Intraoperative lung ultrasound: A clinicodynamic perspective.

In the era of evidence-based medicine, ultrasonography has emerged as an important and indispensable tool in clinical practice in various specialties including critical care. Lung ultrasound (LUS) has a wide potential in various surgical and clinical situations for timely and easy detection of an impending crisis such as pulmonary edema, endobronchial tube migration, pneumothorax, atelectasis, pleural effusion, and various other causes of desaturation before it clinically ensues to critical level. Although ultrasonography is frequently used in nerve blocks, airway handling, and vascular access, LUS for routine intraoperative monitoring and in crisis management still necessitates recognition. After reviewing the various articles regarding the use of LUS in critical care, we found, that LUS can be used in various intraoperative circumstances similar to Intensive Care Unit with some limitations. Except for few attempts in the intraoperative detection of pneumothorax, LUS is hardly used but has wider perspective for routine and crisis management in real-time. If anesthesiologists add LUS in their routine monitoring armamentarium, it can assist to move a step ahead in the dynamic management of critically ill and high-risk patients.

Introduction

Once considered inaccessible, lung ultrasonography has emerged as an indispensible tool in critical care practice. Evidence suggests that clinicians outside radiology can be quickly skilled in limited area of ultrasonography,[12] similarly anesthesiologist can easily adopt lung ultrasound (LUS), in routine anesthesia practice and explore its true potential for perioperative care, allowing the anesthesiologist to diagnose various critical conditions precisely to implement therapeutic measures before the catastrophe ensues.

Adequacy of intraoperative lung function is assessed by monitoring vital parameters, spirometry, and sometimes radiography with variable outcomes.[3] Spirometry and radiography has been acknowledged with low accuracy and limitations.[456] In the last decade, ultrasonography has emerged as a new promising monitoring aid to improve anesthesiologists diagnostic and interventional skills,[7] and moving a step ahead in dynamic management of critically ill and high-risk patients. The aim of this article is to review the scientific basis of LUS (analysis of artifacts generated due to the admixture of air and lung tissue) and its clinical applications in day-to-day anesthesia practice. Search for articles was carried out using search engines such as Google Scholar and PubMed. Words used for the search were “lung ultrasound,” “pneumothorax,” “pulmonary edema,” “atelectasis,” “endobronchial intubation,” “alveolar interstitial syndrome,” and “intraoperative desaturation.”

History of Lung Ultrasound

Pierre Curie and Jacques Curie in 19th century discovered piezoelectric effect of certain crystals (e.g., Quartz), which were later developed by Langevin.[8] In middle of 20th century Dussik brothers[9] described the diagnostic properties of ultrasound and Joyner et al.[10] in 1967 studied utility of thoracic ultrasound for diagnosing pleural effusion. In 1978 La Grange et al.[11] used ultrasound for guiding needle placement in nerve blocks. Lichtenstein and Axler the pioneer of LUS, in the early 90s evaluated the scientific principle of LUS.[7]

Principle of Lung Artifacts

Reflection of the ultrasound beam by an interface between the media of different acoustic impedance is the basis of LUS, succeeding which Lichtenstein[12] gave seven basic principles of lung sonography. No impedance to air was detected in aerated lung. Hence no visible image is generated, but pleura is the only structure, visualized as the hyperechoic horizontal line[13] [Figure 1]. The anatomical configuration of the thorax is such that air and water inherently mix to invoke artifacts. Apart from these intrinsic properties both air and water have divergent gravitational dynamics (air rises and water descends). This pivotal interface classify water rich pathologies as “Dependent disorders” (pleural effusion, atelectasis, and alveolar consolidations), and air rich pathologies as “nondependent disorders” (interstitial syndromes, pneumothorax).[14]

Figure 1

Physical and anatomic basis of echo lung comets. Reflections of the ultrasound beam by the thickened interlobular septa proved comet-tail artifact in patients with extravascular lung water (reprinted with permission from Jambriket al.)

