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CARDIOVASCULAR ANESTHESIA

The Anesthetic Considerations in Patients with Ventricular Assist Devices Presenting for Noncardiac Surgery: A Review of Eight Cases

Department of Anesthesiology, The Mount Sinai School of Medicine, New York, New York

Address correspondence and reprint requests to Marc E. Stone, MD, Assistant Professor of Anesthesiology, The Mount Sinai Medical Center, Department of Anesthesiology, Box 1010, 1 Gustave L. Levy Place, New York, NY 10029-6574. Address e-mail to marc.stone{at}mssm.edu

	Abstract

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IMPLICATIONS: The number of patients supported by ventricular assist devices (VADs) that present for noncardiac surgery is increasing in our institution. Our recent experience with eight such patients is reported, along with a review of the most commonly implanted VADs and the anesthetic implications and considerations for VAD-supported patients undergoing noncardiac surgery.

	Introduction

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The American Heart Association estimates that there are 4,600,000 Americans with congestive heart failure (1). Approximately 550,000 new cases of congestive heart failure are diagnosed every year, and the incidence is approximately 10 per 1000 in patients older than 65 yr (1). Once significant cardiac failure becomes manifest, pharmacological interventions are the cornerstone of therapy. A rapidly progressing technology that has revolutionized the care of patients refractory to pharmacologic therapy is mechanical support of the failing heart.

Depending on the etiology of a patient’s cardiac failure, short- and intermediate-term ventricular assist devices (VADs) can support the circulation in the presence of an acutely failing ventricle as a "bridge to recovery." In cases in which no recovery is expected, a long-term left VAD (LVAD) is implanted as a "bridge to transplantation," if future heart transplantation is planned, or as "destination therapy," if the LVAD is intended to remain in a nontransplant candidate as a permanent management solution for end-stage cardiac failure. Destination therapy remains the subject of clinical trials in the United States. The recently completed REMATCH (randomized evaluation of mechanical assistance for the treatment of congestive heart failure) trial demonstrated a significant survival benefit and improved quality of life in those patients randomized to management with a chronically implanted LVAD compared with optimal medical management (2). Another similar trial, INTREPID (investigation of non-transplant-eligible patients who are inotrope dependent), is in progress. Positive results from large, multicenter trials such as these herald increasing use of LVADs to manage end-stage cardiac failure.

At one time, all patients supported by mechanical VADs were confined to tertiary care medical centers and tethered to large control consoles. This is generally still true for patients supported by short- and intermediate-term LVADs and for those requiring biventricular assistance. However, patients supported by a long-term implantable LVAD, either as a bridge to transplantation or as destination therapy, are usually discharged home. Many of these patients enjoy a return to levels of activity they had not experienced for years. The physical- and emotional-function scores of VAD-supported patients in the REMATCH trial were reportedly similar to those of patients receiving long-term hemodialysis and ambulatory patients with heart failure (2). As more patients with VADs return to their homes after implantation of a long-term device, it is conceivable that these patients may receive health care in facilities outside of the high-level tertiary care center. This is already a reality in Sweden, where the successful introduction of am LVAD program in a nontransplanting center was reported (3).

In our institution, increasing numbers of these devices are already being used, and the number of noncardiac procedures performed on patients with VADs is increasing. The objective of this article is to present our experience with this patient population, to introduce noncardiac anesthesiologists to the basic concepts of these devices, and to describe the anesthetic implications and considerations when caring for a patient already supported by one. This has not previously been a subject of discussion in peer-reviewed, published literature. The concepts underlying the surgical aspects of mechanical ventricular assistance are extensively reviewed in the literature (4–9), as are the anesthetic considerations for the patient undergoing VAD implantation (10).

	Case Report

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We queried our computerized anesthesia record database (CompuRecord; Philips Medical Systems, Andover, MA) for all patients on VAD support who underwent anesthesia for noncardiac procedures at the Mount Sinai Hospital (New York, NY) from September, 1997, to March, 2001. Eight VAD-supported patients were identified. The medical records of the eight patients were reviewed for preoperative duration of VAD support, preoperative diagnosis, proposed procedure, immediate preoperative status, anesthetic technique, details of fluid management, transfusion requirements, pharmacological support, postoperative disposition, and complications.

