James L. Derrick

Closed loop control of anaesthesia: an assessment of the bispectral index as the target of control

We investigated the performance of a closed‐loop system for administration of general anaesthesia, using the bispectral index as a target for control. One hundred patients undergoing gynaecological or general surgery were studied. In 60 patients, anaesthesia was maintained by intravenous infusion of a propofol/alfentanil mixture. In 40, an isoflurane/nitrous oxide based technique was used. For each technique, patients were randomly allocated to receive either closed‐loop or manually controlled administration of the relevant agents (propofol/alfentanil or isoflurane), with an intra‐operative target bispectral index of 50 in all cases. Closed‐loop and manually controlled administration of anaesthesia resulted in similar intra‐operative conditions and initial recovery characteristics. During maintenance of anaesthesia, cardiovascular and electro‐encephalographic variables did not differ between closed‐loop and manual control groups and deviation of bispectral index from the target value was similar. Intra‐operative concentrations of propofol, alfentanil and isoflurane were within normal clinical ranges. Episodes of light anaesthesia were more common in the closed‐loop group for patients receiving propofol/alfentanil anaesthesia and in the manual group for patients receiving isoflurane/nitrous oxide anaesthesia. Convenience aside, the closed‐loop system showed no clinical advantage over conventional, manually adjusted techniques of anaesthetic administration.

Oxygen delivery, oxygen consumption, and gastric intramucosal pH are not improved by a computer-controlled, closed-loop, vecuronium infusion in severe sepsis and septic shock

OBJECTIVE To investigate the influence of the neuromuscular blocking agent vecuronium on oxygen delivery (DO2), oxygen consumption (VO2), oxygen extraction ratio, and gastric intramucosal pH in heavily sedated patients with severe sepsis or septic shock. DESIGN Prospective, randomized, placebo-controlled, cross-over trial. SETTING University hospital intensive care unit. PATIENTS Eighteen mechanically ventilated patients with severe sepsis or septic shock. INTERVENTIONS All patients were heavily sedated. After baseline measurement, a computer-controlled, closed-loop infusion of either vecuronium or saline was initiated and further measurements were made at 40 and 60 mins. The procedure was repeated with the alternative agent after return of neuromuscular function. MEASUREMENTS AND MAIN RESULTS DO2, VO2, intramucosal pH were monitored using pulmonary artery catheters, a gas exchange monitor, and gastric tonometers. Changes from baseline were compared (paired t-test, p = .05). The vecuronium closed-loop infusion achieved T1 between 5% and 15% at 40 mins. There was a significant difference in the changes from baseline for static respiratory compliance in the vecuronium closed-loop infusion group compared with the saline closed-loop infusion group. There was no significant difference in the change from baseline for systemic or pulmonary vascular resistance, DO2, VO2, oxygen extraction ratio, or intramucosal pH. CONCLUSIONS In these patients, vecuronium infusion achieved the targeted level of paralysis and improved respiratory compliance but did not alter intramucosal pH, VO2, DO2, or oxygen extraction ratios. With deep sedation, neuromuscular blockade in severe sepsis/septic shock does not significantly influence oxygen flux and should be abandoned as a routine method of improving tissue oxygenation in these patients.

Expanding ICU facilities in an epidemic: recommendations based on experience from the SARS epidemic in Hong Kong and Singapore

AbstractEpidemics have the potential to severely strain intensive care resources and may require an increase in intensive care capability. Few intensivists have direct experience of rapidly expanding intensive care services in response to an epidemic. This contribution presents the recommendations of an expert group from Hong Kong and Singapore who had direct experience of expanding intensive care services in response to the epidemic of severe acute respiratory syndrome. These recommendations cover training, infection control, staffing, communication and ethical issues. The issue of what equipment to purchase is not addressed. Early preparations should include fit testing of negative pressure respirators, training of reserve staff, sourcing of material for physical modifications to the ICU, development of infection control policies and training programmes, and discussion of triage and quarantine issues.

Isoflurane dosage for equivalent intraoperative electroencephalographic suppression in patients with and without epidural blockade.

