Arterial washin of halothane and isoflurane in young and elderly adult patients

Gas phase diffusion does not limit lung volatile anesthetic uptake rate

BACKGROUND

Inefficiency of lung gas exchange during general anesthesia is reflected in alveolar (end tidal) to arterial (ET-arterial) partial pressure gradients for inhaled gases resulting in an increase in alveolar deadspace. Ventilation-perfusion mismatch is the main contributor to this, but it is unclear what contribution arises from diffusion limitation in the gas phase down the respiratory tree (longtitudinal stratification) or at the alveolar-capillary barrier, especially for gases of high molecular weight (MW) such as volatile anesthetics.

METHODS

The contribution of longtitudinal stratification was examined by comparison of ET-arterial partial pressure gradients for two inhaled gases with similar blood solubility but different molecular weights, desflurane and nitrous oxide (N2O) administered together at 2-3% and 10-15% inspired concentration (FIG) respectively, in seventeen anesthetized ventilated patients undergoing cardiac surgery before cardiopulmonary-bypass. Simultaneous measurements were done of tidal gas concentrations, and arterial and mixed venous blood partial pressures by headspace equilibration, and gas uptake rate calculated using the direct Fick method using thermodilution cardiac output measurement. Adjustment for differences between the two gases in FIG and in lung uptake rate (VG) was made on mass balance principles. A 20% larger ET-arterial partial pressure gradient relative to inspired concentration (PETG-PaG)/FIG for desflurane than for N2O was hypothesized as physiologically significant.

RESULTS

Mean (standard deviation) measured (PETG-PaG)/FIG for desflurane was significantly smaller than that for N2O (0.86 (0.37) versus 1.65 (0.58) mmHg, p<0.0001), as was alveolar deadspace for desflurane. After adjustment for the different VG of the two gases, the adjusted (PETG-PaG)/FIG for desflurane remained less than the 20% threshold above that for N2O (1.62 (0.61) versus 1.98 (0.69) mmHg, p=0.028).

CONCLUSION

No evidence was found in measured end-tidal to arterial partial pressure gradients and alveolar deadspace to support a clinically significant additional diffusion limitation to lung uptake of desflurane relative to N2O.

End-tidal to Arterial Gradients and Alveolar Deadspace for Anesthetic Agents

BACKGROUND

According to the "three-compartment" model of ventilation-perfusion ((Equation is included in full-text article.)) inequality, increased (Equation is included in full-text article.)scatter in the lung under general anesthesia is reflected in increased alveolar deadspace fraction (VDA/VA) customarily measured using end-tidal to arterial (A-a) partial pressure gradients for carbon dioxide. A-a gradients for anesthetic agents such as isoflurane are also significant but have been shown to be inconsistent with those for carbon dioxide under the three-compartment theory. The authors hypothesized that three-compartment VDA/VA calculated using partial pressures of four inhalational agents (VDA/VAG) is different from that calculated using carbon dioxide (VDA/VACO2) measurements, but similar to predictions from multicompartment models of physiologically realistic "log-normal" (Equation is included in full-text article.)distributions.

METHODS

In an observational study, inspired, end-tidal, arterial, and mixed venous partial pressures of halothane, isoflurane, sevoflurane, or desflurane were measured simultaneously with carbon dioxide in 52 cardiac surgery patients at two centers. VDA/VA was calculated from three-compartment model theory and compared for all gases. Ideal alveolar (PAG) and end-capillary partial pressure (Pc'G) of each agent, theoretically identical, were also calculated from end-tidal and arterial partial pressures adjusted for deadspace and venous admixture.

RESULTS

Calculated VDA/VAG was larger (mean ± SD) for halothane (0.47 ± 0.08), isoflurane (0.55 ± 0.09), sevoflurane (0.61 ± 0.10), and desflurane (0.65 ± 0.07) than VDA/VACO2 (0.23 ± 0.07 overall), increasing with lower blood solubility (slope [Cis], -0.096 [-0.133 to -0.059], P < 0.001). There was a significant difference between calculated ideal PAG and Pc'G median [interquartile range], PAG 5.1 [3.7, 8.9] versus Pc'G 4.0[2.5, 6.2], P = 0.011, for all agents combined. The slope of the relationship to solubility was predicted by the log-normal lung model, but with a lower magnitude relative to calculated VDA/VAG.

