Effect of inotropic agents on oxygenation and cerebral perfusion in acute brain injury

Introduction Tissue hypoxia and insufficient energy delivery is one of the mechanisms behind the occurrence of several complications in acute brain injured patients. Several interventions can improve cerebral oxygenation; however, the effects of inotropic agents remain poorly characterized. Methods Retrospective analysis including patients suffering from acute brain injury and monitored with brain oxygen pressure (PbtO2) catheter, in whom inotropic agents were administered according to the decision of the treating physician's decision; PbtO2 values were collected before, 1 and 2 h after the initiation of therapy from the patient data monitoring system. PbtO2 “responders” were patients with a relative increase in PbtO2 from baseline values of at least 20%. Results A total of 35 patients were included in this study. Most of them (31/35, 89%) suffered from non-traumatic subarachnoid hemorrhage (SAH). Compared with baseline values [20 (14–24) mmHg], PbtO2 did not significantly increase over time [19 (15–25) mmHg at 1 h and 19 (17–25) mmHg at 2 h, respectively; p = 0.052]. A total of 12/35 (34%) patients were PbtO2 “responders,” in particular if low PbtO2 was observed at baseline. A PbtO2 of 17 mmHg at baseline had a sensibility of 84% and a specificity of 91% to predict a PbtO2 responder. A significant direct correlation between changes in PbtO2 and cardiac output [r = 0.496 (95% CI 0.122 to 0.746), p = 0.01; n = 25] and a significant negative correlation between changes in PbtO2 and cerebral perfusion pressure [r = −0.389 (95% CI −0.681 to −0.010), p = 0.05] were observed. Conclusions In this study, inotropic administration significantly increased brain oxygenation in one third of brain injured patients, especially when tissue hypoxia was present at baseline. Future studies should highlight the role of inotropic agents in the management of tissue hypoxia in this setting.

Introduction: Tissue hypoxia and insu cient energy delivery is one of the mechanisms behind the occurrence of several complications in acute brain injured patients. Several interventions can improve cerebral oxygenation; however, the e ects of inotropic agents remain poorly characterized.
Methods: Retrospective analysis including patients su ering from acute brain injury and monitored with brain oxygen pressure (PbtO ) catheter, in whom inotropic agents were administered according to the decision of the treating physician's decision; PbtO values were collected before, and h after the initiation of therapy from the patient data monitoring system. PbtO "responders" were patients with a relative increase in PbtO from baseline values of at least %. ]. A total of / ( %) patients were PbtO "responders," in particular if low PbtO was observed at baseline. A PbtO of mmHg at baseline had a sensibility of % and a specificity of % to predict a PbtO responder. A significant direct correlation between changes in PbtO and cardiac output [r = .
Conclusions: In this study, inotropic administration significantly increased brain oxygenation in one third of brain injured patients, especially when tissue hypoxia was present at baseline. Future studies should highlight the role of inotropic agents in the management of tissue hypoxia in this setting. KEYWORDS hemodynamics, acute brain injury, cerebral blood flow, brain oxygenation, inotropic agents Introduction Cerebral hypoxia is a major cause of secondary brain injury that can be observed in a relevant proportion of patients with acute brain injury (ABI), including patients with traumatic brain injury (TBI), subarachnoid hemorrhage (SAH) and intracerebral hemorrhage (ICH) (1)(2)(3)(4)(5). Indeed, low brain tissue oxygenation (PbtO 2 ), which can be measured at bedside using an intraparenchymal probe placed into the "at-risk" areas, has been associated with cerebral anaerobic metabolism, as well as with an increased risk of mortality and poor functional outcome in these patients (6)(7)(8)(9)(10)(11)(12).
In case of reduced CBF, one of the most commonly used intervention is the increase in CPP (i.e., induced hypertension) with normovolemia; in patients suffering from SAH, this intervention can increase regional perfusion in the presence of cerebral vasospasm and neurological deterioration (19). Among TBI patients, this strategy can increase tissue oxygenation and reduce intracranial pressure, if cerebral autoregulation is preserved (14,15,20). In patients with ICH, although it is generally recommended to control blood pressure to avoid hematoma expansion (21), increased CPP was associated with higher PbtO 2 values in those patients with poor clinical condition at presentation (18,22).
Interestingly, the effects of inotropes, such as dobutamine (a β 1 agonist) and milrinone (a phosphodiesterase-3 inhibitor) on brain oxygenation remains unknown. These drugs have been proposed as "second-line" agents to improve CBF and CPP in acute brain injury patients (19, 23), or as a support therapy in those suffering from neurogenic heart failure (24-26). Therefore, the aim of this study was to assess the effect of inotropic agents on PbtO 2 after an acute brain injury.

