Multimodal monitoring in intensive care medicine: Advantages of measuring oxygen partial pressure (pₜᵢO₂)

In unimodal cerebral neuromonitoring, intracranial pressure (ICP) and cerebral perfusion pressure (CPP) are monitored. Multimodal neuromonitoring, or advanced cerebral neuromonitoring, involves the additional monitoring of other physiological parameters of the brain, such as the oxygen partial pressure (pₜᵢO₂) of brain tissue.

By simultaneously monitoring multiple parameters in addition to ICP and CPP, a more comprehensive picture of a patient's condition is obtained. This allows us as physicians to detect complications earlier, derive better and individualized treatment approaches, and ultimately make better treatment decisions.

If oxygen partial pressure is monitored in addition to ICP and CPP, it allows for the analysis of tissue perfusion and oxygenation, which can lead to optimized treatment and better patient outcomes.

By combining multiple monitoring parameters, an individualized assessment of the condition of the injured brain can be made. Additionally monitoring parameters such as ICP (intracranial pressure) and CPP (cerebral perfusion pressure), and also p_tiO₂ (partial pressure of brain tissue oxygen), can help prevent or detect early secondary damages associated with traumatic brain injury (TBI).

Take, for example, an aneurysmal bleed. In addition to the primary damage caused by the bleed itself, secondary effects can also occur, albeit with a delay. There can be a gap of several days between a bleed and the occurrence of secondary harm. A typical example is vasospasm, a narrowing of the blood vessels in the brain, which impairs blood flow and can lead to reduced oxygen supply to the brain tissue, potentially causing ischemia in the affected area. Consequently, the brain tissue does not receive adequate oxygen and nutrients. This is precisely where the advantage of multimodal neuromonitoring can come into play.

Cerebral oxygen deficiency—and the secondary brain damage associated with it—can occur even when ICP (intracranial pressure) and CPP (cerebral perfusion pressure) values are within normal ranges. This is because changes in cerebral oxygen supply or blood flow may only affect ICP and CPP values after a delay. The primary reason for this is cerebral autoregulation.

Mechanical (pressure-dependent) cerebral autoregulation is a compensatory mechanism designed to maintain adequate blood flow to the brain in cases of vessel constriction or reduced blood flow. The blood vessels adapt to maintain blood flow—regardless of fluctuations in mean arterial pressure (MAP). Remembering the calculation of cerebral perfusion pressure (CPP), with intact cerebral autoregulation, blood flow usually remains relatively stable, even with changes in CPP. However, with disrupted autoregulation, seemingly normal CPP values do not necessarily indicate normal cerebral blood flow!

The PRx value ("Pressure Reactivity Index") is suitable for assessing cerebral autoregulation. It is calculated as the correlation coefficient between ICP (intracranial pressure) and MAP (mean arterial pressure). The PRx value ranges between "+1" and "-1". A negative PRx value occurs when MAP increases and ICP decreases, indicating intact pressure autoregulation. Conversely, if MAP and ICP rise simultaneously, resulting in a positive correlation or PRx value, this indicates abnormal cerebral autoregulation, which can lead to negative neural outcomes.

The healthy adaptability of the brain during regular cerebral autoregulation of blood flow ensures that the partial pressure of oxygen is maintained constant by adjusting to the cerebral vascular resistance (CVR). This keeps cerebral blood flow—and thus the partial pressure of oxygen (p_tiO₂)—largely independent of systemic blood pressure. However, this is not the case with dysfunctional autoregulation of blood flow.

CVR no longer responds appropriately to systemic blood pressure, and cerebral blood flow directly follows the fluctuations of systemic blood pressure. Consequently, ptiO2 can no longer be maintained constant through autoregulation and may experience significant fluctuations.

The partial pressure of oxygen in brain tissue reflects the availability of oxygen at a specific location at the cellular level. As a parameter, it expresses the balance between the oxygen delivery from the blood and the oxygen consumption of the brain tissue. It is influenced by changes in capillary blood flow, cerebral blood flow (CBF), and cerebral metabolism.

The results of a retrospective cohort study from the USA in 2005 suggested this possibility. In this study, one cohort was treated with conventional ICP therapy. In the other group, p_tiO₂ was additionally monitored and included in therapeutic decisions. This approach led to a reduction in mortality and fewer discharges of patients to long-term care facilities. While the study was certainly not methodologically perfect, it indicates that the partial pressure of oxygen is an important parameter in neuromonitoring.

In the "BOOST" study from 2017, researchers investigated whether there is a difference in the treatment of traumatic brain injuries when not only ICP (intracranial pressure) and CPP (cerebral perfusion pressure) are monitored but also p_tiO₂ (partial pressure of brain tissue oxygen). For this purpose, two groups or cohorts of patients were analyzed. 

In one group, ICP and CPP were monitored and treated, while in the other, the p_tiO₂ value was additionally monitored. Both cohorts had roughly the same extent of pathological deviations in ICP and CPP values. However, there were significant differences in mortality rates between the groups: The mortality rate for patients treated with conventional ICP and CPP management was 34%. In patients where the partial pressure of oxygen in brain tissue was also monitored, the mortality rate was significantly lower, at 25%.

In view of positive treatment outcomes, it is desirable to be able to monitor multiple parameters for in neurointensive care. It is encouraging that the understanding of the benefits of multimodal neuromonitoring is increasingly becoming established in hospitals.

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