Cerebrovascular responses to carbon dioxide in children anaesthetized with halothane and isoflurane

Age-related carbon dioxide reactivity in children after moderate and severe traumatic brain injury

OBJECTIVE The objective of this study is to assess carbon dioxide reactivity (CO2R) in children following traumatic brain injury (TBI). METHODS This prospective observational study enrolled children younger than 18 years old following moderate and severe TBI. Thirty-eight mechanically ventilated children had daily CO2R testing performed by measuring changes in their bilateral middle cerebral artery flow velocities using transcranial Doppler ultrasonography (TCD) after a transient increase in minute ventilation. The cohort was divided into 3 age groups: younger than 2 years (n = 12); 2 to 5 years old (n = 9); and older than 5 years (n = 17). RESULTS Children younger than 2 years old had a lower mean CO2R over time. The 2-5-year-old age group had higher mean CO2R than younger patients (p = 0.01), and the highest CO2R values compared with either of the other age groups (vs > 5 years old, p = 0.046; vs < 2 years old, p = 0.002). Having a lower minimum CO2R had a statistically significant negative effect on outcome at discharge (p = 0.0413). Impaired CO2R beyond Postinjury Day 4 trended toward having an effect on outcome at discharge (p = 0.0855). CONCLUSIONS Abnormal CO2R is prevalent in children following TBI, and the degree of impairment varies by age. No clinical or laboratory parameters were identified as risk factors for impaired CO2R. Lower minimum CO2R values are associated with worse outcome at discharge.

Perioperative central nervous system injury in neonates

Anaesthetic-induced developmental neurotoxicity (AIDN) has been clearly established in laboratory animal models. The possibility of neurotoxicity during uneventful anaesthetic procedures in human neonates or infants has led to serious questions about the safety of paediatric anaesthesia. However, the applicability of animal data to clinical anaesthesia practice remains uncertain. The spectre of cerebral injury due to cerebral hypoperfusion, metabolic derangements, coexisting disease, and surgery itself further muddles the picture. Given the potential magnitude of the public health importance of this issue, the clinician should be cognisant of the literature and ongoing investigations on AIDN, and raise awareness of the risks of both surgery and anaesthesia.

Gender differences in cerebrovascular reactivity to carbon dioxide during sevoflurane anesthesia in children: preliminary findings

BACKGROUND

Cerebrovascular reactivity to carbon dioxide (CO(2) R) is affected by age, gender and anesthetic agents. While gender differences in CO(2) R are described in adults, there are no such data in children.

AIM

To examine the gender differences in CO(2) R in children during sevoflurane anesthesia.

METHODS

Five girls and five boys <15 years of age and ASA physical status I, undergoing general anesthesia for elective surgery were enrolled. Under steady-state anesthesia with <1.0 MAC sevoflurane, middle cerebral artery blood flow velocity changes were monitored using Transcranial Doppler ultrasound while endtidal carbon dioxide (EtCO(2)) was adjusted from 40 to 30 mmHg (hypocapnia) and then from 40 to 50 mmHg (hypercapnia). CO(2)R was calculated between EtCO(2) ranges 30-40 and 40-50 mmHg. Cerebrovascular resistance (eCVR) was estimated as MAP/Vmca and the change in eCVR (ΔeCVR) between EtCO(2) 30 and 40 mmHg and between EtCO(2) 40 and 50 mmHg was calculated.

RESULTS

There was no gender difference in CO(2)R. However, both CO(2)R and ΔeCVR were lower in the EtCO(2) 40-50 mmHg range compared to EtCO(2) 30-40 mmHg range only in girls (P = 0.01 and P = 0.01, respectively). Vmca increased significantly with increase in CO(2) (P < 0.001) for both boys and girls. The coefficient of nonlinear correlation (r) between Vmca and EtCO(2) was 0.88 in girls vs 0.66 in boys.

CONCLUSION

While there were no gender differences in CO(2)R within the individual EtCO(2) ranges examined, girls but not boys had a significantly lower CO(2)R and ΔeCVR in the higher EtCO(2) range during <1.0 MAC sevoflurane anesthesia.

Cerebrovascular pathophysiology in pediatric traumatic brain injury

BACKGROUND

Traumatic brain injury (TBI) is the leading cause of traumatic morbidity and mortality in children. Although there is increasing information concerning TBI in adults and experimental animal models, relatively little is known regarding cerebrovascular pathophysiology specific to children.

