I-AIM (Indication, Acquisition, Interpretation, Medical Decision-making) Framework for Point of Care Lung Ultrasound

You are on call for anesthesia. The neurosurgeon on call has booked an emergency decompression of a subdural hematoma for an elderly man who fell at home during a syncopal episode. A chest radiograph on admission showed fractures of the sixth to eighth ribs with no evidence of a pneumothorax. Cardiac workup and blood work were noncontributory. On assessment, the man is desaturating on room air despite no previous respiratory history. He is uncooperative and will require general anesthesia and mechanical ventilation.

You are working in the intensive care unit (ICU). A patient with heart failure related to severe systolic dysfunction has steadily increasing work of breathing and oxygen requirements despite noninvasive mechanical ventilation. On physical exam, there is decreased air entry on the right side. A right-sided central line is in place.

You are called to the postanesthesia care unit to see a 60-yr-old patient with a history of asthma who just underwent laparoscopic cholecystectomy. He has persistent wheezing and respiratory distress that has been unresponsive to initial treatment with nebulized beta agonists and intravenous steroids.

Is there any role for lung ultrasound in the management of these patients? How should we perform the ultrasound and what findings should we look for?

Over the past decade, lung ultrasound has emerged as a point of care diagnostic tool that can be applied at the bedside to answer specific clinical questions and guide management. Lung ultrasound is easy to learn, quickly performed, goal-oriented, and has definite, easily recognized findings.^1–3  One of the primary limitations of point of care ultrasound is operator dependency.⁴  Practicing point of care ultrasound applications in an organized, protocol-based fashion is crucial to its clinical effectiveness and to prevent harm.⁴ 

Several protocols for point of care ultrasound have been described in the literature (e.g., Focused Assessment with Sonography for Trauma [FAST], Bedside Lung Ultrasound in Emergency [BLUE], Focused Assessed Transthoracic Echocardiography [FATE]).^5–7  However, they have been developed to address specific clinical situations or organ systems and are not applicable across the spectrum of point of care applications. In contrast, the I-AIM (Indication, Acquisition, Interpretation, Medical decision-making) model, described by Bahner et al.,⁸  presents an intuitive framework and cognitive aid designed to standardize the approach to every point of care exam within a user’s scope of practice, thereby improving the performance of the instrument and avoiding potential harm. The I-AIM framework already has been clinically applied to the FAST⁸  and gastric ultrasound exams.⁹ 

In this article, we present a systematic approach to lung ultrasound, based on the I-AIM framework. We provide a summary of the key terms and patterns associated with lung ultrasound and highlight potential pitfalls to avoid at each stage of the I-AIM protocol. We also present a practical algorithm to structure the interpretation of findings and direct clinical decision-making.

There is a growing body of evidence for the use of lung ultrasound as a diagnostic and monitoring tool (table 1, I-AIM framework for lung ultrasound: Indication).

As a diagnostic tool, the primary clinical indication is the presence of respiratory symptoms (e.g., dyspnea, pleuritic chest pain), or signs (e.g., tachypnea, oxygen desaturation, abnormal findings on respiratory physical examination) where the etiology has not been established.^2,10–13  Another indication is an unclear chest radiography finding, when other imaging modalities (e.g., computed tomography scan) are not practical or available. In these situations, the presence or absence of key sonographic findings may provide objective information with which to focus or revise the differential diagnosis. Lung ultrasound is highly sensitive and specific for the identification of pneumothorax^14,15 ; interstitial syndrome (associated with cardiogenic and noncardiogenic pulmonary edema, pneumonia and pneumonitis, lung contusion, interstitial lung diseases and other pulmonary conditions that cause increased lung density)^2,13,16–22 ; pleural effusions²³ ; and lung consolidation bordering the pleural line.^24–26  However, it does not necessarily discriminate between the various etiologies that may create these sonographic patterns.

As a monitoring tool, lung ultrasound can be used to assess the response to recruitment strategies (positive end-expiratory pressure changes or prone positioning),^27,28  elucidate causes for weaning failure,²⁹  and evaluate response to therapy in patients with pneumothorax, pleural effusions, and interstitial syndrome.^30–32  Lung ultrasound also may play a role in assessing fluid tolerance³³  and have prognostic capabilities in heart failure and end-stage renal disease.^34,35 

According to the I-AIM model,⁸  image acquisition is subdivided into patient, probe, picture, and protocol considerations (table 1, I-AIM framework for lung ultrasound: Acquisition).

