LVAD Efficiency and the Criticality of Accurate Blood Pressure Measurement

By Jodi Hutchins

What is a LVAD? How does it work?

A left ventricular assist device (LVAD), also called a ventricular assist system or VAS, is a mechanical pump that is attached to the left ventricle of the heart to augment the function of the heart’s main pumping chamber. The left ventricle pumps blood from the heart to the rest of the body. [1].

It is important to note that a LVAD is different from an artificial heart. An artificial heart replaces a failing heart completely, whereas a LVAD, such as the one pictures at left developed at Proven Process, assists the heart to help pump more blood to the body with less work. 

The LVAD has both internal and external critical components in order for it to work properly. The internal components often include a pump that rests on or adjacent to the heart’s left ventricle with a tube attached that routes the blood from the left ventricle to the aorta.  Extending from the pump out through the skin is a cable called a driveline. The driveline connects the internal mechanical pump to an external controller and a power source that is worn outside of the body. In order for the mechanical circulatory support to function as needed, the driveline and power source must be connected at all times. The power source can be in the form of a battery, an AC adapter or DC adapter. The controller requires that two sources of power are connected at all times[2]. The typical battery life will provide 4-5 hours of power. Moreover, most controllers have a built in warning system to alert the patient when the source of power is running low.

Why is it used?

LVADs are needed when a patient has reached advanced stages of heart failure and the heart is no longer able to pump enough blood to meet the body’s needs. Although cardiac transplantation still remains the ideal solution for end-stage heart failure, its greatest limit has been the growing number of donor hearts available along with the ever-growing patient waiting lists and stringent eligibility criteria.[3] For these reasons, many doctors at this stage may recommend LVAD implant surgery as an alternative.

LVADs today are used in three different ways. The first is for bridge transplantation. In this situation, a LVAD is implanted as a temporary solution while the patient waits for a heart transplant and may remain in place for several years until a donor heart becomes available. The second way that LVADs are used today is for destination therapy cases. In this situation, if a patient is not a candidate for a heart transplant, a LVAD may be implanted as a permanent solution. This is becoming increasingly common as LVAD technology continues to improve along with the quality of life it offers. The third way that LVADs are used today is for bridge-to-recovery cases. In this situation, a LVAD is implanted so that the patient’s heart can recover its strength with the assistance of the device.

What are the risks?

For patients experiencing end stage heart failure, many are turning to LVADs for destination therapy or bridge transplantation. Because of the significant advances in pump technology, candidate selection, and clinical management, many patients with LVADs are living longer[4]. In addition, due to their improved durability, continuous flow LVADs otherwise known as CF-VAD are predominately replacing pulsatile LVADs.

An essential component of optimal clinical care is the accurate measurement of blood pressure (BP) as well as the recognition and management of hypertension in patients with LVADs. Because hypertension is as an established long-term risk factor for cardiovascular disease, long-term management strategies for the control of BP are becoming increasingly important due to improved outcomes and extended support times for many LVAD patients.

It is important to note, however, that there is increasing evidence that relates hypertension in LVAD patients to adverse outcomes[5]. From 2006 to 2013, a multicenter review examined the association between blood pressure and adverse events of CF-VAD patients. The results from these studies revealed that higher BP was significantly associated with a hemorrhagic stroke, aortic insufficiency, and thromboembolic events[6]. This study was impactful because it was the first to show an association between poorly controlled BP and adverse events in the CF-VAD population. In a separate study that was conducted to evaluate the efficacy and impact on adverse events related to blood pressure control in continuous flow left ventricular assist devices, it found blood pressure control can be achieved in patients with CF LVADs, with the majority of patients requiring only 1 or 2 antihypertensives to achieve a target mean arterial pressure (MAP) of less than 80mmHg, LVAD patients not on any antihypertensive medications experienced higher rates of neurologic events.[7]

Impact on pump output

It is essential to maximize pump output and ensure adequate decompression of the left ventricle for BP control in patients with continuous flow pumps. In addition, blood pressure control is predominantly important in the setting of CF-VADs since these types of devices are more sensitive to afterload than the previous generation of pulsatile flow LVADs. Afterload is defined as the tension developed by the heart during contraction[8]. It represents the load or resistance against which the left ventricle must pump or eject its volume of blood during contraction and indicates how much effort the ventricles must put forth to force blood into systemic circulation. The resistance is produced by the volume already in the vascular system and by the constriction of the vessel walls. Factors that increase afterload include aortic and pulmonary stenosis, systemic and pulmonary hypertension, and high peripheral resistance. In cases of poorly controlled hypertension, an understanding of the physiology of CF-VADs highlights several factors that may contribute to device dysfunction and complications.

