Influence of moderate and profound hyperventilation on cerebral blood flow, oxygenation and metabolism

Assessment of Visual and Brainstem Auditory Evoked Potentials in Patients with Hashimoto's Thyroiditis

BACKGROUND

The present study was aimed to evaluate parameters of visual and brainstem auditory evoked potentials (VEP, BAEP) in euthyreotic Hashimoto's thyroiditis (HT) patients without central nervous system involvement.

METHODS

100 HT patients (92 women, 8 men), mean age 46.9 years, and 50 healthy controls. They underwent a neurological examination, thyroid hormone levels, thyroid autoantibody titers, and brain imaging. Latencies and amplitudes of the N75, P100, and N145 component of VEP and the I-V components of BAEP were analyzed.

RESULTS

The neurological examination revealed in 31 patients signs of increased neurovegetative excitability. Brain resonance imaging showed no abnormalities in HT patients. The mean P100, relative P100, and N145 VEP latencies were significantly longer, and P100 amplitude significantly higher in HT patients than the controls. HT patients also had a longer mean wave BAEP V latency and mean wave III-V and I-V interpeak latencies, and significantly lower mean wave I and V amplitudes. Abnormal VEP and BAEP were recorded in 34% of the patients. There were no statistically significant correlations between the mean VEP parameters and thyroid profile and the applied dose of L-thyroxine. There was a relationship between the level of TSH and the wave BAEP III-V interpeak latency.

CONCLUSIONS

There were changes in the brain's bioelectrical activity in one-third of the patients with HT without nervous system involvement. The increased amplitude of the VEP may indicate increased cerebral cortex activity. Disorders of the brain's bioelectrical activity in the course of HT may be associated with an autoimmune process.

Effects of hypothermia and hypothermia combined with hypocapnia on cerebral tissue oxygenation in piglets

BACKGROUND

Hypothermia and its combination with hypocapnia are frequently associated with anesthesia.

AIMS

The goal was to investigate the effects of hypothermia and hypothermia combined with hypocapnia (hypothermia-hypocapnia) on cerebral tissue oxygenation in anesthetized piglets.

METHODS

Twenty anesthetized piglets were randomly allocated to hypothermia (n = 10) or hypothermia-hypocapnia (n = 10). Cerebral monitoring comprised a tissue oxygen partial pressure (PtO₂ ), a laser Doppler probe, and a near-infrared spectroscopy sensor, measuring regional oxygen saturation (rSO₂ ). After baseline recordings, hypothermia (35.5-36.0°C) with or without hypocapnia (target PaCO₂ : 28-30 mm Hg) was induced. Once treatment goals were achieved (Tr0), they were maintained for 30 minutes (Tr30).

RESULTS

No changes in PtO₂ but a significant increase in rSO₂ (Tr0 (mean difference 8.9[95% CI for difference3.99 to 13.81], P < .001); Tr30 (10.8[6.20 to 15.40], P < .001)) were detected during hypothermia. With hypothermia-hypocapnia, a decrease in PtO₂ (Tr0 (-3.2[-6.01 to -0.39], P = .021; Tr30 (-3.3[-5.8 to -0.80], P = .006)) and no significant changes in rSO₂ occurred. Cerebral blood flow decreased significantly from baseline to Tr0 independently of treatment (-0.89[-0.18 to -0.002], P = .042), but this was more consistently observed with hypothermia-hypocapnia.

CONCLUSIONS

The hypothermia-induced reduction in oxygen delivery was compensated by lowered metabolic demand. However, hypothermia was not able to compensate for an additional reduction in oxygen delivery caused by simultaneous hypocapnia. This resulted in a PtO₂ drop, which was not reflected by a downshift in rSO₂ .

Effects of moderate and severe hypocapnia on intracerebral perfusion and brain tissue oxygenation in piglets

BACKGROUND

Hypocapnia is a common alteration during anesthesia in neonates.

AIM

To investigate the effects of hypocapnia and hypocapnia combined with hypotension (HCT) on cerebral perfusion and tissue oxygenation in anesthetized piglets.

