Biomedical subjects

R R Kennedy

Publications and source records attributed to R R Kennedy.

18 recordsLinked to original sources

PubMed2006

The relationship between calculated effect-site sevoflurane levels and awakening from anaesthesia.

We have previously described a system that displays real-time estimates of effect-site sevoflurane concentrations. Estimates of effect-site levels should be similar to minimum alveolar concentration (MAC) values, which are determined after allowing time for equilibrium. This study aimed to determine estimated effect-site sevoflurane concentrations at awakening from routine anaesthesia and to compare this with published estimates of MAC-awake. If these values were similar; this would validate our approach to the calculation of effect-site concentration. Sixty-five patients undergoing a variety of surgical procedures were observed. Prior to disconnection from the breathing circuit, forward estimates of effect-site sevoflurane were recorded. Patients were observed in the post-anaesthesia care unit and the time at which they responded to command was recorded.Age-adjusted effect-site sevoflurane at the time of awakening was determined. Correlation with patient, surgical and anaesthetic factors including age, gender; ASA status and intraoperative opioid usage were explored. Mean age-adjusted calculated effect-site concentration at awakening was 0.59 (SD 0.27) vol%. This value is within the range of values determined for MAC-awake of sevoflurane. There was no correlation with any of the demographic or anaesthetic factors, but patients undergoing major surgery woke at a significantly lower mean sevoflurane level. These results support the use of effect-site sevoflurane concentration to guide administration of anaesthesia.

Age Factors↗

PubMed2004

Pharmacokinetics of sevoflurane uptake into the brain.

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

Anesthetics, Inhalation↗

PubMed2001

A breathing circuit disconnection detected by anesthetic agent monitoring.

PURPOSE: To describe a case involving a spontaneously breathing patient where a circuit disconnection was detected by a change in monitored anesthetic agent parameters. CLINICAL FEATURES: A patient undergoing shoulder surgery was breathing spontaneously from a circle type anesthesia circuit via a laryngeal mask. A disconnection occurred between the heat and moisture exchanger (HME) and the circle system's Y-piece. As the gas sampling port was integrated into the HME a near normal pattern of CO2 continued to be displayed. The disconnection was noted because of a change in the graphical display of the volatile agent concentration. CONCLUSIONS: Anesthetic circuit disconnection can be difficult to detect, especially in the spontaneously breathing patient. Capnometry may not detect a disconnection on the machine side of the gas sampling port. Changes in oxygen and volatile agent concentrations may provide an early indication of these types of disconnection.

Anesthesiology↗

PubMed2001

Preoperative core temperatures in elective surgical patients show an unexpected skewed distribution.

PURPOSE: To document the preoperative core temperature of adult elective surgical patients. METHODS: A prospective audit obtained sublingual temperatures from 446 adult elective surgical patients on arrival in the preoperative holding area. RESULTS: Temperatures ranged from 35.7 degrees C to 37.8 degrees C with a mean of 36.5 degrees C (0.4 SD). The median was 36.4 degrees C and the mode was 36.1 degrees C. There was a skewed distribution with a clustering of values at the lower end of the range. All recordings were within the accepted normothermic range. CONCLUSION: The asymmetric distribution we observed differs from previously published normothermia data which shows a symmetrical distribution of temperatures. This skewed distribution has not previously been documented and we interpret it as being due to the effect of preoperative cooling factors.

Adolescent↗

PubMed2001

The effect of cardiac output changes on end-expired volatile anaesthetic concentrations--a theoretical study.

Cardiac output is one of the major determinants of the rate of uptake, and therefore the end-expired concentration (F(E')) of volatile anaesthetic agents. The purpose of this theoretical study was to explore the effect of cardiac output changes on F(E') for a range of volatile anaesthetics. A multicompartment model of anaesthetic uptake and distribution which produces constant values of F(E') was used. The minimum detectable change in cardiac output was determined for a variety of anaesthetic agents for four patterns of cardiac output change. The effect of a step change in cardiac output from 5 to 10 l.min(-1) was also recorded. The smallest cardiac output changes (average 33%) were detected with isoflurane. As blood solubility increased or decreased, larger cardiac output changes were needed before they could be detected. With a large step change in cardiac output and with increasing solubility, the final change in F(E') increased but the initial rate of change of F(E') is decreased. A significant cardiac output change will produce a change in volatile anaesthetic uptake. An unexpected change in F(E') should be considered as a possible signal of a sudden cardiac output change. The difference between agents may represent a balance between the amount of agent taken up and the size of the tissue 'sink' for that agent.

