The practices of cardiac electrophysiology and heart failure management intersect more closely each day. The newest way in which this occurs is through the implantation of devices to monitor heart failure status. These devices may be stand-alone devices that monitor fluid status, or devices which offer biventricular pacing and defibrillation combined with monitoring capabilities. Because the EP team may be involved with both implantation and follow-up of these devices, an understanding of their function and capabilities is essential.

Magnitude of the Problem
       The rush to develop methods to monitor for the progression of heart failure has occurred for a number of reasons. First, there are about 1 million hospitalizations per year for heart failure in the United States, making this diagnosis the number one cause of hospital admissions. Secondly, the economic impact of heart failure is estimated to be about $28 billion annually.¹ As the population ages, the financial clout of this one diagnosis on our health care budget will be devastating. The implantation of devices to help monitor and serve as a warning system of imminent problems is being seen as a desirable approach for dealing with this escalating problem. A discussion of devices utilizing intracardiac pressure, impedance monitoring, and heart rate variability follows.

Intracardiac Pressure (Hemodynamic Monitoring)
       The intuitively ideal way to measure pressures in the heart would be directly and continuously. The development of an implantable hemodynamic monitor with a right ventricular lead and generator began in the 1990s and has resulted in multiple clinical trials.
       Data from these implanted devices and clinical data have been collected simultaneously in order to determine a correlation between the data and patient symptomatology. This information subsequently has been utilized to assist in medical treatment decisions, utilizing the devices as a ‘warning’ system for a worsening condition.²
       The results of the COMPASS-HF study³ were reported this year at the American College of Cardiology meeting. COMPASS-HF was a randomized trial examining the use of a continuous ambulatory intracardiac pressure monitor. The Chronicle IHM (implantable hemodynamic monitor) (Medtronic, Inc.) was placed in the upper chest and attached to a lead placed in the right ventricle. Body temperature, intracardiac pressure, patient activity, and heart rate were monitored. Patients were followed for a minimum of six months. Data was transmitted from home via a remote monitor, and health care providers were able to access the information on a website. Hospitalizations were reduced by 41% when pre-study data was compared to that occurring after the study began. Thus, the use of information from this device led to altering of care and averting a large number of hospitalizations. The Chronicle is awaiting FDA approval.

Figure 1.

OptiVol™ Fluid Trends Report (used with permission from Medtronic, Inc.).

Impedance Monitoring
       Impedance is the amount of resistance or opposition to flow of electrical current. A low impedance means that current delivery occurs more easily, while high impedance suggests more difficulty. Since fluids conduct electricity more easily than solids, a decrease in impedance would be found with a greater fluid presence. Applying this theory to the lungs, studies have demonstrated that fluid accumulation in the lungs leads to decreased intra-thoracic impedance and increased conductivity. Measures of LVEDP have been found to correlate inversely with impedance values.^4–8 Data from impedance monitoring has been used to assist in predicting CHF hospitalizations.^9–12
       Transthoracic impedance monitoring involves the emission of low electrical pulses across the thoracic cavity, utilizing the vector between an implanted RV lead and a pulse generator. The device then measures the level of resistance to the pulses.
       A variety of factors have been identified which may impact on the measure of transthoracic impedance.¹³ Reduced impedance is found with lung resistivity, most usually caused by congestion or fluid buildup; tissue mass (chamber dilation and venous congestion); and skeletal tissue and/or pacemaker pocket fluid buildup. Factors increasing impedance include lung resistivity related to reduced pulmonary function; lymphatic drainage which shunts excess fluid from the lungs; fluid redistribution due to the shunting of fluid in the systemic circulation in biventricular failure; and increased muscle resistivity in the area around the pacemaker pocket. In consideration of these various factors, the data from transthoracic impedance monitoring is trended to assess changes with each individual patient.
       In November of 2004, the FDA approved the InSync Sentry™ (Medtronic, Inc.) following results from the MIDHeFT and FAST studies. The Medtronic InSync Sentry is a combined ICD and cardiac resynchronization device which contains the OptiVol™ Fluid Status Monitoring system. This system utilizes intrathoracic impedance measurements to track thoracic fluid status. In addition, a Heart Failure Management Report with 14-month trends of AT/AF Burden, ventricular rate during AF, heart rate variability, patient activity and night/day heart rates is provided. An example of the portion of the Heart Failure Management Report dealing with OptiVol™ Fluid Trends can be seen in Figure 1. Note that the physician is able to select a threshold for notification of decreased impedance for each patient based on their stored values. In Figure 1, the fluid index is shown to be rising as the impedance level drops.
       The reliability of impedance as a monitoring method has been reported in just limited trials. Therefore, in April 2005, Medtronic announced the initiation of several worldwide studies to gather information to further validate this technology.¹⁴ These studies include:
• SENSE-HF (Sensitivity of the InSync Sentry OptiVol Feature for the Prediction of Heart Failure): a prospective study to be conducted in 40 European institutions and 40 US institutions, evaluating sensitivity and predictive value of OptiVol fluid status data;
• FAST Expansion (an extension of FAST): gathering additional data on the OptiVol algorithm in 20 centers worldwide. Some of the patients in this study will receive the ICD without CRT.
• PARTNERS HF (Program to Access and Review Trending Information and Evaluate Corelation to Symptoms in Patients with Heart Failure): a current US study evaluating InSync Sentry in clinical practice.
• European Observational InSync Sentry Study: a European study, collecting data from InSync Sentry patients assessing the value of the OptiVol diagnostic capability.

       Various methods to access the implanted device data are being developed. These methods include use of the internet and wireless transmissions, eliminating the need for the patient to actively transmit information.

Heart Rate Variability
       Heart rate variability (HRV) is the beat-to-beat variation found in heart rate. A reduction in HRV is a marker for reduced vagal activity. This means greater sympathetic influence is being exerted, a situation which is associated with increased risk for MI and sudden cardiac death. Heart rate variability is an indirect indicator of autonomic status, which in the past has been used to capture short-term data. HRV software is incorporated into many of the newer cardiac resynchronization devices developed by the major device companies. This advance has allowed the collection of long-term HRV data collection from the heart failure population.
       A study reported in 2004 included information from 370 patients with implanted cardiac resynchronization devices which contained HRV monitoring capabilities.¹⁵ HRV was lower in patients with high mortality and hospitalization risk. The authors suggested that long-term monitoring of continuous HRV could be a useful tool. Use of HRV with a resynchronization device may be superior because the HRV is calculated from atrial-sensed activity. This makes it possible to distinguish non-sinus aberrancies. Data from this study showed that HRV was stable until days before a hospitalization occurred. It was proposed that the neurohormonal system responds to detected changes before the patient actually experiences symptoms, therefore suggesting that HRV data may be useful for predicting a worsening condition and possibly preventing a hospital admission.

Table 1.

- Data Available from Implantable Devices for Assistance in Heart Failure Management: • Daily heart rate variability • Average heart rate for day and night hours • Patient activity level (captured by use of activity sensor) • Fluid status monitoring

Conclusion
       The development of technologies to assist in monitoring heart failure patients is continuing to evolve. Not only do these devices offer insights into the complex interaction of physiologic phenomena, but they also provide more tools to combat the progression of disease.