Pulsatility and flow patterns across macro- and microcirculatory arteries of continuous-flow left ventricular assist device patients

BACKGROUND

Reduced arterial pulsatility in continuous-flow left ventricular assist devices (CF-LVAD) patients has been implicated in clinical complications. Consequently, recent improvements in clinical outcomes have been attributed to the "artificial pulse" technology inherent to the HeartMate3 (HM3) LVAD. However, the effect of the "artificial pulse" on arterial flow, transmission of pulsatility into the microcirculation and its association with LVAD pump parameters is not known.

METHODS

The local flow oscillation (pulsatility index, PI) of common carotid arteries (CCAs), middle cerebral arteries (MCAs) and central retinal arteries (CRAs-representing the microcirculation) were quantified by 2D-aligned, angle-corrected Doppler ultrasound in 148 participants: healthy controls, n = 32; heart failure (HF), n = 43; HeartMate II (HMII), n = 32; HM3, n = 41.

RESULTS

In HM3 patients, 2D-Doppler PI in beats with "artificial pulse" and beats with "continuous-flow" was similar to that of HMII patients across the macro- and microcirculation. Additionally, peak systolic velocity did not differ between HM3 and HMII patients. Transmission of PI into the microcirculation was higher in both HM3 (during the beats with "artificial pulse") and in HMII patients compared with HF patients. LVAD pump speed was inversely associated with microvascular PI in HMII and HM3 (HMII, r2 = 0.51, p < 0.0001; HM3 "continuous-flow," r2 = 0.32, p = 0.0009; HM3 "artificial pulse," r2 = 0.23, p = 0.007), while LVAD pump PI was only associated with microcirculatory PI in HMII patients.

CONCLUSIONS

The "artificial pulse" of the HM3 is detectable in the macro- and microcirculation but without creating a significant alteration in PI compared with HMII patients. Increased transmission of pulsatility and the association between pump speed and PI in the microcirculation indicate that the future clinical care of HM3 patients may involve individualized pump settings according to the microcirculatory PI in specific end-organs.

A narrative review of e-learning in professional education of healthcare professionals in medical imaging and radiation therapy

OBJECTIVES

This literature review attempts to explore the characteristics of e-learning tools used to develop the qualifications and skills of healthcare professionals in medical imaging and radiation therapy, and to promote the effectiveness and acceptance of e-learning through highlighting the outcomes of its implementation where applicable.

KEY FINDINGS

From the literature search in the PubMed and ResearchGate databases we concluded to 21 articles, which were included in the qualitative synthesis. Acceptance of e-learning tools was confirmed. Also, e-learning can be part of healthcare professionals' blended learning. The acquisition of new or improvement of existing knowledge, the improvement of clinical skills and the increase of the self-confidence of healthcare professionals in their daily practice were recorded, as outcomes of the e-learning implementation. The importance of human-computer interaction for the comprehension of theoretical concepts and practical aspects using multimedia was also captured. No significant findings emerged among the 21 articles against the adoption of the e-learning for the training of healthcare professionals. The Internet is the channel used for synchronous and asynchronous interaction of trainees with instructors.

CONCLUSIONS

We concluded that e-learning is an attractive training method, equally or occasionally more effective than the traditional educational methods for the lifelong training of healthcare professionals in the field of medical imaging and radiation therapy. Also, many collaborative web-based applications provide the necessary means to build an e-learning program, according to the training needs of each professional team.

IMPLICATIONS FOR PRACTICE

This new knowledge corroborates the perspective of e-learning beneficial contribution to remote interaction and collaboration of healthcare professionals in medical imaging and radiation therapy. Collaborative web-based tools are already available to decision makers and stakeholders, who want to develop an e-learning program.

Cross-correlation analysis of pulse wave propagation in arteries: in vitro validation and in vivo feasibility

The stiffness of the arteries is known to be an indicator of the progression of various cardiovascular diseases. Clinically, the pulse wave velocity (PWV) is used as a surrogate for arterial stiffness. Pulse wave imaging (PWI) is a non-invasive, ultrasound-based imaging technique capable of mapping the motion of the vessel walls, allowing the local assessment of arterial properties. Conventionally, a distinctive feature of the displacement wave (e.g. the 50% upstroke) is tracked across the map to estimate the PWV. However, the presence of reflections, such as those generated at the carotid bifurcation, can bias the PWV estimation. In this paper, we propose a two-step cross-correlation based method to characterize arteries using the information available in the PWI spatio-temporal map. First, the area under the cross-correlation curve is proposed as an index for locating the regions of different properties. Second, a local peak of the cross-correlation function is tracked to obtain a less biased estimate of the PWV. Three series of experiments were conducted in phantoms to evaluate the capabilities of the proposed method compared with the conventional method. In the ideal case of a homogeneous phantom, the two methods performed similarly and correctly estimated the PWV. In the presence of reflections, the proposed method provided a more accurate estimate than conventional processing: e.g. for the soft phantom, biases of  -0.27 and -0.71 m · s-1 were observed. In a third series of experiments, the correlation-based method was able to locate two regions of different properties with an error smaller than 1 mm. It also provided more accurate PWV estimates than conventional processing (biases:  -0.12 versus -0.26 m · s-1). Finally, the in vivo feasibility of the proposed method was demonstrated in eleven healthy subjects. The results indicate that the correlation-based method might be less precise in vivo but more accurate than the conventional method.

