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Lung Ultrasound in Critical Care Medicine
Probe selection:
Most suited probe is the micro-convex 2.5- 7 MHz. Phase array probe 2.5-5 MHz can also be used as an extension of the echocardiography examination. The pre-set on the machine is needed to set to lung or pleural examination. Linear array probe has a high spatial resolution and low depth of penetration, which can add value to pleural examination. It is of less value in evaluation parenchymal consolidation and pleural effusion in which higher depth of penetration of ultrasound beam is required.
Probe position:
The lung can be scanned in a transverse dimension (perpendicular to two adjacent ribs and the rib space) or in a longitudinal axis in parallel to the ribs over the rib space. The longitudinal scan is rarely used to assess the extent of aeriation of the lung at a particular lung region as it involves a larger volume of lung to be inspected compared to the transverse approach.
The transverse approach is the most commonly used clinically. Probe is oriented perpendicular to the two adjacent ribs. The marker on the probe corresponds to the left side of screen. This represents the diaphragm and abdominal organs on the right of the screen and the lung and structure above the diaphragm on the left side of the screen.
Image acquisition and getting familiar with the structures:
Lung fields
There are multiple methods being suggested to divide the lung fields into area of interests (AOIs). One of them recommends to divide the anterolateral hemi-thorax into four areas by longitudinal parasternal, anterior axillary and posterior axillary line at the 5th-6th intercostal space. They can be named as L1, L2, L3, L4 and R1, R2, R3, R4 (L- representing for left and R- representing for right side hemi-thoraces) (Figure 1). Lung Ultrasound Score (LUS) also takes the posterior segments L5, L6 and R5, R6) in addition to the traditional segments as described above.
Terminologies
A- Line: - Hyper echoic lines at regular interval corresponding to the distance between transducer and pleural line formed as a result of reverberation artefact. It represents a high gas-volume ratio underneath the parietal pleura which can be present in normal lung, hyperinflation or pneumothorax.
Bat Sign
The ideal image acquisition includes an imaginary structure of a “Bat Wing” called as “Bat Sign”. The echogenic anterior periostia of the adjacent ribs form the wings and the hyperechoic parietal pleura forms the body of the bat. (Figure 2).
Video 1. "Bat Wing" appearance with >2 B lines
Each AOI is evaluated for lung sliding, lung artefacts, consolidation and pleural effusion.
Lung Sliding (LS): -
It represents the movement of the visceral pleura on the parietal pleura with tidal ventilation. It represents that both the pleural surfaces are in touch with each other.
Clinical significance:
Reduced LS: Reduced regional ventilation- Hyperinflation, emphysematous bullae
Absent LS: Absent regional ventilation- Pneumothorax, endobronchial intubation of contralateral side.
To differentiate Absent LS from reduced LS: -
LS present: Tissues superficial to pleura and lung do not move away or towards the probe with tidal breathing which are represented as parallel lines above the pleural lines.If the visceral pleura slides, it represents a sandy pattern on ‘M’ mode called “Sea Shore Sign”
If the visceral pleural is in contact but does not move with tidal breathing (due to either there is minimal or no ventilation to the underneath lung or the visceral pleura is fixed to the parietal pleura and does not slide on it even if it stays in contact with the prior), there will be no sandy pattern underneath the pleural line but the “Lung Pulse” which represents the synchronous movement of it with the movement of heart can be visualized.
If the visceral pleura is not in contact with the parietal counterpart due to pneumothorax, “M” mode would represent straight line both above and below the pleural line “Stratosphere Sign” with a “Lung point” representing the edge of the contact point of visceral and parietal pleural where the “Sea Shore Sign” changes to “Stratosphere Sign”.
Lung Ultrasound Score
Various scores have been used based on ultrasound lung finding. Lung Ultrasound Score (LUS) is described as following: -
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The LUS score ranges from 0-36 (6 regions on each hemi-thorax, total maximum points for each hemi-thorax = 6X3, Total maximum score 2X18=36.
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A LUS of >17 after a successful Spontaneous Breathing Trial (SBT) is highly predictive of post extubation respiratory distress. A LUS <13 is highly predictive of successful weaning from ventilation.
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Similarly, this score has been applied to assess the deleterious effect of fluid resuscitation and guide fluid therapy, monitoring aeriation in patient of ARDS on Extra Corporeal Membrane Oxygenator (ECMO).
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A re-aeriation score has been used to evaluate the effect of antibiotics on lung re-aeriation in patients with Ventilator Associated Pneumonia (VAP) and effect of PEEP on re-aeriation in patients of ARDS.
Structural abnormalities
Pleural effusion
Pleural effusion usually presents as an anechoic or hypoechoic area bounded by the parietal, visceral, rib shadows and the diaphragm. On ‘M’ mode on the pleural line will show a sinusoidal pattern with breathing which represents the floating motion of the lung in the pleural effusion. Presence of particulate matter and septae in the pleural effusion may point towards an exudative pleural effusion and percutaneous drainage of a loculated effusion may not be the ideal management option.
Consolidation
Consolidation can be trans-lobular or non-translobular. The former represents as subpleural echo poor structure with a clear irregular margin called as “Shred Sign”. The later presents as a structure with tissue density.
Dynamic air bronchogram seen as multiple white spots on lung US rules out obstructive atelectasis. Whereas presence of consolidation without any dynamic air-bronchogram can be a feature of obstructive atelectasis which can be benefitted by fibre optic bronchoscopy to remove any secretions.
The Bedside Lung Ultrasound in Emergency (BLUE)-protocol simplified
Other important clinical implications
Use of lung ultrasound in optimizing mechanical ventilation.
Tracheal ultrasound to confirm endotracheal tube position.
Ultrasound to confirm Nasogastric tube position.
