ICU echo
Focused Cardiac Ultrasound for the ICU
Sharon McCartney
Basics of echocardiography:
2 types of resolution:
- Spatial Resolution: the ability to separate points located a very small distance
- Axial resolution: the ability to separate 2 points on the “y-axis”. Determined by the frequency of the transducer. Higher frequency probe=better axial resolution.
- Lateral resolution: the ability to separate 2 points on the “x-axis”. Determined by the “focus”. Best lateral resolution at the focal point.
- Temporal Resolution: ability to assess movement in relation to time. Determined by “frame rate”. 2D imaging will have high frame rates (50Hz or 50 frames per second). When the computer has to process any additional information (ie. color flow Doppler, 3D imaging), the frame rate will then decrease (15-20Hz or 15-20 frames per second). When the frame rate decreases significantly (<15Hz), the image will look “choppy” to the naked eye and pathology can be missed because the image is only being cut into <15 images each second instead of 50+ images each second. (analogous to slice size on a CT scan).
Basics of Image Optimization:
- Gain: gain will enhance all signals (including artifacts and noise). Gain should be set so that blood in the chamber of the heart appears black and myocardial borders appear gray. An image will be “overgained” if the blood appears gray. An image will be “undergained” if the blood appears black, but the myocardial borders are very light gray.
- Time Gain Compensation: will enhance signals selectively along the y-axis of the image. Often used when only one area of the image needs to be brightened or “gained” while leaving the other areas of the image alone.
Equal time gain compensation: Gain equally distributed on y-axis of image
Unequal time gain compensation
The top of the image has more gain than the bottom of the image. Note the knobs are unequally aligned to produce this image.
- Depth: ensure your field if interest is within your ultrasound depth. Increasing your depth beyond your field of interest will increase what you are asking your ultrasound to image and therefore will decrease your frame rate (important if also using color flow Doppler).
Keys to successful image optimization for the novice echocardiographer:
- Use appropriate depth for field of interest
- Set gain for the lowest intensity while still preserving image quality
- Change focal depth to the field of interest to optimize lateral resolution
Focused Cardiac Ultrasound 5 Basic Views:
- Parasternal Long-Axis View (PS LAX):
Transducer Position:
4-5th rib interspace
2-3 inches left of the sternum
Transducer notch towards 10 o’clock/right shoulder
Used to assess LV function and AV/MV pathology
- Parasternal Short-Axis View (PS SAX):
Transducer Position:
4-5th rib interspace
2-3 inches left of the sternum
Transducer notch towards 2 o’clock/left shoulder
From PS LAX view, turn probe so that notch is at 2 o’clock/left shoulder
Used to assess LV function and regional wall motion abnormalities
- Apical 4 Chamber:
Transducer Position:
Place probe at point of maximal impulse (PMI)
Transducer notch pointed towards 3 o’clock/axilla
Used to assess LV and RV function
- Subcostal 4-Chamber:
Transducer Position:
Place probe directly under the xiphoid process
Transducer notch towards 3 o’clock/axilla
Significant pressure may be needed for contact
Used to assess function (RV) and presence of pericardial effusion
- Subcostal Inferior Vena Cava:
Transducer Position:
Place probe under xiphoid process
Transducer notch pointed towards 12 o’clock/patient head
From subcostal IVC view, turn transducer to 12 o’clock.
Used to assess volume status in spontaneously breathing patients
Pitfalls to Avoid in Focused Cardiac Ultrasound:
- Do not estimate LVEF, instead assess if LV function is normal or not normal. LVEF quantification should be reserved for a formal echocardiogram
- Assessment of volume status derived from collapsibility of the IVC should be reserved for spontaneously breathing patients. This assessment should not be derived when patients are on mechanical ventilation
Estimating right atrial pressure using IVC diameter:
- IVC diameter 2.1cm that collapses > 50% with a sniff suggests normal RA pressure of 3 mmHg
- IVC diameter >2.1cm that collapses <50% with a sniff suggests high RA pressure of 15 mmHg
- IVC diameter and collapse that are in between these values suggest RA pressure of 8 mmHg
GREAT high quality video of TTE done in setting of RV strain / PE: click here to open in new page