After attacking a particularly steep hill or finishing a high-speed sprint on our bikes, we often ask each other: “What’s your heart rate?”. It’s really a comparative performance question about who’s in better cardiovascular shape. Invariably, when I answer, the common retort is something like “Yah, but you’re on beta blockers”, as if this medication gave me some sort of advantage. As it turns out, the opposite is true.
I knew through my cardiologist that beta blockers would reduce my already low heart rate by some 10 to 15 beats per minute. Instead of a heart rate of 48 bpm upon waking, my new normal was closer to 35 bpm. I was told that, in compensation, the amount of blood pushed out of my left ventricle into the rest of my body would increase (“stroke volume”). My heart would be therefore more efficient: working less hard but producing the same effect. What I wasn’t told was that my cardiac output and my VO2 Max would be impaired by beta blockers.
VO2 Max is simply the highest rate at which oxygen can be utilized during extreme exercise. It’s a measure of cardiorespiratory fitness, first conceptualized in 1923. Long-distance runners and cross-country skiers and professional cyclists tend to have the high VO2 Max attributes necessary to take in and deliver to the muscles the maximum amount of oxygen. It all depends on:
- the lungs’ ability to capture and diffuse oxygen into the bloodstream,
- the heart’s ability to pump the newly oxygenated blood out,
- the vascular system’s ability to distribute blood to the muscles and
- the muscles’ ability to absorb and utilize the oxygen.
Beta blockers negatively impact the second process, the heart’s ability to pump out the oxygenated blood (i.e., “cardiac output”). In reducing the rate at which the heart pumps, beta blockers reduce cardiac output: fewer beats per minute means less blood pumped out per minute. Stroke volume, the amount of blood pushed out with each heartbeat, has a limited upper capacity due to the size of the ventricles and other factors. At high levels of exercise and oxygen demand, the limitations of heart rate imposed by the beta blocker cannot be compensated by stroke volume. Instead, you work harder, breathe faster and get tired quicker than the people beside you.
Beta blockers can reduce maximum heart rate by 25 to 30% and cardiac output by 15 to 20%, even with a slight increase in stroke volume. VO2 Max can decrease by 5 to 15%. Selective beta blockers (like bisoprolol) that only impact the Beta2 adrenoceptors in the heart have less of an effect on VO2 Max than non-selective beta blockers (like propranolol) that impact the Beta1 and Beta2 adrenoceptors in the heart and vascular system (including the lungs).
While it is saving my life, the beta blocker is a liability when competing against my heart-healthy mates on a bike or running a race. Yet I think I’m going to let them keep believing I have the heart rate advantage. I’d rather keep them on their toes than cutting me some slack on the road.