Methodology

LUS can be performed using any commercially available portable ultrasound machine with acceptable image quality. The high-frequency linear probe (5-12 MHz) is well-suited for pleural analysis while deeper structures can be analyzed with convex and micro-convex probes (2-5 MHz). Conventional B-mode imaging is adequate, and can be aided with M-mode or color Doppler in some difficult situations. Lung pathologies have two main components that is, air and water, which according to gravitational law are either on top (air) or bottom (water), thus probe should be positioned in accordance to suspected pathology. For water containing pathology (pleural effusion) probe should be placed at bases and for air containing pathology (pneumothorax) at apices, scanning should be done in systematic manner for unsuspected pathology (from apex to base and from anterior to lateral chest). Intraoperative LUS can be performed in supine and semilateral position by placing transducer along the longitudinal axis in anterior and lateral upper chest between second to fourth intercostal spaces on both sides of hemithorax.[15] Most practical intraoperative approach for LUS is to scan 4 areas on each hemithorax[16] [Figure 2].

Figure 2

Methods of lung scanning (reprinted with permission from Jambriket al.)

Normal lung semiology

On scanning chest, adjacent ribs produces black shadow while pleura appears as a horizontal hyperechoic line, roughly 0.5 cm below ribs, suggestive of the parietopulmonary interface. These typical ribs and pleural interface illustrate typical “Bat Sign” appearance [Figure 3].

Figure 3

“Bat sign” appearance (small vertical arrows, central arrow is pleura while lateral arrows indicates rib shadow) and A-lines (big horizontal arrows)

Pleura on LUS produces a scintillating synchronized movement with respiration, known as “Lung Sliding” which is a dynamic sign indicative of normal lung anatomy. Movement of pleura in relation to immobile superficial chest wall, produces air artifact as regular repetition of horizontal lines, known as “A-lines,” which is a static sign and physiological artifacts associated with lung sliding.[17] In some clinical situations (morbidly obese, advance age, emphysema, etc.), when lung sliding is not appreciated, “A-lines” can be objectified on M-mode as superficial motionless chest wall layers mimics waves while deeper layer artifacts creates sandy pattern, called “Sea Shore” sign [Figure 4].

Figure 4

“Sea Shore sign” (M-mode, arrow indicates pleural line, above the arrow is chest wall, and sandy is lung parenchyma)

After revealing normal semiology, artifacts used to diagnose pathological conditions are clarified below, which are important to diagnose intraoperative emergencies that warrant immediate intervention. A preoperative lung scanning is must to diagnose any intraoperative aberrations.

Alveolar Interstitial Syndrome

Diverse etiological factors have been found to be associated with perioperative pulmonary edema [Table 1]. Arieff[18] in his study of elective surgery patients, reported the incidence of perioperative pulmonary edema as 7.6%, and mortality due to it to be 11.9%.

Table 1

Factors leading to peri-operative pulmonary edema

Alveolar interstitial syndrome (AIS) is a radiological entity, manifests on LUS as B-lines, which develops due to seepage which of extra fluid into the interstitium, and it correlates with pulmonary edema. The presence of diffuse AIS is highly indicative of pulmonary edema (sensitivity 93.6%, specificity 84%, positive predictive value 87.9%, and negative predictive value 91.3%).[19]

B-line artifact (comet-tail artifact) on LUS are found to be correlated with Kerley B-lines on chest radiography,[15] with extravascular lung water (EVLW),[20] and with severity of diastolic dysfunction.[21]

B-lines are vertical lines originating from pleura and formed due to water thickened or fibrotic interlobular septa,[22] with specific peculiarities [Figure 5]. These are well defined, hyperechoic, laser-like reverberation, extending indefinitely up to the bottom of the screen, erasing A-lines, and is synchronous with lung sliding. In the presence of EVLW, ultrasound beam finds thickened subpleural interlobular septa that are an area of low impedance, with a high acoustic mismatch, causing reflectance of the beam and creates resonance. Such successive reverberations creates the time lag and is interpreted behaves like a persistent source and generating a series of closely spaced interfaces.[23]

Figure 5

“Comet-tail” or B-lines (white arrows)