METHOD

Thirty anesthetized piglets were randomly allocated to groups: moderate hypocapnia (mHC), severe hypocapnia (sHC), and HCT. Cerebral monitoring comprised a tissue oxygen partial pressure and a laser Doppler probe inserted into the brain tissue as well as a near-infrared spectroscopy (NIRS) sensor placed on the skin, measuring regional oxygen saturation. Hypocapnia was induced by hyperventilation (target PaCO₂ mHC: 3.7-4; sHC: 3.1-3.3 kPa) and hypotension by blood withdrawal and nitroprusside infusion (mean blood pressure: 35-38 mm Hg). Data were analyzed at baseline, during (Tr20, Tr40, Tr60) and after (Post20, Post40, Post60) treatment.

RESULTS

Compared to baseline, tissue oxygen partial pressure decreased significantly and equally during all treatments (mean [SD] at baseline: mHC 35.7 [32.45]; sHC: 28.1 [20.24]; HCT 25.4 [10.3] and at Tr60: mHC: 29.9 [27.36]; sHC: 22.2 [18.37]; HCT: 18.4 [9.5] mm Hg). Decreased laser Doppler flow was detected with all treatments at Tr20 (mHC: 0.9 [0.18]; sHC: 0.88 [0.15]; HCT: 0.97 [0.13] proportion from baseline). Independently of group, regional oxygen saturation varied only after reverting and not during treatment. Blood lactate, pH, HCO₃^- , and PaO₂ increased during treatment with no differences between groups.

CONCLUSION

This animal model revealed reduced cerebral blood flow and brain tissue oxygenation during hypocapnia without detectable changes in regional oxygen saturation as measured by NIRS. Changes occurred as early as during moderate hypocapnia.

Studying cerebral hemodynamics and metabolism using simultaneous near-infrared spectroscopy and transcranial Doppler ultrasound: a hyperventilation and caffeine study

Caffeine is one of the most widely consumed psycho-stimulants in the world, yet little is known about its effects on brain oxygenation and metabolism. Using a double-blind, placebo-controlled, randomized cross-over study design, we combined transcranial Doppler ultrasound (TCD) and near-infrared spectroscopy (NIRS) to study caffeine's effect on middle cerebral artery peak blood flow velocity (Vp), brain tissue oxygenation (S_tO₂), total hemoglobin (tHb), and cerebral oxygen metabolism (CMRO₂) in five subjects. Hyperventilation-induced hypocapnia served as a control to verify the sensitivity of our measurements. During hypocapnia (∼16 mmHg below resting values), Vp decreased by 40.0 ± 2.4% (95% CI, P < 0.001), while S_tO₂ and tHb decreased by 2.9 ± 0.3% and 2.6 ± 0.4%, respectively (P = 0.003 and P = 0.002, respectively). CMRO₂, calculated using the Fick equation, was reduced by 29.3 ± 9% compared to the isocapnic-euoxia baseline (P < 0.001). In the pharmacological experiments, there was a significant decrease in Vp, S_tO₂, and tHb after ingestion of 200 mg of caffeine compared with placebo. There was no significant difference in CMRO₂ between caffeine and placebo. Both showed a CMRO₂ decline compared to baseline showing the importance of a placebo control. In conclusion, this study showed that profound hypocapnia impairs cerebral oxidative metabolism. We provide new insight into the effects of caffeine on cerebral hemodynamics. Moreover, this study showed that multimodal NIRS/TCD is an excellent tool for studying brain hemodynamic responses to pharmacological interventions and physiological challenges.

Differing effects when using phenylephrine and norepinephrine to augment cerebral blood flow after traumatic brain injury in the immature brain

Low cerebral blood flow (CBF) states have been demonstrated in children early after traumatic brain injury (TBI), and have been correlated with poorer outcomes. Cerebral perfusion pressure (CPP) support following severe TBI is commonly implemented to correct cerebral hypoperfusion, but the efficacy of various vasopressors has not been determined. Sixteen 4-week-old female swine underwent nonimpact inertial brain injury in the sagittal plane. Intraparenchymal monitors were placed to measure intracranial pressure (ICP), CBF, brain tissue oxygen tension (PbtO2), and cerebral microdialysis 30 min to 6 h post-injury. One hour after injury, animals were randomized to receive either phenylephrine (PE) or norepinephrine (NE) infusions titrated to a CPP>70 mm Hg for 5 h. Animals were euthanized 6 h post-TBI, and brains were fixed and stained to assess regions of cell and axonal injury. After initiation of CPP augmentation with NE or PE infusions, there were no differences in ICP between the groups or over time. Animals receiving NE had higher PbtO2 than those receiving PE (29.6±10.2 vs. 19.6±6.4 torr at 6 h post-injury, p<0.05). CBF increased similarly in both the NE and PE groups. CPP support with PE resulted in a greater reduction in metabolic crisis than with NE (lactate/pyruvate ratio 16.7±2.4 vs. 42.7±10.2 at 6 h post-injury, p<0.05). Augmentation of CPP to 70 mm Hg with PE resulted in significantly smaller cell injury volumes at 6 h post-injury than CPP support with NE (0.4% vs. 1.4%, p<0.05). Despite similar increases in CBF, CPP support with NE resulted in greater brain tissue oxygenation and hypoxic-ischemic injury than CPP support with PE. Future clinical studies comparing the effectiveness of various vasopressors for CPP support are warranted.

