State of the Art in RemoteMonitoring Technology

K.L. Venkatachalam, MDa, Samuel J. Asirvatham, MDb,*

KEYWORDS

� Remote monitoring � Cardiac implantable electronic device � Cardiac monitoring � Telemedicine� Mobile cardiac outpatient telemetry

KEY POINTS

� Remote monitoring of cardiac physiologic parameters, including heart rhythm and volume status, isplaying an increasingly important role in the routine management of cardiac patients.

� This approach has the potential to provide significant cost and time savings to health care providerswhile reducing time for diagnosis and treatment of patients. It also has the ability to significantlyenhance patient convenience by minimizing office visits.

� Integration of routine cardiac health parameters, such as blood pressure and daily weight, alongwith the possibility of monitoring serious changes, such as ST segment deviation, may make rapid,comprehensive cardiac care a reality in the near future.

INTRODUCTION

Although electronic and mechanical technologiesto monitor cardiac status, such as sphygmoma-nometry and electrocardiography, have beenavailable for approximately 150 years, true porta-bility and ability to provide remote monitoringcapabilities depended upon the invention of aviable electronic transistor in 1947. This develop-ment is considered by many technologists to bethe most significant developments of the 20thcentury. Within a few years, reasonably pricedcommercial transistors made of germanium andsilicon were available, and compact electronicamplifiers and analog signal processing circuitfabrication were feasible. The first practical, por-table cardiac pacemaker, built in 1957, was basedon a metronome circuit taken from an electronicsmagazine. The invention of the integrated circuitand the microprocessor represented a giant leapforward in the quest for compact, reliable elec-tronic monitoring systems. Since then, portable

Disclosures: The authors have nothing to disclose.a Department of Medicine, Division of Cardiology, MJacksonville, FL 32224, USA; b Department of MediciMayo Clinic Minnesota, Mary Brigh Building 4-523, 121* Corresponding author.E-mail address: asirvatham.samuel@mayo.edu

Card Electrophysiol Clin 5 (2013) 365–370http://dx.doi.org/10.1016/j.ccep.2013.05.0011877-9182/13/$ – see front matter � 2013 Elsevier Inc. All

rhythm monitoring using on-board storage, im-plantable defibrillators, pressure and volume sen-sors, and mobile cardiac telemetry have beendeveloped, greatly improving the ease and rapidityof diagnosis and treatment. New applications ofthese technologies are revealed on a regular basisand well-done studies have demonstrated theusefulness of these technologies to caregiversand patients. Expert consensus statements havealso been developed to guide clinicians on theuse of these technologies.1

TECHNOLOGY

The heart of any cardiac electronic monitoring sys-tem is the sensor and its associated electronic cir-cuitry. The simplest sensors for monitoring cardiacelectrical activity consist of tabs of silver-silverchloride with a conductive gel to reduce skinimpedance. These can be used to transduce heartrate as well as rhythm and morphology. Polymerversions of these electrodes have also been

ayo Clinic Florida, 4500 San Pablo Road, Davis 7,ne, Division of Cardiology, Saint Mary’s Hospital,6 2nd Street Southwest, Rochester, MN 55902, USA

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developed with adhesive backing and are alsoavailable in hypoallergenic versions, which canbe extremely helpful during long-term monitoringin patients with sensitive skin. These same elec-trodes may be used to monitor respiration by in-jecting small, high-frequency currents throughthe chest and measuring impedance changes,which correlate with respiration rate. Small piezo-electric crystals can also be used as motion sen-sors to detect patient activity level and cancorrelate activity with rhythm, a useful diagnostictool. All of these features may be integrated intoa single, composite, easy-to-apply sensor pack-age. A recent example of such a commercialdevice is the BodyGuardian Remote MonitoringSystem (Preventice, Minneapolis, Minnesota).Microelectromechanical systems (MEMS) tech-

nology has produced a remarkable array of silicon,polymer, ceramic, and metal transducers, whichcanmeasure acceleration (activity) as well as pres-sure in cardiac monitors. These devices can befabricated using lithography and etching tech-niques originally developed for semiconductormanufacturing and can be made exceedinglysmall. Bulk manufacturing also allows for inexpen-sive precalibrated transducers that can be used innoninvasive and invasive applications.2

