Folklore may be right; foxgloves can really trigger cardiac arrest, say scientists

The flowers which are used both as folk medicine and poison for centuries, are notorious for their harmful effects on the human heart. In fact, they are called by names such as ‘dead man’s bells’ and ‘witches’ gloves’ for their poisonous properties.

Scientists say the claims are true as the flower has cardiac and steroidal glycosides which when ingested or absorbed through the skin, can cause gastrointestinal, cardiac and neurological effects.

Let’s understand the effects of foxglove on the human heart.

How does the heart work?

A healthy heart contains cardiac cells which contract and pump blood across the body. The pumping is regulated by tiny electric signals and every cell is actively involved in maintaining this electric activity.

“The membranes of the cardiac cells have lots of different ion channels and transporters which permit charged particles like sodium, potassium and chloride to cross the membrane in a controlled manner,” Hugues Abriel, an ion channel researcher at the University of Bern in Switzerland, told Live Science.

He said the electricity is generated when the movement of ions generates an electrical current and potential difference across the membrane.

The sodium-potassium pump regulates the electric balance as the membrane transporter expends energy to exert sodium ions out of the cells and while pumping potassium ions into the cells. This means the inside of every cardiac cell is negatively charged compared to the outside and maintaining the balance is important for proper heart functioning.

How does foxglove impede heart functioning?

Zhen Wang, a synthetic biologist specialising in plant natural products at the State University of New York at Buffalo says that cardiac glycosides like digoxin, produced by foxglove flowers, bind very tightly to the sodium-potassium pump, and prevent the transporter from pumping the ions.

“Foxgloves contain extremely potent compounds called cardiac glycosides – ‘cardiac’ for their function on the heart muscle and ‘glycosides’ to indicate that these compounds have sugar molecules…to help the body absorb them,” Wang told Live Science.

“All foxglove species produce some amount of these cardiac glycosides, such as digoxin, and they bind very, very tightly to the sodium-potassium pump, inhibiting it so the transporter cannot pump those ions anymore,” he said.

The deactivation of the pump causes the cardiac cells of the heart to beat much harder and faster.

"There's an interplay between the different types of membrane transporters, so inhibiting one means another won't work as well," Abriel told Live Science.

"Hitting the sodium-potassium pump has a knock-on effect which increases the calcium concentration inside the cardiac cells. This rise in calcium concentration acts as a trigger, causing electrical disturbances and making the heart cells contract harder and faster."

Abriel says the process triggers ventricular fibrillation, a dangerous type of arrhythmia, which can lead to sudden cardiac arrest and even death in some cases.

“When the chambers of the heart are not contracting regularly, it can’t pump blood, and the overall effect is as though the heart isn’t beating anymore,” Abriel said.

The silver lining

However, it’s not all bad news as besides having a dangerous effect on the heart, digoxin is also a well-known heart medication and is clinically prescribed for heart failure when all drugs fail, Wang said.

“Heart failure is when the patient’s heart is so weak that it doesn’t pump sufficiently so you want to increase the heart’s pumping force. Although digoxin has serious side effects if overdosed, this is a special case where the benefit of the toxin outweighs the risk and could save the person’s life,” Wang added.