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

Effects of pneumoperitoneum and Trendelenburg position on intracranial pressure assessed using different non-invasive methods

BACKGROUND

The laparoscopic approach is becoming increasingly frequent for many different surgical procedures. However, the combination of pneumoperitoneum and Trendelenburg positioning associated with this approach may increase the patient's risk for elevated intracranial pressure (ICP). Given that the gold standard for the measurement of ICP is invasive, little is known about the effect of these common procedures on ICP.

METHODS

We prospectively studied 40 patients without any history of cerebral disease who were undergoing laparoscopic procedures. Three different methods were used for non-invasive estimation of ICP: ultrasonography of the optic nerve sheath diameter (ONSD); transcranial Doppler-based (TCD) pulsatility index (ICP_PI); and a method based on the diastolic component of the TCD cerebral blood flow velocity (ICP_FVd). The ONSD and TCD were measured immediately after induction of general anaesthesia, after pneumoperitoneum insufflation, after Trendelenburg positioning, and again at the end of the procedure.

RESULTS

The ONSD, ICP_FVd, and ICP_PI increased significantly after the combination of pneumoperitoneum insufflation and Trendelenburg positioning. The ICP_FVd showed an area under the curve of 0.80 [95% confidence interval (CI) 0.70-0.90] to distinguish the stage associated with the application of pneumoperitoneum and Trendelenburg position; ONSD and ICP_PI showed an area under the curve of 0.75 (95% CI 0.65-0.86) and 0.70 (95% CI 0.58-0.81), respectively.

CONCLUSIONS

The concomitance of pneumoperitoneum and the Trendelenburg position can increase ICP as estimated with non-invasive methods. In high-risk patients undergoing laparoscopic procedures, non-invasive ICP monitoring through a combination of ONSD ultrasonography and TCD-derived ICP_FVd could be a valid option to assess the risk of increased ICP.

Nitrous oxide: are we still in equipoise? A qualitative review of current controversies

This review considers the current position of nitrous oxide in anaesthetic practice and balances potential beneficial and disadvantageous effects. The classic adverse characteristics of nitrous oxide, such as diffusion hypoxia, expansion of gas-filled spaces, and postoperative nausea and vomiting, are often cited as reasons to avoid this old drug. Recent concerns regarding neurotoxicity, adverse cardiovascular outcomes, and wound complications have further hardened many practitioners against nitrous oxide. New evidence and underpinning mechanistic data, however, suggest potential beneficial effects on the central nervous system, cardiovascular system, and acute and chronic pain. While we await the outcome of large studies including ENIGMA-II, many clinicians have already decided against this agent. The authors argue that this abandonment may be premature. Clinical Trial Registration None required.

Applications of nitrous oxide for procedural sedation in the pediatric population

OBJECTIVE

The objective of this review was to provide a general descriptive account of the physical properties, end-organ effects, therapeutic applications, and delivery techniques of nitrous oxide (N2O) as used in the arena of procedural sedation.

DATA SOURCE

A computerized bibliographic search regarding the applications of nitrous for provision of sedation and analgesia during procedures with an emphasis on the pediatric population was performed.

RESULTS

The end-organ effects of N2O have been well described in the operating room setting. Aside from its effects on the central nervous system of sedation and analgesia, N2O may alter intracerebral dynamics and alter cerebral blood flow and intracranial pressure especially in patients with altered intracranial compliance. Effects on ventilation include a dose-related depression of ventilatory function and control of upper airway patency. These effects are generally limited in the absence of comorbid diseases and potentiated by other sedative and analgesic agents. The more clinically significant respiratory effect of N2O on ventilatory function is a dose-dependent depression of the ventilatory response to hypoxemia. Hemodynamic effects include a mild direct depressant effect on myocardial function, which in the absence of comorbid cardiac disease is generally compensated by stimulation of the sympathetic nervous system. Nitrous oxide may potentially aggravate pulmonary hypertension. Additional physiologic effects on neurologic and hematologic function may result in inactivation of the enzyme, methionine synthetase. Recent concern has also been raised regarding the potential effects of N2O on immune function and its relationship to perioperative surgical site infections. Given differences in the solubility, N2O will diffuse into and significantly expand gas-filled cavities. Chronic exposure of health care works to N2O is also a concern. Although there are limited data in the literature to clearly substantiate concerns regarding the reproductive toxicity of occupational exposure to N2O, appropriate scavenging and use of other techniques are mandatory. Nitrous oxide has been shown to be effective for a variety of minor surgical procedures such as venipuncture, intravenous cannula placement, lumbar puncture, bone marrow aspiration, laceration repair, dental care, and minor dermatologic procedures. It is generally as effective as midazolam, with several studies demonstrating it to be more effective. However, its utility is not as great in severely painful procedures such as fracture reduction. Demonstrated advantages to parenteral sedation include a more rapid onset and a shorter recovery time with the majority of patients preferring it to over other agents or agreeing to its use for subsequent procedures. The literature also suggests increased success rates with simple procedures such as intravenous cannula placement when compared with placebo. In general, life-threatening adverse events have not been reported. Most common adverse effects include dysphoria and vomiting. For more painful procedures, combination with another agent may be used, and in all cases, topical or infiltrative local anesthesia is recommended.