Miniature thermistors (resistors whose resis-tance decreases in predictable fashion with in-creasing temperature) and thermocouples (metalalloy junctions whose voltage changes predictablywith changes in temperature) are used routinely fortemperature sensing in a variety of medical appli-cations also.Injecting small, high-frequency currents through

external chest electrodes or internal defibrillatorelectrodes can provide additional useful infor-mation on pulmonary congestion by monitoringchanges in absolute impedance. This techniquehas allowed for early warning in the recognitionand treatment of heart failure exacerbation, con-firmed by clinical studies.3,4

The outputs of these sensors (voltage, current,and impedance) need to be amplified, filtered,and processed before they can be interpreted toprovide useful information. The basic buildingblock for signal processing is the electronic instru-mentation amplifier, which provides high levels ofamplification (signal increase) while rejecting noise(unwanted signals) to a significant degree. Theamplified signal is then filtered, initially usinghardware-based filters to limit noise and motionartifact. The amplified, filtered signal is thendigitized with an analog-to-digital converter.5

The digital data may then be stored, manipulated,or transmitted using a variety of techniques(Fig. 1). Memory cost and density have improved

dramatically over the past 20 years, allowing forstorage of significant amounts of high-qualityphysiologic data locally on a medical device.The data may be transmitted back to the clinical

facility using radiofrequency energy from a pa-tient’s device, manually or automatically, usingthe Industrial, Scientific, and Medical band or thecellular Global System for Mobile Communicationsbands. The received data are then subjected tomore powerful digital signal processing tech-niques to clean up the signals and extract smallsignals from noise to allow accurate measure-ment. The measured data are then displayedeither graphically or in tabular form for medicalinterpretation.Increased understanding of cardiac physiology

coupled with the evolution of electronic and med-ical technology has produced a useful set of toolsto remotely monitor important cardiac parametersand deliver that information to appropriate clinicalstaff for prompt follow-up.

STATE OF THE ART IN REMOTE CARDIACPARAMETER MONITORING

Remote cardiac monitoring may be divided intoseveral areas: rhythm monitoring for diagnosticpurposes, pressure and volume monitoring toassess heart failure status, lead and deviceintegrity monitoring, and monitoring efficacy oftherapies, such as ablation procedures fordysrhythmias.

Rhythm Monitoring

Rhythm monitoring for diagnosis of palpitations,near-syncope, and syncope is excellent for corre-lating symptoms and rhythms in the nonmedicalsetting; 24-hour Holter monitors can provide infor-mation on heart rate range and averages, assessfor chronotropic incompetence, and inform onarrhythmia burden, also allowing monitoring oftreatment efficacy. Although the monitoring itselfis remote to themedical facility, the data are storedon board the device and may be downloaded afterthe required duration. Event monitors allow pa-tients to document symptoms andmonitor rhythmsfor up to 4 weeks, with periodic downloadsperformed by the patient through a telephoneconnection. Multilead event monitors, such as theIntelli-Heart monitor (Intelli-Heart Services, LosAngeles, California) can also measure QT intervalsbeat-by-beat for extended periods to assess fordynamic lengthening of QT intervals in suspectedlong-QT syndromes. Periodic monitoring withsuch devices is also used routinely by cliniciansto assess efficacy of therapeutic procedures,such as atrial fibrillation ablation, to determine if

Fig. 1. Block diagram representing a typical cardiac remote monitoring system. The cardiac sensor’s output(voltage, current, pressure, and impedance) is amplified. The amplitude may be adjusted dynamically by thesystem using an automatic gain control circuit before band-pass filtering the signal. It is converted from analogdata to digital information by the analog-digital converter. The digital data are transmitted using radiofrequencyenergy to the data storage system of the device manufacturer. These data are available to qualified health carepersonnel in device clinics via download using an Internet connection. They can then communicate via telephoneto the patient regarding symptoms and therapy.

State of the Art in Remote Monitoring Technology 367

patients are candidates for discontinuation of anti-coagulation. Mobile cardiac outpatient telemetrydevices can continuously transmit rhythm datafrom patients by wireless communication to a cen-tral monitoring station, where trained personnelanalyze and categorize rhythm abnormalities inreal time, alerting the appropriate clinical staff atthe ordering facility of any serious arrhythmias,such as ventricular tachycardia or prolonged sinuspauses.

