The children all underwent heart surgery

The study by researchers at the University of Ulm looked at children who had had a cardiopulmonary bypass.

In this process, the patient's blood is diverted away from the heart and lungs through a machine which infuses the blood with oxygen and pumps it around the body.

This allows surgeons to repair the heart while it is bloodless and motionless.

But, because the patient's blood is in contact with a foreign environment, it stimulates the immune system to cause inflammation throughout the body, known as systemic inflammation, which can prove fatal.

Cytokines are proteins which can act either to increase or decrease systemic inflammation.

Recovery patterns

Researchers measured levels of cytokines and progesterone in children during and after surgery to see if increased levels were linked to the development of systemic inflammation.

Eight girls and 10 boys were studied.

They found the girls had higher levels of cytokine IL-10, both during and after surgery.

This is significant because IL-10 is thought to suppress inflammation.

The finding could explain why girls recover more easily than boys.

Six of the boys, but none of the girls developed multiple organ failure.

No link was found between levels of progesterone and patient's recovery.

Surgery timing

The researchers, led by Andreas Trotter, say their findings require further study.

But they add there can be reasons for treating male and female patients differently.

They suggest that if female sex hormones really protect against the development of systemic inflammation then corrective surgery in girls should be carried out in the first weeks after their birth when their sex hormone levels are higher than those found later in infancy.

The research is published in the Critical Care.

Writing in the same journal, experts from the German Heart Centre in Munich and the Aachen University of Technology, both in Germany say the Ulm research raises many questions.

Dr Marie-Christine Seghaye and colleagues wrote: "Could it be assumed then that female sex, because of the influence of oestrogen, protects against systemic inflammation?

"The question merits further investigation, the outcome of which could have implications for the timing of corrective cardiac surgery in children."

They say the Ulm study is limited because of the wide range of children in the study, and say a larger study is needed.