NIHR Signal Laminar airflow in surgery might not reduce surgical site infections

Published on 11 July 2017

The type of theatre ventilation system used during hip and knee replacement, abdominal or vascular surgery has no effect on the rate of surgical site infections. Prevention of surgical site infection is a complex area with many potential targets for action. So decisions relating to commissioning or decommissioning these systems will need to consider the totality of the evidence alongside the costs.

This systematic review included 12 observational studies comparing wound infection rates following surgery performed in theatres using laminar airflow or conventional ventilation. Laminar ventilation distributes air in parallel layers, forming a “curtain” which has been thought to limit contamination. However, recent studies have raised doubt whether it actually affects surgical infection rates.

Individual study results varied considerably. These reviewers did try to isolate the effect of airflow by controlling for the other factors that also influence infection rates. However, as this is observational data these other factors might have differed between the studies or might not have been recorded. So, the comparisons might not have been strictly “fair”.

These findings do support conventional ventilation in operating theatres as a reasonable option. This could represent a lower investment in operating rooms, releasing NHS resources for other things. But the decision should be made after considering all strategies for reducing infections, such as special air filters, antibiotic protocols or simply restricting human movement.

Why was this study needed?

Surgical site infections increase morbidity, hospitalisation days and costs. In 2011 surgical site infections were the third most common type of healthcare-associated infection in the UK, accounting for 16% of all infections.

Many things influence the development of surgical site infections, including smoking, obesity, intraoperative temperature, glycaemia, or the use of surgical cautery. Personnel movement, antibiotic use and the type of theatre ventilation are also important factors.

Modern operating theatres have high levels of ventilation which swirls in different directions in a turbulent way. Laminar airflow, where the air moves in one direction and in parallel layers, can be used to form invisible curtains around the operating site which keep out potentially contaminated particles. In the 1970s laminar airflow was reported to reduce wound infection compared with conventional ventilation, but these results have not been reproduced in later studies.

This review aimed to review all the existing reliable literature and see if use of laminar airflow ventilation, adjusted for other influences, was linked to reduced surgical site infections.

What did this study do?

This systematic review and meta-analysis included 12 observational studies comparing laminar airflow with conventional ventilation in the operating theatre.

Ten studies looked at wound infections following hip or knee replacement. Three studies included people having abdominal or open vascular surgery. Two studies came from the UK with others from Europe, the US, New Zealand and South Korea.

There were unexplained differences (heterogeneity) in the results. Most data came from national registries or surveillance systems, which may not have collected information on relevant confounders, such as patient characteristics, operative environment and techniques used. Furthermore, several studies also lacked information on the conventional ventilation system used as a comparator to laminar airflow. Therefore, the findings should be interpreted with some caution.

What did it find?

  • Laminar airflow ventilation did not have a statistically significant effect on the risk of infections after hip replacement surgery (odds ratio [OR] 1.29, 95% confidence interval [CI] 0.98 to 1.71). This result came from eight studies covering 330,146 surgical procedures. The wound infection rate was 0.8% in procedures using laminar airflow (1,544 infections) compared with 0.5% using conventional ventilation (671 infections).
  • Neither did the ventilation technology have an effect on the risk of wound infections following total knee replacement (OR 1.08, 95% CI 0.77 to 1.52). This result came from six studies with a total 738 wound infections across 134,368 procedures (event rate 0.6% with laminar flow vs. 0.5% with conventional).
  • Based on three studies, laminar airflow ventilation had no effect on the development of SSIs following 63,472 abdominal or open vascular surgical procedures (OR 0.75, 95% CI 0.43 to 1.33; event rate 2.3% vs. 2.0%).

What does current guidance say on this issue?

The Royal College of Anaesthetists Guidance on the Provision of Anaesthesia Services for Trauma and Orthopaedic Surgery recommends that, in order to reduce the risk of wound infection, major joint replacements and surgery involving bone implants or internal fixation should be carried out in an operating theatre with multiple air changes per hour, but not necessarily laminar flow.

This is reinforced by British Orthopaedic Association guidelines which recommend using dedicated orthopaedic theatres with laminar flow.

What are the implications?

On the basis of the current evidence if baseline infection rates are less than 1%, surgical infection rate may not be reduced further by laminar flow ventilation.

Given the higher capital and running costs of laminar flow systems their role may need to be reviewed. The lack of evidence that they prevent wound infections does not necessarily mean laminar airflow ventilation should be decommissioned if currently in place. There may be a case against installing new equipment.