Patient positioning is critical to the accuracy of lung imaging. The optimal patient position depends on the suspected pathology. For example, if the question regards a pneumothorax, patients should be positioned supine because pleural air collects in the least dependent part of the thorax, or the anterior chest wall, which is highly accessible to ultrasound imaging. The semisitting position places the apical regions of the thorax in the least dependent position, which is much less accessible for imaging due to the presence of the clavicles and may result in missing small pneumothoraces.^15,36 

By contrast, if the primary question pertains to a pleural effusion, the most dependent region of the thoracic cavity is the area of interest. This corresponds to the posterior costophrenic angle provided the patient is in a supine or semisitting position. In rare circumstances, when the patient is placed in prone position or is not subjected to gravitational forces, the findings change dramatically: in the prone position, nonloculated fluid redistributes ventrally whereas in microgravity, it redistributes throughout the thoracic cavity and is visualized in both anterior and posterior windows simultaneously. It also has been shown that the transition from supine to prone position is associated with redistribution of aeration, with improved aeration in the posterior areas and increased loss of aeration in the anterior areas.^28,37 

For lung consolidation and interstitial syndrome, patient position is less crucial and any position allowing complete examination of the lung fields can be used. In situations where serial ultrasounds are performed, it is essential that the scanning position remains consistent to facilitate reliable comparisons and thus patient position should be documented in the report.

To insonate the posterior lung zones, the patient’s torso may be slightly rotated to the contralateral side and the ipsilateral arm abducted. The upper thorax should be exposed and draped, with monitors and dressings repositioned as necessary.

Prospective studies using a variety of ultrasound probes have demonstrated that performance and interpretation of lung sonography is not probe specific.²  Nevertheless, certain probes may be preferred depending on the lung region of interest.³⁶  For instance, visualization of the pleural line is best accomplished with a high frequency (10 to 12 MHz) linear probe, whereas imaging of the supradiaphragmatic region (pleural space and lung) requires a lower frequency (1 to 5 MHz) probe for adequate depth penetration. Either curvilinear, microconvex or phased-array probes may be used.

With respect to probe orientation, either radiology convention (i.e., reference marker to the left of the screen), or cardiology convention (i.e., reference marker to the right of the screen), can be used. Because the probe orientation marker is placed cephalad or toward the right with respect to the patient, the cranial-caudal orientation of the image will be reversed depending on the convention used.

Sonographic examination of the lungs is unique in that it requires systematic analysis of both nonanatomical images (e.g., artifacts as A-lines, B-lines) generated by reflection and reverberation of the ultrasound waves at the interface between aerated lung tissues and fluid-rich structures (e.g., thickened interlobular septa, fluid-filled alveoli, soft tissues), and anatomical visualization of the pleural space and lung parenchyma in the presence of consolidation and effusion. Such lung ultrasound findings have been well described^2,6  and a detailed description is given in table 2, Supplemental Digital Content videos, and at the website http://pie.med.utoronto.ca/POCUS/POCUS_content/lungUS.html. Both the distribution of the abnormalities within the lung zones, and the timing of the findings with respect to symptom onset are significant for later interpretation.

When performing artifact analysis, any machine features designed to decrease artifact production, including compound imaging, speckle reduction, and tissue harmonic imaging need to be deactivated. Some ultrasound machines have a lung exam preset mode that will automatically deactivate these image filters.

The majority of the lung exam utilizes two-dimensional, also called B-mode, imaging. Depth, gain, and focus should be adjusted to optimize the image as needed. Decreasing the gain is particularly important for artifact visualization. In some circumstances, the zoom function may be required to optimize visualization of sonographic air bronchograms.

For the diagnosis of pneumothorax, M-mode imaging may help confirm the presence or absence of pleural movement, especially in situations where it may be reduced (e.g., elderly patients, lung overdistension, severe chronic obstructive pulmonary disease ³⁸ ; Supplemental Digital Content 1, https://links.lww.com/ALN/B515). Some authors also have suggested Power Doppler imaging to help identify lung sliding.³⁹ 

To examine the anterolateral surface of the lung, the probe is placed sagittally on the anterior chest and angulated until the beam is directed perpendicular to the pleural line. An optimal view contains two adjacent ribs with the pleural line visualized between. The presence of an A-line artifact pattern confirms appropriate probe angulation perpendicular to the pleural line. Once the optimal view is identified, the probe must be held completely still to reliably identify the subtle movements of the pleura with respiration (Supplemental Digital Content 2, https://links.lww.com/ALN/B514).

