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M-Mode Ultrasound Applications for the Emergency Medicine Physician
The Journal of Emergency Medicine, Vol. -, No. -, pp. 1–7, 2015 Copyright Ó 2015 Elsevier Inc. Printed in the USA. All rights reserved 0736-4679/$ - see front matter
http://dx.doi.org/10.1016/j.jemermed.2015.06.059
Ultrasound in Emergency Medicine
M-MODE ULTRASOUND APPLICATIONS FOR THE EMERGENCY MEDICINE PHYSICIAN Turandot Saul, MD, RDMS, RDCS, Sebastian D. Siadecki, MD, Rachel Berkowitz, MD, Gabriel Rose, DO, Danielle Matilsky, MD, and Allison Sauler, MD Division of Emergency Ultrasound, Department of Emergency Medicine, Mount Sinai St. Luke’s Hospital, Mount Sinai Roosevelt Hospital, New York, New York Reprint Address: Turandot Saul, MD, Department of Emergency Medicine, Mount Sinai Roosevelt Hospital, 1000 10th Avenue, New York, NY 10019
, Abstract—Background: M-mode or ‘‘motion’’ mode is a form of ultrasound imaging that is of high clinical utility in the emergency department. It can be used in a variety of situations to evaluate motion and timing, and can document tissue movement in a still image when the recording of a video clip is not feasible. Objectives: In this article we describe several straightforward and easily performed applications for the emergency physician to incorporate Mmode into his or her practice, including the evaluation for: 1) pneumothorax, 2) left ventricular systolic function, 3) cardiac tamponade, and 4) hypertrophic cardiomyopathy. Discussion: The emergency physician and other point-of-care ultrasound providers can use this versatile function in the evaluation of patients for a number of critical cardiopulmonary diagnoses. Conclusion: A great deal of important information may be obtained with M-mode imaging through views and measurements that are relatively easy to obtain. Ó 2015 Elsevier Inc.
used for a variety of applications due to its superior temporal resolution. In the ED setting, bedside ultrasound machines are generally selected for their size and portability and may have varying capabilities and quality in terms of spectral, color, or power Doppler. Furthermore, Doppler imaging requires a high level of expertise and can be technically challenging in that the angle of insonation can cause widely varying results. M-mode can provide unique and specific information, is a relatively simple mode to understand and use, and it can assist in a number of important ED diagnoses. M-mode is a one-dimensional (‘‘icepick’’) analysis of the tissue being evaluated. In an M-mode evaluation, echoes from underneath the icepick are displayed across the screen from left to right, creating a distance/time graph with time on the horizontal axis and tissue depth on the vertical axis. From this graph, important information can be assessed, such as the movement of the pleura and the cardiac valves, as well as the timing of the cardiac cycle. The latter is particularly useful because electrocardiographic leads are rarely available in the ED setting. Because it only requires the interpretation of returning echoes from a narrow field, M-mode allows a higher frame rate and thus, better temporal resolution than Bmode ultrasound. Using M-mode, information about tissue movement can also be documented as a still image when the recording of a video clip is not feasible, such
, Keywords—M-mode; ultrasound; pneumothorax; E-point septal separation; cardiac tamponade; hypertrophic cardiomyopathy
INTRODUCTION M-mode or ‘‘motion’’ mode is a form of ultrasound imaging that is of high clinical utility in the emergency department (ED). It can be used in a variety of situations to evaluate motion or timing, and historically has been
RECEIVED: 25 February 2015; FINAL SUBMISSION RECEIVED: 19 June 2015; ACCEPTED: 23 June 2015 1
2
T. Saul et al.
as when there are limitations with equipment or memory space. In this article we describe several straightforward and easily performed applications for the emergency physician to incorporate M-mode into his or her practice, including the evaluation for: 1) pneumothorax, 2) left ventricular systolic function, 3) cardiac tamponade, and 4) hypertrophic cardiomyopathy. DISCUSSION Pneumothorax Ultrasound has been shown to be significantly more sensitive than anterior-posterior radiography and equally sensitive to computed tomography for diagnosing pneumothorax (1–10). The use of M-mode allows for easy visualization of regular lung movement. In the absence of pleural disease and scarring, air in the pleural cavity rises to the anterior chest wall in a supine patient, making the anterior thorax the most sensitive location for pneumothorax evaluation. Using a highfrequency linear or low-frequency curvilinear or phased-array transducer, the probe is placed in the intercostal spaces with the indicator pointing cephalad in the sagittal plane. Ribs will appear as hyperechoic structures with posterior shadowing, whereas the interface of the parietal and visceral pleura will appear as a single horizontal hyperechoic line between the ribs. When the two pleural layers are in contact, the visceral pleura slides along the parietal pleura during respiration. This contact, and therefore the movement and sliding motion of the pleura, is lost when a pneumothorax is present. Evaluation of the pleural line for regular movement may be complicated by several factors, such as motion transmitted from cardiac activity (lung pulse) or chest compressions. M-mode can be used to demonstrate regular lung motion (Figure 1). When the two pleural layers are in contact, the image demonstrates a pattern known as the ‘‘seashore sign.’’ The chest wall tissues superficial to the pleural line remain fairly still over time, like calm water. Deep to the pleural line, a granular or ‘‘sandy’’ appearance is noted from the motion of the lung tissue as it moves beneath the M-mode icepick (Figure 2) (11). When air is present between the pleural layers, as in a pneumothorax, pleural sliding is absent. There is no visible lung movement and the M-mode image will demonstrate the same linear pattern superficial and deep to the pleural line. This is known as the ‘‘barcode’’ sign or, alternatively, the ‘‘stratosphere’’ sign due to its similar appearance to aircraft contrails (Figure 3) (11). Importantly, the absence of regular lung movement does not always indicate the presence of a pneumothorax, as conditions such as massive atelectasis, right main stem intubation, acute respiratory distress syndrome, and
Figure 1. Placement of the M-mode icepick to evaluate the pleura and lung.