Of the eight patients in our series, four were being cared for in the cardiothoracic surgical intensive care unit before surgery, and the remainder were cared for on a surgical ward. The duration of continuing VAD support at the time of the noncardiac surgery ranged from 9 to 114 days, with a bimodal distribution. Five patients had been supported from 9 to 28 days, and three patients had been supported from 74 to 114 days. Five patients were receiving pharmacological inotropic support before surgery. One was mechanically ventilated and critically ill (Patient 5), whereas two were ambulatory with assistance on the surgical floors (Patients 2 and 4). All were adequately anticoagulated with heparin or warfarin per standard protocols for their respective assist devices (see Discussion).

Of the eight patients in this series, a noncardiac anesthesiologist successfully managed six with only minimal input from a cardiac anesthesiologist. All patients in this series were managed with a general anesthetic technique, as appropriate for the procedures performed. Three patients underwent intraabdominal surgery, one underwent a thoracotomy, three underwent procedures related to late wound complications, and one underwent microlaryngoscopy. Four of the eight patients were transfused with packed red blood cells. Three of the patients undergoing procedures requiring significant intraabdominal dissection received fresh frozen plasma, platelets, or both. Detailed patient demographics are summarized in Table 1. Preoperative status, details of the operative procedure, intraoperative management, and postoperative disposition are summarized in Tables 2 and 3. None of the patients in our series required adjustments to device settings during surgery.

	Discussion

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VADs are used when pharmacologic manipulations and intraaortic balloon counterpulsation fail to improve cardiogenic shock or less severe low-output states that are worsening. In general, VADs are pumps that collect blood returning to the heart and eject it downstream of the failing ventricle. Currently available VADs do not provide any respiratory functions; they simply act as pumps that can maintain perfusion of the body in the presence of a failed ventricle.

Typically, for left ventricular support, blood is drained from the left atrium or left ventricular apex to the pump and returned to the ascending aorta. For right ventricular (RV) support, blood is drained from the right atrium or RV to the pump and returned to the main pulmonary artery. The goal is either to decompress the acutely ischemic and failing ventricle (thereby reducing its oxygen demand) so it can recover or to provide long-term support for a chronically failing ventricle as a bridge to transplantation or as destination therapy.

Most of the time, VADs are operated in an automatic "full-to-empty" mode, meaning that the pump will automatically eject as soon as the blood chamber is full. Patient and mechanical factors that result in excessively slow or incomplete pump filling or in prolonged or incomplete pump emptying can lead to decreased pump output. Although different devices from different manufacturers use different strategies to accumulate and eject blood, the principles of VAD function in volume mode are similar. The two most important factors leading to decreased pump output are hypovolemia and increased afterload.

As is the case with the diseased native heart, normal or slightly increased intravascular volume and normal or slightly decreased vascular resistance are necessary for VADs to function optimally. Hypovolemia will delay VAD filling with each pump cycle, which may decrease overall pump output and lead to hypotension. Significantly increased vascular resistances may impede VAD ejection, resulting in prolonged or incomplete chamber emptying, and will also decrease overall pump output. Additionally, incomplete chamber emptying results in stasis of blood in the pump, potentially increasing the risk of thrombotic complications.

The following paragraphs introduce the most commonly implanted VADs and the anesthetic implications for the patient supported by a VAD. Table 4 summarizes the characteristics of several available VADs in routine clinical use.

The Abiomed BVS5000 is used most often to assist the "stunned" ventricle that is expected to recover (e.g., an acute, reversible ischemic insult during cardiotomy, myocardial infarction, or acute myocarditis). The device may also be used to assist the RV in a patient already supported by an LVAD or after failed heart transplantation.

During support, the pump or pumps remain mounted on a pole at the bedside, usually 0–10 cm below the level of the atrium, connected to cannulae from the heart and great vessels as described previously. Abiomed BVS5000 filling is performed by continuous gravity drainage from the heart, and the pump will automatically eject as soon as it is full. Under normal operating circumstances, there are no controls to adjust or variables to set because the BVS5000 is preprogrammed to attempt to provide approximately 5 L/min of flow. As discussed previously, patient and mechanical factors that result in inadequate pump filling (e.g., hypovolemia, pump mounted too high on pole, tamponade, kinked inflow cannula) or prolonged pump emptying (e.g., increased vascular resistance, pump mounted too low on pole, kinked outflow cannula) can lead to decreased pump output. In the case of an LVAD, systemic vascular resistance is the major determinant of afterload, whereas in the case of a right VAD (RVAD), pulmonary vascular resistance (PVR) is the major determinant of afterload.

