Noninvasive Measurement of CO Using Impedance Cardiography in Patients With CHD

This study will test the capability of a non-invasive instrument (the PhysioFlow impedance cardiography instrument) to measure cardiac output in patients with congenital heart disease (CHD). This instrument works by placing electrodes on the skin of a patient and measuring electrical impedance through the chest, which is proportional to blood volume and blood flow at any given time. The instrument has been validated in patients with structurally normal hearts, but in the only two studies using it for patients with CHD, it was deemed too inaccurate for clinical use. The manufacturer of the device would require access to data on the patients in order to improve its accuracy, and that has not been feasible thus far. This study would begin by comparing cardiac output based on the PhysioFlow monitor to standard techniques, then after possible changes to the instrument to enhance accuracy, would test the instrument again in the same way.

The measurement of cardiac output (CO) is important for a wide variety of patients under multiple conditions. At present, gold standard techniques for measurement of CO include velocity encoded phase contrast MRI. Both of these techniques are valid in patients with structural CHD. Disadvantages of these techniques include the requirement of significant time and expertise, and the high cost and anesthesia requirement. Accurate, non-invasive tools to measure CO could be extremely valuable for patients with CHD that require one-time of continuous monitoring of CO, such as during surgery, in intensive care settings, during other diagnostic testing, and during different physiologic states such as sleep and exercise. This study would begin by comparing cardiac output based on the PhysioFlow monitor to cardiac outout by MRI. Impedance cardiography is performed by placing electrodes on the thorax and neck to measure electrical impedance over time. Cardiac output is derived from these measurements. The technique has been studied, validated, and used extensively in adults and has also been shown to be valid in children with structurally normal hearts. In contrast, recent studies of children with CHD have shown a relatively poor agreement between impedance cardiography and both thermodilution technique and velocity encoded phase contrast MRI. The reasons for the poor agreement in CHD patients is not yet understood. There are proprietary algorithms in each impedance device that use the raw data to calculate and report cardiac output. If the physiologic and/or anatomic differences of patients with CHD require changing the algorithm within each system to account for such differences, that needs to be done to each system. In order to do this, owners of any given device (PhysioFlow, NeuMeDx Inc. in this case) need access to the raw data in order to alter the algorithm. This study will involve a concerted effort between investigators at RCHSD, UCSD, and NeuMeDx