Effect of airway masks on physiological parameters of healthcare workers: a clinical trial

Introduction

Since the COVID-19 pandemic occurred in February 2020, the World Health Organization has recommended the use of face masks as one of the preventative measures to reduce COVID-19 transmission. Face masks help protect the wearer’s respiratory environment from droplets that contains the virus, thus reducing the virus’s transmission. Face masks come in several types, such as cloth masks, surgical masks, N95 masks, respirator masks, etc. These masks differ in materials and effectiveness on reducing viral transmission. Masks with more condensed layers have a more protective barrier, thus are more effective in preventing viral transmission. Several types of masks, e.g. the N95 and elastomeric respirator masks, are able to give protection toward airborne agents as well as droplets.¹

Although it can help reduce the spread of COVID-19, wearing a face mask for a relatively long period of time can be uncomfortable. Prolonged use of face masks has been associated with exertion, breathing difficulties, headaches, and even light-headedness. A previous study by Scheid et al.² showed that wearing mask for more than four hours triggers several subjective discomforts.² Furthermore, the study also reported that people with a history of troublesome headaches were more likely to also experience headaches while using a mask for a long period of time.² In addition, Ipek et al.³ showed that the causes of dizziness and headache while using respirators were associated with respiratory alkalosis and hypercarbia.³ Heart rate is expected to increase during physical exertion. Hence, perceived exertion and difficulty in breathing often felt by mask users suggests that it might stimulate an increase in heart rate.

Discomfort from wearing a mask has also been associated with the increased facial skin temperature and humidity inside the mask. However, a previous study using thermal imagery and an infrared thermometer found that while wearing a filtering facepiece respirator (N95 mask), facial skin temperature increased in a manner that was not clinically significant.^2,4 Subjective thermal comfort measured using a visual analog numerical scale while using a respirator also revealed a statistically insignificant difference compared to control.⁵ The increased discomfort caused by prolonged face mask use can potentially reduce the optimal working condition of healthcare workers. Moreover, that discomfort might ultimately cause non-compliance on wearing face mask. This will increase the spread of COVID-19 in healthcare workers and their environment.

In this study, we aimed to evaluate the effect of a surgical mask, N95 mask, and elastomeric respirator on physiological variables including pulse rate, respiratory rate, oxygen saturation, end-tidal CO2, body temperature, Borg scale, talk test, blood lactate, intermittent blood sugar, and subjective indicators in healthcare workers. Everyday workload was reflected in this study by low intensity treadmill test (5.6 km/h). The result of this research can aid in the future regulations regarding mask use for the public.

Methods

Data collection and outcome

After receiving approval from Dr. Saiful Anwar Hospital’s Ethics Commission, this research was carried out. Prior to recruitment, all participants were screened by an author (medical doctor) for illnesses. The form for this screening is available in the extended data.²⁸ The study procedures were demonstrated to the healthcare workers after they passed the health screening. All 10 of the participants passed the health screening, so no one was excluded from the study. All participants provided written and verbal informed consent.

For 30 minutes, each healthcare worker ran on a treadmill (Berwyn) at a low-moderate intensity (5.6 km/h). Four separate treatments were prepared for each healthcare worker. First, they carried out the exercise without a mask. Second, while wearing a surgical mask. Third, they wore an N95 or similar respirator. Lastly, the healthcare workers used a reusable elastomeric respirator. Each treatment was conducted 7-14 days apart (Figure 1). To keep the healthcare workers from falling, the treadmill speed was progressively decreased to zero at the end of each workout. The patients were then instructed to cool down by walking until their pulse rate dropped below 100 beats per minute. All the participants were asked to bring athletic clothing and shoes to be worn during the study.

Figure 1. Flow of the study.

