Comparison of pattern of breathing with other measures of induction of anaesthesia, using propofol, methohexital, and sevoflurane

Muscle power during intravenous sedation

Intravenous sedation is effective to reduce fear and anxiety in dental treatment. It also has been used for behavior modification technique in dental patients with special needs. Midazolam and propofol are commonly used for intravenous sedation. Although there have been many researches on the effects of midazolam and propofol on vital function and the recovery profile, little is known about muscle power. This review discusses the effects of intravenous sedation using midazolam and propofol on both grip strength and bite force. During light propofol sedation, grip strength increases slightly and bite force increases in a dose-dependent manner. Grip strength decreases while bite force increases during light midazolam sedation, and also during light sedation using a combination of midazolam and propofol. Flumazenil did not antagonise the increase in bite force by midazolam. These results may suggest following possibilities; (1) Activation of peripheral benzodiazepine receptors located within the temporomandibular joint region and masticatory muscles may be the cause of increasing bite force. (2) Propofol limited the long-latency exteroceptive suppression (ES2) period during jaw-opening reflex. Thus, control of masticatory muscle contraction, which is thought to have a negative feedback effect on excessive bite force, may be depressed by propofol.

Propofol dose-dependently increases bite force during sedation

PURPOSE

The purpose of this study was to investigate the dose-dependent effects of propofol on cognitive function and muscle power as well as vital functions.

MATERIALS AND METHODS

Twenty volunteers participated in this study. Each subject underwent 2 experiments in a randomized crossover manner (propofol group and control group). After control data were obtained, propofol at predicted effect site concentrations of 0.4, 0.8, 1.2, 1.6, and 2.0 μg/mL was infused in the propofol group using a target controlled infusion system. Heart rate, noninvasive blood pressure, arterial oxygen saturation, respiratory rate, and bispectral index value were monitored. Observer's assessment of alertness/sedation and the correct answer rate of the Stroop color word test were assessed. Muscle power, grip strength and bite force were measured.

RESULTS

In the propofol group, the bispectral index value and observer's assessment of alertness/sedation scale dose-dependently reduced. At the predicted effect site propofol concentration of 2.0 μg/mL, 6 subjects became unconscious. The correct answer rate of Stroop color word test reduced at the predicted effect site propofol concentration of 1.6 and 2.0 μg/mL. Grip strength slightly increased at the predicted effect site propofol concentration of 1.2 μg/mL or less, and bite force dose-dependently increased. At the predicted effect site propofol concentration of 2.0 μg/mL, both muscle powers began to decrease. Bite force dose-dependently increased and reached the maximum at the predicted effect site propofol concentration of 1.6 μg/mL.

CONCLUSION

Although the detailed mechanisms are unknown, propofol dose-dependently increases bite force during minimal and moderate sedation.

Early experience with remote pressure sensor respiratory plethysmography monitoring sedation in the MR scanner

BACKGROUND AND OBJECTIVE

The importance of monitoring the breathing pattern during sedation of children undergoing magnetic resonance scans is indicated in guidelines, but no appropriate magnetic resonance-compatible devices are available. We report preliminary findings from a technique referred to as remote pressure sensor respiratory plethysmography.

METHODS

A data acquisition system was developed, enabling measurement of respiratory rate, plethysmogram amplitude, proportion of inspiratory time over cycle time, thoraco-abdominal phase shift and sigh rate. Correlation between plethysmogram amplitude and tidal volume was investigated on adult volunteers. Twenty-seven children undergoing sedation were monitored with remote pressure sensor respiratory plethysmography, in addition to SPO2 and PetCO2. Differences in monitoring parameters were searched for among three groups: patients who received chloral hydrate only (chloral succeeded, CS group), those who received a supplementation of sodium thiopental (chloral failed, CF group), and those who were sedated with sodium thiopental directly (no chloral, NC group). Correlations were searched for among monitoring parameters, and with total dose of thiopental. The long-term behaviour of respiratory rate, proportion of inspiratory time over cycle time and phase shift was studied.

RESULTS

Plethysmogram amplitude was found to correlate linearly with tidal volume (r>0.92), with a slope varying up to 22%. While 11% of patients did not tolerate the capnometric probe and readings were discontinuous in 26%, all of them tolerated remote pressure sensor respiratory plethysmography belts. Sighs and non-respiratory movements of the torso could be distinguished on remote pressure sensor respiratory plethysmography waveforms. No significant inter-group differences were found in PetCO2, SPO2, respiratory rate and phase shift. Proportion of inspiratory time over cycle time was higher in the NC group when compared to the CS group (0.497+/-0.03 vs. 0.463+/-0.008; P=0.02), the CF group being characterized by intermediate values (0.480+/-0.008); when compared to the CS group, sigh rate was lower in the CF group (0.04+/-0.04 vs. 0.14+/-0.08; P=0.04) and in the NC group (0.06+/-0.05 vs. 0.14+/-0.08, P=0.03). A positive correlation was found between total dose of thiopental and proportion of inspiratory time over cycle time, with r=0.4 and P=0.04. A large baseline variability in phase shift was found. No long-term trends predictive of patient movement could be identified.

CONCLUSIONS

Breathing pattern monitoring is feasible through pneumatic devices, which are well tolerated. The resulting correlation with changes in tidal volume can be better when compared to visual inspection. Proportion of inspiratory time over cycle time and sigh rate convey information related to the state of the sedated patient. These results are not specific to the technology employed, and large-scale studies on the clinical usefulness of breathing pattern monitoring are motivated.