pleural adhesions or blebs can also cause a motionless pleural line (12,13). Because ultrasound can assess only what is directly underneath the probe, the more intercostal spaces that are visualized, the more sensitive the examination. At a minimum, the second to fourth intercostal spaces in the mid-clavicular line should be evaluated bilaterally throughout an entire respiratory cycle. Because it is able to document motion in a still image, an M-mode thoracic evaluation can be used to document regular lung motion when video clip archiving is unavailable. For the purposes of image archiving, documentation, and quality assurance, a single still image of
Figure 2. ‘‘Seashore sign’’. The M-mode finding when the pleural layers are in contact.
M-mode Ultrasound Applications for the Emergency Physician
Figure 3. ‘‘Stratosphere sign’’. The M-mode finding when there is air in the intrapleural space.
a ‘‘seashore’’ sign in M-mode is as adequate as a video clip of lung sliding to confirm regular lung movement. E-Point Septal Separation It is essential for the emergency physician to be able to evaluate left ventricular (LV) systolic function rapidly and reliably, particularly in critically ill patients whose disease process may be undifferentiated and whose hemodynamic status is unclear (14,15). An important and highly practical application of M-mode is in the assessment of LV systolic function via mitral valve E-point septal separation (EPSS). Although the simplest and most widely used method for emergency physicians to estimate LV systolic function is gross visual assessment, this strategy has several important pitfalls. Such assessment is subjective, it is highly operator-dependent, and it requires the sonographer to obtain at least two views of the heart to be able to evaluate the three-dimensional movement using twodimensional imaging. It can also be difficult to estimate the LV systolic function in the presence of regional or asymmetric wall motion abnormalities (15,16). EPSS is a relatively simple measurement and, unlike gross visual estimation, it requires only a single view of the heart. To obtain the EPSS measurement, the sonographer should obtain a parasternal long axis (PLAX) view of the heart with a phased-array low-frequency transducer (17). The M-mode icepick should be placed over the apical tip of the anterior leaflet of the mitral valve (Figure 4). The resultant M-mode tracing will demonstrate the movement of the leaflet toward the interventricular septum. Two characteristic waves created by the mitral valve leaflet will be visualized during each diastole, the larger first wave representing early filling (the E-wave) and the smaller second wave representing the atrial kick (the A-wave). This characteristic pattern is created as the mitral valve opens in early diastole, then starts to drift
3
Figure 4. Parasternal long axis view. Placement of the Mmode icepick over the mitral valve (anterior leaflet) to evaluate the E-point septal separation. IV = interventricular; LV = left ventricle.
closed before it is pushed open again by blood from the atrial contraction. The E-point is the tallest point of the E-wave, and the EPSS is measured as the distance in millimeters between the E-point and the interventricular septum (15). The opening of the anterior leaflet of the mitral valve toward the interventricular septum in diastole requires a pressure difference between the left atrium (LA) and left ventricle. When a patient has poor LV systolic function, there is still a full left ventricle at the end of systole. The pressure gradient between the LA and the left ventricle is not as great and the mitral leaflets will not snap open as vigorously. The EPSS measurement should be 6 mm or less in patients with normal LV systolic function; 6–12 mm suggests moderately depressed LV systolic function and > 12 mm signifies severely decreased LV systolic function (Figures 5, 6) (18). Like any assessment of LV systolic function, the EPSS measurement has limitations. It may not be reliable in patients with valvular abnormalities such as mitral stenosis or aortic regurgitation, severe LV or septal hypertrophy, regional wall abnormalities, or severe LV dilatation. In addition, it may be inaccurate if a true PLAX view is not obtained, or if the view obtained necessitates placement of the M-mode icepick at an angle oblique to the interventricular septum (18). Cardiac Tamponade When a pericardial effusion is identified, there are several sonographic findings suggestive of tamponade, including right ventricular (RV) collapse during diastole and right atrial collapse during late diastole or early systole (19,20). M-mode can be particularly useful in detecting these often-subtle ultrasound findings.
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Figure 5. M-mode demonstrating the E and A wave of the mitral valve (anterior leaflet) and a normal E-point septal separation measurement (4.2 mm). IV = interventricular.
