Real time monitoring of carbon dioxide levels in surgical helmet systems worn during hip and knee arthroplasty

Abstract

Background

Orthopaedic surgical helmet systems (SHS) rely on an intrinsic fan to force clean external air over the wearer. Carbon dioxide (CO₂) is produced through aerobic metabolism and can potentially accumulate inside the SHS. Levels above 2500 ppm have previously been shown to affect cognitive and practical function. Maximum Health and Safety Executive (HSE) 8-h exposure limit is 5000 ppm. There is a paucity of data on real-world CO₂ levels experienced during arthroplasty surgery whilst wearing a SHS.

Objectives

To determine intra-operative levels of CO₂ experienced within SHS.

Methods

CO₂ levels were continuously recorded during 30 elective arthroplasties, both primary and revision. Data was recorded at 0.5Hz throughout the procedure utilising a Bluetooth CO₂ detector, worn inside a surgical helmet worn with a toga gown. Five surgeons contributed real time data to the study.

Results

The average CO₂ level across all procedures was 3006 ppm, with 23 of the cases measured within the surgeons’ helmets having a mean above 2500 ppm, but none having a mean above 5000 ppm. For each procedure, the time spent above 2500 and 5000 ppm was calculated, with the means being 72.6 % and 5.4 % respectively. Minimum fan speed was associated with only a marginally higher mean CO₂ value than maximum fan speed.

Discussion

The use of surgical helmet systems for elective orthopaedic surgery, can result in CO₂ levels regularly rising to a point which may affect cognitive function.

Conclusion

Further research is needed to corroborate these findings however, we recommend that future designs of SHS include active management of exhaust gases, possibly returning to Charnley's original design principles of the body exhaust system.

1. Introduction

Orthopaedic helmets are widely used, particularly in arthroplasty surgery.¹ These systems were first introduced in the 1960s by Sir John Charnley at Wrightington Hospital as body exhaust suits (BES) to keep exhaust and skin scales away from the operative field to reduce infection risk.^2,3 Charnley's system used a reusable sterile toga gown and surgical helmet with built in suction extraction tubing. This system worked on a negative pressure, extraction principle. Current designs of surgical helmets systems (SHS) have evolved away from this principle and now utilise forced air positive pressure ventilation through a battery powered fan on the top of the helmet. However, there is no active management of the exhaust gases. It is assumed exhaust gases flow out of the lower aspect of the gown, below the operative field.

Carbon dioxide (CO₂) is a waste gas from human respiration and can be harmful if inspired at certain levels. As the CO₂ concentration in inspired air rises it can cause headaches, dizziness, confusion, loss of cognitive ability and ultimately loss of consciousness. Since CO₂ is heavier than air, it can pool in reservoirs such as folds in surgical gowns.

CO₂ is classified as a ‘substance hazardous to health’ and in order to reduce symptoms described above, workplace exposure limits (WELs) exist to protect workers health and safety. WELs are defined by a country's health and safety association. Both the British (HSE)⁴ and American (OSHA)⁵ standards for CO₂ WELs include a limit of 8 h exposure over 5000 ppm during a 40-h working week.

Furthermore, there have been many studies into the effects of CO₂ levels on individuals at levels lower than 5000 ppm. For example, levels above 2500 ppm have been shown to produce detrimental effects on cognitive and practical function in airline flight simulators.⁶ At our institution we have noted several instances where surgeons have felt unwell (headache, nausea, light-headedness) during surgery necessitating descrubbing part way through the cases (with resolution of symptoms). It is thought that this may be due to excessive CO₂ build up in the surgical helmet. There have been limited studies on the levels of CO₂ levels within SHS. The aim of this study was therefore to measure CO₂ levels in the surgical helmet during a series of arthroplasty cases.

