Patients With Continuous-Flow Left Ventricular Assist Devices Provide Insight in Human Baroreflex Physiology

Mechanical circulatory support reduces renal sympathetic nerve activity in an ovine model of acute myocardial infarction

PURPOSE

The use of circulatory assist devices has been shown to improve glomerular filtration rate and reduce the incidence of acute kidney injury in patients following acute cardiac pathology. However, the mechanisms of improvement in kidney function are not clear. We tested the hypothesis that mechanical circulatory support would result in a decrease in directly recorded renal sympathetic nerve activity (RSNA) and mediate the improvement in renal blood flow (RBF) in a setting of acute myocardial infarction (AMI)-induced left ventricular systolic dysfunction.

METHODS

An anaesthetized ovine model was used to induce AMI (n = 8) using injections of microspheres into the left coronary artery in one group. The second group did not undergo embolization (n = 6). The effects of mechanical circulatory support using the Impella CP on directly recorded renal sympathetic nerve activity were examined in these two groups of animals.

RESULTS

Injection of microspheres resulted in a drop in mean arterial pressure (MAP) of 21 ± 4 mmHg compared to baseline values (p < 0.05; n = 8). This was associated with a 67% increase in renal sympathetic nerve activity (RSNA; from 16 ± 5 to 21 ± 5 spikes/s; p < 0.05; n = 7). Impella CP support significantly increased MAP by 13 ± 1.5 mmHg at pump level 8 (p < 0.05) in the AMI group. Incremental pump support resulted in a significant decrease in RSNA (p < 0.05) in both groups. At pump level P8 in the AMI group, RSNA was decreased by 21 ± 5.5% compared to pump level P0 when the pump was not on.

CONCLUSION

Our data indicate that the improvement in kidney function following mechanical circulatory support may be mediated in part by renal sympathoinhibition.

Sympathetic vasoconstrictor activity before and after left ventricular assist device implantation in patients with end-stage heart failure

AIMS

Sympathetic overactivity, which predicts poor outcome in patients with heart failure, normalizes following cardiac transplantation. We tested the hypothesis that haemodynamic improvement following left ventricular assist device (LVAD) implantation is also associated with reductions in centrally generated sympathetic activity.

METHODS AND RESULTS

In eight patients with heart failure (two women, six men, age 44-66 years), we continuously recorded electrocardiogram, beat-to-beat finger blood pressure, respiration, and muscle sympathetic nerve activity (MSNA) before and after implantation of the continuous-flow LVAD devices HeartWare HVAD (n = 4) and HeartMate II (n = 2), and the non-continuous-flow device HeartMate 3 (n = 2). LVAD implantation increased cardiac output by 1.29 ± 0.88 L/min (P = 0.060) and mean arterial pressure by 16.2 ± 7.9 mmHg (P < 0.001), while reducing pulse pressure by 25.3 ± 9.8 mmHg (P < 0.001). LVAD implantation did not change MSNA burst frequency (-1.3 ± 7.5 bursts/min, P = 0.636), total activity (+0.62 ± 1.83 au, P = 0.369), or normalized activity (+0.63 ± 4.23, P = 0.685). MSNA burst incidence was decreased (-7.8 ± 9.3 bursts/100 heart beats, P = 0.049). However, cardiac ectopy altered MSNA bursting patterns that could be mistaken for sympatholysis.

CONCLUSION

Implantation of current design LVAD does not consistently normalize sympathetic activity in patients with end-stage heart failure despite haemodynamic improvement.

CORP: Standardizing methodology for assessing spontaneous baroreflex control of muscle sympathetic nerve activity in humans