RV diastolic collapse is highly specific, and is the most recognized sonographic sign of tamponade. Normally, the ventricles expand during diastole as they fill with blood; however, the RV free wall can collapse if intrapericardial pressure exceeds diastolic pressure within the chamber. The cardiac images are relatively easy to acquire, but the phases of the cardiac cycle (systole vs. diastole) can be difficult to discern when viewed dynamically, especially in the setting of tachycardia. Normal RV contraction during systole may easily be mistaken for pathologic diastolic collapse in this setting. Continuous electrocardiographic tracings on the ultrasound screen can be used to discern the phases of the cardiac cycle; however, this is rarely available in the ED setting (21). M-mode makes use of mitral valve opening and closing to discern the phases of the cardiac cycle. It can be easily performed in the PLAX, or subxiphoid cardiac views. With the M-mode icepick placed so that it tran-
Figure 6. M-mode demonstrating the E and A wave and an abnormal E-point septal separation measurement (15.5 mm). IV = interventricular.
T. Saul et al.
Figure 7. Placement of the M-mode icepick over the right ventricular (RV) free wall as well as the mitral valve (anterior leaflet) to evaluate for cardiac tamponade. IV = interventricular; LV = left ventricle.
sects the RV free wall as well as the mitral valve leaflets, the movement of these structures can be compared simultaneously (Figure 7). If the RV free wall moves toward the interventricular septum while the mitral valve is open (diastole), then RV diastolic collapse can be identified (Figure 8). Limitations to this technique are that it may not be reliable in patients with mitral valve abnormalities resulting in abnormal leaflet movement. In addition, patients with severe hypovolemia may exhibit diastolic chamber collapse even in the absence of elevated intrapericardial pressure, whereas elevated right heart pressures (e.g., from pulmonary hypertension or pulmonary embolism) may prevent diastolic collapse of the right ventricle. Thus, the sonographic findings must always be considered in the context of the clinical scenario (21).
Figure 8. M-mode tracing in cardiac tamponade demonstrating right ventricular (RV) free wall collapse (movement toward the interventricular [IV] septum [arrow]) while the mitral valve (arrowhead) is open. LV = left ventricular.
M-mode Ultrasound Applications for the Emergency Physician
5
Hypertrophic Cardiomyopathy Hypertrophic cardiomyopathy (HCM) is another important and time-sensitive diagnosis with findings that can be identified with M-mode ultrasound. Patients with HCM may present to the ED with symptoms such as dyspnea, angina, or exertional syncope (21). The first sonographic criteria for the diagnosis of HCM included B-mode and M-mode findings (22). B-mode echocardiographic assessment includes evaluation for asymmetrical interventricular septal hypertrophy (>15 mm), a ratio of interventricular septum to posterior wall thickness >1.3, and a small LV cavity. LV outflow tract obstruction from a thickened interventricular septum results in two easily identifiable M-mode findings in the PLAX view: systolic anterior motion of the mitral valve (SAM), and premature closure of the aortic valve (23–25). SAM may be due to increased velocity of blood as it passes through a narrower aortic outflow tract or due to drag forces (23,26). To assess for SAM, the M-mode icepick is placed over the anterior leaflet of the mitral valve, similar to the location for EPSS measurement. In this case, however, the systolic phase of the tracing is examined (Figure 9). As opposed to its normal coaptation with the posterior leaflet, the anterior mitral valve leaflet will demonstrate movement toward the interventricular septum as outflow tract obstruction occurs (Figure 10). The severity of SAM can be estimated as mild, moderate, or severe based on the duration of outflow obstruction (27). The second M-mode assessment for HCM is for aortic valve preclosure. The aortic valve usually remains open for the duration of systole; however, in HCM, outflow tract obstruction causes decreased pressure across the aortic valve, leading to early closure. In the PLAX
Figure 9. Placement of the M-mode icepick over the mitral valve (anterior leaflet) to evaluate for systolic anterior motion of the mitral valve. LV = left ventricle.
Figure 10. Systole (box) demonstrating anterior motion (arrow) of the mitral valve (anterior leaflet). IV = interventricular; LV = left ventricular.
view, the M-mode icepick is placed over the aortic valve leaflets (Figure 11) and the tracing will show the aortic valve leaflets closing earlier than normal (28). This will be depicted by early movement toward coaptation of the aortic valve leaflets prior to full descent of the RV free wall at end-systole (Figure 12). In suspected cases, the patient can be asked to perform maneuvers to provoke outflow tract obstruction, such as Valsalva, while the M-mode evaluation is repeated. An important limitation of this technique is that SAM and aortic valve preclosure are each only 61% sensitive when compared to clinical and angiographic diagnosis of HCM (29). SAM has also been found in patients with HCM, but without evidence of obstruction (30). In addition, SAM has also been noted in patients with a number of other conditions, including left ventricular aneurysm, pericardial effusion, and atrial septal defect (31–33). Aortic valve preclosure has also been noted in patients with mitral regurgitation and ventricular septal defect (34). For these reasons, M-mode echocardiographic findings should be used in conjunction with Bmode diagnostic criteria as well as the clinical scenario.
Figure 11. Placement of the M-mode icepick over the aortic valve leaflets to evaluate for preclosure.
