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Purpose: To describe the total artificial heart (TAH) device as a bridge to heart transplantation (BTT), and related physical therapy management, while comparisons to left ventricular assist devices (LVAD) are made. Summary: The SynCardia CardioWest Temporary TAH System is the only TAH approved by the Food and Drug Administration (FDA), Health Canada and Consultants Europe (CE) for BTT. CardioWest implantation involves cardiectomy thus avoiding pulmonary hypertension, right heart failure, inotropic or anti-arrhythmic agents, myocardial and valve related problems. CardioWest has a fixed beat rate and cardiac output is dependent upon venous return and preload. Both TAH and LVADs are adaptive with exercise, increasing cardiac output during activities, allowing for conditioning to occur peripherally. Left ventricular assist devices have portable drivers permitting discharge home, while the CardioWest's large driver console necessitates inpatient therapy. Exercise progression, positioning, and monitoring of exercise intolerance are similar with LVAD and TAH. Ventricular fill volumes in TAH dictate cardiac output and require close attention. Cardiectomy in TAH prevents electrocardiography, telemetry, and native pulse rate monitoring. Conclusion: While mechanical differences exist between TAH and LVAD, physical therapists can provide evidence-based treatment for patients with TAH using previously established guidelines for patients with heart failure and mechanical circulatory support.
The purpose of this paper is to describe the total artificial heart (TAH) device as a bridge to heart transplantation, and related physical therapy (PT) management, while comparisons to left ventricular assist devices (LVAD) are made.
Heart transplantation remains the primary intervention for patients with end-stage heart disease, severe heart failure, and life-expectancy less than one year. As of January 1, 2006, even in the most urgent cases, the percentage of transplant candidates who received a heart at the end of 30, 60, and 90 days was 37%, 54%, and 58% respectively.1 The Organ Procurement and Transplant Network (OPTN) classifies these candidates as “Status 1A.” Patients who are Status IA are typically managed with any of the following: mechanical circulatory support, mechanical ventilation, intra-aortic balloon pump, or continuous infusion of intravenous (IV) inotropes in the intensive care setting. Additionally, Status 1A patients are classified as having a life expectancy of less than 7 days without heart transplantation if they are unable to be managed with the aforementioned devices or medications. The next most urgent category of listed patients is Status 1B. These patients have implanted ventricular assist devices for more than 30 days or receive continuous IV inotropes in a nonintensive care setting. For Status 1B candidates, 12% to 34% received transplants between 30 and 90 days, respectively, of being actively listed as a transplant candidate.1 The remaining patients (21.9%), who are actively listed, will wait at least two years for a donor heart.2
While the supply of donor hearts decreases, the demand is increasing. As of 2005, the OPTN has shifted the selection of patients more towards urgency and survival benefit rather than waiting time alone. Patients using LVAD or TAH as a bridge to transplant have improved survival to and after heart transplantation, improving utilization of donor hearts.3–5 Literature currently exists on physical therapy management for patients with LVADs, with regards to the safety and effectiveness of exercise for this population during bridging to transplant.6–8 However, there is no specific literature regarding physical therapy management in patients who have been implanted with TAH.
Following is an overview of LVAD and TAH device design and PT management considerations. In order to highlight these considerations, as well as the hemodynamic properties of the TAH during PT intervention, a brief case description is included for one patient's hospital course during his bridge to transplant phase with a CardioWest TAH. Descriptive comparisons between the LVAD and TAH will also be made throughout the paper. The goal of this paper is to begin providing a resource for physical therapists for this new TAH device and technology. Posttransplant PT management will not be addressed in this paper.
Appendix 1 provides a comparison of the HeartMate XVE LVAS, the HeartMate II LVAS, and the CardioWest TAH. Please note that specific to the Thoratec Corporation, their preference is to label their devices “left ventricular assist systems,” however for ease of terminology in this paper, these devices will continue to be referred to in the category of LVAD. The two LVADs are chosen for comparison be-cause they are commonly used devices9 and are ones that are used at the author's clinical setting. Figures Figures11 and and22 illustrate the two LVADs respectively and Figure Figure33 illustrates the TAH. Because there is less information available for the TAH, a more detailed description of this device follows.