CONCLUSIONS

Alveolar deadspace for anesthetic agents is much larger than for carbon dioxide and related to blood solubility. Unlike the three-compartment model, multicompartment (Equation is included in full-text article.)scatter models explain this from physiologically realistic gas uptake distributions, but suggest a residual factor other than solubility, potentially diffusion limitation, contributes to deadspace.

[Total intravenous anaesthesia in geriatrics: the example of propofol]

The aim of this review is to analyse the changes in the pharmacology of the elderly patient using, as examples, the existing pharmacokinetics and pharmacodynamics models of propofol and data provided in the literature.

Measurement of Anesthetics in Blood Using a Conventional Infrared Clinical Gas Analyzer

BACKGROUND

Measurement of the partial pressure of volatile anesthetics in blood is usually done using a "headspace equilibration" method with gas chromatography. However, it is not often performed in clinical studies because of the technical, equipment, and logistic requirements. To improve the accessibility of this measurement, we tested the use of a common infrared clinical gas analyzer, the Datex-Ohmeda Capnomac, for this purpose.

METHODS

After characterization of the linearity of the device in measuring the volatile anesthetic concentration in the presence of nitrous oxide, carbon dioxide, and water vapor, blood was tonometered with known concentrations of sevoflurane (actual value between 0.5% and 5.0%) in oxygen and oxygen/nitrous oxide mixtures, as well as mixtures of isoflurane and desflurane in oxygen.

RESULTS

Mean bias (standard deviation) overall for sevoflurane in oxygen relative to the tonometered reference partial pressure was -4.5 (4.8%) of the actual concentration. This was not altered significantly by measurement in 40% oxygen/60% nitrous oxide. For isoflurane and desflurane it was -3.9 (3.3%) and -4.6 (3.8%), respectively, of the actual concentration.

CONCLUSIONS

The accuracy and precision of measurement of volatile anesthetic gas partial pressures in blood by a double headspace equilibration technique, using a clinical infrared gas analyzer, were comparable to that achieved by previous studies using gas chromatography.

Heterogeneity of human adipose blood flow

BACKGROUND

The long time pharmacokinetics of highly lipid soluble compounds is dominated by blood-adipose tissue exchange and depends on the magnitude and heterogeneity of adipose blood flow. Because the adipose tissue is an infinite sink at short times (hours), the kinetics must be followed for days in order to determine if the adipose perfusion is heterogeneous. The purpose of this paper is to quantitate human adipose blood flow heterogeneity and determine its importance for human pharmacokinetics.

METHODS

The heterogeneity was determined using a physiologically based pharmacokinetic model (PBPK) to describe the 6 day volatile anesthetic data previously published by Yasuda et. al. The analysis uses the freely available software PKQuest and incorporates perfusion-ventilation mismatch and time dependent parameters that varied from the anesthetized to the ambulatory period. This heterogeneous adipose perfusion PBPK model was then tested by applying it to the previously published cannabidiol data of Ohlsson et. al. and the cannabinol data of Johansson et. al.

RESULTS

The volatile anesthetic kinetics at early times have only a weak dependence on adipose blood flow while at long times the pharmacokinetics are dominated by the adipose flow and are independent of muscle blood flow. At least 2 adipose compartments with different perfusion rates (0.074 and 0.014 l/kg/min) were needed to describe the anesthetic data. This heterogeneous adipose PBPK model also provided a good fit to the cannabinol data.

CONCLUSION

Human adipose blood flow is markedly heterogeneous, varying by at least 5 fold. This heterogeneity significantly influences the long time pharmacokinetics of the volatile anesthetics and tetrahydrocannabinol. In contrast, using this same PBPK model it can be shown that the long time pharmacokinetics of the persistent lipophilic compounds (dioxins, PCBs) do not depend on adipose blood flow. The ability of the same PBPK model to describe both the anesthetic and cannabinol kinetics provides direct qualitative evidence that their kinetics are flow limited and that there is no significant adipose tissue diffusion limitation.

