Novel characterization method of impedance cardiography signals using time-frequency distributions

The purpose of this document is to describe a methodology to select the most adequate time-frequency distribution (TFD) kernel for the characterization of impedance cardiography signals (ICG). The predominant ICG beat was extracted from a patient and was synthetized using time-frequency variant Fourier approximations. These synthetized signals were used to optimize several TFD kernels according to a performance maximization. The optimized kernels were tested for noise resistance on a clinical database. The resulting optimized TFD kernels are presented with their performance calculated using newly proposed methods. The procedure explained in this work showcases a new method to select an appropriate kernel for ICG signals and compares the performance of different time-frequency kernels found in the literature for the case of ICG signals. We conclude that, for ICG signals, the performance (P) of the spectrogram with either Hanning or Hamming windows (P = 0.780) and the extended modified beta distribution (P = 0.765) provided similar results, higher than the rest of analyzed kernels. Graphical abstract Flowchart for the optimization of time-frequency distribution kernels for impedance cardiography signals.

Autoregressive Modeling with Exogenous Input of Middle-Latency Auditory-Evoked Potentials to Measure Rapid Changes in Depth of Anesthesia

Obtaining an adequate depth of anesthesia is a continuous challenge to the anesthetist. With the introduction of muscle-relaxing agents the traditional signs of awareness are often obscured, or difficult to interpret. These signs include blood pressure, heart rate, pupil size, etc. However, these factors do not describe the depth of anesthesia (DA) in a cerebral activity sense. Hence, a better measure of the DA is required. It has been suggested that Auditory-Evoked Potentials (AEP) can provide additional information about the DA. The general method of extracting AEP is by use of a Moving Time Average (MTA). However, the MTA is time consuming because a large number of repetitions is needed to produce an estimate of the AEP. Hence, changes occurring over a small number of sweeps will not be detected by the MTA average. We describe a system-identification method, an autoregressive model with exogeneous input (ARX) model, to produce a sweep-by-sweep estimate of the AEP. The method was clinically evaluated in 10 patients anesthetized with alfentanil and propofol. The time interval between propofol induction and the time when the Na-Pa amplitude was decreased to 25% of the initial amplitude was measured. These measurements showed that ARX-estimated compared to MTA-estimated AEP was significantly faster in tracing transition from consciousness to unconsciousness during propofol induction (p <0.05).

On-line Analysis of AEP and EEG for Monitoring Depth of Anaesthesia

Achieving and monitoring adequate depth of anaesthesia is a challenge to the anaesthetist. With the introduction of muscle relaxing agents, the traditional signs of awareness are often obscured or difficult to interpret. These signs include blood pressure, heart rate, pupil size, etc. However, these factors do not describe the depth of anaesthesia, (DA), in a cerebral activity sense, hence there is a desire to achieve a better measure of the DA. Auditory Evoked Potentials (AEP) provide two aspects relevant to - anaesthesia: (1) they have identifiable anatomical significance and, (2) their characteristics reflect the way in which the brain reacts to a stimulus. However, AEP is embedded in noise from the ongoing EEG background activity. Hence, processing is needed to improve the signal to noise ratio. The methods applied were moving time averaging (MTA) and ARX-modeling. The EEG was collected from the left hemisphere and analysed by FFT to 1 sec epochs and the spectral edge frequency was calculated. Both the changes in ARX extracted AEP and the spectral edge frequency of the EEG correlated well with the time interval between propofol induction and onset of anaesthesia measured by clinical signs (i.e., cessation of eye-lash reflex). The MTA extracted AEP was significantly slower in tracing the transition from consciousness to unconsciousness.

A Model for Estimation of the Pulmonary Capillary Pressure

To estimate the pulmonary capillary pressure, a theory was introduced by Holloway and coworkers. Based upon this, a mathematical model describing the measured data was developed. Because the physiologic data are embedded in noise and the pulmonary capillary pressure cannot be measured directly, we simulated an extensive series of data. The noise properties of the data were as analyzed to design a signal-processing tool, that cancels the noise from the measured data. The signal processing tool developed for the current application consists of pre-processing with a moving time average filter and post-processing with a neural network. After a verification procedure the tool can be applied to measured data, hence a more reliable measurement of the pulmonary capillary pressure is achieved.

Can the cerebral state monitor replace the bispectral index in monitoring hypnotic effect during propofol/remifentanil anaesthesia?

BACKGROUND

In 2004, the cerebral state monitor, CSM, was launched as a low-cost alternative to the bispectral index, BIS, for monitoring depth of sleep during anaesthesia. We tested whether the two monitors would reflect hypnosis equally during propofol/remifentanil anaesthesia.

METHODS

During laparoscopy or breast/surface surgery, 55 non-paralyzed patients were monitored simultaneously with the BIS and the CSM. Trend curves for the indexes [BIS and cerebral state index (CSI)] were compared for congruence. The difference between the two indexes for the entire course was quantified, and the ability of the two monitors to separate awake from asleep during induction was described.

RESULTS

In the majority of the patients, 87%, there was a good fit between the indexes. There were major deviations in seven patients, in whom CSI indicated that the patients were awake during parts of the course despite clinical sleep, correctly identified with the BIS. Both indexes separated awake from asleep during induction in the individual patient, but the overlap in values between patients was more pronounced for CSI.

CONCLUSION

CSM and BIS show some important differences in measuring hypnotic state during clinical propofol/remifentanil anaesthesia.

Pitfalls and challenges when assessing the depth of hypnosis during general anaesthesia by clinical signs and electronic indices

The objective of this article was to review the present methods used for validating the depth of hypnosis. We introduce three concepts, the real depth of hypnosis (DHreal), the observed depth of hypnosis (DHobs), and the electronic indices of depth of hypnosis (DHel-ind). The DHreal is the real state of hypnosis that the patient has in a given moment during the general anaesthesia. The DHobs is the subjective assessment of the anaesthesiologist based on clinical signs. The DHel-ind is any estimation of the depth of hypnosis given by an electronic device. The three entities DHreal, DHobs and DHel-ind should in the ideal situation be identical. However, this is rarely the case. The correlation between the DHobs and the DHel-ind can be affected by a number of factors such as the stimuli used for the assessment of the level of consciousness or the administration of analgesic agents or neuro muscular blocking agents. Opioids, for example, can block the response to tactile and noxious stimuli, and even the response to verbal command could vanish, hence deeming the patient in a lower depth of hypnosis than the real patient state. The DHel-ind can be disturbed by the presence of facial muscular activity. In conclusion, although several monitors and clinical scoring scales are available to assess the depth of hypnosis during general anaesthesia, care should be taken when interpreting their results.

