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Implementation Case Studies Coronary Artery Bypass Grafts: Six Sigma Breakthrough

Coronary Artery Bypass Grafts: Six Sigma Breakthrough

Six Sigma, 6s, is nothing more, and nothing less, than the use of science to solve problems. Scientific evidence that a breakthrough medical solution is genuine is statistical evidence. Though altruism and evidence influence medical treatments, economic pressure drives improvement. Multi-million dollar savings created by “beating heart” or “off-pump” coronary artery bypass outcomes are a symbolic case in point.

Historically speaking, medical “Six Sigma” style breakthroughs have astonished the world. Near zero death rates related to surgical anesthesia and the polio vaccine’s safety record are but two near perfect success examples. Sir Austin Bradford Hill’s 1951 sentiments sound as fresh as a 21st Century General Electric Six Sigma news release.

“In treating patients with unproved remedies we are, whether we like it or not, experimenting on human beings, and a good experiment well reported may be more ethical and entail less shirking of duty than a poor one.” (Br. Med 2:1088-90, 1951, Hill, 1952).

The ability to consistently replicate experimental outcomes with a high degree of confidence is of paramount importance to everyone in the health care system. Again, off-pump surgical technique provides an ideal compass setting that points the way for current and experimental breakthroughs. Since health care Six Sigma breakthroughs simultaneously improve the profitability, “off-pump” coronary artery bypass grafts (CABG) are as substantive as they are symbolic.

Limited financial resources fostered the early 1980’s development of “beating heart” CABG surgeries in Argentina. Compelling statistical evidence is leading to the reluctant, gradual acceptance of this surgical technique in competitive 2002 health care markets. Patient demand for this lower cost, higher quality procedure has forced, and is forcing, surgeons to master it as a standard of care rather than an experimental method that has yet to be proved.

The classic Six Sigma DMAIC (Define, Measure, Analyze, Improve, and Control) cycle provides a convenient way to summarize this story.


For over 40 years, the use of cardiopulmonary bypass (CPB) pumps defined coronary artery bypass grafting (CABG) procedures.1 Good outcomes and the relative ease of working on an arrested heart led most cardiac surgeons to favor the use of CPB. Adverse and statistically significant blood utilization and neurological side effects associated with on-pump surgeries were accepted.

Though evidence suggested off-pump was safe and advantageous for select patients, the prevailing paradigms of cardiac surgery sustained physician commitment to the on-pump surgical technique. It has taken a decade for surgical practice patterns to emerge that reflected sentiments expressed by researchers in 1992. “Further research should be directed to which subgroups can be operated on to advantage “off-pump” and which, if any, groups of patients should be confined to on-bypass operations.”2

Patterns and pattern identification are key elements in the identification of breakthrough improvements. Database and computing systems accelerate identification when they are included in a closed system feedback loop. Figure 1 illustrates the classic feedback system. Obviously doctors, nurses, allied health professionals, and administrative leaders are the Six Sigma “executives, champions, and Master Black Belt” experts who initiate breakthrough improvement actions.

Figure 1: Closed Loop Feedback System for Six Sigma

Figure 1: Closed Loop Feedback System for Six Sigma

In addition to quantitative feedback measures, qualitative impressions frequently expose opportunities. In the off-pump/on-pump dialogue, one qualitative signal is the long running practice of opinionated debating. Without a commitment to Six Sigma analysis, these are generally sustained without referencing or generating statistical evidence for analysis.

Measure and Analyze

Though surgical practice data is often collected by hand, increasingly this data is automatically entered into databases. Integrated statistical software packages now make it possible to analyze measurement data almost as quickly as they are recorded. Figure 2 shows columns and rows of data for a single cardiac surgeon who, after a number of his patients canceled their scheduled on-pump surgeries in order to have them performed “off-pump” by a different surgeon at a competing hospital, decided to master the skill.