MATERIALS

A review of the pertinent medical literature.

RESULTS

Systemic and cerebral hemodynamic factors such as hypotension, hypoxia, hyperglycemia, and fever are associated with poor outcome in pediatric TBI. Similarly, cerebral autoregulation is often impaired after TBI and may adversely affect outcome, especially if systemic hemodynamics are altered. Furthermore, CO2 vasoreactivity may be altered after pediatric TBI and lead to either cerebral ischemia or hyperemia.

CONCLUSIONS

Understanding the effect of pediatric TBI on the cerebral circulation is needed to potentially develop protocols to improve outcome in this vulnerable population. Specifically, changes in pediatric cerebrovascular physiology and pathophysiology, including CO2 vasoreactivity and pressure autoregulation, must be understood and their mechanism elucidated.

Impact of anesthetic agents on cerebrovascular physiology in children

The role of the pediatric neuroanesthetist is to provide comprehensive care to children with neurologic pathologies. The cerebral physiology is influenced by the developmental stage of the child. The understanding of the effects of anesthetic agents on the physiology of cerebral vasculature in the pediatric population has significantly increased in the past decade allowing a more rationale decision making in anesthesia management. Although no single anesthetic technique can be recommended, sound knowledge of the principles of cerebral physiology and anesthetic neuropharmacology will facilitate the care of pediatric neurosurgical patients.

Cerebral blood flow and autoregulation after pediatric traumatic brain injury

Traumatic brain injury is a global health concern and is the leading cause of traumatic morbidity and mortality in children. Despite a lower overall mortality than in adult traumatic brain injury, the cost to society from the sequelae of pediatric traumatic brain injury is very high. Predictors of poor outcome after traumatic brain injury include altered systemic and cerebral physiology, including altered cerebral hemodynamics. Cerebral autoregulation is often impaired after traumatic brain injury and may adversely impact the outcome. Although altered cerebrovascular hemodynamics early after traumatic brain injury may contribute to disability in children, there is little information regarding changes in cerebral blood flow and cerebral autoregulation after pediatric traumatic brain injury. This review addresses normal pediatric cerebral physiology and cerebrovascular pathophysiology after pediatric traumatic brain injury.

Transcranial Doppler and anesthetics

Transcranial Doppler (TCD) is widely used to investigate the effects of anesthetic drugs on cerebral blood flow. Its repeatability and non-invasivity makes it an ideal, first choice method. Anesthesia providers are required to be conscious of the cerebral hemodynamic effects of drugs given in their practice, especially in neurosurgery and in subjects with impaired brain functions. The purpose of this review is to present the basic concepts of the TCD technique and the effects on cerebral hemodynamics of the most popular anesthetic drugs evaluated using TCD ultrasonography.

Optimizing the intraoperative management of carbon dioxide concentration

PURPOSE OF REVIEW

This review assesses whether there is a carbon dioxide concentration range that provides optimum benefit to the patient intraoperatively. It includes the physiological effects of carbon dioxide on various organ systems in awake and anesthetized individuals and its clinical effects in the ischemia/reperfusion setting. This review will present views on end-tidal or arterial carbon dioxide tension management in the perioperative period.

RECENT FINDINGS

Hypocapnia reduces intracranial pressure and is used by clinicians during acute traumatic brain injury, acute intracranial hemorrhage, and acutely growing brain tumors. There is mounting evidence, however, that hypercapnia improves tissue perfusion and oxygenation. Therefore, clinicians may want to induce mild-to-moderate hypercapnia during reperfusion states such as major vascular surgery, organ transplantation, tissue-graft surgery, and cases managed with low mean arterial pressures to control bleeding. As hypercapnia preserves cerebral blood flow even under relatively low perfusion pressures, it may be beneficial during global reperfusion scenarios. This hypothesis needs to be tested extensively before being considered for clinical applications. From a different perspective, current American Heart Association Guidelines recommend 12-15 breaths/min during cardiopulmonary resuscitation and stress the potential negative role of inadvertent hyperventilation on survival outcome. The importance of this concept is discussed briefly.

SUMMARY

Overall, the benefits of managing carbon dioxide concentration intraoperatively for the maintenance of cardiac output, tissue oxygenation, perfusion, intracranial pressure, and cerebrovascular reactivity are well defined.