Moderate hyperventilation during intravenous anesthesia increases net cerebral lactate efflux

BACKGROUND

Hyperventilation is known to decrease cerebral blood flow (CBF) and to impair cerebral metabolism, but the threshold in patients undergoing intravenous anesthesia is unknown. The authors hypothesized that reduced CBF associated with moderate hyperventilation might impair cerebral aerobic metabolism in patients undergoing intravenous anesthesia.

METHODS

Thirty male patients scheduled for coronary surgery were included in a prospective, controlled crossover trial. Measurements were performed under fentanyl-midazolam anesthesia in a randomized sequence aiming at partial pressures of carbon dioxide of 30 and 50 mmHg. Endpoints were CBF, blood flow velocity in the middle cerebral artery, and cerebral metabolic rates for oxygen, glucose, and lactate. Global CBF was measured using a modified Kety-Schmidt technique with argon as inert gas tracer. CBF velocity of the middle cerebral artery was recorded by transcranial Doppler sonography. Data were presented as mean (SD). Two-sided paired t tests and one-way ANOVA for repeated measures were used for statistical analysis.

RESULTS

Moderate hyperventilation significantly decreased CBF by 60%, blood flow velocity by 41%, cerebral oxygen delivery by 58%, and partial pressure of oxygen of the jugular venous bulb by 45%. Cerebral metabolic rates for oxygen and glucose remained unchanged; however, net cerebral lactate efflux significantly increased from -0.38 (2.18) to -2.41(2.43) µmol min 100 g.

CONCLUSIONS

Moderate hyperventilation, when compared with moderate hypoventilation, in patients with cardiovascular disease undergoing intravenous anesthesia increased net cerebral lactate efflux and markedly reduced CBF and partial pressure of oxygen of the jugular venous bulb, suggesting partial impairment of cerebral aerobic metabolism at clinically relevant levels of hypocapnia.

Parenchymal brain oxygen monitoring in the neurocritical care unit

Patients admitted to the neurocritical care unit (NCCU) often have serious conditions that can be associated with high morbidity and mortality. Pharmacologic agents or neuroprotectants have disappointed in the clinical environment. Current NCCU management therefore is directed toward identification, prevention, and treatment of secondary cerebral insults that evolve over time and are known to aggravate outcome. This strategy is based on a variety of monitoring techniques including use of intraparenchymal monitors. This article reviews parenchymal brain oxygen monitors, including the available technologies, practical aspects of use, the physiologic rationale behind their use, and patient management based on brain oxygen.

The anesthetic effects on vasopressor modulation of cerebral blood flow in an immature swine model

BACKGROUND

The effect of various sedatives and anesthetics on vasopressor modulation of cerebral blood flow (CBF) in children is unclear. In adults, isoflurane has been described to decrease CBF to a lesser extent than fentanyl and midazolam. Most large-animal models of neurocritical care use inhaled anesthetics for anesthesia. Investigations involving modulations of CBF would have improved translatability within a model that more closely approximates the current practice in the pediatric intensive care unit.

METHODS

Fifteen 4-week-old piglets were given 1 of 2 anesthetic protocols: total IV anesthesia (TIVA) (midazolam 1 mg/kg/h and fentanyl 100 μg/kg/h, n = 8) or ISO (isoflurane 1.5%-2% and fentanyl 100 μg/kg/h, n = 7). Mean arterial blood pressure, intracranial pressure (ICP), CBF, and brain tissue oxygen tension were measured continuously as piglets were exposed to escalating doses of arginine vasopressin, norepinephrine (NE), and phenylephrine (PE).

