Forced Air Warming System: Evaluation of Internal System Contamination

Methods

This is a quantitative, descriptive-exploratory laboratory study performed in a large hospital to evaluate microbial contamination in FAW. The institution selected for conducting this study has 22 surgical rooms, an anesthesia room, and an anesthesia recovery room. The operating room sector has 29 Bair Hugger 750^® (BH 750) forced air warming appliances and one Bair Hugger 505^® (BH 505) unit, totaling 30 units. Thus, at least one appliance is allocated to each operating room; despite the fact that they are fixed in these environments, they could be moved if necessary.

The following items were excluded for establishing the sample: The model 505 appliance, three appliances that had not undergone preventive maintenance in January 2017, and one that was out of use. Thus, a random draw of 50% of the FAW of the same model and which had undergone preventive maintenance at the same time was held for the 26 eligible appliances, totaling 13 FAW devices to compose the sample.

Data collection took place in two stages—at two and at six months after the inlet filter of each of the devices was replaced. The last collection six months after the filters were replaced was performed a few days before new maintenance to replace them. Replacement of the inlet filters in the appliances is performed every six months by the supplier company that owns the equipment, according to the guidelines of the operating manual. The last maintenance was performed in January 2017, and data collection took place in March and July 2017.

The samples were collected at night when the appliances were used less. All the evaluated appliances, however, had been used in surgical procedures and cleaned. The FAW appliance is cleaned immediately after use with quaternary ammonia on its external surface. The internal part of the appliance does not undergo any type of cleaning process. The thermal blanket used in the devices is disposable.

Previously trained collectors used surgical masks and gloves to handle the devices and other inputs used in the collection. The devices were moved to a different room for sample collection.

A pilot test for adequacy of the data collection procedure was performed to compare the collection time and the microbial contamination found at one and at 15 minutes of operation. Because no bacterial growth was verified for either time, it was standardized that the collections would be performed in the first minutes of operating the FAW appliances.

The samples for evaluating possible internal contamination of the FAW appliance was performed using plates containing Trypticase Soy Agar (TSA) culture medium. This is a non-selective culture medium that allows the growth of various types of micro-organisms such as bacteria and fungi. Each plate with TSA was positioned 1.5 cm away from the airflow outlet of the FAW hose so that the air coming from inside the appliance directly met the plate. Each collection lasted one minute and was timed using an electronic timer. It should be noted that unheated airflow was selected to preserve the culture medium. Samples were collected in triplicate for each of the 13 appliances.

The samples were then transported in enclosed thermal boxes to the Micro-biological Testing Laboratory of the Nursing School of the University of São Paulo, where they were incubated for 48 hours at a temperature of 36°C ± 0.1°C and checked every 24 hours. The quantitative analysis (colony counting) occurred with the aid of an electronic counter (Quimix).

Data analysis considered microbial contamination of the internal system of the FAW appliance as the dependent variable and the maintenance scheme of the appliance according to the periods of inlet filter replacement as the independent variable. Data were analyzed by absolute numbers and percentages.

Results

In the first step, 13 randomly selected appliances among those available were analyzed, thus generating 39 analyzed samples considering triplicate collections. Among the 39 analyzed samples, only nine (23.1%) presented microbial growth of one or two colonies after 24 and 48 hours of growth (Table 1).

In the second step six months after the inlet filter exchange, the second collection of samples from the same appliances was performed, with the exception of one appliance that was under maintenance and outside the hospital environment. Therefore, the collection was performed using 12 appliances in triplicate, thus totaling 36 analyzed samples.

Among these 36 evaluated samples, only six (16.7%) plates had growth of one or two colonies after 24 and 48 hours growth (Table 1).

Discussion

The results of the present study show that adequate maintenance of FAW produced small microbial growth—i.e., the number of colonies was only one or two per analyzed sample, meaning an average of 1.5 and 1.1 colonies, in 48 hours of incubation. The small growth found enables us to suppose that this contamination would not possibly be enough to cause harm to the patient, and it could even be retained in the thermal blanket, thus not presenting (in principle) any potential risks to the patient.

