Multi-level approach to anaesthetic effects produced by sevoflurane or propofol in humans: 1. BIS and blink reflex †

BACKGROUND

The relative roles of forebrain and brainstem in producing adequate anaesthesia are unclear.

METHODS

We simultaneously analysed the effects of sevoflurane (Group S; n = 18) or propofol (Group P; n = 29) on the bispectral index (BIS) and the first component of the blink reflex (R1). The dose of anaesthetic agent was increased until loss of blink reflex. After discontinuation and reappearance of blink reflex activity, the amount was increased again. The area under curve R1 (area-R1) of the electromyogram of the orbicularis oculi muscle after electrical stimulation of the supraorbital nerve was measured. Using a sigmoid E(max) model and a first-order rate constant k(e0), we characterized the dose-response relationships for BIS and area-R1.

RESULTS

Concentration-dependent depression of BIS and area-R1 was adequately modelled. The concentration that causes an effect midway between minimum and maximum (EC50) for area-R1 was smaller than EC50 for BIS in both groups [0.34 (0.19) vs 1.29 (0.19) vol% and 1.78 (0.65) vs 2.69 (0.67) mug ml(-1); mean (sd)]. At doses of sevoflurane and propofol with equivalent depression of BIS, sevoflurane depressed area-R1 more than propofol. The k(e0) for area-R1 was about half that for BIS in both groups: 0.24 (0.19-0.29) vs 0.48 (0.38-0.60) min(-1) for Group S; 0.28 (0.23-0.34) vs 0.46 (0.40-0.54) min(-1) for Group P, geometric mean (95% CI).

CONCLUSIONS

The blink reflex (brainstem function) is more sensitive to sevoflurane or propofol than BIS (forebrain function). Sevoflurane suppresses the blink reflex more than propofol. Different k(e0)s for blink reflex vs BIS indicate different effect sites.

Multi-level approach to anaesthetic effects produced by sevoflurane or propofol in humans: 2. BIS and tetanic stimulus-induced withdrawal reflex †

BACKGROUND

General anaesthesia could be assessed at two sites: cortical structures and the spinal cord. However, the practicalities of measurement at these two sites differ substantially.

METHODS

We simultaneously analysed effects of sevoflurane (Group S; n = 16) or propofol (Group P; n = 17) on bispectral index (BIS) and the tetanic stimulus-induced withdrawal reflex (TIWR). TIWR was quantified by the area under the curve of the electromyogram of the biceps femoris muscle after electrical stimulation of the sural nerve. After loss of consciousness, TIWR was evoked once per minute. The anaesthetic was increased until TIWR disappeared. After discontinuation of the anaesthetic and reappearance of TIWR, the amount of anaesthetic was increased again. Using a sigmoid E(max) model and a first-order rate constant k(e0), we characterized the dose-response relationships for BIS and TIWR.

RESULTS

Concentration-dependent depression of TIWR was reasonably well modelled for sevoflurane, but poorly for propofol. TIWR was completely suppressed by sevoflurane, but not propofol. Sevoflurane reduced TIWR to 5 mV ms (very weak movement) at 1.68 vol% end-expired concentration [approximately minimum alveolar concentration (MAC value)]. The k(e0)s for TIWR were smaller than those for BIS: 0.25 (0.16-0.39) vs 0.41 (0.33-0.51) min(-1) for Group S; 0.25 (0.22-0.30) vs 0.34 (0.29-0.40) min(-1) for Group P [geometric mean (95% CI)].

CONCLUSIONS

High concentrations of sevoflurane depress TIWR more than propofol. With propofol, we frequently observed a paradoxical behaviour of muscles of the lower leg. TIWR lags behind BIS, indicating different effect sites for two intended anaesthetic effects: unresponsiveness to noxious stimulation and unconsciousness.

Electromyographic assessment of blink reflexes correlates with a clinical scale of depth of sedation/anaesthesia and BIS during propofol administration

BACKGROUND

General anaesthesia is characterized by loss of consciousness, amnesia and obtundation of reflex responses to noxious stimuli. Quantifying the blink reflex may reflect the depression of reflex arches induced by anaesthetics and thus being informative on the anaesthetic state.

