Background: Coronavirus disease 2019 (COVID-19) patients who need intensive medical care often require oxygen ventilation, but the number of ventilation machines is limited, and in some parts of the world, they are not available at all. In addition to patients for whom there is no access to ventilation machines there is also a considerable population of patients for whom ventilation is not sufficient for them to survive a critical state. 

Methods: Here, we propose and test an alternative oxygen supply through accelerated transdermal oxygen delivery. Covering the entire body with liquid fluorocarbons, which can dissolve 20 times more oxygen than water, we hypothesized to increase the contribution of transcutaneous respiration by a sustained amount. 

Results: Experiments applying pure medical grade perfluorodecalin on nude mice did not change their oxygenation in the blood under induced hypoxic conditions compared to control mice. However, increases in blood oxygenation below 2% could not be detected with the applied method.

Conclusions: We could not establish a proof-of-principle for a substantial increase in oxygen supply by transdermal oxygen delivery in mammals. 

Experiments with mice were carried out in strict accordance with the recommendations in the Guide for the Care and Use of Laboratory Animals of the National Institutes of Health. The animal experiment protocol was approved by the State Office for Health and Consumer Protection (permit number: 08/2020). All efforts were made to ameliorate harm to the animals. This was achieved by performing the entire experimental procedure with the mice in anesthesia and by providing an additional pain therapy (details are provided below). 

For this study, we used sex mixed population of CD1nu/nu mice (Charles River, Wilmington, MA, USA) at an age of 50.3±8.5 weeks with a bodyweight of 29.9±3.7 g, which were housed under a 12h/12h day-night rhythm in the animal husbandry of the Institute for Clinical & Experimental Surgery (Homburg, Germany). The animals were kept in individually ventilated cages (IVCs) in groups of 5 animals per cage. They had free access to drinking water and standard pellet food (Altromin, Lage, Germany). No additional inclusion criteria were set. Animals were randomly distributed in 2 groups (control group and treated group). Randomization was done by choosing lots and experiments were performed by alternating measurements of control and treated mice with a cage of animals per (experimental) day. The number of animals per group could not be precalculated because we established this kind of measurements and had no reliable information about the variances of the measured parameters. Therefore, we performed a statistical analysis on a daily base and such decided after we reached a group size of 5 animals to stop the experiments of protocol 1 and after reaching 4 mice for protocol 2.

During the experiment, the mice were anesthetized with isoflurane (5%: initiation, 1%: maintenance of anesthesia). Immediately after anesthesia initiation, the mice received 5 mg/kg carprofen (Rimadyl; Zoetis, Berlin, Germany) i.p. for intraoperative pain therapy. Afterwards, they were fixed on a heating pad in supine position and tracheotomized for the insertion of a catheter for ventilation. Breathing rate and stroke volume could be set on the mouse ventilator (Minivent; Harvard Apparatus, March-Hugstetten, Germany) and was adapted according to the body weight of the mice in the range of 200-270 per minute and 120-140 µL, respectively. The body temperature was kept constant in the range between 37-38^oC, which was monitored with an anal probe (GTH1170, Greisinger Messtechnik, Regenstauf, Germany). The heart rate was documented with the help of a pulse oximeter (MouseSTAT Jr. with Paw sensor, Kent Scientific, Torrington, CT, USA.) The control group contained 5 animals at an age of 47.4±2.4 weeks with a body weight of 28.8±3.7 g and the test group contained the same number of animals (n=5, age: 49.3±3.3 weeks, body weight: 29.4±4.9 g; no significant difference to control group).

All mice in all groups and all protocols were given an initial acclimatization phase as outlined in Figure 1B (-10 to 0 min). Mice of control and treatment groups experienced the same hypoxic protocol. Control mice received no additional treatment whereas mice of the treatment group were brushed with prewarmed (30^oC) perfluorodecalin (medical grade, Pharmpur, Königsbrunn, Germany). This included a full-body application with the exception of the area of the head and the paw to which the sensor of the pulse oximeter was attached. The procedure was continuously renewed in the course of the experiment to compensate the perfluorodecalin evaporation. The oxygenation of the blood was monitored and documented (in addition to the heart rate) with the pulse oximeter. After the oxygen saturation had reached a stationary level, the oxygen content of the ventilation was reduced according to the test protocol (see below). For this purpose, a gas mixer (KM10-2 FLEX, Witt-Gastechnik, Witten, Germany) was used, which mixed oxygen and nitrogen in an adjustable ratio. In addition, the setting was monitored with an oxygen meter (GOX100, Greisinger Messtechnik, Regenstauf, Germany). The mouse group used for protocol 2 consisted of 4 mice at an age of 55±15 weeks with a body weight of 31.9±1.1 g.