Edler and Hertz and the Beginning of Clinical Echocardiography (Ultrasound Cardiography) and M-Mode Imaging

It was the collaboration between cardiologist Inge Edler (1911–2001) and physicist Carl Hellmuth Hertz (1920–1990) (Figure 11-2) at Lund University in Sweden in the early 1950s that is commonly accepted as the beginning of clinical echocardiography.¹³*

Figure 11-2 I. Edler (right) and C. H. Hertz (left) in 1979.

They recorded the first M-mode echocardiogram of the heart in 1953 and are considered the “fathers of clinical echocardiography.”

(From Roelandt JR: Seeing the invisible: A short history of cardiac ultrasound. Eur J Echocardiogr 1:8–11, 2000.)

In a search of a better way to evaluate the mitral valve function before closed mitral commissurotomy, Edler considered the use of something like RADAR (see Table 11-2). He was referred to a young physicist at Lund University, C. Hellmuth Hertz. Hertz was the son of a Nobel Prize winner, and, remarkably, his uncle, Heinrich Hertz, had lent his name to the unit of frequency. Hertz thought that ultrasound (frequencies higher than that audible to the human ear) might be the solution to Edler’s needs.¹⁴ Hertz borrowed an ultrasonic reflectoscope used in nondestructive testing of metals at the ship-building yard in Malmo for a weekend, and Elder tested it on Hertz’s chest and detected movement in the heart. The Siemens Corporation provided them with a reflectoscope in October 1953. They first studied isolated hearts in the laboratory with A-mode (static amplitude), but then devised a recording technique to display motion versus time (M-mode) by photographing the image off the oscilloscope (Figures 11-3 and 11-4). Their machine could produce ultrasound frequencies of 0.5, 1.0, 2.5, and 5.0 MHz, and Hertz opted to use 2.5 in humans as the optimal compromise between penetration and resolution. They first used this machine in a patient on October 29, 1953; they termed it “ultrasound cardiography” (UCG).¹⁹ At first they attributed the reflectors to the posterior wall of the left ventricle and the anterior wall of the left atrium, but, subsequently, Edler demonstrated that the latter came from the anterior leaflet of the mitral valve. He determined this by passing an ice pick in the comparable direction through the chest at the time of an autopsy on a patient he had echoed shortly before death.² He subsequently used UCG to evaluate pericardial effusions and, in 1956, detected a left atrial myxoma (but this was not published until 1960).¹² However, his principal use of UCG was to evaluate mitral stenosis, and he described the value of the E-F slope in quantifying the severity of the stenosis. In the mid-1950s, Edler and Hertz actually experimented with introducing a transducer into the esophagus to overcome the attenuation in lung tissue, but they abandoned this effort because of difficulties in obtaining acoustic coupling between the transducer and the esophageal wall.¹² They published the first review article on UCG in Acta Medica Scandinavia in 1961.²⁰ Hertz left the field of cardiac ultrasound fairly early after he developed ink-jet technology, and Edler made few innovations after 1960.² In 1977, Edler and Hertz were awarded the Lasker Clinical Medicine Research Prize (often referred to as the American Nobel prize in Medicine).

Figure 11-3 The ultrasonoscope initially used by Edler and Hertz for recording their early echocardiograms.

(From Feigenbaum H, Armstrong WF, Ryan T: Feigenbaum’s echocardiography, 6th ed. Philadelphia: Lippincott Williams & Wilkins, 2005.)

Figure 11-4 One of the earliest M-mode echocardiograms of the mitral valve recorded by Edler and Hertz in December 1953.

(From Roelandt JR: Seeing the invisible: A short history of cardiac ultrasound. Eur J Echocardiogr 1:8–11, 2000.)

Others Enter the Field

In the late 1950s, Schmidt and Braun, and Sven Effert in Germany separately began duplicating Elder and Hertz’s work, and in 1959, Effert published the first report identifying an intra-atrial tumor by echocardiography. Between 1961 and 1965, workers in Shanghai and Wuhan, China began to report their use of echocardiography including fetal echocardiography.¹³