The use of spontaneous bursts of muscle sympathetic nerve activity (MSNA) to assess arterial baroreflex control of sympathetic nerve activity has seen increased utility in studies of both health and disease. However, methods used for analyzing spontaneous MSNA baroreflex sensitivity are highly variable across published studies. Therefore, we sought to comprehensively examine methods of producing linear regression slopes to quantify spontaneous MSNA baroreflex sensitivity in a large cohort of subjects (n = 150) to support a standardized procedure for analysis that would allow for consistent and comparable results across laboratories. The primary results demonstrated that 1) consistency of linear regression slopes was considerably improved when the correlation coefficient was above -0.70, which is more stringent compared with commonly reported criterion of -0.50, 2) longer recording durations increased the percentage of linear regressions producing correlation coefficients above -0.70 (1 min = 15%, 2 min = 28%, 5 min = 53%, 10 min = 67%, P < 0.001) and reaching statistical significance (1 min = 40%, 2 min = 69%, 5 min = 78%, 10 min = 89%, P < 0.001), 3) correlation coefficients were improved with 3-mmHg versus 1-mmHg and 2-mmHg diastolic blood pressure (BP) bin size, and 4) linear regression slopes were reduced when the acquired BP signal was not properly aligned with the cardiac cycle triggering the burst of MSNA. In summary, these results support the use of baseline recording durations of 10 min, a correlation coefficient above -0.70 for reliable linear regressions, 3-mmHg bin size, and importance of properly time-aligning MSNA and diastolic BP. Together, these findings provide best practices for determining spontaneous MSNA baroreflex sensitivity under resting conditions for improved rigor and reproducibility of results.

A Case of Abulia From Left Middle Cerebral Artery Stroke in an Adolescent Treated Successfully With Short Duration Olanzapine

INTRODUCTION

Abulia is defined as a pathological state of amotivation, apathy, and global absence of willpower. It presents with a challenging array of overlapping symptoms, making effective identification and treatment difficult.

CASE PRESENTATION

We describe the first known report of an adolescent with a ventricular assist device who developed abulia following a left middle cerebral artery (MCA) stroke who responded successfully to treatment with olanzapine.

DISCUSSION

The neurobiological etiology of abulia is still unclear but is postulated to be related to deficits in the dopaminergic reward circuitry in the frontal-subcortical-mesolimbic regions. There have been reports of poststroke patients with abulia being treated by modulating this dopamine circuitry and in some cases with short-term low-dose olanzapine.

CONCLUSION

Further research is needed to develop a better understanding of the pathophysiology of abulia leading to more effective treatment algorithms including more specific diagnostic tools and effective pharmacological interventions.

Physiology of the Assisted Circulation in Cardiogenic Shock: A State-of-the-Art Perspective

Mechanical circulatory support (MCS) has made rapid progress over the last 3 decades. This was driven by the need to develop acute and chronic circulatory support as well as by the limited organ availability for heart transplantation. The growth of MCS was also driven by the use of extracorporeal membrane oxygenation (ECMO) after the worldwide H1N1 influenza outbreak of 2009. The majority of mechanical pumps (ECMO and left ventricular assist devices) are currently based on continuous flow pump design. It is interesting to note that in the current era, we have reverted from the mammalian pulsatile heart back to the continuous flow pumps seen in our simple multicellular ancestors. This review will highlight key physiological concepts of the assisted circulation from its effects on cardiac dynamic to principles of cardiopulmonary fitness. We will also examine the physiological principles of the ECMO-assisted circulation, anticoagulation, and the haemocompatibility challenges that arise when the blood is exposed to a foreign mechanical circuit. Finally, we conclude with a perspective on smart design for future development of devices used for MCS.

Effects of Prolonged Head-Down Bed Rest on Cardiac and Vascular Baroreceptor Modulation and Orthostatic Tolerance in Healthy Individuals

