INTRODUCTION — Ultrasonography allows rapid acquisition of high-resolution images of anatomic structures in real time. This topic will provide an overview of various perioperative uses of ultrasound. These include echocardiography of the heart and great vessels, examination of pulmonary, gastric and other abdominal structures, guidance for regional anesthetic or vascular cannulation procedures, and rapid diagnosis or confirmation of cause(s) of perioperative hemodynamic instability.
ADVANTAGES AND COSTS — Advantages of perioperative ultrasound use include versatility (eg, diagnosis of pathology, precise needle or catheter placement), rapidity of image acquisition, ability to perform repeated examinations, portability of ultrasound machines and probes, and absence of ionizing radiation risks.
Requirements include initial purchase and maintenance of ultrasound machines and probes, as well as training and availability of skilled perioperative practitioners (typically anesthesiologists) who can perform and interpret perioperative ultrasound images in real time [1-11].
ULTRASOUND EQUIPMENT — A variety of ultrasound machines can be used for perioperative point-of-care ultrasound (POCUS) [12]. Some ultrasound machines provide basic images with or without Doppler, but do not include more advanced capabilities, such as continuous wave Doppler, pulsed wave Doppler, or three-dimensional ultrasound. More sophisticated machines can be adjusted to acquire images appropriate for various diagnostic studies or interventions, as noted below.
Ultrasound transducers include linear, phased array, and curvilinear probes. Linear probes utilize high frequencies and send out ultrasound waves in a linear fashion to generate images that appear on the screen as a rectangle. Linear probes best visualize near (shallow) structures (eg, nerves, blood vessels, pleura). Phased array probes use lower frequencies to better delineate spatial depth of field, activating ultrasound crystals in a sequence across the surface of the probe to generate a "pie-shaped" image on the screen. Phased array probes are typically used to image deeper structures such as thoracic or abdominal organs. Curvilinear probes combine some of the best aspects of each by using lower frequencies to acquire images that have a better depth of field than linear probes, with a wider window than phased array probes. Such curvilinear probes are ideal for imaging intra-abdominal organs.
Details regarding basic principles of ultrasound physics, acquisition of optimal images, and descriptions of available ultrasound machines and transducers are found in other topics describing specific uses:
●(See "Echocardiography essentials: Physics and instrumentation".)
●(See "Principles of ultrasound-guided venous access".)
●(See "Ultrasound for peripheral nerve blocks".)
●(See "Indications for bedside ultrasonography in the critically ill adult patient".)
●(See "Emergency ultrasound in adults with abdominal and thoracic trauma".)
POINT-OF CARE ULTRASOUND (POCUS) — Perioperative point-of-care ultrasound (POCUS) refers to the use of ultrasonography at the patient’s bedside for diagnostic purposes or to aid in performance of a procedure [4]. While comprehensive imaging can be performed and interpreted at the point-of-care, the term POCUS typically refers to a limited qualitative examination that is simple, rapid, and goal-oriented. It is a tool used most often to provide answers to acute “yes or no” clinical questions but can be more sophisticated based on the provider’s qualifications.
Clinical indications — POCUS uses in the perioperative setting include airway, lung, and gastric ultrasound, focused cardiac ultrasound, focused assessment for trauma examination, and guidance during performance of regional, vascular, and pain management procedures (see 'Urgent diagnostic uses' below and 'Elective diagnostic uses' below and 'Procedural uses' below). Clinical benefits of POCUS have been noted for indications that include selected elective diagnostic uses and procedures, and urgent determination of causes of perioperative hemodynamic instability. We agree with expert panel and professional society recommendations for POCUS use for these indications when equipment and expertise are available [13-16]. However, demonstrating consistent positive impact on patient outcomes for these indications has been challenging because POCUS is typically used as a diagnostic and/or monitoring tool when needed rather than routinely, and is often used in conjunction with other types of monitors or interventions [17].
Anesthesiologists in many training programs and practice settings have learned to use perioperative POCUS to assess cardiovascular (see 'Transthoracic echocardiography' below), pulmonary, airway, and abdominal abnormalities [1-9,18,19]. Ease of use, portability, and affordability allow clinicians to complete basic training, practice skills, and develop expertise in use of POCUS relatively quickly. Learning goals and specific skills necessary to achieve competency in the use of POCUS for selected indications have been presented in a framework that includes an understanding of these indications, as well as acquisition, interpretation, and medical decision-making proficiencies [20].
Urgent diagnostic uses — Rapid diagnosis or confirmation of cause(s) of perioperative hemodynamic instability or shock with POCUS is feasible when equipment and expertise are available in perioperative areas which include the obstetrical suite, preoperative areas, after arrival in the OR, or in the immediate postoperative period in the post-anesthesia care units (PACUs) or intensive care units (ICUs) (algorithm 1) [4,21]. A systematic approach such as rapid ultrasound in shock (RUSH) is typically used to examine the heart first, followed by brief imaging of the chest, abdomen, major arteries, and veins, so that "the pump, the tank, and the pipes" are assessed (table 1) [22-28]. (See "Intraoperative management of shock in adults", section on 'Point-of-care ultrasonography'.)
