836 infection control & hospital epidemiology july 2018, vol. 39, no. 7
second week. By November 2016, the protocol was intensified according to the manufacturer by increasing the disinfection interval to once weekly. Bacterial filters were also installed (Pall-Aquasafe disposable water filters for filling the tanks of the HCUs).
Statistical Analysis
Variables were analyzed using the χ2 or Fisher exact test as appropriate. Two-tailed P values <.05 were considered significant. Analyses were performed using SPSS version 22.0.0.0 software (IBM, Armonk, NY).
results
In 2014, contamination with M. chimaera was found in the water tanks of all 3T HCUs and in the HCU30 unit (Table 1). During usage, aerosolization of M. chimaera was initially detected in all 3THCUs but not the HCU30. No aerosolization occurredwith anyHCUthat had been switched off. Air cultures taken directly at the rear of the upper ventilator of contaminated 3T HCUs grew M. chimaera after 13 days, whereas cultures of the lower ventilator took 33 days. Despite intensified disinfec- tion since July 2014 and despite the exchange of the 3THCUs by August 2014, M. chimaera grew in 2 of 42 (5%) water samples from the 3T HCUs in 2015. Following a change to disinfection with peracetic acid, no further M. chimaera were detected. After replacing the 3T HCU with the HCU40 in mid-January 2016, M. chimaera could not be detected in any of the water and air samples obtained from this type of HCU over the complete surveillance period of 15 months (0 of 114). Therefore, we applied epidemiological methods in terms of a “case-control study” to compare the 3T HCUs with the HCU30 and the HCU40. We asked the manufacturers for detailed technical information. Both companies, LivaNova/Sorin and Maquet, kindly provided the requested data (Table 2). During the epi- demiologic workup, we hypothesized that the exhaust air may
substantially contribute to the dissemination of the myco- bacteria. Therefore, we investigated the direction of airflow and measured the exhaust air velocity. The position of the ventila- tors and water tanks as well as the direction of airflow differed substantially among the 3 types of HCUs. We noted 4 main differences in the designs of the otherHCUs compared to the 3T HCU: (1) continuous cooling of the water tank at 4°C, (2) location of cooling ventilators, (3) air flow direction, and (4) the HCU40 includes an electronic alarm to remind the user of the next disinfection cycle (Figure 1). The velocity of the air of themain ventilators for the compressors differed depending on themode of action, with maximal air speed during cooling. Air cooling of the electronic board is solved differently depending on
theHCUtype.Only the 3THCUhas an additional ventilator for the electronic board exhaust air, thereby producing a second source of emitted airflow. This ventilator exhausted air at 75 cm above ground and was located directly on top of the water tank. This air speed was the highest of all measured (2.3–2.4m/s) and remained constantly high after the HCU was switched on. In addition, this airflow was independent of the activity of cooling. The strong airflow of the upper ventilator was visua- lized in smoke dispersal experiments as well (Figure 2). Similarly, the HCU30 also contains a ventilator for cooling of the electronic control board. However, its airflow is directed inward and is not directly in contact with the water tank.
discussion
Currently, only the 3T HCU has been clearly linked to M. chimaera infections of patients through contaminated aerosols.5,6,11 While the HCU30 showed contamination of the device but without aerosolization, only the HCU40 remained negative for M. chimaera during 15 months of intensified surveillance. These findings agree with those of internationally documented cases, which thus far have been associated with the 3T HCU.5 However, the mode of transmission from the water tank and aerosolization to patients by the 3T HCU has
table 1. Microbiological Results of Sampling of Water and Air (During Operation) Samples With M. chimaera Detection, No./Total (%)
Used 3T HCU New 3T HCU HCU30
June 2014– December 2014 Water Air
January 2015–December 2015 Water Air
January 2016–March 2017 Water Air
Total Water Air
8/12 (66) 22/24 (91)
Not in use Not in use
Not in use Not in use
8/12 (66) 22/24 (91)
0/35 (0) 0/19 (0)
2/42 (5) 0/20 (0)
0/6 (0) 0/2 (0)
2/83 (2) 0/41 (0)
7/12 (58) 0/4 (0)
Not in use Not in use
Not in use Not in use
7/12 (58) 0/4 (0)
HCU40
Not in use Not in use
Not in use Not in use
0/64 (0) 0/50 (0)
0/64 (0) 0/50 (0)
P Value
<.01 <.01
… …
NA NA
<.01 <.01
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