The Cerebrovascular Response to Hypocapnia in Children Receiving Propofol

Hypocapnia is used to treat acute increases in intracranial pressure during neurosurgery. Cerebrovascular reactivity to carbon dioxide (CCO(2)R) is preserved above 35 mm Hg ETco(2) in children during propofol anesthesia; however, a plateau effect has been suggested below 35 mm Hg. To further delineate this phenomenon, we measured CCO(2)R by transcranial Doppler (TCD) sonography over small increments in ETco(2) in 27 healthy children. Anesthesia comprised a standardized propofol infusion and a caudal epidural block. A TCD probe was placed to measure middle cerebral artery blood flow velocity (V(mca)). ETco(2) was adjusted between 24 and 40 mm Hg at 1-2 mm Hg increments using an exogenous source of CO(2). There was an exponential relationship between ETco(2) and V(mca) above an ETco(2) value of 30 mm Hg (r = 0.82). However, V(mca) did not change with ETco(2) less than 30 mm Hg (r = 0.06). There were no significant changes in heart rate or arterial blood pressure. We conclude that when contemplating methods to decrease brain volume and intracranial pressure, hyperventilation to ETco(2) values less than 30 mm Hg may not be necessary in children receiving propofol, as no further reduction in cerebral blood flow velocity will be achieved.

The effect of nitrous oxide on cerebral blood flow velocity in children anaesthetised with sevoflurane

To determine the effects of nitrous oxide on middle cerebral artery blood flow velocity (CBFV) during sevoflurane anaesthesia in children, CBFV was measured using transcranial Doppler sonography in 16 ASA I or II children. Anaesthesia consisted of 1.0 MAC sevoflurane in 30% oxygen with intermittent positive pressure ventilation maintaining FEco2 at 38 mmHg (5.0 kPa) and a caudal epidural block using 0.25% bupivacaine 1.0 ml.kg-1. The remainder of the inspired gas was varied in one of two sequences either air/nitrous oxide/air or nitrous oxide/air/nitrous oxide. The results showed that CBFV decreased when nitrous oxide was replaced by air (p = 0.03) and returned to its initial value when nitrous oxide was reintroduced. CBFV increased when air was replaced by nitrous oxide (p = 0.04) and returned to its initial value when air was reintroduced. Mean heart rate and blood pressure remained constant. We conclude that nitrous oxide increases cerebral blood flow velocity in healthy children anaesthetised with 1.0 MAC sevoflurane.

The effect of sevoflurane on cerebral blood flow velocity in children

BACKGROUND

Sevoflurane is a suitable agent for neuroanesthesia in adult patients. In children, cerebrovascular carbon dioxide reactivity is maintained during hypo- and normocapnia under sevoflurane anesthesia. To determine the effects of sevoflurane on middle cerebral artery blood flow velocity (Vmca) in neurologically normal children, Vmca was measured both at different MAC values and at one MAC over a specified time period, using transcranial Doppler sonography.

METHODS

Twenty-six healthy children undergoing elective urological surgery were enrolled (16 patients in part I and 10 in part II). In part I of the study anesthesia comprised sevoflurane 0.5, 1.0 and 1.5 MAC in 30% oxygen and a caudal epidural block. Once steady state had been reached at each sevoflurane MAC level, three measurements of Vmca, mean arterial pressure (MAP) and heart rate (HR) were recorded. In part II of the study patients received sevoflurane 1.0 MAC over a 90-min period, with the same variables being recorded at 15-min intervals.

RESULTS

Vmca did not vary significantly at 0.5, 1.0 and 1.5 MAC sevoflurane. There was a significant decrease in MAP between 0.5 MAC and 1.0 MAC sevoflurane (P < 0.005) and also between 1.0 MAC and 1.5 MAC (P < 0.01). There was no significant change in Vmca over 90 min at 1.0 MAC sevoflurane.

CONCLUSION

Sevoflurane does not significantly affect cerebral blood flow velocity in healthy children at working concentrations.