Anticoagulation with heparin (to maintain an activated coagulation time [ACT] of 180–200 s) is mandatory during Abiomed support to prevent a thrombus from forming on the polyurethane valves in the pump. The level of anticoagulation should be increased (ACT of approximately 300 s) during prolonged periods of low pump flow (<2 L/min) or sustained arrhythmias. Because Abiomed filling and ejection occur asynchronously from the underlying cardiac rhythm, patients on biventricular support may remain surprisingly hemodynamically stable despite otherwise malignant ventricular arrhythmias.

To summarize, Abiomed BVS 5000 output is dependent on the speed of pump filling and the impedance against which it must empty. Therefore, optimal function depends on an adequate intravascular volume status and a normal to slightly low afterload.

The Thoratec^® (Thoratec Laboratories, Pleasanton, CA) is also an external, asynchronous, pneumatically driven, pulsatile circulatory support device that can be used to assist both ventricles or either one separately. There are three main differences between the Abiomed BVS5000 and the Thoratec. The Thoratec can be used for an intermediate period of time, in the range of months, and it is the only Food and Drug Administration (FDA)-approved VAD in routine clinical use that can provide biventricular support for a period long enough to serve as a bridge to transplantation. The Thoratec uses vacuum-assisted filling instead of gravity drainage. Finally, the Thoratec is much more complicated to operate.

Surgically placed inflow and outflow cannulae are tunneled from the heart and great vessels, as described previously, to exit the skin of the upper abdomen, just below the rib cage. The Thoratec pump head rests externally on the abdomen, allowing inspection of its function through the clear plastic casing. Despite the pump location on the abdomen, subsequent midline laparotomy and laparoscopy are not a problem.

Unlike the Abiomed BVS5000, which always functions in an automatic "full-to-empty" mode, attempting to provide 5 L/min of pump output, the Thoratec offers three different modes of VAD function. In the "asynchronous mode," the pump functions in a fixed manner according to user-defined variables (analogous to controlled mandatory ventilation on a ventilator). In the "volume mode," the pump will eject as soon as it is full, similar to the Abiomed. In the "external synchronization mode," the Thoratec provides counterpulsation, which is useful for weaning from VAD support. The majority of the time, a patient presenting to the OR with a Thoratec will be managed in the automatic volume mode.

Operating the Thoratec is more complicated than operating an Abiomed BVS5000 because the user must manually set and occasionally make adjustments to the various variables of VAD function, including the VAD rate, the percentage of the pump cycle spent in systolic ejection, and the vacuum filling and ejection drive pressures. For this reason, the Thoratec should be monitored by someone familiar with its operation and alarm troubleshooting.

Thoratec output can be increased in a number of ways, depending on the clinical situation and mode of operation that is used. In general, fixing problems will usually involve augmenting inadequate pump filling or improving pump emptying, but despite the ability and the temptation, adjusting device settings is not always the appropriate first step. If a patient was previously stable on given settings, one first needs to eliminate a new patient problem, such as hypovolemia, tamponade, or failure of an unassisted ventricle and ensure that the patient has adequate pharmacologic support. If there is no apparent new patient-related problem, one can then increase Thoratec output by various methods: 1) increasing the vacuum pressure, to augment pump filling; 2) decreasing the percentage of time spent in systole, to allow a longer time for pump filling; or 3) increasing drive pressure, to improve pump emptying. Usually, optimization comes from optimization of more than one variable. This may initially seem quite complicated, but with experience, it facilitates achievement of the support needs of a wide range of patients, making the Thoratec an extremely versatile support device.

Anticoagulation during Thoratec support is initially with heparin (ACT, 180–200 s), but once a patient is able to tolerate oral intake, maintenance anticoagulation is with warfarin (INR, 2.5–3.5). Aspirin is added in patients with increased platelet counts. To summarize, just as with the Abiomed, optimal Thoratec function depends on an adequate intravascular volume; however, variables of device filling and emptying can be adjusted when necessary to improve VAD output.