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REFERENCES
Figure 12. Systole (box) demonstrating preclosure (arrow) of the aortic valve prior to full descent of the right ventricular free wall during systole (arrowhead).
Although a variety of Doppler imaging techniques have been proposed as having increased sensitivity for the diagnosis of HCM, despite their limitations, these two M-mode applications are easily performed by the emergency physician and can quickly provide valuable diagnostic information. CONCLUSION The use of M-mode in the ED may assist in diagnoses that would otherwise remain uncertain with traditional B-mode ultrasonography, yet it does not require the specific technology and expertise of Doppler imaging or the equipment necessary for simultaneous electrocardiographic monitoring. In addition, it can provide physiologic information that cannot be obtained any other way. M-mode can be used as an adjunct in the ultrasound diagnosis of pneumothorax to evaluate for regular lung movement. In patients presenting with undifferentiated dyspnea, identification of an abnormal EPSS can suggest left ventricular systolic dysfunction and guide appropriate management such as diuresis and administration of inotropes. In the evaluation of a patient with a pericardial effusion, M-mode may be an indicator of impending cardiac tamponade, allowing the provider time to intervene prior to hemodynamic deterioration. Finally, in cases of suspected hypertrophic cardiomyopathy, M-mode evaluation for SAM or premature closure of the aortic valve may quickly direct the provider to seek timely cardiology consultation. In conclusion, a great deal of important information may be obtained with M-mode imaging through views and measurements that are relatively easy to obtain. The emergency physician and other point-of-care ultrasound providers can use this versatile function to supplement B-mode ultrasonography in the evaluation of patients for a number of critical cardiopulmonary diagnoses.
1. Kirkpatrick AW, Ng AK, Dulchavsky SA, et al. Sonographic diagnosis of a pneumothorax inapparent on plain radiography: confirmation by computed tomography. J Trauma 2001;50:750–2. 2. Blaivas M, Lyon M, Duggal S. A prospective comparison of supine chest radiography and bedside ultrasound for the diagnosis of traumatic pneumothorax. Acad Emerg Med 2005;12:844–9. 3. Sartori S, Tombesi P, Trevisani L, Nielsen I, Tassinari D, Abbasciano V. Accuracy of transthoracic sonography in detection of pneumothorax after sonographically guided lung biopsy: prospective comparison with chest radiography. AJR Am J Roentgenol 2007;188:37–41. 4. Soldati G, Testa A, Sher S, Pignataro G, La Sala M, Silveri NG. Occult traumatic pneumothorax. Chest 2008;133:204–11. 5. Targhetta R, Bourgeois J, Chavagneux R, Balmes P. Diagnosis of pneumothorax by ultrasound immediately after ultrasonically guided aspiration biopsy. Chest 1992;101:855–6. 6. Goodman TR, Traill ZC, Phillips AJ, Berger J, Gleeson FV. Ultrasound detection of pneumothorax. Clin Radiol 1999;54:736–9. 7. Chung MJ, Goo JM, Im J, Cho JM, Cho SB, Kim SJ. Value of highresolution ultrasound in detecting a pneumothorax. Eur Radiol 2005;15:930–5. 8. Garofalo G, Busso M, Perotoo F, De Pascale A, Fava C. Ultrasound diagnosis of pneumothorax. Chest Radiol 2006;222:516–25. 9. Rowan KR, Kirkpatrick AW, Liu D, Forkheim KE, Mayo JR, Nicolaou S. Traumatic pneumothorax detection with Thoracic US: correlation with chest radiograph and CT – initial experience. Radiology 2002;225:210–4. 10. Reissig A, Kroegel C. Accuracy of transthoracic sonography in excluding post-interventional pneumothorax and hydropneumothorax. Comparison to chest radiography. Eur J Radiol 2005;53: 463–70. 11. Lichtenstein DA, Pinsky MR. Pneumothorax and introduction to ultrasound signs in the lung. In: General ultrasound in the critically ill. Heidelberg, Germany: Springer; 2007:105–15. 12. Lichtenstein DA, Menu Y. A bedside ultrasound sign ruling out pneumothorax in the critically ill. Lung sliding. Chest 1995;108: 1345–8. 13. Litchenstein DA, Lascols N, Prin S, Meziere G. The lung pulse: an early ultrasound sign of complete atelectasis. Intensive Care Med 2003;29:2187–92. 14. Randazzo MR, Snoey ER, Levitt MA, Binder K. Accuracy of emergency physician assessment of left ventricular ejection fraction and central venous pressure using echocardiography. Acad Emerg Med 2003;10:973–7. 15. Reardon RF, Joing SA. Cardiac. In: Ma OJ, Mateer JR, Blaivas M, eds. Emergency ultrasound. 2nd edn. New York: The McGraw-Hill Companies, Inc.; 2008:114–5. 16. Moore CL, Rose GA, Tayal V, Sullivan DM, Arrowood JA, Kline JA. Determination of left ventricular function by emergency physician echocardiography of hypotensive patients. Acad Emerg Med 2002;9:186–93. 17. Noble VE, Nelson BP. Cardiac ultrasound. In: Manual of emergency and critical care ultrasound. 