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.
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,
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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