The effect of nitrous oxide on cerebrovascular reactivity to carbon dioxide in children during propofol anesthesia

Nitrous oxide (N(2)O) increases cerebral blood flow when used alone and in combination with propofol. We investigated the effects of N(2)O on cerebrovascular CO(2) reactivity (CCO(2)R) during propofol anesthesia in 10 healthy children undergoing elective urological surgery. Anesthesia consisted of a steady-state propofol infusion and a continuous caudal epidural block. A transcranial Doppler probe was used to measure middle cerebral artery blood flow velocity. Randomization determined the sequence order of N(2)O (N(2)O/air or air/N(2)O) and end-tidal (ET)CO(2) concentration (25, 35, 45, and 55 mm Hg) using an exogenous source of CO(2). At steady state, three sets of measurements of middle cerebral artery blood flow velocity, mean arterial blood pressure, and heart rate were recorded. A linear preservation of CCO(2)R was observed above 35 mm Hg of ETCO(2), irrespective of N(2)O. A decrease in CCO(2)R to 1.4%-1.9% per millimeters of mercury was seen in the hypocapnic range (ETCO(2) 25-35 mm Hg) with both air and N(2)O. We conclude that N(2)O does not affect CCO(2)R during propofol anesthesia in children. When preservation of CCO(2)R is required, the combination of N(2)O with propofol anesthesia in children would seem suitable. The cerebral vasoconstriction caused by propofol would imply that hyperventilation to ETCO(2) values less than 35 mm Hg may not be required because no further reduction in cerebral blood flow velocity would be achieved.

Effect of nitrous oxide on cerebrovascular reactivity to carbon dioxide in children during sevoflurane anaesthesia

BACKGROUND

Sevoflurane and nitrous oxide have intrinsic cerebral vasodilatory activity. To determine the effects of nitrous oxide on cerebrovascular reactivity to carbon dioxide (CCO(2)R) during sevoflurane anaesthesia in children, middle cerebral artery blood flow velocity (V(mca)) was measured over a range of end-tidal carbon dioxide concentrations (E'(CO(2))), using transcranial Doppler (TCD) ultrasonography.

METHODS

Ten children aged 1.5-6 yr were anaesthetized with sevoflurane and received a caudal block. Patients were allocated randomly to receive either air-nitrous oxide or nitrous oxide-air. Further randomization determined the sequence of E'(CO(2)) (25, 35, 45, and 55 mm Hg) and sevoflurane (1.0 then 1.5 MAC or 1.5 then 1.0 MAC) concentrations. Once steady state had been reached, three measurements of V(mca), mean arterial pressure (MAP), and heart rate (HR) were recorded.

RESULTS

Cerebrovascular carbon dioxide reactivity was reduced in the 25-35 mm Hg E'(CO(2)) range on the addition of nitrous oxide to 1.5 MAC, but not 1.0 MAC sevoflurane. A plateau in CCO(2)R of 0.4-0.6% per mm Hg was seen in all groups between E'(CO(2)) values of 45 and 55 mm Hg. Mean HR and MAP remained constant throughout the study period.

CONCLUSIONS

Cerebrovascular carbon dioxide reactivity is reduced at and above an E'(CO(2)) of 45 mm Hg during 1.0 and 1.5 MAC sevoflurane anaesthesia. The addition of nitrous oxide to 1.5 MAC sevoflurane diminishes CCO(2)R in the hypocapnic range. This should be taken into consideration when hyperventilation techniques for reduction of brain bulk are being contemplated in children with raised intracranial pressure.

Cerebral blood flow velocity in children anaesthetized with desflurane

BACKGROUND

Desflurane allows for rapid emergence and changes in depth of anaesthesia which makes it especially suitable for neuroanaesthesia. This study was designed to determine the effects of different desflurane concentrations on cerebral blood flow velocity (CBFV) in healthy children.

METHODS

Twenty children, aged 1-7 years undergoing urological surgery were studied. Anaesthesia was induced with sevoflurane in oxygen. After tracheal intubation, sevoflurane was discontinued and ventilation with desflurane in air/oxygen was initiated and normoventilation maintained. A caudal block was performed. The patients were randomized to receive three different desflurane concentrations (0.5, 1.0 and 1.5 MAC). Fifteen minutes were allowed to reach steady-state at which time CBFV was measured by transcranial Doppler sonography. Mean arterial pressure (MAP) and heart rate (HR) were simultaneously recorded at 1-min intervals.

RESULTS

Cerebral blood flow velocity increased from 0.5 to 1.0 MAC (P < 0.05), but not from 1.0 to 1.5 MAC. HR increased from 0.5 to 1.0 (P < 0.001) and from 1.0 to 1.5 MAC (P < 0.001), whereas the MAP decreased only from 0.5 to 1.0 MAC (P < 0.001).