RESULTS

Baseline CBF was similar in the 2 groups (ISO 38 ± 10 vs TIVA 35 ± 26 mL/100 g/min) despite lower baseline cerebral perfusion pressure in the ISO group (45 ± 11 vs 71 ± 11 mm Hg; P < 0.0005). Piglets in the ISO group displayed increases in ICP with PE and NE (11 ± 4 vs 16 ± 4 mm Hg and 11 ± 8 vs 18 ± 5 mm Hg; P < 0.05), but in the TIVA group, only exposure to PE resulted in increases in ICP when comparing maximal dose values with baseline data (11 ± 4 vs 15 ± 5 mm Hg; P < 0.05). Normalized CBF displayed statistically significant increases regarding anesthetic group and vasopressor dose when piglets were exposed to NE and PE (P < 0.05), suggesting an impairment of autoregulation within ISO, but not TIVA.

CONCLUSION

The vasopressor effect on CBF was limited when using a narcotic-benzodiazepine-based anesthetic protocol compared with volatile anesthetics, consistent with a preservation of autoregulation. Selection of anesthetic drugs is critical to investigate mechanisms of cerebrovascular hemodynamics, and in translating critical care investigations between the laboratory and bedside.

Reviewing the reliability, effectiveness and applications of Licox in traumatic brain injury

AIMS AND OBJECTIVES

To review the pathophysiology, accuracy, effectiveness and use of Licox for brain tissue oxygen monitoring in traumatic brain injury (TBI).

BACKGROUND

The Licox monitoring system allows continuous monitoring of partial pressure of brain tissue oxygen (PbO(2)), brain tissue temperature and intracranial pressure (ICP). The application and effectiveness of the use of Licox in TBI is not clearly explored within the literature.

INCLUSION CRITERIA

A date limit of 1995-2009, English language, all animal and human studies and the following terms were searched: Licox, brain tissue oxygenation, cerebral oxygenation and TBI. MEDLINE database was the primary data source.

EXCLUSION CRITERIA

All paediatric papers were excluded from the search. Studies not related to pathophysiology and management of TBI and brain tissue oximetry in adults were excluded. Data relevant to the subject under consideration were extracted by three independent clinicians to form a narrative report. Studies were critically evaluated using the NHS Public Health Resource Unit's checklist for each study analysed.

CONCLUSIONS

Licox offers new insights into cerebral pathology and physiology. The continuous bedside monitoring provides real-time data that can be used to improve patient management and prognosis in specialist units by trained and experienced staff. More research is required to understand the limitations of this technology and why it is not in widespread use. RELEVENCE TO CLINICAL PRACTICE: A clinical tool that could be utilized more often in the right setting to improve care to patients suffering from TBI by disseminating more information on this unique tool.

Detection of Cerebral Compromise With Multimodality Monitoring in Patients With Subarachnoid Hemorrhage

BACKGROUND:

Studies in traumatic brain injury suggest that monitoring techniques such as brain tissue oxygen (Pbto2) and cerebral microdialysis may complement conventional intracranial pressure (ICP) and cerebral perfusion pressure (CPP) measurements.

OBJECTIVE:

In this study of poor-grade (Hunt and Hess grade IV and V) subarachnoid hemorrhage (SAH) patients, we examined the prevalence of brain hypoxia and brain energy dysfunction in the presence of normal and abnormal ICP and CPP.

METHODS:

SAH patients who underwent multimodal neuromonitoring and cerebral microdialysis were studied. We examined the frequency of brain hypoxia and energy dysfunction in different ICP and CPP ranges and the relationship between Pbto2 and the lactate/pyruvate ratio (LPR).

RESULTS:

A total of 2394 samples from 19 patients were analyzed. There were 149 samples with severe brain hypoxia (Pbto2 ≤10 mm Hg) and 347 samples with brain energy dysfunction (LPR &gt;40). The sensitivities of abnormal ICP or CPP for elevated LPR and reduced Pbto2 were poor (21.2% at best), and the LPR or Pbto2 was abnormal in many instances when ICP or CPP was normal. Severe brain hypoxia was often associated with an LPR greater than 40 (86% of samples). In contrast, mild brain hypoxia (≤20 mm Hg) and severe brain hypoxia were observed in only 53% and 36% of samples with brain energy dysfunction, respectively.

