Effect of N95 Filtering Facepiece Respirator on Venous Blood Carbon Dioxide Levels and Hemodynamic Changes in Health Care Workers

Abstract

Background:

N95 filtering facepiece respirators (FFR) are used by health care workers for prevention of airborne infection, and its use has increased manifolds during COVID-19 pandemic. Prolonged use may result in carbon dioxide (CO₂) accumulation, affect hemodynamics, and blood gas values. Although arterial blood gas values accurately measure the blood CO₂ levels, venous blood gas values also show acceptable correlation.

Aim:

To evaluate the physiological impact of N95 FFRs on health care workers, including hemodynamic changes and venous blood levels of CO₂ during a period of 6 h.

Settings and Design:

Prospective observational study in a tertiary care hospital.

Methods:

The study was conducted on 30 health care workers who performed routine duties while wearing N95 FFR. Venous blood gas values (CO₂, pH, and bicarbonate) and vitals (respiratory rate, heart rate, blood pressure, and saturation) were noted at baseline, 2 (T2), and 6 h (T6) after wearing the mask. Discomfort level was also measured on a Visual Analogue Scale (VAS) of 1–10.

Statistical Analysis:

Repeated measures analysis was done using repeated measures ANOVA or Friedman's test. Group comparisons for continuously distributed data were made using independent sample “t” test or Wilcoxon test.

Results and Conclusion:

Hemodynamic and blood gas values did not change over time. The VAS for discomfort because of respirator use was 1.33 (1.42) at T2 and 2.77 (1.91) at T6. This was a significant increase in discomfort over time (P = 0.001). About 80% of participants experienced discomfort during this period. N95 FFR did not lead to significant alteration in hemodynamics or change in blood gas values after 6 h of continuous usage. However, discomfort significantly increased over time.

INTRODUCTION

N95 filtering facepiece respirators (FFR) are used in hospital settings for prevention of airborne infections since more than a decade. An N95 respirator is a respiratory protective device designed to achieve a very close facial fit and an efficient filtration of airborne particles.[1] It has been used by health care workers during management of respiratory illnesses, and its use increased many folds during the COVID-19 pandemic. It is being used for variable duration and continuous shifts lasting for even for 8–10 h. Centers for Disease Control and Prevention has laid down guidelines for respiratory protection and personal protective equipment (PPE) standards as well as extended use of respirators.[2] Despite the extensive use of N95 FFR, few studies have been done to evaluate the physiological impact on health care workers in whom symptoms such as headache, dizziness, and discomfort have been reported.[3,4,5,6]

Increased carbon dioxide (CO₂) can have deleterious effects on a person. Patients with mild hypercapnia may only present with non-specific headache, mild dyspnoea, tachypnoea, mild cognitive impairment, or somnolence. As higher levels of CO₂ accumulate, patients can become delirious, confused, develop bradypnea, and can ultimately progress to coma.[7] A previous study measured venous blood gas (VBG) parameters after use of N95 FFR, but it was only on 15 volunteers after 4 h of usage and found no increase in CO₂.[8]

The aim of this study was to evaluate the physiological impact of N95 FFRs on health care workers, including hemodynamic changes and blood levels of CO₂ during a period of 6 h. It would help in establishing the safety of these respirators for health care workers and formulating guidelines for the safe duration of its use. The primary objective of the study was to analyze the change in venous blood CO₂ (PvCO₂) levels with the use of N95 respirator at 2 and 6 h compared with baseline. The secondary objectives were to analyze the hemodynamic parameters, oxygen saturation (SpO₂), other blood gas parameters, discomfort level, and the subjective symptoms experienced by the participants.

METHODS

This prospective observational study was undertaken after obtaining ethical clearance from the institutional review board and registration of trial. A total of 30 volunteers from operating room team aged between 18 and 45 years having previous experience of wearing N95 FFR and doing their routine hospital duties of more than 6 h were selected. Smokers, obese (BMI >30 kg/m²), pregnant, suffering from cardio-pulmonary disorders, such as chronic obstructive pulmonary disease, asthma, coronary artery disease, and valvular heart diseases, and with uncontrolled co-morbidities, such as hypertension and diabetes mellitus, were excluded. Procedure was explained to them and written informed consent was taken.

The participants recruited were instructed not to have carbonated drinks on the day of assessment. They had their baseline VBG analysis done to measure pH, PvCO₂, and bicarbonate level. Their baseline vitals at T0 – heart rate (HR), respiratory rate (RR), SpO₂, and blood pressure (BP) were measured. Volunteers were given N95 FFR (3M-N95-8210 valveless mask) and mask fit testing was performed by odor testing of isoamyl acetate. They performed their routine duties while wearing N95 respirator and did not wear face shield or goggles.