CONCLUSIONS

Desflurane in concentrations of 1.0 and 1.5 MAC in children increases CBFV significantly when compared with 0.5 MAC. These changes were associated with a significant increase in HR and decrease in MAP.

Cerebrovascular reactivity to carbon dioxide is preserved during hypocapnia in children anesthetized with 1.0 MAC, but not with 1.5 MAC desflurane

PURPOSE

Maintenance of cerebrovascular reactivity to CO(2) (CCO(2)R) is important during neurosurgical anesthesia. This study was designed to determine the effect of different desflurane concentrations on CCO(2)R in children.

METHODS

Children undergoing urological surgery were enrolled. Anesthesia was induced with sevoflurane in air/oxygen. After intubation, sevoflurane was switched to desflurane. Analgesia was provided with an epidural neuraxial block. Mechanical ventilation was adjusted to an initial EtCO(2) of 30 mmHg. Exogenous CO(2) was used to achieve an EtCO(2) of 40 and 50 mmHg. Patients were randomized to the sequence of desflurane concentration (1.0 and 1.5 MAC) and the EtCO(2). Transcranial Doppler was used to measure middle cerebral artery blood flow velocity (Vmca). Five minutes were allowed to reach steady state after each change in EtCO(2) and 15 min after changing the desflurane concentration.

RESULTS

Sixteen patients were studied. The mean age and weight were 3.5 +/- 1.5 yr and 14.4 +/- 3.1 kg, respectively. Mean arterial pressure remained stable throughout the study, while at an EtCO(2) of 50 mmHg, heart rate decreased at both desflurane concentrations (P < 0.05). At 1.0 MAC, Vmca increased from 30 to 40 mmHg (P < 0.05), but not from 40 to 50 mmHg EtCO(2). At 1.5 MAC, Vmca increased between 30 and 50 mmHg (P < 0.05).

CONCLUSION

CCO(2)R is preserved during hypocapnia in children anesthetized with 1.0 MAC, but not with 1.5 MAC desflurane. The lack of further increase in Vmca at higher EtCO(2) concentrations implies that desflurane may cause significant cerebral vasodilatation in children. This may have important implications in children with reduced intracranial compliance.

The effect of nitrous oxide on cerebral blood flow velocity in children anaesthetised with desflurane

The aim of this study was to determine the effect of nitrous oxide on cerebral blood flow velocity in children anaesthetised with desflurane. Eighteen healthy children scheduled for elective surgery were enrolled into the study. Anaesthesia was induced using sevoflurane, and a caudal block was performed following tracheal intubation. Anaesthesia was maintained with 1 age-adjusted MAC desflurane. A transcranial Doppler probe was used to measure middle cerebral artery blood flow velocity. Each patient was randomised to receive a sequence of either air/nitrous oxide/air or nitrous oxide/air/nitrous oxide in 30% oxygen. Fifteen minutes after each change in the nitrous oxide concentration, three measurements of cerebral blood flow velocity, blood pressure and heart rate were recorded. Neither the addition nor removal of nitrous oxide caused any significant changes in middle cerebral artery blood flow velocity, heart rate or blood pressure. This may be due to a more potent cerebral vasodilatory effect of desflurane in children.

Cerebrovascular carbon dioxide reactivity in children anaesthetized with propofol

BACKGROUND

Propofol, by virtue of its favourable pharmacokinetic profile, is suitable for maintenance of anaesthesia by continuous infusion during neurosurgical procedures in adults. It is gaining popularity for use in paediatric patients. To determine the effects of propofol on carbon dioxide cerebrovascular reactivity in children, middle cerebral artery blood flow velocity was measured at different levels of endtidal (PECO2) by transcranial Doppler sonography.

METHODS

Ten ASA I or II children, aged 1-6 years undergoing elective urological surgery were enrolled. Anaesthesia comprized propofol aimed at producing an estimated steady-state serum concentration of 3 microg x ml-1 and a caudal epidural block. PECO2 was adjusted randomly in an increasing or decreasing fashion between 3.3, 5.2 and 7.2 kPa (25, 40 and 55 mmHg) with an exogenous source of CO2 while maintaining ventilation parameters constant.

RESULTS

Cerebral blood flow velocity increased as PECO2 increased from 3.3 to 5.2 kPa (25-40 mmHg) (P < 0.001) and from 5.2 to 7.2 kPa (40-55 mmHg) (P < 0.001). Mean heart rate and blood pressure did not change significantly.