Orthostatic intolerance commonly occurs after prolonged bed rest, thus increasing the risk of syncope and falls. Baroreflex-mediated adjustments of heart rate and sympathetic vasomotor activity (muscle sympathetic nerve activity – MSNA) are crucial for orthostatic tolerance. We hypothesized that prolonged bed rest deconditioning alters overall baroreceptor functioning, thereby reducing orthostatic tolerance in healthy volunteers. As part of the European Space Agency Medium-term Bed Rest protocol, 10 volunteers were studied before and after 21 days of −6° head down bed rest (HDBR). In both conditions, subjects underwent ECG, beat-by-beat blood pressure, respiratory activity, and MSNA recordings while supine (REST) and during a 15-min 80° head-up tilt (TILT) followed by a 3-min −10 mmHg stepwise increase of lower body negative pressure to pre-syncope. Cardiac baroreflex sensitivity (cBRS) was obtained in the time (sequence method) and frequency domain (spectrum and cross-spectrum analyses of RR interval and systolic arterial pressure – SAP, variability). Baroreceptor modulation of sympathetic discharge activity to the vessels (sBRS) was estimated by the slope of the regression line between the percentage of MSNA burst occurrence and diastolic arterial pressure. Orthostatic tolerance significantly decreased after HDBR (12 ± 0.6 min) compared to before (21 ± 0.6 min). While supine, heart rate, SAP, and cBRS were unchanged before and after HDBR, sBRS gain was slightly depressed after than before HDBR (sBRS: −6.0 ± 1.1 versus −2.9 ± 1.5 burst% × mmHg⁻¹, respectively). During TILT, HR was higher after than before HDBR (116 ± 4 b/min versus 100 ± 4 b/min, respectively), SAP was unmodified in both conditions, and cBRS indexes were lower after HDBR (α index: 3.4 ± 0.7 ms/mmHg; BRS_SEQ 4.0 ± 1.0) than before (α index: 6.4 ± 1.0 ms/mmHg; BRS_SEQ 6.8 ± 1.2). sBRS gain was significantly more depressed after HDBR than before (sBRS: −2.3 ± 0.7 versus −4.4 ± 0.4 burst% × mmHg⁻¹, respectively). Our findings suggest that baroreflex-mediated adjustments in heart rate and MSNA are impaired after prolonged bed rest. The mechanism likely contributes to the decrease in orthostatic tolerance.

Orthostatic Hypotension in Patients With Left Ventricular Assist Devices: Acquired Autonomic Dysfunction

Contemporary left ventricular assist device (LVAD) technology uses nonphysiologic continuous flow to deliver blood into the circulation. This results in a reduction of pulsatility, which is implicated in some of the commonly associated side effects with LVAD therapy, including hypertension and gastrointestinal arterial-venous malformation with related bleeding. A less frequently observed side effect is orthostatic hypotension (OH) in patients supported with LVAD therapy. We present three cases of OH in patients with LVAD, followed by a discussion on how LVAD therapy may induce autonomic dysfunction.

Ventricular flow dynamics with varying LVAD inflow cannula lengths: In-silico evaluation in a multiscale model

Left ventricular assist devices are associated with thromboembolic events, which are potentially caused by altered intraventricular flow. Due to patient variability, differences in apical wall thickness affects cannula insertion lengths, potentially promoting unfavourable intraventricular flow patterns which are thought to be correlated to the risk of thrombosis. This study aimed to present a 3D multiscale computational fluid dynamic model of the left ventricle (LV) developed using a commercial software, Ansys, and evaluate the risk of thrombosis with varying inflow cannula insertion lengths in a severely dilated LV. Based on a HeartWare HVAD inflow cannula, insertion lengths of 5, 19, 24 and 50 mm represented cases of apical hypertrophy, typical ranges of apical thicknesses and an experimental length, respectively. The risk of thrombosis was evaluated based on blood washout, residence time, instantaneous blood stagnation and a pulsatility index. By introducing fresh blood to displace pre-existing blood in the LV, after 5 cardiac cycles, 46.7%, 45.7%, 45.1% and 41.8% of pre-existing blood remained for insertion lengths of 5, 19, 24 and 50 mm, respectively. Compared to the 50 mm insertion, blood residence time was at least 9%, 7% and 6% higher with the 5, 19 and 24 mm insertion lengths, respectively. No instantaneous stagnation at the apex was observed directly after the E-wave. Pulsatility indices adjacent to the cannula increased with shorter insertion lengths. For the specific scenario studied, a longer insertion length, relative to LV size, may be advantageous to minimise thrombosis by increasing LV washout and reducing blood residence time.

