Cardiac Drift is the phenomenon where, at a constant speed, heart rate progressively increases due to dehydration and thermoregulation, forcing us to choose between maintaining pace or staying in the heart rate zone.
- Cardiac Drift is the increase in heart rate for the same mechanical effort.
- It happens because blood volume decreases (sweat) and the body needs to cool down.
- The heart must beat faster to compensate for the smaller amount of blood pumped with each beat.
- If you train by Heart Rate Zones, you must slow down. If you train by pace, you must accept the extra fatigue.
Has it ever happened to you? You went out for a long slow run, settled into your favorite pace, and for the first half-hour everything went smooth as silk: easy breathing, light legs, stable Zone 2 heart rate.
Then, without accelerating a single second per mile, you look at your watch and see your heart rate starting to climb. First by 2 beats, then 5, then 10.
By the end of the workout, despite maintaining the same rhythm, your heart is working as if you were racing.
You ask yourself: “Am I out of shape? Did I eat something heavy? Am I coming down with the flu?”
It might be none of the three. You are simply experiencing Cardiac Drift. It is a completely normal physiological phenomenon, but understanding it is crucial to avoid ruining your workout.
Running at the Same Pace, But the Heart Beats Faster. What’s Happening?
Imagine your cardiovascular system as a plumbing system. To maintain a certain speed, your muscles need a certain amount of oxygen (blood) per minute.
At the beginning of the workout, the system is in equilibrium. But as time passes, internal conditions change.
Cardiac drift usually manifests after about 15-20 minutes of continuous exercise at constant intensity. It is that gap that opens up between the speed line (which remains flat) and the heart rate line (which climbs upward).
It’s not a sensor error: it’s your body fighting to maintain homeostasis.
What Is Cardiac Drift: The Scientific Explanation
To understand drift, we must dust off a simple equation:
Cardiac Output = Heart Rate x Stroke Volume
- Cardiac Output is the total volume of blood pumped in a minute (what the muscles need).
- Heart Rate is the number of beats per minute.
- Stroke Volume is the amount of blood pumped with each single beat.
According to studies like Cardiovascular Drift During Prolonged Exercise, drift happens when Stroke Volume decreases. If the heart pumps less blood with each single beat, it must increase the number of beats (frequency) to keep the total Cardiac Output unchanged and guarantee oxygen to the muscles.
Basically: the heart works harder to achieve the same result.
The Culprits: Dehydration and Heat (Even in Winter)
But why does Stroke Volume decrease? Where does the blood go?
There are two main culprits:
- Dehydration (Reduced Plasma Volume): While you run, you sweat. Part of that fluid comes from blood plasma. Less water in the blood means total blood volume decreases and blood becomes more viscous. Less blood returns to the heart (reduced “venous return”), so each pump is less effective.
- Thermoregulation (Competition for Blood): When body temperature rises, the body needs to cool down. To do so, it sends a large amount of blood to the skin (cutaneous vasodilation) to dissipate heat. This blood is “stolen” from the central circuit.
And this doesn’t just happen in July. It happens even in winter.
Often in the cold, we overdress, sweat without realizing it (because sweat evaporates or is absorbed by layers), and drink less because the thirst stimulus is inhibited by the cold. Result? Dehydration, increase in internal temperature (a tropical microclimate is created under thermal layers), and… cardiac drift.
How to Manage Drift in Training: Slow Down or Ignore It?
This is the million-dollar question. If I see my heart rate rising, what do I do?
It depends on the goal of your workout.
- Goal: Aerobic Building (Zone 2): If your purpose is to improve mitochondrial efficiency and burn fat, you must respect physiology, not the stopwatch. If your heart rate climbs out of your target zone (see our guide on heart rate zones), you must slow down. Even if it means finishing the workout much slower than you started.
- Goal: Race Pace or Tempo Run: If you are training to hold a specific pace (e.g., marathon pace), then ignore the drift (within reasonable limits, say a 5-10% increase). Maintain the pace. Just know that the internal effort will be higher and the required recovery will be longer because metabolically you went beyond what was planned.
Hydration as the Main Weapon to Keep the Heart Stable
We cannot eliminate cardiac drift entirely (it’s physiological), but we can reduce it drastically.
The key is to maintain high blood volume.
- Drink before: Start the workout well-hydrated (hydration starts hours before, not 5 minutes before).
- Drink during: On long runs, don’t wait for thirst. Small, constant sips help maintain plasma volume.
- Manage heat: In summer, wet your head and wrists to help cooling. In winter, dress in layers (“onion style”) and remove a layer as soon as you start to warm up, to avoid excessive sweating.
Remember: your heart isn’t a broken metronome. It’s an intelligent engine that adapts to conditions. Learn to listen to it, and you’ll always know whether to push or ease off the gas.