Beyond intracranial pressure: optimization of cerebral blood flow, oxygen, and substrate delivery after traumatic brain injury
© Bouzat et al.; licensee Springer. 2013
Received: 12 March 2013
Accepted: 25 June 2013
Published: 10 July 2013
Monitoring and management of intracranial pressure (ICP) and cerebral perfusion pressure (CPP) is a standard of care after traumatic brain injury (TBI). However, the pathophysiology of so-called secondary brain injury, i.e., the cascade of potentially deleterious events that occur in the early phase following initial cerebral insult—after TBI, is complex, involving a subtle interplay between cerebral blood flow (CBF), oxygen delivery and utilization, and supply of main cerebral energy substrates (glucose) to the injured brain. Regulation of this interplay depends on the type of injury and may vary individually and over time. In this setting, patient management can be a challenging task, where standard ICP/CPP monitoring may become insufficient to prevent secondary brain injury. Growing clinical evidence demonstrates that so-called multimodal brain monitoring, including brain tissue oxygen (PbtO2), cerebral microdialysis and transcranial Doppler among others, might help to optimize CBF and the delivery of oxygen/energy substrate at the bedside, thereby improving the management of secondary brain injury. Looking beyond ICP and CPP, and applying a multimodal therapeutic approach for the optimization of CBF, oxygen delivery, and brain energy supply may eventually improve overall care of patients with head injury. This review summarizes some of the important pathophysiological determinants of secondary cerebral damage after TBI and discusses novel approaches to optimize CBF and provide adequate oxygen and energy supply to the injured brain using multimodal brain monitoring.
KeywordsTraumatic brain injury Cerebral blood flow Brain oxygen Cerebral metabolism Multimodal monitoring Cerebral microdialysis Transcranial Doppler Neuromonitoring
Pathophysiology and diagnosis
Under physiological conditions, the relationship between CBF and CPP is linked to the cerebral autoregulatory capacity, thus CBF remains constant over a wide range of CPP. After TBI, depending on the nature of the lesion (diffuse vs. focal), there may be a large heterogeneity in brain autoregulatory capacities . Reduction of cerebrovascular reserve and impairment of cerebral autoregulation cause CBF to become increasingly dependent on CPP, thus CBF may be inadequate/insufficient despite CPP is within so-called “normal” ranges (50–70 mmHg) . In addition, apart from the individual relationship between CBF and CPP, secondary elevations of ICP also could contribute to further decrease CPP and aggravate ischemia.
The decrease of CBF after TBI was well documented by different studies using xenon-enhanced computed tomography (CT)  or positron emission tomography (PET)  that revealed heterogeneous regional disturbance of CBF. It also is important to stress the point that reduced CBF does not necessarily mean ischemia. Rather, the balance between CBF and cerebral metabolic rate of oxygen (CMRO2) determines whether the tissue is ischemic or not. For example, the decrease of CBF can be matched by a decrease of cerebral metabolic rate of oxygen (CMRO2), implying adequate CBF and preserved metabolic coupling, as it can be seen with deep sedation. In these circumstances, so-called metabolic autoregulation is the cause of matched reduction of CBF, in the absence of ischemia. Conversely, when CBF reduction is greater than CMRO2 decrease, CBF becomes inadequate to satisfy energy demand, exposing the tissue to hypoperfusion and eventually ischemia. Importantly, low CBF and ischemia might occur despite CPP ≈ 65-75 mmHg . Hence, CPP is not reliable to assess CBF in individual patients and other bedside tools are necessary.
Invasive thermal diffusion flowmetry (TDF) is based on the insertion of an intraparenchymal probe (Thermal Diffusion Probe; Hemedex® Cambridge, MA) generally next to ICP/PbtO2 probes. The technique uses thermal conductivity of the brain tissue and allows measuring regional cerebral blood flow (rCBF) in a quantitative way. The TDF technique was used in TBI to assess cerebral autoregulation and CO2 vasoreactivity and to calculate local cerebral vascular resistance. However, limitations of TDF, such as fever, sensor displacement, and the small area monitored, hindered its clinical application. Vajkoczy et al. showed good agreement between TDF and xenon-CT for regional CBF measurements . Recent studies showed the potential utility of TDF, in combination with PbtO2, to optimize the management of CPP in brain-injured patients. In SAH patients, Muench et al. used TDF to guide medical therapy of delayed cerebral ischemia  and show that MAP augmentation was the only single useful intervention to improve CBF and PbtO2, whilst hypervolemia and hemodilution had only marginal effects at best, supporting recommended practices of induced hypertension alone over “triple-H” therapy for the medical management of delayed cerebral ischemia. After TBI, TDF may guide CPP management at the bedside and identify individual MAP/CPP thresholds . Precise quantification of rCBF with TDF remains highly dependent on stable patient temperature and may be altered significantly in conditions of severe hyperthermia and rapid fluctuations of patient’s temperature. TDF seems a promising tool that may progressively become part of brain multimodality monitoring in the future. However, so far data are limited to small single-centre studies.
