Transcutaneous monitoring of respiratory gases in preterm neonates

The past decades have seen a widespread application of transcutaneous monitoring in neonatology. The noninvasive approach overcomes the painful nature of blood sampling. Our work focused on the technique of simultaneous noninvasive measurement of tcpO2 (transcutaneous partial pressure of oxygen) and tcpCO2 (transcutaneous partial pressure of carbon dioxide). A small sampling chamber was built connecting the carbon dioxide and oxygen sensors to measure the gases diffusing out of the skin. The measurements were based on the initial diffusion rate instead of the mass-transfer equilibrium which allows for a faster response. By using the new method, physical activities, food intake and breathing patterns were shown to have a strong effect on the amount of carbon dioxide and oxygen evolving from the human skin. The measurement parameters such as sampler area, location, position and procedure timing were optimized using design of experiments. The measurements were validated and correlated against an FDA approved commercial transcutaneous monitor (the Radiometer TCM 4.0). The experimental setup was further modified and automated, and then approved for clinical trials at the University of Maryland, School of Medicine in the Neonatal Intensive Care Unit (NICU). The results obtained from a premature baby in the NICU were correlated to the Arterial Blood Gas (ABG) analysis. The noninvasive and rate-based monitoring technique was also extended to monitoring dissolved oxygen and carbon dioxide in cell cultures. We have shown that by measuring the initial diffusion rate we were able to determine the partial pressures of the two gases in the culture. The technique could be readily automated and measurements could be made in minutes. It was tested in demonstration experiments by growing mammalian cells in a T flask and a spinner flask at 37 degree C.The results were validated using optical sensor systems. A dynamic theoretical model based on a three dimensional unit cell representation of the experimental system that includes a single blood vessel was constructed in order to gain a better fundamental understanding of the factors that determine the performance of the new approach. The model employed the finite element numerical method and accounted for the fluid mechanics, mass transfer and reaction kinetics of the system. The model was implemented using COMSOL Multiphysics engineering simulation software