The effect of empagliflozin on muscle sympathetic nerve activity in patients with type II diabetes mellitus

Inhibition of sodium glucose cotransporter 2 with empagliflozin results in caloric loss by increasing urinary glucose excretion and has a mild diuretic effect. Diuretic effects are usually associated with reflex-mediated increases in sympathetic tone, whereas caloric loss is associated with decreased sympathetic tone. In an open-label trial, muscle sympathetic nerve activity (MSNA) (burst frequency, burst incidence, and total MSNA) was assessed using microneurography performed off-treatment and on day 4 of treatment with empagliflozin 25 mg once daily in 22 metformin-treated patients with type II diabetes (mean [range] age 54 [40-65] years). Systolic and diastolic blood pressure (BP), heart rate, urine volume, and body weight were assessed before and on day 4 (BP, heart rate), day 5 (urine volume), or day 6 (body weight) of treatment with empagliflozin. After 4 days of treatment with empagliflozin, no significant changes in MSNA were apparent despite a numerical increase in urine volume, numerical reductions in BP, and significant weight loss. There were no clinically relevant changes in heart rate. Empagliflozin is not associated with clinically relevant reflex-mediated sympathetic activation in contrast to increases observed with diuretics in other studies. Our study suggests a novel mechanism through which sodium glucose cotransporter 2 inhibition affects human autonomic cardiovascular regulation.

Exercise physiology in chronic mechanical circulatory support patients: vascular function and beyond

PURPOSE OF REVIEW

The majority of patients currently implanted with left ventricular assist devices have the expectation of support for more than 2 years. As a result, survival alone is no longer a sufficient distinctive for this technology, and there have been many studies within the last few years examining functional capacity and exercise outcomes.

RECENT FINDINGS

Despite strong evidence for functional class improvements and increases in simple measures of walking distance, there remains incomplete normalization of exercise capacity, even in the presence of markedly improved resting hemodynamics. Reasons for this remain unclear. Despite current pumps being run at a fixed speed, it is widely recognized that pump outputs significantly increase with exercise. The mechanism of this increase involves the interaction between preload, afterload, and the intrinsic pump function curves. The role of the residual heart function is also important in determining total cardiac output, as well as whether the aortic valve opens with exercise. Interactions with the vasculature, with skeletal muscle blood flow and the state of the autonomic nervous system are also likely to be important contributors to exercise performance.

SUMMARY

Further studies examining optimization of pump function with active pump speed modulation and options for optimization of the overall patient condition are likely to be needed to allow left ventricular assist devices to be used with the hope of full functional physiological recovery.

Pulse Oximeter Derived Blood Pressure Measurement in Patients With a Continuous Flow Left Ventricular Assist Device

Currently, blood pressure (BP) measurement is obtained noninvasively in patients with continuous flow left ventricular assist device (LVAD) by placing a Doppler probe over the brachial or radial artery with inflation and deflation of a manual BP cuff. We hypothesized that replacing the Doppler probe with a finger-based pulse oximeter can yield BP measurements similar to the Doppler derived mean arterial pressure (MAP). We conducted a prospective study consisting of patients with contemporary continuous flow LVADs. In a small pilot phase I inpatient study, we compared direct arterial line measurements with an automated blood pressure (ABP) cuff, Doppler and pulse oximeter derived MAP. Our main phase II study included LVAD outpatients with a comparison between Doppler, ABP, and pulse oximeter derived MAP. A total of five phase I and 36 phase II patients were recruited during February-June 2014. In phase I, the average MAP measured by pulse oximeter was closer to arterial line MAP rather than Doppler (P = 0.06) or ABP (P < 0.01). In phase II, pulse oximeter MAP (96.6 mm Hg) was significantly closer to Doppler MAP (96.5 mm Hg) when compared to ABP (82.1 mm Hg) (P = 0.0001). Pulse oximeter derived blood pressure measurement may be as reliable as Doppler in patients with continuous flow LVADs.

Preserved Autonomic Cardiovascular Regulation With Cardiac Pacemaker Inhibition: A Crossover Trial Using High-Fidelity Cardiovascular Phenotyping

BACKGROUND

Sympathetic and parasympathetic influences on heart rate (HR), which are governed by baroreflex mechanisms, are integrated at the cardiac sinus node through hyperpolarization-activated cyclic nucleotide-gated channels (HCN4). We hypothesized that HCN4 blockade with ivabradine selectively attenuates HR and baroreflex HR regulation, leaving baroreflex control of muscle sympathetic nerve activity intact.

METHODS AND RESULTS

We treated 21 healthy men with 2×7.5 mg ivabradine or placebo in a randomized crossover fashion. We recorded electrocardiogram, blood pressure, and muscle sympathetic nerve activity at rest and during pharmacological baroreflex testing. Ivabradine reduced normalized HR from 65.9±8.1 to 58.4±6.2 beats per minute (P<0.001) with unaffected blood pressure and muscle sympathetic nerve activity. On ivabradine, cardiac and sympathetic baroreflex gains and blood pressure responses to vasoactive drugs were unchanged. Ivabradine aggravated bradycardia during baroreflex loading.