Predictive performance of 'Diprifusor' TCI system in patients during upper abdominal surgery under propofol/fentanyl anesthesia

OBJECTIVE

To evaluate the predictive performance of 'Diprifusor' TCI (target-controlled infusion) system for its better application in clinical anesthesia.

METHODS

The predictive performance of a 'Diprifusor' TCI system was investigated in 27 Chinese patients (16 males and 11 females) during upper abdominal surgery under total intravenous anesthesia (TIVA) with propofol/fentanyl. Measured arterial propofol concentrations were compared with the values predicted by the TCI infusion system. Performance was determined by the median performance error (MDPE), the median absolute performance error (MDAPE), the divergence (the percentage change of the absolute PE with time), and the wobble (the median absolute deviation of each PE from the MDPE).

RESULTS

The median (range) values of 14.9% (-21.6%-42.9%) for MDPE, 23.3% (6.9%-62.5%) for MDAPE, -1.9% h(-1) (-32.7%-23.0% h(-1)) for divergence, and 18.9% (4.2%-59.6%) for wobble were obtained from 227 samples from all patients. For the studied population, the PE did not increase with time but with increasing target propofol concentration, particularly following induction. conclusions: The control of depth of anaesthesia was good in all patients undergoing upper abdominal surgical operation and the predictive performance of the 'Diprifusor' target controlled infusion system was considered acceptable for clinical purposes. But the relatively bigger wobble showed that the pharmacokinetic model is not so suitable and requires improvement.

Accuracy of the ‘Paedfusor’ in children undergoing cardiac surgery or catheterization

BACKGROUND

A prototype paediatric propofol target-controlled infusion (TCI) system, the 'Paedfusor' has been developed. This system incorporates a paediatric pharmacokinetic data set and algorithm specific for children in a Graseby 3500 anaesthesia syringe driver. In this study we have evaluated the accuracy of the Paedfusor TCI system in children who underwent either cardiac surgery or cardiac catheterization procedures.

METHODS

Twenty-nine children aged 1-15 yr were investigated. General anaesthesia was provided using propofol administered by the Paedfusor system. Accuracy of the system was evaluated by obtaining up to 9 arterial samples for measurement of propofol concentration both during anaesthesia and in the recovery period. Measured arterial propofol concentrations were then compared with values calculated by the Paedfusor.

RESULTS

The predictive indices of median performance error (MDPE), and median absolute performance error (MDAPE) of the Paedfusor system were found to be 4.1% and 9.7%, respectively and the median value for wobble was 8.3%. These values are much better than those found with the adult 'Diprifusor' system.

CONCLUSION

The Paedfusor performance was found to be within the accepted limits for use as a TCI system.

Evaluation of the predictive performance of a 'Diprifusor' TCI system

The predictive performance of a 'Diprifusor' target controlled infusion system for propofol was examined in 46 patients undergoing major surgery, divided into three age groups (18-40, 41-55 and 56-80 years). Measured arterial propofol concentrations were compared with values calculated (predicted) by the target controlled infusion system. Performance indices (median performance error and median absolute performance error) were similar in the three age groups, with study medians of 16.2% and 24.1%, respectively. Mean values for 'divergence' and 'wobble' were -7.6%.h-1 and 21.9%, respectively. Measured concentrations tended to be higher than calculated concentrations, particularly following induction or an increase in target concentration. The mean (SD) propofol target concentration of 3.5 (0.7) micrograms.ml-1 during maintenance was lower in older patients, compared with higher target concentrations of 4.2 (0.6) and 4.3 (0.7) micrograms.ml-1 in the two younger age groups, respectively. The control of depth of anaesthesia was good in all patients and the predictive performance of the 'Diprifusor' target controlled infusion system was considered acceptable for clinical purposes.

