Non-Invasive Cardiac Output Monitoring
An Israeli company has recently received FDA approval for a bedside non-invasive monitor that can give real-time cardiac output determinations. The system, called the Cheetah Reliant(tm) provides continuous tracking of cardiac output and other important measures of heart function including left ventricular ejection time, heart rate and cardiac contractility. Instead of having to place a Swan-Ganz catheter for invasive monitoring in the intensive care unit or cath lab, the unit relies on enhancement of of a bioimpedance measurement. Bioimpedance based measurements of cardiac output have been based on the relationship between transthoracic voltage and current measured between two electrodes placed on the patient’s chest wall.
While in theory this should work to determine fluid shifts internally, in practice, it has been difficult and unreliable to use for accurate real-time determinations of cardiac output. To overcome the difficulties, Cheetah Medical has developed a new approach using four pairs of double electrode stickers. In each pair, the upper electrode delivers a small alternating current that is sensed through its propagation characteristics by the lower electrodes. This technology increases the signal-to-noise ratio nearly 100 fold. The system then calculates a continuous, real-time cardiac output from the amount of thoracic fluid at any given point in time and the frequency shifts sensed through the electrodes. The validity of non-invasive bioimpedance techniques has been confirmed in early studies of the technology.
Studies have indicated that the Cheetah NICOM (Non-Invasive Cardiac Output Monitoring) is comparable in measurement to use of a Swan Ganz pulmonary artery catheter, but without the inherent difficulty of placement and attendant morbidity. Tests conducted ranged from 6 hours to 24 hours of continuous monitoring and confirmed the ability to detect sudden or gradual changes in cardiac output in a totally non-invasive manner.
The ability to extract important data from a patient non-invasively has been one of the the holy grails of medicine since the development of the x-ray and electrocardiogram were developed over a century ago. While we are still a long way from waving Star Trek’s Dr. McCoy’s wand over the patient and getting an instant readout of the patient’s diagnosis, the technology described may improve the safety and accuracy of patient evaluation, both in the ICU and outpatient settings.