84 Ultrasound
84.1 What this covers
- Modes
- Physics:
- Goal-directed Echo
84.2 Learning objectives
- Modes
- Physics:
- Goal-directed Echo
84.3 Bottom line / summary
- Ultrasound in the ICU Series, ATS
- brightness mode conventional.
- 2d (spatial) map of reflectiveness
- (M)-mode - aka Time-motion mode: gives information on 1 slide through time..
- Note: Duplex using doppler and b-mode
84.4 Approach
- pulse wave doppler sends, then listens after (and not sending at the same time) gets artifacts due to Nyquist effect, also only gets motion vector coming directly to probe.
- Tissue doppler is a variation of this with filters eliminating blood
- Color wave doppler uses pulses and uses info from each pixel to determine a color (still has limitation of aliasing)
- continuous wave doppler continuously sending and listening simultaneously (2 elements / piezoelectric elements) no aliasing
- resolution of ultrasound depends on wavelength (frequency. b/c speed wavelength frequency).
84.5 Red flags / when to escalate
- ##Shock Assessment
84.6 Common pitfalls
- TODO: Capture common errors or missed steps.
84.7 References
- https://www.thoracic.org/professionals/career-development/residents-medical-students/ats-reading-list/adult/ultrasound-in-the-icu.php
- https://www.atsjournals.org/doi/pdf/10.1513/AnnalsATS.202008-948CME
- https://www.thoracic.org/professionals/clinical-resources/critical-care/clinical-education/ultrasound.php
84.8 Source notes
84.8.1 Ultrasound
85 Ultrasound
Ultrasound in the ICU Series, ATS
85.1 Modes
- = brightness mode = conventional. 2d (spatial) map of reflectiveness
(M)-mode - aka Time-motion mode: gives information on 1 slide through time..
85.1.1 Doppler
- pulse wave doppler = sends, then listens after (and not sending at the same time) gets artifacts due to Nyquist effect, also only gets motion vector coming directly to probe.
- Tissue doppler is a variation of this with filters eliminating blood
- Color wave doppler uses pulses and uses info from each pixel to determine a color (still has limitation of aliasing)
- continuous wave doppler = continuously sending and listening simultaneously (2 elements / piezoelectric elements) = no aliasing
Note: Duplex = using doppler and b-mode
85.1.2 Physics:
- resolution of ultrasound depends on wavelength (frequency. b/c speed = wavelength * frequency).
- Speed of sound: air =< lung < fat < soft tissue < bone. US assumes soft tissue speed.
- energy will disperse further from probe - thus, highest resolution = only have what you are looking at in the field of view
85.2 Goal-directed Echo
- RACE (Rapid Assessment by Cardiac Echo): 5 views
- parasternal long
- parasternal short
- apical four- and two-chamber
- subcostal.
- Draping: 2 large towels above and below then just expose the area you need (keeps the gown clean)
- Goals to answer: How is the LV function? how is the RV function, is there a pericardial effusion, and how is volume status.
85.2.1 Apical 4 chamber view
- 6th intercostal space at midclavicular line. Under breast or pectoral crease. Easier if they are left lateral decubitus
- assess RV size, doppler of mitral valve (for LV pressure)
- 5 chamber view = same position, but tilting the view anteriorly (dropping the tail of probe) to visualize LV outflow track as well (can determine LV stroke volume)
- if not getting left atrium, you may be imaging too low (under-side / posterior). Too medial = RV prominent -rotate 90 deg clockwise = apical 2 chamber view.
85.2.2 Parasternal long axis
- less useful? Per ATS ( can measure LV end diastolic diameter to diagnose dilated cardiomyopathy )
- 3rd or 4th intercostal space, probe oriented toward right shoulder
- try to get left ventricle in it’s entirety (if you can between rib shadow). Ideally, also with mitral and aortic valve.
- too much aortic root = (sternum keeps you from being too medial.. so you may be too high)
- if too much apex = can be either too low or too lateral
85.2.3 Parasternal short axis
- rotate 90 degrees clockwise from long axis
- evaluate LV fractional area contraction (surrogate for EF) at papillary muscle level. -evaluate for paradoxical septal motion. (RV dilation + paradoxical septal motion = definition of cor pulmonale) -can tilt field of view toward R shoulder to see pulm trunk = doppler allows RV ejection flow -goal: LV looks circular in the middle of the screen -can tell which part of LV you are in by presence or absence of papillary muscle -too close to apex = looks like apical 4 chamber (elongates)
85.2.4 Subcostal aka Subxiphoid view
-below xiphoid, indicator superior for long axis. Can rotate 90 degrees clockwise for short axis. Probe should be as close to parallel as possible. (can bend their legs to relax rectus abdominus) -look for pericardial effusion (and any collapsing of R heart) -underestimates RV and LV size - liver will be at top of field of view
85.3 Shock Assessment
-Ventricular size: apical 4 chamber or parasternal short. Kissing papillary muscles - can also measure stroke volume variation from apical 5 chamber view (variation with respiratory cycle predicts fluid responsiveness).
85.3.1 IVC size and variation with respiration.
-subxiphoid, but then move perpendicular to skin (probe to head), then move to the patient’s right. Rotate 90 degrees (to longitudenal view), then visualize hepatic vein and right atrium. -If spontaneously breathing, IVC < 2cm more likely CVP less than 10 (though this does not necessarily indicate fluid responsiveness). -Similarly, change 40+% with respiration (both mechanically or not) predicts fluid responsiveness.
85.4 Thoracic Ultrasound
- For pleura, high frequency linear probe should offer good resolution. However, in obese patients or if trying to visualize lung deeper will need low frequency probe.
- mid axillary line is probably good first location (though full exam is 3 frontal planes)
Annals ATS review of pleural findings: https://www.atsjournals.org/doi/pdf/10.1513/AnnalsATS.202008-948CME
A lines: reverberation from pleura, show up as hyperechoic lines parallel to pleura but inside the lung. Does not indicate pathologic process.
B lines: vertically oriented lines that extend from the pleural serface to the max depth of the image and move with lung sliding. Indicates thickened interstitium, most commonly from pulmonary edema.
Consolidation: looks similar to liver. Difficult to differentiate between pneumonia/airspace filling and atelectasis.
Seashore sign: put US in M-mode and expect a speckled appearance. If there is linear pattern both above and below the pleura, that means there is no lung sliding (e.g PTX)
85.5 DVT
- compression US has similar accuracy to duplex (compression + doppler) if done at bedside
- start at inguinal ligament, and compress every 1-2 cm down the common femoral.
- ‘smoke’ in the vein is common in low flow states and does not indicate a thrombosis. Also, be sure you’re not seeing a lymph node (can look similar to thrombosed vein)
85.6 Renal/Urologic Ultrasound
-Technique: obtain transverse + longitudinal view of kidneys and a view of the bladder. -Hydronephrosis: identify anechoic region, apply color doppler to ensure there is no enhancement -Low UOP with foley => differentiate full bladder (catheter problem) from empty bladder (low urine creation)