The theoretical ideal fresh-gas flow sequence at the start of low-flow anaesthesia

A spreadsheet model of a circle breathing system and a 70-kg anaesthetised 'standard man' has been used to simulate the first 20 min of low-flow anaesthesia with halothane, enflurane, isoflurane, sevoflurane and desflurane in oxygen. It is shown that, with the fresh-gas flow set initially equal to the total ventilation and the fresh-gas partial pressure to 3 MAC, the end-expired partial pressure can be raised to 1 MAC in 1 min with desflurane and sevoflurane, 1.5 min with isoflurane, 2.5 min with enflurane and 4 min with halothane. Sequences of lower fresh-gas flow and partial pressure settings are given for then maintaining 1 MAC end-expired partial pressure, with a minimum usage of anaesthetic, e.g. 13 ml of liquid desflurane in 20 min (of which only 33% is taken up by the patient) if the minimum acceptable flow is 11.min-1, or 8 ml (with 57% in the patient) if the minimum is 250 ml.min-1.

Inhalation anaesthesia at the extremes of age: geriatric anaesthesia

No single anaesthetic technique is superior for all elderly patients, although the prognosis is improved if minor surgical procedures are performed with local anaesthesia rather than with general or major regional anaesthesia. If general anaesthesia is required, which specific inhalation anaesthetic can improve the prognosis of the geriatric patient? The low solubility of desflurane and its resistance to biodegradation would seem to recommend its use in the geriatric patient. However, these theoretical advantages remain to be adequately documented.

Peripheral blood flow in the elderly during inhalational anaesthesia

We investigated whether aging altered the peripheral vascular effects of inhaled anaesthetic agents. Forearm blood flow (FBF) was measured in 20 young (18-34 yrs) and 21 healthy elderly (60-79) patients receiving isoflurane or halothane with 66% nitrous oxide (N2O) in oxygen (O2). After etomidate 0.3 mg/kg and vecuronium 0.1 mg/kg, the trachea was intubated and controlled ventilation instituted with 66% N2O in O2. Halothane or isoflurane were administered to achieve end-tidal concentrations of 0.5% halothane or 0.9% isoflurane after 20 min. FBF was measured by venous occlusion plethysmography during the 20 min study period. Induction of anaesthesia with etomidate decreased FBF below baseline (awake) values in both elderly and young; intubation returned FBF to baseline values in the young but not in the elderly. FBF decreased below baseline values in young and elderly patients receiving halothane and in elderly patients receiving isoflurane but not in young patients receiving isoflurane. FBF was significantly greater in young patients receiving isoflurane than halothane after 20 min administration. We conclude that perfusion of forearm muscle and skin is maintained in the young but not in the elderly during anaesthesia with isoflurane/N2O. Perfusion of forearm muscle and skin decreases in both young and elderly patients during anaesthesia with halothane/N2O. The cardiovascular effects of isoflurane/N2O and halothane/N2O did not differ significantly in healthy elderly patients.

Effective concentration 50 for propofol with and without 67% nitrous oxide

The Effective Blood Concentration (EC) of propofol required to prevent response to surgical incision was determined in 65 ASA I or II female patients breathing either 100% oxygen or 67% N2O in oxygen. Propofol was administered via a microcomputer-controlled infusion system programmed to maintain the blood propofol concentration at predetermined target values. The blood propofol concentrations predicted by the micro-computer were validated by measurement of whole blood propofol concentration. Predicted and measured concentrations differed during infusion of propofol, but became similar after discontinuing the infusion for at least 90 s, suggesting that equilibration within the central compartment was incomplete during infusion. The response to the initial incision was observed and probit analysis used to determine the predicted blood concentration at which 50% of patients responded. The predicted EC50 for propofol/N2O/O2 and propofol/O2 was 4.5 micrograms ml-1 and 6.0 micrograms ml-1 respectively, and the measured EC50 propofol/N2O/O2 and propofol/O2 was 5.36 micrograms ml-1 and 8.1 micrograms ml-1, 67% nitrous oxide in oxygen reducing the predicted EC50 by 25% and the measured EC50 of propofol by 33%. The predicted EC may be more representative of the equilibrated concentration in the central compartment and thus reflective of tissue propofol concentrations.