Cardiac Bulldozers, Backhoes, and Blood Pressure

Technological advancement often occurs through the continuous development of small, incremental changes that gradually improve a product. This slow, boring process is occasionally punctuated by the development of “disruptive” technologies as described in The Innovator’s Dilemma by Christensen. Disruptive technologies share some key features. Initially, they are often viewed as inferior products and as such are relegated to “alternative” uses. They may be (but are not always) less expensive than their competition. They are typically ignored by established manufacturers of traditional technology. Yet somehow, they ultimately come to dominate the market, completely wiping out their competition.1 Take, for instance, the development of hydraulic heavy machinery (e.g., bulldozers, excavators). For more than a century after the development of the steam shovel by William Smith Otis in 1837, virtually all excavators used a system of pulleys and cables to operate their “mechanical shovels.” Then, in 1947, a British company, J.C. Bamford, introduced the first-ever hydraulic excavator. Initially, these “backhoes” (as they were called) were small, weak, and had limited reach, making them completely useless to the mining and general excavation industries. Because of these shortcomings, they were marketed to residential contractors and farmers for small jobs, where their small size and affordability were advantages.1 Hydraulic backhoes were derided as cheap, inferior products and were ignored by the majority of established cable shovel makers. Yet they were widely adopted by smaller businesses, helped along by the housing booms that followed 2 wars (World War II, Korea). A virtuous cycle of adoption and continuous improvement led to exponential increases in capacity. By 1966, maximum bucket size had increased by 580%, and by 1973, it had increased by 5500%, far exceeding the needs of even general excavation contractors and leading to the complete dismantling of the heavy machinery industry.1 Despite over 100 years of continued success, only 4 established cable machinery companies ever produced a commercially viable hydraulic excavator, and only 1 of those companies (Link-Belt, Lexington, KY) is in existence today. In this issue of Anesthesia & Analgesia, Smolle et al.2 describe the accuracy of a continuous, noninvasive blood pressure (NIBP) monitor capable of recreating the arterial blood pressure tracing in addition to providing numerical estimates of systolic, diastolic, and mean arterial blood pressure. This device, the CNAP® (CNSystems, Graz, Austria), has all the features of disruptive technology. First, the CNAP, and its American counterpart, the ClearSight system (Edwards Lifesciences, Irvine, CA), will, from a measurement standpoint, always be viewed as inferior products. The availability of intraarterial catheterization means that it is possible to compare the CNAP and related devices with a true gold standard, which simplifies the interpretation of data and allows for meaningful comparisons between devices. This is in contrast to the cardiac output monitoring literature, which is now being littered by method comparison studies with increasing disregard for the concept of an experimental reference standard (or at least a clinical standard)3–5 and which suffers from declining use of what has become a controversial clinical standard.6–8 But this is a double-edged sword, although the CNAP can readily be compared with a reference standard, it will never be more accurate than the reference standard. Second, from the perspective of many payers (insurance companies), the CNAP (which is purchased by either the medical center or the practitioner) is less expensive than intraarterial catheterization, which is a time-consuming, billable procedure that also incurs risk. Third, the volume clamp technique on which both the CNAP and ClearSight systems are based has been in existence since 19679 and has, until recently, been ignored by large, established medical device manufactures. Although one could argue that early adoption was prohibited by inaccuracies, it is hard to imagine that it would take approximately 4 decades for these devices to come to market. On the basis of analysis of 7200 measurements points from 40 critically ill subjects, Smolle et al. found that the CNAP device predicts mean arterial blood pressure with a bias of 4.6 mm Hg and with 95% limits of agreement that range from −8.7 to 17.8 mm Hg. More impressive, trending analysis showed 92% concordance. These results are similar to those produced by Martina et al.,10 who demonstrated comparable limits of agreement in a study of the NexFin® (now ClearSight; BMEYE, Amsterdam, The Netherlands) system in 53 patients undergoing cardiac surgery. The study by Smolle et al. is an important advance in this growing body of data because of the acuity of the patients studied, the large number of measurements, and the trending analysis. Interested readers will likely question whether the CNAP is accurate enough for routine clinical use. The study by Smolle et al. alone will not answer that question. But consider the following: Wax et al.11 compared NIBP from sphygmomanometric readings with invasive arterial blood pressure readings from >15,000 noncardiac anesthetics and found the SD of the difference to be approximately ±12.5 mm Hg at a mean arterial blood pressure of 75 mm Hg (the mode of the distribution). Although the limits of agreement were not calculated, one can estimate (without taking into consideration repeated measures) that the confidence intervals of standard NIBP devices are approximately twice those demonstrated by Smolle et al. (CNAP) and Martina et al. (NexFin/ClearSight) in smaller studies. At the extremes, when accurate measurement of arterial blood pressure matters the most, NIBP performance worsened significantly.11 Note also that the volume clamp technique, which relies on finger cuffs, carries with it no risk of hematoma formation, pseudoaneurysm formation, infection, nerve injury, or permanent arterial occlusion, all of which are rare but some of which are potentially devastating complications of arterial catheterization.12–14 In addition, it can provide a continuous blood pressure tracing within minutes. Some may argue that clinical efficacy is more important than accuracy and prefer to wait for convincing clinical trials before replacing intraarterial blood pressure monitoring with the volume clamp technique. Considering that there are no prospective, randomized, controlled trials demonstrating the superiority of any particular mean arterial blood pressure goal in patients undergoing general anesthesia and the recent publication of the SEPSISPAM trial, which randomly assigned over 700 septic patients to mean arterial blood pressure goals of >65 vs >85 mm Hg and found no difference,15 it is hard to imagine a device trial demonstrating an outcomes difference when the scientific community has yet to prove that blood pressure even matters (this is not to suggest that it does not, only that it has not been demonstrated). So what is next? The CNAP device and other volume clamp monitors would benefit from increased testing, particularly focusing on additional patient populations that might benefit the most, those in whom continuous blood pressure monitoring would be beneficial but in whom arterial blood gases are not typically helpful (e.g., obstetric surgery, orthopedic surgery). More knowledge of how these devices perform in the extremes, as opposed to when blood pressure is normal, would be helpful, as would direct comparisons with automated oscillometric blood pressure cuffs using arterial catheterization as a standard. Finally, understanding why these devices do not work at all in a minority of patients (between the Martina et al.’s and Smolle et al.’s trials, 7 patients were excluded for technical reasons) may provide important insight into how they can be improved. It is only a matter of time until volume clamp devices replace many if not the majority of arterial catheters for the continuous measurement of blood pressure, arterial respiratory variation, and even noninvasive cardiac output monitoring. This technology will likely first gain acceptance in patients for whom the anesthesiologist constantly agonizes about how to monitor blood pressure, because catastrophes (e.g., obstetric bleeding, fat embolism) are rare, but a known, lurking threat. As it continues to improve, the technology will move on to patients in whom arterial catheterization is normally performed for blood pressure monitoring but for whom arterial blood gases are not necessary. Eventually, the decision about whether to continuously monitor blood pressure for every single patient will be a simple question of balancing the financial costs (because the risk to the patient is almost zero) against the perceived clinical benefits. Like the small, rickety tractor-mounted hydraulic excavators that fueled the growth of multinational companies such as John Deere (Moline, IL) and Caterpillar (Peoria, IL), volume clamp devices represent a class of important, highly disruptive devices that will permanently change anesthetic monitoring. DISCLOSURES Name: Robert H. Thiele, MD. Contribution: This author wrote the manuscript. Attestation: Robert H. Thiele approved the final manuscript. This manuscript was handled by: Maxime Cannesson, MD.