Erik Näslund: Non-Invasive Photoplethysmographic Measurements on the Sternum

Abstract

Oxygen is essential for sustaining life, making its monitoring crucial in clinical settings, particularly in critical care. Pulse oximetry, based on photoplethysmography (PPG), is widely used for non-invasive arterial oxygen saturation (SaO2) measurement, typically applied to the finger or earlobe. However, these sites are vulnerable to signal loss during vasoconstriction, such as in shock, cold exposure, or the administration of vasoactive drugs. The sternum, a centrally located bone with a rich blood supply, may be less affected by perfusion disturbances than peripheral sites. This thesis explores the feasibility of using the sternum as a site for non-invasive intraosseous PPG-based monitoring.

The first study, conducted on healthy volunteers subjected to incremental hypoxia, demonstrated that sternal PPG detected the minimum observed arterial saturation significantly faster than conventional pulse oximetry, on average, 28.7 seconds earlier than a finger probe and 6.6 seconds earlier than an ear probe, respectively. However, individual variability of the PPG data required subject-specific analysis.

Since the sternum has a mixed arteriovenous blood flow, a subsequent study aimed to determine the origin of the sternal PPG signal. The results showed that intramedullary sternal oxygen saturation (SsO2) and the PPG signals more closely resembled mixed venous oxygen saturation (SvO2) than SaO2. In the third study, a novel derived parameter, non-invasive SsO2 (nSsO2), was introduced and demonstrated an 87% concordance rate with concomitant alterations in SvO2, suggesting its potential for continuous non-invasive monitoring of changes in venous oxygenation. However, in cases of severe hypovolemia, the correlation weakened, possibly due to altered microcirculatory dynamics in the sternum.

A final study examined the sternal PPG signal under different physiological conditions, including modulation of positive end-expiratory pressure (PEEP), vasodilation, vasoconstriction, and hypovolemia. Despite differences in the compartment composition between the sternum and peripheral soft tissues, the results demonstrated that sternal PPG waveforms responded to hemodynamic alterations in a manner similar to that previously described for peripheral PPG measurements.

The findings of this thesis suggest that intramedullary sternal PPG may offer a useful complement to conventional PPG measurements, particularly for assessing global oxygen balance. However, further research is needed to define and validate its clinical applications.