Because pump output at a given speed is greatly dependent on afterload, continuous flow devices and centrifugal pumps are even more sensitive to afterload than axial flow pumps. [9] When afterload occurs in the form of systemic hypertension, it results in decreased flow, decreased cardiac output, and less effective ventricular unloading. In the setting of high systemic vascular resistance, decreased LVAD flow may also increase stasis and contribute to the risk of device thrombosis, or clotting within the device. Moreover, reducing LVAD ventricular unloading, inducing sub endocardial ischemia, and precipitating ventricular arrhythmias, may acutely affect clinical outcomes and worsen heart failure symptoms in LVAD patients with poorly controlled BP.

Impact on stroke risk

For patients on a long term LVAD, stroke is one of the most devastating outcomes related to the treatment of end stage heart failure. In the original randomized controlled trial of CF-VADs, the incidence of both hemorrhagic and ischemic stroke was 18% at 2 years.[10] The stroke rate has remained constant, despite advances in LVAD technology. According to the Interagency Registry for Mechanically Assisted Circulatory Support (INTERMACS), recent data found a 17% 2-year incidence of stroke.[11]

The most significant risk factor for stroke in the general population is hypertension. For patients with hypertension, the risk of stroke is one and a half times that of patients with BP within normal reference ranges.[12] In fact, patients with severe hypertension have a relative risk of stroke that is nearly four times that of normotensive patients where a systolic BP (SBP) greater than 160 mmHg.[13] It is important to note that data is emerging that indicates an increased risk of stroke in LVAD patients with poorly controlled BP, although the role of typical cardiovascular disease comorbidities such as hypertension in stroke risk in LVAD populations has not been well established.         

Impact on aortic regurgitation

The recognized complication of CF-VAD support that can affect survival and quality of life is aortic regurgitation.  Aortic regurgitation is defined as the backflow of blood from the aorta into the left ventricle resulting in an insufficiency of blood to the aortic valve which may be chronic or acute.[14] In a situation of poorly controlled BP after a LVAD implant, this condition can worsen preexisting aortic insufficiency or lead to de novo aortic insufficiency particularly in the setting of a closed aortic valve or one that rarely opens. A 2010 study on the development of aortic insufficiency in left ventricular assist device-supported patients concluded that aortic insufficiency progresses over time in LVAD support patients, and the patients supported with CF-VADs appeared to develop more aortic insufficiency than those with pulsatile LVADs.[15] Another study was conducted more recently to determine if postoperative blood pressure influenced the development of aortic regurgitation following CF-VAD implantation. The results from that study also identified an association between BP control and the development of aortic regurgitation in patients with CF-VADs.[16] Together, the data suggests that early and aggressive control of BP can aid in the prevention of the development or progression of aortic insufficiency following CF-VAD implantation.

Impact on device thrombosis

Another major complication of CF-VAD support, that is likely related to hypertension, is pump thrombosis.[17] A study suggests that there was an association between pump thrombosis and BP in LVAD patients.[18] After analyzing pump thrombus events in 382 patients who underwent centrifugal CF-VAD implant as a bridge to transplant, they found that a MAP greater than 90 mmHg was a significant risk factor for pump thrombosis. To further clarify the pathophysiology that underlies the association between hypertension and pump thrombosis, further studies are needed.

