Using electrical impedance to monitor heart failure status
“Robert” has been hospitalized five times in the last 10 months. He has battled problems associated with his congenital heart defect since birth. Not a transplant candidate, Robert, in years past, always had found a way to manage medications so that he stayed out of the hospital. But now his chronic heart failure (CHF) has worsened and he feels out of control.
“Lisa,” a professional caregiver, has helped Robert maintain his health status since he came to the long-term living facility in their community. Recently the care team asked Lisa to use a new fluid status monitor that measures “Zo” to assess and help maintain Robert’s fluid status.
Zo, pronounced “Z-oh” or “Z-naught,” is a measure of electrical impedance, which is a very sensitive reflection of fluid. Imagine a lightning bolt. Lightning travels over water more quickly than it does over dry land because there is less impedance to the electrical current in a wet environment than in a dry one. Consequently, when fluid increases, Zo or electrical impedance decreases. When Zo rises, there is less fluid, indicating a drier environment or dehydration.
Zo has been measured in the body since the 1960s. Because of the amount of bioimpedance research, it’s known that healthy normal hydration ranges between 19 and 30 ohms.1 In 1996, Milzman identified that Zo of less than 15 ohms is equivalent to the amount of fluid seen as thoracic infiltrate on a chest x-ray. Because of this finding, Milzman termed Zo measurement as the “noninvasive chest x-ray.” In 1999, at Cleveland Clinic, the Milzman study was replicated with identical results.2
Zo has been measured since the early 1990s using impedance cardiography. Impedance cardiograph monitoring uses four impedance electrodes to create a cylinder of the thorax. The impedance changes in the chest are timed with the cardiac cycle measured via electrocardiogram.
Impedance cardiography offers a noninvasive method for measuring hemodynamic parameters typically obtained via a Swan Ganz catheter or during a heart catheterization. Parameters measured by impedance cardiography provide a suite of hemodynamic values that gauge:
- Contractility or “pump power” of the heart
- Timing, or how long it takes for the left ventricle to fill and push out blood
- Afterload, or the amount of force that the heart is required to pump blood to the peripheral vasculature and back to the heart
- Fluid volume, or Zo
Today, maintaining fluid balance is a key clinical problem for many residents, not just those living with heart failure. In the past, fluid levels have been monitored using very subjective assessment criteria, such as skin turgor, urine color, peripheral edema and abdominal ascites.
Objective fluid assessment criteria such as hematocrit, urine-specific gravity, urine protein levels, weight and lung sounds are often impractical, especially in home and long-term care settings. Many of these fluid assessment parameters also are late indicators of fluid changes. Frequently, weight does not increase in heart failure patients until pulmonary crackles are auscultated. While these assessment parameters facilitate clinical management, their later indication of fluid changes requires a reactive care approach rather than a proactive approach to care.3
Zo measurement is so sensitive to fluid changes that it has been shown to change as early as 12 days prior to weight gain and symptom development in congestive heart failure. Zo’s early indication of fluid changes was illustrated in a 1998 research study.4 Clinicians were blinded to the hemodynamic data; consequently, nurses were unaware that the study patient was experiencing fluid retention as indicated by decreasing Zo.
Zo monitoring also can be used to detect dehydration. Long-term care facilities can be affected by virus outbreaks (for example, the Norovirus), that can cause rapid loss of gastric fluid. Monitoring individuals for dehydration levels is a quick, objective assessment to identify the amount of care needed to reduce hospital or emergency room visits.
Zo can be measured noninvasively using the bioimpedance method, or invasively. In a study conducted at the Queen Mary and Prince of Wales hospitals in Hong Kong, invasive Zo was measured in a blinded study for more than eight months. Eleven patients had a total of 26 hospitalizations for worsening heart failure, with 25 of the admissions caused by congestion and one caused by dehydration.
Zo retrospectively was found to decrease prior to admission over a mean of 11.4 days by 5.9 percent. Zo was found to decrease an average of 7.9 days prior to symptom development. Zo increased by 8 percent three days prior to hospitalization for dehydration. Conclusions of this study state that Zo correlates well with pulmonary wedge pressure and predicts CHF hospitalizations with a high sensitivity.5
By using a noninvasive Zo measurement device, the care team established that Robert’s average Zo was 23-26 ohms. Robert discovered that when he ate salty foods, his Zo decreased by two to four ohms. Because of the quick targeted measurement feedback offered by Zo, Robert became more committed to diet changes.
One day, Robert’s Zo had dropped to 21 ohms. The next day, it was 20 ohms. Robert’s physician was notified. A one-time additional dose of oral diuretic was ordered. One day later, Robert’s Zo had returned to 23 ohms, avoiding an exacerbation and possible trip to the ER.
Noninvasive Zo measurement has been used in clinical practice for several years. In the past, Zo has been typically measured in critical care settings. However, new technology in fluid status monitors allows for the measurement of a single Zo value as opposed to performing an entire hemodynamic profile. This enables home healthcare and long-term care personnel to monitor fluid status objectively and efficiently. Zo measurement provides an assessment tool for clinicians and patients that allow proactive intervention to avoid exacerbation due to fluid changes.
- Visser, D., Lambert (1991) Impedance Cardiography. New York: Springer. Chapter 2, 56-95.
- Peacock, F., Albert, N., White, R. (2000) Bioimpedance monitoring: better than chest xray for predicting abnormal pulmonary fluid? CHF. March/April, 32-35.
- Frantz, A. 2004. Breaking down the barriers to heart failure patient self care. Home Healthcare Nurse Journal, 22(2), 109-114.
- Milzman, D., Hogan C., Han C. (1998) Thoracic base impedance improves the diagnosis of acute congestive heart failure in the emergency department. Annals of Emergency Medicine. Vol.25: 1376-1381.
- Chuen-Man Yu, Li Wang, Elaine Chau, Raymond Chan, Shun-Ling Kong, Robert W. Stadler, Chu-Pak Lau. (2004) Device-Based intra-thoracic impedance correlates with fluid status and provides automated prediction of CHF hospitalization. Poster Presentation 354. Society American Emergency Medicine. September 13, 2004.
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