In many cases, progression of hemodynamic instability to frank shock or cardiac arrest may be avoided by early diagnosis of causes which can include but are not limited to the following [4]:
●Hypovolemia suggested by ultrasound findings that include a small, collapsed inferior vena cava (IVC), a hyperdynamic left ventricle with small end-diastolic size (diameter or area), or variability of aortic pulsatile flow (which evaluates fluid responsiveness) using esophageal Doppler ultrasound. (See "Intraoperative rescue transesophageal echocardiography (TEE)", section on 'Hypovolemic shock' and "Intraoperative fluid management", section on 'Dynamic parameters to assess volume responsiveness'.)
●Cardiac dysfunction diagnosed by noting presence of left or right ventricular dysfunction, regional wall motion abnormalities suggesting myocardial ischemia, or pericardial effusion or tamponade [29]. (See "Intraoperative rescue transesophageal echocardiography (TEE)", section on 'Left ventricular failure' and "Intraoperative rescue transesophageal echocardiography (TEE)", section on 'Right ventricular failure'.)
●Acute respiratory failure or hypoxia due to pulmonary edema, pneumonia, atelectasis, pleural effusion, or pulmonary embolus versus hypoventilation [30,31]. (See 'Lung ultrasound' below.)
●Pneumothorax. In a 2020 meta-analysis of trauma patients with suspected pneumothorax, chest ultrasonography was more sensitive than chest radiograph (CXR) in confirming this diagnosis, with an absolute difference in sensitivity of 0.44 (95% CI 0.27-0.61), although specificity was similar for the two techniques (13 studies with 1271 participants [410 with pneumothorax]) [32]. (See 'Diagnosis of specific abnormalities' below and "Clinical presentation and diagnosis of pneumothorax", section on 'Pleural ultrasonography'.)
●Airway problems such as inappropriate location of the endotracheal tube [33-35]. If urgent cricothyroidotomy or tracheostomy is necessary, ultrasound identification of landmarks (eg, cricothyroid membrane) can be useful [36-39].
●Intra-abdominal abnormalities such as free fluid suggestive of vessel rupture, free air suggestive of a ruptured viscus or a gas-producing organism [40]. (See 'Gastric ultrasound' below and "Emergency ultrasound in adults with abdominal and thoracic trauma", section on 'Abdominal examination'.)
●Pelvic ultrasound abnormalities explaining abnormal urine output. Examples include bladder distention, obstructive hydronephrosis [41], or renal atrophy (eg, in a patient with unknown medical history and absent urine output). (See "Indications for bedside ultrasonography in the critically ill adult patient", section on 'Detection of urinary tract obstruction'.)
●Severe preeclampsia, characterized by pulmonary interstitial syndrome (defined as a bilateral B-line pattern on lung ultrasound (see 'Lung ultrasound' below)) [42]. Although beyond the scope of a POCUS examination, evidence of left ventricular diastolic dysfunction, as well as increased optic nerve sheath diameter, may also be detected using ultrasonography.
Further details regarding urgent use of POCUS are available in other topics:
●(See "Indications for bedside ultrasonography in the critically ill adult patient".)
Depending on the immediate availability of equipment and personnel, transthoracic or transesophageal echocardiography, rather than other POCUS equipment, may be used for urgent assessment of cardiopulmonary status to diagnose causes of hemodynamic instability [4].
●(See 'Transthoracic echocardiography' below.)
●(See "Intraoperative rescue transesophageal echocardiography (TEE)".)
Elective diagnostic uses — POCUS has been employed electively for diagnostic purposes before, during, or after surgical procedures [12,27,43]. Examples include:
●Preoperative period
•Evaluation of signs and symptoms of heart failure [44,45]
•Assessment of the volume and characteristics of gastric contents (see 'Gastric ultrasound' below)
•Diagnosis of obstructive sleep apnea (OSA) [46]
•Assessment of anatomy or pathology that may cause difficulties with laryngoscopy and endotracheal intubation, or with oxygenation, by examination of the neck and chest before induction of general anesthesia [47-49]
●Intraoperative period
•Confirmation of appropriate positioning of the endotracheal tube (ETT) [33-35]
•Confirmation of lung isolation during procedures requiring one lung ventilation by identifying lung sliding in the ventilated lung(s) [35,43,50,51] (see 'Lung ultrasound' below)
●Postoperative period
•Confirmation of bladder distention due to postoperative urinary retention (POUR) in patients who cannot void after surgery (see "Overview of post-anesthetic care for adult patients", section on 'Inability to void')
•Evaluation of the etiology of postoperative hypoxia (high or unexpected oxygen [O2] requirement) in the PACU
Procedural uses
Ultrasound for regional anesthesia — Ultrasound imaging is typically used to guide peripheral nerve block placement. Equipment and techniques are discussed separately. (See "Ultrasound for peripheral nerve blocks".)