Effects of CO2 anaesthesia on central nervous system activity in swine

The objective of the study was to examine the changes in central nervous system (CNS) activity and physical behaviour during induction and awakening from CO2 anaesthesia. Two studies, each using pigs immersed into 90% CO2 gas for a period of 60 s were performed. In study 1, we monitored middle latency auditory evoked potentials (changes in latencies, amplitudes and a depth of anaesthesia index), electroencephalographic parameters (delta, theta, alpha and beta electroencephalographic power and 95% spectral edge frequency) and heart rate; and in study 2, we monitored body movements and arterial and venous partial pressure of CO2 and O2. No behavioural signs of distress were observed during the early part of the induction. The swine exhibited muscular activity from 13-30 s after induction-start as well as during awakening from anaesthesia, possibly because of a transitory weaker suppression of the brain stem than of the cortex. The CNS and blood gas parameters started to change from the very start of induction. The CNS suppression lasted only approximately one minute after the end of the induction period. The two studies indicated a good temporal relationship between changes in amplitude, depth of anaesthesia index, spectral edge frequency, and arterial PCO2 during the induction period.

Comparison of auditory evoked potentials and the A-line ARX Index for monitoring the hypnotic level during sevoflurane and propofol induction

BACKGROUND

Extraction of the middle latency auditory evoked potentials (AEP) by an auto regressive model with exogenous input (ARX) enables extraction of the AEP within 1.7 s. In this way, the depth of hypnosis can be monitored at almost real-time. However, the identification and the interpretation of the appropriate signals of the AEP could be difficult to perform during the anesthesia procedure. This problem was addressed by defining an index which reflected the peak amplitudes and latencies of the AEP, developed to improve the clinical interpretation of the AEP. This index was defined as the A-line Arx Index (AAI).

METHODS

The AEP and AAI were compared with the Modified Observers Assessment of Alertness and Sedation Scale (MOAAS) in 24 patients scheduled for cardiac surgery, anesthetized with propofol or sevoflurane.

RESULTS

When comparing the AEP peak latencies and amplitudes and the AAI, measured at MOAAS level 5 and level 1, significant differences were achieved. (mean(SD) Nb latency: MOAAS 5 51.1 (7.3) ms vs. MOAAS 1: 68.6 (8.1) ms; AAI: MOAAS 5 74.9 (13.3) vs. MOAAS 1 20.7 (4.7)). Among the recorded parameters, the AAI was the best predictor of the awake/anesthetized states.

CONCLUSION

We conclude that both the AAI values and the AEP peak latencies and amplitudes correlated well with the MOAAS levels 5 (awake) and 1 (anesthetized).

Effect of sevoflurane on the mid-latency auditory evoked potentials measured by a new fast extracting monitor

BACKGROUND

Mid-latency auditory evoked potentials (MLAEP) are widely suppressed during general anesthesia and may therefore be useful for assessment of the depth of anesthesia. However, interpretation of amplitudes and latencies in the AEP signal is time consuming. A new monitor (A-line) that quantifies the MLAEP into an index has therefore been developed. The present study aimed to assess the precision of a prototype of the new monitor and to test the hypothesis that the depth of anesthesia index shows a graded response with changing steady-state end-expiratory concentrations of sevoflurane.

METHODS

We studied 10 ASA physical status I or II patients undergoing elective hysterectomy under combined epidural and general anesthesia by sevoflurane. Baseline auditory evoked potentials were recorded in the conscious patient immediately before induction of general anesthesia. Depth of anesthesia indices were recorded before anesthesia and at decreasing end-expiratory steady-state sevoflurane concentrations of 2.0%, 1.5%, 1.0% and 0.5%. All indices were recorded in duplicate 6 s apart. By use of an autoregressive model with exogenous input (ARX-model), the monitor extracted the AEP within 6 s. The depth of anesthesia AEP index calculated in this way was defined as the A-line ARX index (AAI).

RESULTS

Approximately 95% of the differences between repeated recordings were 5 AAI-units or less. A wide interindividual variation was observed at each observation point. AAI at 1%, 1.5% and 2% end-expiratory concentration was significantly less than the baseline AAI obtained before induction of anesthesia (P < 0.001). AAI did not change significantly in the 1-2% concentration range.

CONCLUSION

The new monitor was precise. Attenuation of the A-line ARX-index (AAI) for mid-latency auditory evoked potentials (MLAEP) during general anesthesia was profound. However, the monitor did not show a graded response with changing end-expiratory steady-state concentrations of sevoflurane.

EEG complexity as a measure of depth of anesthesia for patients

A new approach for quantifying the relationship between brain activity patterns and depth of anesthesia (DOA) is presented by analyzing the spatio-temporal patterns in the electroencephalogram (EEG) using Lempel-Ziv complexity analysis. Twenty-seven patients undergoing vascular surgery were studied under general anesthesia with sevoflurane, isoflurane, propofol, or desflurane. The EEG was recorded continuously during the procedure and patients' anesthesia states were assessed according to the responsiveness component of the observer's assessment of alertness/sedation (OAA/S) score. An OAA/S score of zero or one was considered asleep and two or greater was considered awake. Complexity of the EEG was quantitatively estimated by the measure C(n), whose performance in discriminating awake and asleep states was analyzed by statistics for different anesthetic techniques and different patient populations. Compared with other measures, such as approximate entropy, spectral entropy, and median frequency, C(n) not only demonstrates better performance (93% accuracy) across all of the patients, but also is an easier algorithm to implement for real-time use. The study shows that C(n) is a very useful and promising EEG-derived parameter for characterizing the (DOA) under clinical situations.