CONCLUSIONS

In general, N2O is a useful adjunct for procedural sedation. Given the variety of procedures performed in the pediatric patient, ongoing research is required to identify the most appropriate and effective use of this agent. This may be particularly relevant when evaluating its use for procedures associated with significant pain. In these scenarios, the combination of N2O with other agents needs to be evaluated. Given the potential for adverse effects, strict adherence to published guidelines regarding procedural sedation and monitoring is suggested.

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.

Is nitrous oxide use appropriate in neurosurgical and neurologically at-risk patients?

PURPOSE OF REVIEW

To address controversial issues surrounding the use of nitrous oxide as a component of anesthesia in neurosurgical and neurologically at-risk patients.

RECENT FINDINGS

Nitrous oxide has been used as a component of general anesthesia for over 160 years and has contributed to countless apparently uneventful anesthetics in neurologically at-risk patients. Avoidance of nitrous oxide in specific circumstances, such as pre-existing pneumocephalus, during acute venous air embolism, and in patients with disorders of folate metabolism, is warranted. However, various controversies exist regarding the use of this drug in the general neurosurgical population. Specifically, some suggest a possible association between nitrous oxide and the postoperative development of tension pneumocephalus despite lack of data to support this notion. Additionally, data describing alterations of cerebral hemodynamics and metabolism and exacerbation of ischemic neurologic injury by nitrous oxide are inconsistent. Recent data derived from humans having cerebral aneurysm clipping failed to show any long-term adverse effect from the use of nitrous oxide on gross neurologic or cognitive function.

SUMMARY

Except in a few specific circumstances, there exists no conclusive evidence to support the dogmatic avoidance of nitrous oxide in neurosurgical patients.

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.

Hypothalamic injury as a cause of refractory hypotension after sellar region tumor surgery

INTRODUCTION

Fluid-resistant arterial hypotension can result in hypoperfusion of the brain and other organs. Well-known causes of arterial hypotension in neurosurgical practice include cardiac failure, septic shock, adrenal insufficiency, brainstem, and cervical spinal cord damage. Fluid-resistant arterial hypotension can occur in patients with brain edema without damage to brainstem when hypothalamic nuclei suffer. This phenomenon is not a well-documented cause of hypotension.

METHODS

We prospectively investigated 15 cases with clinical syndrome of arterial hypotension in patients following surgery for sellar region tumors. These cases were taken from 1005 patients operated between May 2003 and December 2005. Pulmonary artery catheter was used to investigate hemodynamic profile.

RESULTS

The mechanism of arterial hypotension consisted of decrease of vascular tone (SVRI was 1503 +/- 624 dyn x s x cm(5) x m(2)) and relative hypovolemia (CVP: 4.5 +/- 2.6 torr, PAWP: 7.4 +/- 3.5 torr). In all cases arterial hypotension was corrected with phenylephrine after failure to respond to fluid resuscitation alone. Fluid balance was positive over the next 72 h. Twenty-seven percent of patients had transitory thyroid insufficiency. In these situations dopamine was administrated as symptomatic therapy and dose of thyroid hormone was increased. Mortality was 53%.

CONCLUSION

Hypothalamic damage can result in life-threatening vasodilatory arterial hypotension after sellar region tumor surgery. beta-Sympatomimetics are indicated in cases with thyroid insufficiency.

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 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.