The American Experience

In 1957, engineers John Wild and John Reid at the University of Minnesota described ultrasonic echocardiographic imaging of excised hearts.²¹ When Reid went to the University of Pennsylvania, he joined forces with cardiologist Claude Joyner, building an ultrasonoscope that they used to study mitral stenosis. Their report, which appeared in the journal Circulation in 1963, was the first American publication on clinical echocardiography.²² Harvey Feigenbaum in Indianapolis, author of one of the first (first published in 1972) and still published textbooks of echocardiography, first became interested in echocardiography in 1963 (Figure 11-5). He published his first article on use of ultrasound to diagnose pericardial effusions in 1965,²³ and as mentioned later, he collaborated with Reggie Eggleton to develop a mechanical two-dimensional (2D) scanner. Dr. Feigenbaum also may have been the first to train nonphysicians to do echocardiograms, leading to the development of cardiac sonographers.¹³ Feigenbaum believes that Bernie Segal of Philadelphia was the first to use the term echocardiography in print in an article published in 1966.^24,25 The senior author (E.A.H.) recalls cardiologists presenting their primitive, indistinct, and difficult to interpret M-mode images of the mitral valve at preoperative case conferences in the late 1960s and thinking that echocardiography did not have much future. How wrong he was!

Figure 11-5 Feigenbaum using his early M-mode echocardiography, which used a Polaroid camera to record the echocardiogram.

(From Feigenbaum H, Armstrong WF, Ryan T: Feigenbaum’s echocardiography, 6th ed. Philadelphia: Lippincott Williams & Wilkins, 2005.)

Between the 1950s and the 1970s, M-mode was the only clinically useful format for echocardiography, and many advances and applications were described as the equipment and the skill of the echocardiographers improved, peaking in the late 1970s. But then its role rapidly declined with the development of 2D instrumentation.⁹

Two-Dimensional Scanners

Work to develop real-time 2D scanners began in the 1960s, made possible by advances in sonar and radar technology and circuitry. Early pioneering work was done by the previously mentioned Americans Wild and Reid,²⁶ and Howry and Bliss²⁷ in the 1950s. In the later 1960s, Ebina²⁸ in Japan and Asberg²⁹ reported producing ultrasound-generated tomographic images of thoracic structures in humans. Their clinical application was limited because of transducer size.

In 1968, Somer³⁰ constructed the first electronic phased-array scanner based on the wavefront theory formulated in the 17th century by Huygens.¹¹ In 1971, this was followed by the description by Nicholas Bom³¹ of Rotterdam of an electronic linear scanner (Figure 11-6) that generated a rectangular image and, in 1974, of an electronic phased-array scanner by F. L. Thurstone and O.T. von Ramm at Duke University.^11,32 At the same time, J. Griffith and W. Henry, at the National Institutes of Health, introduced a mechanical sector scanner.^11,33 This was said to be cumbersome to manipulate, and hence shortly thereafter Reggie Eggleton, working with Feigenbaum in Indianapolis, developed a handheld real-time mechanical 2D scanner³⁴ that was more “user friendly” and commercially successful¹³ (Figure 11-7). In 1976 and 1977, Kisslo and colleagues^35,36 at Duke University reported on the clinical use of the von Ramm and Thurston phased-array scanner for 2D echocardiography (2DE).

Figure 11-6 Twenty-element electronic linear two-dimensional transducer developed by N. Bom in Rotterdam circa 1972.

(From Feigenbaum H, Armstrong WF, Ryan T: Feigenbaum’s echocardiography, 6th ed. Philadelphia: Lippincott Williams & Wilkins, 2005.)

Figure 11-7 First practical handheld mechanical real-time sector scanner developed by Eggleton and Feigenbaum in the mid-1970s using a modified Sunbeam electric toothbrush.

(From Feigenbaum H: Evolution of echocardiography. Circulation 93:1321–1327, 1996.)

Development of Vascular and Cardiac Doppler

In 1956, Satomura reported the application of the Doppler principle in the use of ultrasound to measure blood flow velocity.^11,37 However, in 1961, Franklin, at the University of Washington, was the first to measure Doppler shift of frequency, and hence velocity of flow of blood in vessels using a continuous-wave ultrasonic device.³⁸ Pulse-wave Doppler (in which flow at a defined depth could be analyzed) was nearly simultaneously introduced in 1969 and 1970 by P. N. T. Wells of England, P. A. Peronneau of France, and D. W. Baker in the United States.¹¹ In 1974, F. E. Barber, D. W. Baker, and colleagues at the University of Washington introduced the combination of pulse-wave Doppler with 2D scanning to produce the subsequently widely used “Duplex Scanner.”^11,39 Subsequently, in Norway, but at different institutions, J. Holen⁴⁰ and L. Hatle⁴¹ applied the Bernoulli principle to estimate the pressure decline across stenotic mitral and aortic valves, respectively.