In selected patients (eg, those with obesity or difficult anatomy), preprocedure ultrasound scanning of the spine before neuraxial anesthesia may be useful to identify the intervertebral space for needle placement and to estimate the depth required to reach the subarachnoid or epidural space. (See "Spinal anesthesia: Technique", section on 'Preprocedure ultrasonography' and "Epidural and combined spinal-epidural anesthesia: Techniques", section on 'Preprocedure ultrasonography'.)
Unexpected findings during use of ultrasound for regional anesthesia have led to definitive treatment. Examples include free fluid in the abdomen from the hip joint following hip arthroscopy or free fluid in the peritoneal space or pericardial sac in trauma patients [52-54]. (See "Emergency ultrasound in adults with abdominal and thoracic trauma".)
Ultrasound for vascular cannulation
●Peripheral and central venous cannulation – Intravascular venous access into central (or peripheral) veins is the most common perioperative use of ultrasound. Ultrasound can identify clotted vasculature, which may prevent potential complications. The internal jugular (IJ) vein is most commonly cannulated using ultrasound guidance for central venous catheter (CVC) placement (movie 1 and movie 2) [55,56]. Guidance for needle, wire, and catheter placement includes static ultrasound imaging for vessel localization before puncture, as well as dynamic (ie, real-time) imaging to guide the needle into the lumen of the vein. Further details regarding technique are discussed in other topics. (See "Principles of ultrasound-guided venous access" and "Central venous access: General principles", section on 'Use of ultrasound'.)
Also, transesophageal echocardiography (TEE) in an anesthetized patient, or modified subcostal transthoracic echocardiography (TTE) views have been used to identify correct wire location during central venous access [57].
Furthermore, ultrasound may be used to detect mechanical complications occurring after CVC placement in the IJ (or subclavian) vein. In one multicenter study that included more than 750 patients with a CVC, malposition was present in 3.3 percent, while pneumothorax occurred in 0.7 percent [58]. The investigators noted agreement between ultrasound and chest radiography for a correct diagnosis in 99 percent of these cases.
●Intra-arterial cannulation – Ultrasound imaging may be employed to identify and guide intra-arterial catheter placement when palpation of the arterial pulse is difficult (eg, small arteries, peripheral vascular disease, obesity). Ultrasound equipment and specific techniques for this use are discussed separately. (See "Intra-arterial catheterization for invasive monitoring: Indications, insertion techniques, and interpretation", section on 'Use of ultrasound guidance'.)
ECHOCARDIOGRAPHY
Transesophageal echocardiography — Transesophageal echocardiography (TEE) is the most commonly used modality to evaluate cardiac anatomy, physiology, and function in the operating room. Societal recommendations suggest use of TEE performed by qualified echocardiographers for intracardiac procedures (eg, cardiac valve repair or replacement), thoracic aortic surgery, and when either the planned surgical intervention or the patient's underlying physiology may result in severe hemodynamic, pulmonary, or neurologic compromise during cardiac or noncardiac surgery [59].
●Cardiac surgical procedures – Uses of TEE during specific cardiac surgical procedures are discussed separately. (See "Anesthesia for cardiac surgery: General principles", section on 'Prebypass transesophageal echocardiography' and "Anesthesia for cardiac surgery: General principles", section on 'Postbypass transesophageal echocardiography'.)
●Noncardiac surgical procedures – Perioperative uses of TEE during noncardiac surgery are discussed separately. (See "Intraoperative transesophageal echocardiography for noncardiac surgery".)
Unplanned rescue TEE can be employed to diagnose and manage severe perioperative hemodynamic instability that persists despite corrective therapy. (See "Intraoperative rescue transesophageal echocardiography (TEE)".)
Transthoracic echocardiography — While TEE is the most common application of echocardiography in the OR, transthoracic echocardiography (TTE) has also been incorporated into perioperative care [3,27,29,44,60-63].
