Airway Gastric and Pulmonary Ultrasound

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Along with serving as a screening tool for difficult laryngoscopy, airway ultrasound assists in selection of endotracheal tube (ETT) size, confirmation of ETT placement and  procedural guidance for percutaneous cricothyroidotomy, 

While  clinical airway screening tests (e.g., Mallampati, lip bite test) intend to predict difficult airways, none have a 100% predictive value.  Several airway ultrasound scans have at least moderate correlation with difficult intubation (Cormack Lehane grade ≥ 2) including this skin to hyoid distance scan.  The distance from skin to hyoid bone with a cut-off point at 1.19 cm could be predicted with a positive predictive value of 36.3%  and a negative predictive value of 91.6%.

  The distance from  the skin to anterior commissure of the vocal cords had similar value to DSE though less robust especially across genders. The distance from skin to anterior commissure of the vocal cords had a cu-off value of 0.82 cm with a PPV of 35.6% and a NPV of 87.7%. Based on these and other results ultrasonography for preoperative airway assessment is now recommended in anesthesiology curriculum training

The thyroid cartilage acts as the starting cephalad reference point in the transverse "TACA" scanning sequence (Thyroid-Airline-Cricoid-Airline) to systematically locate the cricothyroid membrane. First, the prominent, triangular thyroid cartilage is visualized. Moving caudally, the hyperechoic linear CTM can be found. More caudally, the hypoechoic lateral edges of the cricoid cartilage will appear to converge anterior to the CTM. Scanning cranially, the CTM can then be identified again  Structures identified in this transverse view at the level of the thyroid cartilage include Tc, thyroid cartilage; VL, vocal ligaments; Ac, arytenoid cartilage; Sm, sternocleidomastoid muscle.

Point-of-care ultrasound (POCUS) scanning of the cricothyroid membrane provides rapid, real-time identification of anterior neck anatomy to guide emergency front-of-neck access and airway blocks. Key in this scan is first identifying the thyroid cartilage, then the cricoid cartilage, then finding the CTM with its prominent air-mucosal interface. Cricothyroid membrane in transverse view. Sm, sternocleidomastoid muscle; CTm, cricothyroid membrane

The endotracheal tube is visualized with either a "double tract" characteristic or filling the cuff with saline. First the trachea or cricoid cartilage is visualized using transverse and sagittal plane. The esophagus should also be interrogated for the ET tube along with "lung sliding" which is evidence of lung inflation and deflation.

In the "Tool or Toy" category, I am gong to go out on a limb and say this may be a redundant scan. I am sure the senior clinician who took the image of the ET tube in the esophagus knew where it was. Regardless the empty esophagus appears as a flat, multi-layered structure usually situated to the left of the trachea. Seeing a tube inside this structure confirms an esophageal intubation The "Double Track" or "Sign of the Ring": When a tube enters the esophagus instead of the trachea, the scan shows a dilated, rounded structure next to the trachea containing a distinct double hyperechoic line or air artifact (often called the double-track sign)

 In comparing three prandial states in the same reluctant CRNA, here   

we have a normal gastric scan at 8 AM before coffee. The patient is positioned RLD 25-45 degrees, the gastric antrum is imaged by tilting the probe between 11 and 1 o’clock. Notice the distinct “bulls eye” pattern, with the liver and aorta in plane along with the superior mesenteric artery.

After a 30" period the same person was scanned in the same position and plane after consuming a left over soft taco. There is a frosted glass appearance with ring down from the solid food in the gastric antrum. No measurement is necessary of the volume in the antrum as this would be a cancellectomy.

Using a BodyWorks Eve simulator, empty and fluid-filled gastric antrum scans are toggled back and forth. There s a stark difference between the bull's eye/target with the empty antrum compared to the anechoic, thin walled fluid filled antrum. If scanning with a Butterfly of other handheld/console system that allows cross sectional area no calculation is necessary. If using a system with simple calipers/distance measurements the anterior-posterior and cranial-caudal dimensions are assessed.  

  Volume estimation based on a CSA of the gastric antrum has been the focus of multiple recent investigations. A clinically relevant ‘‘cut-off’’ value of CSA would be one that differentiates a baseline volume (Grade 0 or Grade 1 antrum or <1.5 mLkg-1) from a greater-than-baseline condition (Grade 2 antrum or >1.5 mLkg-1). This value of antral CSA could be a simple surrogate measure that could facilitate the interpretation of examination findings when clear fluid is observed in the antrum. Based on this metric a CSA less than 8 cm2 in women or 10 cm2 in men is acceptable.

Point of Care ultrasound (POCUS) of the lungs, also known as lung ultrasound (LUS), has emerged as a technique  that allows for pulmonary assessment  with greater accuracy and speed compared to conventional techniques such as chest x-ray and auscultation.  Lung ultrasound is dependent on the interpretation of ultrasound artifacts with unique signatures that allow interpretation of pathologies such as pneumothorax, pulmonary edema, effusion, pneumonia and even failed intubation. Visceral and parietal pleura appear as hyperechoic and movement or "lung sliding" represents ventilation. Consolidation and "B" lines represent pathology.

In this simulator based example we have B-lines (bright vertical laser like artifacts) that signal fluid or thickening  in the lung tissue. This could represent pulmonary edema or interstitial disease as with COVID. In addition consolidation is present. Needless to say interpretation will depend on clinical presentation.

Start with a pencil grip, 2nd-3rd intercostal space midclavicular. Orientation marker towards the top, looking for pneumothorax and pulmonary edema. Landmarks are the ribs, lung and pleural lines between two ribs. Artifacts include pleural sliding, A-lines and the absence/presence of B-lines. Here we see sliding, no B-lines so no pneumothorax, pulmonary edema/interstitial lung disease or consolidation. 

  M-mode ultrasound diagnoses a pneumothorax by showing a "barcode sign," which indicates that the lung is not moving against the chest wall. The 

Seashore Sign (Normal Lung) using M-mode displays distinct layers. The stationary chest wall above the pleura looks like parallel straight lines ("sky"), and the moving lung tissue below the pleural line creates a granular, sandy pattern (the "beach“). The Barcode Sign (Pneumothorax) displays uniform, parallel horizontal lines both above and below the pleural line (stratosphere sign), meaning there is no pleural motion because air separates the lung from the chest wall.

A normal scan (seen here as part of a FATE exam using a phased array transducer) shows healthy, air-filled lungs. Here the lung is seen as an air-filled "lung curtain" with 0-3 "B Lines". The diaphragm is seen next to the liver with no signs of consolidation, lung collapse or pleural effusion. 

  The spine sign on a lung ultrasound is the abnormal appearance of the thoracic spine bones above the diaphragm, which confirms a pleural effusion (fluid collection around the lung). Here we have the jellyfish sign as a bonus. With normal lungs air inside the lung blocks sound waves, so the spine cannot be seen above the diaphragm. However with pleural fluid liquid acts as a clear window that transmits sound waves deeply, allowing the vertebral bones of the spine to show on the scan. Effusion size is estimated by the number of visible vertebrae above the diaphragm. When two vertebrae are seen the effusion is considered small (roughly 250 ml); when 2-4 vertebrae are seen the effusion is considered moderate (roughly 500 ml), and when 4-6 vertebrae are seen the effusion is considered large (roughly 1 liter).