Study design
This is a single center retrospective analysis of prospectively collected data conducted at Erasme University Hospital, Brussels, Belgium. All adults patients (>18 years old) admitted to the Intensive Care Unit of Erasme hospital from January 2015 to February 2022, due to acute brain injury (SAH, TBI or ICH), who survived at least 24 h were eligible for inclusion. The study protocol was approved by the local ethics committees (SRB2022006). Data collection and analyses were carried out in accordance with relevant scientific and ethical guidelines and regulations.

Inclusion criteria and exclusion criteria
We included eligible patients who were monitored with intracranial pressure (ICP) and PbtO 2 and who received inotropic treatment (i.e., either by dobutamine or milrinone) during monitoring, according to the treating physician's decision. We excluded patients who had malfunctioning/unreliable PbtO 2 readings or unavailable data.

Patient's management
Current guidelines for the management of TBI (20), SAH (27), and ICH (21) were implemented in clinical practice; invasive multimodal neuro-monitoring, including PbtO 2 , was included according to a recent consensus (28) and considered as "standard of care" for acute brain injury patients with a Glasgow Coma Score (GCS) <9 and requiring intracranial pressure (ICP) monitoring. PbtO 2 catheter was preferably placed into the "at risk" area, i.e., close to the injured/contused area in TBI and ICH patients and in the region at risk (vascular territory of the artery harboring the aneurysm) or with demonstrated initial or delayed hypoperfusion on brain imaging for SAH.

Data collection
Physiological variables as well as ICP and PbtO 2 , were measured in real-time and collected prospectively. Cerebral perfusion pressure (CPP) was calculated as the difference between mean arterial pressure (MAP) and ICP; MAP was zeroed at level of left atrium. Intracranial hypertension was defined by the observation of ICP values above 20 mmHg for at least 5 min at any time. Brain tissue hypoxia was defined as a PbtO 2 < 20 mmHg. At baseline (immediately before the start of inotropic therapy), 1 and 2 h after the start of inotropes, the 20min mean value of the following variables was collected: PbtO 2 , MAP, heart rate (HR), ICP, CPP, body temperature, arterial oxygen saturation (SaO 2 ), central venous oxygen saturation (SvcO 2 ), cardiac output (CO), pH, PaCO 2 , PaO 2 , blood lactate and blood glucose levels. Vasopressors and inotropic doses were also recorded. We also calculated the PbtO 2 defined as the difference between PbtO 2 values at 2 h and at baseline; similarly, the CO, ICP, CPP, and PaCO 2 were also computed.
We also recorded demographics and the presence of comorbidities, the Sequential Organ Failure Assessment (SOFA) (29) and the Acute Physiology and Chronic Health Evaluation (APACHE) II scores (30) on admission, as well as the GCS score (31) on admission, the use of mechanical ventilation, vasopressor and renal replacement therapy (RRT). The occurrence of neurological complications, such as epilepsy, hydrocephalus, rebleeding, delayed cerebral ischemia and intracranial hypertension, was collected.
Hospital mortality and the Glasgow Outcome Scale (GOS) (32) at 6 months was also reported, either collected from the medical charts or via the general practitioner. Unfavorable neurological outcome was defined as GOS of 1-3.

Outcomes
The primary outcome of the study was the changes in PbtO 2 within 2 h after the initiation of inotropic therapy. PbtO 2 increase was calculated as the difference between the highest mean PbtO 2 values, measured at 1 or 2 h after the initiation of inotropic therapy, and the baseline PbtO 2 value; PbtO 2 "decrease" was identified as a difference value <0, "stable" PbtO 2 as a difference = 0 and "increase" as a difference >0. A "significant" increase was defined as > 20% from the baseline. Patients that experienced a significant increase were defined as "responders." Secondary outcomes included the identification of factors associated with a significant PbtO 2 increase.

Statistical analysis
Descriptive statistics were computed for all variables. Normality was assessed using the Kolmogorov-Smirnov test. Non-gaussian continuous variables were described as median and interquartile range [IQRs] and compared using Mann-Whitney test (independent variables) or Friedman/Wilcoxon test (repeated measures of related variables); normally distributed variables were expressed as mean (±SD) and compared using Student t-test. Categorical variables were described as proportions (%) and compared using Chi square or Fisher's exact test. A logistic regression of physiological variables at baseline was conducted to assess factors associated with a significant PbtO 2 increase after inotropic therapy. The independence of errors, presence of multicollinearity and of influential outlier assumptions were checked; none were violated. The area under the receiver operator characteristic (AUROC) was computed to identify the optimal cut-off value (i.e., best sensitivity and specificity-Youden's index) of baseline PbtO 2 , baseline ICP and baseline CPP to predict a significant increase of PbtO 2 after inotrope infusion. We performed a correlation analysis using Spearman's test between PbtO 2 and CO, ICP, CPP, and PaCO 2 . The R coefficient and the 95% confidence interval were computed for all correlations. We also performed a linear regression analysis with PbtO 2 as the dependent variable and CO, ICP, CPP, and PaCO 2 as covariates. All statistical analyses were performed using SPSS 27.0 for MacIntosh. A p-value <0.05 was considered significant.