Interpretation of B-lines and should be done cautiously for diagnosis and evaluation of AIS, and ideally scanning should be done in eight regions of anterior and lateral chest wall. Scanning of 28 rib technique given by Jambrik et al.[15] should be implemented for semi quantification of the interstitial syndrome. In an emergency situation, scanning of anterior two regions may suffice for rapid detection of the interstitial syndrome. This test is defined negative, when either B-lines are absent, solitary or <3 in number of isolated scanned field or present only in last intercostal space above diaphragm (which may be a normal variant observed in 27% healthy subjects[22]). When the distance between two B-lines is <7 mm or count of B-lines are >3 in each scanned area, it is considered positive B-profile.[24] If these B-profiles are present in two or more areas bilaterally or all over the lung surface, it is specific of AIS. Picano et al.[14] gave the scoring system to yield a score of B-lines, denoting the extent of EVLW, score ≥6 is suggestive of a mild degree of AIS while score ≥30 indicates severe AIS [Table 2]. Twenty-eight rib scanning technique to yield a score of B-lines is most informative, but impractical intraoperatively and can be used in postoperative period. Eight area scanning technique given by Volpicelli et al.[16] is more practical approach for intraoperative use.

Table 2

Scoring of B-lines

Clinical Implication

Intraoperative intravascular volume assessment is one of the most difficult tasks and is pivotal to the perioperative outcome. Superiority of goal-directed therapy to restrictive fluid strategy is still debatable,[25] and even advanced hemodynamic monitoring aids (both static and dynamic) used for intraoperative fluids assessment, come with variable inference. None of monitors for fluid assessment is gold standard for intraoperative use with uniformity in all patients, as the preference depends on ASA physical status of patients, the extent of surgery, and major fluid shifts.[2627] Furthermore, dynamic indices have their own limitations in presence of arrhythmias and patients on spontaneous breathing. Sometimes, patients may be at responsive part of Fick's cardiac output curve, and are unable to tolerate the transfused fluid, probably due to leaky lung capillaries, which can be detected as B-lines, hence EVLW. LUS detects quiescent pulmonary edema much earlier before the patient desaturates or clinically evident heart failure ensues. Thus, it can keep real-time vigilance on fluid transfusion and its tolerance. LUS aids in restricting fluid, planning to decongest the lung and following the response of therapeutic intervention. Before interpreting and concluding LUS findings, these scans should be compared with preoperative scans, as these focal or diffuse B-lines can be seen in pneumonia, atelectasis, pulmonary contusion, pulmonary infarction, neoplasms, and pleura disorders.

Pneumothorax

Pneumothorax is a potentially serious perioperative complication, due to numerous surgical, iatrogenic, and medical etiologies.[28] It presents as acute desaturation and cardiorespiratory instability, and if not diagnosed timely can progress to life-threatening tension pneumothorax.

Pneumothorax is a nondependent pathology, thus can be located in least dependent areas of chest.[2930] Transducer probe is first placed longitudinally in third and fourth intercostal space, and then gradually scanning these spaces from anterior to inferior areas of the chest between parasternal to the midclavicular line.[31] The focus should be emphasized to locate parietal pleura, which usually lies 0.5 cm below the rib shadow.[32]

Sonographic signs of pneumothorax

Sonographic diagnosis of pneumothorax is not straight forward, but requires sequential recognition and exclusion of different artifacts in protocol-based manner. Though LUS artifacts, appear complex to identify, even brief training can allow easy recognition of pneumothorax.[33] Identification of following four sonographic signs: Absence of lung sliding, B-lines, and lung pulse and the presence of lung point are required for diagnosis.[17]

Lung sliding

Lung sliding has a negative predictive value of 100% in pneumothorax.[32] Absence of lung sliding can also be found in numerous other clinical scenarios such as adult respiratory distress syndrome (ARDS), massive atelectasis, pleural adhesions, endobronchial intubations, cardiopulmonary resuscitations, and phrenic nerve palsy[34] Due to these various confounding conditions, the specificity of LUS in critical scenarios varies from 91% in general population,[31]78% in all critically ill patients, and 60% in ARDS patients.[35] Positive predicted value of LUS is 22% in all critically ill patients.[36] Lung sliding appears as a blush of color under the pleura in color Doppler mode, whenever in doubt, it can be used to confirm lung sliding.[37] It can also be objectified on M-mode as multiple horizontal lines known as “Stratosphere” sign[13] [Figure 6].