Macrovascular disturbances and reduced/inadequate CBF are not the only determinants of secondary ischemia. Impairment of the microvascular circulation also could play a key role in the constitution of secondary brain damages. Evidence for post-TBI microcirculatory dysfunction is both experimental and clinical. Marked disturbances in microcirculatory blood flow can be due to swelling of astroglial foot processes and compression of surrounding capillaries [13–15], or the formation of thrombi in the cerebral microcirculation . In addition, electron microscopy showed endothelial edema, vacuolization and pinocytic vesicles [13, 14, 17]. Microvascular edema might cause increased barriers for oxygen diffusion with reduction of cellular oxygen delivery despite the absence of frank cerebral ischemia . Microvascular damage accounts for the inability of pericontusional tissue to increase the oxygen extraction fraction (OEF = SaO2-SvO2/SaO2) in response to reduction of CBF induced by hyperventilation. Considering capillaries architecture as a central role for oxygen delivery to the cell , heterogeneity of red blood cell transit time into capillaries could lead to hypoxia in the injured brain, despite normal CBF.
The direct measurement of PbtO2 is now increasingly recognized as part of the bedside neuromonitoring after severe TBI  and can be used to detect brain hypoxia. Low PbtO2 has been reported to be associated with worse outcome after TBI [20, 21], independently of brain hypoperfusion detected by ICP/CPP monitoring .
Laser Doppler flowmetry (LDF) is an invasive fiber-optic laser probe that can be inserted into brain parenchyma to measure regional perfusion of a tissue volume of approximately 1 mm3. Contrary to TDF, LDF provides only qualitative—but not quantitative—measurements of microvascular perfusion . Routine use of this optical technique remains complex and artifacts, such as heterogeneity of microvascular architecture, probe motion, room temperature, and strong external light and sound may compromise data quality .
Brain activation and any augmentation of energy demand are followed by an increase in CBF and OEF. What is peculiar to the brain is its “avidity” for glucose utilization. Raichle and colleagues demonstrated that neuronal activation was followed by an increase in CBF and glucose utilization (≈30%) without a proportionate increase of OEF and cerebral metabolic rate of oxygen (CMRO2; ≈6%): this phenomenon is known as brain metabolic uncoupling. Experimental and clinical studies have shown that glucose utilization may increase dramatically after TBI, in the absence of oxygen or CBF limitation (cerebral hyperglycolysis) [26–28]. This may lead to a reduction of cerebral glucose below the critical level and to a state of brain energy dysfunction or crisis [28, 29]. In addition, cerebral oxidative metabolism may also be impaired due to mitochondrial dysfunction, thereby resulting in reduction of CMRO2 up to 50% in the acute period after TBI . Altogether these findings support the view that nonischemic mechanisms are implicated in the pathophysiology of TBI.
Bedside clinical approach
Cerebral blood flow optimization
Determining the optimal CPP
Despite its limitations, the measure of ICP for the calculation of CPP is essential to the management of TBI at the bedside. Low CPP was well correlated with poor outcome in several studies [31, 32]. All of these studies determined a critical ischemic threshold of CPP between 50 and 60 mmHg. Accordingly, and due to lack of benefit in increasing CPP to higher levels , actual recommendations from the Brain Trauma Foundation suggest a target of CPP between 50 and 70 mmHg . However, recent clinical studies in TBI patients have repeatedly demonstrated that the so-called “optimal” CPP (i.e., the CPP threshold below which secondary ischemia occurs) differs individually and might vary over time within each single patient [6, 8, 35]. Arterial blood pressure/intracranial pressure (ICP)-derived pressure reactivity index (PRx) could be used to assess cerebrovascular pressure reactivity . This index relies on spontaneous changes of arterial blood pressure and is calculated using a time correlation method. PRx, when averaged over specific CPP thresholds, demonstrated a U-shaped curve suggesting a specific relationship with the level of CPP. Although its utility can be debated, PRx may be useful in determining optimal CPP in individual patients. The reader also can refer to recent reviews on this particular topic .