2nd edn. New York, NY: Cambridge University Press; 2011:63–9. 18. Secko MA, Lazar JM, Salciccioli LA, Stone MB. Can junior emergency physicians use E-point septal separation to accurately estimate left ventricular function in acutely dyspneic patients? Acad Emerg Med 2011;18:1223–6. 19. Nagdev A, Stone M. Point-of-care ultrasound evaluation of pericardial effusions: does this patient have cardiac tamponade? Resuscitation 2011;82:671–3. 20. Mallemat H. Right heart. In: Mallin M, Dawson M, eds. Bedside ultrasound, volume 2 [eBook]. Lexington, KY: Emergency Ultrasound Solutions; 2013. 21. Wein MN, Dec GW, Lilly LS. The cardiomyopathies. In: Lilly LS, ed. Pathophysiology of heart disease. 4th edn. Philadelphia: Lippincott Williams & Wilkins; 2007:258–65. 22. Maron BJ, McKenna WJ, Danielson GK, et al. ACC/ESC clinical expert consensus document on hypertrophic cardiomyopathy: a
M-mode Ultrasound Applications for the Emergency Physician
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report of the American College of Cardiology Task Force on Clinical Expert Consensus Documents and the European Society of Cardiology Committee for Practice Guidelines (Committee to Develop an Expert Consensus Document on Hypertrophic Cardiomyopathy). J Am Coll Cardiol 2003;42:1687–713. Losi MA, Nistri S, Galderisi M, et al. Echocardiography in patients with hypertrophic cardiomyopathy: usefulness of old and new techniques in the diagnosis and pathophysiological assessment. Cardiovasc Ultrasound 2010;8:7. Maron BJ, Towbin JA, Thiene JA, et al. Contemporary definitions and classification of the cardiomyopathies. Circulation 2006;113: 1807–16. Elliot P, Andersson B, Arbustini E, et al. Classification of the cardiomyopathies: a position statement from the European Society of Cardiology Working Group on Myocardial and Pericardial Disease. Eur Heart J 2008;29:270–6. Afonso LC, Bernal J, Bax JJ. Echocardiography in cardiomyopathy: the role of conventional and emerging technologies. JACC Cardiovasc Imaging 2008;1:787–800. Pollick C, Rakowski H, Wigle ED. Muscular subaortic stenosis: the quantitative relationship between systolic anterior motion and the pressure gradient. Circulation 1984;69:43–9.
7 28. Williams LK, Frenneaux MP, Steeds RP. Echocardiography in hypertrophic cardiomyopathy diagnosis, prognosis, and role in management. Eur J Echocardiogr 2009;10:9–14. 29. Doi YL, McKenna WJ, Gehrke J, Oakley CM, Goodwin JF. M mode echocardiography in hypertrophic cardiomyopathy: diagnostic criteria and prediction of obstruction. Am J Cardiol 1980;45:6–14. 30. King JF, DeMaria AN, Miller FF, Hilliard GK, Zelis R, Mason DT. Markedly abnormal mitral valve motion without simultaneous intraventricular pressure gradient due to uneven mitral-septal contact in idiopathic hypertrophic subaortic stenosis. Am J Cardiol 1974;34:360–6. 31. Greenwald J, Yap JF, Franklin M, Lichtman AM. Echocardiographic mitral systolic motion in left ventricular aneurysm. Br Heart J 1975;37:684–90. 32. Prakash F, Greenfield RS, Aronow WS. Echocardiographic pseudoidiopathic hypertrophic subaaortic stenosis in a patient with pericardial effusion. J Clin Ultrasound 1977;5:350–1. 33. Tajik AJ, Gau GT. Echocardiographic ‘‘pseudo-IHSS’’ in atrial septal defect. Chest 1972;62:324–5. 34. Feigenbaum H. Echocardiography. Philadelphia: Lea & Febiger; 1976.
http://dx.doi.org/10.1016/j.jemermed.2015.06.059
Ultrasound in Emergency Medicine
M-MODE ULTRASOUND APPLICATIONS FOR THE EMERGENCY MEDICINE PHYSICIAN Turandot Saul, MD, RDMS, RDCS, Sebastian D. Siadecki, MD, Rachel Berkowitz, MD, Gabriel Rose, DO, Danielle Matilsky, MD, and Allison Sauler, MD Division of Emergency Ultrasound, Department of Emergency Medicine, Mount Sinai St. Luke’s Hospital, Mount Sinai Roosevelt Hospital, New York, New York Reprint Address: Turandot Saul, MD, Department of Emergency Medicine, Mount Sinai Roosevelt Hospital, 1000 10th Avenue, New York, NY 10019
, Abstract—Background: M-mode or ‘‘motion’’ mode is a form of ultrasound imaging that is of high clinical utility in the emergency department. It can be used in a variety of situations to evaluate motion and timing, and can document tissue movement in a still image when the recording of a video clip is not feasible. Objectives: In this article we describe several straightforward and easily performed applications for the emergency physician to incorporate Mmode into his or her practice, including the evaluation for: 1) pneumothorax, 2) left ventricular systolic function, 3) cardiac tamponade, and 4) hypertrophic cardiomyopathy. Discussion: The emergency physician and other point-of-care ultrasound providers can use this versatile function in the evaluation of patients for a number of critical cardiopulmonary diagnoses. Conclusion: A great deal of important information may be obtained with M-mode imaging through views and measurements that are relatively easy to obtain. Ó 2015 Elsevier Inc.
