Broadband microwave permittivity measurements of blood for hydration monitoring.

PhD ThesisIn the UK, dehydration and malnutrition affects vulnerable patients, costing the NHS an estimated £13 billion. Whilst a patient’s hydration levels is usually determined by fluid balance or plasma osmolality methods, these have a slow turnaround time and are invasive. New approaches that are non-invasive, real-time and easy to use need to be developed to improve dehydration prevention. The present study modified an NPL broadband microwave permittivity system by inverting the dielectric probe to allow for the broadband permittivity measurement of microliter fluid samples. The system was extensively characterised using solutions of NaCl and pure water. While some calibration artifacts were introduced, overall the measured permittivity response was only 3% below the expected absolute permittivity. To determine if blood permittivity changed during dehydration, blood permittivity measurements were taken from healthy athletes (who cycled at a high intensity in a 35°C, 40% humidity environment for 80 min) and compared to changes in blood osmolality and body mass. We demonstrated a correlation existed between blood osmolality and permittivity at a range of frequencies, suggesting the technique has the potential to monitor hydration in sportspeople. Finally, to determine exactly which components were affecting the permittivity profile during dehydration, a range of NaCl, bovine HCT and BSA concentrations in physiological osmotic solutions were measured at frequencies between 0.5 GHz to 20 GHz using our inverted probe method. We demonstrated a strong linear correlation between the real and imaginary permittivity when varying concentrations of HCT, BSA and osmolality. However, while little difference was found between the frequency-permittivity profile of HCT and BSA for both the real and imaginary permittivity, significant difference existed compared to NaCl, with a window existing at 20 GHz where measurements could be made independent of NaCl. Overall microwave dielectric measurements were found to be suitable for measuring changes in blood hydration