Assessment of a Non-Invasive Brain Oximeter in a Sheep Model of Acute Brain Injury

Cerebrovascular Responses in a Patient with Lundberg B Waves Following Subarachnoid Haemorrhage Assessed with a Novel Non-Invasive Brain Pulse Monitor: A Case Report

Subarachnoid haemorrhage (SAH) can trigger a range of poorly understood cerebrovascular responses that may play a role in delayed cerebral ischemia. The brain pulse monitor is a novel non-invasive device that detects a brain photoplethysmography signal that provides information on intracranial pressure (ICP), compliance, blood flow and tissue oxygen saturation. We monitored the cerebrovascular responses in a patient with Lundberg B waves following a SAH. The patient presented with a Fischer grade 4 SAH that required urgent left posterior communicating artery aneurysm coiling and ventricular drain insertion. On hospital day 4 oscillations or spikes on the invasive ICP were noted, consistent with Lundberg B waves. Brain pulse monitoring demonstrated concurrent pulse waveform features consistent with reduced brain compliance and raised ICP over both brain hemispheres. Oxygen levels also demonstrated slow oscillations correlated with the ICP spikes. Brief infrequent episodes of reduced and absent brain pulses were also noted over the right hemisphere. Our findings suggest that the brain pulse monitor holds promise for early detection of delayed cerebral ischemia and could offer insights into the vascular mechanisms at play.

The Influence of Extracerebral Tissue on Continuous Wave Near-Infrared Spectroscopy in Adults: A Systematic Review of In Vivo Studies

Near-infrared spectroscopy (NIRS) is a non-invasive technique for measuring regional tissue haemoglobin (Hb) concentrations and oxygen saturation (rSO₂). It may be used to monitor cerebral perfusion and oxygenation in patients at risk of cerebral ischemia or hypoxia, for example, during cardiothoracic or carotid surgery. However, extracerebral tissue (mainly scalp and skull tissue) influences NIRS measurements, and the extent of this influence is not clear. Thus, before more widespread use of NIRS as an intraoperative monitoring modality is warranted, this issue needs to be better understood. We therefore conducted a systematic review of published in vivo studies of the influence of extracerebral tissue on NIRS measurements in the adult population. Studies that used reference techniques for the perfusion of the intra- and extracerebral tissues or that selectively altered the intra- or extracerebral perfusion were included. Thirty-four articles met the inclusion criteria and were of sufficient quality. In 14 articles, Hb concentrations were compared directly with measurements from reference techniques, using correlation coefficients. When the intracerebral perfusion was altered, the correlations between Hb concentrations and intracerebral reference technique measurements ranged between |r| = 0.45-0.88. When the extracerebral perfusion was altered, correlations between Hb concentrations and extracerebral reference technique measurements ranged between |r| = 0.22-0.93. In studies without selective perfusion modification, correlations of Hb with intra- and extracerebral reference technique measurements were generally lower (|r| < 0.52). Five articles studied rSO₂. There were varying correlations of rSO₂ with both intra- and extracerebral reference technique measurements (intracerebral: |r| = 0.18-0.77, extracerebral: |r| = 0.13-0.81). Regarding study quality, details on the domains, participant selection and flow and timing were often unclear. We conclude that extracerebral tissue indeed influences NIRS measurements, although the evidence (i.e., correlation) for this influence varies considerably across the assessed studies. These results are strongly affected by the study protocols and analysis techniques used. Studies employing multiple protocols and reference techniques for both intra- and extracerebral tissues are therefore needed. To quantitatively compare NIRS with intra- and extracerebral reference techniques, we recommend applying a complete regression analysis. The current uncertainty regarding the influence of extracerebral tissue remains a hurdle in the clinical implementation of NIRS for intraoperative monitoring. The protocol was pre-registered in PROSPERO (CRD42020199053).

Assessment of a Non-Invasive Brain Pulse Monitor to Measure Intra-Cranial Pressure Following Acute Brain Injury

Background

Intracranial pressure (ICP) monitoring requires placing a hole in the skull through which an invasive pressure monitor is inserted into the brain. This approach has risks for the patient and is expensive. We have developed a non-invasive brain pulse monitor that uses red light to detect a photoplethysmographic (PPG) signal arising from the blood vessels on the brain's cortical surface. The brain PPG and the invasive ICP waveform share morphological features which may allow measurement of the intracranial pressure.

Methods

We enrolled critically ill patients with an acute brain injury with invasive ICP monitoring to assess the new monitor. A total of 24 simultaneous invasive ICP and brain pulse monitor PPG measurements were undertaken in 12 patients over a range of ICP levels.

Results

The waveform morphologies were similar for the invasive ICP and brain pulse monitor PPG approach. Both methods demonstrated a progressive increase in the amplitude of P2 relative to P1 with increasing ICP levels. An automated algorithm was developed to assess the PPG morphological features in relation to the ICP level. A correlation was demonstrated between the brain pulse waveform morphology and ICP levels, R²=0.66, P < 0.001.

Conclusion

The brain pulse monitor's PPG waveform demonstrated morphological features were similar to the invasive ICP waveform over a range of ICP levels, these features may provide a method to measure ICP levels.

Trial Registration

ACTRN12620000828921.

Assessment of a Non Invasive Brain Oximeter in Volunteers Undergoing Acute Hypoxia

INTRODUCTION

Research in traumatic brain injury suggests better patient outcomes when invasive oxygen monitoring is used to detect and correct episodes of brain hypoxia. Invasive brain oxygen monitoring is, however, not routinely used due to the risks, costs and technical challengers. We are developing a non-invasive brain oximeter to address these limitations. The monitor uses the principles of pulse oximetry to record a brain photoplethysmographic waveform and oxygen saturations. We undertook a study in volunteers to assess the new monitor.

PATIENTS AND METHODS

We compared the temporal changes in the brain and skin oxygen saturations in six volunteers undergoing progressive hypoxia to reach arterial saturations of 70%. This approach provides a method to discriminate potential contamination of the brain signal by skin oxygen levels, as the responses in brain and skin oxygen saturations are distinct due to the auto-regulation of cerebral blood flow to compensate for hypoxia. Conventional pulse oximetry was used to assess skin oxygen levels. Blood was also collected from the internal jugular vein and correlated with the brain oximeter oxygen levels.

RESULTS

At baseline, a photoplethysmographic waveform consistent with that expected from the brain was obtained in five subjects. The signal was adequate to assess oxygen saturations in three subjects. During hypoxia, the brain's oximeter oxygen saturation fell to 74%, while skin saturation fell to 50% (P<0.0001). The brain photoplethysmographic waveform developed a high-frequency oscillation of ~7 Hz, which was not present in the skin during hypoxia. A weak correlation between the brain oximeter and proximal internal jugular vein oxygen levels was demonstrated, R2=0.24, P=0.01.

CONCLUSION

Brain oximeter oxygen saturations were relatively well preserved compared to the skin during hypoxia. These findings are consistent with the expected physiological responses and suggest skin oxygen levels did not markedly contaminate the brain oximeter signal.