Cerebral tissue oxygenation measured by two different probes: challenges and interpretation

Abstract

Purpose

Cerebral tissue oxygenation (PbrO₂) is most frequently monitored using a Licox CC1.SB system (LX, Integra Neuroscience, France) but recently a new probe—the Neurovent-PTO (NV)—was introduced by a different manufacturer (Raumedic, Germany). There are no prospective data on how these probes compare in clinical routine. We therefore compared both probes in comatose patients suffering from traumatic brain injury (TBI) or subarachnoid haemorrhage (SAH) during dynamic changes of inspirational oxygen fraction (FiO₂) and mean arterial pressure (MAP).

Methods

PbrO₂ in 11 patients was recorded continuously using an LX and NV probe placed side by side into the same cerebrovascular region. Once a steady baseline value was reached FiO₂ was increased by 20% for 10 min. Once the baseline values were re-established MAP was increased by 20 mmHg for 10 min. Evaluation was performed using a four-parameter logistic function and Bland–Altman analyses.

Results

PbrO₂ values of both probes differed significantly at all times. The LX probe reacted significantly faster to changes in FiO₂ and MAP. Limits of agreement ranged between −32.1 and 20.0 mmHg. Mean LX values were 6.1 mmHg lower than NV values.

Conclusions

Since the examined patient cohort was rather small, this study’s results are preliminary. However, they suggest that LX and NV probes measure different PbrO₂ values in routine monitoring as well as during phases of dynamic changes in FiO₂ and MAP. These data therefore do not support the view that both probes can be used interchangeably.

Appendix

For modelling the time course of PbrO₂ during an intervention, a four-parameter logistic function according to Pinheiro and Bates [12] was used in the following modified form:

$${\text{PbrO}}_2 \left( t \right) = {\text{PbrO}}_{2,{\text{start}}} + {\text{PbrO}}_{2,{\text{change}}} - \frac{{{\text{PbrO}}_{2,{\text{change}}} }} {{1 + {\text{e}}^{{{\left( {t_{0.5} - t} \right)} \mathord{\left/ {\vphantom {{\left( {t_{0.5} - t} \right)} \Phi }} \right. \kern-\nulldelimiterspace} \Phi }} }}.$$

Parameters in this exponential function are:

• PbrO_2,start, the asymptotic starting value at the beginning of an intervention.

• PbrO_2,change, the asymptotic change reached at the end of an intervention.

• t _0.5, the time at the inflection point of the curve at midway between the asymptotes.

• Φ, a scale parameter measured in seconds on the time axis. When the time (t) after the start of the intervention equals t _0.5 + Φ, the response is PbrO_2,change/(1 + e⁻¹) or 0.7310586 × PbrO_2,change, roughly three-quarters of the change in PbrO₂. Higher values of Φ indicate a slower change in PbrO₂.