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This calendar year, 2013, marks the 50th anniversary of the publication of an article which, unbeknown to the authors, was the first report of what would become one of the most widely used and researched tests in clinical cardiology throughout the world during the 20th century—the Bruce protocol exercise treadmill test (ETT).1 This editorial describes the historical background to the introduction of the ETT, the rationale for a multistage protocol, the first account of the ‘Bruce protocol’ and provides a brief discussion on the late Professor Robert Bruce himself.
Although it had long been known that evaluating a symptomatic patient during exertion might disclose evidence of coronary artery disease, before the advent of the Bruce treadmill test there was no safe, standardised and validated stress protocol that could be used to monitor cardiovascular haemodynamic changes in exercising patients. Master's two-step test,2 a submaximal exercise test for diagnosing ‘coronary insufficiency’ (in which ECGs were recorded during and after a patient repeatedly ascended and descended two steps) was often used, but it was too strenuous for some patients, inadequate for simultaneous assessment of cardiac and respiratory function during exercise and, measured by the energy expenditure per unit of weight, stressed the underweight patient considerably more than the overweight patient.1 Submaximal stress tests were the preferred stress technique; the concept of testing patients to their maximum exercise capacity was almost unthinkable, owing to the perceived risk of complications, a perception reinforced by reports of major complications even during submaximal stress testing.3
Nonetheless, Bruce recognised that many patients were not reliably or reproducibly stressed. Consequently, he tested initially a single-stage treadmill test in thousands of normal volunteers and subsequently, in cardiac patients, proving its feasibility, safety and reproducibility.4 This stress was only moderate for normal subjects and cardiac patients with mild disease, but was maximal for severely affected cardiac patients. He was among the first to identify the prognostic importance of exertion-induced hypotension, using catheterisation studies to demonstrate that the underlying mechanism was a fixed stroke volume (due to significant cardiac compromise), often secondary to critical coronary artery disease or severe cardiomyopathy.5
Bruce still realised, however, that the single-stage methodology did not have the potential to stress all patients maximally. Having demonstrated the safety of a maximal single-stage stress test, Bruce suggested that ‘the principle of stressing more strenuously the patients with mildly impaired cardiac disease, whose working capacity was most difficult to appraise, became justifiable’.1 Bruce then set about developing a multistage treadmill test in which each individual could attain a self-determined point of maximal exertion.
The Bruce protocol treadmill test
In 1962, the American Academy of Pediatrics organised a symposium on exercise fitness tests owing to concerns about the poor physical fitness of America's youth. In view of his initial work on the single-stage treadmill test, Dr Bruce was an invited participant.6 As a result, Bruce and colleagues published their research—the first report of a multistage treadmill protocol—in a special supplement of Pediatrics in 1963.1 They assessed normal subjects and cardiac patients and demonstrated the feasibility, safety and potential clinical utility of their multistage protocol, which would in time evolve into the modern seven-stage ‘Bruce protocol’. Each stage lasted 3 min, after which the treadmill speed and inclination was increased. He chose 3 min stages as this provided ‘the optimal compromise between requirements for physiological adaptations and minimal time for expeditious testing’.1
The original protocol comprised four stages: stage 1 started at a speed of 1.7 mph and a 10% inclination, stage 2 had a speed of 3.4 mph and 14% inclination, stage 3 had a speed of 5.0 mph and 18% inclination and stage 4 had a speed of 6.0 mph at 22% inclination to continue until exhaustion. By this means, all patients started with a gentle, submaximal stress and then gradually progressed to increasing workloads. He described results from almost 300 subjects, showing that the majority of normal subjects could reach stage 3 but very few patients could do so. This test incorporated continuous monitoring of heart rate, blood pressure and the ECG and also used a face mask and valve system to collect expired air and assess maximal oxygen intake.
Bruce believed in assessing not just the haemodynamic changes on exertion but also the workload and exercise ability of the individual, and was among the first to promote the concept of ‘functional aerobic exercise capacity.’ Indeed, Bruce deemed this and the blood pressure response to exercise as equally important as ECG and heart rate changes that occurred during the test. This study established many facts that are still true today—that the average maximal pulse rate was lower in older normal subjects than in younger normal subjects, that minor degrees of ST depression occurred at maximal exertion in normal individuals without significant coronary disease, that patients can develop ventricular arrhythmias during recovery as well as on exertion and that patients with ventricular aneurysms can develop ST segment elevation during exertion.1 The last two points were used to highlight the importance of continuous supervision by an experienced doctor, an aspect of the test often underemphasised today. In later years a gentler, ‘modified Bruce protocol’ was used for determining functional capacity and prognosis in, among others, survivors of myocardial infarction.7
Why the >85% age-predicted maximum heart rate cut-off point?