BP goals in CF-VAD patients

Various BP target ranges have been proposed regarding the management of patients with LVADs. According to the research and studies conducted, there is recognition that BP control is important and guidelines for BP control in LVADs exist. However, there is a lack of evidence to support current BP recommendations. A hypertension adverse event, as defined by The INTERMACS, as new onset SBP > 140mmHg, or diastolic BP > 90mmHg for pulsatile pumps and a mean BP > 110mmHg for continuous flow pumps. [19] It is important to note that expert opinions over the years have progressively recommended lower goals for BP as evidence about the adverse effects of hypertension in LVAD patients grows. Following a 2009 review of ventricular assist devices and the challenges of outpatient management, it suggested a MAP goal of 70-90mmHg.[20] Another study based on the clinical management of continuous flow left ventricular assist devices in advanced heart failure cases recommends a goal MAP range of 70-80mmHg.[21] According to the ISHLT guidelines, it acknowledges that there is no strong evidence base for BP targets with CF-VADs and it advises a target MAP less than 80 mmHg provided that the adverse effects of low BP can be avoided.[22] (https://www.escardio.org/Guidelines/Clinical-Practice-Guidelines/Pulmonary-Hypertension-Guidelines-on-Diagnosis-and-Treatment-of) In order to achieve optimal BP control, current guidelines also recommend the titration of standard heart failure pharmacotherapy with a beta blocker, ace inhibitor, or angiotensin receptor blocker (ARB) in order to achieve optimal BP control.

Challenges in BP measurement in CF-VAD patients

Patients with CF-VADs can present unique challenges regarding the measurement of BP and the management of hypertension because patients with CF-VADs often do not have a palpable pulse. As a result, traditional BP measurement by auscultation or automated cuff is less reliable. The ideal method in measuring BP for patients with CF-VADs is the arterial line. However, it is an invasive procedure and not practical for routine outpatient use. A test was conducted to determine the ideal methodology to assess systemic blood pressure in patients with continuous flow left ventricular assist devices where they measured BP by various methods with arterial lines in the immediate post-operative period. They found that a Doppler probe, compared to automatic cuff, palpation, and auscultation, detected a pressure in almost all patents that was accurate and correlated with arterial catheter mean BP.[23]

Many efforts and studies are underway to identify an easier and more reliable method to measure BP for patients with LVADs. As pulse oximetry is widely available in healthcare settings, a recent group examined sphygmomanometry combined with finger pulse oximetry as a unique method to non-invasively measure BP in CF-VAD patients, however this technique may warrant further consideration.[24] A slow cuff deflation device was compared to four other methods of BP measurement in another recent study that included a standard automated BP cuff, and Doppler, and arterial line. It concluded that the slow cuff deflation device was successful, reliable, and valid when compared to arterial line, and offered a significant advantage over Doppler in that it is inexpensive and can be used in the home setting.[25]

Conclusions

It has been determined that as the number of people with mechanical circulatory support continues to grow, hypertension is now an established risk factor for adverse outcomes in patients with CF-VADs. Additionally, more stringent BP goals may prevent device complications like stroke, pump thrombosis, and aortic insufficiency, as evidence suggests. MAP should be maintained between 70-80 mmHg, striking a balance between tight BP control in the long term management of LVAD patients to avoid the side effects of low BP. Fortunately, LVAD patients may have additional benefits in terms of the management of heart failure and the establishment of a favorable environment for potential myocardial recovery by following the same pharmacotherapy regimen that can be utilized to lower BP. Although Doppler measurement of BP is the current standard of care, it is generally accepted to be the most reliable and is the most consistent method in the setting of continuous flow, but is not without its limitations. There are fewer consensuses on whether the obtained Doppler pressure represents MAP or SBP. In the setting of continuous flow, efforts are underway to develop easier, more accessible noninvasive methods to measure BP.

About the Author

Jodi Hutchins of Regulatory Matters Consulting is an Independent Regulatory and Quality Consultant with over 15 years of global medical device registration experience. She held her most recent position for 9 years, as QA/RA Director for a worldwide distributor of medical devices.



[5] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4385742/

Wasson LT, Yuzefpolskaya M, Wakabayashi M, et al. Hypertension: an unstudied potential risk factor for adverse outcomes during continuous flow ventricular assist device support. Heart Fail Rev 2015;20:317-22. https://www.ncbi.nlm.nih.gov/pubmed/25870369

[PMC free article] [PubMed]

[6] https://www.ncbi.nlm.nih.gov/pubmed/25870369

Saeed O, Jermyn R, Kargoli F, et al. Blood pressure and adverse events during continuous flow left ventricular assist device support. Circ Heart Fail 2015;8:551-6. [PubMed]

[7] https://www.ncbi.nlm.nih.gov/pubmed/24075484/

Lampert BC, Eckert C, Weaver S, et al. Blood pressure control in continuous flow left ventricular assist devices: efficacy and impact on adverse events. Ann Thorac Surg 2014;97:139-46. [PubMed]