Acute effects of fentanyl on breathing pattern in anaesthetized subjects

BACKGROUND

The predominant effect of opioids on respiratory pattern during anaesthesia is an increase in the duration of expiration (an effect on 'timing'), but there may also be changes in tidal volume (an effect on 'drive'). Timing and drive are controlled by separate neuronal systems, but are infrequently considered individually. The effects of opioids on breathing are not well characterized clinically because changes in carbon dioxide and anaesthetic levels usually occur at the same time, and can obscure the effects of the opioid.

METHODS

To study these effects in isolation, we established stable mild hypercapnia in female patients breathing spontaneously during sevoflurane anaesthesia, and then gave fentanyl 0.5 microg kg(-1) i.v. End-tidal carbon dioxide and sevoflurane concentrations were maintained constant, and the changes in timing of inspiration, expiration and tidal volume were measured.

RESULTS

The duration of inspiration increased by 30%, and the duration of expiration increased by 95%. Tidal volume increased in proportion to inspiratory duration, and the pattern of flow during the breath was recognizably changed, with a reduction in the rate of increase of flow at the onset of inspiration.

CONCLUSIONS

Small doses of opioid given when anaesthesia and carbon dioxide are stable affect respiratory timing predominantly, but in addition changes in the pattern of motor output can be detected.

Randomized comparison of three methods of induction of anaesthesia with sevoflurane † †Presented in part at the Anaesthetic Research Society meeting held in Cardiff, UK, July 2002, and published in abstract form in Br J Anaesth 2002; 89: 673P

BACKGROUND

Rebreathing will occur if a low gas flow and a Mapleson D circuit are used to induce anaesthesia with a volatile anaesthetic agent. This has the advantage that it allows ventilation to be sustained when consciousness is lost, and specific manoeuvres such as breath-holding or vital capacity breaths are not needed to facilitate induction of anaesthesia. However, if the fresh gas flow were too small, this would slow induction by limiting the rate of delivery of the anaesthetic agent. To assess the impact of fresh gas flow and rebreathing, we compared induction using sevoflurane 8% given by three different methods.

METHODS

We randomly allocated 65 patients to receive induction of anaesthesia from either a Mapleson A breathing system with a fresh gas flow of 9 litre min(-1) (group A9), a Mapleson D system with a fresh flow of 6 litre min(-1) (group D6) or a Mapleson D system with a fresh flow of 3 litre min(-1) (group D3). We measured times for induction, end-tidal sevoflurane and end-tidal carbon dioxide.

RESULTS

The median (quartiles) induction times were 58 (45, 72), 50 (42, 65) and 64 (52, 92) s in the groups A9, D6 and D3 respectively. Induction of anaesthesia took longer (P<0.01) and was more variable in group D3. In this group, end-tidal sevoflurane concentration at the time of induction of anaesthesia was lower (P<0.05). In group A9, end-tidal carbon dioxide was less (P<0.05).

CONCLUSIONS

In adult patients allowed to breathe normally, prompt and consistent inhalation induction of anaesthesia with sevoflurane is obtained when fresh gas flow is limited to 6 litre min(-1) from a Mapleson D circuit, but smaller flows are impractical.

The comparative effects of sevoflurane and methohexital for electroconvulsive therapy

The standard anesthetic agent for electroconvulsive therapy (ECT) has been methohexital. We compared sevoflurane, a short-acting halogenated anesthetic, to methohexital for induction in ECT. Twelve subjects received sevoflurane or methohexital on alternating treatment days. Seizure duration, time to administering ECT, emergence and recovery, as well as several hemodynamic measures were recorded. A total of 69 treatments were analyzed. When sevoflurane was used, seizure durations recorded by observation and by EEG, were shorter by 10 and 23 seconds, respectively. With sevoflurane, seizure duration remained, however, within a clinically acceptable range. Methohexital allowed faster administration of ECT and discharge from the recovery room (3.8 vs. 6.2 minutes and 40.8 vs. 47.0 minutes, respectively). No difference in the post-ECT hemodynamic changes was found between the two treatments. We conclude that, when indicated, sevoflurane could provide a suitable alternative treatment option to methohexital, but some limitations, including shortened seizure duration and potential side effects, should be kept in mind.

Loss of volition and pain response during induction of anaesthesia with propofol or sevoflurane

We compared the time to reach two anaesthetic end-points during induction of anaesthesia with a potent inhalation agent (sevoflurane) and an i.v. agent (propofol). We used a method to ensure steady breathing during inhalation induction, and measured loss of tone in the outstretched arm and loss of response to a painful stimulus. Thirty-eight female patients (age 39 (9) yr, weight 65 (11) kg, and height 165 (8) cm) (mean (SD)) were randomly allocated to receive either propofol or sevoflurane. The predicted induction dose of propofol, estimated from age and weight for each patient, was given at a rate of 1% of the induction dose per second, to a possible maximum of 2.5 times the predicted induction dose. Sevoflurane was given with an inhaled concentration of 8%, which was anticipated to cause loss of arm tone within 90-120 s. After loss of consciousness, we applied a painful electrical stimulus to a finger at 15-s intervals and measured the time to loss of motor response. The median times and interquartile values for loss of arm tone were 105 (88-121) s for sevoflurane and 65 (58-80) s for propofol. This was equivalent to 0.65 of the ED(50) of propofol. The time to loss of response to pain was 226 (169-300) s for sevoflurane. The variances of these three measurements were not significantly different, indicating that these dose-response relationships were similar. In contrast, only 11 of the patients given propofol lost the response to pain after 2.5xED(50) had been given. These results support previous evidence of substantial differences between anaesthetic end-points, and show that this evidence can be obtained using a simple and rapid method.