Drinking coffee before a blood pressure test can affect the results.
Coffee is popular around the world. In the United States, over 50 percent of people drink coffee, jointly spending some $40 billion a year on the beverage.
Studies show that caffeinated coffee can acutely increase blood pressure, but decaffeinated coffee does not. Caffeine seems to be a major factor in affecting blood pressure, and experts suggest that as such, it is also a trigger for cardiovascular events.
However, people who consume coffee and caffeine regularly are not thought to face such a risk, because they develop a tolerance. This suggests that the intervals at which people drink coffee are of some significance.
However, for occasional rather than regular consumers, it might be a different story.
How does coffee affect a blood pressure test?
A team from Western University and Lawson Health Research Institute in London, Ontario, in Canada, came up with some surprising results when they measured the effect of occasional coffee consumption on blood pressure, and especially how coffee consumption impacts the action of calcium channel blockers.
Calcium channel blockers are a type of medication for lowering blood-pressure. They are commonly prescribed for patients with hypertension. Calcium channel blockers, such as felodipine, relax and widen the blood vessels, making it easier for blood to flow. In this way, they reduce blood pressure.
Led by Dr. David Bailey, a Lawson Scientist and researcher at Western's Schulich School of Medicine and Dentistry, the team wanted to find out what would happen to the blood pressure if a person abstained from caffeine long enough to eliminate the caffeine from the blood.
They expected to see a higher blood pressure the next time a person drank coffee, because eliminating caffeine and then consuming it again could cancel out the pressure-lowering effects of felodipine.
Drinking coffee after a break raises blood pressure
To test their theory, they invited 13 people with an average age of 52 with normal blood pressure to participate in an experiment.
They carried out three tests on the subjects, separated in time by 1 week. Before each test, the people consumed no coffee, caffeine-containing products, or other items such as alcohol, grapefruit, marmalade, tobacco, and medications for 48 hours.
At intervals of a week, the participants then took the following, and then they had their blood pressure taken:
- Two 300 milliliter cups of black coffee
- The maximum recommended dose of felodipine (10 mg)
- The coffee plus a dose of felodipine.
Results showed that, after the participants avoided coffee for only 2 days, enough caffeine was eliminated from the body so that the next time they drank coffee, their blood pressure rose.
After just one cup of coffee, the participants in the coffee-only group experienced the greatest increase in blood pressure. The blood pressure rose within an hour after drinking the coffee, and it lasted for several hours.
Combining coffee with felodipine led to higher levels of blood pressure than taking only felodipine. This could be because the caffeine blocks the positive effect of the drug on the blood vessels, the researchers say.
They note that a morning cup of coffee could affect the diagnosis and treatment of hypertension, or high blood pressure.
"Even one cup of coffee containing a relatively low amount of caffeine remarkably compromised the anti-hypertensive effect of this drug at the maximum recommended dose. If you wanted to overcome the effect of the coffee, you had to double the dose of this anti-hypertensive drug which could increase the risk of unwanted excessive drug effects, particularly during the period when coffee is not consumed."
Dr. David Bailey
Dr. Bailey is concerned that if a patient drinks coffee just before visiting their doctor, it could complicate diagnosis and treatment. The acute increase in blood pressure could lead to over-prescription of antihypertensive drugs.
He points out that between 15 and 20 percent of people who drink coffee do so only occasionally. People who drink coffee twice a week or less may have an occasional rise in blood pressure. In some people, the rise can be greater than in others.
Dr. Bailey notes that national and international guidelines regarding high blood pressure do not take into account the impact of coffee, probably because of a lack of evidence.
He hopes that further studies will provide more data, and that occasional coffee drinkers will become more aware of the risks they face.