percent of heart rate reserved used =
= 100% * ((heart rate during stage) – (heart rate at rest)) / ((peak heart rate) – (heart rate at rest))
percent of metabolic reserve used =
= 100% * ((METs for stage) – (METs at rest)) / ((METs at peak) – (METs at rest))
where:
• peak heart rate = (220 – (age in years))
• Usually METs are estimated based on the protocol and stage used; they are not directly measured.
• If the patient is evaluated during peak exercise, then the metabolic reserve portion equals 1.
chronotropic index =
= (heart rate reserve used) / (metabolic reserve used)
Interpretation:
• The normal chronotropic index is 1.0 (95% CI 0.8 to 1.3).
• A chronotropic index < 0.8 indicates a low chronotropic index
Factors not affecting the chronotropic index (Lauer, 1999, page 525):
(1) physical fitness
(2) exercise time
(3) level of functional capacity
(4) exercise protocol
(5) stage of exercise protocol
