Intrathecal dexmedetomidine attenuates hypercapnic but not hypoxic cerebral vasodilation in anesthetized rabbits

Anesthesia Blunts Carbon Dioxide Effects on Glymphatic Cerebrospinal Fluid Dynamics in Mechanically Ventilated Rats

BACKGROUND

Impaired glymphatic clearance of cerebral metabolic products and fluids contribute to traumatic and ischemic brain edema and neurodegeneration in preclinical models. Glymphatic perivascular cerebrospinal fluid flow varies between anesthetics possibly due to changes in vasomotor tone and thereby in the dynamics of the periarterial cerebrospinal fluid (CSF)-containing space. To better understand the influence of anesthetics and carbon dioxide levels on CSF dynamics, this study examined the effect of periarterial size modulation on CSF distribution by changing blood carbon dioxide levels and anesthetic regimens with opposing vasomotor influences: vasoconstrictive ketamine-dexmedetomidine (K/DEX) and vasodilatory isoflurane.

METHODS

End-tidal carbon dioxide (ETco2) was modulated with either supplemental inhaled carbon dioxide to reach hypercapnia (Etco2, 80 mmHg) or hyperventilation (Etco2, 20 mmHg) in tracheostomized and anesthetized female rats. Distribution of intracisternally infused radiolabeled CSF tracer 111In-diethylamine pentaacetate was assessed for 86 min in (1) normoventilated (Etco2, 40 mmHg) K/DEX; (2) normoventilated isoflurane; (3) hypercapnic K/DEX; and (4) hyperventilated isoflurane groups using dynamic whole-body single-photon emission tomography. CSF volume changes were assessed with magnetic resonance imaging.

RESULTS

Under normoventilation, cortical CSF tracer perfusion, perivascular space size around middle cerebral arteries, and intracranial CSF volume were higher under K/DEX compared with isoflurane (cortical maximum percentage of injected dose ratio, 2.33 [95% CI, 1.35 to 4.04]; perivascular size ratio 2.20 [95% CI, 1.09 to 4.45]; and intracranial CSF volume ratio, 1.90 [95% CI, 1.33 to 2.71]). Under isoflurane, tracer was directed to systemic circulation. Under K/DEX, the intracranial tracer distribution and CSF volume were uninfluenced by hypercapnia compared with normoventilation. Intracranial CSF tracer distribution was unaffected by hyperventilation under isoflurane despite a 28% increase in CSF volume around middle cerebral arteries.

CONCLUSIONS

K/DEX and isoflurane overrode carbon dioxide as a regulator of CSF flow. K/DEX could be used to preserve CSF space and dynamics in hypercapnia, whereas hyperventilation was insufficient to increase cerebral CSF perfusion under isoflurane.

EDITOR’S PERSPECTIVE

Efficacy and safety of dexmedetomidine infusion for patients undergoing awake craniotomy: An observational study

Background:

The goal of awake craniotomy is to maintain adequate sedation, analgesia, respiratory, and hemodynamic stability and also to provide a cooperative patient for neurologic testing. An observational study carried out to evaluate the efficacy of dexmedetomidine sedation for awake craniotomy.

Materials and Methods:

Adult patients with age >18 year who underwent awake craniotomy for intracranial tumor surgery were enrolled. Those who were uncooperative and had difficult airway were excluded from the study. In the operating room, the patients received a bolus dose of dexmedetomidine 1 μg/kg followed by an infusion of 0.2–0.7 μg/kg/h (bispectral index target 60–80). Once the patients were sedated, scalp block was given with bupivacaine 0.25%. The data on hemodynamics at various stages of the procedure, intraoperative complications, total amount of fentanyl used, intravenous fluids required, blood loss and transfusion, duration of surgery, Intensive Care Unit (ICU), and hospital stay were collected. The patients were assessed for Glasgow outcome scale (GOS) score and patient satisfaction score (PSS).

Results:

A total of 27 patients underwent awake craniotomy during a period of 2 years. Most common intraoperative complication was seizures; observed in five patients (18.5%). None of these patients experienced any episode of desaturation. Two patients had tight brain for which propofol boluses were administered. The average fentanyl consumption was 161.5 ± 85.0 μg. The duration of surgery, ICU, and hospital stays were 231.5 ± 90.5 min, 14.5 ± 3.5 h, and 4.7 ± 1.5 days, respectively. The overall PSS was 8 and GOS was good in all the patients.