The SynCardia CardioWest Temporary Total Artificial Heart System is the only TAH approved by the Food and Drug Administration (FDA), Health Canada and Consultants Europe (CE) Mark for bridge to transplantation. As of May 5, 2008, Centers for Medicare and Medicaid Services (CMS) has approved coverage of TAHs, including the Cardio West. The device is being implemented in 12 certified centers nationwide, including the Cleveland Clinic.10 When implanted, the CardioWest replaces the native ventricles and valves. Patients with a TAH do not require inotropic or antiarrhythmic agents and avoid pulmonary hypertension, right heart failure, and myocardial and valve related problems. The CardioWest is indicated for patients with biventricular failure, cardiogenic shock, multiple organ failure, and/or those at risk for LVAD implantation. It occupies the space of the removed heart and valves, making a surgical pocket unnecessary.
The CardioWest weighs 160 grams and occupies 400 cm3 of space. The lower limit of body surface area necessary for proper fit is considered to be 1.7 m2, suitable for most adults and some larger adolescents; although similar results have been reported for patients with body surface area as low as 1.5 m2.11 The two independent polycarbonate ventricles of the CardioWest have a total volume of 750 ml with unidirectional inflow and outflow valves. It is pulsatile and pneumatic. Polyurethane diaphragms separate blood from air pressure pulses controlled by an external driver. A driver delivers pneumatic pulses through drive lines into the air chambers of the ventricles, distending the diaphragms and ejecting blood. Seven foot pneumatic drive lines exiting the body are Dacron velour covered to allow tissue ingrowth and avoid spread of infection along the lines.12 The driver console contains two compressed air tanks and a backup power supply (battery) for mobility; the battery is automatically activated in any instance where the hospital's air and power supply are interrupted. A laptop computer on the console calculates and provides noninvasive monitoring information. The computer screen displays the device rate, stroke volume, cardiac output, drive pressure and flow waveforms, cardiac output trends, and patient related alarms. The large size of the console prevents patients from being discharged from the hospital (Figure (Figure4).4). A portable and wearable driver allowing hospital discharge is currently being used in Europe.13
The CardioWest has the shortest blood flow path of any device, reducing the risk of thromboembolism.14 It also has the largest inflow conduit area allowing for high cardiac output, with flows up to 9.5 L/min. The large inflow diameter and short blood flow distance through the device reduces central venous pressure, while high cardiac output increases systemic pressure, improving end organ perfusion and function.14 The device has a range of 100 to 130 beats per minute and a 70 mL ventricle. It is typically set to partial fill and runs at a fixed percentage of systole and a fixed beat rate of 120 to130 beats per minute with a stroke volume of 50 – 60 mL. The beat rate remains at the preset value and does not fluctuate until the value is reset on the device. The 70 mL ventricle is not fully filled to accommodate increases in venous return. With exercise or volume loading, cardiac output automatically increases as in a normal heart (Appendix 1).
Studies have been favorable for the CardioWest TAH. It has been shown to reduce the occurrence of stroke and transient ischemic attack during bridging to transplant in comparison to LVAD or BiVentricular Assist Devices (BIVAD) devices, with competitive rates of bleeding and infection.14 The CardioWest currently has the highest survival to transplant rate of any device.4 In 2004, Copeland and colleagues4 reported 79% of patients using the CardioWest bridged to transplant versus 46% of controls not receiving mechanical circulatory support. Survival at one and 5 years after transplant was 86% and 64% for study patients versus 69% and 34% of controls. Overall one year survival among implanted patients was 70% as compared to 31% of controls. The study also reported that 75% of patients were out of bed within one week of implantation of the CardioWest, and that patients bridging to transplant with the CardioWest experienced improvements in quality of life and mobility (defined as walking greater than 100 feet within two weeks).
A primary difference between LVAD and TAH is that currently, the only approved use for TAH is for bridge to transplant therapy whereas the Heartmate XVE can be used for either bridge (temporary) or destination (permanent) therapy (Appendix 1). Functionally the TAH console is larger and somewhat more cumbersome to mobilize a patient with and may require more technical support. All devices are adaptive with exercise and have the ability to increase flow or cardiac output during activities that allow for conditioning to occur peripherally.8,15,16
Monitoring the patient is similar for TAH and LVAD with the following exceptions: Since a patient with a TAH has undergone a cardiectomy, there is no native heart rate or monitoring via telemetry. If necessary or in the ICU setting, native heart rates and rhythms in patients with LVADs may be monitored via telemetry. Blood pressure for patients with TAH or LVAD can be taken manually, although for the HeartMate II, a mean arterial pressure (MAP) may be taken via doppler ultrasound in the event that peripheral pulses become diminished. The continuous motor and flow of blood through the Heart Mate II device may diminish peripheral pulses particularly in cases where left ventricular contribution to flow is compromised such as when the ventricle is flaccid or in fibrillation.
