According to a Health and Disability Commission report, it was then determined that Ms A had suffered a brain injury caused by a lack of oxygen during her cardiac arrest.
She was taken off life support four days after being admitted to hospital.
Deputy Health and Disability Commissioner Carolyn Cooper investigated whether the MidCentral District Health Board and the ENT registrar, identified as Dr C, had provided Ms A with an appropriate standard of care.
She found the health board and Dr C had both breached the Code of Health and Disability Services Consumers' Rights by failing to provide services with reasonable care and skill.
She also found that the DHB, which has since been taken over by the national agency Te Whatu Ora – Health New Zealand, failed to provide the required level of co-operation between clinicians.
In the words of head and neck surgeon Chris Thomson of Christchurch, who provided advice to the inquiry: "Many of the pressures placed on (Dr C) in this situation were the likely result of an over-stretched and under-resourced hospital system with an apparent unavailability of senior staff at a critical time."
A DHB review of Ms A's care found that Dr C gave her lorazepam, which can be used to manage agitation, and the sedative midazolam after she became agitated and refused instructions and help from nurses to control her bleeding nose.
These medications were on top of her regular prescription for antipsychotic medication to treat schizoaffective disorder and bipolar disorder.
The review found that these contributed to an "accumulative peak" of medication in her system which increased her risk of respiratory problems, to which she was also prone.
Dr C did not consult a senior medical officer on the use and dosage of midazolam.
Ms A was placed in an unmonitored bed with no planned one-on-one nursing care and no line of sight to the main ED central station after being given the midazolam.
"The ED has ongoing challenges with the space and design of current facilities, which are a risk for patient safety," Cooper said.
Cooper also noted that during the inquiry it was noted that some staff were reluctant to call upon more senior clinicians too frequently.
Cooper recommended that MidCentral Health use her report for staff training.
She acknowledged that Dr C had made changes to his practice, including seeking support from senior doctors, and recommended that he also write an apology.
Lyn Horgan, Chief Operating Officer at Te Whatu Ora MidCentral, said they acknowledged and accepted the findings of the report and extended sympathies to the woman's whānau, to whom they had formally apologised.
"Since the incident we have implemented all the … recommendations, including changes to processes and facilities to minimise the risk of incidents like this occurring," she said.
These changes included ensuring a senior staff member must be consulted if a doctor is unfamiliar with, or not confident in administering, sedatives.
The ED and Medical Assessment and Planning unit was being expanded, and more senior staff had been recruited.
These included five senior medical officers and three nurse practitioners, with a budget for five nurse practitioners alongside a number of new roles.