Previous investigations have observed microbial contamination in the internal structure of FAW, raising questions about the emission of contaminants into the environment, and especially near the surgical incision, associating the use of these forced air appliances to a possible increase in the risk of SSI and in forming convection currents [10–13]. Nonetheless, there is no conclusive evidence to determine the direct relationship between the use of FAW systems and the occurrence of SSI [6].

In this sense, a recent study performed in Europe evaluated microbial accumulation on internal surfaces of 23 FAW devices by smears on the internal air passage surface, finding samples of coagulase-negative staphylococci, fungi, and micro-cocci, respectively, inside 74%, 26%, and 9% of the appliances. The evaluated FAW appliances, however, did not present maintenance records, and it was not possible to evaluate the periodicity of filter system replacement or other preventive maintenance procedures [10]. Similarly, another investigation evaluating the possible contamination of the internal structures of FAW found the presence of Staphylococcus aureus, coagulase-negative staphylococci, and methicillin-resistant S. aureus, respectively, in 13.5%, 3.9%, and 1.9% of the analyzed appliances [11]. The cited studies, however, did not analyze SSI rates related to the use of contaminated equipment [10,11].

A review study examined evidence on the use of ultra-clean ventilation and its possible interactions with any type of patient heating device, particularly FAW, and possible relative increases in SSI. The authors concluded that the use of FAW generates a disturbance in the airflow of the operating environment, which can lead to an accumulation of contaminants near the surgical site; however, it found no significant increases in SSI rates after its use. In addition, the study also claimed that there are insufficient clinical trials evidencing an increase in SSI, which could nullify the effectiveness of implementing FAW in preventing peri-operative hypothermia [14].

Further, the institution selected to conduct the present research relies on the use of High Efficiency Particulate Arrestance (HEPA) filters. This type of ultra-clean ventilation in the operating rooms ensures a significant reduction in the number of airborne particles in the surgical center, as well as near the surgical incision site, reducing 99.97% of particles with a size of >0.3 mcm [14]. Thus, the air used by FAW appliances not only goes through a filtration system in the external environment, but also an inlet filter inside the equipment.

Another aspect to be considered is related to the fact that the micro-organisms most frequently responsible for SSI are S. aureus, coagulase-negative staphylococci, Escherichia coli, Enterococcus faecalis, Pseudomonas aeruginosa, Enterobacter spp., and Klebsiella spp. [4], all larger than 0.5 mcm, so that the ideal filtration system for FAW should be smaller than this size, thus ensuring retention of larger size particles.

A review of the literature analyzing 15 studies on the role of FAW in the occurrence of SSI showed that the evaluated investigations had methodologic limitations related to the sample number, information regarding maintenance, or also on the potential for bias because of financing received from companies. The authors concluded by highlighting the importance of using equipment in the prevention of peri-operative hypothermia, and also suggested that manufacturers develop devices that allow convenient cleaning of the internal airflow path. Finally, they claim that there is insufficient evidence showing that micro-organisms potentially allocated in the internal system of the devices are related to SSI cases [6].

A combination of appropriate product selection according to the clinical reality in which it will be employed associated with adequate adherence to the manufacturers' recommendations regarding the equipment maintenance, and adequate continuing education of the health team on the maintenance of body temperature in the peri-operative period [15] seems to be best for success of the intervention.

Considering the risks and the benefits, it seems that maintenance of peri-operative normothermia for preventing complications, among them SSI [3,4,6,8,9,15,16], is more advantageous than the potential risks associated with possible contamination [10,11]. In this study, considering the precautions related to indoor air quality and proper maintenance of the equipment in the analyzed institution, it is advisable to maintain the recommendation to use the equipment while following international guidelines [3,4,8], because these devices can aid effectively in prevention of hypothermia.

As limitations of this study, we can point out that analysis between the presence of microbial growth and the occurrence of SSI, or between identification by genus and species of the types of isolated micro-organisms was not performed.

Conclusions

This study showed small microbial growth of culture after 48 hours after filter replacement. New investigations that correlate the findings of micro-biology analysis and the occurrence of SSI should be conducted.