CONCLUSION:

Our data demonstrate that ICP and CPP monitoring may not always detect episodes of cerebral compromise in SAH patients. Our data suggest that several complementary monitors may be needed to optimize the care of poor-grade SAH patients.

Cerebral Blood Flow and Metabolism Following Decompressive Craniectomy for Control of Increased Intracranial Pressure

OBJECTIVE

Decompressive craniectomy (DC) is a common practice for control of intracranial pressure (ICP) following traumatic brain injury (TBI), although the impact of this procedure on the fate of operated patients is still controversial.

METHODS

Cerebral blood flow (CBF) and metabolic rates were monitored prospectively and daily as a surrogate of neuronal viability in 36 TBI patients treated by DC and compared with those of 86 nonoperated patients. DC was performed either on admission (n = 29) or within 48 hours of admission (n = 7).

RESULTS

DC successfully controlled ICP levels and maintained CBF within a normal range although the cerebral metabolic rate of oxygen (CMRO2) was significantly lower in this group. In 7 patients, pre- and postoperative recordings showed a significant ICP decrease that correlated with CBF augmentation but not with concurrent improvement of CMRO2 that remained particularly low. Logistic regression analysis of all investigated variables showed that DC was not associated with higher mortality despite more severe injuries in this group. However, operated patients were 7-fold more likely to have poor functional outcomes than nonoperated patients. Good functional outcome was strongly associated with higher CMRO2 but not with higher CBF values. CMRO2 levels were significantly lower in the DC group, even after adjustment for injury severity, and showed a progressive and sustained trend of deterioration significantly different from that of the non-DC group.

CONCLUSION

These results suggest that DC may enhance survival in the presence of severe brain swelling, although it is unlikely to represent an adequate answer to mitochondrial damage responsible for cellular energy crisis and edema.

Does adherence to treatment targets in children with severe traumatic brain injury avoid brain hypoxia? A brain tissue oxygenation study

OBJECTIVE

Most physicians rely on conventional treatment targets for intracranial pressure, cerebral perfusion pressure, systemic oxygenation, and hemoglobin to direct management of traumatic brain injury (TBI) in children. In this study, we used brain tissue oxygen tension (PbtO2) monitoring to examine the association between PbtO2 values and outcome in pediatric severe TBI and to determine the incidence of compromised PbtO2 in patients for whom acceptable treatment targets had been achieved.

METHODS

In this prospective observational study, 26 children with severe TBI and a median postresuscitation Glasgow Coma Scale score of 5 were managed with continuous PbtO2 monitoring. The relationships between outcome and the 6-hour period of lowest PbtO2 values and the length of time that PbtO2 was less than 20, 15, 10, and 5 mmHg were examined. The incidence of reduced PbtO2 for each threshold was evaluated where the following targets were met: intracranial pressure less than 20 mmHg, cerebral perfusion pressure greater than 50 mmHg, arterial oxygen tension greater than 60 mmHg (and peripheral oxygen saturation > 90%), and hemoglobin greater than 8 g/dl.

RESULTS

There was a significant association between poor outcome and the 6-hour period of lowest PbtO2 and length of time that PbtO2 was less than 15 and 10 mmHg. Multiple logistic regression analysis showed that low PbtO2 had an independent association with poor outcome. Despite achieving the management targets described above, 80% of patients experienced one or more episodes of compromised PbtO2 (< 20 mmHg), and almost one-third experienced episodes of brain hypoxia (PbtO2 < 10 mmHg).

CONCLUSION

Reduced PbtO2 is associated with poor outcome in pediatric severe TBI. In addition, many patients experience episodes of compromised PbtO2 despite achieving acceptable treatment targets.

[Monitoring of tissue oxygen pressure (PtiO2) in cerebral hypoxia: diagnostic and therapeutic approach]

One of the main causes of secondary cerebral injury is cerebral hypoxia, basically of ischemic origin. However, cerebral tissue oxygenation depends on multiple physiological variables and cerebral hypoxia may be caused by an alteration of any one of them. Although several methods of continuous cerebral oxygenation monitoring of neurocritical patients have been developed, direct and continuous measurement of the oxygen pressure in the cerebral tissue (PtiO2) has been a reality in the handling of the neurocritical patients over recent years. This technique is highlighted by its reliability and value of the information that it provides. This present article presents a review of the most outstanding aspects of the PtiO2 monitoring and proposes a protocol for the interpretation of this monitoring technique. This algorithm attempts to facilitate the identification of the different types of different cerebral hypoxia and of the correct therapeutic choice in the complex decision making process in neurocritical patients at risk of cerebral hypoxia.