Middle-latency auditory evoked potentials during induction of thiopentone anaesthesia in pigs

A method is described for measuring middle-latency auditory evoked potentials (MLAEP) in consciously awake, non-sedated pigs during the induction of thiopentone anaesthesia (0.6 ml/kg, 2.5% thiopentone solution). It was done by using autoregressive modelling with an exogenous input (ARX). The ability to perceive pain during the induction was compared with (1) the changes in latencies and amplitudes of the MLAEP, (2) the change in a depth of anaesthesia index based on the ARX-model and (3) the change in the 95% spectral edge frequency. The pre-induction MLAEP was easily recordable and looked much like the one in man, dogs and rats. The temporal resolution in the ARX method was sufficiently high to describe the fast changes occurring during induction of thiopentone anaesthesia. As previously reported from studies in man, dogs and rats, induction of thiopentone anaesthesia resulted in significantly increased latencies and decreased amplitudes of the MLAEP trace as well as in a significantly reduced depth of anaesthesia index and spectral edge frequency. None of the changes, however, related well to the ability to react to a painful stimulus. Whether an ARX-based depth of anaesthesia index designed especially for pigs might be better than the present index (designed for man) for assessing depth of anaesthesia must await the results of further studies.

[Comparison of an auditory evoked potentials index and a bispectral index versus clinical signs for determining the depth of anesthesia produced by propofol or sevoflurane]

OBJECTIVES

To evaluate an anesthetic depth index (ADI) obtained from auditory evoked potentials and a bispectral EEG index (BIS) in comparison with clinical assessment of anesthetic depth using the modified observer's assessment of awareness/sedation scale (MOAA/SS), for induction of anesthesia with propofol or sevoflurane as the only agent.

PATIENTS AND METHODS

The ADI and BIS were recorded simultaneously in this prospective study and compared to the MOAA/SS during the anesthetic induction of 26 adults undergoing elective heart surgery. Assignment of patients to two groups was random. Group A (n = 13) patients were induced with propofol (target dose 5 micrograms.ml-1 in 5 min). Induction in group B (n = 13) was with sevoflurane (8% tidal volume). A scheme of awake-sleeping-awake-sleeping was followed. The means of the two indexes were compared (Mann-Whitney test) one minute before the patient slept (awake) and one minute later (sleeping), and the evolution of the indexes was compared during awake/sleep and sleep/awake phase changes and while the patients were in a stable sleep phase. The sensitivity and specificity of each index was analyzed in function of the MOAA/SS. We also analyzed the time elapsing from the moment the patient fell asleep (MOAA/SS 2) until the two indexes reached published reference values (ADI = 38, BIS = 60).

RESULTS

After induction with propofol (group A) the ADI fell to 29.2 +/- 11.7 and the BIS fell to 63.5 +/- 13.4. After induction with sevoflurane (group B) the ADI fell to 33.8 +/- 14.9 and the BIS to 66.8 +/- 15. The ADI value that best discriminated between arousal and sleeping (sensitivity 100%) was 38; the BIS value that best discriminated was 60. The responses to sound in decibels (dB) during "awake/sleeping" and "sleeping/awake" phases were, respectively, -3.8 dB and -4.5 dB for the ADI and -1.5 dB and -0.8 dB for the BIS. With the patient in stable sleep, response to the two indexes was at -0.79 dB. In group A, the ADI detected MOAA/SS 2 significantly earlier (ADI 13.1 +/- 30 s; BIS 56 +/- 36 s; p < 0.05). No patient reported remembering the study period.

CONCLUSIONS

Monitoring anesthetic depth with the ADI or BIS was technically easy and effective for detecting whether patients were awake or sleeping. The ADI response was faster and identified awake/sleeping and sleeping/awake phase changes better than did the BIS.

Changes in rapidly extracted auditory evoked potentials during tracheal intubation

BACKGROUND

One of the problems encountered in assessment of the hypnotic level during anesthesia is the extraction of a consistent and reliable measure online and close to real time. Hemodynamic parameters such as heart rate and blood pressure are not, at least with the traditional single parameter versus time presentation, adequate for ensuring an optimal level of anesthesia, especially when using neuromuscular blocking agents (NMBA). In the literature, it has been demonstrated that auditory evoked potentials (AEP) are able to provide two aspects relevant to determining level of anesthesia: firstly, they have identifiable anatomical significance and, secondly, their characteristics reflect the way the brain perceives a stimulus.

METHODS

The aim of this study was to evaluate the AEP index based on a system identification model, the autoregressive model with exogenous input (ARX-model), and to compare it to the classical method, the moving time average (MTA). The ARX enables the extraction within 15-25 sweeps, depending on the signal-to-noise ratio (SNR), whereas MTA typically needs 250-500 sweeps. The hypothesis of the present study was that since the ARX-model extracts the AEP faster than the MTA-model, the former should be able to detect changes during the brief, intense stimulus of endotracheal intubation. Twelve female patients scheduled for gynecological surgery were included in the study. Anesthesia was initiated with thiopentone and maintained with isoflurane and alfentanil. The AEP was mapped into an index (AEP-index) normalized to 100 when the individual was awake and decreasing to an average of 25 during thiopentone induced anaesthesia. The results were compared to those obtained by MTA-extracted AEP.

RESULTS

During tracheal intubation 9 patients showed an increase in the ARX-extracted AEP-index larger than 15, and 6 of these patients showed an increase larger than 25 (mean increase=33, SD=18). The MTA-extracted AEP-index showed only one patient with an increase larger than 15. The ARX-extracted AEP changed significantly faster than the MTA-extracted AEP.

CONCLUSION

The ARX-extracted AEP-index increases during tracheal intubation. There is a significant difference between the ARX-extracted AEP and the traditional MTA-extracted AEP, in terms of response time. In order to trace short-lasting changes in the hypnotic level by AEP, the AEP should be extracted by a method with a fast response such as the ARX-model.

Identification of causal relations between haemodynamic variables, auditory evoked potentials and isoflurane by means of fuzzy logic

The aim of this study was to identify a possible relationship between haemodynamic variables, auditory evoked potentials (AEP) and inspired fraction of isoflurane (ISOFl). Two different models (isoflurane and mean arterial pressure) were identified using the fuzzy inductive reasoning (FIR) methodology. A fuzzy model is able to identify non-linear and linear components of a causal relationship by means of optimization of information content of available data. Nine young female patients undergoing hysterectomy under general anaesthesia were included. Mean arterial pressure (MAP), heart rate (HR), end-tidal expired carbon dioxide (CO2ET), AEP and ISOFl were monitored with a sampling time of 10 s. The AEP was extracted using an autoregressive model with exogenous input (ARX model) which decreased the processing time compared with a moving time average. The AEP was mapped into a scalar, termed the depth of anaesthesia index (DAI) normalized to 100 when the patient was awake and descending to an average of 25 during loss of consciousness. The FIR methodology identified those variables among the input variables (MAP, HR, CO2ET, DAI or ISOFl) that had the highest causal relation with the output variables (ISOFl and MAP). The variables with highest causal relation constitute the ISOFl and MAP models. The isoflurane model predicted the given anaesthetic dose with a mean error of 12.1 (SD 10.0)% and the mean arterial pressure model predicted MAP with a mean error of 8.5 (7.8)%.