Color-Flow (Doppler) Echocardiography

In 1978, the Swiss-born M. A. Brandestini, working at the University of Washington, described a method (based on pulse-wave Doppler) of color encoding flow velocity data and superimposing it on M-mode images.¹³ His initial observations were extended by Namekawa et al⁴² in Japan who described, in 1982, color Doppler flow imaging (CFI) utilizing autocorrelation to produce real-time images of flow in 2D sector scans. Bommer and Miller in the United States⁴³ and Omoto working with the Namekawa group⁴⁴ described clinical use of 2D color Doppler flow imaging.^9,11

Prelude to Perioperative Transesophageal Echocardiography

The first reported use of intraoperative echocardiography (IOE) was in 1972 when Johnson and his colleagues⁴⁵ at the University of Colorado reported on the use of intraoperative epicardial M-mode echocardiography to evaluate the results after open mitral commissurotomy. In 1976, Wexler and Pohost, of Massachusetts General Hospital, wrote a review article in the journal Anesthesiology on noninvasive techniques for hemodynamic monitoring, in which they described the basic principles and the potential application to anesthesia of the relatively new technique of “echocardiography.”⁴⁶ At that time, this was limited to M-mode technology, but they anticipated that in the future 2D sector scanning would overcome some of the limitations of M-mode scanning. In that same year, at the annual meeting of the American Society of Anesthesiologists (ASA) in October in San Francisco, Rathod and colleagues,⁴⁷ at Cook County Hospital and Loyola University in Chicago, and Paul Barash and colleagues,⁴⁸ at Yale University, described use of transthoracic M-mode echocardiography to monitor the effects of anesthetics on cardiac function during anesthesia in 20 adults and 13 children, respectively. They concluded that this technique was a useful method to assess cardiac function, and Barash et al⁴⁸ predicted that it “has the potential to be a useful tool for clinical anesthesiology that may supplant the invasive monitors currently available.”

In 1978, Strom and colleagues at Albert Einstein College of Medicine in Bronx, New York, published their experience with use of intraoperative epicardial M-Mode scanning during cardiac surgery,⁴⁹ which led to their investigations of transesophageal echocardiography (TEE; see later).

When superior handheld 2D transducers became available, they began to be used epicardially during cardiac surgery after gas sterilization or wrapping them in sterile sheaths. In the early 1980s, Spotnitz and colleagues,⁵¹ at Columbia University College of Medicine in New York City (NYC), reported use of intraoperative epicardial 2D scanning after CPB to detect intracardiac air⁵⁰ and to assess left ventricular ejection fraction before and after cardiac surgery. In 1984, Goldman and colleagues, at Mount Sinai Medical Center in NYC, reported the use of intraoperative epicardial contrast 2DE to evaluate regurgitation after mitral valve surgery⁵² and to assess myocardial perfusion during cardiac surgery.⁵³

Intravascular Ultrasound

The development of TEE had its roots in the search for alternative ultrasound windows because of difficulties encountered in obtaining good ultrasound signals through the chest using the early insensitive transthoracic transducers.⁷ In the early 1960s, this led to the investigation of intravascular probes. In 1960, Cieszynski⁵⁴ inserted a single-element transducer on a catheter into the jugular vein of dogs. In 1963, Omoto et al⁵⁵ reported obtaining static cross-sectional images in patients by slowly rotating a single-element transducer inserted into the right atrium,⁷ and a year later, Kimoto et al⁵⁶ reported obtaining C-scans of the atrial septum in humans from an intravascular catheter.⁹ In 1968, Carleton and Clark⁵⁷ reported similar studies. 1970, Reggie Eggleton⁵⁸ mounted four elements on a catheter and created the first cross-sectional images of intracardiac structures by computer reconstruction of images obtained by slow rotation and ECG triggering,⁵ and in 1972, Nicholas Bom,⁵⁹ in Rotterdam, described a real-time intra- cardiac scanner using a 32-element circular electronically phased-array transducer placed at the tip of a 9 F-catheter. Thereafter, interest faded as sophisticated TEE probes became available, only to re-emerge in the late 1980s (see later).

