Ask The Doctors: June 2019
Q: I want to determine my target heart rates for exercise but have found wide differences in online calculator results. What is the best way? (Note: Dr. Rocco addressed this issue in April, but is explaining these calculations differently in response to readers' questions.)
A: The American Heart Association recommends a minimum of 150 minutes of moderate activity or 75 minutes of vigorous activity weekly. Moderate is often defined as 50 to 70 percent of maximally predicted heart rate (MPHR); vigorous is 70 to 85 percent. You can calculate MPHR by subtracting your age from 220 (Fox formula), but this often underestimates MPHR. The Tanaka formula (206.9 – (0.67 x age)) is more precise, yet in women, the Gulati Formula (206 – (0.88 x age)) may be more accurate.
A simple way to determine target heart rate (THR) is to multiply the MPHR by the percentage (e.g., 70 percent target is 0.70 x MPHR). If you are age 65, MPHRs using the Fox, Tanaka and Gulati formulas are 155, 163 and 149 respectively. Using this method, the 70 percent THRs would be 109, 114 and 104. A further modification is the Karvonen formula, [(Max HR – Resting HR) x % intensity] + Resting HR. Using this method and a resting HR of 70, the 70 percent THRs would be 130, 135 and 125.
The largest discrepancies exist between the simple percentage and the Karvonen methods for THR. Different online calculators use different formulas and give different results. Combining a gender-specific estimate of MPHR with the Karvonen formula gives the most accurate results-but even the best formulas are based on estimates from population studies. These should be used only as guidelines and adjusted by any symptoms and your level of perceived exertion. Furthermore, many factors such as medications and presence of heart disease influence estimates of exercise targets. In these cases, direct measurement of MPHR with a stress test and lower starting target is more appropriate.
Q: I have both high blood pressure and atrial fibrillation. Is there a connection?
A: Risk factors associated with atrial fibrillation (A-fib) include valvular heart disease, coronary artery disease, heart failure and, yes, high blood pressure (HBP). HBP and A-fib often coexist. In the Framingham study, the presence of HBP increased the risk for A-fib by 50 percent in men and 40 percent in women, ranking fourth after heart failure, aging and valvular heart disease. On the other hand, HBP is present in about 60 to 80 percent of patients with A-fib.
The propensity to develop A-fib increases when HBP and obesity coexist. Obstructive sleep apnea, which is present in up to 50 percent of people with HBP, is also associated with an increased risk of A-fib.
Uncontrolled HBP causes changes in the heart. It must pump harder to push blood out, which leads to structural changes such as muscle thickening (hypertrophy) and stretching of the heart chambers. It also contributes to hardening of the arteries and affects kidney function. All these can create a favorable environment for A-fib development.
HBP also impacts some of the adverse effects of A-fib. About three in every four people who have a first stroke have HBP, and the arrhythmia is present in about one in five strokes. High BP also predisposes to serious bleeding events, especially intracranial hemorrhage, further compounding the risk of commonly used blood thinners in A-fib.
HBP is the most important modifiable risk factor for A-fib. Adequate blood pressure treatment is an essential part of overall management for A-fib prevention, rhythm control, and heart failure and stroke prevention. A recent study also noted that in middle-aged patients with Afib, the risk of dementia can be reduced by controlling HBP.