Microdialysis applications in neuroscience

BACKGROUND

Microdialysis is a technique to monitor extracellular changes in living tissue. Substances present in the extracellular space, such as neurotransmitters and metabolites transported between cells and capillaries in the extracellular fluid (ECF), are major object.

RESULTS

Since its introduction to the research of the nervous system, microdialysis has become a popular method for the measurements of brain chemistry and greatly affected in the fields of neuropharmacology, neuroanatomy and neurophysiology. Most of published papers using microdialysis have focused on the area of neuroscience, recently more biomedical application.

CONCLUSION

In this review, we focused on cerebral microdialysis as a monitoring tool for physiologic and pathophysiologic changes in chemical processes in the brain. Then we presented the principle and various applications of cerebral microdialysis.

The use of hyperventilation therapy after traumatic brain injury in Europe: an analysis of the BrainIT database

OBJECTIVE

To assess the use of hyperventilation and the adherence to Brain Trauma Foundation-Guidelines (BTF-G) after traumatic brain injury (TBI).

SETTING

Twenty-two European centers are participating in the BrainIT initiative.

DESIGN

Retrospective analysis of monitoring data.

PATIENTS AND PARTICIPANTS

One hundred and fifty-one patients with a known time of trauma and at least one recorded arterial blood-gas (ABG) analysis.

MEASUREMENTS AND RESULTS

A total number of 7,703 ABGs, representing 2,269 ventilation episodes (VE) were included in the analysis. Related minute-by-minute ICP data were taken from a 30 min time window around each ABG collection. Data are given as mean with standard deviation. (1) Patients without elevated intracranial pressure (ICP) (< 20 mmHg) manifested a statistically significant higher P(a)CO(2) (36 +/- 5.7 mmHg) in comparison to patients with elevated ICP (> or = 20 mmHg; P(a)CO(2): 34 +/- 5.4 mmHg, P < 0.001). (2) Intensified forced hyperventilation (P(a)CO(2) < or = 25 mmHg) in the absence of elevated ICP was found in only 49 VE (2%). (3) Early prophylactic hyperventilation (< 24 h after TBI; P(a)CO(2) < or = 35 mmHg, ICP < 20 mmHg) was used in 1,224 VE (54%). (4) During forced hyperventilation (P(a)CO(2) < or = 30 mmHg), simultaneous monitoring of brain tissue pO(2) or S(jv)O(2) was used in only 204 VE (9%).

CONCLUSION

While overall adherence to current BTF-G seems to be the rule, its recommendations on early prophylactic hyperventilation as well as the use of additional cerebral oxygenation monitoring during forced hyperventilation are not followed in this sample of European TBI centers.

DESCRIPTOR

Neurotrauma.

Brain changes to hypocapnia using rapidly interleaved phosphorus-proton magnetic resonance spectroscopy at 4 T

Substantial controversy persists in the literature concerning the physiologic consequences hypocapnia, or low partial pressure of carbon dioxide (PaCO(2)). Invasive animal studies have demonstrated large pH increases (>0.25 U), phosphocreatine (PCr) decreases (>30%), and adenosine triphosphate (ATP) decreases (>10%) after hyperventilation (HV) (20 mm Hg PaCO(2)). However, using magnetic resonance spectroscopy, HV studies in awake humans have demonstrated only small pH changes ( approximately 0.05 U) and no changes in PCr or ATP. It remains important to ascertain whether this failure to detect PCr changes in human studies reflects a true absence of changes, or a limitation in data fidelity. The present study used a rapidly interleaved phosphorus-proton spectroscopy acquisition from large samples at high magnetic field (4 T), to measure pH, PCr, inorganic phosphate, beta-ATP, and lactate changes with high temporal and signal sensitivity. Five of six subjects had usable data. During 20 mins HV, PaCO(2) reached a minimum at 16 mins (17 mm Hg); however, the maximum pH change (+0.047) peaked earlier (14 mins). Maximal lactate increases were measured at 15 mins. By 10 mins, maximum changes were observed for PCr (-3.4%) and inorganic phosphate (+6.4%). No changes in beta-ATP were observed. The peak in pH, despite continued decreases in PaCO(2), suggests active buffering during HV. These data, and the small magnitude of early PCr and inorganic phosphate changes, do not support substantial energy compromise during HV. Other mitigating factors, such as anesthesia-induced deregulation of the cerebrovasculature, might have contributed to the exaggerated metabolic changes observed in previous animal investigations.