●Focused cardiac ultrasound – Focused cardiac ultrasound (FOCUS) is a qualitative surface cardiac ultrasound examination, performed as an adjunct to physical examination to recognize specific ultrasonic signs indicating a narrow list of likely diagnoses in specific clinical settings [3,29,44]. Cardiac abnormalities that can be rapidly diagnosed during a FOCUS examination include global biventricular systolic dysfunction, significant valvular abnormalities, pericardial effusion or tamponade, or likely hypovolemia (based on ventricular chamber size). Use of FOCUS during preoperative cardiovascular risk assessment has also been proposed for patients with suspected severe left ventricular systolic dysfunction [45]. In a 2019 meta-analysis of studies of FOCUS as a supplement to clinical evaluation (compared with clinical examination alone), sensitivity (but not specificity) for correct diagnosis of left ventricular dysfunction or valvular abnormalities was improved by addition of FOCUS [64]. Notably, FOCUS examination can be performed with a handheld ultrasound machine with limited capabilities by an operator who does not necessarily have the extensive training for performance and interpretation of a comprehensive quantitative TEE or TTE examination [65,66]. (See "Indications for bedside ultrasonography in the critically ill adult patient", section on 'Thoracic ultrasonography' and "Indications for bedside ultrasonography in the critically ill adult patient", section on 'Training and competence'.)
●Comprehensive transthoracic echocardiography examination – A more comprehensive perioperative TTE examination using standard parasternal, apical, and subcostal windows can be performed and interpreted by a fully qualified echocardiographer when indicated. This can be a useful alternative to TEE in selected patients (eg, those with an indication for diagnostic or procedural echocardiographic guidance if esophageal pathology precluding placement of a TEE probe is present) [60]. TTE has also been employed for preoperative assessment of suspected abnormalities in cardiac function (eg, valvular heart disease, left ventricular dysfunction) or volume status [67]. (See "Transthoracic echocardiography: Normal cardiac anatomy and tomographic views".)
LUNG ULTRASOUND
General considerations — Lung ultrasonography can be used to rapidly diagnose and manage causes of acute respiratory distress that may be encountered in the perioperative period (eg, pneumothorax, pleural effusion, pneumonia, cardiogenic pulmonary edema, pulmonary embolus, esophageal or endobronchial intubation, or exacerbation of asthma or chronic obstructive pulmonary disease [COPD]) (algorithm 2) [27,68-75]. In many cases, the diagnostic accuracy of ultrasonography is equal or superior to chest radiography or computed tomography (CT) [76,77].
When equipment and trained personnel are available, lung ultrasonography may be used as a primary tool for evaluation of persistent intraoperative hypoxemia, together with lung auscultation and checking the administered gas mixture, mechanical ventilator settings, and the anesthesia machine for possible malfunctioning (table 2) [78].
Training in image acquisition and interpretation is required to master lung and pleural ultrasonography, but ease of use, rapidity, repeatability, and reliability have made this a useful perioperative tool. In urgent situations, very rapid determination of the most likely cause of acute respiratory distress is possible using only selected areas on the left and right hemithorax for examination [74]. One multicenter study noted that 100 trainees were able to correctly classify normal or diseased lung regions (ie, normal aeration, interstitial-alveolar syndrome, lung consolidation) in 80 percent of more than 2500 critically ill patients [79]. Trainees required approximately 12 minutes to complete the examination, while experts required 8 to 10 minutes.
Technique — The technique for performing lung ultrasound is described below [70]:
●Place the patient in a supine position since pleural air is located in the least dependent portion of the lung. Thus, the anterior chest is examined in a supine patient, particularly if pneumothorax is suspected.
●Place a linear array probe in the third or fourth intercostal space in the mid-clavicular line, with the probe perpendicular to the chest in a parasagittal orientation (parallel to the long axis of the body). Two ribs are visualized on either side of the ultrasound image.
●Identify the bright, hyperechoic line which represents the pleura. It will be located underneath as well as between the ribs.
●If pneumothorax is not immediately diagnosed (see 'Findings and implications' below), obtain at least two additional views (in the mid-axillary and posterior axillary locations of each side of the thorax). Thus, abnormalities can be detected in either lung.
Further discussion of imaging techniques, patient positioning, and ultrasound machine settings is available in a separate topic. (See "Bedside pleural ultrasonography: Equipment, technique, and the identification of pleural effusion and pneumothorax".)
Normal findings — Normal lung ultrasound findings are shown in the movie, and in a table with links to additional video ultrasound images (movie 3 and table 3). These normal images have an A-line pattern indicating good aeration of the lungs. Ultrasound waves are almost completely reflected at the interface between the pleura and the aerated lung, generating a hyperechoic (ie, bright) horizontal stripe called the pleural line in these normal images [80]. Under the pleural line are regularly spaced reverberation artifacts called A lines (ie, A pattern). Also seen are small focal densities at the level of the pleural line representing microatelectasis, and short bright vertical artifacts (termed “Z” lines). In addition, lung movements from breathing result in dynamic horizontal movements along the pleural line occurring in synchrony with ventilation (lung sliding), and a normal rhythmic movement of the pleura occurring in synchrony with the cardiac rhythm (lung pulse).
