M. chimaera transmission in the OR: Progress from Switzerland

Hugo Sax and his colleagues first reported the link between heater-cooler units (HCUs) and an outbreak of invasive Mycobacterium chimaera infections, and today they published a follow up investigation that strongly supports their hypothesis regarding mode of transmission.

I encourage you to read the article for yourself, but it describes a set of experiments using smoke dispersal and particle counts to demonstrate that the HCU tested (Sorin Model 3T) both dispersed smoke and increased particle counts over the surgical field in a OR with ultraclean airflow ventilation (also known as laminar flow). As would be expected, the dispersal of smoke and particles was greatest when the unit exhaust was oriented toward the surgical field, but higher particle counts were noted even when the exhaust was oriented away. Finally, settle plates were used to show that M. chimaera could be isolated up to 5 meters away from a contaminated HCU while it was in operation (this experiment was performed in a non-ventilated test room, so in essence determined how far the aerosols carried when no air currents were being generated (other than by the HCU itself). 

The video below shows how smoke dispersal through the HCU was carried by the OR ventilation system to directly over the surgical field (scary!). 

The authors point out that airflow systems designed to keep OR air clean may fail to protect against, or paradoxically contribute to, the dispersal of HCU exhaust over the surgical field. While I find this an interesting explanation for why some patients may be at higher risk for exposure and infection than others, the main story here is still that there is a device in the OR that produces a microbial aerosol:

“Heater–cooler units are stand-alone devices that contain a tank that holds filtered tap water, which serves as transfer fluid to control the temperature of patient blood and cardioplegia solution……The tank features filling and overflow tubes, heating and cooling coils, probes, and stirring devices. It is not airtight [emphasis mine]. The space beneath the tank holds a radiator to dissipate superfluous heat produced through water cooling. The efficacy of this radiator is increased by a fan that ensures a constant airflow through ventilation grid openings on either side of the heater–cooler unit housing.”

In short, this HCU seems designed to produce contaminated aerosols once it becomes colonized. Thus the only two options are to get the units out of the OR, or find a way to keep them clean. On this point, there is another key tidbit from this article: 

“…M. chimaera has regrown in some of the monthly water cultures from most factory-new heater–cooler units delivered in January 2014 after 1 year of operation on a daily water-change with filtered tap water scheme…”

This is consistent with what we’ve learned from others dealing with this issue: there seems to be no reliable way to keep these units free of contamination, or to decontaminate them once they are contaminated. 

Thus the most important sentence in this article is the last one: 

“…all heater–cooler units should be reliably separated from air that can gain access to sterile areas and instruments...”

How to accomplish this is the current challenge: we took the approach of removing the units entirely from the OR, while the authors report that “their exhaust air is now captured by a secondary housing and reliably channeled to the operating room exhaust.” 

Until a better-designed (i.e. safe) HCU is widely available, hospitals are in dire need of guidance about how to eliminate this insidious risk to their patients.