A central venous catheter (CVC) machine used for haemodialysis in kidney patients.

Safety risk of air embolus associated with central venous catheters used for haemodialysis treatment

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 aims to improve patient safety by supporting healthcare staff in the safe use of central venous catheters to access a patient’s blood supply. Specifically, it looks into the use of tunnelled haemodialysis central venous catheters, which are a type of central line.

A tunnelled dialysis catheter is a hollow plastic tube which is used with patients who need long term haemodialysis, which is treatment on a dialysis machine. It is usually inserted into a vein in the neck and sits in a large vein in a patient’s chest. The end of the tube that sits outside the patient’s body is separated into two lumens (tubes), each with a clamp on it and a cap on the end. One of the lumens is for the blood to flow out of the body and through the dialysis machine, where it is filtered and returned to the body via the second lumen. This process happens continuously during haemodialysis treatment.

In this investigation, the focus is on permanent haemodialysis catheters, although similar risks of an air embolism also apply to a temporary haemodialysis catheter. An air embolism occurs when air bubbles enter a patient’s vein and block it; this can cause serious harm or death.

The uncapping and unclamping of a haemodialysis catheter (without attaching a syringe), by a user not familiar with the medical device, task or the equipment required, may allow air to get into a person’s bloodstream, resulting in an air embolus.

A haemodialysis catheter has a wide bore (internal diameter) compared to other types of central line, to allow a high volume of blood to flow through. It is this wide bore that increases the risk of a large air embolus, compared with narrower catheters.

The investigation used a real patient safety incident, referred to as ‘the reference event’, to examine aspects of the management of, and access to, haemodialysis catheters to take blood samples.

The reference event involved Joan, who had a cardiac arrest caused by an air embolus after her haemodialysis catheter was uncapped, unclamped, and left open to air. This took place during a procedure to take blood culture samples to test for a possible line infection (where bacteria or viruses enter the bloodstream).

This investigation’s findings, safety recommendations and safety observations aim to prevent the future occurrence of an air embolus following uncapped and unclamped haemodialysis catheters, and to improve care for patients across the NHS.

Reference event

Joan was 75 years old at the time of the incident. She was taken to hospital from a nurse-led satellite haemodialysis unit, where she had been receiving regular treatment. Joan reported feeling unwell and was found to have low blood pressure and a suspected diabetic toe ulcer infection.

A plan was made in the emergency department to start Joan on antibiotics, and she was admitted to hospital for ongoing treatment. Following a short stay on a medical assessment unit, she was transferred to an older persons’ ward for further investigations and treatment.

Microbiologists informed the clinicians caring for Joan that bacteria had grown in her initial blood culture samples which was unlikely to have arisen from her toe ulcer. Microbiologists advised taking more blood cultures from Joan’s haemodialysis catheter to exclude an infection on her line and recommended a change in the type of antibiotic treatment.

The following day, Joan’s care was reviewed by a multidisciplinary team on the older persons’ ward, and the task of taking the blood cultures was discussed. A medical student asked if they could carry out the task, under the supervision of a junior doctor who had volunteered to take the blood cultures. This was agreed by a specialist trainee doctor, after a run-through of the steps involved in the task.

The junior doctor and medical student approached Joan to discuss taking the blood cultures. They started unwrapping the gauze covering over the end of the haemodialysis catheter, and realised it looked different to other central lines they had seen previously. After an unsuccessful attempt to seek nursing support/guidance, they decided to continue with the task. The cap of the line was removed and while the medical student was cleaning the port, the junior doctor unclamped the line. After approximately 4 minutes, Joan collapsed and suffered a cardiac arrest (her heart stopped). She was successfully resuscitated and transferred to the intensive care unit but died 2 days later from multi-organ failure and sepsis.

A subsequent coroner’s inquest concluded that a possible contributory cause of Joan’s death was the cardiac arrest, which was ‘probably secondary to air embolus’.

The national investigation

The Department of Health and Social Care notified HSIB of the incident relating to an ‘uncapped central line’, after it had been issued with a Prevention of Future Deaths report by HM Coroner. The matters of concern relating to this investigation included medical school training regarding taking blood samples from central lines; the assessment of doctors’ competencies to undertake procedures; and guidance on the safety risks when taking blood from central lines.

HSIB launched an initial scoping investigation in August 2021, which determined that the patient safety concern met the criteria for investigation. HSIB’s Chief Investigator authorised a national investigation.

A national investigation was undertaken to explore the factors that affect the ability of staff to safely access haemodialysis catheters. The focus was on this specific type of central line because of the increased safety risk of an air embolus associated with a wider bore line.

The national investigation focused on:

  • Identifying the factors within the healthcare system that influence awareness and understanding of patient safety risks associated with the management of haemodialysis central venous catheters.
  • Evaluating the current mechanisms for the mitigation of patient safety risks associated with haemodialysis central venous catheters.


  • The investigation found there are currently no long-term haemodialysis catheters on the UK market, or being developed, that have integrated ‘safety-valves’.
  • Manual clamps on haemodialysis catheters rely on people ensuring that the clamp is on before accessing the haemodialysis catheter ports and do not mitigate against design-induced error.
  • A review of patient safety risks associated with other haemodialysis devices (for example, fistulas) showed several mitigations which are not consistently used for haemodialysis catheters. These include a coloured patient wristband, line labelling, alert cards being carried by the patient and educating patients/family members.
  • The training and education of all grades of medical staff has not been consistent in relation to the risks of catheter-related air embolism.
  • There is currently no recognised national training or national training guidelines regarding the safe access of haemodialysis catheters.
  • Incidents appear to be under-reported to the Medicines and Healthcare products Regulatory Agency, due to misconceptions about ‘human error’ being the cause, rather than the design of the equipment.
  • The Medicines and Healthcare products Regulatory Agency, in partnership with NHS England, have explored integrated incident reporting system possibilities. While a recent funding bid to support full development for an in-service solution was unsuccessful, the organisations are committed to drive this project forward.
  • There is a general lack of literature on, and knowledge of, catheter-related air embolism in relation to access when the catheter is in situ (in position), rather than during insertion or removal of the catheter.