RESUMO
BACKGROUND: Brain tissue oxygenation (PbtO2) in traumatic brain injury (TBI) is known to be dependent on cerebral blood flow (CBF) which remains difficult to assess during the very early phase of TBI management. This study evaluates if blood flow velocity measurement with 2D color-coded transcranial Doppler (TCD) can predict cerebral hypoxic episodes in moderate-to-severe TBI measured with a PbtO2 probe. METHODS: This is a prospective observational study of serial TCD measurements to assess blood flow velocity and its association with PbtO2. Measurements were done bilaterally on the middle cerebral artery (MCA) early after the insertion of PbtO2 monitoring, daily for 5 days and during dynamic challenge tests. Physiological parameters affecting PbtO2 and Doppler velocities were collected simultaneously (PaO2, PaCO2, hemoglobin [Hb] level, intracranial pressure, and cerebral perfusion pressure [CPP]). RESULTS: We enrolled 17 consecutive patients with a total of 85 TCD studies. Using 2D color-coded TCD, signal acquisition was successful in 96% of the cases. Twenty-nine (34%) TCD measures were performed during an episode of cerebral hypoxia (PbtO2 ≤ 20 mmHg). For early episodes of cerebral hypoxia (occurring ≤ 24 h from trauma), all Vmean < 40 cm/s were associated with an ipsilateral PbtO2 ≤ 20 mmHg (positive predictive value 100%). However, when considering all readings over the course of the study, however, we found no correlation between PbtO2 and MCA's mean blood flow velocity (Vmean). Vmean is also positively correlated with PaCO2, whereas PbtO2 is also correlated with PaO2, CPP, and Hb level. CONCLUSIONS: Early TCD measurements compatible with low CBF (mean velocity < 40 cm/s) detect brain tissue hypoxia early after TBI (≤ 24 h) and could potentially be used as a screening tool before invasive monitoring insertion to help minimize time-sensitive secondary injury. Various factors influence the relationship between Vmean and PbtO2, affecting interpretation of their interaction after 24 h.