METHODS

The relation between the electrically evoked blink reflexes and the depth of sedation and anaesthesia induced with intravenous propofol was investigated. Twenty patients received propofol by target-controlled infusion to create a stepwise deepening of sedation and anaesthesia. Depth of anaesthesia was assessed using the observer's assessment of anaesthesia and sedation (OAAS) scale, and by bispectral EEG analysis (BIS). Probit analysis was used to estimate the predicted propofol effect site concentrations producing unconsciousness, no response to noxious stimulation, and loss of blink reflex components.

RESULTS

Latency of the first (R1) and second (R2) blink component increased, whereas duration and area decreased with increasing depth of sedation and anaesthesia. A reasonably strong correlation between OAAS and the areas of R1 and R2 components was found (Spearman's rho = 0.92 and 0.89). The areas of R1 and R2 and the OAAS also correlated with BIS (Spearman's rho = 0.91, 0.88 and 0.90). EC(50) and EC(95) for loss of R1 were 2.8 (95% CI: 2.5-3.2) micro g/ml and 4.6 (95% CI: 4.1-5.5) micro g/ml, respectively.

CONCLUSIONS

Our results suggest that the differential sensitivity of the components of the blink reflex could be useful in monitoring depth of sedation and light levels of anaesthesia during the administration of propofol. Both OAAS and BIS correlate similarly with the blink reflex components.

Cerebrotendinous xanthomatosis. Controversies about nerve and muscle: observations in ten patients

Neuromuscular characteristics were documented in ten patients with biochemically and genetically confirmed cerebrotendinous xanthomatosis. An array of genotypes was found in these patients. Only one patient complained of muscle weakness, while clinical signs of peripheral neuropathy were present in six patients. Electromyogram showed predominantly axonal neuropathy in seven patients. Neurogenic changes were seen in muscle biopsies of nine patients. Sural nerve biopsies of three patients showed features of axonal neuropathy. In addition, in one patient, extensive onion bulb formation was seen, which is indicative of a primarily demyelinating process. Five patients had normal mitochondrial respiratory chain enzyme activity. It is concluded that myopathy is not a feature of cerebrotendinous xanthomatosis and that the most prominent neuromuscular abnormality is sensorimotor axonal polyneuropathy.

Body position and late postoperative nocturnal hypoxaemia

Thirteen patients were monitored for nocturnal body position (supine vs. side) and arterial oxygen saturation pre-operatively and on the second postoperative night after major abdominal surgery. The number of positional changes were significantly decreased after operation (p < 0.05) with a trend towards more time spent in the supine position (p = 0.1). Individual mean arterial oxygen saturation decreased postoperatively (p < 0.05) but without a difference between the supine and side positions (p = 0.9). Pre-operatively, episodic desaturations were significantly more frequent in the supine position than on the side (p < 0.05) but not postoperatively. Pain was the most frequent reason for decreased nocturnal movements.

Measurement of muscle fiber conduction velocity in humans: techniques and applications

Different methods of measuring human muscle fiber conduction velocity (MFCV) are outlined. A major division can be made between invasive (needle) and noninvasive [surface electromyographic (EMG)] recording techniques. The benefits and limitations associated with the different approaches are discussed. The conduction velocity of individual muscle fibers are measurable with needle recordings on routine EMG apparatus. Velocities measured with electrical stimulation of single fibers or bundle of fibers are in general lower than recordings made during voluntary contraction. With surface EMG, an average estimate of the MFCV from many fibers and motor units can be determined with computerized data analyses--both frequency and time domain approaches. The cross-correlation method is widely used to calculate the time lag between two EMGs recorded along the muscle. Using the surface EMG-based methods, MFCV can be calculated at all contraction levels. Thus, the change in sarcolemmal function during local muscle fatigue can be assessed. Physiological factors influencing the MFCV and the utility of the MFCV estimation in clinical practice are discussed. Compared to nerve conduction velocity studies, the clinical interest in MFCV has been limited.