used for a variety of applications due to its superior temporal resolution. In the ED setting, bedside ultrasound machines are generally selected for their size and portability and may have varying capabilities and quality in terms of spectral, color, or power Doppler. Furthermore, Doppler imaging requires a high level of expertise and can be technically challenging in that the angle of insonation can cause widely varying results. M-mode can provide unique and specific information, is a relatively simple mode to understand and use, and it can assist in a number of important ED diagnoses. M-mode is a one-dimensional (‘‘icepick’’) analysis of the tissue being evaluated. In an M-mode evaluation, echoes from underneath the icepick are displayed across the screen from left to right, creating a distance/time graph with time on the horizontal axis and tissue depth on the vertical axis. From this graph, important information can be assessed, such as the movement of the pleura and the cardiac valves, as well as the timing of the cardiac cycle. The latter is particularly useful because electrocardiographic leads are rarely available in the ED setting. Because it only requires the interpretation of returning echoes from a narrow field, M-mode allows a higher frame rate and thus, better temporal resolution than Bmode ultrasound. Using M-mode, information about tissue movement can also be documented as a still image when the recording of a video clip is not feasible, such
, Keywords—M-mode; ultrasound; pneumothorax; E-point septal separation; cardiac tamponade; hypertrophic cardiomyopathy
INTRODUCTION M-mode or ‘‘motion’’ mode is a form of ultrasound imaging that is of high clinical utility in the emergency department (ED). It can be used in a variety of situations to evaluate motion or timing, and historically has been
RECEIVED: 25 February 2015; FINAL SUBMISSION RECEIVED: 19 June 2015; ACCEPTED: 23 June 2015 1
2
T. Saul et al.
as when there are limitations with equipment or memory space. In this article we describe several straightforward and easily performed applications for the emergency physician to incorporate M-mode into his or her practice, including the evaluation for: 1) pneumothorax, 2) left ventricular systolic function, 3) cardiac tamponade, and 4) hypertrophic cardiomyopathy. DISCUSSION Pneumothorax Ultrasound has been shown to be significantly more sensitive than anterior-posterior radiography and equally sensitive to computed tomography for diagnosing pneumothorax (1–10). The use of M-mode allows for easy visualization of regular lung movement. In the absence of pleural disease and scarring, air in the pleural cavity rises to the anterior chest wall in a supine patient, making the anterior thorax the most sensitive location for pneumothorax evaluation. Using a highfrequency linear or low-frequency curvilinear or phased-array transducer, the probe is placed in the intercostal spaces with the indicator pointing cephalad in the sagittal plane. Ribs will appear as hyperechoic structures with posterior shadowing, whereas the interface of the parietal and visceral pleura will appear as a single horizontal hyperechoic line between the ribs. When the two pleural layers are in contact, the visceral pleura slides along the parietal pleura during respiration. This contact, and therefore the movement and sliding motion of the pleura, is lost when a pneumothorax is present. Evaluation of the pleural line for regular movement may be complicated by several factors, such as motion transmitted from cardiac activity (lung pulse) or chest compressions. M-mode can be used to demonstrate regular lung motion (Figure 1). When the two pleural layers are in contact, the image demonstrates a pattern known as the ‘‘seashore sign.’’ The chest wall tissues superficial to the pleural line remain fairly still over time, like calm water. Deep to the pleural line, a granular or ‘‘sandy’’ appearance is noted from the motion of the lung tissue as it moves beneath the M-mode icepick (Figure 2) (11). When air is present between the pleural layers, as in a pneumothorax, pleural sliding is absent. There is no visible lung movement and the M-mode image will demonstrate the same linear pattern superficial and deep to the pleural line. This is known as the ‘‘barcode’’ sign or, alternatively, the ‘‘stratosphere’’ sign due to its similar appearance to aircraft contrails (Figure 3) (11). Importantly, the absence of regular lung movement does not always indicate the presence of a pneumothorax, as conditions such as massive atelectasis, right main stem intubation, acute respiratory distress syndrome, and
Figure 1. Placement of the M-mode icepick to evaluate the pleura and lung.
pleural adhesions or blebs can also cause a motionless pleural line (12,13). Because ultrasound can assess only what is directly underneath the probe, the more intercostal spaces that are visualized, the more sensitive the examination. At a minimum, the second to fourth intercostal spaces in the mid-clavicular line should be evaluated bilaterally throughout an entire respiratory cycle. Because it is able to document motion in a still image, an M-mode thoracic evaluation can be used to document regular lung motion when video clip archiving is unavailable. For the purposes of image archiving, documentation, and quality assurance, a single still image of
Figure 2. ‘‘Seashore sign’’. The M-mode finding when the pleural layers are in contact.