Diagnosis of specific abnormalities — Lung ultrasound findings indicating specific diagnoses are shown in the tables, with links to video ultrasound images of those findings (table 3 and table 4) [70,74,80,81]. (See "Indications for bedside ultrasonography in the critically ill adult patient", section on 'Thoracic ultrasonography'.)
●Pneumothorax – Evaluation for possible pneumothorax is the most common application of lung ultrasound during the intraoperative period. Pneumothorax is suspected if adequate mechanical ventilation cannot be achieved, unexplained hypoxemia is detected, or asymmetrical breath sounds are noted, particularly after difficulty with central venous catheter (CVC) placement, blunt or penetrating chest trauma, or in a patient with risk factors for spontaneous pneumothorax (eg, Ehlers-Danlos syndrome, Marfan syndrome).
Pneumothorax is suggested by the absence of lung sliding on the affected side (the presence of lung sliding effectively excludes the possibility of pneumothorax on that side) (movie 4) [70,72,78,82]. Particular care must be taken not to misinterpret visualization of the pericardium in the left chest as being "lung sliding," since pericardial movement occurs in synchrony with the cardiac rhythm (lung pulse). Other causes of absent lung sliding include absence of ventilation (eg, due to breath holding or endobronchial intubation on the opposite side) or significant pleural adhesions [83]. Pneumothorax is also verified by the presence of lung point (image 1 and table 4), which is very specific for pneumothorax because it represents the transition point at which partially collapsed lung contacts the parietal pleura during respiration, and by the absence of lung pulse [70,78,83-87]. The B line pattern is absent on the affected side, although A lines remain present. (See "Bedside pleural ultrasonography: Equipment, technique, and the identification of pleural effusion and pneumothorax", section on 'Evaluation for pneumothorax' and "Clinical presentation and diagnosis of pneumothorax", section on 'Diagnostic imaging'.)
If available, lung ultrasound should always be used before needle placement for decompression of suspected tension pneumothorax. In the intraoperative setting, the sterile surgical field may preclude a comprehensive thoracic ultrasound, but at least a portion of the anterior lung fields can usually be evaluated during surgery.
●Pleural effusion – Patients with a pleural effusion have an anechoic (fluid) collection between the parietal and visceral pleura (ie, the base-diaphragmatic interface or the most dependent portion of the thorax, depending on the patient's position) (image 2 and table 4) [70,78]. Diagnostic evaluation and imaging of the effusion and the technique of thoracentesis are discussed in separate topics:
•(See "Imaging of pleural effusions in adults", section on 'Ultrasonography'.)
•(See "Ultrasound-guided thoracentesis".)
●Pneumonia or acute respiratory distress syndrome – In patients with a primary lung injury process such as acute respiratory distress syndrome (ARDS) or consolidation (ie, pneumonia, atelectasis), lung ultrasound imaging may show variable lung sliding patterns (normal, reduced, or absent), presence of lung pulse, and irregularly spaced B lines with irregular pleural morphology (ie, heterogeneous hyperechoic lung), with the B pattern being a focal or multifocal pattern of distribution [70,71,78]. Three or more B lines per intercostal space indicate moderate loss of lung aeration (B1 or interstitial pattern) (figure 1). B1 pattern with three or more B lines per intercostal space indicates moderate loss of aeration. Multiple coalescent B lines per intercostal space indicate tissue-like increased density with almost complete loss of aeration (B2 or consolidation pattern) (figure 1) [80,81]. (See "Indications for bedside ultrasonography in the critically ill adult patient", section on 'Evaluation of the etiology of cardiopulmonary failure'.)
●Cardiogenic pulmonary edema – In patients with cardiogenic pulmonary edema, a bilateral diffuse and homogenous gravity-dependent B-line pattern is typical (image 3 and movie 5), with smooth pleural morphology [69,70,78,88,89]. Lung sliding is present, while the A-line pattern may be absent. Also, the “comet-tail artifact” (ie, B lines) may be seen arising from the lung wall interface and extending perpendicular from the surface to the deeper portion of the lung; this is a reverberation artifact caused by areas of interstitial edema on the visceral pleura (image 4 and image 5) [90]. The presence of this artifact rules out pneumothorax (as with sliding pleura) and helps to differentiate cardiogenic pulmonary edema from an acute exacerbation of asthma or COPD as a cause of respiratory distress [91-93]. (See "Indications for bedside ultrasonography in the critically ill adult patient", section on 'Evaluation of acute cardiopulmonary failure' and "Emergency ultrasound in adults with abdominal and thoracic trauma", section on 'Pleural examination'.)
●Esophageal or endobronchial intubation – Lung ultrasonography has high diagnostic value for identifying esophageal or endobronchial intubation [34,78]. Transtracheal ultrasonography can be employed to confirm correct placement of an endotracheal tube in the trachea. This technique is 98.7 percent sensitive (95% CI 97.8-99.2) and 97.1 percent specific (95% CI 92.4-99.0) [73].
