Autoregulation monitoring and outcome prediction in neurocritical care patients: Does one index fit all?

Abstract

Indexes PRx and Mx have been formerly introduced to assess cerebral autoregulation and have been shown to be associated with 3-month clinical outcome. In a mixed cohort of neurocritical care patients, we retrospectively investigated the impact of selected clinical characteristics on this association. Forty-one patients (18-77 years) with severe traumatic (TBI, N = 20) and non-traumatic (N = 21) brain injuries were studied. Cerebral blood flow velocity, arterial blood pressure and intracranial pressure were repeatedly recorded during 1-h periods. Calculated PRx and Mx were correlated with 3-month clinical outcome score of modified Rankin Scale (mRS) in different subgroups with specific clinical characteristics. Both PRx and Mx correlated significantly with outcome (PRx: r = 0.38, p < 0.05; AUC = 0.64, n.s./Mx: r = 0.48, p < 0.005; AUC = 0.80, p < 0.005) in the overall group, and in patients with hemicraniectomy (N = 17; PRx: r = 0.73, p < 0.001; AUC = 0.89, p < 0.01/Mx: r = 0.69, p < 0.005; AUC = 0.87, p < 0.05). Mx, not PRx, correlated significantly with mRS in patients with heart failure (N = 17; r = 0.69, p < 0.005; AUC = 0.92, p < 0.005), and in non-traumatic patients (r = 0.49, p < 0.05; AUC = 0.79, p < 0.05). PRx, not Mx, correlated significantly with mRS in TBI patients (r = 0.63, p < 0.01; AUC = 0.89, p < 0.01). Both indexes did not correlate with mRS in diabetes patients (N = 15), PRx failed in hypocapnic patients (N = 26). Both PRx and Mx were significantly associated with 3-month clinical outcome, even in patients with hemicraniectomy. PRx was more appropriate for TBI patients, while Mx was better suited for non-traumatic patients and patients with heart failure. Prognostic values of indexes were affected by diabetes (both Mx and PRx) and hypocapnia (PRx only).

Keywords: Cerebral autoregulation; Cerebral blood flow; Cerebrovascular pressure reactivity; Modified Rankin Scale; Stroke; Traumatic brain injury.

Fig. 1

Physiologic model conception of PRx. If cerebrovascular pressure reactivity (CVR) is intact (upper line), small cerebral vessels dilate in response to decreasing ABP, resulting in an increased cerebral blood volume. In regards to the pressure–volume curve of brain [–24], this causes an increase of ICP, i.e. ABP and ICP are negatively correlated. If CVR is disturbed (lower line), ABP decrease is passively followed by constriction of small vessels. This causes a decrease of cerebral blood volume, and, therefore, causes a decrease of ICP. ABP and ICP are positively correlated. Conversely, in the case of increasing ABP, a negative correlation between ABP and ICP is generally associated with intact CVR, while a positive correlation between both signals indicates impaired CVR

Fig. 2

Signal recording of a 71-year-old patient with hemorrhagic stroke, heart failure, and a 3-month outcome mRS score of 4. CBFV, ABP and ICP have been recorded for 3450 s. CPP was calculated by ABP–ICP. In the lower channel, signal correlation coefficients are indicated either by circles (between CBFV and CPP, for Mx calculation) or by squares (between ABP and ICP, for PRx calculation) and moving average curves of five consecutive correlation coefficients are drawn. The signals CBFV and CPP showed strictly parallel fluctuations, while signal changes of ABP and ICP were clearly opposed. Accordingly, the indexes strongly differed: Mx was 0.31, indicating impaired CA, while PRx was −0.77, indicating intact cerebrovascular reactivity. The moderately severe outcome (mRS score = 4) better fits to the Mx value. ABP arterial blood pressure, CA cerebral autoregulation, CPP cerebral perfusion pressure, CBFV cerebral blood flow velocity, ICP intracranial pressure, mRS modified Rankin Scale

Fig. 3

Three-month outcome in patients with high Mx and with low Mx. In patients with low Mx (Mx < 0.2; N = 23) the distribution of mRS scores (upper bar) was shifted towards lower scores (indicating better outcome) if compared to the mRS scores of the patients with high Mx (Mx ≥ 0.2; N = 13; lower bar). In seven patients mRS was either 2 or 1, in all of them Mx was low. The difference between outcome distributions of both groups was significant (p < 0.005; Mantel–Haenszel test). mRS modified Rankin Scale, 0 no symptoms–6 death)

Fig. 4

PRx and Mx plotted versus modified Rankin Scale (mRS). In the subgroup of 36 patients with known 3-month outcome, higher index values corresponded to poorer outcome. mRS scores were significantly correlated with PRx (r = 0.38, p < 0.05), and even stronger correlated with Mx (r = 0.48, p< 0.05)