On-line analysis of middle latency auditory evoked potentials (MLAEP) for monitoring depth of anaesthesia in laboratory rats

In laboratory animals as well as in human beings a depth of anaesthesia, where the subject has no pain or recall of events from the surgery, should be provided. Haemodynamic parameters such as heart rate and blood pressure are not a guarantee for an optimal depth of anaesthesia, especially when using neuromuscular blocking agents (NMBA). A number of studies suggest that the Middle Latency Auditory Evoked Potentials (MLAEP) contain information about the state of consciousness in humans. The purpose of this study was to examine whether the AEP could serve as an indicator of depth of anaesthesia in rats. The AEP was elicited with a click stimulus and monitored in an 80 ms window synchronised to the stimulus. The AEP was extracted applying an Auto Regressive Model with Exogenous Input (ARX-model) from which a Depth of Anaesthesia Index (DAI) was calculated. DAI was normalised to 100 while awake and decreasing gradually to a level between 50 and 20 as the rat was anaesthetised. Nine rats were anaesthetised and included in the study. Four doses of Hypnorm vet. and Dormicum were given as a total, each with 5 minutes interval. Clinical signs of the level of anaesthesia were observed simultaneously with the AEP. The results showed that in four rats DAI decreased to a level below 30 while anaesthetised. In the remaining five rats the AEP was only decreased to a level below 45. The results indicated that a simple dosing regimen based on weight was unable to give the same depth of anaesthesia in individual rats. The decrease in the DAI correlated well with the loss of stimulus response. In conclusion, MLAEP could be used as an indicator of depth of anaesthesia in rats during Hypnorm vet. and Dormicum administration. However studies applying other anaesthetic drugs should be carried out, before a conclusion of the general utility of the method can be made.

On-line analysis of AEP and EEG for monitoring depth of anaesthesia

Achieving and monitoring adequate depth of anaesthesia is a challenge to the anaesthetist. With the introduction of muscle relaxing agents, the traditional signs of awareness are often obscured or difficult to interpret. These signs include blood pressure, heart rate, pupil size, etc. However, these factors do not describe the depth of anaesthesia, (DA), in a cerebral activity sense, hence there is a desire to achieve a better measure of the DA. Auditory Evoked Potentials (AEP) provide two aspects relevant to anaesthesia: (1) they have identifiable anatomical significance and, (2) their characteristics reflect the way in which the brain reacts to a stimulus. However, AEP is embedded in noise from the ongoing EEG background activity. Hence, processing is needed to improve the signal to noise ratio. The methods applied were moving time averaging (MTA) and ARX-modeling. The EEG was collected from the left hemisphere and analysed by FFT to 1 sec epochs and the spectral edge frequency was calculated. Both the changes in ARX extracted AEP and the spectral edge frequency of the EEG correlated well with the time interval between propofol induction and onset of anaesthesia measured by clinical signs (i.e., cessation of eye-lash reflex). The MTA extracted AEP was significantly slower in tracing the transition from consciousness to unconsciousness.

Spectral analysis of cyclic fluctuations in haemodynamic parameters in critically ill patients

In critically ill patients haemodynamic parameters are being routinely monitored. All of the fluctuations in blood pressures cannot be visualised since on most monitors the time window is too short and trend curves do not have a sufficient time resolution. Therefore, frequency analysis was applied to an 800-second window. Systemic artery pressure, central venous pressure and pulmonary artery pressure curves of 6 patients were sampled with a frequency of 40 Hz. The signals were transformed into the frequency domain by the Fast Fourier Transform method. Bispectral analysis was applied to determine the origin of higher frequencies. There were three main frequencies present: heart stroke rate, respiratory frequency and a slow frequency (< 0.05 Hz), which was equal to the used infusion rate (2-10 ml/h) of vaso-active drugs. Continuous infusion of short-acting vaso-active drugs delivered by pulsatile diaphragm pumps to produce slow significant fluctuations in especially the arterial blood pressures (range: 5-40 mmHg). The periodicity of these slow fluctuations is not visualised during routine monitoring, so the observer may misinterpret the cause of changes in blood pressure and make inappropriate clinical decisions. A solution for detection of such slow waves is Fast Fourier Transform combined with bispectral analysis.

Plasma noradrenaline and ageing: effects of smoking habits

We tested the hypothesis that long-term smoking is responsible for increased plasma noradrenaline (NA) in elderly healthy subjects. Thirty-nine subjects were studied both at rest and during exercise: 10 young non-smokers (median age 24 years, range 21-33), 10 young smokers (30.5 years, 27-36), 10 elderly non-smokers (64 years, 52-75) and nine elderly smokers (62 years, 56-68). The young and elderly subjects had smoked for an average of 15 years and 46.8 years respectively. Plasma NA was significantly elevated in elderly long-term smokers compared with elderly non-smokers, young non-smokers and young smokers in both supine and sitting positions (supine: 1.06 +/- 0.24 vs. 0.71 +/- 0.22, 0.53 +/- 0.12, and 0.70 +/- 0.29 nmol L-1 respectively; sitting: 3.01 +/- 0.83 vs. 2.07 +/- 0.77, 1.89 +/- 0.52 and 2.25 +/- 0.47 nmol L-1 respectively). Plasma adrenaline did not differ among the groups. At submaximal exercise (60 W), plasma NA was significantly elevated in the elderly smokers compared with the other groups, owing to the elevated basal values. Increments in plasma NA at 60, 100 and 140 W were correlated with the relative exertion and not influenced by smoking. Plasma NA increased more in young subjects than in the elderly during maximal work load (21.7 +/- 8.0 vs. 13.4 +/- 5.4 nmol L-1) and correlated with the peak O2 uptake. Total blood volume was not different among the four groups and correlated inversely with basal plasma NA. It is concluded that long-term smoking may contribute to increased basal plasma NA concentrations and probably also increased sympathetic nerve activity in elderly healthy subjects, whereas smoking has little if any effect on plasma NA responses induced by exercise. Interindividual variability in basal plasma NA concentrations may in part be explained by differences in blood volume.