For examination of the posterior and supradiaphragmatic regions of the lung, the probe is placed longitudinally in the midaxillary line with the beam directed posteriorly. A slight counterclockwise rotation allows the probe footprint to lie obliquely over an intercostal space and eliminate the rib shadows from the image. An optimal image contains the diaphragm and the liver or spleen to the right of the display with the spine and kidney visualized in the far field. Visualization of the spine confirms that the posterior costophrenic angle has been imaged; if more anterior structures are visualized, such as the colon or inferior vena cava, the probe should be angled more posteriorly to ensure that a small pleural effusion has not been missed.⁴⁰  The probe is held still while the patient performs a deep inspiration-expiration to assess for descent of the lung. If a pleural effusion or consolidation is identified, the probe is swept in a posterior to anterior direction and moved cranially to assess its full extent (Supplemental Digital Content 3, https://links.lww.com/ALN/B518 and Supplemental Digital Content 4, https://links.lww.com/ALN/B519). A detailed stepwise approach to scanning the lung at the anterior chest wall and posterior and supradiaphragmatic regions of the lung is given in table 1.

A complete lung exam will insonate the anterior, lateral, and posterior surfaces of the lung bilaterally from the anterior and lateral chest walls. The most commonly used protocols are an eight-zone protocol (four chest areas per side based on a division of each hemithorax into anterior/lateral and upper/lower zones) and a twelve-zone protocol (six chest areas per side, based on a division of each hemithorax into anterior/lateral/posterior and upper/lower zones—anterior chest identified by anterior axillary line; lateral chest by anterior axillary line and posterior axillary line; posterior chest by posterior axillary line and spine—not including scapular area).^2,27  In emergency situations, however, the lung exam may be abbreviated to rapidly diagnose or exclude life-threatening acutely intervenable causes of respiratory failure (one anterior chest zone to evaluate for pneumothorax and interstitial syndrome, a lateral zone for interstitial syndrome, and one lower posterior site to evaluate for hemothorax and massive effusion; fig. 1).^2,6 

The lung is a dynamic organ; therefore, images ideally are captured as videos of at least one to two respiratory and several cardiac cycles in length. Occasionally a breath-hold (e.g., to assess lung pulse) or a deep breathing maneuver (e.g., to assess the curtain sign), would aid the accuracy of the exam (table 2). Attention to the respiratory and cardiac cycles as opposed to absolute duration ensures that severely bradypneic/apneic or bradycardic patients will not be misdiagnosed with a pneumothorax. In bradypnea/apnea there may be little, or complete lack of, lung sliding while in bradycardia the lung pulse may be missed, thereby falsely raising suspicion for a pneumothorax.

Image storage is essential for comparison with future images, quality assurance, education, and medico-legal purposes. In addition, a written report should be generated (Supplemental Digital Content 5, https://links.lww.com/ALN/B513).

Image interpretation should follow a logical stepwise approach including: (1) assessment of image quality and adequacy for interpretation, with correct ultrasound settings and inclusion of relevant anatomical structures; (2) establishment of the presence or absence of suspected findings; (3) generation of an ultrasonographic differential diagnosis; (4) progression to further scanning if needed, including utilization of other point of care applications (e.g., focused cardiac ultrasound, vascular ultrasound, diaphragmatic and abdominal ultrasound) (table 1, I-AIM framework for lung ultrasound: Interpretation; fig. 2).

Image quality should be maximized by attention to machine settings, probe location and angulation, inclusion of appropriate anatomical structures, and recording of an optimal video length. Images of inadequate quality may not be amenable to interpretation. For instance, nonperpendicular probe orientation may artificially display the pleural line as fragmented and nonhomogenous and/or fail to demonstrate the expected artifact pattern (Supplemental Digital Content 2, https://links.lww.com/ALN/B514).