CONCLUSIONS

This study demonstrates that cerebrovascular CO2 reactivity is maintained over PECO2 values of 3.3, 5.2 and 7.2 kPa (25, 40 and 55 mmHg) in healthy children anaesthetized with propofol.

Propofol decreases cerebral blood flow velocity in anesthetized children

PURPOSE

Propofol, by virtue of its favourable pharmacokinetic profile, is suitable for maintenance of anesthesia by continuous infusion during neurosurgical procedures in adults. It is gaining popularity for use in pediatric patients. To determine the effects of propofol on cerebral blood flow in children, middle cerebral artery blood flow velocity (Vmca) was measured at different levels of propofol administration by transcranial Doppler (TCD) sonography.

METHODS

Twelve ASA I or II children, aged one to six years undergoing elective urological surgery were randomized to receive one of two propofol dosing regimens. Half of the patients received propofol in an escalating fashion, initially targeting an estimated steady-state serum concentration of 3 microg x mL-1, which was then doubled. The other half received propofol designed initially to target the high concentration followed by the lower one. In each child anesthesia was induced and maintained with propofol according to the protocol, rocuronium was given to facilitate tracheal intubation, and a caudal epidural block was performed. A TCD probe was placed appropriately to measure Vmca. Cerebral blood flow velocity (CBFV), mean arterial pressure (MAP) and heart rate (HR) were recorded simultaneously at both levels of propofol administration.

RESULTS

Twelve patients were studied. At the higher estimated target serum propofol concentration there were significant decreases in Vmca (17%, P < 0.001), MAP (6%, P < 0.002) and HR (8%, P < 0.05) when compared to the lower targeted concentration.

CONCLUSION

This study shows that a higher rate of propofol infusion is associated with lower CBFV and MAP values in children. Propofol's cerebral vasoconstrictive properties may be responsible for this finding.

Cerebrovascular carbon dioxide reactivity in children anaesthetized with sevoflurane

BACKGROUND

To determine the effects of sevoflurane on cerebrovascular carbon dioxide reactivity (CCO2R), middle cerebral artery blood flow velocity (CBFV) was measured at different levels of PE'CO2 by transcranial Doppler sonography in 16 ASA I or II children, aged 18 months to 7 yr undergoing elective urological surgery.

METHODS

Anaesthesia comprised 1.0 MAC sevoflurane and air in 30% oxygen delivered through an Ayre's T piece by intermittent positive-pressure ventilation, and a caudal epidural block with 0.25% bupivacaine 1.0 ml kg(-1) without epinephrine. PE'CO2 was randomly adjusted to 25, 35, 45 and 55 mm Hg (3.3, 4.6, 5.9 and 7.2 kPa) with an exogenous source of CO2, while maintaining ventilation variables constant.

RESULTS

CBFV increased as PE'CO2 increased from 25 to 35, and to 45 mm Hg (P<0.001), but did not increase significantly with an increase in PE'CO2 from 45 to 55 mm Hg. Mean heart rate and arterial pressure remained constant.

CONCLUSION

CCO2R is preserved in healthy children anaesthetized with 1.0 MAC sevoflurane.

[The specificity of neurosurgical anesthesia for the child]

Anaesthesia for paediatric neurosurgical procedures presents an interesting challenge to the anaesthesiologist. The child is not simply a small adult. At birth the central nervous system (CNS) development is incomplete and will not be mature until the end of the first year of life. Because of this delay in the maturation of the CNS, several specific pathophysiological and psychological differences ensue. Although one has little control on the child primary lesion, the selection of an anaesthetic technique designed to protect the perilesional area and the recognition of perioperative events and changes may well have a profound effect in the reduction or prevention of significant morbidity. Current neuroanaesthestic practice is based on the understanding of cerebral anatomy and physiology. Paediatric neuroanaesthesiologists must face the added challenge of the physiological differences between developing children and their adult counterparts.