Autoregressive modeling with exogenous input of middle-latency auditory-evoked potentials to measure rapid changes in depth of anesthesia

Obtaining an adequate depth of anesthesia is a continuous challenge to the anesthetist. With the introduction of muscle-relaxing agents the traditional signs of awareness are often obscured, or difficult to interpret. These signs include blood pressure, heart rate, pupil size, etc. However, these factors do not describe the depth of anesthesia (DA) in a cerebral activity sense. Hence, a better measure of the DA is required. It has been suggested that Auditory-Evoked Potentials (AEP) can provide additional information about the DA. The general method of extracting AEP is by use of a Moving Time Average (MTA). However, the MTA is time consuming because a large number of repetitions is needed to produce an estimate of the AEP. Hence, changes occurring over a small number of sweeps will not be detected by the MTA average. We describe a system-identification method, an autoregressive model with exogeneous input (ARX) model, to produce a sweep-by-sweep estimate of the AEP. The method was clinically evaluated in 10 patients anesthetized with alfentanil and propofol. The time interval between propofol induction and the time when the Na-Pa amplitude was decreased to 25% of the initial amplitude was measured. These measurements showed that ARX-estimated compared to MTA-estimated AEP was significantly faster in tracing transition from consciousness to unconsciousness during propofol induction (p < 0.05).

Optimum time for neostigmine reversal of atracurium-induced neuromuscular blockade

PURPOSE

The aim of the study was to determine the optimum time for administration of neostigmine during recovery from atracurium-induced neuromuscular blockade.

METHODS

The study comprised 103 patients anaesthetised with midazolam, fentanyl, thiopentone, halothane, and nitrous oxide. Relaxation was induced with atracurium 0.5 mg. kg-1 and maintained with supplements of 0.15 mg. kg-1. The ulnar nerve was stimulated with train-of-four (TOF) and double burst stimulation (DBS). Evoked MMG responses were recorded. Patients were randomized to spontaneous recovery (n = 20) or to assisted recovery by neostigmine (0.07 mg.kg-1) at varying intervals (6-50 min) from the last atracurium dose (n = 83).

RESULTS

The reversal time (time from administration of neostigmine to TOF ratio 0.7) was always < 13 min, when T1 (first twitch in TOF) was detectable or when D1 (first twitch in DBS) was > 5%. Total assisted recovery time (time from last supplemental atracurium dose to TOF ratio 0.7) increased with increasing T1 and D1 twitch heights (P < 0.05). The curve fitted to the scattergram with total assisted recovery time vs time from last atracurium supplement to neostigmine administration decreased to reach a minimum after which it increased to approach the line of identity. The minimum of the curve (total assisted recovery time 30.7 min) was reached when neostigmine was given 18.6 min after last atracurium supplement. At this time the T1 and D1 twitch height averaged 4 and 8% respectively. If prolongation of the minimum total recovery time of 2.5% is accepted, neostigmine can be given at T1 and D1 twitch height values of 0 to 8% and 4 to 15%, respectively.

CONCLUSION

The optimum time for neostigmine administration, taking both the reversal time and total recovery time into consideration, is when 0 < T1 < 8% or when 5 < D1 < 15%. Giving neostigmine at more profound degrees of blockade prolongs reversal time, while giving neostigmine later in the recovery phase prolongs total recovery time.

Evaluation of dynamic performance in liquid-filled catheter systems for measuring invasive blood pressure

Invasive blood pressure measurement is used in patients with unstable haemodynamics. The demand of the accuracy of these measurements is high. The reliability of the reproduced signal strongly depends on the measurement system's dynamic characteristic-its resonance frequency and damping factor. These characteristics were examined with the frequency response method, which is valuable for second and higher order systems. Most of the pressure measuring systems in use in clinical practice have low damping factor (0.1-0.2), which causes high overshoot in systolic pressure values (up to 13%), since putting all the measuring components in a chain reduces the dynamic properties of a single component and the resonance frequency drops drastically from over 100 Hz to even below 10 Hz. One of the solutions to increase the damping ratio is to insert a damping device R.O.S.E. parallel to the tubing. The resonance frequency remains the same, the damping factor increases to around 0.5. Systems with higher damping factors (0.5-0.7) have lower overshoot (1-2%), therefore the blood pressure measurements are more accurate.

Sympathoadrenal activity and psychosocial stress. The significance of aging, long-term smoking, and stress models

Recent studies have indicated that the increase in plasma norepinephrine and sympathetic activity with aging in healthy subjects is largely due to long-term cigarette smoking. In patients who have or have had duodenal ulcer the increase in plasma norepinephrine with age was markedly increased. These patients as a group perceive their lives somewhat more stressful than the general population and they tend to die prematurely due to smoking-associated diseases. These patients may select dysfunctional coping strategies like smoking, which may result in organ pathologies and a compensatory increase in plasma norepinephrine. No close correlation has been established between plasma epinephrine and "ill health." High plasma epinephrine levels may have a deleterious effect on the cardiovascular system in elderly subjects during certain conditions. In a population study, we found, however, that low resting plasma epinephrine levels were associated with an unfavorable survival rate. We speculate that an inadequate response to psychosocial stress and the choice of dysfunctional coping strategies may be more harmful and cause more "ill health" than hypersecretion of stress hormones like epinephrine and cortisol, which has been the traditional view. We suggest that there are different stress states. Stress hormones like epinephrine and cortisol may play a major role during situations like combat, illness, and strenuous exercise. In response to psychosocial stress, dysfunctional coping strategies are, however, largely responsible for harmful effects of stress.