PvCO₂ levels were again measured after 2 and 6 h of continuous use of N95 respirator. All the blood samples were taken by withdrawing blood from cubital vein. HR, BP, SPO₂, and RR were measured at 2 and 6 h (T2 and T6, respectively). Discomfort level during respirator use was recorded at 2 and 6 h using Visual Analog Scale (VAS)[9] (score of 1–10). Subjective symptoms if any such as headache, shortness of breath, dizziness, thirst, facial pressure, pain, or others were recorded during the study period. Any volunteer who removed their mask during their duty of 6 h or was unable to complete the study was noted and analyzed till the time they wore the mask.

Statistical analysis

Power analysis for repeated measures ANOVA was conducted in G-POWER to determine a sufficient sample size using an alpha of 0.05, a power of 0.80, an effect size of 0.25 (moderate effect size) using Cohen's convention, and two tails. Based on the aforementioned assumptions, the desired sample size was 28. Thus, an optimal sample size of 30 subjects was taken. SPSS v23 (IBM Corp.) was used for data analysis. Group comparisons for continuously distributed data were made using independent sample “t” test when comparing two groups. If data were non-normally distributed, Wilcoxon test was used for these comparisons. Repeated measures analysis was done using repeated measures ANOVA if the data were normally distributed, or Friedman's test if data were skewed. Statistical significance was kept at P < 0.05.

RESULTS

A total of 30 volunteers participated in the study and completed the desired duration of 6 h. Thus, all the participants were included for the final analysis. The work profile of the participants was as follows: 12 anesthetists, seven surgeons, six operation theatre (OT) nursing staff, and five OT technical staff. Table 1 shows the baseline characteristics of the participants.

Table 1.

Baseline characteristics of the participants

Variables
Age (years) mean±SD||Min-Max	28.87±5.85||21.00-43.00
Age (years)
21-30	22 (73.3%)
31-40	6 (20.0%)
41-50	2 (6.7%)
Gender
Male	21 (70.0%)
Female	9 (30.0%)
BMI (kg/m2) mean±SD||Min-Max	23.85±3.38||18.50-29.50
Pulse rate (BPM) ± SD	85.43±14.27
Systolic BP (mmHg) ± SD	125.53±9.85
Diastolic BP (mmHg) ± SD	77.73±8.61
MAP (mmHg) ± SD	92.50±6.99

MAP=Mean arterial pressure, BP=Blood pressure, BMI=Body mass index, BPM=Beats per minute

The hemodynamic parameters did not change significantly over time [Figure 1]. The change in respiratory parameters (RR and SpO₂) and blood gas values have been shown in Table 2. There was no significant change in parameters at T2 and T6. Five participants (16.7%) had increase in PvCO₂ by more than 10%, whereas two (6.67%) had increase of more than 20%. The VAS for discomfort because of respirator use was 1.33 (1.42) at T2 and 2.77 (1.91) at T6. This was a significant increase in discomfort over time (P = .001). Majority of participants (80%) experienced subjective symptoms, such as headache, breathing difficulty, facial pressure, thirst, and others. The frequency of these symptoms is shown in Figure 2. Other symptoms that few participants reported were facial heat, pain, and nasal block.

Figure 1.

Hemodynamic parameters during 6 h of N95 mask usage (SBP = systolic blood pressure, DBP = diastolic blood pressure, MAP = mean arterial pressure)

Table 2.

Respiratory parameters and blood gas values during 6 h

Parameter	Baseline (T0)	2 h (T2)	6 h (T6)	P
Respiratory rate (/min)	17.23 (2.86)	17.83 (3.17)	18.00 (3.90)	0.65
SpO2 (%)	98.73 (1.17)	98.80 (1.16)	98.63 (1.27)	0.59
pH	7.33 (0.03)	7.34 (0.03)	7.33 (0.04)	0.99
PvCO2 (mmHg)	51.97 (5.79)	51.63 (7.28)	52.73 (5.75)	0.91
Bicarbonate (mEq/L)	27.42 (2.77)	27.22 (2.78)	28.01 (2.64)	0.79

PvCO₂=venous CO₂ levels, SpO₂=oxygen saturation Values in are in mean (SD)

Figure 2.

Percentage of participants experiencing different types of symptoms

Post hoc analysis was done to assess the relationship of the subjective symptoms with physiological and blood gas parameters. None of the subjective symptoms correlated with the change in hemodynamic parameters or blood gas value of CO₂ or pH. The VAS discomfort score also did not correlate with the change with the physiological variables. Two patients had VAS of 6 and one patient had maximum VAS of 7. In these three patients, one patient had increase in PvCO₂ of 24% (46–57 mmHg), one patient had 9% (58–64 mmHg), and one patient had just 3% increase. No patient in our study had any episode of desaturation, hypotension, bradycardia, or any other complication.