Direct cerebral oxygenation monitoring--a systematic review of recent publications

This review has been compiled to assess publications related to the clinical application of direct cerebral tissue oxygenation (PtiO2) monitoring published in international, peer-reviewed scientific journals. Its goal was to extract relevant, i.e. positive and negative information on indications, clinical application, safety issues and impact on clinical situations as well as treatment strategies in neurosurgery, neurosurgical anaesthesiology, neurosurgical intensive care, neurology and related specialties. For completeness' sake it also presents some related basic science research. PtiO2 monitoring technology is a safe and valuable cerebral monitoring device in neurocritical care. Although a randomized outcome study is not available its clinical utility has repeatedly been clearly confirmed because it adds a monitoring parameter, independent from established cerebral monitoring devices. It offers new insights into cerebral physiology and pathophysiology. Pathologic values have been established in peer-reviewed research, which are not only relevant to outcome but are treatable. The benefits clearly outweigh the risks, which remains unchallenged in all publications retrieved. It is particularly attractive because it offers continuous, real-time data and is available at the bedside.

Comparison of moderate hyperventilation and mannitol for control of intracranial pressure control in patients with severe traumatic brain injury--a study of cerebral blood flow and metabolism

OBJECTIVE

To compare the respective effects of established measures used for management of traumatic brain injury (TBI) patients on cerebral blood flow (CBF) and cerebral metabolic rates of oxygen (CMRO2), glucose (CMRGlc) and lactate (CMRLct).

METHODS

Thirty-six patients suffering from severe traumatic brain injury (TBI) were prospectively evaluated. In all patients baseline assessments were compared with that following moderate hyperventilation (reducing PaCO2 from 36 +/- 4 to 32 +/- 4 mmHg) and with that produced by administration of 0.5 gr/kg mannitol 20% intravenously. Intracranial and cerebral perfusion pressure (ICP, CPP), CBF and arterial jugular differences in oxygen, glucose and lactate contents were measured for calculation of CMRO2, CMRGlc and CMRLct.

RESULTS

Following hyperventilation, CBF was significantly reduced (P < 0.0001). CBF remained most often above the ischemic range although values less than 30 ml x 100 gr(-1) x min(-1) were found in 27.8% of patients. CBF reduction was associated with concurrent decrease in CMRO2, anaerobic hyperglycolysis and subsequent lactate production. In contrast, mannitol resulted in significant albeit moderate improvement of cerebral perfusion. However, administration of mannitol had no ostensible effect either on oxidative or glucose metabolism and lactate balance remained mostly unaffected.

CONCLUSIONS

Moderate hyperventilation may exacerbate pre-existing impairment of cerebral blood flow and metabolism in TBI patients and should be therefore carefully used under appropriate monitoring. Our findings rather support the use of mannitol for ICP control.

[Monitoring of cerebral oxygenation with SvjO(2) or PtiO(2)]

Jugular venous oxygen saturation (SvjO(2)) monitoring has been developed in order to detect cerebral ischaemia. The interpretation of SvjO(2) values remains nevertheless complex, and should be associated with cerebral haemodynamic multimonitoring with ICP and transcranial Doppler. With the hypothesis of a constant cerebral oxygen consummation, and also with a constant haematocrit, SvjO(2) variations correlates with cerebral blood flow variations. After a brain trauma, an SvjO(2)<50% or>75% is associated with a bad prognosis. To maintain SvjO(2)>50% constitutes a reasonable therapeutic objective, but the benefice associated with such a strategy has not been validated. Oxygen partial pressure (PtiO(2)) in the brain parenchyma may be monitored in the non-lesioned area (usually frontal) in order to detect a global cerebral ischaemia, or in the penumbra of a cerebral lesion in order to detect a local ischaemia. The values associated with an ischemic risk are not fully defined and may be under 10-15 mmHg. A concomitant metabolic monitoring by cerebral microdialysis is of importance to fully address the real cerebral local ischaemic burden. Scientific studies are mainly focused on patients with a brain traumatism. Nor SvjO(2), nor PtiO(2) monitoring have at present been demonstrated to be associated with a clinical benefit, and their use should be restricted to scientific research.

