Overview of perioperative diagnostic uses of ultrasound

INTRODUCTION — Ultrasonography allows rapid acquisition of high-resolution images of anatomic structures in real time. This topic will provide an overview of perioperative diagnostic uses of ultrasound. These include examination of lung, airway, gastric and other abdominal structures, as well as echocardiography of the heart and great vessels. Elective perioperative uses and use for rapid diagnosis or confirmation of cause(s) of perioperative hemodynamic instability are discussed.

Further details regarding used of bedside ultrasonography in patients with critical illness or traumatic injuries in other settings are available in separate topics:

●(See "Indications for bedside ultrasonography in the critically ill adult patient".)

●(See "Emergency ultrasound in adults with abdominal and thoracic trauma".)

Specific uses of ultrasound to provide imaging guidance during performance of regional anesthetic or vascular cannulation procedures are multiple separate UpToDate topics.

GENERAL CONSIDERATIONS

Ultrasound equipment — A variety of ultrasound machines can be used for perioperative point-of-care ultrasound (POCUS) [1]. 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, three-dimensional ultrasound, or strain imaging. More sophisticated machines can be employed to acquire images appropriate for various diagnostic studies or interventions, but such advanced equipment is not required for basic point-of-care (POC) imaging.

Ultrasound transducers include linear, phased array, and curvilinear probes. Linear probes use 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 "Basic principles of ultrasound-guided venous access".)

●(See "Ultrasound for peripheral nerve blocks", section on 'Ultrasound equipment'.)

Ultrasound equipment incorporating virtual reality enhanced imaging and artificial intelligence support are being developed and will shortly enter clinical practice [2-4].

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.

Anesthesiologists in many training programs and practice settings have learned to use perioperative POCUS to assess cardiovascular, pulmonary, airway, and abdominal abnormalities, as outlined in the sections below [5-16]. 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 [14,17].

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 [5-8,10-15,18-20].

PERIOPERATIVE POINT-OF-CARE ULTRASOUND: OVERVIEW — Point-of-care ultrasound (POCUS) can be used at the patient's bedside or in the operating room (OR) for diagnostic purposes and/or to aid in performance of a procedure [4,8,14,21]. While comprehensive imaging can be performed and interpreted in these settings, 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.

Urgent diagnostic uses — Urgent uses of POCUS in the perioperative setting include focused cardiac ultrasound and examination of the airway, lungs, abdomen, and/or pelvis to diagnose pathology, determine causes of hemodynamic instability, or confirm suspected complications. We agree with expert panel and professional society recommendations for POCUS use for these indications when equipment and expertise are available [14,22-24]. 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 [25].

●Determining causes of hemodynamic instability – 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 including obstetrical suites, preoperative areas, ORs, or in the immediate postoperative period in a post-anesthesia care unit (PACU) or intensive care unit (ICU). Diagnosis of hypovolemia, cardiac tamponade, tension pneumothorax, acute myocardial infarction, pulmonary embolus, severe left ventricular (LV) or right ventricular (RV) dysfunction, and likely causes of respiratory failure with hypoxia are possible using cardiac, pulmonary, and abdominal ultrasound techniques [4,8,21,26,27]. A systematic approach such as rapid ultrasound in shock (RUSH) may be 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) [4,8,21,27-33]. In many cases, progression of perioperative hemodynamic instability to frank shock or cardiac arrest may be avoided by early diagnosis and treatment of the cause. (See "Intraoperative management of shock in adults", section on 'Point-of-care ultrasonography'.)

Examples include diagnosis of:

•Hypovolemia – (See 'Cardiac ultrasound' below.)

•Cardiogenic causes of shock such as severe left or right ventricular dysfunction, pericardial tamponade, or LV outflow tract obstruction – (See 'Cardiac ultrasound' below.)

•Arrhythmias, with ability to distinguish causes such as "true pulseless electrical activity", "pseudo-pulseless electrical activity", or arrhythmias likely to respond to defibrillation (eg, ventricular fibrillation) [21] – (See "Intraoperative advanced cardiac life support (ACLS)", section on 'Initial resuscitation'.)

