The OptiVol Fluid Index and average daily impedance measurements are shown in Figure 2.
The index is reset if the daily impedance readings become higher than the impedance trend, thus reducing the risk of an alarm when the lungs are drying out. The physician can activate the patient alert in the device and when a threshold is crossed the device will audibly alarm. In other words, as the lungs become wetter or stay wetter the index increases. The OptiVol Fluid Index™ represents the accumulation of consecutive day-to-day differences between the daily and reference impedance. The device can be interrogated in detail by a technician, or at a more basic level by either the patient or physician using a simple hand-held device. This measurement is made multiple times each day and recorded for up to 14 months. Transthoracic impedance is measured between the coil of the right ventricular lead and the can of the cardiac resynchronisation therapy (CRT) device or implanted cardioverter defibrillator (ICD) (see Figure 1). It is now possible to add the facility to monitor intrathoracic impedance to either cardiac resynchronisation devices or implantable cardioverter defibrillators using OptiVol™ technology, which is unique to Medtronic. Intrathoracic impedance falls as the amount of fluid in the lungs increases, due to fluid being a good conductor of electrical current, 7-9 raising the possibility that this technology may be clinically useful in providing an early warning of decompensation. Implantable Intrathoracic Impedance Monitoring Various, more sophisticated, methods of assessing heart failure control have been proposed, including serial measurement of B-type natriuretic peptide, 5 implantable haemodynamic monitors 6 and implantable intrathoracic impedance monitors. Such measures are far from perfect - in chronic heart failure many patients may not have marked clinical signs or symptoms, despite high and possibly rising pulmonary capillary wedge pressures. Physicians rely on the subjective assessment of exercise tolerance and breathlessness, changes in body weight and clinical examination to detect increasing dependent oedema or lung crackles. Unfortunately, the monitoring of heart failure syndrome control in clinical practice is usually relatively unsophisticated. 2 Recent efforts to reduce the risk of re-hospitalisation have largely focused on multidisciplinary disease management teams working with patients to ensure adherence to medication and the early detection of signs of decompensation, particularly in the early period after hospital discharge. In-hospital mortality was reported as 6.9% and by 12 weeks the mortality had risen to 13.5%, with a risk of re-hospitalisation of 24%. 2 Such admissions are usually long, with an average duration of 11 days. 1 A pan-European survey has shown that up to 65% of patients hospitalised with heart failure have a past history of chronic heart failure. Management of heart failure consumes 1% to 2% of the healthcare budget in developed countries, with the largest share due to the cost of hospitalisations.