Patient acceptance of external patches and thebulk of the attached electronic devices usuallylimit the use of such products to a few weeks ofmonitoring. In cases where symptomatic epi-sodes are infrequent (eg, syncopal spells everyfew months), implantable loop recorders havedemonstrated their usefulness unequivocally.6

These leadless devices, implanted with a quickand simple surgical procedure in the left chestwall, can remain implanted for more than 3 yearsand provide valuable information on symptom/rhythm correlation. Such devices are also increas-ingly used for diagnosing atrial fibrillation as thecause of some cases of cryptogenic stroke. Anongoing Spanish trial, CRYPTONITE, will becompleted in 2013.

Heart Failure Monitoring

Patient volume status and assessment of intra-cardiac pressures remotely have received intensescrutiny from heart failure specialists over the pastseveral years. Approximately 1 million heart failurehospitalizations occur in the United States annu-ally, with a 30-day readmission rate of 27%,7

placing an enormous burden on health care re-sources. Several early trials, including Telemoni-toring to Improve Heart Failure Outcomes8 andTelemedical Interventional Monitoring in HeartFailure,9 were not able to demonstrate a positiveimpact of telemonitoring on heart failure–relatedrehospitalizations or mortality. Dedicated intra-cardiac pressure monitors, such as the right ven-tricular pressure sensor in the Chronicle device(Medtronic, Minneapolis, Minnesota) and the leftatrial pressure sensor in HeartPOD (St. Jude Med-ical, Minneapolis, Minnesota), provide indirect ordirect assessment of left ventricular filling pres-sures. The initial results of the Chronicle OffersManagement to Patients with Advanced Signsand Symptoms of Heart Failure trial, which usedthe Chronicle device, did not find a significant dif-ference in heart failure events between the inter-vention and control groups. Subsequent analysis

Venkatachalam & Asirvatham368

suggested, however, that persistently high fillingpressures recorded by Chronicle, irrespective ofsymptoms, placed people at higher risk for hos-pitalization.10 An observational study, Hemody-namically Guided Home Self-Therapy in SevereHeart Failure Patients (HOMEOSTASIS), in NewYork Heart Association Class III and Class IV pa-tients, using the HeartPOD, had a lower risk ofacute decompensation or death.11 Accurate mea-surement of pulmonary artery pressure usinga MEMS sensor implanted in the pulmonary arteryshowed a 30% reduction in heart failure hospita-lizations in New York Heart Association Class IIIpatients.12 This study (CHAMPION [CardioMEMSHeart Sensor Allows Monitoring of Pressure toImprove Outcomes in NYHA Class III Patients])also showed that a comprehensive approach topatient management using the pressure data,with guidelines on medication adjustment pro-vided to clinicians, improved the chance of goodoutcomes in these patients. OptiVol (Medtronic,Minneapolis, Minnesota) technology measuresheart failure exacerbation by monitoring changesin lung impedance, which can alert clinicians toimpending episodes of heart failure, promptingearly treatment.13

Heart failure management systems have alsobeen developed (Latitude, Boston Scientific,St. Paul, Minnesota) that integrate daily weightmeasurement with daily blood pressure and symp-tom self-reports. The digital outputs of each of theseparate devices thatmake up this systemconnectto the Latitude communicator via a short-range ra-diofrequency communication system (Bluetooth),and the composite data can be retrieved by a pa-tient’s clinician to evaluate treatment efficacy andassess for impending heart failure exacerbation.

Device and Lead Integrity Monitoring

Despite the exceedingly stringent working envi-ronment of pacemaker and defibrillator leads(100,000 flexions per day for 20–30 years), devicemanufacturers have developed reliable systemsoverall. There have been a few instances, how-ever, of lead failures and lead integrity problemsthat have led to advisories as well as lead recalls,with a significant burden on patients and clini-cians. Algorithms to monitor lead integrity re-motely have now been incorporated into thedevice software of most implanted cardiacdevices, with the ability to alert patients as wellas physicians about potential lead problems trig-gered by excessive noise, significant impedancechanges, or large changes in sensed voltages.The implications of all of these data sent by pa-tients manually and automatically to pacemaker

clinics can be huge. Transmissions not requiringany action may be processed rapidly but clinicallyimportant transmissions can dramatically increasethe workload on any given day in a clinic.14,15