Catecholamine responsiveness in human lymphocytes evaluated by intracellular free calcium measurements

Intracellular free calcium concentration [f(Ca2+)i] and cyclic adenosine monophosphate (cAMP) in lymphocytes and basal plasma noradrenaline (NA) were measured in nine healthy male subjects (age 22-72 years). Lymphocytes were stimulated with isoproterenol and the plant lectin concanavalin A (Con A). Con A induced a dose dependent increase in f(Ca2+)i without increasing inositol lipid turnover. The mechanism by which Con A mobilized f(Ca2+)i is not clear, but we found that the level of prostaglandin E2, a metabolite from arachidonic acid, increased after stimulation with Con A, indicating the possibility of arachidonic acid released by phospholipase A2. Isoproterenol inhibited the Con A-induced calcium mobilization in a dose dependent manner in lymphocytes stimulated with both isoproterenol and Con A. This inhibitory effect of isoproterenol is most probably mediated via cAMP. Both isoproterenol induced increase in cAMP and inhibition of calcium mobilization were significantly correlated to basal plasma NA. Taken together our data indicate that the lymphocyte response to a specific stimulus may depend not only on the strength of this stimulus, but also on the level of sympathetic nerve activity.

Stabilization of the neuromuscular response when switching between different modes of nerve stimulation at surgical degrees of neuromuscular blockade

During general anesthesia, different modes of nerve stimulation are used for estimation of the degree of neuromuscular blockade. When switching between the different modes, it is important to know whether the preceding mode influences the responses to the succeeding mode, and if so, for how long. The object of our study was to determine the number of stimulations required for stabilization of the muscular response when switching between double-burst stimulation (DBS) applied every 20 sec, train-of-four (TOF) applied every 12 sec, and posttetanic count (PTC) at surgical degrees of neuromuscular blockade. A total of 33 women were anesthetized with fentanyl, thiopental, halothane, and nitrous oxide. A constant degree of neuromuscular blockade was maintained at a twitch height of 4 to 11% of the control twitch height using a continuous infusion of atracurium. The ulnar nerve was stimulated supramaximally at the wrist, and the contraction in the adductor pollicis was measured mechanomyographically. At surgical degrees of neuromuscular blockade, only the first twitch response to TOF stimulation (T1) and the first twitch response to DBS stimulation (D1) are consistently present. When switching from DBS to TOF, 4 to 7 stimulations (56 to 92 sec) were required for stabilization of the T1 response. When switching from TOF to DBS, 3 stimulations (36 to 52 sec) were required for stabilization of the D1 response, and, finally, when switching from PTC to DBS, 5 to 11 stimulations (81 to 201 sec) were required for stabilization of the D1 response. Stabilization of D1 was faster following TOF than following PTC (p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Basal and exercise-induced changes in plasma catecholamines: relationship to age, smoking habits and sampling times

Contrary to earlier investigations, we have previously shown that the initial increase in venous plasma noradrenaline (NA) during exercise did not differ in young and elderly non-smoking subjects exercising at the same relative workload (75%), and that the greatest increase at maximal workload was observed in the young subjects. In order to determine whether the observed discrepancy could be explained by long-term smoking, plasma NA and adrenaline (A) were evaluated in eight elderly male smokers (mean age 67 years). The results were compared with data reported previously [Jensen et al. (1994) Exercise-induced changes in plasma catecholames and neuropeptide Y: relation to age and sampling times. J Appl Physiol, 76, 1269-1273] in seven young (mean age 27 years) and seven elderly (mean age 61 years) non-smoking male subjects. As shown earlier, basal plasma NA was significantly increased in elderly smokers compared to young and elderly non-smokers. During exercise at 75% of maximal work capacity, no difference was observed between elderly smokers and non-smokers. No difference in plasma A was found between elderly smokers and non-smokers either at rest or during exercise. These findings indicate that the exercise-induced increase in plasma NA did not differ in elderly smokers and non-smokers despite increased resting levels in the former group, and that the increase in the elderly subjects was not augmented compared to that in young subjects.

The influence of nitrous oxide on propofol dosage and recovery after total intravenous anaesthesia for day-case surgery

We studied the influence of nitrous oxide on the maintenance dose of propofol and recovery characteristics in 42 patients, aged 18-62 years, ASA 1 or 2, scheduled for day case inguinal herniotomy. Using a double-blind, randomised design, patients received anaesthesia with propofol-alfentanil-vecuronium-oxygen and either nitrous oxide or room air (FIO2 = 0.30). The rate of propofol infusion was adjusted depending on anaesthetic depth as judged using standard clinical criteria; alfentanil was administered on a weight basis. Patients' lungs were manually ventilated after tracheal intubation and muscle relaxation was reversed at the end of surgery with neostigmine and atropine. A series of psychomotor tests was performed pre-operatively and 30 and 120 min postoperatively. The mean maintenance doses of propofol were 0.084 mg.kg-1.min-1 in the N2O group and 0.088 mg.kg-1.min-1 in the air group (p = 0.97). In the nitrous oxide group the mean (SD) interval to spontaneous eye opening was 13.1 (7.3) min compared to 8.1 (4.9) min in the air group (p = 0.01). Similarly, the interval until obtaining a standardised response was 13.5 (5.3) min and 9.8 min (5.4) in the nitrous oxide and air groups, respectively (p = 0.04). The addition of nitrous oxide to propofol-alfentanil-vecuronium anaesthesia does not reduce propofol requirements and prolongs early recovery compared to air.

Increased transcapillary escape rate of albumin in elderly subjects due to long-term smoking habits

Transcapillary escape rate of albumin (TER) expressed as percentage decrease in specific activity of plasma albumin per hour was measured in 44 healthy male subjects, 10 young non-smokers (median age 24.0 years, range 21-33), 10 young smokers (30.5 years, range 27-36), 10 elderly non-smokers (64.0 years, range 51-75) and 14 elderly smokers (61.5 years, range 54-69). In addition urinary albumin excretion rates were measured in 15 of the elderly subjects. The subjects were investigated after overnight fasting and abstention from tobacco. Median TER was 7.33% (interquartile range 5.82-9.90) in the elderly smoking subjects. This value was significantly elevated as compared to elderly non-smokers, 4.47% (2.93-5.83), young non-smokers, 3.70% (2.24-6.30) and young smokers, 4.25% (2.80-4.44). A statistically non-significant (P = 0.064) tendency of a higher urinary albumin excretion rate was observed in elderly smokers. This agrees with other investigations showing slightly increased albuminuria in smokers compared with non-smokers. The differences were not related to differences in blood pressure, plasma cholesterol, weight, or abuse of alcohol in the elderly subjects. Diabetic subjects with an increased TER to levels comparable with levels in long-term smokers have increased urinary albumin excretion rates, which seems to be greater than in long-term smoking subjects. It is suggested that increased TER in long-term smokers may indicate a vascular dysfunction affecting either capillary permeability or endothelial surface area.