The effect of low concentrations of halothane on the cerebrovascular circulation in young children

To determine the effect of halothane on cerebral blood flow velocity measured by transcranial Doppler, 23 healthy young children were studied during surgery. Anaesthesia was induced with thiopental, fentanyl and vecuronium, and maintained with halothane in 70% nitrous oxide in oxygen. A continuous epidural anaesthesia with 0. 25% bupivacaine was performed. End-tidal carbon dioxide pressure, temperature, heart rate and systolic blood pressure were kept constant. Three minimal alveolar concentrations (MAC; 0.5, 1.0 and 1. 5) of halothane were administered in stepwise increases. The cerebral blood flow velocity increased significantly at 1.0 (p < 0. 01) and 1.5 MAC (p < 0.001) compared with the value at 0.5 MAC. No further change in cerebral blood flow velocity was seen between 1.0 and 1.5 MAC. These data show that maximal changes in cerebral blood flow velocity are obtained at 1.0 MAC and that further increases in halothane concentration do not modify the cerebral circulation. It is suggested that young children differ from adults in that the maximal effect of halothane occurs at lower concentrations.

Cerebral blood flow velocity after mannitol infusion in children

PURPOSE

There is conflicting evidence as to whether the effect of mannitol on brain bulk arises from haemodynamic, rheologic, or osmotic mechanisms. If mannitol alters cerebral haemodynamics by inducing vasoconstriction, this change should be reflected in cerebral blood flow velocity (CBFV) in the middle cerebral artery (MCA). The purpose of this study was to evaluate the effect of mannitol on CBFV in children.

METHODS

Children scheduled for intracranial surgery were enrolled. After a loading dose of 10 micrograms.kg-1 of fentanyl, general anaesthesia was maintained with fentanyl (3 micrograms.kg-1.hr-1), 66% nitrous oxide, and isoflurane (0.2-0.5% inspired). Mean and systolic CBFV (Vm and Vs) and pulsatility index (PI) were recorded with a transcranial Doppler (TCD) directed at the M1 segment of the MCA. Mannitol was administered, 1 gm.kg-1 iv over 15 min. The osmolality (Osm), haematocrit (Hct), mean arterial pressure (MAP), heart rate (HR), and TCD variables were recorded before and 15, 30, 45, and 60 min after the mannitol infusion.

RESULTS

Mannitol infusion resulted in an increase in Osm and decrease in Hct (P < 0.05). Heart rate, MAP and arterial carbon dioxide tensions did not change (P > 0.05) during the measuring period. The Vm did not vary from baseline. The Vs and PI both increased briefly (P < 0.01 at 15 min and P < 0.05 at 30 min) after the mannitol, suggesting an increase in resistance distal to the MCA.

CONCLUSION

The time course of CBFV changes produced by mannitol corresponds with previous animal data concerning cerebrovascular tone. Our results suggest that mannitol briefly increases cerebrovascular resistance and thereby diminishes cerebral blood volume.

Effects of laryngoscopy and tracheal intubation on cerebral and systemic haemodynamics in children under different protocols of anaesthesia

The effects of laryngoscopy and tracheal intubation on cerebral and systemic haemodynamics were studied in 30 children. The objective was to identify conditions in which the alterations of cerebral and systemic haemodynamics were minimal. The children were intubated after muscular relaxation and following 10 min of mechanical ventilation with end-tidal halothane concentrations of 1.5%, 2.0% and 2.5%, respectively. With 1.5%, 2.0% and 2.5% end-tidal halothane, the mean flow velocity increased by 26%, 19% and 5%, the mean blood pressure by 14%, 10% and 1%, and the heart rate by 26%, 8% and 5% respectively after intubation. Adverse effects of laryngoscopy and tracheal intubation on cerebral haemodynamics in children can be avoided by adequate anaesthetic protocols.

Cerebral blood flow velocity patterns during cardiac surgery utilizing profound hypothermia with low-flow cardiopulmonary bypass or circulatory arrest in neonates and infants