Confirmation that relevant anatomical structures are present is also essential for accurate image interpretation. For instance, imaging the lower posterior lung without including the spine may miss a small pleural effusion because the beam has not reached the most dependent region of the lung⁴⁰  (Supplemental Digital Content 4, https://links.lww.com/ALN/B519). Likewise, an image at the anterior chest wall that does not include two ribs flanking the pleural line may result in mistaking the hyperechoic line of the rib as the pleural line and falsely diagnosing a pneumothorax due to the absence of movement (table 1, I-AIM framework for lung ultrasound: Acquisition).

Regardless of the likelihood of the diagnostic hypothesis, interpretation of lung ultrasound findings should proceed in an organized fashion. Our approach begins with an assessment of the pleural line interface (fig. 2). This interface represents the boundary between the intercostal muscles/parietal pleura and the visceral pleura/most superficial alveolar units and is a key branch point in the discovery of the most likely diagnoses (Supplemental Digital Content 2, https://links.lww.com/ALN/B514). It appears on lung ultrasound as: (1) fluid interface between parietal and visceral pleura; (2) A-line artifact pattern (aerated lung or air in the pleural space); (3) increased lung density, ranging from a B-line artifact pattern to a solid interface (increased lung weight or decreased lung aeration due to deflation or collapse).

The finding at the pleural line interface provides immediate feedback on the accuracy of the pretest hypothesis and refines the differential diagnosis depending on the specific pattern (fig. 2). Fluid beneath the pleural line usually represents a pleural effusion. Ultrasound is limited in further differentiating the type of fluid (except when mobile echoic particles or septa are identified),²  but it can provide an estimate of effusion size⁴¹ (Supplemental Digital Content 3, https://links.lww.com/ALN/B518; Supplemental Digital Content 4, https://links.lww.com/ALN/B519). An A-line pattern is further categorized by the presence or absence of lung movements (lung sliding and/or pulse) and the presence or absence of vertical artifacts. In the absence of both movements and artifacts, pneumothorax is highly likely, with the caveat that certain lung conditions (e.g., severe chronic obstructive pulmonary disease, bullous disease, lung overdistension) can create a similar sonographic pattern (Supplemental Digital Content 1, https://links.lww.com/ALN/B515). When lung motion is present at the pleural line, an A-line pattern represents normal lung density and the differential is focused on pathologies not necessarily visible on lung ultrasound (e.g., pathologies not reaching the pleural line, pulmonary vascular diseases, effect of high positive end-expiratory pressure). Increased lung density on lung ultrasound can be considered as a continuum of sonographic findings, from a progressively more severe interstitial syndrome (B-line patterns, see table 2) to a consolidated appearance of the lung parenchyma.⁴²  The extensive differential diagnosis can be narrowed by specific sonographic findings (e.g., diffuse vs. focal pattern, B-lines distribution—gravity dependent or independent, changes in characteristics of lung sliding and pulse, presence of pleural and subpleural line abnormalities)^2,17,43  and by the clinical context (table 3; Supplemental Digital Content 6, https://links.lww.com/ALN/B516; Supplemental Digital Content 7, https://links.lww.com/ALN/B517).⁴⁴ 

Before proceeding, two caveats should be mentioned. Complete lung ultrasound requires images at multiple locations on the chest wall. A given image is reflective of the pathology at that specific location and is not necessarily representative of the entire organ. Further, multiple pleural line interface patterns may be observed within the same lung. A patient in respiratory distress following chest trauma, for example, may have an A-line pattern in the anterior lung zones (normal lung or pneumothorax), a B-line pattern in the lateral zones (area of contused lung), and fluid in the lower posterior zones (hemothorax).

Point of care lung ultrasound must be performed and interpreted within a clinical context, defined by the history, physical exam, and other standard radiologic and laboratory testing. The clinical context informs physicians’ initial hypothesis regarding the diagnosis, which determines the pretest probability of a finding¹⁶  (table 1, I-AIM framework for lung ultrasound: Medical Decision-making).

