Point-of-care ultrasound (POCUS) is an increasingly available tool and an essential skill for anyone caring for critically ill patients. It can be invaluable in the preintubation hemodynamic assessment of a critically ill patient with a physiologically difficult airway. Chapters 6, 7, 8 focus on the underpinnings of developing a strategy for the physiologically difficult airway, and Chapter 18 addresses optimizing the patient with the physiologically difficult airway. This chapter describes how to obtain those key ultrasound images to inform that strategy and manage the physiology. Specifically, we will discuss gastric imaging to assess for risk of aspiration, lung and pleural space imaging that may impede preoxygenation, and focused cardiac imaging for hemodynamic assessment. Chapter 10 discusses upper airway ultrasound for preintubation identification of upper airway anatomy and identifying the cricothyroid membrane, and preextubation evaluation for laryngeal edema. This chapter is not a comprehensive treatise on ultrasound or echocardiography. The reader is presumed to have at least basic familiarity with ultrasound physics, knobology, basic image acquisition, and references are provided for those interested in further reading.

Aspiration remains one of the most common complications of airway management worldwide.^1,2 Nil per os (NPO) status cannot be guaranteed in emergency airway management as it generally is prior to elective anesthesia. While patient positioning and RSI decrease the incidence of aspiration, they are not guaranteed to prevent aspiration.^3,4 Gastric ultrasonography provides a rapid and reliable method to qualitatively, and quantitatively, assess gastric contents that may benefit from decompression prior to induction to further reduce the risk of aspiration.^4,5,6

The patient is placed in the supine position, however right lateral decubitus or Trendelenburg positions can be used as well. A curvilinear probe is placed in the craniocaudal orientation in the subxiphoid (SX) midline position, with the indicator toward the head, and swept laterally in either direction to identify the gastric antrum. The antrum is an easily identified, relatively superficial structure, which provides an excellent acoustic window to the stomach contents. Other structures that may be visualized in this view are the liver, pancreas, small intestine, aorta, and the superior mesenteric vessels (Fig. 11.1).

Depending on the contents of the antrum, it may be collapsed or distended. A collapsed antrum and associated visible antral wall in supine position (and if feasible, confirmed in a right lateral position) confers a low risk of gastric contents aspiration. A distended antrum, which will typically have a thinner wall than in a collapsed state, needs further investigation about its contents and estimated volume. Hyperechoic contents denote liquid contents (Fig. 11.2). These may contain air bubbles if there is effervescence in the ingested contents. Solid components are visible as heterogeneous contents (if mixed with liquids) or may obstruct the posterior wall and structures and give a frosted glass appearance (Fig. 11.2).

Blood in the stomach may be visible as any of the above and in suspected gastrointestinal bleeding, decompression along with anticipation of potential large volume of regurgitation should be done with ramped-up positioning, use of RSI, and availability of large bore suction catheters.

The difference between an empty stomach and a large amount of gastric contents can be made by volumetric assessment. Cross-sectional area of the antrum can be measured and a predictive
table used to estimate volume.⁶ Volume may also be calculated by multiplying pi (Π) with the anterior-posterior diameter with the craniocaudal diameter and dividing by 4.

An area of more than 3.6 cm² is associated with an increased risk of aspiration and warrants decompression with a gastric tube prior to induction.^5,6

Focused lung and pleural ultrasonography may be utilized for identifying risk factors for rapid desaturation, and insights into opportunities to mitigate those risks. These include unidentified pneumothorax and large pleural fluid collections. Various other pulmonary pathologies may be identified during lung ultrasonography, such as dense consolidations, which inform the airway management strategy and preparation. However, these may not be modifiable in the peri-intubation period but represent areas of increased risk for desaturation or hemodynamic collapse, such as dense consolidations with unrecruitable lung parenchyma despite preoxygenation with NIPPV or HFNO. Lung ultrasonography depends frequently on artifacts generated by the pleural interface with either air or fluid, and thus, a high-frequency probe with a small footprint, such as the linear probe gives easy visualization of the superficial structures. Published protocols may guide the operator about the sites to investigate,^7,8 but typically pneumothoraces are best seen anteriorly near the apices, and effusions/hemothoraces and consolidations are best seen posteriorly or laterally near the bases and diaphragm.

A pneumothorax is visible as “A lines” with an absence of lung sliding with or without presence of a lung point. In the M-mode, this can be seen as the barcode sign instead of the normally expected seashore sign (Fig. 11.3). Evacuation of a pneumothorax may decrease the risk
of peri-intubation desaturation and/or cardiopulmonary collapse. Large pleural collections whether simple, infected, or hemorrhagic will reduce the functional residual capacity of the lung, which may decrease the efficiency of preoxygenation and contribute to peri-intubation hypoxia (Fig. 11.4). Drainage of these pleural collections allows for increased FRC and may improve the ventilation-perfusion mismatch to lower the risk of peri-intubation hypoxia, or obviate the need for intubation and invasive mechanical ventilation in some circumstances. Consolidations appear as semisolid structures on ultrasound and are often referred to as “hepatization” as they take on the appearance of the liver. Identifying these consolidations may provide an opportunity to change positioning or use more advanced methods of preoxygenation to recruit any recruitable lung parenchyma (Fig. 11.4, Video 11.1). We prefer a phased array probe (i.e., “cardiac probe”) for pleural fluid and consolidation assessment, although curvilinear and linear probes may also be used but result in a narrower field of view. Thorough reviews of lung and pleural ultrasonography are widely available.^9,10

Postintubation cardiovascular collapse occurs in up to half of critically ill patients.^{11,12,13,14,15} While detailed echocardiographic assessment is unnecessary, a goal-directed echocardiographic assessment facilitates a rapid assessment of a patient’s peri-intubation hemodynamic milieu. The information gained is used to formulate an informed mitigation strategy to reduce this risk. The basic cardiac views—apical four and five chambers (A4C, A5C), parasternal long axis (PLAX), parasternal short axis (PSAX), and subxiphoid (SX) views are best utilized for these assessments using a phased array, low-frequency probe. Non-crystal-based ultrasounds which are becoming increasingly common use a singular probe for all applications but do not yet offer capabilities such as tissue Doppler. Specific goal-directed questions for echocardiography as they relate to managing the physiologically difficult airway include: