pericardium effusion
        • Inferior pericardium should be visualized in its entirety and checked for pericardium effusion
        • Sweep anterior to posterior via fanning
        • In supine patients, effusions will accumulate posteriorly so fluid pocket seen will increase in size as scan is swept posteriorly
        • Note: false positive of epicardial fat pad
          • Usually brighter and will disappear in posterior
      • LV function
        • Change in size and wall thickening can be seen

  • IVC

    • 90 degree counterclockwise from subxiphoid position (indicator toward head; cephalad)
      • May need to slide to patient’s right slightly
    • Example views:
    • Findings/Investigations:

      • Respiratory variability

        • IVC size changes with respiration; maximal at expiration and minimal at inspiration
        • 50% collapse with normal respiration

      • IVC diameter

        • Freeze image and measure at largest diameter (at end of inspiration) 1-2 cm laterally from where first hepatic vein inserts
        • Normal = 2.1 cm

      • Pathology: Plethoric (no collapse + large diameter) vs. flat IVC (collapses completely with inspiration)

      • Shock patients (note: not binary yes/no as with effusion, must correlate clinically for assess volume status)

        • Hypovolemia
          • Flat IVC
          • ≥50% respiratory variability
        • Equivocal
          • Plethoric IVC (>2.1 cm)
          • ≥50% respiratory variability
          • Not very useful for fluid status assessment
        • Increased right heart pressure/obstructive
          • Plethoric IVC (>2.1 cm)
          • <50% respiratory variability

Tips/Tricks

  • Draping
    • Towel across chest/abdomen
    • Above drape for parasternal, below drape for A4C and subxiphoid
  • Check depth properly
  • Optimize pt position
    • Bend knees to relax abdomen for subxiphoid view
    • Roll pt to left for heart to get closer to surface
  • May need to coach breathing
    • Parasternal best viewed in full expiration
    • Subxiphoid best viewed with full inspiration
  • Confirm finding in multiple views