•With esophageal intubation, there will be bilateral absence of lung sliding (highly unusual in other pathologic lung conditions). However, lung movement synchronous with mechanical or hand ventilation may be noted since the motion of the insufflated stomach can translate to the lung.
•With endobronchial intubation, findings are VERY similar to those for unilateral pneumothorax, including absence of lung sliding on the non-intubated side with presence of lung sliding on the side of the endobronchial intubation. A lung point (as described above) helps distinguish pneumothorax from endobronchial intubation. In addition, a non-intubated lung may still demonstrate a lung pulse (which is absent in pneumothorax). Accurate diagnosis also depends on a high index of suspicion for endobronchial intubation (eg, increased depth of intubation >23 to 24 cm), an unusually short patient, or development of symptoms after placement of the patient in steep Trendelenburg position.
●Pulmonary embolism – In patients with pulmonary embolism, lung ultrasound imaging may identify peripheral wedge-shaped abnormalities, a pleural line interrupted by small peripheral consolidations, and absent B lines, with normal lung sliding, lung pulse, and A-line pattern [70,94,95]. Lung ultrasound is not typically employed to assist with diagnosis of suspected pulmonary embolism unless the patient is hemodynamically stable. Other more sensitive and specific modalities are employed for unstable patients (eg, transesophageal echocardiography [TEE], helical CT scan). (See "Indications for bedside ultrasonography in the critically ill adult patient", section on 'Investigational' and "Clinical presentation, evaluation, and diagnosis of the nonpregnant adult with suspected acute pulmonary embolism", section on 'Investigational'.)
●Exacerbation of asthma or COPD – Although lung ultrasound cannot be used to specifically diagnose asthma or COPD exacerbation, a sonographic A pattern without consolidations or pleural effusions in the posterolateral thoracic regions had 78 percent sensitivity and 94 percent specificity for this diagnosis in one study of patients with acute dyspnea or respiratory failure [75].
GASTRIC ULTRASOUND — Gastric ultrasound has been employed to assess the volume and character of gastric contents, thereby supplementing preoperative fasting guidelines in selected situations (table 5) [96-100] (see "Preoperative fasting in adults"). In a randomized trial that included 80 ultrasound examinations, the diagnostic accuracy of gastric ultrasound was high for detection of a full stomach (ie, any solid or >1.5 mL/kg of clear fluid), with a sensitivity of 100 percent (95% CI 93-100 percent) and specificity of 98 percent (95% CI 95-100 percent) [101].
Although there are no absolute indications for performing gastric ultrasound, examination can be used to reassure clinicians that the volume of stomach contents in "at risk" patients is sufficiently low to minimize aspiration risk or, conversely, support a decision to delay elective surgery in a patient with evidence of increased gastric volume in situations such as:
●Concern regarding patient compliance with fasting guidelines before surgery
●Unknown fasting history (eg, trauma, cognitive impairment, communication barrier)
●Likely delay in gastric emptying (eg, pregnancy, neuromuscular disease, pain, chronic opioid use, diabetes mellitus)
Technique — The technique for performing gastric ultrasound is described below (figure 2) [96]:
●The patient is initially imaged in the supine position. In this position, imaging has good specificity but low sensitivity for identification of gastric contents. The patient is then imaged in the right lateral decubitus position, causing the liquid and solid stomach contents to be located against the pylorus filling the gastric antrum. In this position, air will typically be in the non-dependent body and cardia of the stomach. Raising the upper section of the bed to a 45° angle may improve qualitative assessment of gastric fluid contents [102]. Prior gastric surgery or a large hiatal hernia may interfere with image acquisition in any position.
●A low-frequency (2 to 5 mHz) curvilinear probe is employed for scanning the epigastrium in a sagittal plane. Sweeping left and right along the subcostal margin allows identification of the gastric antrum at the level of the abdominal aorta. (The aorta appears as a relatively thick-walled vascular structure with pulsatility in systole only.) The antrum is a hollow viscus between the liver anteriorly and the pancreas posteriorly. It has five distinct layers, including the distinctive echolucent muscularis layer, and its contents accurately represent the contents of the remainder of the stomach (picture 1).
Findings and implications
●The composition of gastric contents is assessed:
•Empty stomach – Antrum is flat or collapsed and antrum walls are thick (figure 3).
•Solids – Antrum is round and distended, there is a mix of air and particulate matter that interferes with ultrasound transmission, and it is difficult to image distal to the solid contents. This creates what is sometimes termed the "frosted glass" appearance (figure 4 and figure 5).
•Liquids – Antrum is round and distended, and hypoechoic fluid can be seen in the lumen of the antrum. This may be seen as a "starry night" pattern, with air bubbles suspended in liquid (figure 6 and figure 7).
