Heart rate as a measure of exercise intensity or capacity in patients taking cardiovascular drugs

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Heart rate as a measure of exercise intensity/capacity in patients taking cardiovascular drugs.

by Gavin Curry

Exercise and cardiovascular drugs such as beta-Blockers have differing effects on cardiac output and heart rate. Cardiovascular drugs such as beta-blockers can cause a reduction in heart rate of around 20-30% (Easton and Thompson, 1977). This can have an impact on an individual’s ability to perform without becoming fatigued at higher exercise intensities.

Beta-Blocking drugs cause a decrease in HR and cardiac output at rest and during exercise, a decrease in myocardial contractility and a decrease in coronary and muscle blood flow (Easton and Connoly, 1996). Subsequently patients using heart rate as the determining factor for exercise intensity could be in danger as they are exercising beyond their capabilities. This is because their heart rate will not go above a specific level because of the cardiovascular drugs affects limiting on stroke volume and heart rate.

Heart rate as a measure of exercise intensity or capacity in patients taking cardiovascular drugs
Other methods, such as the Borg rating of perceived exertion or the Karvonen formula, are used to measure exercise intensity. These are effective as they do not rely on the subjects exercising heart rate directly as a measure of exercise intensity.

The Borg rate of perceived exertion works on a scale from 1-10 or 6-20 with the higher numbers representing close to exhaustion. Although this is a subjective measure, a person's exertion rating may provide a fairly good estimate of the actual heart rate during physical activity (Borg, 1998). This study also found a high correlation between 10 times the rate of perceived exertion and estimating the actual heart rate.

The Karvonen formula is used to derive the ideal exercise training zone through a formula that helps to determine target heart rate zone from the resting heart rate(karvonen et al., 1957). This formula can be used as a method of projecting the target heart rate, again the disadvantage of this technique is that beta-Blockers can inhibit the heart rate so much that it would be impossible to for subjects to get their heart rate within the target area (Martin et al., 1989).

The problem with taking cardioselective beta-Blockers such as atenolol, which act solely on the cardiovascular system, is that they suppress the maximum heart rate more than non-cardioselective drugs, although all beta-blockers reduce maximum heart rate to some extent (Head, 1999).

To conclude, using a rate of perceived exertion would be more beneficial for determining an ideal exercise training zone rather than using heart rate. This is because it takes into account the effects of cardiovascular drugs on the lowering of heart rate. This would therefore be a lot safer and ensure the subject is exercising at a level they feel comfortable with.


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