Anesthetics, Inhalation↗

PubMed2001

The effect of cardiac output changes on end-tidal volatile anaesthetic concentrations.

Cardiac output is a major determinant of the rate of uptake of volatile anaesthetic agents. Theoretical work suggests a 30% change in cardiac output will produce a measurable change in end-tidal volatile agent. We present three patients in whom significant changes in haemodynamic parameters, consistent with a large fall in cardiac output, were accompanied by an increase in end-tidal volatile agent concentrations. The changes in end-tidal volatile agent concentrations were comparable in magnitude but in the opposite direction to the changes in end-tidal CO2. Clinically, an unexplained change in end-tidal concentration of a volatile agent may signify a large change in cardiac output.

Adult↗

PubMed1998

Disconnect alarm failure in detection of common gas outlet disconnection.

This laboratory study was prompted by two paediatric cases where low pressure alarms and capnography failed to detect common gas outlet disconnection when using a T-piece pump ventilator. A carbon dioxide producing model lung was ventilated using the Clare ventilator (a T-piece pump type ventilator) via an Ayres T-piece. The T-piece used has 3 mm diameter fresh gas tubing. Common gas outlet disconnects were carried out using varying fresh gas flows and ventilatory patterns. At fresh gas flows of less than 8 l/min, the low pressure alarm of the Clare ventilator failed to sound following common gas outlet disconnection. Oxygraphy showed a rapid rise in inspired oxygen. End-tidal CO2 rose, whilst inspired CO2 remained at zero. This experiment demonstrates that the Clare ventilator's low pressure alarm detects common gas outlet disconnection poorly when used with an Ayres T-piece with narrow fresh gas tubing. Graphical representation of airway oxygen content has merit as an alerting monitor for common gas outlet disconnection.

Anesthesiology↗

PubMed1995

A modified Trigg's Tracking Variable as an 'advisory' alarm during anaesthesia.

We wished to assess the accuracy of a modified form of Trigg's Tracking Variable (TTV) at detecting the onset of changes in systolic blood pressure. A computer model generated systolic blood pressures which changed to a new value after period of stability. A separate algorithm based on TTV indicated when a 'significant' change had been detected. This signal occurred when TTV had exceeded a set limit (0.8-0.99) a predetermined number of times (1-10). Five anaesthetists were shown 40 sets of data generated by same the computer model and asked to indicate the onset of changes. The greatest number of changes (94.1%) were correctly identified when TTV exceeded 0.92 on 4 consecutive determinations. The onset of the trend was detected after an average delay of 140 s. The anaesthetists correctly detected 85% of the changes after an average delay of 162 s. There was no statistically significant difference between the anaesthetists and the algorithm, although only one anaesthetist performed better than the modified TTV. The modified TTV detected changes in simulated invasive systolic blood pressures faster and more accurately than four of a group of five anaesthetists. Such simple trend detection systems may be useful as 'advisory' alarms.

Algorithms↗

PubMed1994

Infusing liquid anaesthetics into a closed circuit.

Closed circuit anaesthesia offers the user many advantages but practical problems impede its widespread use. When conventional vaporizers are employed adequate amounts of agent cannot be delivered into a totally closed circuit during the early stages of an anaesthetic. Direct injection, or infusion, of liquid anaesthetic into the circuit overcomes this problem. The standard method for injecting agents directly into circuits is that described by Lowe and Ernst. Their system can be approximated to a series of constant rate infusions, as is frequently done for propofol, and forms the basis of our technique. For anaesthesia without nitrous oxide, liquid isoflurane should initially be infused into the circuit at a rate in ml/hr of 14 + 0.4 x weight in kg. After five minutes the infusion rate is reduced to 20% of this value. These rates are altered in the light of measured concentrations and clinical responses.

