Accumulation and transport of microbial-size particles in a pressure protected model burn unit: CFD simulations and experimental evidence

Abstract

Background: Controlling airborne contamination is of major importance in burn units because of the high susceptibility of burned patients to infections and the unique environmental conditions that can accentuate the infection risk. In particular the required elevated temperatures in the patient room can create thermal convection flows which can transport airborne contaminates throughout the unit. In order to estimate this risk and optimize the design of an intensive care room intended to host severely burned patients, we have relied on a computational fluid dynamic methodology (CFD).

Methods: The study was carried out in 4 steps: i) patient room design, ii) CFD simulations of patient room design to model air flows throughout the patient room, adjacent anterooms and the corridor, iii) construction of a prototype room and subsequent experimental studies to characterize its performance iv) qualitative comparison of the tendencies between CFD prediction and experimental results. The Electricité De France (EDF) open-source software Code_Saturne® (http://www.code-saturne.org) was used and CFD simulations were conducted with an hexahedral mesh containing about 300 000 computational cells. The computational domain included the treatment room and two anterooms including equipment, staff and patient. Experiments with inert aerosol particles followed by time-resolved particle counting were conducted in the prototype room for comparison with the CFD observations.

Results: We found that thermal convection can create contaminated zones near the ceiling of the room, which can subsequently lead to contaminate transfer in adjacent rooms. Experimental confirmation of these phenomena agreed well with CFD predictions and showed that particles greater than one micron (i.e. bacterial or fungal spore sizes) can be influenced by these thermally induced flows. When the temperature difference between rooms was 7°C, a significant contamination transfer was observed to enter into the positive pressure room when the access door was opened, while 2°C had little effect. Based on these findings the constructed burn unit was outfitted with supplemental air exhaust ducts over the doors to compensate for the thermal convective flows.

Conclusions: CFD simulations proved to be a particularly useful tool for the design and optimization of a burn unit treatment room. Our results, which have been confirmed qualitatively by experimental investigation, stressed that airborne transfer of microbial size particles via thermal convection flows are able to bypass the protective overpressure in the patient room, which can represent a potential risk of cross contamination between rooms in protected environments.

Figure 1

Patient room and anteroom layout. Top view of intensive care room for severely burned patients (size L5.95 × W4.3 × H2.9 m) and anterooms layout.

Figure 2

CFD simulations. CFD snap-shot of the treatment room, anteroom and corridor under standard operating conditions with source contamination arising from the patient table. Colors represent the fraction of source contamination, with highest concentrations in red (10^-3) and lowest in blue (0.0). Dotted-line circles indicate zones of high airborne contamination.

Figure 3

CFD simulation of thermal convective flows. CFD images demonstrating the thermal convective flows between rooms that have different temperatures. Arrows indicate direction and the relative air speed by their length. Colors correspond to temperature, red being the highest and blue the lowest. Large dark arrows are superimposed on the flow field lines to help visualize the overall air flows. Figure 3a simulates a temperature difference of 7°C while 3b one of 2°C.

Figure 4

Experimental validation of CFD findings. Sequential particle measurement at 1 min interval during and after spiking with inert particles (dashed area). 4a: spiking the operating table, showing accumulation of particle in the upper part of the room (30 cm from the ceiling, black circles) compared to the lower part (30 cm from the floor, open circles). 4b: spiking the entry anteroom, showing transfer of particles from the upper part of the entry anteroom (black squares) towards the lower part of the room (open circles) when the door is opened (arrows).