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Ann Thorac Surg 1993;55:1534-1539
© 1993 The Society of Thoracic Surgeons

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Articles

Factors confounding impedance catheter volume measurements in vitro

Department of Surgery, Columbia University, College of Physicians and Surgeons, New York, New York USA

Accepted for publication October 5, 1992.

^_* Address reprint requests to Dr Spotnitz, Department of Surgery, Columbia University, College of Physicians and Surgeons, 630 W 168th St, New York, NY 10032.

The impedance catheter allows continuous measurement of ventricular volume. External influences have been described as causing parallel shifts in impedance-measured volumes; however, factors affecting impedance measurements in a nonparallel manner have not been fully characterized. Accordingly, an impedance catheter was placed inside a latex balloon into which known volumes of normal saline solution were injected. Conductive and noncenductive materials were individually placed within the balloon. Impedance was measured with materials touching (T) or not touching (NT) the catheter. Impedance-measured volumes were plotted versus actual volumes. Compared with the line of identity (LID), a statistical difference (p < 0.05) was found in the slopes in the presence of metallic objects only. These included a pacing lead (T, NT) (m_T = 1.32, m_NT = 1.29 versus m_LID = 1.00), titanium (T) (m_T = 1 68 versus m_LID = 1.00), and aluminum (NT) (m_NT = 0.72 versus m_LID = 1.00). These changes in slope indicate nonparallel effects on impedance that confound the ability of the impedance catheter to determine volumes in vitro. These observations imply that serial calibration of both the slope constant (α) and the intercept (parallel conductance) of impedance may be necessary for in vivo measurements of ventricular volume based on impedance in the presence of metallic objects.

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