Results
Characteristics of the study participants  (18/35, 51%). The median GCS on admission was 9 (3-13) and the most common complication was the occurrence of intracranial hypertension. Ninety-one percent of patients experienced at least one episode of brain tissue hypoxia over the ICU stay. All patients received dobutamine as the studied inotrope except one patient who received milrinone. Overall, in-hospital mortality was 57%; 71% of patients had an unfavorable neurological outcome at 6 months.

Inotropic agents and brain oxygenation
An increase in PbtO 2 was observed in 22/35 patients (63%); of those, 12 (55% of patients with PbtO 2 increase and 34% of the whole cohort) experienced a significant PbtO 2 increase from baseline (responders). In 4 patients, PbtO 2 did not change while it decreased in 9 patients (26%).
There was a non-significant change in PbtO 2 after the initiation of the inotropic agents, as shown in Table 1 and Figure 1 (p = 0.052). Also, a significant increase in heart rate and in cardiac output was observed after inotropic agents' administration; no changes in CPP values were observed. The doses of dobutamine are reported in Table 2; milrinone was given at 0.375 mcg/kg * min.
There was a significant direct correlation between PbtO 2 and CO [r

PbtO responders
Main differences in collected variables at baseline between responders and non-responders are presented in Supplementary Table 2. Responders had more frequently seizures and rebleeding than non-responders; also, PbtO 2 and CPP were significantly lower, while ICP higher, than non-responders at baseline. No significant changes in measured variables over time were observed between groups, except a higher requirement in norepinephrine in the "responders" group when compared to the others (Supplementary Table 3). No statistically significant difference in hospital mortality and neurological outcome at 6 months between responders and non-responders was observed (Supplementary Table 2); no multivariable analysis was performed due to the limited sample size. Among survivors (n = 15), responders [GOS 4 (4, 5)] had numerically higher GOS at 6 months than non-responders [GOS 4 (3, 4)-p = 0.30]. No differences in mFisher and WFNS score on admission between responders and no responders were observed (Supplementary Table 2).
In a multivariable model, lower baseline PbtO 2 was associated with higher probability of a significant PbtO 2 increase after inotropic treatment ( Table 2). The AUROC for baseline PbtO 2 (Figure 2) to predict a significant PbtO 2 increase was 0.85 (95% CIs 0.70-1.00). The baseline PbtO 2 cut-off that better discriminated between responders and non-responders was 17 mmHg, with a sensibility of 84% and a specificity of 91% to predict a significant PbtO 2 increase. The AUROC for CPP and ICP and response to predict a significant PbtO 2 increase were 0.72 (95% CIs 0.50-0.94-best cut-off of 99 mm Hg) and 0.76 (95% CIs 0.60-0.92-best cut-off of 14 mmHg), respectively. In SAH patients, a multivariable model adjusted for poor grade on admission (WFNS 4 or 5) and mFisher, lower baseline PbtO 2 was associated with a higher chance of response to inotropes (Supplementary Table 4).

