Depth of anaesthesia monitoring: time to reject the index?

Depth of anaesthesia monitors can fail to detect consciousness under anaesthesia, primarily because they rely on the frontal EEG, which does not arise from a neural correlate of consciousness. A study published in a previous issue of the British Journal of Anaesthesia showed that indices produced by the different commercial monitors can give highly discordant results when analysing changes in the frontal EEG. Anaesthetists could benefit from routinely assessing the raw EEG and its spectrogram, rather than relying solely on an index produced by a depth of anaesthesia monitor.

Effects of sevoflurane versus propofol on cerebral autoregulation during anaesthesia for robot-assisted laparoscopic prostatectomy

Robot-assisted laparoscopic prostatectomy requires a pneumoperitoneum combined with steep Trendelenburg positioning, and these conditions can be associated with impairment of cerebral autoregulation. The objective of this study was to determine if choice of anaesthetic agent affects the preservation of cerebral autoregulation during robot-assisted laparoscopic prostatectomy. We randomly assigned 30 patients to maintenance of general anaesthesia with either propofol or sevoflurane. Cerebral autoregulation was tested by administration of intravenous phenylephrine to increase mean arterial pressure from approximately 80 mmHg to 100 mmHg while assessing cerebral blood flow using transcranial Doppler ultrasonography. Autoregulation was first tested in the supine position and then approximately once every hour after Trendelenburg positioning. The main outcome measure was the result of the final autoregulation test prior to completion of surgery. At that time, we found cerebral autoregulation to be significantly impaired in six of the 15 patients receiving sevoflurane and none of the 15 patients receiving propofol (P = 0.02). However, it should be noted that some patients in the propofol group had impaired autoregulation on earlier tests. In conclusion, we found that autoregulation during robot-assisted laparoscopic prostatectomy is less likely to be impaired with propofol compared to sevoflurane anaesthesia, particularly towards the end of the surgery.

Correlation and agreement between the TEG® 5000 and the TEG® 6s during liver transplant surgery

The TEG® 5000 and novel TEG® 6s measure the viscoelasticity of whole blood during in vitro clot formation. The two devices measure similar coagulation variables but utilize distinctly different technologies. This study aimed to determine the correlation and agreement between the thrombelastographic parameters obtained by the two devices during liver transplant surgery. We obtained blood samples at six predefined intervals during the surgery of 10 consecutive patients. Two operators proficient in the use of the TEG® 6s and TEG® 5000 systems performed thrombelastographic measurements on each sample: non-citrated TEG® 5000, citrated TEG® 5000 and citrated TEG® 6s. Agreement and correlation were assessed using Bland Altman plots and Lin’s concordance correlation. There was considerable inter-device variability for the different parameters measured by the TEG® 5000 and TEG® 6s devices. Acceptable agreement was observed when results were within the normal reference ranges. However, with increasing coagulopathy, agreement was poor and results could not be considered interchangeable. Although each of the three tests appeared reliable for qualitative detection of abnormalities of clot formation during liver transplant surgery, we found their quantitative results were not interchangeable.

The evidence supporting WHO recommendations on the promotion of hand hygiene: a critique

OBJECTIVE

To examine the quality of the evidence relied upon by the World Health Organisation (WHO) in promoting hand hygiene with campaigns such as "Save Lives: Clean Your Hands".

RESULTS

The quality of evidence in the studies quoted by the WHO evidence document is highly variable and the methods used limited. In some of the quoted studies, hand hygiene was the primary outcome, rather than the clinically significant outcome of hospital acquired infection (HAI). When HAI was the primary outcome, it was often poorly defined and reported with scant detail. There was wide variation in the hand hygiene compliance achieved in the intervention studies. The majority of studies where the intervention was a campaign to promote hand hygiene used historical control data with variable attempts to account for the fact that HAI rates may have been declining prior to the hand hygiene intervention. The results from trials with a contemporaneous control were conflicting.

Relative hypotension in the beach-chair position: effects on middle cerebral artery blood velocity

