BANGKOK - The health of the King of Thailand, who at 88 is the world's longest reigning monarch, is unstable, after he underwent haemodialysis, according to the Royal Palace.

In a statement issued Sunday night, King Bhumibol Adulyadej's medical team said he received a haemodialysis Saturday to drain cerebrospinal fluid from the brain, adding that his blood pressure recorded drops during the treatment.

Several tests were conducted following a rapid pulse and fall in blood pressure on Sunday, which revealed increased blood acidity and a marked decrease in the blood flowing into the lower left part of the heart, added the statement.

He was administered medication that helped slow down his pulse and improve blood pressure, said the doctors.

While his health remains unstable, the king is under careful medical observation and has been advised to suspend all royal duties.

Fragile health has led King Bhumibol to remain admitted at Bangkok's Siriraj Hospital almost without a break for over a year now, and palace authorities have issued 37 health bulletins on his condition.

The last public appearance of the monarch was on Jan. 11, when he paid a visit to the Royal Chitralada Palace for a few hours.

The condition of King Bhumibol's health is a taboo subject in the country due to the lese-majeste laws that shield the Royal Family against all kinds of accusations and comments.

Bhumibol, on the throne since Jun 9, 1946, is the only King most Thais know and is revered as an almost divine being - a symbol of unity and guide of the nation.