In this study, it was found that the pO2 value decreased after eight hours of surgical masks usage in healthy volunteers, the pH value increased in both mask groups and the lactate value decreased in N95 mask group. However, it was determined that cognitive functions were affected in N95 mask group compared to the surgical mask group. During the COVID-19 pandemic, severe psychological changes including anxiety and depression were frequently observed accompanying to significant negative changes in cognitive functions such as memory weakness, focusing problems, and irritability in intensive care workers [2–4]. As in aviation, flight and underwater diving environments, especially in environments where protective masks are used due to pandemics, there are those who say that chronic hypoxia at different levels has negative effects on the central nervous system, especially on cognitive functions, as well as those who state that it has positive effects [4, 5].
In our study, it was observed that moderate hypoxia developed in some of the volunteers as a result of the eight-hour study. The vpO2. 2nd measurement in both single use three-ply surgical mask and N95 mask group was found to be low compared to vpO2-1st measurement at the end of eight hours. In relation, the vpO2 values of the intensive care workers using two different types of masks were lower at the end of eight-hour period than the value at the beginning of the study. Studies evaluating blood gas analysis revealed a strong correlation between venous and arterial blood gas samples for pCO2, pH values, and a weak correlation for pO2 and satO2 values. They stated that venous blood gas can still provide information about respiratory functions [6, 7].
Despite showing no change in vpCO2 level in N95 mask group in the present study, vpCO2 was found to be significantly lower at the end of eight hours in workers using three-ply surgical masks. This result suggests that three-ply surgical masks may have allowed CO2 leakage from the mask, and that those who use N95 masks may have re-breathed some of their exhailed air. Studies have shown that hypercarbia under mask initially developed during the adaptation process to the mask which is rapidly returned to normal levels. In the present study, this mechanism could be explained by the involvement of the compensatory respiratory dynamics [8, 9]. Despite the pH values of the volunteers used both types of masks were found to be higher after eight hours, this difference was not considered clinically significant. Similar to the measured pH value, the decline in lactate level in N95 mask group was also not accepted as a clinically significant decrease.
Furthermore, Robberge et al. conducted a study on ten healthcare workers using filter mask for one hour, and they reported that carbon dioxide and oxygen levels were found to be reached significantly above the standards accepted for the workplace. They concluded that using a valve mask did not improve the pCO2 level [9].
Li et al. reported that there was a decrease in tidal volume and some cardiopulmonary parameters in volunteers using surgical mask and no mask. They stated that the decrease in those parameters suggests that muscle mitochondria may reduce the ability to use oxygen thus affecting exercise capacity [8].
Blood gas analysis revealed negative effects of long-term use of protective masks on the central nervous system, cardiovascular system and respiratory system [3, 8, 9]. The present study demonstrated that vpO2-2nd measurement in both mask groups was lower than the vpO2-1st measurement, and the presence of long-term intermittent low-level hypoxia had negative effects on cognitive and physical functions as reported in previous studies [8, 10–13]. In this study, volunteers declared cognitive dysfunction such as “I am forgetful”, “I am having trouble on focusing”, “I have distraction problem” ranged from 33.3–54.8%. In addition, the rate of those who mentioned “I get tired quickly”, “I am nervous”, and “I am not as patient as before” was between 33.3% and 66.7%. Impairment in cognitive functions and negative behavioral changes were remarkable in intensive care workers. In this regard, Champod et al. found that intermittent hypoxia had a negative effect on learning and memory [13]. It is known that the duration of mask use by healthcare workers was long and it continued recursively at varying intervals over time during COVID-19 pandemic. It has been stated in different studies that the possibility of developing intermittent and long-term hypoxia and hypercarbia could not be underestimated [3, 4, 9].
The most remarkable study among the studies conducted to date is the study of Vakharia et al. on sixty volunteers [14]. The authors conducted two groups, the three-ply mask and N95 mask group in which brain oxygenation was evaluated with cerebral blood oxygenation level dependent magnetic resonance imaging (BOLD MR) at the beginning of the shift and at the end of the six-hour shift. Headache was the most common symptom in both groups in their study. In addition, clinical symptoms such as nausea, vomiting, dizziness, blurred vision, drowsiness and fatigue were observed more frequently in N95 group, but also in the group using both types of masks [14].
In our study, it was observed that painkillers were used at a rate varying between 38.1% and 42.9% among all mask groups in healthcare workers compared to before during the pandemic. The pathophysiology of cerebral hypoxia is complex and multifactorial, and the severity and duration of hypoxia are important. In this context, hypoxia is defined under two headings as acute hypoxic hypoxia and chronic hypoxic hypoxia [15–17]. A broad definition can be made for the duration of acute hypoxia and chronic hypoxia from seconds, hours, days and years. Cerebral vascular vessels respond with a change in vessel diameter when the pO2 level decreases below 50 mmHg and in hypercapnia. Carbon dioxide is a potent vasodilator by leading to dilatation of the pial vessels, and a decrease in vascular resistance resulting an increase in cerebral blood flow (CCA). Vasodilation also occurs in the intracranial and extracranial arteries, carotid and vertebral arteries. In chronic hypoxia, blood is directed to vital centers in the brain stem. While cellular oxygenation is maintained, other non-vital brain regions remain relatively hypoxic [18].
Cerebral oxygenation is affected in the frontal lobe and anterior cingulate gyri. These regions manage the compensation mechanism for cognitive functions. The clinical mechanism of confusion was also explained in the study of Vakharia et al. by the demonstration of insufficient blood redistribution and compensation using BOLD MR in seven subjects[14, 15, 18]. However, it is seen that there is still no consensus on what and how to use the definition of "intermittent hypoxia" [10, 11]. It has been stated that the effect of intermittent hypoxia on many organ systems is related to various variables (hypoxemia severity, hypoxia duration, number of cycles/day, intermittent hypoxia pattern-intermittent hypoxia-intermittent, consecutive days, total number of days) [8, 10].
When the neurophysiological basis of the effects of hypoxia on cognitive functions is examined, it has been reported that it causes some disorders ranging from metabolic homeostasis to the deterioration of neural integrity by causing molecular damage, depending on the severity of hypoxia. Cognitive functions such as attention, information storage, retrieval as well as processing speed are supported by many large-scale neural networks. The primary and sensory cortex with some of parietal and frontal regions join this network anteriorly. It has been shown that these regions are affected in hypoxic and ischemic memory disorders. It has been reported in various studies that there are regions of the brain that are sensitive to oxygen deficiency, especially those that are closely related to cognitive functions [5, 19, 20]. The effects of hypoxia on central nervous and cardiovascular system, and the cellular changes that have been shown to affect the mental state, may be sufficient to explain the response of the intensive care workers that we have observed in our study as "I get tired more quickly".
In addition to these, it is not difficult to say that heavy working conditions may have an accumulated stress effect during the pandemic process. Mecit et al. showed an increase in burnout syndrome where anxiety and depression was observed during the pandemic process. Similarly in previous studies, the remarkable number of people those indicated that they were angry and not as patient as they used to be in the present study, supports the existence of psychological fluctuations ranging from anxiety to burnout syndrome during the pandemic [3].
There are some limitations in the study. First, the number of patients and nationality is focused on one and small limiting the generatability of the findings of the present study. Second, arterial blood gas analysis might obtain detailed results compared to venous gas analysis however arterial access is an an invasive procedure thus restricting the feasibility of the procedure.