CDU Contributing Editor

With the exploding interest in impedance cardiography (ICG) for the noninvasive tracking of patients' hemodynamic parameters, including cardiac output, stroke volume and systemic vascular resistance, market leader CardioDynamics International (San Diego, California) has seen its revenues soar. But this success also has allowed several companies that previously disappeared from this market niche to suddenly reappear, armed with "new and improved" ICG technologies. Whether any of these pose any sort of significant threat to CardioDynamics, the niche market leader, is questionable, but the challenge is nonetheless interesting.

Practically, it would seem to be hard for anyone to overtake or pose any formidable risk to CardioDynamics, which already has achieved not only the miniaturization of its product, but the implementation of it as an OEM module, which has been integrated into the Solar product line from GE Medical Systems Information Technologies (Milwaukee, Wisconsin). CardioDynamics is now working on a universal external OEM device that will also interface with monitoring companies that wish to keep ICG external. So companies offering only a standalone product may find that the market, particularly the OEM market of other monitoring equipment vendors, has moved beyond what they are offering. However, some think there is always room for a better mousetrap.

Even with CardioDynamics' impressive growth, the noninvasive hemodynamics diagnostic and monitoring market is enormous and barely penetrated. So there theoretically is plenty of room in the market for many companies if they can offer truly different and superior technology. One company making such claims is Sorba Medical Systems (Brookfield, Wisconsin). Sorba is planning to unveil its new Steorra ICG monitor at next month's annual meeting of the American Association for Respiratory Care (AARC; Dallas, Texas), in San Antonio, Texas.

Sorba is not an impedance cardiography newcomer. It has been in the ICG market space since 1988, when it introduced its first ICG product, the Model CIC-1000. That model required a cart full of equipment, and its sheer size, weight and cost made it inconvenient to use at the patient's bedside, particularly in the emergency department or the space-cramped intensive care unit setting. As a result, the company remained small and did not achieve critical mass in the marketplace. It did capture some share of the domestic research market, but not too much revenue. Sorba's small size and lack of marketing resources limited the market awareness of the CIC-1000. Its inability to shrink the product to a more portable and convenient package that could be carried from patient to patient also limited growth.

Impedance cardiography works by determining the stroke volume and then multiplying it by the patient's heart rate (HR). The technology available for determining heart rate for a wide range of patients is fairly mature and robust, but the ability to obtain precise and accurate stroke volume in the presence of physiological noise is another matter. That is important because any inaccuracy in the determination of SV will be multiplied by the HR, which can range from 50 beats per minute to more than 200 bpm during exercise or tachycardia events. This multiplication of the error in SV can make the final cardiac output calculation somewhat inaccurate. Fortunately, if the errors are consistent, so are the trends of calculated cardiac output, but that is a big "if." This technical issue about accuracy has limited the appeal of ICG as a replacement for invasive methods of determining cardiac output during the early years it was available.

Over the last few years, ICG companies have developed more sophisticated and accurate means of making the stroke volume measurement in subsequent generations of their products. As they did, and as clinical studies documented the improvements, and the full extent of the risks of the alternative invasive procedures became clearer, the market for ICG in the U.S. became viable.

Market interest came too late for companies like Renaissance, which went into bankruptcy, and caught other companies – including Sorba – between a product generation change and thus unprepared to complete with a small, portable product. Now Sorba has completed its next-generation product, the Steorra ICG monitor, which it will introduce at the AARC meeting. Steorra is nothing like its predecessor, the CIC-1000. Apart from the obvious physical differences of a truly portable package (in place of a cart) that weighs less than 10 pounds and can be supported on anything from an overbed table to a shelf or a bed mattress, Steorra also appears to have important engineering advances over the previous model.

The Steorra ICG is available with an optional NIBP module, or alternately can accept BP readings from a separate, external invasive blood pressure monitor. The addition of blood pressure increases the scope of the hemodynamic profile that can be derived for the patient. The key to the accuracy is the company's Ensemble Averaging approach.