2. Methods

Inspired CO₂ was continuously monitored using a Bluetooth carbon dioxide detector (SP1000 ambient air CO₂ meter, TPI Europe, Crawley, West Sussex, UK), fixed within a Flyte Surgical helmet (Stryker, Kalamazoo, MI, USA). The position of fixation was chosen so that the detector was close to the surgeon's nose and mouth (Fig. 1). The Bluetooth detector recorded data at a frequency of 0.5 Hz with a range of 0–6000 ppm and resolution of 1 ppm. Data was captured during 30 hip and knee arthroplasty surgeries. Ambient CO₂ was also recorded throughout the cases using an Extech CO210 desktop carbon dioxide data logger with a resolution of 1 ppm (Teledyne FLIR Systems, Wilsonville, OR, USA). As an additional control, CO₂ levels were measured in two individuals whilst wearing a helmet but without the toga covering.

Fig. 1.

Showing fixation position of Bluetooth CO2 monitor within surgical helmet.

Operative data was captured during primary and revision hip and knee arthroplasty cases, amongst 5 surgeons. Only the all-in-one Flyte Surgical Helmet and Togas (Stryker, Kalamazoo, MI, USA) were used as this is current practice at our unit based on a belief that it offers improved prevention of operative field contamination. Data was recorded for 30 arthroplasties measuring CO₂ levels in the surgeon's helmet. In addition, we collected data during 3 arthroplasties measuring CO₂ levels in the assistant's helmet. In 12 of the operations, the fan speed setting (max/min) was recorded. CO₂ levels during surgery were then used to calculate mean levels and percentage time above 2500 ppm and 5000 ppm. These levels being chosen to reflect thresholds at which cognitive impairment has been shown (2500 ppm) and the UK HSE safe 8-h exposure limit of 5000 ppm.

The study was approved by the hospital research and development department (ref RD58).

Statistical analysis was performed in Microsoft Excel. Parametric data was compared using one-tailed unpaired t-tests assuming unequal variance.

3. Results

The mean ambient CO₂ levels measured during the 30 cases was 427–493 ppm (min-max). The CO₂ levels measured whilst wearing a helmet without the hood was mean 854 ppm with levels not exceeding 2500 ppm. The surgical time had a range of 36–207 min with a mean of 88.6 min. Surgeries included 21 primary hip arthroplasty, 4 primary knee arthroplasty, 4 revision hip arthroplasty and 1 revision knee arthroplasty. The mean BMI of the surgeons was 25.73 with a range of 19.5–35. All surgeons were non-smokers and had a mean of 157.5 min of exercise per week with a range of 60–360.

During all 30 cases measured, CO₂ levels were measured that exceeded 2500 ppm and 23 out of 30 cases had a mean CO₂ level above 2500 ppm. The overall mean CO₂ concentration of all cases exceeded 2500 ppm (Table 1). The 2-min moving mean of CO₂ concentrations throughout operations demonstrates significant portions of time where CO₂ levels exceed 2500 ppm (Fig. 2). In most cases, the CO₂ concentration exceeded 2500 ppm for the majority of the operation time (Fig. 3).

Table 1.

Analysis of CO₂ concentrations within surgical helmets throughout operations.

	Mean	Standard Deviation	Maximum
Overall CO2concentration (ppm)	3006.9	1214.1	8884.0
Percentage of time the CO2concentration is ≥ 2500 (%)	72.6	23.8	94.7
Time the CO2concentration takes to reach ≥ 2500 (min)	5.1	7.8	36
Percentage of time the CO2concentration is ≥ 5000 (%)	5.4	8.9	32.1
Time the CO2concentration takes to reach ≥ 5000 (min)	27.2	20.3	88

Fig. 2.

Showing the 2-min moving mean of CO₂ concentrations throughout all 30 cases.

Fig. 3.

Distribution of time below 2500 ppm, between 2500 ppm and 5000 ppm and above 5000 ppm for each of the 30 cases.