•Acute respiratory failure or hypoxia due to pulmonary edema, pneumonia, atelectasis, pleural effusion, or pulmonary embolus versus hypoventilation [34,35] – (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]) [36]. (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 [4,37-39] – (See 'Confirming endotracheal tube placement' below.)

•Intra-abdominal abnormalities – Visualization of free intraperitoneal fluid, suggestive of vessel rupture or abscess, or free air suggestive of a ruptured viscus or gas-producing organism are possible with abdominal ultrasound [40] – (See "Emergency ultrasound in adults with abdominal and thoracic trauma", section on 'Abdominal examination' and "Indications for bedside ultrasonography in the critically ill adult patient", section on 'Abdominopelvic ultrasonography'.)

•Severe preeclampsia with pulmonary edema [41] (see 'Lung ultrasound' below). Although beyond the scope of a POCUS examination, evidence of LV diastolic dysfunction, as well as increased optic nerve sheath diameter, may also be detected using ultrasonography in parturients with preeclampsia [14]. (See "Preeclampsia: Clinical features and diagnosis".)

Details regarding these urgent uses of POCUS are discussed in the sections below and in other topics:

•(See "Indications for bedside ultrasonography in the critically ill adult patient".)

•(See "Evaluation of and initial approach to the adult patient with undifferentiated hypotension and shock", section on 'Point-of-care ultrasonography'.)

•(See "Emergency ultrasound in adults with abdominal and thoracic trauma".)

●Diagnosing complications after interventional or surgical procedures

•Complications of central venous catheter (CVC) insertion – Ultrasound may be used to detect suspected mechanical complications that may occur after attempted or apparently successful placement of a CVC in an internal jugular (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 [42]. The investigators noted agreement between ultrasound and chest radiography for correct diagnosis in 99 percent of these cases.

•Unexpected findings during ultrasound imaging for regional anesthesia – In rare cases, performance of regional anesthetic techniques has led to unexpected diagnoses and urgent definitive treatment [43-45]. An example is noting free fluid in the abdomen from the hip joint following hip arthroscopy [45]. (See "Emergency ultrasound in adults with abdominal and thoracic trauma".)

Elective diagnostic uses — POCUS has been employed electively for diagnostic purposes before, during, or after surgical procedures [1,32,46]. Examples include:

●Preoperative period

•Evaluation of cardiovascular pathology such as ventricular dysfunction, cardiac valve pathology, or abnormal intravascular volume status – (See 'Assessment of preoperative cardiovascular risk' below.)

•Diagnosis of a potentially difficult airway or obstructive sleep apnea (OSA) – (See 'Screening for a difficult airway' below.)

•Assessment of the volume and characteristics of gastric contents – (See 'Gastric ultrasound' below.)

•Identification of preoperative frailty by examination of quadriceps depth and psoas muscle area [47] – (See "Overview of prehabilitation for surgical patients", section on 'Frailty'.)

●Intraoperative period

•Confirmation of appropriate positioning of the endotracheal tube (ETT) – (See 'Confirming endotracheal tube placement' below.)

•Confirmation of lung isolation during procedures requiring one lung ventilation by identifying lung sliding in the ventilated lung(s) [39,46,48,49] – (See "One lung ventilation: General principles" and 'Lung ultrasound' below.)

●Postoperative period

•Confirmation of bladder distention due to postoperative urinary retention (POUR) – (See 'Pelvic and bladder ultrasound' below and "Overview of post-anesthetic care for adult patients", section on 'Inability to void'.)

•Evaluation of likelihood of pulmonary complications in the early postoperative period [50,51]. A quantitative lung ultrasound score of the number of B line artifacts has been used to detect respiratory disorders, with a scale ranging from 0 to 36 (18 per lung for a total potential score up to 36) [52]. In one prospective observational study of 149 patients who underwent major abdominal surgery, point-of-care (POC) lung ultrasound was performed on postoperative day one [50]. Scores >12 on postoperative day one predicted development of pulmonary complications before postoperative day ten (defined as a composite outcome that included acute respiratory failure, pneumonia, acute respiratory distress syndrome, cardiogenic edema, atelectasis, reintubation, or major pleural effusion), with a sensitivity of 0.54 (95% CI 0.40-0.67) and a specificity of 0.77 (95% CI 0.67-0.85).

