How I do it: feasibility of a new ultrasound probe fixator to facilitate high quality stress echocardiography

Probefix is a non-invasive tool which provides lengthy and stable fixation of an ultrasound probe to the body. The Probefix consists of several parts that together enable fixation of the ultrasound probe to the patient’s chest for continuous echocardiographic examination (Fig. 1). The device is attached to the patient’s chest with two chest straps (Fig. 2). The horizontal and vertical chest strap may be adjusted to the patients’ chest size by the velcro tape. The external holder of the fixator has three rings to attach the flexible chest straps. The external holder has three silicon feet by which it is placed onto the patient’s chest. Within the external holder an internal holder fixates the ultrasound probe. Nonetheless, the probe angle can be altered with a relative high freedom of degrees in any direction, thus enabling rotating from, for example, the apical four chamber to two- and three chamber position. The internal holder can also be positioned up- and downward in the external ring, thereby affirming the probe towards the chest. The probe itself is held in the internal ring by a blue elastic ring, which is moulded to match the shape of the specific ultrasound probe. By using different elastic rings, ultrasound probes of different sizes and manufacturers can be fixated in the universal Probefix.

Stress echocardiography requires a treadmill or bicycle ergometer with a dedicated system for stress protocols. The ergometer can be either upright or supine, depending on the available equipment (Figs. 3 and 4). During bicycle stress tests, a 12-lead ECG can be recorded simultaneously, combining a standard ergometer test and stress echocardiography (Fig. 3). Alongside the exercise module, an echocardiographic machine is positioned. Connected to the ultrasound machine is a conventional 2D or 3D brightness mode (B-mode) ultrasound probe.

Before positioning the ultrasound probe at the ideal acoustic window, the probe is loosely connected to the Probefix, without fixating the device to the patient’s body. This set up, allows for accurate, manual positioning of the ultrasound probe at the optimal position. When the desired acoustic window is identified, the Probefix can be fixated to the patient’s chest with the two Velcro straps. The horizontal chest strap is positioned first and the vertical chest strap is fixated next. Both should be firmly attached to the patient’s chest. The internal holder of the Probefix can then be pushed towards the chest to achieve a firm position, while rotating the ultrasound probe is still feasible.

During stress echocardiography, 2D or 3D B-mode images of the left ventricle may be obtained during various stages of physical exercise. Alternatively, pulsed wave or continuous wave Doppler recordings may be obtained over the cardiac valves. The ultrasound probe can be rotated and angulated to acquire all desired imaging planes (e.g. the apical two chamber, apical three chamber or apical four chamber view). Several parameters can be assessed, such as, ventricular function, valvular and subvalvular gradients and regurgitant flows. Implementation of a 3D ultrasound probe is even more convenient, as multiple 2D imaging planes can be recorded without rotating the probe.

We performed SE with the Probefix in healthy volunteers and in patients from our outpatient clinic. Exclusion criteria were pharmacological stress testing and the necessity to perform imaging in both apical and parasternal view, because during stress test it is very challenging to relocate the Probefix. All patients that received exercise induced SE, for the assessment of valvular disease or CRT device optimization in January and February 2018 were asked for participation (n = 6). In addition we asked six healthy volunteers (colleagues) to undergo SE with Probefix. All volunteers signed informed consent prior to stress testing. Stress tests were performed at supine or upright bicycle ergometer and stress protocol were adjusted to the subjects physical fitness (Figs. 3 and 4). Study subject were tested until maximal exercise levels were achieved. In the healthy volunteers we performed imaging in either short axis view or apical view (depending on the best image quality) and we assessed whether during SE the image quality was sufficient for wall motion analysis. In patients from our outpatient clinic we used local imaging protocols according to the objectives of the diagnostic test.

Stress tests were performed on an Ergoline bicycle (Ergoline GmbH, Bitz, German) ergometer. For echocardiographic examination GE Vivid7 or Philips iE33 ultrasound machines were used, with 2D echo probes (M5Sc, GE Healthcare, Milwaukee, USA and X5-1, Philips Medical Systems, Best, The Netherlands). The study complied with the Declaration of Helsinki and the protocol was issued by the local Medical Research Ethics Committees (MREC/METC) as non-WMO (Medical Research Involving Human Subjects Act) research.

The inherent limitations associated with a small, pilot study should be acknowledged. Nevertheless, this study is the first to describe a new technique for facilitating continuous ultrasound measurements. The superiority of continuous echocardiographic measurements during exercise over post-exercise testing has previously been demonstrated by the studies of Dagianti et al. and Badruddin et al. [5, 6].

We demonstrate the feasibility of Probefix in five out of six healthy volunteers and four out of five patients from our outpatient clinic. There are however also limitations to its’ implementation. Firstly, due to the fixation of the echo probe, it is challenging to change from e.g. the parasternal view to the apical view during exercise testing. Fixating two Probefix devices to the patients’ chest could be a solution to this problem, however this is not possible in all patients due to lack of space on the patients’ thorax. Secondly, due to poor baseline echocardiographic windows, SE with Probefix will not be achievable in all patients. As a result of these limitations we believe that the primary application of the Probefix are echocardiographic imaging protocols in which only one view (either apical or parasternal) is required. Consequently, patients with valvular disease and patients whom undergo diastolic function testing are especially suitable for SE with the Probefix. In this patient group we strongly believe that Probefix facilitates continuous monitoring of cardiac function during SE. Because Probefix removes the necessity for sonographers to manually hold the probe during continuous stress testing, this could also reduce the risk of repetitive strain injuries. However more research is needed to test this hypothesis. As Probefix is a new device scientific proof of its added value is of importance. This article explores the possibilities of Probefix in the field of (stress) echocardiography. A trial on the efficacy of this device must be executed.

We demonstrated the feasibility of Probefix in the field of SE. In addition to achieving high quality, stable, hands-free continuous echocardiographic monitoring during exercise, the Probefix has far reaching potential. Continuous monitoring of cardiac function with Probefix could be implemented for the assessment of contractile reserve for response prediction to CRT or for monitoring of myocardial function and hemodynamics under certain physiological or pharmacological conditions [3, 9]. Another application for Probefix could be continuous monitoring of cardiac output at the intensive care department or during cardiac and non-cardiac surgery. A recent case report by our hospital demonstrated for the first time the successful treatment of severe mitral regurgitation through transthoracic echocardiography-guided MitraClip placement [10]. Transcatheter MitraClip placement is traditionally performed using transesophageal echocardiography. However, a history of esophagectomy left the patient involved unfit for transesophageal echocardiography. In special cases like this, the Probefix could facilitate continuous transthoracic echocardiographic measurements. At the same time, the exposure to ionizing radiation of the cardiologist is reduced. As a safe distance to the radiation beam can be secured during measurements.