The proof of the success of this approach is in the clinical results and data presented to the FDA. The accuracy of Steorra depends on Sorba's proprietary RTea technology. The data that Sorba presented to the FDA included validation against thermodilution cardiac output in patients undergoing the heart catheterization procedure or the repair of abdominal aortic aneurysm. Volunteers also were studied in a physiology research lab.

Among the early adopters of Sorba's new technology was James Leibsohn, MD, interventional cardiologist at St. Luke's Medical Center (Milwaukee, Wisconsin), who described the Sorba device as "an invaluable tool in the management of congestive heart failure in the clinical setting." With the Steorra ICG monitor, he said "we are now able to accurately determine peripheral resistance and cardiac output in an outpatient setting and, for the first time, adjust medications precisely on the basis of accurate hemodynamic measurements."

Richard Summers, MD, associate professor of emergency medicine at the University of Mississippi (Oxford, Mississippi), said invasive critical care methods, such as Swan-Ganz catheter placement, "are frequently impractical or too risky for routine use in the emergency department." But the Sorba noninvasive impedance cardiograph "provides an acceptable and safe alternative method for providing much of the needed information concerning the hemodynamic state of these emergency room patients."

ICG measurements also proved useful in research settings. Peter Hanson, MD, cardiologist at the University of Wisconsin Hospitals (Madison, Wisconsin) said he and his colleagues have found the Sorba system "particularly well suited to both research and clinical studies involving non-steady state activities, such as: isometric exercise, postural stress and evaluation of baroreflex function."

To be effective, reliable and reproducible, the technology of ICG must differentiate the cardiac related change in impedance from the basal transthoracic impedance change, and thereby rid the ICG signal of its respiratory artifact component. Muscle movement or artifact also must be removed or the small cardiac component is easily obliterated. That is where the Steorra ICG primarily differentiates itself from other ICG technologies.

The key is to use a technology that accurately identifies the segment of the ICG waveform that relates to the mechanical ejection (stroke volume) in an accurate, noise-insensitive way that is repeatable. Sorba uses "R-Wave Triggered, Ensemble Averaging" (RTea). The benefit of the RTea approach, according to Sorba President Jill Barney, is that its accuracy increases as a 4th power of the number of samples (cardiac cycles). RTea allows the Steorra to make accurate and reproducible measurements of stroke volume even in the presence of heart rate dysrhythmias and/or low stroke volumes, both of which can be challenging to the accuracy of other approaches to calculating ICG, according to Barney.

Ensemble averaging is a sophisticated mathematical technique that has been applied to a number of signal processing applications. Because it exponentially improves the signal-to-noise ratio of the data sample, it is critical to obtain accurate and reproducible measurement of stroke volume in the Steorra ICG device.

Prior to processing by the ensemble averager, the ICG data are "qualified" – that is, each waveform must meet acceptance criteria based on amplitude, duration and timing to be included in the ensemble. This process effectively removes aberrant, noisy data from further processing which might distort the ensermble average.

Sorba's niche and advantage, according to Barney, are that measurements of ejection time and peak change are much more accurate when determined on the ensemble averaged waveform than when the same points are determined on each individual waveforms and then averaged, simply because normal mathematical averaging does not improve the signal-to-noise ratio. In contrast to simple mathematical averaging, RTea does improve signal-to-noise significantly, so that Steorra can provide reliable and reproducible results that even in noisy environments or low cardiac output states, according to Sorba.

Will the novel technology and slick packaging of the Steorra be enough to attract market attention and earn Sorba a piece of the rapidly expanding impedance cardiography market? It depends on how much interest Sorba can muster for its new device, and whether it can ultimately offer a viable OEM board for suppliers to incorporate into their own monitoring systems. Sorba must identify companies currently designing their products and sell them on the solution. This is not an easy task, even if the technology is good. Another alternative would be for one of the monitoring companies with a broader product line to purchase Sorba, along with its new technology. If Sorba can make either of these things happen, it will bring some healthy competition to a market that is huge but has a miniscule current penetration.