Digital Clips: Ultrasound Video Recording and Current Recording Technology for the storage of Echocardiographic Images
©Walter Rasmussen, R.D.C.S.
A digital clip is an ultrasound video recording of one or more cardiac cycles using computer hardware and software rather than videotape. The clip is initially stored on a hard-drive and then transferred to an archiving computer. Playback of a clip displays cardiac cycles repeatedly, giving the appearance of a seamless, endless recording.
Ultrasound scanners acquire, store and transmit medical images digitally using an industry standard called DICOM, short for: Digital Imaging and Communications in Medicine. Once the sonographer has finished an exam, the images are transmitted either manually or automatically to a server called PACS, (Picture Archiving and Communication System). Each digital clip or frozen image contains information that tells the software of a remote viewing station what type of image it is and relevant details about that allow for measurement and image manipulation. It is the responsibility of the sonographer to assure that the echocardiographic study is transmitted to the correct PACS and that the images are immediately available to those who interpret the images and write the report.
The correct functioning of automated digital clipping for Echocardiography is highly dependent upon the quality of the ECG hookup and the shape of the QRS complex. Typically, the machine is programmed to start and stop the clip at some point in the QRS complex, usually the peak of the R wave. If the machine does not obtain a clear ECG signal, the resultant clips will be incomplete or recorded at the wrong points in the cardiac cycle.
For the purposes of an echocardiogram, placement of the color-coded electrodes on the chest does not have to conform to the standards for a twelve lead ECG. It is sufficient to know that the white electrode is negative and in most cases, is suitable to be placed somewhere on the right side of the anterior chest, typically just above the right pectoral muscle. A green colored electrode is the ground and can be placed anywhere on the body because it does not produce an ECG deflection. A black or red electrode is positive and can be placed anywhere between the heart and the white electrode, preferably along the patient’s axis of ventricular depolarization, which is usually the margin of the right and left rib cage. The taller and narrower the R wave, the more likely that good triggering of the clip will be obtained. A tall R wave indicates that the electrodes are aligned with the heart’s electrical axis.
Common problems encountered with this automated capture function are caused by:
- A tall P wave being mistaken for the R wave, causing double triggering.
- Low QRS voltage that makes the signal look like artifact causing the machine to clip erratically or default to timed clips.
- Excessive muscle tremor or noise on the baseline of the ECG.
- Wide QRS complexes and paced rhythms, making analysis the QRS complex difficult for the capture software.
Many of the above problems can be solved by either accessing the physio controls and switching the lead or changing the location of the electrodes on the chest. A tall P wave can be remedied by moving the white electrode away from the atria, e.g.: more inferior on the chest and/or to the left or right. If the QRS is too short, the ECG gain can be increased however, if the baseline noise or muscle tremor are also magnified, it may cause false triggering and a better signal can be obtained by changing the axis of the electrodes 90 degrees, (make the selected positive and negative electrodes more horizontal or vertical on the body).
A wide QRS with a sagging S-T segment such as is seen with bundle branch block or paced rhythms can be challenging to the automated system and moving the electrodes or switching the leads to make the QRS less bizarre-looking or switching the machine to timed captures such as two to four second clips will usually aid in correcting the problem.
When to use Single Beat Clips:
When the heart rhythm is regular, it is preferable to clip just one cardiac cycle. This makes detailed evaluation of cardiac motion easier because the observer can study many distinct areas of the heart as they are repeatedly displayed with the minimization of extraneous movement caused by respiration and unsteady positioning of the transducer.
It is often important to time the acquisition at a point in the natural respiratory cycle when the heart is not drifting, either at the end of exhalation or the peak of inhalation because an unsteady target can cause the image to jump where the loop is spiced together, making interpretation of motion more difficult. In some instances, it may be necessary to ask the patient to briefly halt respiration while the clip is being recorded.
When to use Multiple Beat Clips:
In the majority of patients, the ideal single beat clip is obtainable however, it is often necessary to record more than one cardiac cycle when the heart rhythm is irregular, such as in atrial fibrillation and when there are frequent premature beats. It is also necessary to record multiple beats when respiration causes extreme changes in either cardiac position or pressure relationships.
When the heart rhythm is atrial fibrillation, the diastolic portion of the cardiac cycle varies, causing changes in stroke volume from beat to beat. In this case, the recording of 2 to 3 beats per clip allows multiple cardiac cycles to be used for wall motion analysis and averaging of volume measurements for the calculation of ejection fraction.
Effort should be made to exclude PVC’s or PAC’s from the study by waiting for periods of normally-paced beats to appear so that normal wall motion can be observed. It is also important to try to include a normal beat that does not immediately follow a premature beat because the diastolic portion of the cardiac cycle is prolonged due to the compensatory pause. This is best achieved by recording a three or more beat clip and selecting a sequence where there at least two normal beats in a row.
In a patient with frequent intermittent paced, or premature beats, it is also useful to record samples of both the normal and abnormal beats, in order for the reader to observe how the alternate paths of conduction affect cardiac function. If there are runs of tachycardia, a long clip demonstrating the event will aid in a more complete assessment of the patient’s pathology.
When a patient is in normal sinus rhythm during one portion of the exam and then has long periods of frequent premature atrial or ventricular beats in another part of the exam, the best strategy is to wait for the regular rhythm to return, rather than recording a study with a lot of mixed rhythms which will affect Doppler velocities and two-dimensional volumes and their derived calculations.
Multiple beat clips are also useful in the following cases:
Pulmonary Hypertension and Cardiac Tamponade: A longer clip should be recorded when the patient has abnormal septal motion due to RV volume/pressure overload and there is intermittent respiratory variation of wall motion. In these conditions, it is helpful to turn-on the respiration monitor which is located in the Physio-Control menu of most current ultrasound machines.
Atrial-Septal Aneurysm: Observation of the movement of a highly mobile atrial septum can necessitate a clip of several beats duration since respiration often influences its movement.
Shunt Study: Assessment for ASD or PFO involves injection of agitated saline into a peripheral vein and the observation of the micro-bubbles as they move through the heart.
Congenital Heart Defects: Slowly panning the transducer while recording a long clip of up to ten seconds can be useful for demonstrating anatomical transitions.
Inferior vena cava: Recording IVC collapse involves making adjustments in the length of the clip so that it contains at least one respiratory cycle, (several seconds or about 4-6 beats).
Chaotic Motion: Structures which exhibit chaotic motion are much better illustrated with a long clip. These include vegetations, mobile tumors, thrombi, ruptured chordae tendinae and flail valve leaflets.
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