In all patients with either TAH or LVAD, device rate, flow and volume, blood pressure, oxygen saturation, and symptoms of orthostasis and exercise intolerance [rating of perceived exertion (RPE)] should be monitored throughout each treatment session.17 Physical therapy should be withheld or terminated and the medical team notified when: the patient is short of breath or reports exercise intolerance, systolic blood pressure is less than 80 mmHg or decreases by more than 20 mmHg, flow is less than 3 L/min, reduced volumes, the device alarms, and if the therapist notices neurological changes or bleeding. In LVAD recipients, other causes for concern and course of action are chest pain and palpitations, which are not considerations in a patient with TAH as the native heart has been removed. With the HeartMate XVE, care needs to be taken not to occlude the vent filter. The filter protects the motor from any contamination that could be introduced through the vent adapter located on the drive line. Occlusion of the filter can impair the pneumatic drive lines and ultimately LVAD function. Nursing needs to be notified immediately if fluid enters the filter.
Infection, especially along the drivelines exiting the body, is a major issue9 for patients with LVAD or TAH. Good hand washing on the part of the clinician is essential, along with patient education regarding wearing of an abdominal binder and avoiding disturbance of the drivelines. Patients should avoid sleeping, turning to and lying on the side of the body where the drive lines exit, and prone positioning.
The CardioWest, HeartMate XVE and II devices each have audio and visual alert for alarms. HeartMate devices alarm when issues are detected related to low flows or device rates, low voltage of batteries, and disruptions of the system controller, driver or power cables. Therapists working with patients implanted with the HeartMate II should also monitor the pump power and pulsatility index (PI), both found on the system monitor. Pump power is a direct measurement of the work of the pump. Pulsatility index is an average of the magnitude of pump flow pulses over 15 second intervals; it represents LV filling and native cardiac pulsatility. Higher PI values indicate greater ventricular filling and pulsatility (the pump is providing less support) and lower values indicate less ventricular filling and pulsatility (the pump is providing greater support to the ventricle) (Appendix 1). Pulsatility index should not vary considerably during rest. Significant increases in PI (gradual or abrupt) may indicate the presence of a thrombus in the pump. Significant drops in PI may indicate a decrease in native heart function or circulating blood volume and need to be evaluated. The CardioWest will alarm with disturbances of driveline pressure, low or left/right imbalances of cardiac output, activation of reserve air tanks or low air tank pressure, and faults related to the monitoring computer.
There are other safety concerns as patients with either LVAD or TAH become increasingly mobile with progression of ambulation in PT. Therapists ambulating patients outside of their rooms need to ensure that batteries and air tanks (where applicable) are charged and full (Appendix 1). The proper back up or emergency equipment such as an extra controller, hand pump, or power base unit (where applicable) need to travel with the patient (Appendix 1). Generally, patients have been educated in and are aware of the proper procedures required for mobility. Physical therapists can enhance safety and empower patients, who are becoming increasingly independent as they bridge to transplant or prepare for discharge home, by reinforcing patient education each treatment session regarding mobility and exercise safety.
At the time of this writing, two patients at the author's facility have undergone TAH implantation with one of those patients successfully receiving orthotopic heart transplantation and being discharged home. Provided below is a brief synopsis of his clinical course highlighting specific PT management considerations in relation to the TAH with a focus on hemodynamic monitoring and exercise response. Prior to reviewing the medical record and documenting pertinent aspects for this paper, informed consent was obtained from the patient.
The patient is a 61-year-old previously independent and active white male with a past medical history of ischemic cardiomyopathy, coronary artery disease, congestive heart failure (CHF), mitral valve disorder, myocardial infarction, atrial fibrillation, and ejection fraction of 10% and a surgical history of coronary artery bypass grafting (1992), percutaneous transluminal coronary angioplasty, and automated implantable cardioverter-defibrillator. He was admitted to an outside hospital via the emergency department with shortness of breath. After admission, the patient experienced respiratory failure and full cardiac arrest. He was intubated and administered inotropic therapy of levophed and dopamine. Once extubated, and weaned off of levophed and dopamine, dobutamine was initiated. The patient was then transferred to the Cleveland Clinic with a diagnosis of cardiogenic shock and as a potential heart transplantation candidate. Six days after admission he underwent implantation of the CardioWest TAH.