Figure 6

“Stratosphere sign” of pneumothorax

B-Lines

It is one of the key sign to be checked for disregarding diagnosis of pneumothorax and has an indirect interpretation, as the presence of B-lines indicates intact/adherent pleura. The presence of even single B-line can safely be used to rule out pneumothorax with 100% true negative rate.[38]

Lung pulse

Lung pulses are tenuous, rhythmic, vertical movement of visceral upon parietal pleura synchronous with cardiac pulsations, and are caused by transmission of cardiac oscillations through motionless lung and can be objectified on M-mode[3439] [Figure 7]. Lung pulse is also evident in massive atelectasis, consolidations, endobronchial intubations, One lung ventilation or simply in apneic breath holdings.[34] The presence of lung pulse can safely rule out pneumothorax in the absence of lung sliding with sensitivity of 93%.[34] Conversely absence of lung pulse with absence of lung sliding goes in favor of pneumothorax.

Figure 7

Lung pulse (arrows shows transmitted heart pulsations)

Lung point

Lung point represents transitional area of absence of lung sliding to area of presence of lung sliding, or absence of B-lines to area of presence of B-lines, or is the point from motionless pleura to normal respiratory pattern [Figure 8]. This lung point should be mapped from anterior inferior area to lateroinferior area, where it can confidently be used to define the physical limits of pneumothorax.[3640] Thus more lateral the lung point more extensive is the pneumothorax. Lung point is a pathognomonic sign of pneumothorax with 100% specificity.[35] Unfortunately, the sensitivity of this sign is very low, as there is no lung point in complete retraction of lung, and sometimes it is not possible to define desired chest area intraoperatively. That's why it is recommended to incorporate all LUS sign in the sequential and protocol-based manner before reaching any conclusion as given by Volpicelli.[41]

Figure 8

Lung point (arrows show interface of normal lung from pneumothorax)

Clinical Implications

The exact incidence of intraoperative pneumothorax is not known, and it varies from 0.01% in laproscopic surgeries[42] to 14% in mechanical ventilation.[43] Its incidence is likely to upsurge in modern clinical practice due to increased use of invasive monitoring and newer modes of mechanical ventilation. The diagnosis of pneumothorax in an anesthetized patient is always difficult, due to the subdued clinical presentation and its resemblance to bronchospasm. In an unexplained intraoperative desaturation, with or without cardiorespiratory instability, pneumothorax should be suspected and ruled out especially if the patient is undergoing procedures prone to pneumothorax. Pneumothorax once suspected should be confirmed, and chest X-ray is always the preferred method in diagnosis of pneumothorax, but recently, LUS has emerged as a new bedside diagnostic tool for diagnosis of pneumothorax, and various studies have proven its utility in emergency department,[44] critical care[3245] and for intraoperative diagnosis of pneumothorax.[28] Sensitivity and specificity of LUS in the diagnosis of pneumothorax had been reported to vary from 86-100% to 97-100%, respectively, whereas 28-75% and 100% reported for chest X-ray.[4446] LUS can be used to quantify pneumothorax and to evaluate its extension for planning the therapeutic modality (conservative management vs. intercostal drainage [ICD] insertion), especially in clinical scenario where bedside computed tomography (CT) scan is not a feasible option. Serial LUS scans can be used to evaluate changes in volume and extension of pneumothorax, and hence aids in decision making for removal of ICD.

Atelectasis

Atelectasis is one of the frequently encountered postoperative pulmonary complication (PPC), with wide spectrum of causes related to perioperative events. General anesthesia causes atelectasis within the first few minutes of induction in the most dependent part of lung.[4748] Approximately, 15-20% of lung tissue near diaphragm or about 10% of total lung mass in supine posture may develop atelectasis during surgery, which may extend up to 50% of lung tissue in open heart surgery.[49]

Lung Semiology of Atelectasis

Scanning for atelectasis should start by placing the probe on lateral and inferior chest longitudinally, eventually should move anteriorly in a supine posture, to outline the extent of atelectasis. Sensitivity and specificity of LUS to detect atelectasis is 93% and 100%, respectively.[34] The absence of lung sliding with lung pulse and standstill cupola (absence of lung expansion) are the early signs of atelectasis.[50] With progressive absorption of air, there is a loss of volume leading to a hypoechoic pattern known as static air bronchogram (late sign).[50] Static air bronchogram is the most valuable sign to diagnose atelectasis and to differentiate from consolidation. Air trapped in bronchus of consolidated lung, appears either linear or lentil sized punctiform image, produces centrifugal inspiratory movement (dynamic bronchogram), and is pathognomonic of nonretractile consolidation.[51]

