Investigation of oxygen saturation derived from cardiac pulsations measured on the adult head using NIR spectroscopy

When the human brain goes diving: using near-infrared spectroscopy to measure cerebral and systemic cardiovascular responses to deep, breath-hold diving in elite freedivers

Continuous measurements of haemodynamic and oxygenation changes in free living animals remain elusive. However, developments in biomedical technologies may help to fill this knowledge gap. One such technology is continuous-wave near-infrared spectroscopy (CW-NIRS)-a wearable and non-invasive optical technology. Here, we develop a marinized CW-NIRS system and deploy it on elite competition freedivers to test its capacity to function during deep freediving to 107 m depth. We use the oxyhaemoglobin and deoxyhaemoglobin concentration changes measured with CW-NIRS to monitor cerebral haemodynamic changes and oxygenation, arterial saturation and heart rate. Furthermore, using concentration changes in oxyhaemoglobin engendered by cardiac pulsation, we demonstrate the ability to conduct additional feature exploration of cardiac-dependent haemodynamic changes. Freedivers showed cerebral haemodynamic changes characteristic of apnoeic diving, while some divers also showed considerable elevations in venous blood volumes close to the end of diving. Some freedivers also showed pronounced arterial deoxygenation, the most extreme of which resulted in an arterial saturation of 25%. Freedivers also displayed heart rate changes that were comparable to diving mammals both in magnitude and patterns of change. Finally, changes in cardiac waveform associated with heart rates less than 40 bpm were associated with changes indicative of a reduction in vascular compliance. The success here of CW-NIRS to non-invasively measure a suite of physiological phenomenon in a deep-diving mammal highlights its efficacy as a future physiological monitoring tool for human freedivers as well as free living animals. This article is part of the theme issue 'Measuring physiology in free-living animals (Part II)'.

Reproducibility of retinal oximetry measurements in healthy and diseased retinas

PURPOSE

Retinal oximetry (RO) has been established as a non-invasive method to analyse oxygen saturation in retinal vessels. The aim of our study was to determine the reproducibility of RO images in healthy and in diseased retinas.

METHODS

A total of 61 right eyes (244 RO images) in 61 subjects (35♀, 26♂) were examined: 22 controls, 18 patients with glaucoma and 21 patients with inherited retinal diseases (IRDs). Four test-retest RO images were obtained in each subject. Oxygen saturation was measured with the oxygen saturation measurement tool of the Retinal Vessel Analyser (RVA; IMEDOS Systems UG, Jena, Germany). The test-retest standard deviation within the subject's (±SDw ) measurements (the mean vessel oxygen saturation in retinal venules and arterioles), its coefficient of variation (CoV) and the intraclass correlation coefficients (ICC) were analysed.

RESULTS

The average test-retest SDw in venules was ±2.52% (CoV = 4.35%) and in arterioles was ±1.67% (CoV = 1.76%). Among controls, glaucoma eyes and eyes with IRDs, the test-retest SDw in venules were ±2.33% (CoV = 4.48%), ±2.85% (CoV = 4.71%) and ±2.43% (CoV = 3.90%) (SDw p = 0.366 (CoV p = 0.452); one-way anova). The test-retest SDw in arterioles were ±1.65% (CoV = 1.80%), ±1.83% (CoV = 1.92%) and ±1.54% (CoV = 1.56%), respectively [SDw p = 0.762 (CoV p = 0.686)]. The ICCs in venules were 0.76 in controls, 0.69 in patients with glaucoma and 0.82 in patients with IRD. The ICCs in arterioles were, respectively, 0.92, 0.70 and 0.93.

CONCLUSION

The reproducibility of RO in healthy, as well as in diseased retinas, is excellent. In the glaucoma group, the lower standard deviation between subjects (SDb ) for arterioles contributes to the lower ICCs. Nevertheless, the measurements of oxygen saturation in arterioles seem more reliable when compared to venules.

A method to calculate arterial and venous saturation from near infrared spectroscopy (NIRS)

For adequate development and functioning of the neonatal brain, sufficient oxygen (O2) should be available. With a fast sampling (f(s) > 50 Hz) continuous wave NIRS device, arterial (SaO2) and venous (SvO2) saturation can be measured using the physiological fluctuations in the oxyhemoglobin (O2Hb) and total hemoglobin (tHb) concentrations due to heart action and respiration. Before using this technique in a neonatal setting, the method was verified on adult volunteers (n=7) by decreasing inspired oxygen down to an arterial saturation of 70% using a pulse oximeter as reference. NIRS optodes were placed on the left forehead; the pulse oximeter sensor was placed on the right forehead. The experiments were repeated with different optode spacings. SaO2 and SvO2 were determined using the ratio between the O2Hb and tHb value in the amplitude spectrum at the heart rate and respiration rate, respectively. A good agreement between calculated SaO2 and reference SaO2 from pulse oximetry was found (bias range -3.5% to 5.2%, SD of the residuals 1.3% to 3.5%). Optode spacing of 15 mm yielded a negative bias compared to optode spacing of 45 mm. It was not always possible to calculate SvO2 because the respiration peak could not always be detected.