Beginning at postoperative day (POD) 7, when PT was consulted and the initial PT examination was completed, the patient received PT services through the majority of his bridge to heart transplantation. Table Table11 outlines the results of the initial examination that was completed at bedside in the ICU. Therapists working with the patient had completed a training course by the SynCardia Company regarding the device, its operation, and patient safety. However because of the recent initiation and clinical implementation of this device at the Cleveland Clinic, no specific PT guidelines for patients with TAH had been established. Therefore the rehabilitation team decided to use the pre-existing guidelines, referenced in the literature, that are implemented for patients with cardiac dysfunction, including those receiving LVAD placement.6–8,17–22
Department guidelines were followed such as the observance of activity orders and sternal precautions along with careful progression of activity and monitoring for symptoms of exercise intolerance. Activity orders from the physician were “out of bed to chair as tolerated.” Medications that the patient was initially prescribed included Hydralazine (Apresoline), a direct peripheral vasodilator and Amlodipine (Norvasc), a calcium channel blocker used for vasodilation.23 Over time, the patient was transitioned to Hydralazine and Lisinopril (Zestril or Prinivil), an angiotensin converting enzyme (ACE) inhibitor, used to minimize fluid retention and vasoconstriction.23 He also took Propoxyphene (Darvon) as needed for pain. Documentation regarding the medication adjustment was not clear; however for purposes of comparison, as a result of the cardiectomy and TAH implantation, medications need to be directed peripherally as opposed to patients who receive LVAD may still receive centrally acting cardiovascular agents such as positive inotropes.
During the examination, at rest and with activity, the therapist assessed for signs of orthostasis, monitoring left and right fill volumes, cardiac output, heart rate, and blood pressure.6,7,18 Monitoring fill volumes are unique to patients with TAH as these volumes will help dictate cardiac output. Cardiac output, heart rate, and fill volumes are displayed on the computer screen, while blood pressure was monitored by arterial line readings (Table (Table2).2). Once the patient was transferred out of the ICU, subsequent blood pressure measurements were taken manually with a cuff. Care was taken not to disturb the abdominal location of the exiting drive lines, which the patient reported were painful. Patient education focused on sternal precautions, symptom awareness, and mobility safety with drive lines including the use of an abdominal binder and positioning to avoid kinking of drive lines or trauma to the surgical site.
Evaluation findings associated with the initial examination were impairments of gait, balance, activity tolerance, pain, and knowledge deficit. Despite the TAH implantation, the evaluation findings are consistent with those reported in the literature for patients with LVAD placement.8,19 The consulting therapist's initial plan of care included a treatment frequency of 6 days per week to address these impairments and functional limitations. Goals were to prevent the deleterious effects of immobility, improve his functional limitations with bed mobility, transfers, ambulation, pain management and safety awareness, and ultimately maximize his strength and endurance in preparation for heart transplantation. Since this was a novel experience for the consulting therapist, the decision making and plan of care for this patient was based on literature for patients with LVADs.8,17,19
The first 3 weeks of PT consisted of active and active-assisted range of motion (ROM) exercises, bed mobility progression, sitting and standing activities to increase tolerance to upright, progressive ambulation, and instruction for gentle active ROM exercises that the patient could perform on his own as tolerated in supine and sitting.8,18 During week 3 after implantation of the CardioWest, the patient progressed to being able to perform standing therapeutic exercises and ambulating 150 feet with a wheeled walker and contact guard assist from the PT. An additional health care worker was needed to push the console while the patient performed gait training. Hemodynamic measurements and responses are noted in Table Table2.2. Thorough warm-ups consisting of seated and standing exercises preceded each episode of ambulation in PT. Though the patient was never symptomatic, it was noted that at times, if the patient sat down in a slumped position immediately after ambulation without any cool down activity, his left cardiac output would drop to or just below 4.0 liters per minute. From week 3, onwards cardiac output prior to and during gait training averaged from 5 to 7 liters (Table (Table2).2). More gradual cool downs and postural awareness were incorporated into treatment sessions. Cool downs consisted of a shorter, lower intensity walk after endurance training and/or gentle seated lower extremity exercises prior to termination of activity or exercise. Postural awareness addressed the patient's tendency to “slump” while seated, or improve thoracic positioning while seated. These measures reduced incidences of post-activity drops in left cardiac output during PT.