Transesophageal Ultrasound (Doppler and M-Mode)

Side and Gosling⁶⁰ were the first to use transesophageal (continuous-wave) Doppler (transducer mounted on a steerable gastroscope) to assess flow in the heart and aorta in 1971. This was followed by similar reports by Olson and Shelton in 1972. In 1974 and 1975, Duck et al⁶¹ and Daigle et al⁶² used transesophageal pulsed Doppler to measure flow in the thoracic aorta.⁷ However, true TEE is said to have its embryonic beginnings in 1976 when Lee Frazin (a Chicago cardiologist) et al⁶³ reported recordings of M-mode echocardiograms of the aortic root and valve, mitral valve, and left atrium from the esophagus by attaching a nonfocused 3.5-mHz transducer attached to a 3-mm coaxial cable (Figure 11-8). They reported superior recordings compared with the transthoracic approach in 38 patients.⁶³ Following this lead, Matsumoto and Oka,⁶⁴ at Albert Einstein College of Medicine, fabricated a stiff transesophageal probe supporting an M-mode transducer (Figure 11-9). They first used this in 1979 to measure ventricular dimensions and volume during mitral valve surgery in a 65-year-old woman.⁶⁴ They subsequently reported intraoperative transesophageal M-mode monitoring in 21 patients undergoing cardiac surgery.⁶⁵ At that time, M-mode TEE was difficult to interpret except by the extremely sophisticated clinician and hence was not widely adopted. However, the development of a new generation of gastroscopes with steerable tips in the late 1960s, onto which echo transducers could be mounted, had a significant positive impact on the development of TEE by facilitating direct contact with the wall of the esophagus,⁷ overcoming the problem encountered by Edler and Hertz a decade earlier.

Figure 11-8 First clinically used transesophageal ultrasound probe used by Frazin and colleagues in the mid-1970s. Consisted of a nonfocused, 3.5-MHz M-mode transducer mounted on the end of a 3-mm coaxial cable.

(From Frazin L, Talano, Stephanides L, et al: Esophageal echocardiography. Circulation 54:102–108, 1976, Figure 1.)

Figure 11-9 First “homemade” transesophageal M-mode probe used to monitor patients during cardiac surgery by Matsumoto and Oka in the Bronx, NY. The shaft consisted of a No. 4 sternal wire covered with a vinyl esophageal stethoscope catheter (A) KB-aerotech 3.5 MHz transducer; (B) Handmade cable.

(From Oka Y, Goldiner PL: Transesophageal echocardiography. Philadelphia: Lippincott, 1992, Figure 1. Originally from Matsumoto M, Oka Y, Lin YT, et al: Transesophageal echocardiography for assessing ventricular performance. N Y State J Med 79:19–21, 1979, Figure 1.)