Comparison of cardiac output measurement techniques: thermodilution, Doppler, CO2-rebreathing and the direct Fick method

Simultaneously measured cardiac output obtained by thermodilution (TD), transcutaneous suprasternal ultrasonic Doppler (DOP), CO2-rebreathing (CR) and the direct Fick method (FI) were compared in eleven healthy subjects in a supine position (SU), a sitting position (SI), and during sitting exercise at a workload of 50 W (EX). The agreements between the techniques, two by two, were expressed as the bias calculated as the averaged differences between the techniques. Precision was expressed as the standard deviation of the bias. The overall agreement (bias +/- precision) between TD, DOP and CR respectively and FI were 2.3 +/- 1.6, -0.1 +/- 1.4, and -0.2 +/- 1.1 l/min. TD overestimated cardiac output consistently in SU, SI and EX. DOP was in-accurate during EX and agreed well with FI in SU and SI. CR agreed closely with FI in SI and EX, but values were underestimated in SU. The overall agreement between DOP and CR, respectively, and TD were 2.5 +/- 2.2 and 2.6 +/- 1.6 l/min. The overall agreement between DOP and CR was 0.1 +/- 1.6 l/min. In conclusion, TD overestimated cardiac output compared to the other techniques and the poor agreement has to be taken into consideration especially in measures of low values. The precision of DOP and CR against FI seems to be within clinically acceptable limits, and these methods may provide interchangeable alternatives to the invasive Fick method.

[Inhalation therapy with nitric oxide--an evaluation of a dosage system and clinical examples]

In 1987 it was discovered that nitric oxide is a very potent vasodilator and in fact a natural mediator for vasodilatation. The effects of inhalation of nitric oxide on pulmonary vascular resistance were described in 1988 and a new therapeutic possibility for artificially ventilated patients with pulmonary hypertension has arisen. However, the administration of nitric oxide has to be performed with high accuracy and without reaching toxic levels of nitrogendioxide. We have evaluated a system using conventional air and oxygen rotameters for administration of nitric oxide via a ventilator. A comparison of the chemiluminescence technique with a technique based on fuel cells was also performed. With the system it was possible to administer nitric oxide with high accuracy within the range of 10-60 ppm without exceeding values for nitrogendioxide above 0.4 ppm. The analysis of nitric oxide and nitrogendioxide based on electrochemical cells proved to be comparable with the chemiluminescence technique within the same range. Nitric oxide inhalation was used in six patients with severe hypoxia and seemed to have a favorable effect on most of the patients.

Percentage of NK-cells in peripheral blood in resting normal subjects is negatively correlated to plasma adrenaline

The purpose of the present study was to examine, if there was a correlation between natural killer cells in blood and plasma catecholamines in a resting situation. Lymphocyte subsets, especially, the NK-cells (CD3-CD56+), plasma adrenaline and noradrenaline were determined in peripheral blood from healthy male subjects resting in the supine position. Median age was 32 years. A negative correlation was observed between resting plasma adrenaline and the percentage of (CD3-CD56+) mononuclear cells (p = 0.048, r = -0.61). Previous studies have shown however, that adrenaline may increase the number of natural killer cells in blood within minutes. We suggest that adrenaline may have a dual effect on NK-cells: an acute effect by which NK-cells are mobilized from depots and a chronic effect, which decreases the number of lymphocytes and especially NK-cells in peripheral blood.

Exercise-induced changes in plasma catecholamines and neuropeptide Y: relation to age and sampling times

Plasma norepinephrine (NE), epinephrine (E), and neuropeptide Y (NPY) were evaluated in seven young (median age 29 yr, range 21-31) and seven elderly (median age 61 yr, range 51-72) healthy subjects. They were all nonsmokers and had never smoked. Basal values and the initial increase in plasma NE and E during exercise at 75% of maximal work load were identical in the two groups. After 5 min of exercise at this load, plasma NE and E remained constant in the elderly but were still increasing in the young subjects. Plasma NE and E increased to significantly higher values after 15 min in the young subjects: 1.68 +/- 0.18 vs. 0.92 +/- 0.11 ng/ml (NE) and 0.136 +/- 0.012 vs. 0.061 +/- 0.013 ng/ml (E). Basal plasma NPY was highest at rest and increased only during exercise in the young subjects. Our results demonstrate that in nonsmokers aging is not associated with increased plasma NE levels. The blood pressure response to exercise was the same in both groups, suggesting that the increase in the sympathetic activity was adequate in the elderly. Alternatively, the higher increase in sympathetic activity in the young subjects may be attributed to their higher work capacity.

A model for estimation of the pulmonary capillary pressure

To estimate the pulmonary capillary pressure, a theory was introduced by Holloway and coworkers. Based upon this, a mathematical model describing the measured data was developed. Because the physiologic data are embedded in noise and the pulmonary capillary pressure cannot be measured directly, we simulated an extensive series of data. The noise properties of the data were as analyzed to design a signal-processing tool, that cancels the noise from the measured data. The signal processing tool developed for the current application consists of pre-processing with a moving time average filter and post-processing with a neural network. After a verification procedure the tool can be applied to measured data, hence a more reliable measurement of the pulmonary capillary pressure is achieved.

Effect of psychosocial stress and age on plasma norepinephrine levels: a review

Forearm venous plasma norepinephrine and muscle sympathetic nerve activity, which are closely correlated, both increase with age. Recent studies have shown that the increase in plasma norepinephrine with aging is confined to long-term cigarette smokers, whereas values in elderly nonsmokers or young smokers were not different from values obtained in young nonsmokers. The mechanism of the increase in plasma norepinephrine in long-term smokers has not been clarified, but cardiac output and plasma volume were not different in smokers and nonsmokers. In patients who have or have had a duodenal ulcer, the increase in plasma norepinephrine with age is markedly increased. These patients tend to die prematurely because of smoking-associated diseases. These patients may select dysfunctional coping strategies like smoking, which may result in organ pathologies and a compensatory increase in plasma norepinephrine. No close correlation has been established between plasma epinephrine and "ill health." High plasma epinephrine levels may have a deleterious effect on the cardiovascular system in elderly subjects during certain conditions. In a population study, however, we found that low resting plasma epinephrine levels were associated with an unfavorable survival rate. We speculate that an inadequate response to psychosocial stress and the choice of dysfunctional coping strategies may be more harmful and cause more "ill health," than hypersecretion of stress hormones like epinephrine and cortisol, which has been the traditional view.

