On a monitor above the surgeons' heads, a red and green '0' indicates Junior's heart has been stopped. The medics get to work quickly, their fingers moving carefully around the ten or so wires protruding out of the three-year-old's chest. After a while, the heart jumps back to life.

More than a million babies worldwide are born every year with a congenital heart defect. Of these, one tenth will not live to see their first birthday. While corrective surgery is normally performed within weeks of the birth, in Tanzania and other developing countries, poverty and a lack of cardiac specialists or facilities mean many go untreated.

Junior is being operated on in Muhimbili National Hospital in the Tanzanian port city of Dar es Salaam. It is the only medical centre in the vast, east African country of 50 million that is equipped for open-heart surgery. His mother is sitting in the corridor outside the operating theatre, waiting anxiously to find out if her son has survived the high-risk but life-saving surgery.

He is one of dozens of children being treated by a team of specialists from the UK-based NGO Muntada Aid. The group of 35 surgeons, doctors and nurses fly out to developing countries to carry out surgeries, provide medical equipment and train locals to diagnose and treat children with congenital heart disease as part of the charity's Little Hearts project.

"We treat around 65 to 85 patients per mission," says Dr Jameel Al-Ata, a paediatric cardiologist from Saudi who is leading the Tanzania mission. Since the Little Hearts project started in 2008, the team have operated on or catheterised more than 3,000 patients. "We target countries with poor cardiac service – we have been to Kazakhstan, Yemen, Egypt, Syria, Jordan, Bangladesh. Our target is the poor child with congenital heart disease who cannot get the service."

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Surgeons operate on a young boy in Muhimbili National HospitalIBT

"We train the people we go to," Al-Ata says. "We sometimes see how much assistance the locals can give us and we cut that from our team – not because we want to make our team smaller, but because we like to use them to help us, so they can learn, too."

In Tanzania, there are only a handful of surgeons able to perform open heart surgery. Although sub-Saharan Africa shoulders more than 24% of the global disease burden, it is home to just 3% of the global health workforce. And despite relative domestic stability in the country, many still live in poverty – a major barrier to accessing healthcare.

Frora is from Shinyanga, a city in northern Tanzania. She has travelled more than 600 miles for her four-year- old son, Innocent, to be treated. He is one of the lucky ones: he received surgery several days ago.

"The problem was with his breathing, he was crying, he could not do anything. He was sleeping a lot, he was sweating," Frora says, sitting at the end of Innocent's hospital bed.

"Flying hospital"

Congenital heart disease is particularly prevalent in developing countries, in part, because of a lack of maternal healthcare. While there is no obvious cause, the mother having certain infections such as rubella and poorly controlled type 1 and type 2 diabetes can increase the risk of heart defects.

Because Muhimbili is the only hospital which can provide paediatric heart surgery in Tanzania, parents have travelled from across the country with the hope of curing their sons and daughters. Twenty children have arrived from the island of Zanzibar, off the Tanzanian coast.

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Frora travelled 600 miles so her son could have surgery for a congenital heart defectIBT

"First, our waiting list is long – you are talking 500 patients on the list by the end of last year," says Dr Mohamed Janabi, head of the cardiac centre at Muhimbili. "And the majority of all patients – around 70% of them – cannot afford to pay. The third, which is the most important thing, is that we have a mass training of five, 10, 15 doctors at the same time – in the theatre, the cath lab or in intensive care. It means we are not going to become mission-dependent," Janabi says.

Health insurance is still low in Tanzania. Because of this, many people rely on direct payment at the point of use of health care. In most cases, this denies the poorest access to much-needed treatment. "Though the coverage is not where we want it, it is growing," Janabi adds. "I wish the coverage would reach more people because then we can just operate."

Not all of the children at Muhimbili need open-heart surgery. The hospital is also equipped with a 'cath lab' – where the team can do less-invasive heart operations using a probe, balloons and stents passed through a catheter.

Innocent had open-heart surgery to fix his condition. Dr Mohammad Shihata, one of the surgeons who treated him, says it is like a "flying hospital" – but Tanzania poses certain challenges not usually faced in developed countries.

"A lot of people have been missed for so long, or haven't had the chance to get access to healthcare, so they are really delayed. The human body is not built to sustain low levels of oxygen for prolonged periods of time, or too much volume load to one side of the heart," he says.

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Hundreds of patients are on the waiting list at Muhimbili, the only hospital equipped for cardiac surgeryIBT

Another challenge is a shortage of blood in banks – so much so that the head of the intensive care unit and the charity's project manager donate blood after one patient develops complications after surgery.

"You cannot help everybody"

With so many sick children in desperate need of care, it is difficult not to let emotion get in the way. Dr Khalid Kamal Al-Hroub, the head of intensive care, says you have to brush feelings to one side. "Sometimes with emotion you can panic and lose it. But it is better to hold up and be calm," he says. He is approaching his 40th mission and has travelled across Africa and the Middle East. "If you don't do this surgery, they will die," Al-Hroub explains.

In his ICU team is Zeba Butt, a 29-year-old nurse from Lahore, Pakistan. "I don't feel any difficulty emotionally because I know I'm working to make things better, so I should not be emotional. If I am emotional, how can I do my job?"

Her face crumples slightly when she talks about working in her native country – which has one of the highest rates of children born with congenital heart disease in the world. Each year, between 40,000 and 50,000 children are born with heart defects, but it has just eight paediatric heart surgeons. "I have pity for my nation," Butt says. "I want the help to go there."

It is impossible for the team to treat all of the children with heart defects in Tanzania, which is one of the most difficult challenges. "The families are really appreciative. But then towards the end, when you start to close the mission, you still have some people on your list," Dr Hayam Aboremsh, a cardiologist from Jeddah who works alongside her husband in the team. "It is a bit emotional because you cannot help everybody."

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Halima with her two-year-old son Shabani, who received surgery for his heart defectIBT

For the lucky ones, the surgery is a lifeline. But others must rely on the team's long-term goal – to train enough Tanzanian medics so they don't need outside help.

Two-year-old Shabani was diagnosed with congenital heart disease when he was four months old. His mother, Halima, noticed his pulse and breathing was fast and took him to the local clinic. He was referred to Muhimbili. "I heard there were operations being done, I was told my son would be included," she says. "Now he is good. He is happy. "

"Before he was very weak but now I think he will be good. He will go to school, he can do anything because now he is not sick at all. Now he is recovering, we say thanks."

IBTimes UK travelled to Muhimbili National Hospital courtesy of Muntada Aid.