Interpretation
Wearing a mask can create a hard-to-ventilate space providing low oxygen environment that can have physiologic consequences. As an example, hypoxia may stimulate erythropoietin secretion, which in turn increases reticulocytes production4. The reticulocytes will eventually mature to red blood cells, manifested as elevated hemoglobin levels. A study conducted by Vij et al., investigated the effect of chronic hypoxia on hematologic parameters. The study followed 40 healthy men’s hemoglobin, fibrinogen, platelets and white blood cells concentration at sea level and at the following 3- and 13-months sojourn at high altitude. The study’s results suggest that prolonged stay at high altitude leads to elevated hemoglobin and fibrinogen levels and a lower platelet and white blood cells levels3. Due to the function of hemoglobin as an oxygen carrier throughout the cardiovascular system, higher levels of hemoglobin lead to greater Vo2-max capacity, enabling better physical performance. A common practice in the elite athletes’ field is training in high altitude, in an intent of utilizing this phenomenon5. Due to geographical limitations, not all athletes are able to train in high altitude, possibly preventing them from gaining high-altitude training advantages. For this reason, elevation-masks were developed in a purpose of imitating the oxygen deprivation conditions present in high-altitude. Theses masks use a valve system to reduce the amount of airflow to the lungs. In this study we report the same results as did Vij et al. Hemoglobin and fibrinogen levels increase while platelets and white blood cells levels decrease. A possible explanation is that wearing a mask throughout the day during the COVID-19 pandemic has the same hematologic effect as does the elevation-masks.
We have found higher post-partum hemorrhage rates during the mask-wearing period, and a lower platelet count. The American College of Obstetricians and Gynecologists classifies platelet count less than 70 k/microliter as a risk factor for post-partum hemorrhage. Nevertheless, Shravya et al. reported a twofold greater likelihood of post-partum hemorrhage among women with mild thrombocytopenia (platelet count 100–149 k/microliter)6. Our results may reflect the same consequences of mild thrombocytopenia on post-partum hemorrhage rates.
The elevated hemoglobin levels may be a plausible explanation to our reported lower rates of preterm birth and composite neonatal outcome. Hemoglobin concentrations greater than 14.6 g/dL at prenatal visits were previously associated with increased risk of preterm birth, stillbirth and growth restriction7,8. Conversely, relative elevations of hemoglobin levels can protect against anemia during pregnancy, previously associated with adverse obstetrical outcomes including preterm birth, low birth weight, stillbirth and neonatal mortality9,10. These results can potentially reflect an effect of anemia correction due to hemoglobin elevation. Additionally,
A proposed underlying mechanism for the pregnancy outcomes is the induction of heme oxygenase-1 (HO-1) during hypoxia11. HO-1 induction has been shown to improve pregnancy outcome, reduce the rate of placental mediated complications12 and of spontaneous preterm birth13.
Reported preterm birth rates during the COVID-19 pandemic are scarce, and mainly include women infected with COVID-1914. A previous report regarding preterm births during the pandemia among women who were mostly not infected with COVID-19 did not find a difference in preterm birth rates15. A possible explanation for the difference between the two studies are different populations studied and different sample sizes.
Mask wearing may also have significant impact on high risk populations such as elderlies, patients suffering from cardiac and hematologic diseases, as well as patients with lung diseases such as chronic obstructive pulmonary disease and asthma. First and foremost, these populations may bare increased sensitivity to altered blood composition, as described, and consequent cardiac activity. Additionally, though not evaluated in our study, the presence of a mask may also increase air-flow resistance, thereby deteriorating spirometric measurements.