Anesthesia, Closed-Circuit↗

PubMed1994

Vaporization of isoflurane by liquid infusion.

We investigated the vaporization of liquid isoflurane when infused directly into a circuit. Pooling of isoflurane occurred within the circuit tubing at infusion rates used during clinical practice when constant gas flows were used. Despite pooling, the concentration of isoflurane was linearly related to infusion rate. Cyclical gas flow, such as that seen in a circle system, increased vaporization so that pooling occurred only at the higher infusion rates used during the first five minutes of totally closed circuit anaesthesia. There were no major differences in pooling or the maximum concentration of isoflurane reached between 26 gauge needle and droplet administration of isoflurane: however the maximum concentration was reached more quickly by droplet administration. We conclude that direct infusion of liquid isoflurane into an anaesthetic circuit will result in complete vaporization during maintenance anaesthesia.

Anesthesia, Closed-Circuit↗

PubMed1993

Solubility characteristics of the ideal agent for measurement of cardiac output by soluble gas uptake methods.

Measurements of the rate of uptake of soluble gases such as nitrous oxide or acetylene has been used as the basis of a method of cardiac output estimation since 1912. Using theoretical models of single, multiple and rebreathing techniques of cardiac output measurement, we determined the effect of solubility in blood and lung tissue on the changes in the final expired concentration of the gas. Decreasing lung tissue solubility increased the sensitivity of all three models to cardiac output changes. When the lung tissue/blood partition coefficient was 1, the optimum blood/gas partition coefficients were 2.6 for the single-breath model, 4.1 for the three breath-model and 3.5 for the rebreathing model. A selection of gases, including volatile anaesthetic agents was studied using the same models. Under most conditions, enflurane approximated most closely to the requirements for an ideal agent for use in this technique of cardiac output measurement.

Anesthetics↗

PubMed1993

Analysis of uncertainty in theoretical methods of cardiac output measurement using the "Monte Carlo" technique.

We have developed three models which describe the relationship between cardiac output and the uptake of volatile anaesthetic agents, based on the Fick equation, and determined if these models could provide useful methods of cardiac output measurement. Because many variables are involved in the calculation of cardiac output using these methods, a "Monte Carlo" simulation was performed to investigate the combined effect of uncertainties in several variables on the resultant cardiac output estimate. We found that the single-breath model was most accurate when the inspired concentration was large, while the rebreathing model was better with smaller inspired concentrations. The three-breath model was the least accurate under all conditions studied. Volatile anaesthetics were generally more accurate than nitrous oxide, with both enflurane and halothane more accurate than isoflurane. The "Monte Carlo" technique provides a valuable tool for analysis of errors in measurement methods.

Anesthetics↗

PubMed1993

Gas uptake from an unventilated area of lung: computer model of absorption atelectasis.

A computer model of gas uptake from an area of nonventilated lung, such as a pulmonary lobe with an occluded bronchus or an alveolus with an occluded airway, is presented. Previous analyses have assumed that when an inert gas is present, equilibration of O2 and CO2 with mixed venous blood is sufficiently rapid to be treated as instantaneous. This is valid for insoluble gases such as N2 or He when the fractional concentration of inspired O2 (FIO2) is < or = 0.6 but is invalid for a relatively soluble gas such as N2O. When a mixture of O2 and an inert gas is breathed, the time for an area of unventilated lung to collapse depends on the solubility of the inert gas and FIO2. When the solubility is low (N2 or He), collapse takes longer than when 100% O2 is breathed, and the lower the FIO2 the longer the time to collapse. When the gas is more soluble (N2O) and FIO2 is > 0.3, collapse is more rapid than when 100% O2 is breathed.

Absorption↗

PubMed1992

Anaesthesia and the 'inert' gases with special reference to xenon.