[Cerebral ischemic threshold in clinical practice]

The ischemic threshold is reached when the availability of oxygen in the cerebral tissue does not cover oxygen requirement. For a patient sedated, with constant PaO(2) and haemoglobin, the cerebral blood flow (CBF) global and local is the essential factor to maintain such a balance. At a cellular level, ischemia occurs when the CBF is below 20-25 ml/min. However, this threshold probably varies with the patient and also within the normal or perilesional tissue. A cerebral perfusion pressure (CPP) of 60 mmHg, recommended for a cerebral perfusion allowing a sufficient CBF for normal brain, does not prevent ischemia. Monitoring aimed to control parameters of the aerobic metabolism (PtiO(2), SjO(2) and microdialysis) and to detect the ischemic threshold allows to adapt the CPP to each patient and continuously.

The role of tissue oxygen monitoring in patients with acute brain injury

Cerebral ischaemia is implicated in poor outcome after brain injury, and is a very common post-mortem finding. The inability of the brain to store metabolic substrates, in the face of high oxygen and glucose requirements, makes it very susceptible to ischaemic damage. The clinical challenge, however, remains the reliable antemortem detection and treatment of ischaemic episodes in the intensive care unit. Outcomes have improved in the traumatic brain injury setting after the introduction of progressive protocol-driven therapy, based, primarily, on the monitoring and control of intracranial pressure, and the maintenance of an adequate cerebral perfusion pressure through manipulation of the mean arterial pressure. With the increasing use of multi-modal monitoring, the complex pathophysiology of the injured brain is slowly being unravelled, emphasizing the heterogeneity of the condition, and the requirement for individualization of therapy to prevent secondary adverse hypoxic cerebral events. Brain tissue oxygen partial pressure (Pb(O2) monitoring is emerging as a clinically useful modality, and this review examines its role in the management of brain injury.

[Cerebral microdialysis as a clinical tool]

Microdialysis is the only technique available for cerebral metabolic monitoring in the clinical setting. By the mean of a probe inserted in the brain, it provides an extracellular space sampling. Values of various substrates including cerebral glucose, lactate, pyruvate, glycerol or glutamate can be obtained at the bedside at intervals between minutes and hours. Values are critically dependent on the flow of the perfusion liquid and reflect a highly localized cerebral metabolism. Cerebral microdialysis improves our understanding of acute neurological events such as intracranial hypertension or decrease in brain tissue oxygen pressure. Cerebral microdialysis can be used for detection of ischaemia, especially after malignant stroke or vasospasm complicating subarachnoid haemorrhage. In these cases, it may influence the therapeutic management. Moreover, it permits the assessment of metabolic changes after therapeutic interventions. Finally, some markers (like lactate/pyruvate ratio) are related to outcome, especially after traumatic brain injury.

Hypocapnia related changes in pain-induced brain activation as measured by functional MRI

Stress, acute pain and chronic pain may often result in hyperventilation (HV) which produces hypocapnia. The aim of this fMRI-study was to investigate the influence of hypocapnia on cortical activation during noxious stimulation in 14 healthy volunteers. The intensity of voluntary HV was controlled by capnometry Three tasks were performed in the fMRI sessions: (I) three 3-min HV periods with 7-min periods of recovery in between; (II) mechanically induced phasic pain stimulation--pain task (PT); (III) tapping--motor task (MT). The last two of these protocols were performed under normocapnic and hypocapnic conditions. HV decreased the fMRI signal by 3-7% in all regions of the cortex and subcortical nuclei. This decrease was most prominent in the opercular, frontal and temporal areas. When the PT was performed during hypocapnia a strong reduction in cluster sizes and lower t-values in S1 and insular cortex were found. In contrast MT was accompanied by an increase in cluster sizes and higher t-values. From this we conclude that hypocapnia significantly influences the BOLD signal in nociceptive and motor systems, indicating that either the coupling between the BOLD effect and neuronal processing changed or that the activity in the cortical network which represents the pain processing is decreased. These effects should be considered for functional brain imaging studies on the nociceptive system.