When anaesthetising patients for arthroscopic shoulder surgery, it is common practice to sit the patient in the beachchair position and to optimise arthroscopy by allowing relative hypotension. There is little published information regarding the cerebral haemodynamic effects of hypotension in the sitting position during general anaesthesia. In this study, 19 patients scheduled for shoulder surgery were anaesthetised with desflurane. Phenylephrine and/or remifentanil were used to control blood pressure. Cerebral haemodynamics were assessed by monitoring middle cerebral artery blood velocity with transcranial Doppler, and by invasive arterial pressure monitoring with the transducer kept level with the external auditory meatus. Pressure and velocity waveforms were analysed to calculate apparent zero flow pressure and resistance area product. Cerebral haemodynamics in patients anaesthetised supine at the pre-induction blood pressure were compared with haemodynamics while seated at 45 degrees with hypotension. According to our routine practice, blood pressure management was guided by non-invasive measurement of systolic pressure using an arm cuff Changing from supine/normotensive to sitting/hypotensive caused mean arterial pressure at the auditory meatus to decrease 47 +/- 7% and middle cerebral artery blood velocity to decrease 22 +/- 7%. In the beach-chair position, systolic pressure was 96 +/- 10 mmHg in the arm and 76 +/- 10 mmHg at the auditory meatus (P < 0.0001). Both resistance area product and apparent zero flow pressure decreased, suggesting decreases in cerebrovascular resistance and critical closing pressure. Although there was some evidence of an autoregulatory response, middle cerebral artery blood velocity decreased when relative hypotension was induced in patients anaesthetised with desflurane in the beach-chair position.

The effects of hypocapnia and the cerebral autoregulatory response on cerebrovascular resistance and apparent zero flow pressure during isoflurane anesthesia

BACKGROUND

Simultaneous recordings of arterial blood pressure (ABP) and middle cerebral artery blood velocity can be used to calculate the apparent zero flow pressure (aZFP). The inverse of the slope of the pressure-velocity relationship is known as resistance area product (RAP) and is an index of cerebrovascular resistance. There is little information available regarding the effects of vasoactive drugs, arterial carbon dioxide (Paco(2)), and impaired cerebral autoregulation on aZFP and RAP during general anesthesia. During isoflurane anesthesia, we investigated the effects of hypocapnia and the effects of a phenylephrine infusion, on aZFP and RAP.

METHODS

Radial ABP and transcranial Doppler middle cerebral artery blood velocity signals were recorded in 11 adults undergoing isoflurane anesthesia. A phenylephrine infusion was used to increase ABP and ventilation was adjusted to control Paco(2). Cerebral hemodynamic variables were compared at two levels of mean ABP (approximately 80 and 100 mm Hg) and at two levels of Paco(2): normocapnia (Paco(2) 38-43 mm Hg) and hypocapnia (Paco(2) 27-34 mm Hg). Two aZFP analysis methods were compared: one based on linear regression and one based on Fourier analysis of the waveforms.

RESULTS

At the lower ABP, aZFP was 23 +/- 11 mm Hg and 30 +/- 13 mm Hg (mean +/- sd) with normocapnia and hypocapnia, respectively (P < 0.001) and RAP was 0.76 +/- 0.97 mm Hg x s x cm(-1) and 1.16 +/- 0.16 mm Hg x s x cm(-1) with normocapnia and hypocapnia, respectively (P < 0.001). Similar effects of hypocapnia were seen at the higher ABP. With normocapnia, isoflurane impaired cerebral autoregulation and aZFP did not change with the increase in ABP. With hypocapnia, cerebral autoregulation was not significantly impaired and increasing ABP was associated with increased aZFP (from 30 +/- 13 to 35 +/- 13 mm Hg, P < 0.01) and increased RAP (from 1.16 +/- 0.16 to 1.52 +/- 0.20 mm Hg x s x cm(-1), P < 0.001). Calculation of the relative contributions of aZFP and RAP to the cerebral hemodynamic responses indicated that changes in RAP appeared to have a greater influence than changes in aZFP. The mean difference between the two methods of determining aZFP (Fourier-regression) was 0.5 +/- 3.6 mm Hg (mean +/- 2sd).

CONCLUSIONS

During isoflurane anesthesia, two interventions that increase cerebral arteriolar tone, hypocapnia and the autoregulatory response to increasing ABP, were associated with increased RAP and increased aZFP. The effect of changes in RAP appeared to be quantitatively greater than the effects of changes in aZFP. These results imply that arteriolar tone influences cerebral blood flow by controlling both resistance and effective downstream pressure.

A Randomized Crossover Comparison of the Effects of Propofol and Sevoflurane on Cerebral Hemodynamics during Carotid Endarterectomy

Background

Intravenous and inhalational anesthetic agents have differing effects on cerebral hemodynamics: Sevoflurane causes some vasodilation, whereas propofol does not. The authors hypothesized that these differences affect internal carotid artery pressure (ICAP) and the apparent zero flow pressure (critical closing pressure) during carotid endarterectomy. Vasodilation is expected to increase blood flow, reduce ICAP, and reduce apparent zero flow pressure.

Methods

In a randomized crossover study, the gradient between systemic arterial pressure and ICAP during carotid clamping was measured while changing between sevoflurane and propofol in 32 patients. Middle cerebral artery blood velocity, recorded by transcranial Doppler, and ICAP waveforms were analyzed to determine the apparent zero flow pressure.