M-mode Ultrasound Applications for the Emergency Physician
Figure 3. ‘‘Stratosphere sign’’. The M-mode finding when there is air in the intrapleural space.
a ‘‘seashore’’ sign in M-mode is as adequate as a video clip of lung sliding to confirm regular lung movement. E-Point Septal Separation It is essential for the emergency physician to be able to evaluate left ventricular (LV) systolic function rapidly and reliably, particularly in critically ill patients whose disease process may be undifferentiated and whose hemodynamic status is unclear (14,15). An important and highly practical application of M-mode is in the assessment of LV systolic function via mitral valve E-point septal separation (EPSS). Although the simplest and most widely used method for emergency physicians to estimate LV systolic function is gross visual assessment, this strategy has several important pitfalls. Such assessment is subjective, it is highly operator-dependent, and it requires the sonographer to obtain at least two views of the heart to be able to evaluate the three-dimensional movement using twodimensional imaging. It can also be difficult to estimate the LV systolic function in the presence of regional or asymmetric wall motion abnormalities (15,16). EPSS is a relatively simple measurement and, unlike gross visual estimation, it requires only a single view of the heart. To obtain the EPSS measurement, the sonographer should obtain a parasternal long axis (PLAX) view of the heart with a phased-array low-frequency transducer (17). The M-mode icepick should be placed over the apical tip of the anterior leaflet of the mitral valve (Figure 4). The resultant M-mode tracing will demonstrate the movement of the leaflet toward the interventricular septum. Two characteristic waves created by the mitral valve leaflet will be visualized during each diastole, the larger first wave representing early filling (the E-wave) and the smaller second wave representing the atrial kick (the A-wave). This characteristic pattern is created as the mitral valve opens in early diastole, then starts to drift
3
Figure 4. Parasternal long axis view. Placement of the Mmode icepick over the mitral valve (anterior leaflet) to evaluate the E-point septal separation. IV = interventricular; LV = left ventricle.
closed before it is pushed open again by blood from the atrial contraction. The E-point is the tallest point of the E-wave, and the EPSS is measured as the distance in millimeters between the E-point and the interventricular septum (15). The opening of the anterior leaflet of the mitral valve toward the interventricular septum in diastole requires a pressure difference between the left atrium (LA) and left ventricle. When a patient has poor LV systolic function, there is still a full left ventricle at the end of systole. The pressure gradient between the LA and the left ventricle is not as great and the mitral leaflets will not snap open as vigorously. The EPSS measurement should be 6 mm or less in patients with normal LV systolic function; 6–12 mm suggests moderately depressed LV systolic function and > 12 mm signifies severely decreased LV systolic function (Figures 5, 6) (18). Like any assessment of LV systolic function, the EPSS measurement has limitations. It may not be reliable in patients with valvular abnormalities such as mitral stenosis or aortic regurgitation, severe LV or septal hypertrophy, regional wall abnormalities, or severe LV dilatation. In addition, it may be inaccurate if a true PLAX view is not obtained, or if the view obtained necessitates placement of the M-mode icepick at an angle oblique to the interventricular septum (18). Cardiac Tamponade When a pericardial effusion is identified, there are several sonographic findings suggestive of tamponade, including right ventricular (RV) collapse during diastole and right atrial collapse during late diastole or early systole (19,20). M-mode can be particularly useful in detecting these often-subtle ultrasound findings.
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Figure 5. M-mode demonstrating the E and A wave of the mitral valve (anterior leaflet) and a normal E-point septal separation measurement (4.2 mm). IV = interventricular.
RV diastolic collapse is highly specific, and is the most recognized sonographic sign of tamponade. Normally, the ventricles expand during diastole as they fill with blood; however, the RV free wall can collapse if intrapericardial pressure exceeds diastolic pressure within the chamber. The cardiac images are relatively easy to acquire, but the phases of the cardiac cycle (systole vs. diastole) can be difficult to discern when viewed dynamically, especially in the setting of tachycardia. Normal RV contraction during systole may easily be mistaken for pathologic diastolic collapse in this setting. Continuous electrocardiographic tracings on the ultrasound screen can be used to discern the phases of the cardiac cycle; however, this is rarely available in the ED setting (21). M-mode makes use of mitral valve opening and closing to discern the phases of the cardiac cycle. It can be easily performed in the PLAX, or subxiphoid cardiac views. With the M-mode icepick placed so that it tran-
Figure 6. M-mode demonstrating the E and A wave and an abnormal E-point septal separation measurement (15.5 mm). IV = interventricular.
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Figure 7. Placement of the M-mode icepick over the right ventricular (RV) free wall as well as the mitral valve (anterior leaflet) to evaluate for cardiac tamponade. IV = interventricular; LV = left ventricle.
sects the RV free wall as well as the mitral valve leaflets, the movement of these structures can be compared simultaneously (Figure 7). If the RV free wall moves toward the interventricular septum while the mitral valve is open (diastole), then RV diastolic collapse can be identified (Figure 8). Limitations to this technique are that it may not be reliable in patients with mitral valve abnormalities resulting in abnormal leaflet movement. In addition, patients with severe hypovolemia may exhibit diastolic chamber collapse even in the absence of elevated intrapericardial pressure, whereas elevated right heart pressures (e.g., from pulmonary hypertension or pulmonary embolism) may prevent diastolic collapse of the right ventricle. Thus, the sonographic findings must always be considered in the context of the clinical scenario (21).