Due to the observational nature of the evidence it remains unclear if this finding may be influenced by confounding factors that could be alternative explanations for the findings. A broader view of all prevention practices in surgery might be advisable before commissioning decisions are made.

Citation and Funding

Bischoff P, Kubilay NZ, Allegranzi B, et al. Effect of laminar airflow ventilation on surgical site infections: a systematic review and meta-analysis. Lancet Infect Dis. 2017;17(5):553-61.

There was no funding source for this study.

Bibliography

British Orthopaedic Association. A national review of adult elective orthopaedic services in England. London: British Orthopaedic Association; 2015.

Health Protection Agency. English National Point Prevalence Survey on Healthcare-associated Infections and Antimicrobial Use, 2011: Preliminary data. London: Health Protection Agency; 2012.

Royal College of Anaesthetists. Guidance on the Provision of Anaesthesia Services for Trauma and Orthopaedic Surgery. London: Royal College of Anaesthetists; 2016.

Weinstein RA, Bonten MJ. Laminar airflow and surgical site infections: the evidence is blowing in the wind. Lancet Infect Dis. 2017;17(5):472-73.

Why was this study needed?

Surgical site infections increase morbidity, hospitalisation days and costs. In 2011 surgical site infections were the third most common type of healthcare-associated infection in the UK, accounting for 16% of all infections.

Many things influence the development of surgical site infections, including smoking, obesity, intraoperative temperature, glycaemia, or the use of surgical cautery. Personnel movement, antibiotic use and the type of theatre ventilation are also important factors.

Modern operating theatres have high levels of ventilation which swirls in different directions in a turbulent way. Laminar airflow, where the air moves in one direction and in parallel layers, can be used to form invisible curtains around the operating site which keep out potentially contaminated particles. In the 1970s laminar airflow was reported to reduce wound infection compared with conventional ventilation, but these results have not been reproduced in later studies.

This review aimed to review all the existing reliable literature and see if use of laminar airflow ventilation, adjusted for other influences, was linked to reduced surgical site infections.

What did this study do?

This systematic review and meta-analysis included 12 observational studies comparing laminar airflow with conventional ventilation in the operating theatre.

Ten studies looked at wound infections following hip or knee replacement. Three studies included people having abdominal or open vascular surgery. Two studies came from the UK with others from Europe, the US, New Zealand and South Korea.

There were unexplained differences (heterogeneity) in the results. Most data came from national registries or surveillance systems, which may not have collected information on relevant confounders, such as patient characteristics, operative environment and techniques used. Furthermore, several studies also lacked information on the conventional ventilation system used as a comparator to laminar airflow. Therefore, the findings should be interpreted with some caution.

What did it find?

  • Laminar airflow ventilation did not have a statistically significant effect on the risk of infections after hip replacement surgery (odds ratio [OR] 1.29, 95% confidence interval [CI] 0.98 to 1.71). This result came from eight studies covering 330,146 surgical procedures. The wound infection rate was 0.8% in procedures using laminar airflow (1,544 infections) compared with 0.5% using conventional ventilation (671 infections).
  • Neither did the ventilation technology have an effect on the risk of wound infections following total knee replacement (OR 1.08, 95% CI 0.77 to 1.52). This result came from six studies with a total 738 wound infections across 134,368 procedures (event rate 0.6% with laminar flow vs. 0.5% with conventional).
  • Based on three studies, laminar airflow ventilation had no effect on the development of SSIs following 63,472 abdominal or open vascular surgical procedures (OR 0.75, 95% CI 0.43 to 1.33; event rate 2.3% vs. 2.0%).

What does current guidance say on this issue?

The Royal College of Anaesthetists Guidance on the Provision of Anaesthesia Services for Trauma and Orthopaedic Surgery recommends that, in order to reduce the risk of wound infection, major joint replacements and surgery involving bone implants or internal fixation should be carried out in an operating theatre with multiple air changes per hour, but not necessarily laminar flow.

This is reinforced by British Orthopaedic Association guidelines which recommend using dedicated orthopaedic theatres with laminar flow.

What are the implications?

On the basis of the current evidence if baseline infection rates are less than 1%, surgical infection rate may not be reduced further by laminar flow ventilation.

Given the higher capital and running costs of laminar flow systems their role may need to be reviewed. The lack of evidence that they prevent wound infections does not necessarily mean laminar airflow ventilation should be decommissioned if currently in place. There may be a case against installing new equipment.

