Air movement in Laminar Flow With Different Patient Warming Systems

Forced Air Warming (FAW) Versus Conductive Fabric Blankets (CFB): A Randomized Trial of Laminar Flow Disruption in Orthopedics


Laminar ventilation protects the surgical site from airborne pathogens. However, ventilation performance is fragile and can be compromised by flow obstructions and thermals. Patient warming systems represent a potential source of ventilation disruption via the waste heat they generate. For equivalent patient warming, forced air warming (FAW) generates ≈800 watts of waste heat compared to ≈100 watts with conductive fabric blankets (CFB). We compared the effects of these alternative systems on ventilation performance during a mock hip replacement surgery.

Methods: In an orthopedic operating theatre (0.5m/s airflow and 19.0ºC), a mannequin was draped in the lateral position and operated on by a surgeon and anesthesiologist at a table of comfortable height (95cm) with lights in their normal position. A replicated factorial experiment was used to randomly assign the warming system (FAW or CFB) and anesthesia screen position to no screen (drapes lying-over-patients head), half screen (1.5-meters-tented), or Full screen (2.0-meters-tented). Neutrally buoyant soap bubbles were created using a mix of helium and air and these were introduced under the drape near the mannequin’s mouth. Laminar ventilation performance was assessed by counting the number of bubbles reaching the surgical site via photography.

Results: Using bubbles present in the operative field as the assessment of contamination there was no contamination when using CFB. When using FAW there was a mean of 13.6 bubbles with a half screen, 0.6 with no screen and 0.2 with a full screen. A significant increase in laminar ventilation disruption was detected using Poisson regression for FAW versus CFB with no screen (6 versus 0 bubbles; p<0.01) and half screen set ups (146 versus 0; p<0.01); ventilation disruption was non-significant when the full anesthetic screen was used(2 versus 0; p=0.16).

Conclusion: FAW, with high waste heat, disrupts laminar ventilation performance and mobilized potentially contaminated under-drape air against the downward laminar flow and into the surgical site; CFB, with low waste heat load had no mobilizing effect. The common combination of FAW with a half anesthesia screen was associated with the highest contamination. The authors can clearly demonstrate the effect using video footage.