By postoperative week 6, the patient progressed to ambulation at supervision level without an assistive device and to a distance greater than 1,000 feet with the hemodynamic response shown in Table Table2.2. Hemodynamically, between weeks 3 and 7, the patients resting blood pressure went from 150/50 to 134/82, while his peak blood pressure went from 170/80 to 138/72. The therapist referred the patient to Phase I Cardiac Rehabilitation (Cardiac Rehab); a supervised and monitored exercise program for ambulatory inpatients that require endurance progression, which is provided by a team of exercise physiologists at the Cleveland Clinic. The patient began daily treadmill training with Cardiac Rehab on POD 42, and the PT plan of care was reduced in frequency to 3 days per week. Treadmill training continued throughout the remainder of the patient's bridge to transplant phase. Over the course of 5 weeks, he progressed in level treadmill walking from a speed of 0.8 mph for 10 minutes to walking at 1.5 to 2.0 mph for 65 minutes. During these Cardiac Rehab sessions, despite the increases in exercise intensity and frequency, his peak blood pressure remained steady in the 130s (systolic) and 60 to 70s (diastolic). While the patient underwent endurance training with Cardiac Rehab, his PT sessions focused on gait and balance training via walking and standing exercises, and strength training via seated and standing exercises with the addition of one pound ankle weights.20
After 10 weeks in PT, the patient was ambulating and safely performing strengthening exercises independently, and had transitioned to progressive treadmill training with Cardiac Rehab. Physical therapy essentially “signed off,” and followed the patient at a distance should any issues arise. After 83 days of bridging with the CardioWest, and 27 days actively listed as Status 1A, the patient underwent successful heart transplantation.
Because of the recent initiation and clinical implementation of this device at this author's facility, no PT guidelines had been established for patients with TAH. The patient's presentation however was similar to those referenced in the literature for patients with heart failure and circulatory assist devices.8,19 Therefore, the early mobilization of our patient was appropriate for the sequence suggested by Humphrey and colleagues for patients with LVAD.7 This sequence consisted of careful monitoring of device flows and fill volumes, signs of orthostasis, and avoidance of positioning or activity causing stress to drive line insertion site.6,7,18 Ongoing assessment and treatment of the patient was carried out with monitoring of hemodynamics and symptoms of exercise intolerance. Because the patient's functional limitations were consistent with most cardiac patients seen at this facility, the majority of this discussion will focus on the hemodynamic response in relation to the TAH device. Comparison of blood pressure responses from the initial examination to week 3 onwards cannot be made because of the different measurements conducted (ie, arterial line to cuff measurements).
With reference to the specific features of the TAH, the heart rate is fixed with increased cardiac output occurring as a result of increasing preload. As stated earlier in the paper, the 70 ml ventricle is not fully filled at rest in order to accommodate more venous return during exercise, thus being able to adapt to a certain amount of workload. This is demonstrated in Table Table22 with regards to the heart rate, beginning in week 3, remaining fixed at 120 beats per min-ute throughout the course of rehabilitation while the peak cardiac output increases from 5.6 liters/min. to 6.8 liters/min (weeks 3-11). Interestingly enough, in weeks 3 through 5, the increased cardiac output, both at rest and with activity, did not result in a concurrent increase in systolic blood pressure at rest and during activity. In weeks 5 through 9, there appeared to be a plateau in both blood pressure and cardiac output at rest and during activity despite an increase in endurance as noted both by the treadmill parameters and duration of activity.
Interpretations of these hemodynamic responses are limited in this single retrospective case review. However, it appears that the TAH mechanism, such as a fixed heart rate and maximal stroke volume, maybe responsible for some of the cardiovascular responses measured during exercise with this patient. Additionally as result of early mobilization, initiation of progressive ambulation and exercise training with physical therapy; peripheral skeletal muscle adaptation and associated vasodilation may have occurred, possibly accounting for the relative plateau in blood pressure and cardiac output from weeks 5–9.18,22,24 Vascular response may also be influenced pharmacologically by the vasodilators that the patient was receiving as part of his medical management.