CARDIAC ULTRASOUND

Transesophageal echocardiography — Transesophageal echocardiography (TEE) is the most commonly used ultrasound modality to evaluate cardiac anatomy, physiology, and function in operating room (OR) settings.

●Noncardiac surgery – Depending on the availability of equipment and personnel, TEE, rather than point-of-care ultrasound (POCUS), may be used to urgently assess cardiopulmonary status to diagnose causes of hemodynamic instability during noncardiac surgery [8,53]. Details regarding elective or emergency intraoperative use of TEE during noncardiac surgery are available in separate topics:

•(See "Intraoperative transesophageal echocardiography for noncardiac surgery".)

•(See "Intraoperative rescue transesophageal echocardiography (TEE)".)

●Cardiac surgery – Uses of TEE during 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'.)

Societal recommendations suggest routine use of TEE performed by qualified echocardiographers for selected 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 surgery [54].

Focused transthoracic cardiac ultrasound (FOCUS) — While TEE is the most common application of echocardiography in the OR, FOCUS has also been incorporated into perioperative care [7,27,32,55-61].

Determining causes of hypotension — 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 [7,27,55,57,61]. FOCUS is used to rapidly diagnose cardiac abnormalities that cause hypotension and assess the effects of treatments such as administration of fluids or vasoactive agents [27,55]. Examples include (algorithm 1) (see "Indications for bedside ultrasonography in the critically ill adult patient", section on 'Thoracic ultrasonography'):

●Significant hypovolemia (based on a small ventricular chamber size with small end-diastolic size [diameter or area]) as well as a small, collapsed inferior vena cava (IVC), or variability of aortic pulsatile flow (which evaluates fluid responsiveness) using esophageal Doppler ultrasound [27,55]. Details are discussed in separate topics:

•(See "Evaluation of and initial approach to the adult patient with undifferentiated hypotension and shock", section on 'Point-of-care ultrasonography'.)

•(See "Intraoperative fluid management", section on 'Dynamic parameters to assess volume responsiveness'.)

●Severe left, right, or biventricular dysfunction (image 1 and image 2 and image 3 and figure 1 and image 4) – (See "Echocardiographic recognition of cardiomyopathies", section on 'Dilated cardiomyopathy' and "Echocardiographic assessment of the right heart".)

●Regional wall motion abnormalities (RWMAs) suggesting myocardial ischemia – (See "Role of echocardiography in acute myocardial infarction", section on 'Use of echocardiography' and "Role of echocardiography in acute myocardial infarction", section on 'Location and extent of MI'.)

●Left ventricular (LV) outflow tract obstruction [62] – (See "Echocardiographic recognition of cardiomyopathies", section on 'Hypertrophic cardiomyopathy' and "Anesthesia for patients with hypertrophic cardiomyopathy undergoing noncardiac surgery".)

●Pericardial effusion or tamponade (movie 1 and movie 2 and movie 3) – (See "Cardiac tamponade", section on 'Echocardiography' and "Echocardiographic evaluation of the pericardium" and "Emergency ultrasound in adults with abdominal and thoracic trauma", section on 'Pericardial and limited cardiac examination'.)

●Pulmonary embolism (PE) suggested by reduced right ventricular (RV) contractility or pulmonary hypertension (image 5 and movie 4). Recognition of a floating thrombus in the right atrium or ventricle, or of a clot in transit also support a diagnosis of PE. (See "Right ventricular myocardial infarction", section on 'Echocardiography' and "Clinical presentation, evaluation, and diagnosis of the nonpregnant adult with suspected acute pulmonary embolism", section on 'Echocardiography'.)

