Published Wednesday, 14 November 2012
The baby girl was born in Northern Ireland on 6 October 2008, with a hole in her heart and a further heart defect.
She died at Birmingham Children's Hospital one month after her first birthday.
Hayley underwent palliative surgery at the Royal Victoria Hospital inBelfast, but was then flown to Birmingham for corrective heart surgery.
She died there 12 days after being transferred from an intensive care unit to a general ward.
Her mum Paula Stevenson, 40 - who is originally from Northern Ireland, but now lives in Australia - had told an earlier hearing that she believed her daughter could have been saved if staff had listened to her concerns at the time.
At the end of the inquest, Coroner Aidan Cotter said on Wednesday: "The failures by a number of the staff at Birmingham Children's Hospital were serious but not gross."
Speaking after the verdict was recorded, Ms Stevenson said she had been waiting three years to speak up.
Nobody listened to me while Hayley was dying and nobody listened to me when Hayley was dead.
While Hayley's heart surgery was a success, she spent 17 days in Birmingham Hospital's Paediatric Intensive Care Unit before being moved onto a normal ward.
Her mum raised concerns about the little girl's breathing, but says she was ignored.
On 9 November 2008, an X-ray revealed that Hayley's right lung had collapsed. She was given physiotherapy the following day but, on the morning of 11 November, both lungs collapsed and she went into cardiac arrest.
She was pronounced dead after doctors spent 20 minutes trying to save her, the inquest heard.
"Our entire family has been completely devastated by what happened and to this day we continue to grieve for Hayley," Ms Stevenson said.
"I still cannot understand how trained medics could ignore the fact that she was slowly deteriorating before their eyes.
"My parents and I never left Hayley's bedside during the entire time she was in hospital and it was obvious to us, despite our lack of medical training, that she was a very sick little girl who needed help."
In an internal review of Hayley's death, the NHS found that a "hierarchy" among medics deterred junior staff from referring Hayley back to a paediatric intensive care unit in the days before she died.
Ms Stevenson, who called for the NHS to set up rapid response teams to give patients' families a right to an urgent second opinion, added: "It is to their shame that not one of them saw fit to ask for an urgent second opinion."
© UTV News