•Suspensions (or milk) – Antrum is round and distended, and hyperechoic fluid ("hepaticized") fluid is seen in the lumen of the antrum (figure 5).
●The volume of gastric contents is estimated by measuring the cross-sectional area of the antrum, which has a linear correlation with gastric volume. The steps are to:
•Trace the antrum at the level of the aorta in the right lateral decubitus position. The antrum is traced around its outside edge (image 6).
•Use the following formula to estimate gastric volume from the traced cross-sectional area [97]:
Volume (mL) = 27 + 14.6 x right lateral decubitus cross-sectional area – 1.28 x age (y)
In pregnant patients, the antral cross-sectional area is measured if fluid is visible; cut-off values for risk of pulmonary aspiration are 608 mm2 and 960 mm2 in the semirecumbent and right lateral semi-recumbent positions, respectively [103].
●Gastric volume is graded according to risk of aspiration of gastric contents during induction of general anesthesia [104]:
•Grade 0 – Antrum is empty in supine and right lateral decubitus positions (low risk).
•Grade 1 – Antrum is empty in the supine position, clear fluid is visible in the right lateral decubitus position, and gastric volume is estimated at <1.5 mL/kg (ie, 100 mL in the average adult), which is normal in a fasted patient (low risk).
•Grade 2 – Clear fluid is visible in the antrum in the supine and right lateral decubitus position, and volume >1.5 mL/kg (high risk).
ULTRASOUND USE DURING THE COVID-19 PANDEMIC — While only urgent and emergency surgical and other interventional procedures have been performed during the novel coronavirus disease 2019 (COVID-19) pandemic, elective procedures have been resumed in many institutions. Perioperative ultrasonography is widely used by anesthesia providers before, during, and after many types of surgery to guide regional anesthetic or vascular cannulation procedures, and to evaluate cardiovascular, pulmonary, and gastrointestinal pathology.
Precautions to minimize infection risk — During the COVID-19 pandemic, strategies to minimize infection risks during the use of perioperative ultrasound vary according to local resources and institutional protocols [105,106]. In regions with a high COVID-19 infection rate, all patients undergoing elective surgery may be screened and tested for COVID-19 infection. (See "COVID-19: Perioperative risk assessment and anesthetic considerations, including airway management and infection control", section on 'Preoperative evaluation during the pandemic'.)
Specific strategies and precautions to minimize infection risks for the ultrasonographer, equipment, and the environment during ultrasound procedures in a patient with confirmed or suspected COVID-19-positive status include [105-108]:
●Avoidance of transesophageal echocardiography (TEE), which is an aerosolizing procedure, in COVID-19-positive patients unless the findings are likely to be critically important [107-112]. In many cases, focused cardiac ultrasound (FOCUS) using transthoracic echocardiography (TTE) may be used as an alternative (see 'Transthoracic echocardiography' above) [113,114]. If TEE is indicated, personal protective equipment (PPE) appropriate for performance of high-risk aerosolizing procedures [108] is used for patients with known or suspected COVID-19-positive status, and in all patients in regions with a high COVID-19 infection rate. Further discussion is available in separate topics:
●Consider dedicated ultrasound machine for COVID-19-positive patients.
●Removal of all nonessential equipment typically used to facilitate ultrasound examinations (eg, alternative transducers, extra tubes of ultrasound gel, paperwork) before entry in to room, and avoid use of electrocardiography (ECG) pads and cable if possible.
●Plastic ultrasound probe cover.
●Proper protection of other ultrasound equipment, according to institutional resources and protocols. This may include plastic covers for the ultrasound machine, screen, and parts of equipment that may be touched and are difficult to clean (eg, knobs, transducer ports) [115].
●Use of a single ultrasonographer and performance of an abbreviated pathology-directed examination to minimize clinician exposure.
●Careful donning and doffing of appropriate PPE prior to endotracheal intubation or other aerosolizing procedures in patients with suspected or confirmed COVID-19-positive status if being performed with ultrasonographer in the room. (See "COVID-19: Perioperative risk assessment and anesthetic considerations, including airway management and infection control", section on 'PPE during airway management or aerosol generating procedures'.)
●Proper cleaning of equipment, which includes wiping of all exposed equipment surfaces and the ultrasound probe (including handle, cable, and connector) with a hospital-approved disinfectant, before placement of the probe in a closed container for transport and further cleaning required by local protocols.
Diagnostic uses of ultrasound in COVID-19-positive patients — Point-of-care ultrasound (POCUS) can be used to diagnose common causes of poor oxygenation or hemodynamic instability during the perioperative period in critically ill COVID-19 patients (eg, hypovolemia, septic shock, poor left or right ventricular function, pericardial effusion, pulmonary edema, and pulmonary or other vascular embolic phenomena) including cardiac arrest [108,113,116-122]. (See 'Point-of care ultrasound (POCUS)' above.)