Bedside measurement of PbtO2 also can be used as a surrogate marker of CBF. In agreement with recent data indicating that the venous fraction within cortical microvasculature exceeds 70%, it is suggested that PbtO2 predominantly reflects venous PO2. Among the factors affecting PbtO2, the effect of decreased cerebral perfusion pressure (CPP) and CBF has been the most studied . PbtO2 appears to correlate well with regional CBF and the relationship follows the autoregulation curve regulating CBF along a wide range of mean arterial pressure (MAP) . The fact that increase in PbtO2 can be obtained with CPP and MAP augmentation further supports the notion that PbtO2 is a good marker of CBF and cerebral ischemia in certain conditions. Rosenthal and colleagues, however, using parenchymal TDP and PbtO2 monitoring, showed that PbtO2 more appropriately reflects the product of CBF and arteriovenous oxygen tension difference (AVTO2) . Based on the formula PbtO2 = CBF ∙ AVTO2, reduced PbtO2 occurs frequently because of low CBF. However, PaO2 also is an important determinant of PbtO2 and additional pathological events (e.g., impaired lung function  or impaired O2 extraction due to increased gradients for oxygen diffusion in injured brain tissue ) might reduce PbtO2, in the absence of reduced CBF. Hemoglobin concentration also affects PbtO2 and reduced hemoglobin concentration below 9 g/dl may aggravate brain hypoxia .
Measuring CBF velocities with TCD also could help physicians to manage CPP. A PI above 1.4 and a diastolic CBFV below 20 cm.sec-1 are characteristic TCD markers of ischemia after severe TBI , as illustrated in Figure 2B. In centers with clinical expertise, TCD is increasingly used to assess brain compliance, CPP and impending cerebral ischemia in patients in whom invasive ICP is not available. For example, TCD has been used in the early phase in the emergency room to detect brain ischemia non-invasively in TBI patients , before ICP monitoring was inserted. TCD also can be used to test brain autoregulation and vasoreactivity to CO2.
A reduction in PaCO2 results in a proportional reduction of CBF and cerebral blood volume. While this could be temporarily useful in decreasing elevated ICP, it must be underlined that prolonged aggressive hyperventilation (PaCO2 < 25–30 mmHg) is deleterious for patients with TBI by decreasing CBF below the ischemic threshold [48, 49]. Regular measurements of PaCO2 with arterial blood gas analysis and continuous monitoring of end-tidal CO2 (EtCO2) must be achieved in every TBI patients to manage CBF variations secondary to PaCO2. Again, PbtO2 can be used to manage PaCO2 at the bedside [41, 50]. Using CO2 reactivity, TCD also detects brain perfusion changes related to PaCO2 noninvasively and may be used to evaluate cerebral vasoreactivity and to tailor individual PaCO2 after TBI .
Brain oxygen supply
Cerebral blood flow is one of the most important determinants of brain oxygen delivery. As discussed previously, arterial blood pressure and CPP are the major modifiable variables of brain oxygenation. This implicates that manipulating blood pressure and CPP may be the first and often the most effective intervention to optimize CBF and oxygen supply to injured brain tissue. Second-tier interventions to improve PbtO2 despite CPP modifications include optimization of systemic oxygenation (lung protective ventilation and maintenance of strict normoxia)  and red blood cell transfusion if hemoglobin is below 9 g/dl  (Figure 4).
However, other mechanisms may reduce brain tissue oxygenation. Among these, diffusion-limited oxygen delivery plays a key role and might explain why PbtO2 can be reduced despite oxygen delivery (PvO2) and CBF are normal . In clinical practice, this may explain why in some circumstances brain tissue hypoxia can occur despite ICP/CPP being within normal ranges . Without anemia or hypoxia, low PbtO2 despite adequate CBF probably reflects microcirculatory dysfunction and pericapillary edema . This has important implications for the management of PbtO2 and the response of PbtO2 to therapeutic interventions.
One strategy to force diffusion barriers is to increase the fraction of dissolved oxygen with hyperoxia. All studies have demonstrated a robust effect of increasing FiO2 on brain oxygenation . Whether this increase of PbtO2 is beneficial for patients remains controversial, probably due to the ability of the brain to use oxygen, i.e., the oxidative metabolism. Therefore, hyperoxia could benefit some patients in distinct cerebral areas . However, considering the absence of strong evidence, this strategy is not recommended in TBI patients.
Erythropoietin (Epo) is a promising neuroprotective treatment in experimental models of TBI  and exerts significant cerebral antiedematous effect . In an experimental model of diffuse TBI, it was recently shown that Epo, administrated intravenously up to 30 minutes after TBI, not only reverses cerebral edema but also significantly restores brain tissue oxygenation to normal levels . Electronic microscopy revealed a decrease of astrocytic end-foot swelling, which could improve red blood cell transit time. Hence, Epo, given its combined effects on brain edema, perfusion, and oxygenation may be particularly promising for the treatment of TBI. The ongoing “Epo-TBI” study will clarify therapeutic potentials of Epo in patients with TBI (clinialtrials.gov: NCT00987454).