Power cavitation-guided blood-brain barrier opening with focused ultrasound and microbubbles

Image-guided monitoring of microbubble-based focused ultrasound (FUS) therapies relies on the accurate localization of FUS-stimulated microbubble activity (i.e. acoustic cavitation). Passive cavitation imaging with ultrasound arrays can achieve this, but with insufficient spatial resolution. In this study, we address this limitation and perform high-resolution monitoring of acoustic cavitation-mediated blood-brain barrier (BBB) opening with a new technique called power cavitation imaging. By synchronizing the FUS transmit and passive receive acquisition, high-resolution passive cavitation imaging was achieved by using delay and sum beamforming with absolute time delays. Since the axial image resolution is now dependent on the duration of the received acoustic cavitation emission, short pulses of FUS were used to limit its duration. Image sets were acquired at high-frame rates for calculation of power cavitation images analogous to power Doppler imaging. Power cavitation imaging displays the mean intensity of acoustic cavitation over time and was correlated with areas of acoustic cavitation-induced BBB opening. Power cavitation-guided BBB opening with FUS could constitute a standalone system that may not require MRI guidance during the procedure. The same technique can be used for other acoustic cavitation-based FUS therapies, for both safety and guidance.

In vivo repeatability of the pulse wave inverse problem in human carotid arteries

Accurate arterial stiffness measurement would improve diagnosis and monitoring for many diseases. Atherosclerotic plaques and aneurysms are expected to involve focal changes in vessel wall properties; therefore, a method to image the stiffness variation would be a valuable clinical tool. The pulse wave inverse problem (PWIP) fits unknown parameters from a computational model of arterial pulse wave propagation to ultrasound-based measurements of vessel wall displacements by minimizing the difference between the model and measured displacements. The PWIP has been validated in phantoms, and this study presents the first in vivo demonstration. The common carotid arteries of five healthy volunteers were imaged five times in a single session with repositioning of the probe and subject between each scan. The 1D finite difference computational model used in the PWIP spanned from the start of the transducer to the carotid bifurcation, where a resistance outlet boundary condition was applied to approximately model the downstream reflection of the pulse wave. Unknown parameters that were estimated by the PWIP included a 10-segment linear piecewise compliance distribution and 16 discrete cosine transformation coefficients for each of the inlet boundary conditions. Input data was selected to include pulse waves resulting from the primary pulse and dicrotic notch. The recovered compliance maps indicate that the compliance increases close to the bifurcation, and the variability of the average pulse wave velocity estimated through the PWIP is on the order of 11%, which is similar to that of the conventional processing technique which tracks the wavefront arrival time (13%).

An inverse approach to determining spatially varying arterial compliance using ultrasound imaging

The mechanical properties of arteries are implicated in a wide variety of cardiovascular diseases, many of which are expected to involve a strong spatial variation in properties that can be depicted by diagnostic imaging. A pulse wave inverse problem (PWIP) is presented, which can produce spatially resolved estimates of vessel compliance from ultrasound measurements of the vessel wall displacements. The 1D equations governing pulse wave propagation in a flexible tube are parameterized by the spatially varying properties, discrete cosine transform components of the inlet pressure boundary conditions, viscous loss constant and a resistance outlet boundary condition. Gradient descent optimization is used to fit displacements from the model to the measured data by updating the model parameters. Inversion of simulated data showed that the PWIP can accurately recover the correct compliance distribution and inlet pressure under realistic conditions, even under high simulated measurement noise conditions. Silicone phantoms with known compliance contrast were imaged with a clinical ultrasound system. The PWIP produced spatially and quantitatively accurate maps of the phantom compliance compared to independent static property estimates, and the known locations of stiff inclusions (which were as small as 7 mm). The PWIP is necessary for these phantom experiments as the spatiotemporal resolution, measurement noise and compliance contrast does not allow accurate tracking of the pulse wave velocity using traditional approaches (e.g. 50% upstroke markers). Results from simulations indicate reflections generated from material interfaces may negatively affect wave velocity estimates, whereas these reflections are accounted for in the PWIP and do not cause problems.

Heart rate variability may identify patients who will develop severe bradycardia during spinal anaesthesia

BACKGROUND AND OBJECTIVES

The reported incidence of cardiac arrest during spinal anaesthesia is 6.4+1.2 per 10,000 patients. Many of these arrests occurred in healthy young patients during minor surgery. This raises the question of whether some of them were avoidable. We investigated the value of Heart Rate Variability (HRV) to identify patients prone to developing severe bradycardia during spinal anaesthesia.

METHODS

Eighty ASA I-II patients, 21-60 years of age, undergoing elective surgery under spinal anaesthesia were studied. The HRV was assessed for 25 min before the spinal block. Two spectral components of HRV were calculated: a low-frequency (LF) and a high-frequency (HF) component. Patients were grouped according to whether bradycardia did or did not develop during spinal anaesthesia.

RESULTS

Nineteen patients developed severe bradycardia (<45 b.p.m.). The mean value of HF before spinal anaesthesia was significantly increased in the bradycardic group (P<0.05). The correlation between baseline heart rate (HR(baseline)) and minimum heart rate and LF, HF during spinal anaesthesia was significant (P<0.01). A receiver operator curve characteristic analysis showed a sensitivity and specificity of HF and HR(baseline) of 65% and 74%, respectively, to predict bradycardia <45 b.p.m. after spinal anaesthesia.

CONCLUSIONS

The present study shows that HF and clinical factors such as patient's HR(baseline) could identify patients prone to developing severe bradycardia during spinal anaesthesia.