Discussion
In this study including mostly SAH patients, we observed that the administration of inotropic agents was associated with an increase in brain oxygenation in 63% of patients; however, only half of those experienced a significant PbtO 2 increase from baseline. The response to inotropic agents was greater in patients with lower PbtO 2 values at baseline. There was a positive correlation between PbtO 2 and CO and a negative correlation between PbtO 2 and CPP.
The relationship between cardiac output and cerebral blood flow is complex and difficult to study in clinical practice (33). The use of inotropes to improve brain hemodynamics and oxygenation comes from the hypothesis of a so-called "cardio-cerebral coupling" (34, 35), where an increase in cardiac  output could promote a proportional increase in CBF, in particular in case of impaired autoregulation and ischemia (36), both being common features of acute brain injury (37). We observed a positive correlation of PbtO 2 changes with CO changes, suggesting the higher was the effects on global hemodynamics, the highest the changes in brain oxygenation. Also, CPP was reduced by the administration of dobutamine, probably due to its vasodilatory via β 2 receptors, but this did not result in impaired cerebral hemodynamics. This might suggest the importance of systemic hemodynamics monitoring to adequately understand the effects of increasing CO on brain oxygenation (38) and the relevance of an "hyperdynamic" approach, rather than "hypertensive" one, to manipulate cerebral hemodynamics in ABI patients. Previous studies conducted in SAH patients found that fluid boluses to improve CO yielded a positive correlation between changes in cardiac output and PbtO 2 (39-41); to the best of our knowledge, this is the first study to assess the impact of inotropic agents on brain oxygenation in ABI patients. In experimental model of SAH, administration of dobutamine increased global  CBF, which was initially reduced because of an induced brain injury, and resulted into a lower prevalence of DCI and better functional outcomes (42). Moreover, dobutamine may provide, through its effects on 2-receptors, some vasodilatory effects and protect vascular smooth cell muscle against the occurrence of vasospasm in an in vitro study (43). Furthermore, adding dobutamine and nimodipine to the management of cerebral vasospasm in SAH patients compared to nimodipine alone tended to improve the outcome of these patients (43). Similarly, a study by Levy et al. found that dobutamine administration was able to clinically reverse vasospasm in 78% of patients, together with a significant increase in cardiac output (44). Also, in SAH patients with cerebral vasospasm, dobutamine effectively and significantly increased CBF, measured with Xenon-CT scan, independently from changes in blood pressure and similarly to norepinephrine (45). As for dobutamine, intravenous milrinone can also improve cardiac function after SAH (46) and reverse symptoms due to clinical deterioration caused by DCI in most cases (47), although no data on cerebral oxygenation are available.
It has been suggested that cerebral hemodynamics after an acute brain injury could be assessed using non-invasive monitoring, such as cerebral blood flow velocities or cerebral autoregulation indices. Nevertheless, cerebral autoregulation or CBF velocities cannot evaluate the adequacy of tissue oxygenation. As CPP target is routinely based on PbtO 2 or other monitoring tools using vasopressors, our findings suggest that tissue hypoxia can be present in some ABI patients in the absence of elevated ICP and that systemic targets, including CPP or CO, should be individualized according to cerebral demand (15). As inotropic agents are not superior to norepinephrine alone to improve outcome in ABI patients at risk of cerebral ischemia (48), our results suggest that inotropic agents could be considered as a "secondline" therapy to improve brain oxygenation when initial CPP optimization with vasopressors has failed. Interestingly, the more significant effects on tissue oxygenation were observed in those patients with baseline tissue hypoxia, as reported for CPP augmentation or red blood cells transfusions (15,17,39).
This study has several limitations. First, this study has an important selection bias since the decision to start inotropes was based on the attending physician' opinion in the absence of an established protocol and due to its retrospective nature we could not determine what were the criteria used. Moreover, we included a very small number of patients, since inotropic treatment were used only in selected cases; therefore, this study is only hypothesis generating. Second, this is a single center study, which limits the generalization of our results. Third, we had only one patient who received milrinone and no comparison between the effects of different inotropes was possible. Additionally, milrinone was used in a lower dose than usually recommended for the treatment of refractory DCI (bolus of 0.1-0.2 mg/kg and continuous infusion of 0.75 mcg/kg/min up to 2.5 mcg/kg/min) (49,50); however, in the case included in our cohort, this dose was sufficient to increase CO and improve PbtO 2 . Forth, we did not assess the clinical impact of inotropic administration, only its physiological effects on brain oxygenation; whether optimizing PbtO 2 in TBI patients improves outcome remains to be demonstrated. Fifth, we did not take into consideration the exact positioning of the probes, i.e., some of those might have been placed into normally appearing areas, where no potential benefits from hemodynamic manipulation would have been expected. Sixth, we could not assess the correlation between cardiac output and PbtO 2 changes in the subgroup analysis because of several missing CO values. Seventh, responders required higher doses of epinephrine after the start of inotropic treatment; therefore, we cannot exclude that the improvement in PbtO 2 was due to the increased dose of vasopressors in addition to inotropic agents. Similarly, there was a slight increase in CO 2 concomitant to the inotropic administration, which can also induce vasodilation and acts as important confounder. Finally, most of patients had SAH; whether this strategy is effective also in other forms of acute brain injuries remains to be further studied.

Conclusions
In this study, the administration of inotropic agents could increase brain tissue oxygenation in nearly 65% of patients; however, only half of them had a significant PbtO 2 increase. Baseline tissue hypoxia could predict this significant PbtO 2 increase after the initiation of therapy. The effects on brain oxygenation were proportional to changes in cardiac output. Further prospective studies are needed to explore the impact of inotropic therapy on brain oxygenation and outcome in this setting.

Data availability statement
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

Ethics statement
The studies involving human participants were reviewed and approved by Erasme Hospital Ethics Committee. The Ethics Committee waived the requirement of written informed consent for participation.