Lung ultrasound is only one piece of the diagnostic puzzle and will have one of four possible effects on the pretest diagnostic hypothesis:

• (1) Lung ultrasound findings may corroborate and support (e.g., patient with chest trauma; pretest hypothesis is pneumothorax; lung ultrasound reveals no sliding, no pulse and presence of a lung point. Lung ultrasound supports the high pretest probability of pneumothorax);

• (2) Lung ultrasound findings may mislead (e.g., elderly patient with fever and dyspnea; pretest hypothesis is pneumonia; lung ultrasound findings are consistent with the appearance of normal lungs. In reality, the computed tomography scan of the chest shows a large consolidation. Lung ultrasound is misleading due to its inability to detect consolidation that has not reached the pleural line, giving the false impression that respiratory pathology is absent)^25,26 ;

• (3) Lung ultrasound findings may be inconsequential, not supporting but not changing the hypothesis/plan (e.g., ICU patient; pretest hypothesis of acute respiratory distress syndrome; lung ultrasound reveals diffuse B-line pattern and mild to moderate pleural effusion. Lung ultrasound findings not completely specific for acute respiratory distress syndrome but also unlikely to change clinical management);

• (4) Lung ultrasound findings may completely change the pretest diagnosis (e.g., patient in the emergency department with wheezing; pretest hypothesis is asthma exacerbation; lung ultrasound reveals diffuse B-line pattern bilaterally suggesting alternative diagnosis of pulmonary edema).

It is apparent from the above examples that lung ultrasound, like other diagnostic tests, can lead to false positive or false negative results. Therefore, positive results should always be confirmed before reacting (e.g., confirmed in another view or ordering a different test, if appropriate) while negative results do not necessarily definitively exclude the diagnosis in question.²⁵  Further, when a sonographic diagnosis is made in a patient with a low pretest probability of that disease, a more advanced test should be considered for confirmation.

Finally, the clinical context should remain the primary determinant of patient management. A small pneumothorax may require intervention if the patient requires general anesthesia and mechanical ventilation for a surgical procedure, whereas a large pleural effusion may not require drainage in a patient requiring minimal respiratory support.

Now, you can return to the three patients with dyspnea and use lung ultrasound as part of their clinical work-up, applying the I-AIM framework.

Case 1: Lung ultrasound revealed the absence of lung sliding, lung pulse, and vertical artifacts on the anterior left hemithorax. A lung point also was identified more laterally. Left pneumothorax was therefore diagnosed and a chest tube inserted preoperatively (fig. 3, A and B; Supplemental Digital Content 1, https://links.lww.com/ALN/B515).

Case 2: Lung ultrasound revealed a large right pleural effusion that was drained under ultrasound guidance; no pneumothorax was detected. Intubation was averted (fig. 3C; Supplemental Digital Content 3, https://links.lww.com/ALN/B518; Supplemental Digital Content 4, https://links.lww.com/ALN/B519).

Case 3: Lung ultrasound revealed multiple bilateral B-lines. A subsequent focused cardiac ultrasound showed hypertrophic cardiomyopathy and the diagnosis of cardiac asthma secondary to heart failure with preserved ejection fraction was made (fig. 3D; Supplemental Digital Content 6, https://links.lww.com/ALN/B516).

In conclusion, we present the application of the I-AIM framework to a specific point of care ultrasound exam. As has been demonstrated in other applications, the I-AIM framework is intuitive, structured, and generalizable. There are some limitations to the model with respect to adequate validation of the design, identification of benefits and risks, and its dependency on the English language. Nevertheless, we believe it has practical bedside potential as the adoption of point of care ultrasound becomes more widespread.

The authors acknowledge Jean YiChun Lin, M.Sc.B.M.C., Gordon Tait, Ph.D., and Massimiliano Meineri, M.D., from the Toronto General Hospital Department of Anesthesia Perioperative Interactive Education (http://pie.med.utoronto.ca/index.htm), Department of Anesthesia, University Health Network and University of Toronto, Toronto, Canada, and Emanuele Pivetta, M.D., M.Sc., from the Cancer Epidemiology Unit, Department of Medical Sciences, University of Turin, and the Division of Emergency Medicine and High Dependency Unit, AOU Cittá della Salute e della Scienza, Turin, Italy, for the generous sharing of their educational material.

Supported by the University Health Network, Toronto, Ontario, Canada (Drs. Kruisselbrink, Chan, and Goffi), the Department of Anesthesia, University of Toronto, Ontario, Canada (Drs. Kruisselbrink and Chan) and the Interdepartmental Division of Critical Care Medicine, University of Toronto, Toronto, Ontario, Canada (Drs. Abrahamson and Goffi).

The authors declare no competing interest.