The sensor has been designed by doctors at the University Hospital Southampton (UHS), in Hampshire.
The device was implanted during a simple 45-minute procedure, using a small catheter which is placed in a vein at the top of the leg.
With this innovative method, the doctors hope to enable more rapid interventions, helping keep people well for longer and easing financial pressures on the NHS.
“The procedure is part of a cutting-edge international research study which intends to prove this new way of monitoring and treating heart failure patients is safe and effective," said a UHS spokeswoman.
“The unique technology is a sensor about the size of a pen lid which is designed to monitor the amount of fluid in the body – elevated levels can give an indication of worsening heart failure."
The device is implanted into the inferior vena cava (IVC), which carries oxygen-depleted blood from the abdomen back to the heart.
It works by continuously measuring...