Conclusion:

The use of dexmedetomidine infusion with regional scalp block in patients undergoing awake craniotomy is safe and efficacious. The absence of major complications and higher PSS makes it close to an ideal agent for craniotomy in awake state.

Dexmedetomidine as an anaesthetic adjuvant in patients undergoing intracranial tumour surgery: a double-blind, randomized and placebo-controlled study †

BACKGROUND

Dexmedetomidine (DEX) has been shown to provide good perioperative haemodynamic stability with decreased intraoperative opioid requirements. It may have neural protective effects, and thus may be a suitable anaesthetic adjuvant to neurosurgical anaesthesia.

METHODS

Fifty-four patients scheduled for elective surgery of supratentorial brain tumour were randomized to receive in a double-blind manner a continuous DEX infusion (plasma target concentration 0.2 or 0.4 ng ml(-1)) or placebo, beginning 20 min before anaesthesia and continuing until the start of skin closure. The DEX groups received fentanyl 2 microg kg(-1) at the induction of anaesthesia and before the start of operation, the placebo group 4 microg kg(-1), respectively. Anaesthesia was maintained with nitrous oxide in oxygen and isoflurane.

RESULTS

The median times from the termination of N2O to extubation were 6 (3-27), 3 (0-20) and 4 (0-13) min in placebo, DEX-0.2 and DEX-0.4 groups, respectively (P<0.05 anova all-over effect). The median percentage of time points when systolic blood pressure was within more or less than 20% of the intraoperative mean was 72, 77 and 85, respectively (P<0.01), DEX-0.4 group differed significantly from the other groups. DEX blunted the tachycardic response to intubation (P<0.01) and the hypertensive response to extubation (P<0.01). DEX-0.4 group differed in the heart rate variability from placebo (93 vs 82%, P<0.01).

CONCLUSIONS

DEX increased perioperative haemodynamic stability in patients undergoing brain tumour surgery. Compared with fentanyl, the trachea was extubated [corrected] faster without respiratory depression.

Spatial flow-volume dissociation of the cerebral microcirculatory response to mild hypercapnia

The spatial and temporal response of the cerebral microcirculation to mild hypercapnia was investigated via two-photon laser-scanning microscopy. Cortical vessels, traversing the top 200 microm of somatosensory cortex, were visualized in alpha-chloralose-anesthetized Sprague-Dawley rats equipped with a cranial window. Intraluminal vessel diameters, transit times of fluorescent dextrans and red blood cells (RBC) velocities in individual capillaries were measured under normocapnic (PaCO2= 32.6 +/- 2.6 mm Hg) and slightly hypercapnic (PaCO2= 45 +/- 7 mm Hg) conditions. This gentle increase in PaCO2 was sufficient to produce robust and significant increases in both arterial and venous vessel diameters, concomitant to decreases in transit times of a bolus of dye from artery to venule (14%, P < 0.05) and from artery to vein (27%, P < 0.05). On the whole, capillaries exhibited a significant increase in diameter (16 +/- 33%, P < 0.001, n = 393) and a substantial increase in RBC velocities (75 +/- 114%, P < 0.001, n = 46) with hypercapnia. However, the response of the cerebral microvasculature to modest increases in PaCO2 was spatially heterogeneous. The maximal relative dilatation (range: 5-77%; mean +/- SD: 25 +/- 34%, P < 0.001, n = 271) occurred in the smallest capillaries (1.6 microm-4.0 microm resting diameter), while medium and larger capillaries (4.4 microm-6.8 microm resting diameter) showed no significant changes in diameter (P > 0.08, n = 122). In contrast, on average, RBC velocities increased less in the smaller capillaries (39 +/- 5%, P < 0.002, n = 22) than in the medium and larger capillaries (107 +/- 142%, P < 0.003, n = 24). Thus, the changes in capillary RBC velocities were spatially distinct from the observed volumetric changes and occurred to homogenize cerebral blood flow along capillaries of all diameters.