MONITORING EFFICACY OF THERAPY

Monitoring patients for extended periods afterablation for atrial fibrillation is a promising newarea for use of remote monitors, which could beexceptionally useful in determining long-term effi-cacy of various ablation procedures, the need forlong-term anticoagulation in selected patients,and correlation between nonatrial fibrillation-related symptoms and rhythms. Patients withpreexisting dual-chamber pacemakers or defibril-lators can continue routine monitoring of their de-vices and evaluation of atrial high rate episodes,which may point to atrial flutter or atrial fibrillationrecurrence. An alternative, not currently reim-bursed, is to implant a loop recorder for 3 yearsof data. A noninvasive approach involves externalcardiac monitors placed every few months for 7 to10 days to sample patient rhythms to detect silentepisodes of atrial fibrillation.Manufacturers of all implanted defibrillators and

some implantable pacemakers offer wirelessremote monitoring. Routine device parametersmeasured include battery status, lead integritychecks, capture and sensing, and arrhythmia epi-sodes. The data are automatically retrieved fromthe device by a home monitor that then transmitsthe information via a telephone connection (wiredor mobile) to the manufacturer’s servers. Thedata can then be downloaded via the Internet byindividual pacemaker clinic staff and displayedon commercial software such as Paceart (Med-tronic, Minneapolis, Minnesota). Patients mayalso activate a download based on symptoms,with requested callbacks from the appropriatepacemaker clinic. This has definitely improved pa-tient convenience although there has also beensome resistance to the use of this technology bypatients. Simplification of the communicationprocess (technology) and the realization that thisis an effective way to monitor their devices shouldincrease patient compliance.

THE FUTURE

Remote cardiac monitoring has entered anexciting phase in its development, with rapidtechnologic advance supported by increasingclinician and patient awareness and acceptance.As more wireless bandwidth becomes availableto the medical community, more useful datacan be downloaded quickly, improving perceived

State of the Art in Remote Monitoring Technology 369

convenience. Smartphone applications have al-ready been developed to allow patients to monitorpulse rate (with no additional hardware) or recordrhythms (with small data acquisition hardware at-tachments) and transmit the information to theirclinicians for rapid response to symptoms. Thisincreases the burden on clinicians, and mecha-nisms to handle the significant increase in clinicaldata need to be developed before it becomesoverwhelming. Compact hardware solutions toimprove the reliability of rate and rhythm moni-toring with sophisticated software algorithmsproviding immediate diagnoses with reasonableaccuracy are being developed and will be avail-able to general cardiac health care consumersin the near future. An exciting addition to thisdevelopment is the possibility of monitoring STsegment deviations associated with cardiacischemia in high-risk patients.16 This was testedin 2 pilot studies (DETECT and Cardiosaver) in37 patients with an implanted device, theAngelMed Guardian implantable ischemia detec-tion system (Angel Medical Systems, Shrewsbury,New Jersey), designed to measure ST segmentdeviations on a beat-by-beat basis and comparethem to a moving average. Over a mean follow-up of 1.5 years, 4 patients had ST segment shifts(7 events) related to heart rate changes, prompt-ing an alarm from the device, urging the patientsto be evaluated at a local emergency departmentfor true supply-related ischemic events. The meanalarm-to-door time was 26.5 minutes, substan-tially less than the 144 minutes observed inthe general ST elevation myocardial infarctionpopulation. This technology, when improved tominimize false alarms, has the potential to signifi-cantly reduce time-to-treatment in high-risk car-diac patients and could be incorporated intostandard implantable defibrillator designs in thenot-too-distant future.

Overall, the future looks bright for cardiacremote monitoring. Confirming that such technol-ogy, when used appropriately for patient manage-ment, can reduce health care costs in the long runwill also provide the impetus for reasonable finan-cial reimbursement. Clinicians deluged with theanticipated data from all this remote cardiac moni-toring will have to work out approaches to providesmooth workflow in device clinics to minimize frus-tration in patients and health care staff while effi-ciently handling the true clinical emergenciesrevealed by the monitoring.

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