The Cooper Institute
 

Founded in 1970 by the "Father of Aerobics"
Kenneth H. Cooper MD, MPH

 
 

Know your Cardiorespiratory Fitness Level and Coronary Artery Calcium Score

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Friday, Feb 02, 2018

Knowing your Cardiorespiratory Fitness Level and Coronary Artery Calcium Score Helps Predict the Future Risk of Cardiovascular Disease Outcomes: The Cooper Center Longitudinal Study

Just about everyone who follows The Cooper Institute knows that cardiovascular disease (CVD) is far and away the leading cause of death among U.S. adults. Nearly 30 years ago, landmark data from the Cooper Center Longitudinal Study showed for the first time that a low level of cardiorespiratory fitness (CRF) was independently associated with increased CVD mortality risk in both men and women. Over the past two decades, a large body of evidence has also shown that coronary artery calcification (CAC) score is another strong predictor of future CVD events such as heart attack, stroke, coronary artery bypass surgery, balloon angioplasty, and stent placement. Briefly, CAC score is determined by using computed tomography (CT) scanning. The walls of normal, healthy coronary arteries contain no calcium; this type of patient would thus have a CAC score of zero. A CAC score greater than zero indicates the presence of plaque within the coronary arteries, the higher the CAC score, the greater the risk of a CVD event. CAC score is an especially strong predictor of future CVD events when combined with traditional risk factors such as blood cholesterol, glucose, and triglyceride level, resting blood pressure, smoking, family history, age, and obesity.

Although CRF and CAC each contribute strongly to prediction of future CVD events, surprisingly little is known regarding how CRF impacts CVD risk across different categories of CAC. In other words, let’s say two men had similar CAC scores and similar values for traditional risk factors, but had different levels of CRF. Would their future CVD risk be the same or would it be different? With this in mind, researchers sought to examine these relationships in a sample of 8,425 generally healthy Cooper Clinic men between the ages of 30 and 80 who were examined between 1998 and 2007. Their comprehensive exam included a maximal treadmill exercise test to measure CRF, a scan to determine CAC score, as well as careful measurement of the more traditional cardiovascular disease risk factors mentioned earlier. The men were then followed for an average of 8.4 years, during which time 383 fatal and non-fatal CVD events occurred. Now, let’s take a look at the results.
   
In Figure 1, the rate of CVD events is shown in 4 different CAC categories. While men with CAC scores of 0 had a very low rate of CVD events (1.3 events per 1000 person-years*), men with CAC scores of >400 had a much higher rate (18.9 events per 1000 person-years). Based on previous studies, this finding was expected. 

Shown below in Figure 2 is where things really begin to get interesting! Here we are looking at the risk of CVD events up to the age of 70 across CRF level in the 4 CAC categories over a 15 year period following the baseline exam. In the Figure, CRF is expressed in units called METs, and no, that’s not a baseball player from New York City! Just know that the higher the MET level, the higher the level of CRF. During this analyses, many factors that might have ‘muddied the waters’ were carefully controlled for. These factors included age, smoking, body mass index, systolic blood pressure, blood glucose level, history of diabetes, blood cholesterol level, and statin therapy. After taking all of these factors into account, you can see that within each CAC category, the risk of CVD events decreased across increasing levels of CRF. In other words, within any given CAC category, as CRF increased, the risk of a CVD event decreased. The decrease in risk across CRF was more pronounced among men with higher CAC scores. As the Figure shows, the lowest risk was seen in very highly fit men with CAC scores of 0, while the highest risk was seen in very low fit men with CAC scores >400. What is important to reinforce is that higher levels of CRF provide some degree of protection against CVD events in all 4 CAC categories.    



These results suggest that improving CRF may help to decrease the risk of future CVD events, regardless of which CAC category the patient is in. These findings strengthen the position of the American Heart Association that CRF should be measured or estimated during physical exams, and should be added to the list of clinical vital signs. 

You may be wondering why this study only included men. By and large, Cooper Clinic women are an extremely healthy group; they were excluded from this study simply because of a very small number of CVD events. You may also be wondering if you are a candidate for a CAC scan. Your primary care physician would make this determination based on your age, gender, family history of heart disease, and additional factors. 

*Rates of events are often reported per 1000 person-years of data. Here are a couple of examples: If one patient is followed for one year, then we would have 1 x 1 = 1 person-year of data. If 100 patients are followed for 10 years, then we would have 100 x 10 = 1000 person-years of data.  

Reference
Radford, N., DeFina, L., Leonard, D., Barlow, C., Willis, B., Gibbons, L., Gilchrist, S., Khera, A., Levine, B. (2018). Cardiorespiratory fitness, coronary artery calcium, and cardiovascular disease events in a cohort of generally healthy, middle-aged men: Results from the Cooper Center Longitudinal Study. Circulation, doi: 10.1161/CIRCULATIONAHA.117.032708. [Epub ahead of print]