●Identification of arrhythmias. This includes distinguishing between true pulseless electrical activity (absence of cardiac motion) versus pseudo pulseless electrical activity (cardiac motion with failure to eject adequately) or an arrhythmia that should be defibrillated (eg, ventricular fibrillation) or cardioverted (eg, atrial fibrillation or flutter) [21,63]. (See "Intraoperative advanced cardiac life support (ACLS)", section on 'Treat the etiology of the cardiac arrest' and "Intraoperative advanced cardiac life support (ACLS)", section on 'Arrhythmias'.)

Assessment of preoperative cardiovascular risk — Use of FOCUS during preoperative cardiovascular risk assessment has also been proposed for selected patients such as those with suspected ventricular dysfunction, valvular heart disease, or abnormal intravascular volume status (eg, hypovolemia) [64-66]. 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 LV dysfunction or valvular abnormalities was improved by addition of FOCUS [66]. 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 transthoracic echocardiography (TTE) examination [67,68]. (See "Indications for bedside ultrasonography in the critically ill adult patient", section on 'Training and competence'.)

Use in patients with contraindications for TEE — FOCUS is not intended to be a comprehensive examination, and available equipment may lack capabilities for complete analysis [61]. However, a more comprehensive perioperative TTE examination can be performed using standard parasternal, apical, and subcostal windows, with interpretation by a fully qualified echocardiographer. Such a complete examination can be a useful alternative to TEE in selected patients when a diagnostic or procedural echocardiographic guidance is indicated, but esophageal pathology precluding placement of a TEE probe is present [56]. (See "Transthoracic echocardiography: Normal cardiac anatomy and tomographic views".)

LUNG ULTRASOUND

Indications and general considerations — Pulmonary and pleural ("lung") ultrasound 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) [8,32,52,69-76]. In many cases, the diagnostic accuracy of ultrasonography is equal or superior to chest radiography or computed tomography (CT) [50,52,77,78]. (See "Clinical presentation and diagnosis of pneumothorax", section on 'Diagnostic imaging'.)

When equipment and trained personnel are available, point-of-care (POC) lung ultrasonography may be used as a primary tool for evaluation of persistent intraoperative hypoxemia (table 2), together with lung auscultation and checking the anesthesia machine for possible malfunctioning, the administered gas mixture, and mechanical ventilator settings [79].

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 [75]. 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 [80]. Trainees required approximately 12 minutes to complete the examination, while experts required 8 to 10 minutes.

Technique — The technique for performing lung ultrasound to identify abnormalities in either lung is described below [71]:

●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 suspected but 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.

Normal findings — Normal lung ultrasound findings are shown in the movie, and in a table with links to additional video ultrasound images (movie 5 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 [81]. 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) [71,75,81,82]. (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 (see "Bedside pleural ultrasonography: Equipment, technique, and the identification of pleural effusion and pneumothorax", section on 'Evaluation for pneumothorax'). 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). 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.

Pneumothorax is suggested by:

•Absence of lung sliding on the affected side (the presence of lung sliding effectively excludes the possibility of pneumothorax on that side) (movie 6) [71,73,79,83]. 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 [84].

•Presence of lung point (image 6 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.

•Absence of lung pulse [71,79,84-88].

•Absence of B line pattern on the affected side, although A lines remain present.

●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 7 and table 4) [71,79]. Details regarding diagnostic evaluation and imaging of the effusion and thoracentesis techniques for treatment are discussed separately:

•(See "Bedside pleural ultrasonography: Equipment, technique, and the identification of pleural effusion and pneumothorax", section on 'Identification of pleural effusion using ultrasonography'.)

•(See "Imaging of pleural effusions in adults", section on 'Ultrasonography'.)

Performance of ultrasound-guided thoracentesis for treatment of pleural effusion is discussed in a separate topic. (See "Ultrasound-guided thoracentesis".)

●Lung injury – Lung injury with increased lung density and loss of aeration may be caused by (see "Indications for bedside ultrasonography in the critically ill adult patient", section on 'Evaluation of the etiology of cardiopulmonary failure'):

•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), findings on lung ultrasound imaging may include [71,72,79]:

-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. Three or more B lines per intercostal space indicate moderate loss of lung aeration (B1 or interstitial pattern) (figure 2). Multiple coalescent B lines per intercostal space indicate tissue-like increased density with almost complete loss of aeration (B2 or consolidation pattern) (figure 2) [81,82].