Figure 8. M-mode tracing in cardiac tamponade demonstrating right ventricular (RV) free wall collapse (movement toward the interventricular [IV] septum [arrow]) while the mitral valve (arrowhead) is open. LV = left ventricular.
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Hypertrophic Cardiomyopathy Hypertrophic cardiomyopathy (HCM) is another important and time-sensitive diagnosis with findings that can be identified with M-mode ultrasound. Patients with HCM may present to the ED with symptoms such as dyspnea, angina, or exertional syncope (21). The first sonographic criteria for the diagnosis of HCM included B-mode and M-mode findings (22). B-mode echocardiographic assessment includes evaluation for asymmetrical interventricular septal hypertrophy (>15 mm), a ratio of interventricular septum to posterior wall thickness >1.3, and a small LV cavity. LV outflow tract obstruction from a thickened interventricular septum results in two easily identifiable M-mode findings in the PLAX view: systolic anterior motion of the mitral valve (SAM), and premature closure of the aortic valve (23–25). SAM may be due to increased velocity of blood as it passes through a narrower aortic outflow tract or due to drag forces (23,26). To assess for SAM, the M-mode icepick is placed over the anterior leaflet of the mitral valve, similar to the location for EPSS measurement. In this case, however, the systolic phase of the tracing is examined (Figure 9). As opposed to its normal coaptation with the posterior leaflet, the anterior mitral valve leaflet will demonstrate movement toward the interventricular septum as outflow tract obstruction occurs (Figure 10). The severity of SAM can be estimated as mild, moderate, or severe based on the duration of outflow obstruction (27). The second M-mode assessment for HCM is for aortic valve preclosure. The aortic valve usually remains open for the duration of systole; however, in HCM, outflow tract obstruction causes decreased pressure across the aortic valve, leading to early closure. In the PLAX
Figure 9. Placement of the M-mode icepick over the mitral valve (anterior leaflet) to evaluate for systolic anterior motion of the mitral valve. LV = left ventricle.
Figure 10. Systole (box) demonstrating anterior motion (arrow) of the mitral valve (anterior leaflet). IV = interventricular; LV = left ventricular.
view, the M-mode icepick is placed over the aortic valve leaflets (Figure 11) and the tracing will show the aortic valve leaflets closing earlier than normal (28). This will be depicted by early movement toward coaptation of the aortic valve leaflets prior to full descent of the RV free wall at end-systole (Figure 12). In suspected cases, the patient can be asked to perform maneuvers to provoke outflow tract obstruction, such as Valsalva, while the M-mode evaluation is repeated. An important limitation of this technique is that SAM and aortic valve preclosure are each only 61% sensitive when compared to clinical and angiographic diagnosis of HCM (29). SAM has also been found in patients with HCM, but without evidence of obstruction (30). In addition, SAM has also been noted in patients with a number of other conditions, including left ventricular aneurysm, pericardial effusion, and atrial septal defect (31–33). Aortic valve preclosure has also been noted in patients with mitral regurgitation and ventricular septal defect (34). For these reasons, M-mode echocardiographic findings should be used in conjunction with Bmode diagnostic criteria as well as the clinical scenario.
Figure 11. Placement of the M-mode icepick over the aortic valve leaflets to evaluate for preclosure.
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REFERENCES
Figure 12. Systole (box) demonstrating preclosure (arrow) of the aortic valve prior to full descent of the right ventricular free wall during systole (arrowhead).
Although a variety of Doppler imaging techniques have been proposed as having increased sensitivity for the diagnosis of HCM, despite their limitations, these two M-mode applications are easily performed by the emergency physician and can quickly provide valuable diagnostic information. CONCLUSION The use of M-mode in the ED may assist in diagnoses that would otherwise remain uncertain with traditional B-mode ultrasonography, yet it does not require the specific technology and expertise of Doppler imaging or the equipment necessary for simultaneous electrocardiographic monitoring. In addition, it can provide physiologic information that cannot be obtained any other way. M-mode can be used as an adjunct in the ultrasound diagnosis of pneumothorax to evaluate for regular lung movement. In patients presenting with undifferentiated dyspnea, identification of an abnormal EPSS can suggest left ventricular systolic dysfunction and guide appropriate management such as diuresis and administration of inotropes. In the evaluation of a patient with a pericardial effusion, M-mode may be an indicator of impending cardiac tamponade, allowing the provider time to intervene prior to hemodynamic deterioration. Finally, in cases of suspected hypertrophic cardiomyopathy, M-mode evaluation for SAM or premature closure of the aortic valve may quickly direct the provider to seek timely cardiology consultation. In conclusion, a great deal of important information may be obtained with M-mode imaging through views and measurements that are relatively easy to obtain. The emergency physician and other point-of-care ultrasound providers can use this versatile function to supplement B-mode ultrasonography in the evaluation of patients for a number of critical cardiopulmonary diagnoses.
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