Correlation of Intrathoracic Impedance Measure With Blood Plasma Volume in Congestive Heart Failure

Congestive heart failure (CHF) affects 5 million people in the United States, with an additional 550,000 cases diagnosed each year. Despite major recent strides in treatment, a diagnosis of CHF still carries significant annual morbidity and mortality. The resultant economic burden associated with the management of this disease has been estimated at $40 billion per year, with 60% - 75% resulting from inpatient management(1). Emergency room visits, frequent outpatient encounters and the hemodynamic monitoring and intensive care associated with inpatient management contribute significantly to this total cost.

Current outpatient heart failure management programs focus on lifestyle changes, pharmacotherapy, and disease awareness, emphasizing self identification of signs and symptoms of volume overload. Despite the best efforts of all involved, frequent unscheduled visits to the outpatient clinic or emergency department remain common, even in those patients participating in formal management programs. Accurate identification of elevated filling pressures and intravascular volume prior to the significant worsening of symptoms remains an elusive goal of management.

Although elevated jugular venous pressure and the presence of a third heart sound have been shown to have prognostic value in patients with heart failure(2), physical exam findings of elevated filling pressures do not reliably correlate with invasive hemodynamics(3).

Invasive measurement of filling pressures and cardiac output with a pulmonary artery catheter has been the gold standard for hemodynamic assessment for decades. However, this method carries significant inherent risks of placement and maintenance and is not well-suited for frequent assessment in the outpatient setting. Development of a method that would accurately and reliably identify hypervolemia and elevated filling pressures prior to the onset of symptoms may allow earlier intervention and have a significant impact on quality of life, morbidity, and mortality in patients with CHF.

Blood volume analysis has been used for over 100 years in the assessment of plasma volume and red cell mass. Earlier techniques relied on dilution of various dyes and were based on the concept that the concentration of a known quantity of dye was inversely proportional to the volume of blood into which it was injected. Later, using the same concept, 51Cr labeled red blood cells and 125I labeled albumen, were developed to measure the red cell and plasma compartments respectively. Although the dual isotope method is considered the gold standard, it is cumbersome and requires at least 6 hours for completion.