Results

ICAP increased when changing from sevoflurane to propofol, causing the mean gradient between arterial pressure and ICAP to decrease by 10 mmHg (95% confidence interval, 6-14 mmHg; P&lt;0.0001). Changing from propofol to sevoflurane had the opposite effect: The pressure gradient increased by 5 mmHg (95% confidence interval, 2-7 mmHg; P=0.002). Ipsilateral middle cerebral artery blood velocity decreased when changing from sevoflurane to propofol. Cerebral steal was detected in one patient after changing from propofol to sevoflurane. The apparent zero flow pressure (mean+/-SD) was 22+/-10 mmHg with sevoflurane and 30+/-14 mmHg with propofol (P&lt;0.01). There was incomplete drug crossover due to the limited duration of carotid clamping.

Conclusions

Compared with sevoflurane, ipsilateral ICAP and apparent zero flow pressure are both higher with propofol. Vasodilatation associated with sevoflurane can cause cerebral steal.

A comparison of the oropharyngeal leak pressure between the reusable Classic laryngeal mask airway and the single-use Soft Seal laryngeal mask airway

We tested the oropharyngeal leak pressure with the reusable laryngeal mask airway and the single-use Soft Seal laryngeal mask airway. These two types of laryngeal mask airway (LMA) have a similar design but the reusable LMA cuff is made from silicone whereas the Soft Seal LMA cuff is polyvinylchloride. Thirty-five healthy subjects were anaesthetized and paralyzed and the two types of Soft Seal LMA were compared in a blinded randomized cross-over trial. The oropharyngeal leak pressure was significantly higher with the Soft Seal than the reuable (21 +/- 7.6 and 16 +/- 6.7 cm H2O respectively, P = 0.002). However, in four subjects the oropharyngeal leak pressure was higher with the reusable by > 4 cm H2O. We concluded that the reusable LMA may provide a better seal in some individuals but that, on average, the Soft Seal provides a higher oropharyngeal leak pressure than the reusable LMA.

The Effect of Hypocapnia on the Autoregulation of Cerebral Blood Flow During Administration of Isoflurane

Isoflurane impairs autoregulation of cerebral blood flow in a dose-related manner. Previous investigations in several other conditions have demonstrated that impaired autoregulation can be restored by hyperventilation. We hypothesized that hypocapnia may restore cerebral autoregulation impaired by isoflurane anesthesia. We administered isoflurane in 100% oxygen to 12 healthy patients aged 21-59 yr scheduled for elective nonneurological surgery. Isoflurane end-tidal concentration was individualized at 0.1% to 0.2% less than that required to induce short periods of isoelectric electroencephalogram. This resulted in an end-tidal isoflurane concentration of 1.6% +/- 0.2% (mean +/- sd) corresponding to an age-adjusted minimum alveolar anesthetic concentration multiple of 1.4. Mean arterial blood pressure was reduced to <80 mm Hg, by infusion of remifentanil if required. Cerebral autoregulation was assessed by infusing phenylephrine to increase mean arterial blood pressure to 100 mm Hg while monitoring middle cerebral artery blood flow velocity with transcranial Doppler ultrasonography. The change in flow velocity was used to calculate the autoregulation index (ARI). The ARI ranges between 0 and 1 and an ARI < or =0.4 indicates significantly impaired autoregulation. Autoregulation was tested twice in randomized order: once during normocapnia (Paco(2) 38-43 mm Hg) and once during hypocapnia (Paco(2) 27-34 mm Hg). The median (interquartile range) ARI was 0.29 (0.23-0.64) during normocapnia and 0.77 (0.70-0.78) during hypocapnia (P < 0.005). Of the 12 subjects, autoregulation was significantly impaired in 8 subjects during normocapnia and none during hypocapnia (P = 0.001). Hypocapnia restored cerebral autoregulation in normal subjects during isoflurane-induced impairment of autoregulation.

Pharmacokinetics of sevoflurane uptake into the brain

Two recent studies have examined the pharmacokinetics of sevoflurane in adults. Lu et al.(Pharmacokinetics of sevoflurane uptake into the brain and body, Anaesthesia 2003; 58: 951-6) observed that jugular bulb sevoflurane concentration initially rose unexpectedly rapidly and then approached arterial concentrations unexpectedly slowly, suggesting that a blood-brain diffusion barrier exists. They also observed a large alveolar-arterial sevoflurane gradient, suggesting that an alveolar-arterial diffusion barrier exists. Nakamura et al. (Predicted sevoflurane partial pressure in the brain with an uptake and distribution model: Comparison with the measured value in internal jugular vein blood. Journal of Clinical Monitoring and Computing 1999; 15: 299-305) found no diffusion barriers. We used a computer model to analyse both data sets and show that the observations of Lu et al. can be explained by contamination of jugular samples with extracerebral blood. It is possible that the alveolar-arterial gradients observed by Lu et al. are due to discrepancies in conversions between blood concentrations and gas partial pressures. Our study suggests that there is no blood-brain diffusion barrier for sevoflurane and that the data of Lu et al. must be interpreted with caution.