Clinical Application

Postoperative atelectasis, a self-limiting lung aberration, can progress to a critical level in high-risk patients. Initial presentation is hypoxemia, which can even progress to pneumonia as a continuum of intraoperative atelectasis.[5253] Mild to moderate perioperative atelectasis usually occurs in nearly half of elective surgical patients, approximately 20% patients may develop severe atelectasis (oxygen saturation <81% up to 5 min) intraoperatively[54] and 13% in the Postanesthesia Care Unit.[55] The frequency of PPC were comparable to cardiac complications in patients undergoing abdominal surgeries (9.6% vs. 5.7%, respectively)[56] and was associated with increased length of hospital stay, increased cost of treatment, and approximately one-fourth of mortality within 6 days of surgery.[52]

Conventional chest radiography does not detect minimal atelectasis in the early postoperative period unless it becomes massive.[57] CT scan is a gold standard tool to detect minimal atelectasis, but unfeasible in the operation theater,[5859] LUS is a reliable, real-time alternative tool to detect and review the progression of atelectasis. Apart from its diagnostic use, it can also help in the evaluation of the effectiveness of therapeutic measures taken for its prevention.[60]

Pleural Effusion

A pleural effusion is a manifestation of various diseases, commonly associated with respiratory and nonrespiratory pathology. Pleural effusion can be asymptomatic to full blown respiratory failure, depending on the amount of effusion. Since, the sentinel work of Joyner et al.[10] for diagnosis and assessment of pleural effusion in 1967, LUS as a diagnostic modality is underutilized, probably due to more inclination of clinicians toward chest radiography[3] and faith in accuracy of CT scan. Real-time ultrasonography offers more effectual and expedient diagnosis of pleural effusion in the supine position than traditional radiography,[61] with comparable diagnostic accuracy to CT scan.[62]

Methodology and Semiology of Pleural Effusion on Lung Ultrasound

In supine posture, USG probe should be placed longitudinally on the inferior chest near diaphragm in mid or posterior axillary line. Sizeable effusion can be easily visualized, but the patient can be further scanned posteriorly in supine position or in dependent posture. In accordance to gravitational principle, scanning should start with localization of diaphragm and looking for a pattern of pleural effusion, the probe should then move anterior and cephalic to outline the extent of effusion. The sonographic pattern of pleural effusion varies depending on consistency of effusion, and can be subclassified from anechoic in transudate to echoic, complex nonseptated, complex septate, or homogenous in exudates, empyema or postoperative organized clots.[63] Pleural effusion is confined by four boundaries formed superiorly by pleural line (parietal pleura), inferiorly by smooth and regular lung line (visceral pleura) and laterally by upper and lower rib in the form of a sharp, that's why it is also known as “Sharp” sign [Figure 9]. “Sinusoidal sign” is a dynamic sign, which depicts sinusoidal movement on M-mode reflecting the compressibility and distensibility of effusion with respiration [Figure 10]. The presence of these two signs has 97% diagnostic specificity.[62] Compared to CT scan as standard diagnostic modality, sensitivity, and specificity of ultrasound is 93% in detecting the minimal pleura effusion.[3]

Figure 9

“Sharp Sign” (white arrow shows parietal pleura and black arrow visceral pleura, in between two are pleural effusion)

Figure 10

Sinusoidal sign (on M-Mode arrow shows the approximation of visceral pleura to parietal pleura with inspiration)

LUS provides a prompt and visual diagnosis of pleural effusion in several perioperative clinical scenarios.[64] LUS has more advanced and better proficiency in quantifying the volume of pleural effusion, describing its nature, and locating the area for thoracocentesis as compared to radiograph. Volume up to 525 ml can be missed on traditional radiography as false positive results can be produced.[65]

Massive to large symptomatic pleural effusions with compromised pulmonary functions are treated preoperatively. In patients with moderate pleural effusion and compromised cardiac reserve, dilemma persists regarding their preoperative optimization. ICD insertion before surgery is still a debatable issue. Asymptomatic patients with more than 1 L of effusion, have to be clinically assessed by positioning them in lateral decubitus posture, on the side opposite to effusion, worsening of preexisting dyspnea or new onset of dyspnea, warrants chest drain insertion preoperatively.[66] Radiologists rather recommend an ultrasound to quantify the volume and the need of chest drain. To overcome such dubious situations, intraoperative LUS can be decisive for further management. Fifty percent of patients will not require chest drain if the inspiratory expansion of the interpleural space is ≥15 mm on LUS.[13] Correlation of clinical test with LUS makes anesthetist more decisive regarding the placement of preoperative chest drain, intraoperative lung recruitment strategy and need for postoperative ventilator support.