The patient's achievements in exercise performance at 9 weeks was somewhat consistent with findings by Morrone and colleagues8 for patients with LVAD, where after 6 to 8 weeks of conditioning, only minimal improvements in exercise tolerance and functional capacity occurred. Since this patient's response to exercise is consistent with literature for patients with LVAD, it appears that following these established parameters in the literature is appropriate for patients with TAH. However, given the limitation of a single case report and the mechanical differences between LVAD and TAH, further study on these physiological aspects is warranted in the future to fully determine management guidelines.
The incidences of low left cardiac output during PT appeared to be related to improper cool down (ie, sitting abruptly) after endurance training, or positioning. Poor thoracic posture while seated may have stressed the drive lines exiting the body, compromising TAH function. Improper cool down, coupled with being managed with vasodilators, could have led to pooling of the blood in the lower extremities, and a sudden decrease in venous return to the TAH. Since the TAH relies on venous return to increase cardiac output, then these drops in cardiac output appear to be both reasonable and preventable as long as the clinician keeps the mechanical aspects of the device in mind.
Ventilatory limitation was not a likely factor in his training as oxygen desaturation was not an issue with the patient. However ventilatory adaptations may have played a role in this patients exercise reponse.25 No formal measurements of pre-rehabilitation and post-rehabilitation ventilatory function were conducted and would be a beneficial factor for future study particularly based on recent literature review by Arena et al26 describing the diagnostic benefits of examining ventilatory parameters during exercise in patients with heart failure. Central cardiac adaptations would not have occurred due to the mechanical device replacing the native heart.
Standardized tests and scales were not used. Literature regarding the rehabilitation of patients with CHF or with LVAD suggests using the Borg RPE ratings to assess exercise tolerance and the Six Minute Walk Test to safely progress exercise in this population.7,19,21 In the future, therapists at this facility should implement these suggestions, with patients who have these diagnoses, to further promote evidence-based practice. Additionally more thorough documentation of vital sign response during specific activities with physical therapy, such as seated exercises or balance activities, need to occur to further investigate and understand the physiological responses that are associated with this device.
Given these limitations and the lack of PT literature regarding TAH and the recent initiation and implementation of the TAH in this facility, examination and intervention for this patient with TAH relied on similar parameters to that of patients with LVAD. Surgical precautions for patients who are status post median sternotomy applied in this patient situation, as did the exercise safety guidelines for the treatment of patients with CHF21,25 or denervated heart.18
The CardioWest TAH is proving to be an effective and increasingly prevalent mechanical bridge to heart transplantation. While mechanical differences exist between TAH and LVAD, physical therapists can provide evidence-based treatment to this population using previously established guidelines6–8,18–22 for patients with heart failure and mechanical circulatory support. As more and more patients receive TAH implantation, further evidence supporting physical therapy management of bridge to heart transplantation with TAH should continue to be gathered and documented.
The authors wish to thank Dean Jeris, PT, and Stephanie Liebert, MPT, Clinical Team Leader, both at the Cleveland Clinic Neurological Institute, for their assistance in managing this patient during his course of therapy and providing clinical consult for this manuscript.
|HeartMate XVE LVASa||HeartMate II LVASb||CardioWest TAHc|
|Size||1150 g||375 g||160 g|
|SV, Flow/CO||83 mL, 10 L/min||124 mL, 10 L/min||70 mL, 9.5 L/min CO|
|Operating Modes||Fixed Speed||Fixed beat rate, % systole and left and right driving pressures|
|Vitals and Parameters to Monitor||Pump Rate, Flow, SV, Mode, Native HR if on telemetry, BP, SpO2 RPE||Pump Flow, Speed, Power, Pulsatiliy Index, Native HR if on telemetry, BP or MAP, SpO2, RPE||Left Fill, Right Fill, Left CO, Right CO, HR, BP, SpO2, RPE|
|PT Considerations and Patient Safety|
SV=stroke volume, CO=cardiac output, BTT=bridge to transplant, DT=destination therapy, BSA=body surface area, Pt=patient, LV=left ventricular, MAP=mean arterial pressure, HR=heart rate, BP=blood pressure, SpO2=saturation of peripheral oxygen, RPE=rate of perceived exertion, PBU=power base unit