Sum: Cardiac capacity (Max heart rate – Minimum heart rate), which can be measured via any wearable heart rate monitor provides a good approximation of cardiovascular fitness – the single best predictor of cardiovascular risk and related mortality.


Heart disease continues to be the leading cause of mortality globally. And despite the many advances in treatment, mortality rates have either plateaued or started to rise again, due to the ongoing change in lifestyle that leads to heart disease.

We’ve known for a while that cardiovascular fitness (aka VO2Max, cardio-respiratory fitness) is the single best predictor of cardiovascular risk and related mortality. A study from Cleveland Clinic that tracked 122K patients over 24 years found that:

Increased cardiorespiratory fitness was directly associated with reduced long-term mortality, with no limit on the positive effects of aerobic fitness

The risk associated with poor cardiorespiratory fitness was comparable to or even exceeded that of traditional clinical risk factors, such as cardiovascular disease, diabetes and smoking.

Source: Cleveland Clinic Research News Release, October 19, 2018

The American Heart Association has therefore published a policy statement recommending that cardiovascular fitness be measured as a vital sign in clinical practice.

For a full review of VO2Max, go here: https://hack.fitness/vo2max-the-most-vital-of-vital-signs/

VO2Max / cardiovascular fitness is not easy to measure – the gold standard lab test (also known as a “stress test”) requires that you wear an oxygen mask while you progressively increase your running or biking speed until you reach exhaustion / max out. And this is expensive – average price of a single test is $150.

So I invented a novel method that made it easier to measure VO2Max – obtaining near lab quality accuracy (in initial testing) without using face masks. However this still required an actual test on an exercise bike – the idea of paying money to do a test on a bike in a distant location ended up being a barrier to mainstream adoption.

Now there are methods to estimate VO2Max — many wearable devices – Garmin, Fitbit, Apple Watch for example provide an estimate. And while they typically claim high accuracy, in reality the estimates suffer from a number of deficiencies that reduce their applicability to a broad population.

1/ The validation studies are typically conducted amongst young, healthy, active individuals — not people at risk of heart disease. Firstbeat’s technology, which powers the Gamin watch amongst others, was validated amongst marathon runners.

2/ They rely on estimates of Max HR using age / gender based formulas. As we age, there is wider variation in Max HR versus what the formula would predict.

3/ Max HR is also significantly affected by use of medications (statins for example and certain types of blood pressure meds) – given that roughly half the US adult population is on meds for lifestyle disease, this is a big impediment. And this is precisely the population who could most benefit from a measure of cardiovascular fitness!

4/ Also, these estimates seem to work better for people with VO2Max values in the middle, but tend to under-estimate for those who are very fit and over-estimate for sedentary problems (the classic challenge of “curve-fitting”).

5/ Importantly, these estimates are pretty sensitive to accurate HR measurement — which is not always guaranteed in wrist worn wearables using optical sensors to measure heart rate.

As a result of all these limitations, when I evaluated one of these VO2Max algorithms (the NASA method), which claims 81% fit – on the Revvo dataset (which spans a wide spectrum of age and fitness levels) the fit is much lower at 0.32.

image2

So I’ve continued to look for ways to simplify this so that anyone with interest in staying fit / healthy can measure themselves to take corrective action.

Introducing cardiac capacity

Let’s step back a bit to understand what VO2Max / cardiovascular fitness really is. This talk by Dr. Marialice Kern provides a pretty comprehensive overview of the metric.

 

As you’ll see, VO2Max is the max oxygen the heart can supply and muscles can uptake to provide energy. This is captured in the follow equation (called the Fick’s equation):

VO2Max = Cardiac Output (Max heart rate X Stroke Volume) * Muscle Oxygen Uptake

Max Heart Rate = self-explanatory
Stroke Volume = quantum of blood that the heart pumps out per beat.
Max Oxygen Uptake = how much of the oxygen that reaches muscles is actually used.

Exercise leads to an increase in stroke volume and/or oxygen update (for relatively stable Max HR) to then increase VO2Max.

Max HR can be directly measured using a heart rate monitor. Stroke Volume typically requires an ultrasound to measure – however we know that as VO2Max increases, resting heart rate tends to drop. Since the heart pumps out more blood per beat (i.e. higher stroke volume) it doesn’t have to beat as frequently to supply the same amount of oxygen to organs. Therefore a lower resting heart rate should indicate higher stroke volume (other things being equal) .

So if we were to calculate the different between Max HR and Min HR, then we would get an approximate indication of cardiac output — let’s call this metric Cardiac Capacity.

In large part, changes in cardiac output tend to go hand in hand with changes in muscle oxygen uptake (there are of course exceptions to this — the human body is a complex system — but for now let’s assume they are directionally consistent over time).

If these assumptions are valid, cardiac capacity on its own should be able to roughly predict an individual’s VO2Max / cardiovascular fitness. And so when I ran the analysis on the same fitness test sample from Revvo (n=759 usable records out of >1400 test subjects), the resulting correlation is a meaningful 0.55 — significantly higher than what we saw earlier for the estimated VO2Max.

image1

And based on the known categories of VO2Max, it’s possible to similarly classify levels of cardiac capacity into fitness zones, that help interpret the score. (Note: Cardio Fitness = Cardiac Capacity).

Cardio Fitness
Revvo, Inc. 2019, All Rights Reserved.

How reliable is this metric? I.e. how closely does this correlate with changes in VO2Max? Below is just my VO2Max and Cardiac Capacity scores tracked over a few weeks. As you’ll see both cardiac capacity (cardio fitness) and VO2Max peak in the early part of the chart and then drop down slightly. Cardiac capacity seems to fluctuate a bit more than VO2Max — though there is directional consistency.

Screen Shot 2019-08-29 at 3.32.17 PM

Of course this is just n of 1, we need a whole lot more data before we can be confident this could be a reliable measure of cardiovascular fitness.

Limitations

There are a range of factors that can influence the cardiac capacity scores – some of the key ones are below.

1/ Max HR is really a function of how hard you work out — if you were working out at a relaxed pace this week you may not get anywhere close to your real max.

2/ Min HR is ideally measured at rest and not just before starting a workout (as it is in the examples above and in the dataset used for the calculations — so this fit may improve or decline if we had true min HR).

3/ Different sports elicit different heart rate ranges — for example typically running elicits a higher max HR than cycling for example.

So there’s work to be done to judge how well this metric accommodates the variety of different sports & activities that people engage in. Though some of these limitations can be mitigated by taking a reasonably wide time range (over a week for example rather than session by session).

Next step — collect more data to further validate / refine this metric. If you’re interested in beta testing this metric to help refine / improve, please sign-up via the form below.

 

 

We’re close to wrapping up work on an iOS version that reads heart rate data from your Strava profile (so once you integrate it with Strava, it will automatically spit out your metrics every week). Working on integration with other wearable devices.

Appreciate hearing your thoughts / comments!


Disclaimer: The Content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.

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