Xenon has many of the properties of the ideal anaesthetic agent and has been proposed as a suitable replacement for nitrous oxide in routine clinical anaesthesia. Xenon, krypton and argon are chemically inert under most circumstances, yet all have anaesthetic properties. Xenon is of particular interest because it is the only 'inert' gas which is an anaesthetic under normobaric conditions. Because of this property, xenon has an important place in the history of the development of theories of anaesthetic action and of concepts such as MAC. Cost is likely to be a major impediment to the regular use of xenon.

Anesthesia, Inhalation↗

PubMed1991

Effect of acute versus chronic Trichinella pseudospiralis infections on systemic cell-mediated immunity.

Infection of the mouse with Trichinella pseudospiralis is accompanied by pronounced suppression of host inflammatory response. This study examines the effects of infection with this parasite on several key elements in cell-mediated immunity. Early down-regulation of host granulomatous response to subcutaneously implanted cotton string and delayed-type hypersensitivity (DTH) response to trinitrochlorobenzene (TNCB) was followed later during infection by normalization of these parameters compared to that seen in uninfected mice. Cytotoxic T lymphocyte responses to tumor-specific antigens expressed on the syngeneic P91 mastocytoma were depressed early following infection with T. pseudospiralis relative to that seen in uninfected mice but were similar in these two groups during the later stages of infection. Down-regulation of the components of cell-mediated reaction examined herein accompanied the presence of migratory larvae in the host.

Acute Disease↗

PubMed1991

Data recording and trend display during anaesthesia using 'MacLab'.

A single screen display of variables monitored during anaesthesia may be ergonomically superior to the 'stack' of monitors seen in many anaesthetising locations. A system based on a MacLab (Analogue Digital Instruments) analogue-to-digital convertor used in conjunction with a Macintosh computer was evaluated. The system was configured to provide trend displays of up to eight variables on a single screen. It was found to be a useful adjunct to monitoring during anaesthesia. Advantages of this system are low cost, flexibility, and the quality of the software and support provided. Limitations of this and other similar systems are discussed.

Analog-Digital Conversion↗

PubMed1990

Alterations in the longevity and fecundity of adult Trichinella pseudospiralis related to method of isolation of infective larvae.

The infectivity of Trichinella pseudospiralis infective larvae was reduced significantly following exposure to low pH or a combination of 1% pepsin at low pH compared to that for larvae isolated in phosphate-buffered saline (PBS) at pH 7.0. Reduction of host gastric pH by administration to mice of sodium bicarbonate solution in PBS was accompanied by an increase in the infectivity of larvae isolated in 1% pepsin/HCl (P/HCl) compared to that for worms inoculated into hosts given PBS alone. Fewer adult worms developing from larvae isolated in P/HCl became established in the host small bowel than was seen with larvae isolated in PBS; moreover, the fecundity in vitro of adult worms developing from P/HCl-isolated larvae was reduced below that for adults developing from larvae isolated from host muscle in PBS. More adult worms were recovered following infection of immune hosts with PBS-isolated larvae than were recovered from immune mice challenged with larvae isolated in P/HCl. Similar findings were observed in mice immunized by infection with Trichinella spiralis and challenged with T. pseudospiralis larvae isolated in either P/HCl or PBS. Immunization of mice with T. pseudospiralis larvae isolated by either method and challenged with larvae of T. spiralis resulted in recovery of similar percentages of the challenge inoculum.

Animals↗

PubMed1990

Infectivity of Trichinella pseudospiralis isolated from carrion.

The reproductive capacity index for Trichinella pseudospiralis infective larvae was similar for worms isolated from mouse carcasses on the day upon which mice were killed (day 0: 104.4 +/- 18.6 [mean +/- SD]) and on day 5 following mouse death (106.1 +/- 23.6), but was reduced for worms recovered from carcasses on day 10 postkill (PK: 22.7 +/- 5.7). Larvae isolated from mouse carcasses held at 24 C after day 10 PK were not infective. The percentage of viable worms (tightly coiled or moving) isolated from carrion was similar on days 0 and 5 PK but had declined to 40.4% by day 10 PK and showed a further reduction to 11.8% for worms isolated from carrion on day 15 PK. Viable worms were not recovered from carcasses after day 15 PK.

Animals↗