The Novacor LVAS^® (World Heart, Ottawa, Canada) and the Heartmate LVAS^® (Thoratec Corporation, Woburn, MA) are fully implantable LVADs intended as long-term bridges to transplantation for patients with end-stage cardiomyopathy or for those whose left ventricle fails to recover despite support with a short- or intermediate-term device, such as the Abiomed or Thoratec. Implantability and electrically powered mechanical ejection strategies were a major advance in VAD technology compared with the previous generations of VADs that required the supported patient to be in the hospital, tethered to a large control module and source of compressed air.

Long-term LVADs are completely implanted in the preperitoneal space or posterior rectus sheath and are connected to the heart and ascending aorta by cannulae as described previously. The only external component is a power cable or driveline that is tunneled through the abdominal wall and exits the skin of the lower abdomen to plug into the system controller and power converter (often worn in a specially designed vest). These implantable LVADs have extremely sophisticated control algorithms that enable device output to be responsive to physiologic needs. As long as intravascular volume and RV function are adequate (see Discussion), there is no need to adjust any of the settings.

Although the Novacor requires continuous long-term anticoagulation with warfarin (international normalized ratio, 2.5–3.5), the Heartmate incorporates an effective antithrombogenic surface composed of sintered titanium microspheres. Most patients supported by a Heartmate receive no anticoagulants at all, although some centers maintain their patients on aspirin, occasionally in combination with dipyridamole.

Development and clinical trials of both the Heartmate and the Novacor began in the mid 1970s. The original, pneumatic version of the Heartmate received its FDA approval in November 1994, and the device quickly established itself in transplant centers. The Novacor finally received its FDA approval in September 1998, coincidentally on the same day as the current electric version of the Heartmate was approved. Because the Heartmate device was available 4 yr earlier than the Novacor, it has been implanted in approximately twice as many patients as the Novacor worldwide. According to World Heart, which owns the Novacor, the device has currently been implanted in >1300 patients worldwide. The Heartmate was recently acquired by Thoratec Corporation, which, as of the time of this writing, has not yet released updated statistics for 2001. The maximum duration of trouble-free support for a Heartmate is approximately 17–18 mo, whereas a single Novacor can apparently last an average of 3–4 yr. Bridge to transplantation is currently by far the most common indication for implantation of these devices. Both the Novacor and the Heartmate have a successful bridge-to-transplantation rate of 60%–80% (6–8).

Preoperative Anesthetic Considerations
As illustrated by the patients in our series, VAD-supported patients present for noncardiac surgery with widely varying clinical conditions. Some are critically ill patients from intensive care units, whereas others may be ambulatory patients in good condition. The clinician must specifically evaluate the patient for neurological deficits (often resulting from thromboembolic events or hypoperfusion) and ascertain the extent of major organ dysfunction (e.g., renal or hepatic insufficiency), which is very common in this population. Any further deterioration in the perioperative period may preclude full recovery or disqualify a patient from later heart transplantation.

With the exception of the Heartmate, maintenance of therapeutic levels of anticoagulation is imperative for extracorporeal circulation through these devices (because of the risk of thromboembolism), and both the surgeon and anesthesiologist must be prepared for increased intraoperative bleeding. Heparin infusions or warfarin maintenance should not routinely be discontinued before surgery. Anticoagulation is compatible with most surgical procedures; neurosurgical procedures, however, may pose a particular problem. The surgeon, the anesthesiologist, and the physician managing the VAD should agree on a coagulation management scheme that is compatible with both safe conduct of the noncardiac surgical procedure and continued extracorporeal circulation through the VAD. It is reasonable to administer small amounts of fresh frozen plasma, guided by frequent measurements of activated partial thromboplastin time or ACT, to decrease the level of anticoagulation to the lower limit of the manufacturers’ recommendation to minimize excessive bleeding. Caution is advised to avoid thromboembolic complications. At no time should anticoagulation ever be fully reversed in a VAD-supported patient. One potential advantage of the Abiomed in this regard is that developing thrombi would be directly visible on the device’s artificial polyurethane valves.