Sympathetic activity increases with age. Relationship to blood flow volume and long-term smoking

Basal venous plasma noradrenaline and muscle sympathetic nervous activity both increase with age. It has been reported that during exercise venous plasma noradrenaline increased more in elderly subjects both at the same relative as well as the same absolute work load, compared to younger subjects. Recent studies have shown that increased basal levels of plasma noradrenaline in elderly subjects were confined to elderly long-term smokers, whereas values in elderly non-smokers and young smokers were not different from values obtained in young non-smokers. At present, the pathophysiological mechanism of increased levels of basal plasma noradrenaline in long-term smokers has not been clarified, but cardiac output and total blood volume were not different in smokers and non-smokers. This may be due to a reduced sensitivity of alpha-adrenoceptors to noradrenaline or a compensatory mechanism to vascular dilatation induced by chronic smoking. Reported values of increases in plasma noradrenaline during exercise are conflicting. However, recent studies have shown that the increase in plasma noradrenaline was not different in young and elderly subjects when correcting for the generally lower physical fitness in the latter group. During exercise at relatively high work loads, plasma noradrenaline increased more in young than in elderly subjects. This may be explained by lack of a continuous increase in plasma noradrenaline during exercise in elderly subjects.

Venous plasma noradrenaline increases with age: correlation to total blood volume and long-term smoking habits

Venous plasma noradrenaline (NA), cardiac index, total blood volume, and other haemodynamic parameters were measured in 12 young (median age of 29, range 21-37 years) and 10 elderly (median age of 68, range 55-85 years) healthy male subjects in the resting supine and sitting positions. Cardiac index was equal in the two groups and did not correlate to plasma NA. Plasma NA was significantly elevated in the elderly subjects in the sitting position (2.47 +/- 0.28 vs. 1.80 +/- 0.13 nmol l-1, P = 0.038). Elevated plasma NA levels were confined to elderly long-term smokers. Sitting plasma NA was significantly correlated to total blood volume corrected for body weight, r = -0.720, P = 0.0002, but with no difference in blood volume between smokers and non-smokers. It is concluded, that long-term smoking may result in elevated plasma NA levels seen in elderly subjects. It is suggested, that this is a compensatory mechanism to vascular dilation induced by chronic smoking.

Long-term smoking increases transcapillary escape rate of albumin

Transcapillary escape rate of albumin was measured in 11 young and 10 healthy, elderly male subjects. Approximately half of the subjects were chronic cigarette smokers. Transcapillary escape rate of albumin expressed as percentage of decrease in specific activity of plasma albumin per hour averaged 12.3% in long-term smokers. This value was significantly elevated and twice as high as values measured in young smokers and in young and elderly non-smokers. It is concluded that capillary permeability to albumin is increased in long-term smokers. Further studies are warranted to examine the pathophysiological mechanism and to what extent this abnormality is reversible.

Sham-feeding decreases cardiac output in normal subjects

The cardiovascular effect of sham-feeding was measured in seven healthy non-obese human subjects by the Fick principle using the carbon dioxide rebreathing method. The subjects were resting in the sitting position and were exposed to the sight and smell but not the taste of a breakfast meal. Cardiac output decreased significantly from a mean value of 4.0 1 min-1 to 3.5 1 min-1 during sham-feeding (Friedman, P = 0.004). The cardiac output returned to basal values in all seven subjects when the sensory stimulus was removed. The decrease in cardiac output was due to a decreased stroke volume, whereas heart rate and blood pressure did not change. The mechanism of the decrease in cardiac output during sham-feeding remains to be established.

The family routines inventory: theoretical origins

This paper describes the conceptual foundation underlying the development of the Family Routines Inventory, a new instrument designed to measure the extent of predictability or routinization in the daily life of a family. The primary impetus for the development of this instrument is a theoretical perspective in which family routines are regarded as behaviors which may protect the health and well-being of family members by providing stability and continuity during periods of stressful change.

The family routines inventory: development and validation

This report builds upon pertinent theoretical considerations regarding the nature and importance of routinization within families and describes the development and validation of a standardized inventory to measure family routines. The Family Routines Inventory (FRI) measures 28 positive, strength-promoting family routines, those observable, repetitive behaviours which involve two or more family members and which occur with predictable regularity in the daily life of a family. The 28 routines were selected from an extensive list of 104 routines obtained through family interviews. Scoring options for the inventory were identified based on their face validity and consistency with the underlying theoretical construct. The inventory was subsequently administered to a diverse group of families for reliability and validity testing. This testing identified an optimal scoring method (frequency score) for the inventory and revealed that the Family Routines Inventory, which measures the extent and importance of routinization within a given family, appears to be a reliable and valid measure of family cohesion, solidarity, order and overall satisfaction with family life.

The family routines inventory: development and validation

This report builds upon pertinent theoretical considerations regarding the nature and importance of routinization within families and describes the development and validation of a standardized inventory to measure family routines. The Family Routines Inventory (FRI) measures 28 positive, strength-promoting family routines, those observable, repetitive behaviours which involve two or more family members and which occur with predictable regularity in the daily life of a family. The 28 routines were selected from an extensive list of 104 routines obtained through family interviews. Scoring options for the inventory were identified based on their face validity and consistency with the underlying theoretical construct. The inventory was subsequently administered to a diverse group of families for reliability and validity testing. This testing identified an optimal scoring method (frequency score) for the inventory and revealed that the Family Routines Inventory, which measures the extent and importance of routinization within a given family, appears to be a reliable and valid measure of family cohesion, solidarity, order and overall satisfaction with family life.

Beyond biology: a curriculum in methods of analysis for clinicians

Over the past five years a three-month interdisciplinary seminar curriculum for postgraduate physicians entering a Robert Wood Johnson Clinical Scholars Program has been developed. While emphasizing an epidemiologic paradigm of analysis, the format facilitates collegial learning among fellows and faculty from diverse fields in the biometric, social, and management sciences. Questions for discussion are drawn largely from the on-going clinical concerns of the participants. This approach has provided a useful basis for both problem assessment and the development of independent research. Educational strategies and devices from this effort appear to be applicable to other contexts and illustrate ways of exploring beyond the predominantly biological models of contemporary medical education.