We did not observe any cases with a mean CO₂ level above 5000 ppm. However, in 20 out of the 30 cases CO₂ levels exceeded 5000 ppm. In an example case (Fig. 4), points at which CO₂ levels exceeded 5000 ppm correlated with activities such as acetabular reaming and cementation of the femur.

Fig. 4.

The 2-min moving mean of the CO₂ concentration in an example case (A) and the corresponding times of activities within the operation (B).

Mean CO₂ concentrations were demonstrated to be slightly lower in operations with the surgical helmet fan set at maximum compared to minimum with statistical significance (Table 2). Mean CO₂ concentrations were also lower in assistants' surgical helmets compared to the surgeons’ helmets with statistical significance (Table 2).

Table 2.

Comparisons of CO₂ concentrations within surgical helmets of either the surgeon or assistant throughout operations with the fan set at maximum or minimum.

	Mean overall CO2 concentration	Mean percentage time CO2 concentration ≥2500	Mean percentage time CO2 concentration ≥5000
Fan speed max	2924.1	65.7	2.9
Fan speed min	3112.5	73	4.3
p values	p < 0.05a	p = 0.359	p = 0.265
Surgeon	3000.9	72.6	5.4
Assistant	2447.8	37.9	3.6
p values	p < 0.05a	p = 0.154	p = 0.263

^aIndicates statistically significant difference (p < 0.05).

4. Discussion

The main outcome of this study is that CO₂ levels measured throughout a series of hip and knee arthroplasty cases consistently exceed 2500 ppm within the SHS. On average the CO₂ was above 2500 ppm for 70 % of the time during surgery. Therefore, CO₂ levels in real world operating environments are shown to regularly exceed levels demonstrated in previous studies to affect cognitive function e.g. decision making, problem resolution, speed of addition.6, 7, 8, 9, 10, 11 For example, performance in flight simulators tests decreased when CO₂ levels were increased from 700 ppm to 1500ppm–2500 ppm.⁶ The odds of successfully completing a manoeuvre were 1.52 times higher and 1.69 times higher at 1500 ppm and 700 ppm respectively, compared to 2500 ppm. Peaks in CO₂ level were seen to occur during more physically demanding periods of arthroplasty surgery for example reaming the acetabulum and broaching the femur. Furthermore, levels exceeded the HSE and OSHA limit of 5000 ppm for 27 % of the operating time. Whilst these limits relate to 8 h exposure over a 40-h working week, a simple calculation based on a mean 1.5 h operative time indicates that if a surgeon was performing more than 19 joint arthroplasties per week they could exceed these recommendations. Levels below 2500 ppm have also demonstrated effects on individuals such as headaches, dizziness and fatigue in previous studies.12, 13, 14, 15 Therefore, low levels of CO₂ may still have effects on a surgeon that could indirectly affect their performance.

Some form of protection is always required during this type of surgery and there is clearly a balance between the level of protection of the patient and the surgeon and the level of CO₂. Studies of CO₂ levels inside standard surgical masks have measured an average of inhaled and exhaled air at the nasolabial junction in which CO₂ concentrations exceeded 2500 ppm when using a surgical mask and exceeded 5000 ppm when using a KN95 respirator or a FFP2 respirator (Rhee 2021, Martellucci 2022). However, these methods differ from our study as we measured CO₂ concentrations away from the face to reflect reservoir concentrations within the surgical helmet which should be more representative of the inhaled CO₂ concentration. Limited studies have been completed to analyse CO₂ levels within the SHS. A similar study by Jones et al., 2004¹⁶ observed even higher CO₂ levels than exhibited in our study. However, only 10 arthroplasties were completed, all by the same surgeon. In contrast to a study by Patel et al., 2017,¹⁷ increasing the fan speed exhibited only a small reduction in overall CO₂ concentration and did not affect the percentage of time the CO₂ concentration was over 2500 ppm. Therefore, the relevance of changing the fan speed seems to be minimal. Although we observed a significant reduction in overall CO₂ concentration in the assistant's helmet compared to the surgeon's helmet there was no statistically significant difference in the percentage of time the CO₂ concentration was over 2500 ppm. Given that there were only 3 assistant data sets, with considerable variation between them, the relevance of this may be minimal. A previous study comparing CO₂ levels within different SHS systems found that all four helmets (Depuy Sterile View, Stackhouse Freedom Aire, Stackhouse Freedom Mark and Stryker Steri-Sheild) in different configurations failed to meet the OSHA 8-h exposure limits during light exercise, apart from the Freedom Aire with high fan setting which still exceeded 2500 ppm (Rubman 1998). Therefore, this suggests this may be an issue with all types of SHS, regardless of the make and design.

