Tuesday, July 31, 2007
Nocturnal Oxygen Desaturation (Personal Experience)
One of the ways doctors can tell if a pulmonary exacerbation is occurring, is to monitor the oxygen saturation in the blood. A device called a pulse oximeter is used to calculate how much of the blood is oxygenated. A pulse oximeter works by sending two different wavelengths (i.e. colors) of light from a probe through a person's fingertip. The device then calculates how much of each color of light is absorbed, and compares the two. The result is a ratio of oxygenated hemoglobin to arterial blood. The ratio is expressed as a percentage.
The expected healthy range is 96-100%. It is important to remember that this percentage fluctuate in the course of a day, so there is no reason for concern if the numbers dip ever so slightly. However, if desaturation, or, oximeter readings of less than 90% occur frequently and persistently, a doctor may recommend having an overnight pulse oximetry study done.
It is not uncommon for people with a chronic respiratory disease, like cystic fibrosis, to experience what is called nocturnal oxygen desaturation (NOD). Symptoms of NOD include morning headaches, restless sleep, poor sleep due to difficulty in breathing. Researchers have also seen that desaturation is more likely to occur in patients who have moderate to severe lung damage as indicated by FEV1.
My lung function had been lower than usual and I was hospitalized a number of times in the last 12 months. I wanted to keep better track of my own vital signs at home, so I purchased a small pulse oximeter. I took readings from it first thing each morning, and prior to my nebulized medications in the evening. I began to notice that my percentage was hovering around 93% for days at a time. Occasionally in the evenings it would drop down to 88% or lower. Although it didn't stay that low, it was notable enough for me to bring it to the attention of my doctor. My doctor ordered an overnight pulse oximetry study. Overnight oxygenation studies are usually done when sleep apnea is suspected, or when there is a potential need for supplemental oxygen.
The nice part about the study is that I was able to do it in the comfort and privacy of my own own. Unfortunately, the first night I tried the test, the device was so uncomfortable that I never really fell into a deep enough sleep to get accurate "sleep study" results. The finger sensor was quite tight and kept causing me to lose feeling in my fingertip. I told my doctor that the results of that night's test were likely to be inaccurate, since I never truly fell completely asleep.
A week later I was given the device again. This time I had a more comfortable device and slept pretty well with it. As I lay in bed watching the digital display, one of the last numbers I remembered seeing was 85%.
The technician who brought the pulse oximeter to my home told me that to get a good amount of data, I should try to get at least 6 hours worth logged in. I think I got in at least 7. Since it was the weekend and I wouldn't have to return the oximeter until Monday, I went ahead and wore it for an additional night, just to make sure there was enough data to review. By that time I had become a pro at sleeping with the sensor clipped to my index finger.
My doctor had the results at my latest appointment. The computer interprets all the logged data and calculates how much of the time I was in the "safe zone" and how many times and for how long I dropped into desaturation. Desaturation is what happens when there is some sort of pulmonary obstruction (such as decreased lung function in cystic fibrosis) causing the blood to be lower in oxygenated hemoglobin than it should.
My test results showed that 99 percent of the time I was in the safe zone. The safe zone is considered 90-95%. Healthy is 96-100%. It was determined that I do not need supplemental oxygen at this time.
For More Information
Nocturnal Ventilatory Support for CF Patients
Pulse Oximetry (Wikipedia definition)
Home Measurement of Oxygen Saturation in CF Patients
NOD and Spirometry in CF