Transesophageal Two-Dimensional Imaging

Although some primitive 2D TEE mechanical scanners and linear were described between 1977 and 1980 (e.g., Hisanaga et al⁶⁶ and DiMagno et al⁶⁷), it was not until Jacques Souquet⁶⁸ (Figure 11-10), working in conjunction with the Varian Corporation, developed their phased-array transducer (Figure 11-11) mounted on the end of a gastroscope for their 2D sector scanner system (Figures 11-12 and 11-13) that TEE became a practical reality. These new TEE probes were evaluated clinically by cardiologists Michael Schluter and Peter Hanrath in Hamburg, Germany,⁶⁹ and their preliminary results were highlighted at an international conference held in Hamburg in 1981.⁸ The early subsequent use of TEE took different directions in the United States and Europe. Cardiologists in Germany and the Netherlands rapidly began using TEE in awake patients to aid in the diagnosis of a variety of cardiac pathologies. In the United States, cardiologists seemed reluctant to adopt this new technology.⁶ In the early 1980s, Peter Hanrath sent prototypes (first M-mode transducers and then with the 2D transducers mounted on gastroscopes) to cardiologists James Seward at Mayo Clinic and Nelson Schiller (Figure 11-14) at the University of California in San Francisco (UCSF), who passed them on to anesthesiologists. Schiller⁷⁰ spoke with William Hamilton, Chair of the Department of Anesthesiology at UCSF, who put him in contact with two young faculty members, Michael Cahalan and Michael Roizen. At that time (1981–1983), a cardiology fellow from Hanrath’s group in Germany, Peter Kremer (Figure 11-15), came to UCSF and collaborated with Cahalan (Figure 11-16) and Roizen to investigate these new TEE instruments. At the 1982 annual meeting of the ASA, they started the American TEE “revolution” when they presented their results in monitoring cardiac and vascular surgery patients with this new TEE probe, displaying the high-quality recordings and images obtained (Figures 11-17 and 11-18), and describing its usefulness in assessing filling and function of the left ventricle, and in detecting myocardial ischemia and intracardiac air.^71,72 Subsequently, they reported its superiority to PAC in assessing filling in the operating room⁷³ and usefulness in evaluating the hemodynamic changes during anaphylaxis⁷⁴ and during surgery for pheochromocytoma.⁷⁵ Topol and colleagues at Johns Hopkins University demonstrated its usefulness in determining the cause of hypotension immediately after CPB⁷⁶ and documented improvement in myocardial dysfunction immediately after coronary revascularization.⁷⁷ Cucchiara et al,⁷⁸ at the Mayo Clinic, described its usefulness in detecting air embolism during neurosurgery, whereas the UCSF group described its value in monitoring during vascular surgery^79,80 and its superiority over ECG in detecting myocardial ischemia.⁸¹ In 1984, the Hamburg group and Cahalan published an anatomic analysis of six standard TEE views,⁸² which was expanded on a few years later by the Mayo Clinic group.⁸³ Meanwhile, the group at Mount Sinai Medical Center in NYC (Martin Goldman, Joel Kaplan, Daniel Thys, Zaharia Hillel, and Steven Konstadt) was quick to adopt and evaluate perioperative TEE, following the experience of Kaplan working with the early Diasonic TEE probes in the early 1980s.^84–88 In 1987, Cahalan et al⁸⁹ and Clements and deBruijn⁹⁰ at Duke University wrote review articles on the use of TEE in anesthesiology, whereas the group at Mayo Clinic reported on the reproducibility of measurements obtained during intraoperative TEE.⁹¹ In that same year (1987), the second edition of this text edited by Kaplan on Cardiac Anesthesia contained for the first time a 63-page thoroughly referenced chapter on Intraoperative Echocardiography.⁹²

Figure 11-10 Jacques Souquet, PhD, a contemporary portrait. French ultrasound engineer working in both the United States and Europe, largely responsible for the design of the miniaturized phased-array two-dimensional transducer used in the early transesophageal echocardiography probes.

(From Image: A Conversation with…Jacques Souquet, PhD. Available at: http://www.rt-image.com/A_Conversation_with_Jacques_Souquet_PhD_The_diagnostic_capabilities_of_ShearWave/content=8504J05E48BE588440B6967644A0B0441.)

Figure 11-11 First 32-element 3.5-MHz phased-array transducer developed by Jacquet Souquet that was fitted on an Olympus gastroscope by Souquet.

(From Stumper O, Sutherland GR: Transesophageal echocardiography in congenital heart disease. London: Edward Arnold, 1994, Figure 1.2.)

Figure 11-12 An early Souquet transesophageal echocardiography probe (Souquet phased-array transducer mounted on gastroscope) as used by Hanrath, Schluter, and Kremer in the early 1980s.

(From Schluter M, Langenstein BA, Polster J, et al: Transesophageal cross-sectional echocardiography with a phased array transducer system. Technique and initial clinical results. Br Heart J 48:67–72, 1982, Figure 1A.)

Figure 11-13 The ability to adjust the orientation of the transducer by the external controls on the gastroscope is a key innovative feature of the gastroscope mounted transesophageal echocardiography probe.

(From Schluter M, Langenstein BA, Polster J, et al: Transesophageal cross-sectional echocardiography with a phased array transducer system. Technique and initial clinical results. Br Heart J 48:67–72, 1982, Figure 1B.)

Figure 11-14 Nelson Schiller, MD, in 1983.

He was the head of echo cardiology at the University of California San Francisco (UCSF) and was instrumental in promoting the work of Cahalan, Roizen, and Kremer and supporting their efforts to win acceptance of their publications.

(Photograph courtesy M. K. Cahalan.)

Figure 11-15 Peter Kremer, the cardiology fellow from Hanrath’s group in Germany, who brought the probes to the University of California San Francisco (UCSF) and worked with them there for 2 years. Shown standing before a poster presentation of their work at a cardiology meeting circa 1982.