Patients will be transported to the OR with the VAD running on battery power. It must be ensured that the device is connected to a stable power source upon arrival in the OR to prevent electrical failure and catastrophic outcomes (e.g., device thrombosis).

Strict sterile technique and appropriate prophylactic antibiotics should be used for all invasive procedures. Infection of a VAD is catastrophic, because these are very large foreign bodies that cannot be sterilized with antibiotic therapy. It is inadvisable for the surgeons to directly prepare the devices, cannulae, and drivelines with povidone-iodine or polar organic solvents, such as 100% alcohol, because these solutions may degrade plastic device components. Drivelines and cannulae are excluded from the surgical field by the use of a standard, sterile, adherent, clear plastic incise drape, such as a Steri-Drape^® (3M Corp., St. Paul, MN).

Intraoperative Anesthetic Considerations
Currently available VADs do not oxygenate or eliminate carbon dioxide from the blood. One should use the same considerations, indications, and criteria for tracheal intubation and extubation as in the patient without a VAD. Spontaneous ventilation is a desirable goal, because it will facilitate venous return to the device, but the decision to use positive pressure ventilation must be based on the individual needs of the patient. As outlined previously, only one of the patients in our series was intubated and mechanically ventilated before coming to the OR (Patient 5), and one had a tracheostomy but was not mechanically ventilated (Patient 4). Not every patient on a VAD will need to remain intubated at the end of the case, as illustrated by several of the patients in our series.

Any patient with an implanted, long-term device should be considered a "full stomach" because of the size and location of the device (preperitoneal). Precautions against pulmonary aspiration, including rapid-sequence induction with cricoid pressure, are prudent in this population.

VAD control consoles continuously display the device output (usually an average of every four beats). Thus, invasive hemodynamic monitoring is not mandatory for all procedures.

Arterial catheters are inserted for blood pressure monitoring when the surgical procedure is anticipated to produce large swings in blood pressure or when frequent arterial blood gas analyses are required. Although several patients in our series had arterial catheters in situ, Patient 7 had an arterial catheter inserted because of the nature of the procedure. Patients 2, 4, 6, and 8 had their blood pressures monitored noninvasively at the discretion of the attending anesthesiologist.

It must be appreciated that the monitored pulse rate will reflect VAD ejection and may not be the same as the electrocardiogram-derived heart rate. This may have implications for the vital signs recorded on the anesthesia record, especially when automated anesthesia record-keeping systems are in use.

Central venous pressure (CVP) monitoring should be considered if large fluid shifts are anticipated, because there is a 20%–30% incidence of RV failure in patients on isolated LVAD support (11,12). This increased risk of RV failure exists for a variety of reasons, depending on the LVAD-supported patient’s clinical situation. Increased left-sided output from the VAD will result in increased venous return to the right side of the heart. Decompression of the left side of the heart by an LVAD causes a leftward shift of the interventricular septum, resulting in altered RV geometry, increased RV compliance, and decreased RV contractility (11). Sometimes, these factors alone are enough to cause RV failure in patients with preexisting RV compromise. Although a properly functioning LVAD will reduce RV afterload and often improve RV function in patients with normal PVR, patients with fixed, increased PVR may actually experience an increase in RV afterload because of increased right-sided flows (12). Additionally, in our experience, significant tricuspid regurgitation occasionally results from dilation of the tricuspid annulus during LVAD support. For all these reasons, patients with left ventricular failure and significant preexisting RV compromise are usually placed on biventricular support at the time of VAD placements. Although CVP monitoring is of questionable accuracy in patients with an RVAD, central access may still be useful for drug and volume infusions. In the absence of a CVP monitoring line, inadequate RVAD filling (indicated by slow RVAD rate and poor RVAD output) may be suggestive of hypovolemia.

In general, pulmonary artery catheters (PACs) provide little useful information in the patient with an LVAD (that continuously displays left-sided output); however, they may be of some use in guiding therapy in patients with pulmonary hypertension at risk of RV failure. PACs are impossible to place in a patient with a functioning RVAD. In the absence of a PAC, suddenly inadequate LVAD ejection (indicated by suddenly increased residual volumes, often accompanied by an audible alarm) may be suggestive of acutely increased systemic vascular resistance. Transesophageal echocardiography is the intraoperative monitor of choice if there is concern about failure of an unassisted ventricle. The use of transesophageal echocardiography for late follow-up and diagnosis of problems in this population has been described (13,14).