To examine the effects of low-flow cardiopulmonary bypass (CPB) and circulatory arrest (PHCA) on cerebral pressure-flow velocity relationships, we studied 32 patients (< 9 mo of age) undergoing corrective cardiac procedures. Pressure-flow velocity relationships were studied during profound hypothermia (nasopharyngeal temperature < 20 degrees C). Cerebral blood-flow velocity (CBFV) was measured in the middle cerebral artery using transcranial Doppler sonography. The anterior fontanel pressure (AFP) was measured using an intracranial pressure monitor. Cerebral perfusion pressure (CPP) was calculated (mmHg) as mean arterial pressure (MAP) minus AFP. Nasopharyngeal temperature, PaCO2 and haematocrit were controlled during the study period. Alpha-stat acid-base management was employed. The CBFV measurements were made continuously over a range of CPP as pump flow (Q) was decreased to low-flow or to circulatory arrest and again during the subsequent increase in Q and CPP to normal. As Q and CPP were increased after a period of low-flow CPB during which period detectable CBFV was present, the CBFV was greater at any given CPP than prior to the low-flow state (P < 0.05). However, after PHCA a higher CPP (P < 0.05) was necessary to re-establish detectable CBFV and at any given CPP the CBFV was less than prior to PHCA (P < 0.05). Seventeen patients underwent low-flow CPB during which CBFV became non-detectable (7 +/- 1 cm.sec-1). In 12 of these patients the pattern of recovery of CBFV was the same as that observed after low-flow CPB whereas the remaining five (29%) demonstrated a pattern of recovery identical to the ones recorded after PHCA. We conclude that after PHCA a higher CPP is necessary to re-establish and maintain detectable CBFV. Furthermore, during low-flow CPB, patients where CBFV becomes non-detectable and show a pattern of CBFV recovery similar to PHCA, cessation of cerebral perfusion must be considered.

Cerebrovascular stability during isoflurane anaesthesia in children

The aims of this study were firstly, to determine the effect of various concentrations of isoflurane on cerebrovascular circulation and secondly, to examine the time-response characteristics of the drug on cerebral blood flow velocity in anaesthetized children. Thirty-two ASA physical status I or II patients aged one to eight years and scheduled for urological surgery were studied. Anaesthesia was induced with thiopentone 5 mg.kg-1 and fentanyl 2 micrograms.kg-1. Muscle relaxation was provided with vercuronium 0.1 mg.kg-1. Tracheal intubation was performed in all cases. Anaesthesia was maintained with isoflurane in a mixture of air and oxygen to produce an inspired oxygen fraction (FIO2) of 0.3. Ventilation was adjusted to maintain normocapnia. A caudal or lumbar epidural catheter was inserted before skin incision and a continuous bupivacaine, without epinephrine, infusion established. During the first part of this study, the initial isoflurane concentration for 24 patients was randomized and age-adjusted to 0.5 MAC, 1.0 MAC, or 1.5 MAC. After steady-state was reached, the subsequent isoflurane MAC concentration was randomized by either raising or lowering it from the initial concentration. In the second part of this study, the time-response effect of isoflurane was examined. Eight patients received 1.0 MAC isoflurane over 90 to 150 min. Temperature, heart rate, and systolic blood pressure were unchanged throughout the study. Cerebral blood flow velocity (CBFV) and resistance index (RI+), a measure of cerebrovascular resistance, were measured in the M1 segment of the middle cerebral artery (MCA) with a 2 MHz transcranial Doppler monitor.(ABSTRACT TRUNCATED AT 250 WORDS)

Transcranial Doppler sonography: nitrous oxide and cerebral blood flow velocity in children

To determine the effect of nitrous oxide (N2O) on cerebral blood flow velocity (CBFV) and cerebrovascular resistance index (RI+) in children, ten ASA physical status I or II patients aged one to eight years old, scheduled for urological procedures, were studied. Anaesthesia was induced with thiopentone 2 mg.kg-1, fentanyl 5 micrograms.kg-1 and diazepam 0.3 mg.kg-1. Muscular relaxation was ensured by using vecuronium 0.1 mg.kg-1. After tracheal intubation, anaesthesia was randomly assigned to either a mixture of air in oxygen (N2/O2) or 70% N2O in oxygen (N2O/O2) producing an FIO2 of 30%. Three sets of measurements of CBFV and RI+ were made with both gas mixtures. The CBFV and RI+ were measured in the middle cerebral artery (MCA) with a transcranial Doppler monitor. Measurements were made while using the initial gas mixture, then the second gas mixture was administered, and finally, the patient again was given the initial gas mixture. A continuous caudal epidural or lumbar epidural block was performed before skin incision. Neuromuscular blockade was maintained with vecuronium 0.05 mg.kg-1. Temperature, heart rate, end-tidal CO2, arterial oxygen saturation, haematocrit and arterial blood pressure were maintained constant. Ventilation was adjusted to achieve normocapnia. The CBFV increased when 70% N2/O2 was replaced by 70% N2O/O2 (P less than 0.05) while the CBFV decreased when 70% N2/O2 was readministered (P less than 0.05). Likewise, the CBFV decreased when 70% N2O/O2 was replaced by 70% N2/O2 (P less than 0.05) while the CBFV increased when 70% N2O/O2 was readministered (P less than 0.05).