Due to the observational nature of the evidence it remains unclear if this finding may be influenced by confounding factors that could be alternative explanations for the findings. A broader view of all prevention practices in surgery might be advisable before commissioning decisions are made.

Citation and Funding

Bischoff P, Kubilay NZ, Allegranzi B, et al. Effect of laminar airflow ventilation on surgical site infections: a systematic review and meta-analysis. Lancet Infect Dis. 2017;17(5):553-61.

There was no funding source for this study.

Bibliography

British Orthopaedic Association. A national review of adult elective orthopaedic services in England. London: British Orthopaedic Association; 2015.

Health Protection Agency. English National Point Prevalence Survey on Healthcare-associated Infections and Antimicrobial Use, 2011: Preliminary data. London: Health Protection Agency; 2012.

Royal College of Anaesthetists. Guidance on the Provision of Anaesthesia Services for Trauma and Orthopaedic Surgery. London: Royal College of Anaesthetists; 2016.

Weinstein RA, Bonten MJ. Laminar airflow and surgical site infections: the evidence is blowing in the wind. Lancet Infect Dis. 2017;17(5):472-73.

Effect of laminar airflow ventilation on surgical site infections: a systematic review and meta-analysis

Published on 22 February 2017

Bischoff, P.,Kubilay, N. Z.,Allegranzi, B.,Egger, M.,Gastmeier, P.

Lancet Infect Dis , 2017

BACKGROUND: The role of the operating room's ventilation system in the prevention of surgical site infections (SSIs) is widely discussed, and existing guidelines do not reflect current evidence. In this context, laminar airflow ventilation was compared with conventional ventilation to assess their effectiveness in reducing the risk of SSIs. METHODS: We searched MEDLINE, Embase, Cochrane Central Register of Controlled Trials, and WHO regional medical databases from Jan 1, 1990, to Jan 31, 2014. We updated the search for MEDLINE for the period between Feb 1, 2014, and May 25, 2016. We included studies most relevant to our predefined question: is the use of laminar airflow in the operating room associated with the reduction of overall or deep SSI as outcomes in patients of any age undergoing surgical operations? We excluded studies not relevant to the study question, studies not in the selected languages, studies published before Jan 1, 1990, or after May 25, 2016, meeting or conference abstracts, and studies of which the full text was not available. Data were extracted by two independent investigators, with disagreements resolved through further discussion. Authors were contacted if the full-text article was not available, or if important data or information on the paper's content was absent. Studies were assessed for publication bias. Grading of recommendations assessment, development, and evaluation was used to assess the quality of the identified evidence. Meta-analyses were done with RevMan (version 5.3). FINDINGS: We identified 1947 records of which 12 observational studies were comparing laminar airflow ventilation with conventional turbulent ventilation in orthopaedic, abdominal, and vascular surgery. The meta-analysis of eight cohort studies showed no difference in risk for deep SSIs following total hip arthroplasty (330 146 procedures, odds ratio [OR] 1.29, 95% CI 0.98-1.71; p=0.07, I2=83%). For total knee arthroplasty, the meta-analysis of six cohort studies showed no difference in risk for deep SSIs (134 368 procedures, OR 1.08, 95% CI 0.77-1.52; p=0.65, I2=71%). For abdominal and open vascular surgery, the meta-analysis of three cohort studies found no difference in risk for overall SSIs (63 472 procedures, OR 0.75, 95% CI 0.43-1.33; p=0.33, I2=95%). INTERPRETATION: The available evidence shows no benefit for laminar airflow compared with conventional turbulent ventilation of the operating room in reducing the risk of SSIs in total hip and knee arthroplasties, and abdominal surgery. Decision makers, medical and administrative, should not regard laminar airflow as a preventive measure to reduce the risk of SSIs. Consequently, this equipment should not be installed in new operating rooms. FUNDING: None.

Expert commentary

Disentangling what myriad factors affect surgical infection rates is difficult; rates of infection are (thankfully) low, and many variables need to be accounted for. So how confidently can we ditch laminar flow in theatres of the future? Whilst the data in this study is observational (lower quality), a considerable number of patients have been examined, and there is unlikely to be the enthusiasm or resources for a contemporary definitive randomised controlled trial. Whilst it may be inherently counterintuitive to replace a more advanced technology for a simpler one, in the case of laminar flow, it's probably time we did.

Dr David Bosanquet, Surgical Registrar, University Hospital of Wales

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  •   Commissioning, Health management, Infections, Perioperative care, Surgery