Timing of the parameters is shown in Figure 2. Each test was conducted once by each participant. Pulse rate and oxygen saturation was measured using a Withleof® handheld pulse oximeter. Capnography was used to measure respiratory rate and end-tidal CO₂. Meanwhile, body temperature was measured using an infrared thermometer (Omron). Borg rating of perceived exertion (Borg Scale) was used to measure the physical sensations experienced by the healthcare workers.⁶ The scale starts from 6 indicating no exertion at all to 20 indicating maximal exertion. These measurements were obtained before exercise, 3 minutes, 10 minutes, 20 minutes, and 30 minutes into the exercise, and then 3 minutes and 30 minutes after resting (Figure 2). A talk test was conducted according to previous studies.^7,8 The healthcare workers joined in a conversation with the examiner, then the examiner determined how the subject talked. The range of talk test measurement is shown in Figure 3. For the purpose of statistical analysis, the intensity of the exercise (light, moderate, and vigorous) was represented by 1, 2, and 3, respectively. If the participant could easily carry the conversation with the examiner, the physical activity then classified into light activity, etc. Blood lactate was measured by taking the participants’ capillary blood while running, using a blood lancet, which was then inserted to Accutrend® Plus Lactate. Blood glucose levels were also measured by taking the capillary blood and inserted to Accutrend® Plus Glucose.

Figure 2. Timing of pulse rate, body temperature, and subjective sensations.

Figure 3. Talk test measurement range.

All participants were given a form that contains a subjective indicators scale. They were asked to rate each subjective indicator from 0 (not at all) to 10 (strongly) before exercise and then after they finished the exercise. The subjective indicators scale (Figure 4) was adopted from a previous study by Li et al.⁴

Figure 4. Subjective sensations scale.

Results

Discussion

The increased need for oxygen during physical exercise affects heart rate, which is reflected by the pulse rate in this study. Increased sympathetic response during physical exercise causes an increase in pulse rate.⁹ This is shown in this study by the result of time effect in pair-wise comparison which means with each time period, the value of pulse rate differs. However, the difference between pulse rate between control and tested mask conditions did not show any statistically significant difference, which was in accordance with the results of several previous studies.^5,10–13 Contrary to our findings, there were several studies that showed significant differences in the use of filtering facepiece respirators, where the N95 mask condition showed a statistically significant difference in pulse rate compared to surgical mask condition. However, in that study, the treadmill speed tested was higher compared to this study, which was at 6.4km/h. The higher treadmill speed might have caused the significant difference seen between the two masks.⁴

The increase of oxygen demand also affects the respiratory physiology. The physiological impact that occurs while using a mask is hypothesized to be caused by the filter media inside, which blocks the flow of air into the respiratory tract.¹⁴ Thus, the more layers a mask has, the higher the resistance it will cause while breathing. Therefore, the respiratory rate is expected to increase as well. In this study, there was no significant difference between the various tested masks compared with controls.

In this study, treadmill exercise did not cause the healthcare workers to reach a hypoxic state, with the lowest SpO₂ median of 96%. Previous studies have shown that physical exercise causes SpO₂ to decrease as oxygen demand increases.¹⁵ Coupled with the possibility of an increase in respiratory resistance caused by wearing masks, the SpO₂ is expected to decrease in the tested mask conditions compared to the control conditoin. However, in this study, the SpO₂ values were not significantly different statistically compared to the control condition. The reason behind this might be due to the intensity of exercise carried out in this study (moderate intensity) did not cause an increase in oxygen demand that will significantly reduces SpO₂.

The other parameter of respiratory physiology are end-tidal CO₂, capillary oxygen saturation, and the Borg scale. When PtcCO₂ value increases, the body will compensate by increasing the respiratory rate.^16,17 In this study, there was no statistically significant difference between the tested mask conditions and the control condition during physical exercise. This result is in accordance with previous studies.^10,11,13,18