DISCUSSION

The results of this study show that there was no significant change in physiological parameters (hemodynamic and respiratory) or blood gas values after 6 h of continuous N95 mask use. However, majority of respondents complained of subjective symptoms of discomfort with significant increase in scores for discomfort with increasing time.

A time period of 6 h was selected for the assessment as a hospital shift for health workers wearing PPE lasts from 6 to 8 h at most places. Previous studies have only assessed the physiological burden of N95 for short periods of time, which may not convey the true burden in clinical setting. A similar study was carried out by Yalciner et al.[8] on VBG analysis and no significant change was found, corroborating with our results. However, they analyzed the VBG samples before and after 4-h shift on only 15 volunteers. A minimum of 6-h study period resembles the clinical setting more accurately. Furthermore, we measured the VBG at 2-h interval after wearing the mask to analyze any acute physiological changes.

Although increased CO₂ levels during N95 use has been reported as a concern,[10] our study did not find any increase in PvCo2 after 6 h of mask usage. Elevation in inspired CO₂ was found in a study by Özdemir et al.[11] after 30 min of N95 usage in 12 health care workers. In another study on 11 volunteers, CO₂ levels inside the mask were raised 10 fold on wearing N95 masks for 15 min.[12] This level was, however, less than the National Institute for Occupational Safety and Health limits. Similarly, in a study by Roberge et al.,[13] dead space CO₂ inside the mask was found to be significantly higher than ambient air in 10 health care workers. However, there was no change in physiological variables and transcutaneous CO₂ levels after 1 h of exertion. Despite the rebreathing present, it does not translate into increased CO₂ levels. This may be because of increased minute ventilation that can be a combination of increased RR or tidal volume. Although the RR did not increase, tidal volume was not measured in our study.

Discomfort and exertion from wearing N95 masks have been studied before. Shenal et al.[6] found that discomfort increases with time, whereas exertion does not increase. Radonovich et al.[4] assessed the tolerability of different respirator masks on 27 volunteers. They concluded that people were unwilling to wear masks for the entire 8-h shift even with interposed break periods. In our study, there was also a significant increase in discomfort over time; however, all the volunteers completed the 6 h of study duration. Eighty percent of participants reported subjective symptoms on wearing the mask, with headache, difficulty in breathing, and facial pressure being most commonly reported (40%). This is in accordance with study by Lim et al.,[5] where 37% of the health care workers reported headache. In a study on 154 health workers, headache developed in 80% after wearing N95 mask and this was accompanied by alterations in cerebral hemodynamics that included increase in mean flow velocity and decrease in pulsatility index.[14] These symptoms could be attributed to hypoxia or hypercapnia, although this was not found in our results. Also, the symptoms reported by health care workers did not have any correlation with physiological or blood gas variables measured in our study. N95 respirators have been found to result in increase of airflow resistance by more than 100% that can be cause of perception of difficulty in breathing and may result in fatigue.[15] However, this correlation needs to be objectively verified and these symptoms should not be ignored as it may result in suboptimal patient care.[16]

Although there were no significant changes in physiological variables, nor did any health worker developing hypercapnia or respiratory acidosis, three participants had severe discomfort (VAS ≥6) and one developed dizziness. There has been a report of severe hypercapnia (from 42 to 70 mmHg CO₂) and respiratory acidosis with headache in health care worker after 6-h shift while donning PPE.[10] Therefore, we must remain cautious as such sporadic cases may occur and usage of N95 FFR remains an occupational hazard.

Limitations

First, VBG analysis was done instead of the arterial blood gas (ABG) analysis that would have measured the true CO₂ and oxygen levels. Although VBG by peripheral route can be used instead of ABG, it may not reflect the trending changes in PCO₂ accurately.[17] The difference in CO₂ values between arterial and venous samples can range from 4.4 to 6 mmHg. Although arterial and PvCO₂ show acceptable correlation, this difference can increase and become unpredictable in patients with haemodynamic instability and respiratory failure.[18,19] In our study, venous sampling was performed as the volunteers were healthy with no underlying lung pathology and PvCO₂ values might have correlated with arterial levels. Multiple arterial punctures would have been painful with risk of hematoma and thrombosis. Second, the health volunteers recruited worked in the OT complex. Other areas such as intensive care unit, emergency room, and wards may have different level of physical exertion and same results may not be applicable to them. The volunteers in our study did not wear PPE kit during their duties, whereas N95 usage with PPE kit would more ideally simulate the working conditions of a COVID-designated area.

CONCLUSION

Wearing N95 respirators does not lead to increase in CO₂ or change in blood gas parameters and hemodynamics after 6 h of usage. However, discomfort and symptoms such as headache, facial pressure, and difficulty in breathing are commonly present that may lead to poor compliance and inefficiency in performing duties while managing patients with infectious respiratory viral illness.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.