Intrathecal Neostigmine Prevents Intrathecal Clonidine from Attenuating Hypercapnic Cerebral Vasodilation in Rabbits

We previously demonstrated that lumbar intrathecal alpha(2) agonists attenuate hypercapnia-induced cerebral vasodilation. The combination of intrathecal clonidine and neostigmine is being investigated as pain therapy. The effects of their combination on cerebrovascular reactivity are unknown. We allocated rabbits anesthetized with pentobarbital to two groups: (a) clonidine (normal saline followed 30 min later by clonidine 2 microg/kg, both into the lumbar intrathecal space; n = 6), and (b) neostigmine-pretreatment (neostigmine 2 microg/kg followed 30 min later by clonidine 2 microg/kg, both into the lumbar intrathecal space; n = 6). We then evaluated the hypercapnia-induced changes in pial arteriolar diameter in these two groups using the closed cranial window preparation. The pial arteriolar dilator response to hypercapnia was significantly attenuated in the clonidine group (14% +/- 4%, 4% +/- 4%, 6% +/- 6%, and 5% +/- 7% for before and 30, 60, and 90 min, respectively). Neither normal saline nor neostigmine alone induced any change in the cerebral reactivity to hypercapnia. Pretreatment with neostigmine completely prevented the clonidine-induced attenuation of the hypercapnic cerebral vasodilation attenuated by intrathecal clonidine (16% +/- 7%, 15% +/- 6%, 12% +/- 6%, and 16% +/- 8%, respectively).

Effects of ketamine on isoflurane- and sevoflurane-induced cerebral vasodilation in rabbits

Although ketamine has been reported to have little effect on the cerebral circulation when used with other anesthetics, its effect on the cerebral vascular response to volatile anesthetics, which increase cerebral blood flow in a concentration-dependent manner, remains obscure. A closed cranial window was prepared in 15 pentobarbital-anesthetized adult rabbits. The cerebral pial arteriolar alteration induced by either isoflurane (n = 8) or sevoflurane (n = 7) at 0 (before volatile anesthetic), 0.33, 0.67, and 1.0 minimum alveolar concentration (MAC) was measured under three consecutive conditions: intravenous infusion with saline, with ketamine, and with ketamine plus l-arginine. Ketamine reduced the vasodilation induced by 0.67 (120 +/- 9% versus 113 +/- 9%; P <.05) and 1.0 MAC isoflurane (136 +/- 11% versus 118 +/- 10%; P <.05), but l-arginine did not restore the isoflurane-induced cerebral vasodilation. In rabbits inhaling sevoflurane, the degree of cerebral vasodilator response was smaller than that by isoflurane, and the cerebral vasodilation was comparable whether in the presence or absence of ketamine (with or without l-arginine). In conclusion, ketamine reduces isoflurane-induced cerebral vasodilation, apparently independently of nitric oxide formation, while sevoflurane-induced cerebral vasodilation is not significantly affected by ketamine.

L-arginine and nitroglycerin restore hypercapnia-induced cerebral vasodilation in rabbits after its attenuation by ketamine

Although it has been reported that ketamine attenuates hypercapnia-induced cerebral vasodilation, the mechanism remains unknown. Because nitric oxide is involved in cerebral CO2 reactivity, we studied the effects of L-arginine and nitroglycerin on ketamine-mediated attenuation of vascular responses to hypercapnia. Under pentobarbital anesthesia, 16 rabbits underwent closed cranial window preparation. Hypercapnic challenges were repeated after IV saline, ketamine (10 mg/kg, followed by 20 mg x kg(-1) x h(-1)), or ketamine plus either L-arginine (150 mg/kg, followed by 100 mg x kg(-1) x h(-1); n = 8) or nitroglycerin (5 microg x kg(-1) x min(-1) infusion; n = 8). Ketamine reduced hypercapnia-induced cerebral vasodilation (1.27%/mm Hg +/- 0.45%/mm Hg [saline] versus 0.82%/mm Hg +/- 0.53%/mm Hg [ketamine]: P < 0.05), but L-arginine restored reactivity (1.28%/mm Hg +/- 0.73%/mm Hg: P < 0.05 versus ketamine), as did nitroglycerin (1.14%/mm Hg +/- 0.73%/mm Hg [saline] versus 0.56%/mm Hg +/- 0.63%/mm Hg [ketamine]: P < 0.05, and 1.15%/mm Hg +/- 0.74%/mm Hg [ketamine plus nitroglycerin]: P < 0.05 versus ketamine). This indicates that ketamine attenuates cerebral CO2 reactivity, at least in part, via suppression of nitric oxide-cyclic guanosine monophosphate mechanisms in the cerebral vasculature.

IMPLICATIONS

The attenuation of cerebral vasodilation to hypercapnia seen under ketamine anesthesia is reversed by L-arginine or nitroglycerin infusion.