Brain energy supply
Cerebral microdialysis (CMD) has largely contributed to a better understanding of the pathophysiology of acute brain dysfunction after TBI and was introduced recently as an additional bedside neuromonitoring tool in this context. CMD consists in the placement of an intraparenchymal probe that has on its tip a semi-permeable dialysis membrane. A cerebrospinal fluid-like solution, infused through this catheter, allows hourly sampling of patients’ brain extracellular fluid into microvials for bedside analysis . CMD provides continuous monitoring of cerebral energy metabolism at the bedside and the measurement of absolute and dynamic changes of brain extracellular concentrations of glucose, pyruvate, and lactate. Additional markers, such as glutamate (excitotoxicity) and glycerol (membrane integrity), can be measured [57, 58]. The clinical utility of CMD is mainly to detect impending ischemia/hypoxia and to assess the energetic state of the injured human brain [19, 59, 60]. Abnormal lactate/pyruvate ratio (LPR) is defined by a LPR > 25 , and this threshold is associated with worse outcome after TBI . Others also used LPR > 40 as marker of cell energy crisis . In clinical practice, values >35-40 are used to start therapeutic interventions. Elevated LPR can be due to ischemia/hypoxia: in these circumstances, elevated lactate is accompanied by low pyruvate, as well as low cerebral glucose and brain oxygen. “Ischemic/hypoxic” LPR elevations can reach very high levels, well above 40. A second pattern also can be seen, where elevated lactate/normal-to-high pyruvate is seen and indicates the cause of LPR elevation is nonischemic in nature and can be due to hyperglycolysis  or mitochondrial dysfunction . In this second “nonischemic” pattern levels of LPR are often only slightly elevated (LPR 40–50). Elevated LPR is therefore not only a marker of ischemia, but rather reflects the metabolic state of the tissue.
Brain glucose supply and blood glucose control
Alternative energy substrates
Glucose utilization involves two different pathways. Oxidative phosphorylation is the energy-generating biochemical process whereby pyruvate, produced by glycolysis, is oxidized to CO2 and H2O with the production of 30 moles of ATP. This process requires oxygen. Glycolysis is the energy-generating biochemical process whereby glucose is converted to pyruvate and lactate with the net production of 2 moles of ATP. This process is nonoxidative, producing lactate .
Evidence from in vitro and in vivo studies demonstrates that lactate is an important energy substrate for neurons , particularly in conditions of hypoxia . Recently, evidence of brain lactate utilization in humans with acute brain injury has been suggested [61, 71]. Exogenous administration of lactate with the use of sodium lactate perfusions improves cerebral performance during intense exercise  and in diabetic patients , with sparing of brain glucose . Preliminary studies in TBI patients suggest that sodium lactate solutions may be a future therapeutic strategy, potentially more effective to lower ICP than mannitol .
ICP/CPP monitoring is important after TBI since CBF is highly dependent on CPP below the lower limit of cerebral autoregulation, i.e., CPP <50 mmHg. Above the ischemic threshold, tailoring CPP for each patient needs a more comprehensive approach guided to brain multimodal monitoring to target CBF, oxygen delivery, and supply of brain energy substrates individually. Intraparenchymal PbtO2 reflects the complex interaction between CBF and oxygen delivery/consumption by the injured brain. As a result, PbtO2-targeted therapy might help managing CPP and prevent secondary cerebral ischemia and can be considered as a useful addition to standard ICP monitoring. Transcranial Doppler also can help to diagnose reduced CBF and elevated ICP at the bedside noninvasively and may help to direct in the early phase the need for additional intracranial monitoring and for emergent surgical interventions. Cerebral microdialysis provides essential information on brain metabolism and the availability of main energy substrates (mainly glucose) and also is potentially useful to detect secondary delayed cerebral ischemia and manage blood glucose control. Beyond ICP monitoring, contemporary management of severe TBI patients is increasingly based upon a more comprehensive clinical approach that must not be limited to ICP/CPP therapy but also includes the individual optimization of CBF, oxygen, and energy substrate delivery guided by bedside brain multimodal monitoring.
Cerebral blood flow
Cerebral blood flow velocity
Cerebral blood volume
Cerebral metabolic rate of oxygen
Cerebral perfusion pressure
Laser Doppler flowmetry
Mild cerebral artery
Magnetic resonance imaging
Mean transit time
Oxygen extraction fraction
Arterial partial pressure of oxygen
Brain tissue oxygenation pressure
Positron emission tomography
Traumatic brain injury
Thermal diffusion flowmetry.
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