Atrial fibrillation (AF) is especially common among older adults. It is estimated that about 60,000 New Zealanders have been diagnosed and more remain unaware they have the condition. This is concerning because AF increases the risk of suffering a stroke and heart failure, and it has been linked to dementia.
In a healthy person, electrical signals keep the heart rate at a steady rhythm. With AF, those electrical signals are chaotic and this results in the heartbeat becoming irregular, which means blood is not pumped efficiently.
The causes of atrial fibrillation aren’t always clear, although some lifestyle factors play a part, including long-term heavy drinking and being overweight. Often, the first sign is the sensation of heart palpitations or a racing “fluttering” heartbeat and, at the beginning, episodes may come and go, lasting for minutes, hours or a few days. But in many cases, this progresses to persistent AF.
Exactly how that affects blood flow to the brain is being investigated by researchers in Auckland. They are using MRI scans to measure exactly what is happening to the blood and how it affects the structure of the brain.
“There are two underlying mechanisms that we believe may be involved,” says University of Auckland researcher Harvey Walsh. “One is that the atrium of the heart is not contracting properly and you get a reduction of blood being pumped with each beat, which leads to reductions in brain blood flow.
“The other theory is that in AF you get low-grade inflammation in the body and this can damage the endothelial cells lining the blood vessels. Those cells play an important role in causing blood vessels to dilate and increasing brain blood flow when it is needed.”
Participants in the study are shown a visual stimulus, such as a flashing chequerboard, designed to stimulate a change in blood flow to a region of the brain known as the visual cortex. That change is measured to find out if and how it is different in those who are experiencing atrial fibrillation.
“We also do a CO₂ [carbon dioxide] reactivity test,” says Walsh. “When you give someone CO₂, you cause an increase in brain blood flow. The MRI allows us to see what is happening and which areas of the brain are affected.”
Previous research overseas, using the less-precise measuring technique of ultrasound, suggests that blood flow is decreased in those with AF. However, the new MRI study is still underway, so it is too early to draw any firm conclusions.
Researchers in Auckland are also about to embark on another study using inhaled sodium nitrate to help the blood vessels dilate more. It is hoped that this may lead to a treatment to restore blood flow in AF patients and ensure their brains aren’t deprived of essential nutrients and oxygen. “The research around dementia suggests that the symptoms you see in old age were developing 20 or 30 years earlier,” says Walsh. “So this could be hugely transformative in reducing their risk of neurological disease.”
Both studies are still recruiting for participants.
Atrial fibrillation is the most common type of irregular heartbeat. Women have lower rates, but are more likely to suffer adverse consequences. People are particularly at risk if they are over 65, have high blood pressure, underlying heart disease or untreated sleep apnoea. It is also thought that AF can sometimes run in families. Certain things may trigger an episode, such as fatigue, stress, smoking and some medications. Symptoms can differ, but as well as the sensation of a racing heart, they may include breathlessness, dizziness, fatigue, difficulty exercising and chest discomfort.
Treatments are available, so it is important to get a diagnosis. A doctor may recommend a blood-thinning medication to lower the risk of stroke, as well as drugs to slow the heart rate and lower blood pressure. There are also treatments to restore the heart rate to a normal rhythm and keep it there.
“People often aren’t diagnosed until they see a doctor for something else and have an ECG,” says Walsh. “In the UK recently, they had a trial in a supermarket where they put sensors on the trolleys and identified people who weren’t aware they had the condition.”