In numerous textbooks published in the late 20th century, a generation of medical students was taught that people performing ETT must achieve 85% of their age-predicted maximum heart rate in order to have worked at an adequate stress. Indeed, in many cardiology departments around the world, it was (and still is) commonplace for an ETT to be terminated once 85% of the predicted maximum heart rate has been achieved, irrespective of symptoms status or degree of fatigue.8 One may wonder where such a precise figure came from; in fact, this cut-off point was arbitrarily chosen—not by Bruce himself—as there remained concern about imposing maximal stress upon patients. Thus, submaximal stress tests were considered by many to be safer than continuing to the point at which the patient could physically exercise no more, as it was feared this would increase the risk of major complications, including ventricular arrhythmias, myocardial infarction and even, cardiac arrest.
This point merits special attention because the introduction of this cut-off point was completely at odds with the purpose of the multistage ETT as envisaged by Bruce, who believed in symptom-limited stress testing in order to study a number of physiological and haemodynamic parameters during exercise, not just the heart rate. To Bruce's relief, the practice of rate-limited rather than symptom-limited treadmill testing was explicitly discouraged in subsequent recommendations on exercise testing.8 ,9 Indeed, more recent research has shown that stopping exercise once the 85% target heart rate is achieved results in significant underestimation of both exercise capacity and myocardial ischaemia.10
Treadmill testing in contemporary cardiology
Use of the Bruce protocol ETT as the first-line investigation in patients with suspected coronary artery disease is falling in developed countries, directly as a result of the significant growth of non-invasive imaging technologies, which have been shown to have better diagnostic accuracy than ETT. Indeed, in some countries, such as the UK, the use of the ETT for this purpose is discouraged.11 Nonetheless, the treadmill undoubtedly retains relevance in 21st century practice—it is still widely used for screening symptomatic patients for coronary disease. It is also often used during exercise stress echocardiography and exercise perfusion scintigraphy, for assessing symptoms and blood pressure response in patients with hypertrophic cardiomyopathy, as part of cardiopulmonary exercise testing and for serial assessment of functional capacity in patients with valvular heart disease. Furthermore, the wealth of prognostic data accumulated with the ETT has ensured its continued use for medical insurance purposes and the assessment of a number of professional groups, ranging from airline pilots to high-performance athletes. Although the Bruce protocol underwent some modifications after its first description (eg, conversion from four-stage to seven-stage) and alternative treadmill protocols were developed,12–14 the multistage Bruce protocol remains the most frequently used.
And finally the man—who was Dr Bruce?
Robert Arthur Bruce, born in 1916, graduated from the University of Rochester Medical School in 1943 and, just 7 years later, became the first chief of cardiology at the newly formed University of Washington School of Medicine in Seattle, where he spent the rest of his illustrious career. His first publication on the use of a treadmill test was in 1949, when he investigated dyspnoea in beryllium workers and concluded that the ETT was ‘a technique…satisfactory for the investigation of dyspnoea in clinical patients’.15 After his landmark 1963 paper1 which first described the multistage protocol, he helped initiate the Seattle Heart Watch programme in 1971. This generated a database of over 10 000 patients over the following decade, as the ETT was widely employed in the community in ambulatory cardiac patients and in apparently healthy individuals.16 ,17 He became professor emeritus in 1987. Sadly, he developed chronic lymphatic leukaemia about 10 years before his death and, after a gradual deterioration from spinal stenosis, died peacefully at home in Seattle in February 2004.18 Although the ‘father of exercise cardiology’ has died, his name lives on—and will continue to do so—for generations of future doctors using the ‘Bruce protocol’ treadmill test.
Contributors BNS conceived the idea for the article, performed the literature searches and wrote the manuscript.
Competing interests None.
Provenance and peer review Not commissioned; externally peer reviewed.
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