•Cardiogenic pulmonary edema – In patients with cardiogenic pulmonary edema, findings include [70,71,79,89,90]:

-A bilateral diffuse and homogenous gravity-dependent B-line pattern is typical (image 8 and movie 7), with smooth pleural morphology.

-Presence of lung sliding, but the A-line pattern may be absent.

-A "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 9 and image 10) [91]. 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 [92-94].

•Interstitial alveolar syndrome – Interstitial pulmonary edema (ie, interstitial alveolar syndrome) may develop in healthy term parturients with maximum risk during the early postpartum period [78], and develops before overt pulmonary edema in women with preeclampsia [95]. Findings may include:

-The presence of multiple B-lines (with three or more B-lines in a particular lung field defined as a positive lung region and the presence of two or more positive lung regions defined as diffuse B-line pattern) [78].

-The number of single and confluent B-lines can be recorded to calculate a score ranging from 0 to 32, with the total B-line score values in this study correlated with diastolic cardiac dysfunction, high blood pressure values, and presence of breathlessness [95].

●Pulmonary embolism – In patients with pulmonary embolism, lung ultrasound imaging may identify [71,96,97]:

•Peripheral wedge-shaped abnormalities

•A pleural line interrupted by small peripheral consolidations

•Absent B lines, with normal lung sliding, lung pulse, and A-line pattern

However, 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'.)

Focused lower extremity ultrasound to detect deep venous thrombosis aids in confirmation of pulmonary embolism. Details are discussed separately. (See "Indications for bedside ultrasonography in the critically ill adult patient", section on 'Detection of deep venous thrombosis'.)

●Esophageal or endobronchial intubation – Transtracheal ultrasonography can be employed to diagnose incorrect placement or migration of an endotracheal tube (ETT):

•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 nonintubated 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 nonintubated 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.

•Correct placement of an ETT can be confirmed as noted below (see 'Confirming endotracheal tube placement' below). Ventilation can be further confirmed by using lung ultrasonography to visualize the presence of bilateral sliding of the visceral pleura against the parietal pleura [4,39,79].

●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 [76].

●Postoperative diaphragmatic dysfunction – In the postoperative period, POC lung ultrasound has been used to detect diaphragmatic dysfunction (defined as diaphragmatic excursion <10 mm) in patients at high risk for this complication. In one study of 75 patients who had undergone thoracic surgery, diaphragmatic dysfunction was more common after open thoracotomy (29 of 35 patients [83 percent]) compared with video-assisted thoracoscopic (VATS) surgery (22 of 40 patients [55 percent]) [51]. In this study, postoperative diaphragmatic dysfunction was a risk factor for development of pulmonary complications such as hypoxemia, bronchospasm, atelectasis, new pulmonary infiltrates, or pleural effusion (odds ratio [OR] 5.5, 95% CI 1.9-16.3).

AIRWAY ULTRASOUND

Screening for a difficult airway — Assessment of the airway by ultrasonographic examination of the neck and chest has been used as a supplemental tool for patients anatomical and pathological findings that may lead to difficulty with laryngoscopy, tracheal intubation, or ventilation via mask or supraglottic airway (SGA) [4,98-101]. (See "Management of the difficult airway for general anesthesia in adults", section on 'Recognition of the difficult airway'.)

Furthermore, if obstructive sleep apnea (OSA) is suspected during examination, extra precautions to prevent postoperative respiratory complications are prudent [102]. (See "Surgical risk and the preoperative evaluation and management of adults with obstructive sleep apnea".)