Figure 2: Sample Database

Figure 2: Sample Database

The computerized analysis of length of stay data from this spreadsheet, Figure 3, reflects similar findings contained in 443 peer-reviewed articles published on the on-pump/off-pump subject since 1992. Patients who undergo “off-pump” CABG surgeries experience dramatically lower lengths of stay. Computerized health science library searches can help confirm or refute local data analysis results. This double check can be completed in only a few hours. Literature searches used to cross check inferences are a value added service physicians appreciate.

Figure 3: Lengths of Stay Differences Between On-pump and "Off-pump" Surgeries

Figure 3: Lengths of Stay Differences Between On-pump and "Off-pump" Surgeries

A quality control chart, figure 4, provides another view of the impact off-pump surgical technique brings to the quality of patient care. As the average length of stay shrinks, so does variation around the mean.

Figure 4: Reductions in Patient Lengths of Stay Related to Off-pump CABG Surgeries

Figure 4: Reductions in Patient Lengths of Stay Related to Off-pump CABG Surgeries

The surgeon’s database was stratified to facilitate a 3-dimensional statistical analysis to consider the effect a number of other factors might have had on length of stay outcomes. An example is cube plot shown in Figure 5. The Cartesian coordinate system’s cube is an ideal graphic for presenting multidimensional statistical evidence. The numbers contained in the rectangular boxes at the cube’s corners are average values. Even a novice can interpret the results at a glance.

All four of the shortest lengths of stay related to CABG are located on the cube’s left plane. The shortest average length of stay, 1.875, was a result of an off-pump surgery with a male patient with ICD code 36.11. All of the longer lengths of stay are located on the cube’s right plane. The longest average length of stay, 6.875, was the effect of on pump surgeries for men with ICD code 36.12. Though three factors are presented simultaneously, the only statistically significant factor related to a lower length of stay is a surgery performed “off-pump”.  In comparing the surgeon against himself, “off-pump” surgeries had a statistically significant lower length of stay at the 95% level of confidence. Even when comorbidities, patient age, and other confounding variables were considered, “off-pump” surgeries surfaced as the main effect responsible for shorter lengths of stay.

Figure 5: Comparing Surgeon Against Himself

Figure 5: Comparing Surgeon Against Himself


Sixteen years of experience in promoting breakthrough improvements in health care quality and productivity teach an important lesson. Before changes occur in physician or hospital practice, benefits must be translated into a compelling financial story. Though this reality can be disheartening for caregivers who put patient safety first, leaders must prioritize accounting if they expect to see system wide improvements take place.

Simulation modeling using spreadsheet applications are relatively easy tools to master. The psychological impact of seeing 1,000 or more iterations of multivariate spreadsheet practice scenarios in a matter of moments is significant. More often than not, spreadsheet simulations are persuasive.

Figure 6 shows the variation in financial impact one surgeon can have on a hospital’s CABG income. The low end of the distribution suggests that by mastering the “off-pump” procedure for the majority of his patients, an additional $448,000 in revenue would be generated. On the high end of the distribution, this change could produce as much as $1.45 million.

Figure 6: Value of Spreadsheet Simulation Tools

Figure 6: Value of Spreadsheet Simulation Tools

Actual results fell near the center of the prediction parameters. Savings were achieved through lower nursing care costs and overhead. “off-pump” patients avoided adverse side effects while the hospital enjoyed improved profitability. These results are classic hallmarks of a Six Sigma style breakthrough.


The final step in the Six Sigma DMAIC process is to standardize breakthroughs and hold the gains. Discipline is as important to success here as it is with each of the other steps.

Leadership and culture determine the rate of adoption for breakthroughs in productivity and quality. When senior leaders are disciplined and when they role model the use of science, statistical analysis, and systematic experimentation breakthrough improvements occur. Six Sigma culture evolves along with the breakthroughs. The degree of success in every Six Sigma breakthrough is directly related to the level of commitment that is demonstrated by senior leadership.


1. Pfister, Albert J., Zaki, M. Salah, et al. Coronary Artery Bypass without Cardiopulmonary Bypass. Ann of Thorac Surg 1992; 54:1085-92.
2. Pfister, Albert J., Zaki, M. Salah, et al. Coronary Artery Bypass without Cardiopulmonary Bypass. Ann of Thorac Surg 1992; 54:1085-92.

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