A nurse presses a button on a patient monitor in a critical care unit.

The use of an appropriate flush fluid with arterial lines

HSIB legacy content

HSIB legacy content

This investigation was carried out by the Healthcare Safety Investigation Branch (HSIB). Find out more about HSIB legacy.


This investigation aimed to understand the risks for patients associated with blood sampling from arterial line systems used in adult critical care.

An arterial line is a system used to continuously monitor a patient’s blood pressure and intermittently monitor blood glucose levels by taking a blood sample from the arterial line. A thin tube called a cannula is inserted into an artery, usually in the person’s forearm, and tubing is used to attach a device called a transducer. This is connected to a bag of fluid (the flush fluid) and the pressure change of the fluid within the connecting plastic tubing transmits to an electrical monitoring device. This device displays the blood pressure within the artery as a continuous wave line on the monitoring screen. Arterial lines are used in other areas of healthcare and the findings of this investigation may also be relevant to these areas.

The selection and attachment of the incorrect flush fluid is a recognised risk in the use of arterial lines. When a flush fluid contains glucose rather than saline (0.9% sodium chloride) and a blood sample from an arterial line is contaminated, a false high blood glucose is recorded. This will mislead the clinician who may start the patient on insulin, which controls blood sugar levels. This may lead to unrecognised and dangerous levels of hypoglycaemia (low blood glucose levels), which can lead to the patient going into a coma. Careful monitoring of blood glucose levels has become established practice for critical care patients. It provides the information clinicians need to understand whether treatment is appropriate to correct an imbalance in blood glucose levels. Blood glucose monitoring using an arterial blood sample is recommended in critical care environments.

As an example, which is referred to as ‘the reference event’, the investigation reviewed the care of a patient named Keith. Keith received treatment to drain his gallbladder but then became very unwell. He was admitted to a critical care unit with significantly low blood pressure and sepsis (a reaction to infection that causes a person’s body to damage its own tissues and organs). An arterial line was inserted, and the incorrect flush fluid was connected to the transducer. The use of the incorrect flush fluid led to the contamination of the blood samples taken from the site of the arterial line, which consequently misled clinicians to give Keith an unnecessary and potentially harmful treatment.

This investigation’s findings, safety recommendations and safety observations aim to demonstrate the risk associated with the set-up of the arterial monitoring line and use of the correct flush fluid and to improve care for patients across the NHS.

Reference event

Keith was 66 years old and was recently retired. After feeling unwell for 25 days and attending medical appointments to investigate his gallbladder, he was admitted to hospital. Following an investigation and the drainage of his gallbladder his condition deteriorated. He was admitted to the critical care unit late in the year of 2020 with sepsis and very low blood pressure. Keith was medically unstable and required numerous medications and devices to be attached to him to enable accurate monitoring of his condition.

The insertion of the arterial line was completed around 16:20 hours. The doctor inserted a cannula into an artery in Keith’s arm, while a nurse went to look for the correct equipment, an arterial line (marked red) and a 500ml bag of saline (sodium chloride 0.9%) flush fluid. Saline is the recommended flush fluid for an arterial line and other fluids should not be used for this purpose.

The nurse looked first within the ‘lines’ drawers at the bedside (intended to include all necessary equipment for the insertion of an arterial line). When unable to find the correct arterial line transducer set, the nurse looked in two other areas of the critical care unit. They also collected the bag of saline flush fluid from the drug cupboard.

The nurse was aware that the cannula had been inserted and the blood would clot and block the line without the arterial line and saline flush fluid attached. They returned to the bedspace with a blue line central venous transducer set. The line and bag of flush fluid were checked and attached. The risk of attaching a blue transducer line was recognised by staff, as a blue transducer line is intended to be connected via a vein and can indicate it is safe to administer medication, which would not be safe through an arterial transducer line.

The need to change the line was prioritised by staff but the nature of Keith’s condition created several competing tasks. Keith’s blood glucose levels were found to be low. A different nurse went to retrieve a bag of glucose so that a glucose infusion (where glucose in a liquid solution is delivered via a patient’s vein) could be commenced to increase Keith’s blood glucose levels. Unable to locate the required strength of glucose, the nurse returned with two alternative strengths for the doctor to select for treatment. While collecting the glucose the nurse also found and returned with the correct red arterial transducer line. The red arterial transducer line and remaining bag of glucose were left together by the bed.

The other nurse interrupted their current task and collected both items to replace the blue transducer line that was in position with the correct red arterial transducer line. They replaced the central venous transducer set (blue line) and the correct saline flush fluid with the arterial transducer set (red line) and the incorrect bag of fluid containing glucose as the flush fluid.

Subsequent blood samples taken from Keith’s arterial line were contaminated with the flush fluid containing glucose. This led clinicians to conclude that Keith was suffering from high blood glucose levels and at risk of further harm. An infusion of insulin, the required treatment for high blood glucose, was administered for approximately 8 hours. This treatment reduced Keith’s blood glucose levels to below the recommended limit. The incorrect fluid was identified the next day, during morning safety checks completed by the nurse taking over Keith’s care. The treatment of insulin was stopped, and glucose administered to correct the blood glucose levels.

Keith had a brain scan on the same day, which concluded at that time there was no neurological damage associated with the abnormal blood glucose levels. He was discharged from the critical care unit 15 days after his admission following care for his underlying condition. Sadly, Keith died of COVID-19 later during his hospital stay.

The national investigation

HSIB was contacted by the Department of Health and Social Care in response to receiving a prevention of future deaths notice from a coroner’s investigation into the death of a woman aged 57 years. The investigation was completed on the 7 September 2020 and concluded that the use of a solution containing glucose instead of saline, the recommended fluid to flush an arterial line, contributed to the patient’s death.

HSIB identified Keith’s case as a similar patient safety incident. Although the event did not result in the death of a patient, it involved a similar sequence of events and the same error was identified.


The key findings from the investigation include:

  • The physical layout and design of the clinical and storage areas will influence how reliably staff are able to select and collect similar-looking equipment and medication.
  • The labelling of bags of fluids, similar looking medications and manufacturers’ packaging reduce the reliability of selecting the correct flush fluid in the context of a critical care unit with time pressures and high workloads.
  • The procurement and design of arterial transducer line equipment, the pressure infusion bags and transducer, do not assist in the identification of the incorrect flush fluid or prevent contamination from the flush fluid of a blood sample taken from the arterial line. Alternative equipment, for example transparent pressure infusion bags and closed arterial transducer lines, are currently available to the NHS. These may reduce the risk but are not routinely in use.
  • Challenges in the provision of a consistent suitable workforce and high workloads have a detrimental effect on the safety controls currently relied upon to avoid or identify the risk of using the wrong flush fluid. Safety checks and training lack resilience to organisational pressures regularly experienced within critical care units.
  • There can be a delay in identifying the contamination with glucose of an arterial line blood sample due to a normalisation and acceptance that critically ill patients may have altered blood glucose levels and require insulin treatment, and a perceived low risk associated with the use of a flush fluid.
  • The design of systems to record and monitor information relevant to the arterial transducer line system and blood glucose levels do not easily alert staff to the potential use of the wrong flush fluid.
  • Recommendations issued over the last 14 years by national safety bodies and professional healthcare organisations to address the safety of blood sampling associated with arterial lines have not been effectively implemented.


We have published a blog on The importance of equipment design in patient safety to help with understanding some of the key findings from this report.


Tell us what you think about this investigation using our online feedback form.

Investigation report