Also, lung ultrasound can be used to diagnose likely COVID-related pulmonary abnormalities, assess perioperative pulmonary status, and guide ventilation management [106,116,117,121,123-127]. Common findings include confluent and inferior lung field lesions, thickened and/or irregular pleural lines, presence of subpleural consolidations, and air bronchograms [113,116,117,121,125,126]. In some centers, lung ultrasound is preferred over chest radiography or computed tomography for diagnosis and monitoring of COVID-19 patients (eg, abdominal ultrasound for free fluid), since its use minimizes the need for intrahospital transfers [113,116,126,127]. (See 'Lung ultrasound' above and "COVID-19: Epidemiology, clinical features, and prognosis of the critically ill adult", section on 'Clinical features in critically ill patients'.)
Other uses of POCUS in critically ill COVID-19 patients include confirmation of endotracheal tube position after intubation, confirmation of appropriate positioning of a central venous catheter (CVC), evaluation for pneumothorax after CVC placement, diagnosis of deep venous thrombosis, or the use of TEE to guide placement of catheters for extracorporeal membrane oxygenation (ECMO) [108,113,121,128].
SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Venous access" and "Society guideline links: Use of point-of-care echocardiography and ultrasonography as a monitor for therapeutic intervention in critically ill patients".)
SUMMARY AND RECOMMENDATIONS
●A variety of ultrasound machines and transducers for perioperative uses are available. Advantages include versatility (eg, diagnosis of pathology, precise needle or catheter placement), rapidity of image acquisition, ability to perform repeated examinations, portability of ultrasound machines and probes, and absence of ionizing radiation risks. Costs include initial purchase and maintenance of equipment and training and availability of skilled clinicians to perform and interpret ultrasound images. (See 'Ultrasound equipment' above and 'Advantages and costs' above.)
●Point-of-care ultrasound (POCUS) refers to the use of ultrasonography at the patient’s bedside for diagnostic or therapeutic purposes, and is typically a limited qualitative examination that is simple, rapid, and goal-oriented (see 'Point-of care ultrasound (POCUS)' above). Examples include:
•Urgent diagnosis of causes of perioperative hemodynamic instability or shock (algorithm 1) (see 'Urgent diagnostic uses' above)
•Elective diagnostic purposes before, during, or after a surgical procedures (see 'Elective diagnostic uses' above)
•Aid in performance of procedures such as regional anesthesia or vascular cannulation (see 'Procedural uses' above)
●Transesophageal echocardiography (TEE) is the most common application of echocardiography in the operating room. Elective and emergency uses of TEE during cardiac and noncardiac surgical procedures are described elsewhere:
•(See "Anesthesia for cardiac surgery: General principles", section on 'Prebypass transesophageal echocardiography' and "Anesthesia for cardiac surgery: General principles", section on 'Postbypass transesophageal echocardiography'.)
•(See "Intraoperative transesophageal echocardiography for noncardiac surgery".)
•(See "Intraoperative rescue transesophageal echocardiography (TEE)".)
●Transthoracic echocardiography (TTE) may be used to conduct a qualitative focused cardiac ultrasound (FOCUS) to rapidly diagnose global biventricular systolic dysfunction, significant valvular abnormalities, pericardial effusion or tamponade, or likely hypovolemia (based on ventricular chamber size). (See 'Transthoracic echocardiography' above.)
●Lung ultrasonography may be used in the perioperative setting to rapidly diagnose and manage causes of acute respiratory distress that may be encountered in the perioperative period (eg, pneumothorax, pleural effusion, pneumonia, cardiogenic pulmonary edema, pulmonary embolus, exacerbation of asthma or chronic obstructive pulmonary disease, or esophageal or endobronchial intubation) (algorithm 2). (See 'Lung ultrasound' above.)
●Gastric ultrasound can be used in selected perioperative patients to assess the volume and character of gastric contents, thereby supplementing preoperative fasting guidelines in selected situations. (See 'Gastric ultrasound' above.)
●Specific strategies and precautions have been developed to minimize infection risks for the ultrasonographer and equipment during perioperative use of ultrasound in patients with confirmed or suspected novel coronavirus disease 2019 (COVID-19). (See 'Precautions to minimize infection risk' above.)
●For COVID-19 patients, lung ultrasound can be used to assess pulmonary status and guide management of intraoperative ventilation, while POCUS can be used to diagnose common causes of poor oxygenation or hemodynamic instability (eg, hypovolemia, septic shock, poor left or right ventricular function, pulmonary emboli). (See 'Diagnostic uses of ultrasound in COVID-19-positive patients' above.)
ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Nathaniel M Birgenheier, MD, who contributed to an earlier version of this topic review.
