Biomedical subjects
Publications and source records attributed to F C Forrest.
MultiMed is a telemedicine and tele-education project to extend access to a sophisticated medical simulation facility in Bristol. It will allow remote users to undertake simulated medical scenarios and to access a reference database, for a comprehensive remote learning experience. The first phase will focus on the area of anaesthesia and all remote users will be based in the UK.
We used a technique of measuring Xenon133 washout (XeW) from the alveolar space to evaluate transfer of gas from the acinus (Mackenzie et al., J. Appl. Physiol. 68: 2013-2018, 1990) during 2 min of apnea, 2 min of tracheal insufflation with oxygen (TRIO) and 90 sec of intermittent positive pressure ventilation (IPPV) in 6 anesthetized and paralyzed dogs. Xenon133 dissolved in saline was injected into an occluded acinar region through a pulmonary artery catheter, and XeW was measured by gamma scintillation scanning. With this technique, XeW during apnea represents the contribution of cardiogenic oscillations in regional flow. The XeW rate constant (min-1 +/- SE) was 0.37 +/- 0.03 during apnea. This was not different (P > 0.05) with TRIO (0.29 +/- 0.04). With IPPV, the rate constant increased to 3.49 +/- 0.39, faster than with either apnea or TRIO (P < 0.001). We conclude that: (1) TRIO does not increase convective gas transfer from the acini compared to apnea; and (2) transfer of gas out of the acini due to cardiogenic oscillations is a very small portion of the total gas eliminated during IPPV.
We have used Median Power Frequency (MPF) to study changes in the electroencephalogram during propofol infusions in 52 women about to undergo gynaecological surgery. Patients were allocated to receive propofol by one of nine different manually-controlled infusion schemes designed to achieve and maintain a stable blood propofol concentration between 1.0 and 6.0 micrograms ml-1, covering a range of states between conscious sedation and full anaesthesia. We recorded the changes in MPF and the response to clinical signs of loss of consciousness at these different doses and concentrations of propofol. Using probit analysis, we derived MPF values corresponding to 50% and 95% suppression of response to verbal (9.3 Hz and 6.8 Hz), eyelash (8.9 Hz and 6.7 Hz) and venepuncture (5.7 Hz and 3.0 Hz) stimuli. Likewise, we obtained dose and concentration requirements for propofol to suppress these stimuli. The mean (95% confidence intervals) ED50 (5.8 (3.5-6.8) mg kg-1 h-1) and ED95 (8.3 (7.1-16.9) mg kg-1 h-1) propofol doses for suppression of consciousness were similar to the values for suppression of the eyelash reflex (6.2 (5.3-6.8) mg kg-1 h-1 and 8.6 (7.8-10.8) mg kg-1 h-1, respectively). The EC50 for loss of consciousness was a propofol concentration of 2.3 (1.8-2.7) micrograms ml-1 and for 50% suppression of MPF was 3.1 (2.7-3.5) micrograms ml-1. The dose required for 50% suppression of MPF was 7.1 (6.2-8.0) mg kg-1 h-1. After 30 min, at blood propofol concentrations > 4.0 micrograms ml-1, consistent with stable anaesthesia, the mean MPF was 5.6 (4.5-6.3) Hz.
We directly measured the effect of progressive pulmonary edema on gas transfer from the acinus by injecting 133Xe dissolved in saline through a pulmonary artery catheter into an acinar region with occluded blood flow and measuring 133Xe washout by gamma scintillation scanning. We measured washout in six anesthetized paralyzed dogs during mechanical ventilation with O2 before and after injection of 0.6 mg/kg of oleic acid into the right atrium, which induces severe pulmonary edema within 2 h. Changes in the elastance and resistance of the lung were also calculated from measurements of airway flow, airway pressure, and esophageal pressure. Before injection of oleic acid, the monoexponential rate constant for 133Xe washout was 3.6 +/- 1.4 (SE) min-1; from this we estimated that the rate of gas transfer of 133Xe from the acini was 1.0 l/min. The rate constant decreased gradually after the injection and was correlated with increases in elastance and resistance (r = -0.66) and decreases in alveolar PO2 (r = 0.71). At 2 h after injection, the rate constant (1.2 +/- 0.8 min-1) was lower than control (P < 0.01), and the rate of gas transfer of 133Xe from the acini was < 0.32 l/min. We conclude that resistance in the acini is increased during pulmonary edema and that it is correlated, in the oleic acid model, with changes in overall lung mechanical properties.
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