CONCLUSIONS

HCN4 blockade with ivabradine reduced HR, leaving physiological regulation of HR and muscle sympathetic nerve activity as well as baroreflex blood pressure buffering intact. Ivabradine could aggravate bradycardia during parasympathetic activation.

CLINICAL TRIAL REGISTRATION

URL: http://www.clinicaltrials.gov. Unique identifier: NCT00865917.

Restoration of Pulsatile Flow Reduces Sympathetic Nerve Activity Among Individuals With Continuous-Flow Left Ventricular Assist Devices

Background—

Current-generation left ventricular assist devices provide circulatory support that is minimally or entirely nonpulsatile and are associated with marked increases in muscle sympathetic nerve activity (MSNA), likely through a baroreceptor-mediated pathway. We sought to determine whether the restoration of pulsatile flow through modulations in pump speed would reduce MSNA through the arterial baroreceptor reflex.

Methods and Results—

Ten men and 3 women (54±14 years) with Heartmate II continuous-flow left ventricular assist devices underwent hemodynamic and sympathetic neural assessment. Beat-to-beat blood pressure, carotid ultrasonography at the level of the arterial baroreceptors, and MSNA via microneurography were continuously recorded to determine steady-state responses to step changes (200–400 revolutions per minute) in continuous-flow left ventricular assist device pump speed from a maximum of 10 480±315 revolutions per minute to a minimum of 8500±380 revolutions per minute. Reductions in pump speed led to increases in pulse pressure (high versus low speed: 17±7 versus 26±12 mm Hg; P <0.01), distension of the carotid artery, and carotid arterial wall tension ( P <0.05 for all measures). In addition, MSNA was reduced (high versus low speed: 41±15 versus 33±16 bursts per minute; P <0.01) despite a reduction in mean arterial pressure and was inversely related to pulse pressure ( P =0.037).

Conclusions—

Among subjects with continuous-flow left ventricular assist devices, the restoration of pulsatile flow through modulations in pump speed leads to increased distortion of the arterial baroreceptors with a subsequent decline in MSNA. Additional study is needed to determine whether reduction of MSNA in this setting leads to improved outcomes.

Simulation of Apical and Atrio-aortic VAD in Patients with Transposition or Congenitally Corrected Transposition of the Great Arteries

Purpose

VADs could be used for transportation of the great arteries (TGA) and for congenitally corrected transposition (ccTGA) treatment. A cardiovascular numerical model (NM) may offer a useful clinical support in these complex physiopathologies. This work aims at developing and preliminarily verifying a NM of ccTGA and TGA interacting with VADs.

Methods

Hemodynamic data were collected at the baseline (BL) and three months (FUP) after apical (atrio-aortic) VAD implantation in a TGA (ccTGA) patient and used in a lumped parameter NM to simulate the patient's physiopathology. Measured (MS) and simulated (SIM) data were compared.

Results

MS and SIM data are in accordance at the BL and at FUP. Cardiac output (l/min): BL_m = 2.9 ± 0.4, BL_s = 3.0 ± 0.3; FUP_m = 4.2 ± 0.2, FUP_s = 4.1 ± 0.1. Right atrial pressure (mmHg): BL_m = 21.4 ± 4.1, BL_s = 18.5 ± 4.5; FUP_m = 13 ± 4, FUP_s = 14.8 ± 3.6. Pulmonary arterial pressure (mmHg): BL_m = 56 ± 6.3, BL_s = 57 ± 2, FUP_m = 37.5 ± 7.5, FUP_s = 35.5 ± 5.9. Systemic arterial pressure (mmHg): BL_m = 71 ± 2, BL_s = 74.6 ± 2.1; FUP_m = 84 ± 9, FUP_s = 81.9 ± 9.8.

Conclusions

NM can simulate the effect of a VAD in complex physiopathologies, with the inclusion of changes in circulatory parameters during the acute phase and at FUP. The simulation of differently assisted physiopathologies offers a useful support for clinicians.