A new study suggests that the design of a particular heater-cooler unit used in cardiac surgeries raises the risk for invasive Mycobacterium chimaera infection.
Hundreds of patients have been sickened in a global outbreak of difficult-to-diagnose M. chimaera infections over the past decade. The infections have mainly been linked to the leading heater-cooler unit (HCU) brand, the LivaNova 3T. Physicians use HCUs with heart-lung machines to regulate a patient’s body temperature during cardiothoracic surgeries and other procedures.
Experts assume the LivaNova devices were contaminated at the German plant where they were manufactured — water outside the plant was found to harbor the same M. chimaera clone — but Richard Kuehl, MD, infectious diseases physician at University Hospital Basel in Switzerland and colleagues said M. chimaera has also been isolated from water tanks of other HCU models, raising questions about why 3T HCUs have been associated with infections.
Kuehl and colleagues examined three HCUs used at University Hospital of Basel, taking monthly water and air samples, analyzing the design of the devices and measuring exhaust airflow. They found four main differences between the LivaNova 3T and the other two devices:
3T HCUs kept water at an ambient temperature, potentially allowing M. chimaera to grow faster, whereas water in the other two devices — the HCU30 and HCU40, manufactured by Maquet — was held between 2°C and 4°C (35°F and 39°F).
3T HCUs directed air flow horizontally from two rear air vents, whereas the other units blew air from the sides to the floor.
Electronics were positioned directly on top of the water tank in 3T HCUs and were cooled by a ventilator that blew air at a high speed from directly above the water tank. Previous studies showed that unsealed holes in 3T water tanks caused aerosolized water to lead into this upper compartment. In the other models, the electronics are positioned away from the tank
The HCU40 includes an electronic reminder to notify the user if the next disinfection cycle is due, a feature not available on 3T HCUs.
Kuehl and colleagues determined that the upper ventilator blowing potentially contaminated air from directly above the water tank was “the main culprit” that amplified the risk for M. chimaera in surgeries where 3T HCUs were used.
“The much shorter time to positivity for air cultures taken close to the upper ventilator compared to the ones from the lower ventilator underlines this assumption,” they wrote.
Experts have recommended that hospitals either remove their 3T HCUs from the OR altogether and run them from a separate room or house them in a box or cage to separate them from OR air. But Kuehl and colleagues said HCUs could remain in the OR provided they are designed to minimize aerosol production and to keep any aerosols away from the ventilator air stream.
“It could be argued that it is a statistical phenomenon and that all patients worldwide were contaminated through the 3T HCU because it is by far the most [commonly] used type of HCU worldwide, with [around] 60% of the market share,” they wrote. “Nevertheless, this study provides strong evidence that the 3T HCU has the largest propensity to aerosolize M. chimaera due to its machine design. Further studies are needed to exclude M. chimaera aerosolization through other HCUs.”
In a statement, LivaNova told Infectious Disease News that it has begun modifying 3T HCUs to include internal sealing and a vacuum system designed to reduce the risk for possible dispersion of aerosols during surgery. Upgrades on existing 3T HCUs began in Europe last August, and the company anticipates expanding the program globally as it receives regulatory approvals.
It offers free deep cleaning services for 3T HCUs in many countries — including the United States — to any hospitals that have reported confirmed M. chimaera contamination. In countries where the design modification has been approved, 3T HCUs that are being cleaned also receive the modification before they are returned.
In a program started in the U.S., the company also will loan new 3T devices to hospitals that have old devices at no charge pending regulatory approval of the new modifications.
“LivaNova continues to work closely with regulatory agencies around the world to address this important issue of heater-cooler use during open-heart surgery, and remains committed to ensuring continued clinician access to this important device that enables lifesaving cardiac surgery,” the company said. – by Gerard Gallagher
Disclosures: The authors report no relevant financial disclosures.