Confirming endotracheal tube placement — Transtracheal ultrasonography can be employed to confirm correct placement of an endotracheal tube (ETT) 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) [74]. Confirmation of proper ETT positioning is accomplished by placing the ultrasound probe transversely across the suprasternal notch [4,37-39]. Images of correct ETT placement appear as a single air-filled structure with acoustic shadowing, while an esophageal intubation appears as two air-filed structures with acoustic shadowing (image 11) [74,103]. Use of ultrasonography to confirm proper ETT placement is particularly valuable when end-tidal CO₂ (ETCO₂) is low (eg, in the setting of low cardiac output) rendering confirmation with waveform capnography or color indicators of CO₂ inadequate or challenging [104]. If the ETT has been inserted too far into a mainstem bronchus, the ETT cuff balloon may be visualized at the level of the sternal notch in a longitudinal view [39].

In addition to standard bilateral auscultation of the lungs, ventilation can be further confirmed by the presence of bilateral sliding of the visceral pleura against the parietal pleura, as described above [4,39,79]. (See 'Normal findings' above.)

Guidance for emergency invasive airway placement — If emergency establishment of an invasive airway becomes necessary (eg, urgent cricothyroidotomy or tracheostomy), ultrasound identification of landmarks (eg, cricothyroid membrane) can be useful, and may be more accurate than external palpation is some circumstances [4,105-111].

ABDOMINAL ULTRASOUND

Gastric ultrasound — Gastric ultrasound has been employed to assess the volume and character of gastric contents, thereby supplementing preoperative fasting guidelines in selected clinical scenarios when the presence of gastric content is uncertain (table 5) [112-122] (see "Preoperative fasting in adults"). A randomized trial that included 80 ultrasound examinations noted that 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) [123].

There are no absolute indications for performing gastric ultrasound [124]. However, 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). Other possible causes of delay in gastric emptying include certain neuromuscular diseases, pain, chronic opioid use, and diabetes mellitus (particularly for patients taking glucagon-like peptide-1 receptor agonists [122]).

Technique — The technique for performing gastric ultrasound is described below (figure 3) [112]:

●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 nondependent 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 [125]. 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 4).

•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 5 and figure 6).

•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 7 and figure 8).

•Suspensions (or milk) – Antrum is round and distended, and hyperechoic fluid ("hepaticized") fluid is seen in the lumen of the antrum (figure 6).

●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 12).

•Use the following formula to estimate gastric volume from the traced cross-sectional area [113]:

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 semirecumbent positions, respectively [126].

●Gastric volume is graded according to risk of aspiration of gastric contents during induction of general anesthesia [127]:

•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).

Uses in shock or trauma — Transesophageal echocardiography with transgastric abdominal ultrasonography (TGAUS) has been used to diagnose less common causes of hemodynamic instability in the intraoperative setting (eg, inferior vena cava [IVC] thrombus, renal tumor extending into the IVC, hemoperitoneum, aortic dissection extension under the diaphragm) [40].

Also, abdominal ultrasound is an important initial screening examination in trauma and other critically ill patients, allowing visualization of free peritoneal fluid, free fluid suggestive of vessel rupture or abscess, or free air suggestive of a ruptured viscus or gas-producing organism. Details are discussed in separate topics:

●(See "Emergency ultrasound in adults with abdominal and thoracic trauma", section on 'Abdominal examination'.)

●(See "Evaluation of and initial approach to the adult patient with undifferentiated hypotension and shock", section on 'Point-of-care ultrasonography'.)

PELVIC AND BLADDER ULTRASOUND

●Diagnosis of postoperative bladder distention – Postoperative bladder distention can be confirmed as the cause of postoperative urinary retention (POUR) in patients unable to void after surgery. Details regarding this examination are discussed in a separate topic. (See "Overview of post-anesthetic care for adult patients", section on 'Inability to void'.)

●Other causes of oliguria – Other causes of oliguria/anuria or urinary tract obstruction can be diagnosed with pelvic ultrasound views (eg, obstructive hydronephrosis, renal atrophy, presence of blood or urine in the pelvic peritoneum) [128]. Details are discussed in separate topics:

•(See "Indications for bedside ultrasonography in the critically ill adult patient", section on 'Detection of urinary tract obstruction'.)

•(See "Emergency ultrasound in adults with abdominal and thoracic trauma", section on 'Pelvic view'.)