The sinusoidal sign also predicts the safe removal of chest drains when the interpleural expansion is more than 15 mm with inspiration in association with stable hemodynamic parameters. Hemothorax and empyema are echoic and can be septated, nonseptated, or organized. It requires precise chest drain positioning for optimal drainage. Real-time sonography not only increases the success rate to place proper drain but also prevents procedure related complications or re-explorations.[67] LUS can also be used to check the patency of chest drain. When the lung line approximates pleural line it indicates near complete lung expansion, but with nonapproximating lung line to pleural line and static air column, suggests drain blockage and warrants chest drain to be changed.

Intraoperative Desaturation

There are a number of causes of intraoperative desaturation, which requires early recognition and prompt management to prevent any catastrophe. Depending on criteria used to define desaturation, heterogeneity of sample, demographics of patients, anesthesia equipment's error, and complexity of surgical procedures, the incidence of intraoperative desaturation varies from 0.5% to 2.74%, whereas postoperative desaturation varies from 0.32% to 55%.[68] Probability of intraoperative desaturation is maximum at maintenance (or induction) of general anesthesia and is accountable for approximately 50% of all intraoperative desaturations.[69]

Runciman et al. in 1993 applied checklist with mnemonic “COVER ABCD — A SWIFT” for intraoperative crisis management for desaturation by analysis of 2000 incidents reports. Of all incidents “COVER” would diagnose and correct in the 60% of cases.[70]

Szekely et al. in 2005 re-evaluated the algorithm's role in crisis management and concluded that “COVER” includes approximately 41% of all causes of intraoperative desaturation, whereas ABCD includes approximately 48% of all causes of intraoperative desaturation, remaining 11% does not come under either of the category. Fifth of these desaturations were due to endobronchial intubations and were late to diagnose.[69]

Various causes of desaturation have been discussed previously and can be diagnosed with the help of LUS by stepwise use of “BLUE” protocol given by Lichtenstein.[36] There are various measures to confirm proper endotracheal intubation; some of the traditionally used are bilateral chest movement, five point auscultation, tidal volume loops, and endotracheal tube cuff maneuvers.[71] Confirmation of tube placement with EtCO2 is among the most reliable method, but still 7% false negative failure rate (tube in trachea but capnography indicates esophageal intubation) and 3% false positive rate (tube in esophagus but capnography indicates tracheal intubation) was reported during intraoperative period.[72] Fiberoptic bronchoscopy remains the gold standard for confirmation of proper tube placement.[73]

Endobronchial intubation or dislodgement of the endotracheal tube can be easily identified on LUS, and can be decisive in ruling out these as a cause of acute desaturation. Bilateral lung sliding is highly specific and indicates the correct tube placement and when lung sliding is inconclusive, other signs can be applied. In order to diagnose the causes of intraoperative desaturation, the first sign to look for is lung sliding, followed by an assessment of other lung signs. Diagnosis of other causes of acute desaturation (pulmonary edema, pneumothorax, bronchospasm, or pulmonary embolism) can be elucidated according to Table 3.

Table 3

Intraoperative diagnosis of desaturation by LUS

If echocardiography (for cardiac assessment) is incorporated with LUS, the source behind most of the acute desaturations could be clarified. LUS according to BLUE protocol exhibits high concordance with CT scan[36] and radiography[3] for schematic diagnosis of acute intraoperative desaturation, it can be easily repeated whenever required and is informative regarding the progression or regression of diagnosed etiology.

Conclusion

LUS has already established its utility in intensive care and emergency situations. After reviewing in detail the shortcomings of standard methods still being used and the scope of LUS in intraoperative anesthetic management, it is clear that LUS is an indispensible tool. In modern anesthesia practice incorporation of LUS can help the clinicians to manage critical situation swiftly in real-time.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.