Echocardiography
With its ability to provide comprehensive evaluation of myocardial, valvular, and hemodynamic performance, echocardiography is the first imaging technique to enter the mainstream of intraoperative patient monitoring (Table 26-1) (Perrino AC, Popescu WM, Skubas NJ. Echocardiography. In: Barash PG, Cullen BF, Stoelting RK, Cahalan MK, Ortega R, Stock MC, eds. Clinical Anesthesia. Philadelphia: Lippincott Williams & Wilkins; 2013:723–761). In conjunction with the National Board of Echocardiography, the American Society of Anesthesiologists has established a second certification pathway in basic perioperative echocardiography.
I. Principles and Technology of Echocardiography
Echocardiography generates dynamic images of the heart from the reflections of sound waves.
Physics of Sound. In clinical echocardiography, a mechanical vibrator, known as the transducer, is placed in contact with the esophagus (transesophageal echocardiography [TEE]), the skin (transthoracic echocardiography), or the heart (epicardial echocardiography) to create tissue vibrations.
Properties of Sound Transmission in Tissue. Echocardiographic imaging relies on the transmission and subsequent reflection of ultrasound energy back to the transducer.
Instrumentation
Transducers. Ultrasound transducers use piezoelectric crystals to create a brief pulse of ultrasound.
Beam Shape. The ultrasound transducer emits a three-dimensional ultrasound beam similar to a movie projection. The beam is narrow in the near field and then diverges into the far field zone.
Resolution. Three parameters are evaluated when assessing the resolution of an ultrasound system: the resolution of objects lying along the axis of the ultrasound beam (axial resolution), the resolution of objects horizontal to the beam’s orientation (lateral resolution), and the resolution of objects lying vertical to the beam’s orientation (elevational resolution).
Table 26-1 Application of Intraoperative Echocardiography
Guide placement of intracardiac and intravascular catheters and devices
Assessment of the severity of valve pathology
Immediate evaluation of a surgical intervention
Rapid diagnosis of acute hemodynamic instability
Directing appropriate therapies
Signal Processing. To convert echoes into images, the returning ultrasound pulses are received, electronically processed, and displayed.
Image Display. Ultrasonic imaging is based on the amplitude and time delay of the reflected signals.
With B-mode echocardiography, the amplitude of the returning echoes from a single pulse determines the display brightness of the representative pixels.
Motion-mode (M-mode) echocardiography provides a one-dimensional, single-beam view through the heart but updates the B-mode images at a very high rate, providing dynamic real-time imaging. M-mode echocardiography remains the best technique for examining the timing of cardiac events.
Two-dimensional (2-D) echocardiography is a modification of B-mode echocardiography and is the mainstay of the echocardiographic examination.
Spatial versus Dynamic Image Quality. These selections determine whether sector size, spatial resolution, or dynamic motion is best displayed. A common approach is to focus each part of the examination on a given structure of interest and select the imaging plane that best delineates the structure in the near field. When the maximal frame rate is desired, M-mode echocardiography is chosen.
II. Two-Dimensional and Three-Dimensional Transesophageal Echocardiography
TEE is the favored approach to intraoperative echocardiography. In the operating room, TEE is useful because the probe does not interfere with the operative field and can be left in situ, providing continuous, real-time hemodynamic information used to diagnose and manage critical cardiac events. TEE is also useful when the transthoracic examination is limited by various
factors (obesity, emphysema, surgical dressings, prosthetic valves) and for examining cardiac structures that are not well visualized with TEE (left atrial [LA] appendage).
factors (obesity, emphysema, surgical dressings, prosthetic valves) and for examining cardiac structures that are not well visualized with TEE (left atrial [LA] appendage).
Probe Insertion. The TEE probe is inserted in the anesthetized patient in a manner similar to insertion of an orogastric tube. For improved image quality, the stomach is emptied of gastric contents and air before probe insertion. The TEE probe is advanced beyond the larynx and the cricopharyngeal muscle (∼25–30 cm from the teeth) until a loss of resistance is appreciated. At this point, the TEE probe lies in the upper esophagus, and the first cardiovascular images are seen.
Transesophageal Echocardiography Safety. When performed by qualified operators, TEE has a low incidence of complications. Various studies have suggested an association between swallowing dysfunction after cardiac surgery and the use of intraoperative TEE. Postoperative swallowing dysfunction is associated with pulmonary complications.
Contraindication to TEE Probe Placement (Table 26-2). To maintain the safety profile of TEE, each patient should be evaluated before the procedure for signs, symptoms, and history of esophageal pathology. The most feared complication of TEE is esophageal or gastric perforation.
Probe Manipulation. Image acquisition depends on precise manipulation of the TEE probe. By advancing the shaft of the probe, the probe position can be moved from the upper esophagus to the midesophagus and into the stomach. The shaft can also be manually rotated to the left or right. By using the large knob on the probe handle, the head of the probe can be anteflexed (turning the knob clockwise) and retroflexed (turning the knob counterclockwise). The smaller knob, located on top of the large knob, is used to tilt the head of the probe to the right or left. Using the electronic switch on the probe handle, the operator can rotate
the ultrasound beam from 0 degrees (transverse plane) to 180 degrees in 1-degree increments.
Table 26-2 Contraindications to Transesophageal Echocardiography Probe Placement
Esophageal strictures, rings, or webs
Esophageal masses (malignant tumors)
Recent bleeding of esophageal varices
After radiation of the neck
Recent gastric bypass surgery
Orientation. An understanding of the basic rules of imaging orientation is essential to echocardiographic interpretation.
The ultrasound beam is always directed perpendicular to the probe face. An easy way to understand this orientation is to place your right hand in front of your chest with your palm facing down, your thumb oriented left, and your fingers oriented anterior right. The scan lines that generate the TEE image start at your fingers and sweep toward your thumb.Related posts:
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