There are a number of limitations to this study which must be considered while assessing possible conclusions. Although this is the first study to measure CO₂ in SHS during surgery, the sample size was relatively limited, especially when measuring fan speed. Additionally, the CO₂ monitor was only in one position and was not able to differentiate between inspired and expired CO₂. Therefore, the CO₂ concentrations measured may not be fully representative of the inspired CO₂ levels. However, due to the enclosed nature of the SHS and thus the relatively small volume of air within them, it is unlikely that this would make a substantial difference. Clearly, we have not measured surgical performance or cognitive ability in this study, however previous studies6, 7, 8, 9, 10, 11 have clearly demonstrated the well-known effect of CO₂ level of human performance. Finally, many different variables that could affect the CO₂ levels were not measured such as the surgeon's fitness and BMI. Therefore, further research in this area is needed to verify our results. This could focus on the completion of cognitive tests whilst wearing the SHS compared to without the SHS. CO₂ levels could also be assessed during surgery with the use of assistive technology, e.g., robotic surgery, which reduce the physical exertion of surgeon could also be assessed. Larger scale research should be completed to validate the reproducibility of the results observed.

The first generation of surgical helmet and suit systems utilised negative pressure extraction to remove exhaust gases and skin scale in order to reduce infection risk (body exhaust system- BES).² The utility of BES in reducing the infection rate in arthroplasty was proven in a large-scale randomised trial.³ Current positive pressure surgical helmet system (SHS) designs force air in but do not manage outflow. There has been considerable debate in the literature regarding the potential benefits of modern SHS^1,18, 19, 20, 21, 22 however a recent large-scale registry study from New Zealand has reported significant infection risk reduction with their use in total knee arthroplasty.²³ Our observations suggest that CO₂ levels may have the potential to effect patient outcomes in that even small effects on surgeon performance may potentially influence the surgical outcome. It is likely that any effect is small, given that SHS have been in use for decades and this is one of the first papers to highlight this possible issue. However, due to the excellence in outcomes for the vast majority of patients, the field of arthroplasty is one of marginal gains and SHS that more effectively manage the CO₂ level of the surgeon should be considered.

In considering potential design changes we would argue caution. Simply introducing more powerful intake fans would increase airflow and likely reduce CO₂ level but also increase the positive pressure in the suit leading to greater escape of contaminated air through seams and cuffs into the operative field. It is our view that design considerations to improve CO₂ management should also lead to improvements in management of the flow of airborne skin scales from the surgeon. Perhaps there is a need to return to a design principle based on Charnley's successful BES.

In conclusion, this is the first study to demonstrate that during real world arthroplasty surgeries, the levels of CO₂ within current generation of SHS are higher than those known to effect cognitive performance and can exceed occupational guidelines. Further research in this area is warranted.

Author contributions

TB – Conceptualization, Data curation, Methodology, Investigation, Writing – review & editing, CW – Investigation, Methodology, Data curation, Formal analysis, Writing – original draft, JB - Investigation, Methodology, Data curation, Formal analysis, Writing – original draft, RB – Formal analysis, Writing – original draft, editing

Funding statement

No funding was required for this study.

Guardian/patient consent

No consent required.