Body temperature is very tightly regulated by thermoreceptors located in the hypothalamus. Physical exercise can trigger vasodilation which causes blood vessels to dilate to allow greater blood flow to the skin and ultimately increases skin temperature.¹⁹ In this study, body temperature was described by skin temperature measured using an infrared thermometer placed on the forehead. The moderate intensity exercise in this study was not expected to trigger heat stress which stimulates an increase in skin temperature. This is consistent with the results of this study which showed that until the 30^th minute, the control condition did not show an increase in body temperature greater than 37.5 °C. Similar result can be seen in the tested mask conditions, where in the post-hoc analysis there was no significant difference compared with control, which means that the use of masks does not cause heat-stress in moderate-intensity physical exercise. Previous study by Kim et al. obtained similar results where the measurement of rectal temperature (p= 0.519) and overall temperature (p= 0.654) were similar in the FFR (filtering facepiece respirator) mask group and the control condition.²⁰ In the study by Li et al. comparing the skin temperature in the use of N95 masks with surgical masks, there was a significant difference where the skin temperature in the surgical mask group was lower than that of the N95 mask.⁴ However, in that study the mask was used for longer period of time, namely for 100 minutes, and at a higher treadmill speed at 6.4km/h. This result showed that the FFR mask has the potential to cause greater metabolic stress when used for a longer period of time and at higher workload or exercise intensity.

The scores for the Borg scale did not differ significantly between the control and the other tested masks. However, the median results of the Borg scale measurement increased with time, and decreased at rest, and this result was statistically significant. This shows that the value of the Borg scale, which describes exertion efforts, was influenced more by the length of time exercising than the use of mask. Several previous studies also found the similar result. A study by Roberge et al. compared Borg scale scores on a treadmill test using a filtering facepiece respirator at two different speeds (2.74 km/h vs. 4.03 km/h). The results of this study showed a significant difference in the scores for exertion (p=0.01).^5,11,12,20

The concentration of serum lactate is a represent of anaerobic metabolism.^20,21 The results of this study indicated that lactate concentration increases after 15 minutes of exercise compared to the pre-exercise measurement even though when compared between the test mask groups, there was no significant difference. Thus, it can be concluded that the use of surgical, N95, or elastomeric respirator does not cause an increase or decrease in anaerobic metabolism compared to using no mask during the treadmill test. A study by Lassing et al.²³ showed that in exercise using constant load, the concentration of blood lactate in the control and surgical masks group was not significantly different (p= 0.26).²³

The talk test has been used before in several studies to determine exercise intensity.^16,24,26 Exercise with moderate intensity will result in the ability of the healthcare workers to speak comfortably without gasping or the need to alter their speed.²⁵ Compared to control, there was a statistically different result in elastomeric respirator group (Table 2). However, this difference was not clinically meaningful since in the mean of every group was in the range of light to moderate. The talk test has been proposed to be comparable with lactate threshold in measuring exercise intensity.¹⁶ The findings in this study are in accordance with that, in which the value of blood lactate and talk test represented moderate exercise intensity in all group. This finding showed that using a mask did not increase exercise load, despite the presence of filtering layers.

The cardiovascular system will respond to the exercise with an increase of cardiac output to deliver the oxygen and glucose to the muscles that play important roles in the metabolic process during physical exercise.^21,22 In this study, the results of the blood glucose measurements before exercise compared to 3 minutes after rest showed no significant difference. There are no studies yet that have examined the effects of using various masks on random blood glucose levels. This may be caused by the intensity of physical exercise. In addition, the duration of exercise was 30 minutes. This duration period might be too short for it to cause hypoglycemia. In addition, the ideal measurement of glucose consumption is to measure it directly on muscle cells i.e using nanobiosensor.²⁶

The results of the subjective indicators values before and after physical exercise showed no significant differences between the various masks, except for the tight sensation (p= 0.0017) with the highest mean rank in the N95 mask group. A previous study by Li et al.⁴ comparing the use of N95 masks and surgical masks showed significant differences in all subjective sensations with higher scores in the N95 mask group.⁴ In this study, masks were tested for 100 minutes. Compared to other masks, the wearer of an N95 mask has the highest probability in feeling subjective discomforts. However, with a longer period of mask use, other types of masks also might also cause discomforts. Hence, studies with a longer period of mask use should be conducted in the future.

Conclusions

The effect of surgical masks, N95 masks, and elastomeric respirators on the cardiopulmonary and metabolic response during 30 minutes of low-moderate intensity exercise is negligible and generally well tolerated by healthy healthcare workers.