(Photograph courtesy M. K. Cahalan)

Figure 11-16 Michael K. Cahalan, Professor Anesthesiology, University of Utah.

Pioneered use of transesophageal echocardiography (TEE) perioperatively (together with P. Kremer and M. Roizen) at the University of California in San Francisco in 1982 and helped introduce this modality to cardiologists, surgeons, and anesthesiologists in the United States. He facilitated the collaboration of anesthesiologists with echocardiologists (American Society of Echocardiography) and assisted with the development of postgraduate courses, guidelines, and certifying examinations in perioperative TEE.

(Photograph courtesy M. K. Cahalan.)

Figure 11-17 High-fidelity transesophageal M-mode echocardiogram from the first Hanrath-Souquet probe used in surgery at the University of California San Francisco (UCSF) circa 1981. AW, anterior wall; DD, diastolic distance; et, ejection time; PW, posterior wall; SD, systolic distance.

(Courtesy M. K. Cahalan.)

Figure 11-18 Two-dimensional (2D) image (transgastric midpapillary short-axis view) recording from the first 2D Hanrath-Souquet transesophageal echocardiographic probe used in surgery at the University of California San Francisco (UCSF) circa 1982. alp, anterior lateral papillary muscle; LV, left ventricle; pmp, posterior medial papillary muscle; RV, right ventricle.

(Courtesy M. K. Cahalan.)

In 1986, Goldman and colleagues,⁹³ at Mount Sinai Medical Center in NYC, reported their experience with a new 2D TEE probe that also provided Doppler color-flow imaging (Aloka and Irex), as did a Japanese group.⁹⁴ That same year, the Hewlett-Packard Corporation also introduced a color-flow Doppler TEE probe; and in 1987, deBruijn and Clements, together with pioneering echocardiographer, Joseph Kisslo at Duke University, reviewed their early experience with this new technology.⁹⁵ In that same year, pulse-wave Doppler was added to 2D TEE probes,⁹⁶ although several years earlier the Hamburg group had described results of use of a TEE probe that combined pulse-wave Doppler with M-mode TEE.⁹⁷ In 1989, biplane probes (Figure 11-19) first became available,⁹⁸ although prototypes had been described earlier,⁹⁹ and soon thereafter, multiplane single-transducer TEE probes¹⁰⁰ and smaller diameter probes suitable for pediatric use¹⁰¹ appeared. In 1992, Pandian and colleagues, at Tufts-New England Medical Center, reviewed their early clinical experience with a 5-MHz phased-array multiplane TEE probe (“OmniPlane”).¹⁰²

Figure 11-19 Photograph of single-plane (horizontal or transverse) and biplane (transverse and vertical) probes. The advent of biplane probes greatly increased the views available but were eventually superseded by multiplane probes.

(From Matsuzaki M, Toma Y, Kusukawa R: Clinical applications of transesophageal echocardiography. Circulation 82:709–722, 1990, Figure 1.)

A survey conducted of Society of Cardiovascular Anesthesiologists (SCA) members in early 1988 provides a “snap-shot” of the state of practice of IOE at that time¹⁰³; 20% of the responders reported that IOE studies were being performed at their institutions. The majority of responders were practicing in large teaching hospitals. Anesthesiologists were involved 41% of the time and cardiologists in 33%, although in one third of the former group, cardiologists always assisted. TEE technique was used in 51%, epicardial in 37%, and transthoracic in 22% of cases. At the time of the survey, six manufactures made 2D TEE transducers—Hewlett-Packard, Aloka Toshiba, Diasonics, General Electric, Acusonics, and Hoffrel, but only the first three provide color-flow TEE. Most responders reported using either Diasonics (46%) or Hewlett-Packard (42%); 90% used the 2D mode, 75% M-mode, 60% pulsed-Doppler, 58% color-flow Doppler, and only 23% continuous-wave Doppler, whereas 97% believed that IOE had been or could have been helpful. The authors of the survey noted that few academic medical centers offered formal instruction and certification in IOE, and that a definite need for training had been identified.

Along this same line, in an editorial published in 1989, Kaplan¹⁰⁴ urged caution and restraint in the adoption of this new and complex technology that had the potential to cause numerous complications, and emphasized the need for anesthesiologists who aspire to be echocardiographers to obtain a high level of training.

In the late 1980s, the group at Duke University demonstrated the utility of routine epicardial echocardiography during surgery for congenital heart disease.¹⁰⁵

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