The anesthesiologist must consider the effect that surgical positioning will have on venous return, because an adequate circulating blood volume is an important factor in maintaining device output. As discussed previously, the faster the pump fills, the faster will be the pump rate and, therefore, the pump output. Management must be individualized, and inotropes, vasodilators, and vasopressors should be used as necessary to maintain optimal hemodynamics. For example, Patient 1 in our series was weaned off vasopressor support with volume loading, whereas Patient 5 required initiation of a norepinephrine infusion to maintain hemodynamic stability.

The anesthetic technique and drugs chosen should be appropriate for the planned operation. The anticoagulation required with most VADs contraindicates many forms of regional anesthesia, and a general anesthetic is usually the most appropriate choice. However, sedation with local skin infiltration or a Bier block can be used for appropriate procedures. The Heartmate does not require formal anticoagulation, and some Heartmate patients may be acceptable candidates for regional techniques. There are no specific drugs that are contraindicated because of the VAD itself. As long as there is adequate intravascular volume, VAD function will not be depressed, regardless of what drug is used for the induction or maintenance of anesthesia. With the exception of Patient 5 (who initially tolerated only scopolamine, small doses of fentanyl, and muscle relaxant), the patients in our series tolerated balanced general anesthesia with a variety of conventional drugs, including midazolam, fentanyl, propofol, ketamine, remifentanil, nitrous oxide, isoflurane, sevoflurane, and desflurane. Muscle relaxation was achieved with pancuronium, vecuronium, rocuronium, or cisatracurium (individualized for each patient, considering any coexisting renal or hepatic dysfunction), and reversal was accomplished with neostigmine and glycopyrrolate. Just as in any sick patient, appropriate consideration must be given to a potentially dysfunctional unassisted ventricle, and the anesthetic technique should be tailored accordingly by continuously optimizing intravascular volume status, systemic vascular resistance and PVR, and inotropic support.

The hemodynamic significance of arrhythmias is dependent on the rhythm and which ventricle or ventricles are supported. VAD ejection is asynchronous with the underlying cardiac rhythm, and patients on biventricular support will often remain hemodynamically stable in the face of otherwise malignant arrhythmias. Regardless, malignant ventricular arrhythmias should be terminated whenever possible (see below). Rate-controlled atrial fibrillation may be well tolerated. Many patients with permanent, implanted LVADs have automatic implanted cardiac defibrillators because loss of RV function during a ventricular arrhythmia will prevent LVAD filling. Standard Advanced Cardiac Life Support (ACLS) protocols (with the exception of chest compressions that could cause potential dislodgement of intracardiac cannulae) should be used when needed, and malignant arrhythmias should be electrically or pharmacologically terminated. Extracorporeal devices (the Abiomed and the Thoratec) and the well shielded Novacor will not be affected by defibrillation or the electrocautery. Unfortunately, the Heartmate is not well shielded and may be reset to a fixed-rate mode by the electrocautery and potentially damaged by external defibrillation. When feasible, the use of bipolar electrocautery is recommended.

In conclusion, it may be anticipated that patients supported by a VAD will present for anesthetics for diagnostic procedures and noncardiac surgery with increased frequency in the foreseeable future. Where available, consultation with cardiac anesthesia colleagues is highly desirable; however, in our experience, adherence to standard anesthesia principles and practices is the core of patient management approaches. Provided that circulating blood volume remains adequate and hemodynamics are continuously optimized (as they typically are for every anesthetized patient), it is unlikely that adjustments to device settings will need to be made in the perioperative period. Nevertheless, careful monitoring and titration of anesthetic and pharmacological support therapy is necessary to safely anesthetize patients with VADs.

	References

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (3) Citing Articles via Google Scholar Google Scholar Articles by Stone, M. E. Articles by Reich, D. L. Search for Related Content PubMed PubMed Citation Articles by Stone, M. E. Articles by Reich, D. L. Related Collections Cardiovascular Heart Monitoring (Cardiac)