Cerebral hemodynamics immediately following carotid occlusion

During carotid endarterectomy, we routinely monitor internal carotid artery pressure (P(ICA)) and middle cerebral artery flow velocity (V(MCA)). P(ICA) has been previously shown to accurately reflect pressure at the origin of the middle cerebral artery, even during times of rapidly changing pressure such as occurs with sudden occlusion of the common carotid artery. We retrospectively analyzed pressure recordings around the time of carotid cross clamping in 29 consecutive carotid endarterectomy operations. Suitable transcranial Doppler recordings of V(MCA) were available from eight of the operations. Comparing the cardiac cycle prior to cross clamping with the first complete cardiac cycle after cross clamping, the mean P(ICA) fell from 93 mm Hg to 62 mm Hg and the mean V(MCA) fell from 41 cm x sec-1 to 25 cm x sec-1. Over the subsequent 10 seconds, there was a further decrease in P(ICA) to 51 mm Hg (P <.0001), while V(MCA) changed in the opposite direction, increasing to 32 cm x sec-1 (P <.01). The patients with the greatest decrease in P(ICA) immediately on cross clamping also had the greatest additional decrease over the following 10 seconds (r = 0.74). The increase in V(MCA) during the first 10 seconds after carotid occlusion is well recognized and is presumed to be due to autoregulatory vasodilatation. The simultaneous decrease that we observed in P(ICA) indicates an increase in the pressure gradient along the collateral vessels, which is to be expected during a period of increasing flow along those vessels.

Graded hypercapnia and cerebral autoregulation during sevoflurane or propofol anesthesia

BACKGROUND

Hypercapnia abolishes cerebral autoregulation, but little is known about the interaction between hypercapnia and autoregulation during general anesthesia. With normocapnia, sevoflurane (up to 1.5 minimum alveolar concentration) and propofol do not impair cerebral autoregulation. This study aimed to document the level of hypercapnia required to impair cerebral autoregulation during propofol or sevoflurane anesthesia.

METHODS

Eight healthy subjects received a remifentanil infusion and were anesthetized with propofol (140 microg. kg-1. min-1) and sevoflurane (1.0-1.1% end tidal) in a randomized crossover study. Ventilation was adjusted to achieve incremental increases in arterial carbon dioxide partial pressure (Paco2) until autoregulation was impaired. Cerebral autoregulation was tested by increasing the mean arterial pressure (MAP) from 80 to 100 mmHg with phenylephrine while measuring middle cerebral artery flow velocity by transcranial Doppler. The autoregulation index, which has a value ranging from 0 to 1, representing absent to perfect autoregulation, was calculated, and an autoregulation index of 0.4 or less represented significantly impaired autoregulation.

RESULTS

The threshold Paco2 to significantly impair cerebral autoregulation ranged from 50 to 66 mmHg. The threshold averaged 56 +/- 4 mmHg (mean +/- SD) during sevoflurane anesthesia and 61 +/- 4 mmHg during propofol anesthesia (P = 0.03). Carbon dioxide reactivity measured at a MAP of 100 mmHg was 30% greater than that at a MAP of 80 mmHg.

CONCLUSIONS

Even mild hypercapnia can significantly impair cerebral autoregulation during general anesthesia. There is a significant difference between propofol anesthesia and sevoflurane anesthesia with respect to the effect of hypercapnia on cerebral autoregulation. This difference occurs at clinically relevant levels of Paco2. When inducing hypercapnia, carbon dioxide reactivity is significantly affected by the MAP.

The communicating vein and its use in the radial forearm free flap

Thrombosis is the most frequent cause of failure in microvascular free-tissue transfer. The large communicating vein of the cubital fossa connects the deep and superficial venous drainage of the radial forearm free flap (RFFF). This vein allows the surgeon to simultaneously drain both systems by means of the large veins of the cubital fossa. We prospectively collected data on the venous anatomy of the cubital fossa in 40 consecutive RFFFs over a 3-year period. We then retrospectively reviewed available data from the 14 cases preceding the cases in the prospective series. At least 78% of our patients had a communicating vein that facilitated dual venous drainage; 87% of our RFFFs were drained by both the superficial and deep venous systems, and 90% of our RFFFs had two or more venous anastomoses. We had no RFFF failures in our series of 54 flaps. We present our venous anatomy findings in this series of forearms as well as the venous anastomoses of our 54 patients. The surgi- cal-flap harvest, including the communicating vein and its use, may provide an advantage in the dependability and quality of venous outflow.