It was 1948. An epidemic of cardiovascular disease was killing Americans and no one knew why. The causes of heart disease and stroke were not yet understood.
The National Heart Institute decided to try to understand the problem.
What if, they thought, we study an entire town for their entire lives? Find out everything about them. Physical exams. Habits. Family histories. Study them before they get sick? These questions launched into an ambitious, longitudinal study that came to be known as the Framingham Heart Study. Virtually everything you know about heart health today has its roots in that research.
How long did the study last? Here’s the kicker. It’s still going on.
Back in 1948 when doctors began examining 5209 inhabitants of Framingham, MA, they never could have envisioned that their study, 70 years later, would continue across three generations. Nor could they have imagined that the research tools would now include wearable tech to measure blood oxygenation (spO2) at home.
But that’s what happened.
That’s right, in 1971, the children of the original participants were added, 5124 of them. Then in 2002, the study added a third generation, the grandchildren of the original cohort (and diversified the pool with the addition of Omni participants).
Since its inception, the Framingham Study has identified every factor we know to be implicated in cardio health: smoking, blood pressure, cholesterol, diabetes, obesity, sleep, exercise, and apnea. Now, this renowned study has turned its attention to next-gen tech, concluding that home measure of spO2 was “feasible, yields reliable results” and calling wearable devices “a convenient alternative to traditional measures of sleep”.
How do we breathe?
The level of oxygen in our blood is a key indicator of overall health. We don’t give it much thought. But what happens when we breathe? The air we breathe at sea level is about 21% oxygen. We extract this oxygen from the air. As oxygenated air enters our lungs, it passes down through our alveoli to the capillaries outside the heart. This oxygen passes into our blood, and we expel carbon dioxide as a waste product.
The role of hemoglobin
Hemoglobin (a protein in the red blood cells) helps move the oxygen into the blood. It’s what makes our blood red. We’ve known that hemoglobin carries oxygen since 1840. Each hemoglobin molecule can carry four oxygen molecules. We measure the level of oxygen in our blood by counting this hemoglobin. Oxygen saturation (also called SpO2 or SaO2) is the amount of oxygenated hemoglobin compared to the total amount of hemoglobin in our blood.
So, what’s normal?
Our heart is a magnificent organ. It takes tired, depleted blood and re-vivifies it, pumping in oxygen. Without this oxygen, you wouldn’t even be able to hold your phone or stay upright in your chair. So what’s normal? A healthy person has a SpO2 (blood oxygenation) of over 95%. Any measure below 90% is serious.
Can it be changed?
Factors that compromise your body’s ability to maintain sufficient oxygen levels include internal problems (diseases such as COPD or asthma) and external events like altitude or poor air quality. Some changes are transient, like an infection or pneumonia. Some are permanent. Supplemental oxygen can be used to raise levels in severely oxygen compromised individuals. Exercise and a healthy lifestyle can increase blood oxygenation for nearly everybody.
Living at high altitude actually changes the composition of your blood, as your body manufactures extra red blood cells to compensate for the lower oxygen levels in the environment. This is why many elite athletes train at altitude.
There are obvious consequences of having too little oxygen in your blood. Lack of energy for example. Shortness of breath. Eventually, without intervention, death. But there are subtle changes too, for example, poor sleep, cognitive deficits, and weight gain.
Some variation in SpO2 is normal, especially as we sleep. During REM sleep, our breathing slows so desaturation (decreased oxygen) is routine. But some changes signify a larger problem. Obstructive sleep apnea (OSA) is a common sleep disorder that causes interrupted sleep and decreased blood oxygen.
The severity of your sleep apnea is measured as a combination of your AHI (apnea-hypopnea index, or how many times you awake per hour). Less than five times an hour is considered minimal or even normal: sufferers of severe apnea awaken more than 30 times per hour. Small wonder they are exhausted. (Between 15 and 30 events is considered moderate, 5 to 15 is deemed mild.)
“Oxygen is necessary for life,” says Dr. Neil Kline of the American Sleep Association, “Sleep apnea is characterized by repetitive pauses in breathing. When we stop breathing, oxygen does not enter our lungs.” Obesity increases the risk for OSA, explains Dr. Kline, because “with increased weight comes additional neck and throat-area tissue which can obstruct, or block, the flow of air from outside of the body to inside of the body.”
“Individuals with untreated OSA run much higher risks of developing hypertension, diabetes, obesity, coronary artery disease, and metabolic syndrome,” says Dr. Jeffrey Durmer, MD Ph.D., Chief Medical Officer of FusionHealth. “The impact of repeated drops in oxygen due to apneas and hypopneas throughout the night includes higher levels of inflammatory hormones (called cytokines and interleukins) as well as cortisol and the dysregulation of glucose metabolism.”
These drops in oxygen are called oxygen desaturation and can be measured by a pulse oximeter. “When you combine these drops in oxygen with the obligate surge in sympathetic nervous system activity (the fight-or-flight response) associated with choking throughout sleep,” says Dr. Durmer, “This is why people suffering with untreated OSA have a 2-5 times higher rate of catastrophic events like stroke, heart attack, and heart failure.”
New research presented at the 2017 ATS International Conference found that patients with OSA are at a higher risk of developing atrial fibrillation.
“Early identification equals early intervention,” says respiratory therapist turned hospital executive Joe Musto, “If a patient’s SpO2 drops below their ‘normal’ range they can take action, i.e., implement breathing exercises, take any prescribed medication, seek medical attention if necessary.” Knowing what’s “normal” for you is key. That’s why a self measure of spO2 at home is such a boon to health.
Then vs. Now
Seventy years ago, cardiovascular disease was the number one cause of mortality, accounting for 1 in 2 U.S. deaths. Yet its causes were so little understood that H.G. Bruenn said: “most Americans accepted early death from heart disease as unavoidable.”
So much has changed.
Cardiovascular disease is now a leading preventable cause of death. That’s good news! The Framingham Study continues, shedding light on the complicated interplay between genetics, obesity, dementia, sleep, social connectedness, and heart disease. Each new discovery increases our knowledge and, with it, our power. Framingham has spawned countless research-based interventions. Armed with these findings, we are empowered to maximize our health at home.