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

●General considerations – A variety of ultrasound machines and transducers are available for perioperative use. 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 ensuring 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) – POCUS refers to the use of ultrasonography at the patient's bedside for diagnostic or therapeutic purposes, typically with a limited qualitative examination that is simple, rapid, and goal-oriented. Perioperative clinical indications and uses include (see 'Perioperative point-of-care ultrasound: Overview' above):

•Urgent diagnosis of causes of perioperative hemodynamic instability or complications of interventional or surgical procedures. (See 'Urgent diagnostic uses' above.)

•Elective diagnostic uses before, during, or after a surgical procedures. (See 'Elective diagnostic uses' above.)

●Cardiac ultrasound

•Transesophageal echocardiography (TEE) – 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 in separate topics:

-(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)".)

•Focused transthoracic cardiac ultrasound (FOCUS) – FOCUS is a qualitative surface cardiac ultrasound examination that can be used in the perioperative setting to:

-Rapidly diagnose cardiac abnormalities that cause hypotension (eg, hypovolemia, severe left or right ventricular dysfunction, regional wall motion abnormalities indicating ischemia, left ventricular outflow tract obstruction, pericardial effusion or tamponade, pulmonary embolism, arrhythmias (algorithm 1)), and assess the effects of treatments such as administration of fluids or vasoactive agents – (See 'Determining causes of hypotension' above and "Evaluation of and initial approach to the adult patient with undifferentiated hypotension and shock", section on 'Point-of-care ultrasonography'.)

-Assess preoperative cardiovascular risks – (See 'Assessment of preoperative cardiovascular risk' above.)

●Lung ultrasound – Lung ultrasound can be used to rapidly diagnose and aid in management of perioperative causes of respiratory distress or pulmonary complications such as pneumothorax, pleural effusion, lung injury (eg, pneumonia, acute respiratory distress syndrome, cardiogenic pulmonary edema, interstitial alveolar syndrome), pulmonary embolus, endobronchial or esophageal intubation, exacerbation of asthma or chronic obstructive pulmonary disease, or diaphragmatic dysfunction (algorithm 2). (See 'Lung ultrasound' above.)

●Airway ultrasound – Ultrasonographic examination of the neck and chest can be used to assess a potentially difficult airway, confirm correct placement of an endotracheal tube, or identify landmarks (eg, cricothyroid membrane) during invasive airway placement. (See 'Airway ultrasound' above.)

●Abdominal ultrasound

•Gastric ultrasound – Gastric ultrasound can be used to assess the volume and character of gastric contents, thereby supplementing preoperative fasting guidelines in selected clinical scenarios when the presence of gastric content is uncertain. (See 'Gastric ultrasound' above.)

•Uses in shock or trauma – Abdominal ultrasound can be used to visualize free peritoneal fluid suggestive of vessel rupture or abscess, or free air suggestive of a ruptured viscus or gas-producing organism. Details are discussed in separate topics. (See "Emergency ultrasound in adults with abdominal and thoracic trauma", section on 'Abdominal examination' and "Evaluation of and initial approach to the adult patient with undifferentiated hypotension and shock", section on 'Point-of-care ultrasonography'.)

●Pelvic and bladder ultrasound

•Postoperative bladder distention – Bladder distention can be confirmed as the cause of urinary retention in patients unable to void after surgery. Details are discussed in separate topics. (See "Overview of post-anesthetic care for adult patients", section on 'Inability to void'.)

•Other causes of oliguria – Other causes of oliguria/anuria or urinary tract obstruction (eg, obstructive hydronephrosis, renal atrophy, blood or urine in the pelvic peritoneum) can be diagnosed using pelvic ultrasound. Details are discussed in separate topics. (See "Indications for bedside ultrasonography in the critically ill adult patient", section on 'Detection of urinary tract obstruction' and "Emergency ultrasound in adults with abdominal and thoracic trauma", section on 'Pelvic view'.)

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Nathaniel M Birgenheier, MD, who contributed to an earlier version of this topic review.