We conducted a prospective, randomized, controlled trial to establish the effect of epidural blockade on isoflurane requirements for equivalent intraoperative electroencephalographic (EEG) suppression. Fifty patients undergoing abdominal hysterectomy received combined epidural and general anesthesia or general anesthesia alone with isoflurane and alfentanil. Isoflurane was administered by computer-controlled closed-loop feedback to maintain an EEG 95% spectral edge frequency of 17.5 Hz, a target chosen on the basis of a pilot study. In epidural patients, end-tidal isoflurane concentration (F E′ISO) was 0.19% smaller (95% confidence interval [CI], −0.32% to −0.06%;P < 0.01), mean arterial blood pressure was 17 mm Hg lower (95% CI, −24 to −9 mm Hg;P < 0.0001), and body temperature was 0.4°C lower (95% CI, −0.7 to 0°C;P < 0.05) than in controls. EEG bispectral index (BIS) was 4 points higher (95% CI, 1 to 7;P < 0.05). EEG median frequency and heart rate were similar in both groups. Epidural patients were 76% more likely (95% CI, 58% to 94%;P < 0.001) to require metaraminol for hypotension and were 28% more likely (95% CI, 3% to 53%;P < 0.05) to require glycopyrrolate for bradycardia. After surgery, the time to eye opening in epidural patients was 2.3 min shorter (95% CI, −4.2 to −0.5 min;P < 0.05). Time to eye opening correlated better with F E′ISO in the last 30 s of anesthesia (F E′ISO = 0.07 × time to eye opening + 0.31;r2 = 0.59;P < 0.0001) than with BIS from the same period (BIS = 64 − 1.25 × time to eye opening;r2 = 0.22;P < 0.001) (P < 0.0001). To maintain similar intraoperative spectral edge frequency, patients receiving combined epidural and general anesthesia require 21% less isoflurane than those receiving general anesthesia alone. This smaller isoflurane dose is associated with faster emergence from anesthesia.

The Application of a Modified Proportional-Derivative Control Algorithm to Arterial Pressure Alarms in Anesthesiology

Objective. We have developed an arterial pressure alarm system based on a modified proportional-derivative (PD) controller algorithm, and prospectively tested its ability to predict significant hypotensive episodes, defined as systolic arterial pressure <80 mmHg, in comparison to conventional limit alarms. Methods. The alarm algorithm was tuned to detect hypotension using selected invasive arterial pressure traces taken from ten patients who had large intra-operative arterial pressure changes. The algorithms performance was then tested prospectively in comparison to conventional limit alarms and median filtered limit alarms, set at 85 mmHg and 90 mmHg, for its ability to predict hypotensive episodes in a further 100 patients who required invasive arterial pressure monitoring. Results. For the PD alarm algorithm, onset times for significant hypotensive episodes were between those of limit alarms set at 85 mmHg and 90 mmHg. Offset times were similar to the 85 mmHg limit alarms. The false positive rate was 34% compared with 45–64% for the other alarms (p < 0.01). Using our definitions, there was one false negative in the PD group, being a 15 second drop in observed arterial pressure, when a non invasive blood pressure cuff was inflated above the arterial line. Conclusions. An arterial pressure alarm system design based on a closed loop control algorithm offered improved performance over conventional limit alarms and in addition provided a graded output of severity of the hypotension.

Sampling intervals to record severe hypotensive and hypoxic episodes in anesthetised patients

Objective. To define the longest sampling interval which will faithfully record the time course of episodes of severe hypotension and hypoxia in anesthetised patients. Methods. Electronic anesthetic records of 1501 patients were analyzed retrospectively for hypotension where measured systolic blood pressure fell to 60 mmHg or less, and for hypoxia where SpO2 fell to 80% or less. The onset and offset times, maximum rate of change and spectral content were calculated for each episode. Results. These episodes commonly were rapid in onset and offset. The longest sampling interval to accurately represent these data was calculated to be 36 s for invasive blood pressure and 13 s for pulse oximetry. Conclusions. Our current anesthetic record is inadequate to record many of the severe changes that we observed. One minute recording intervals, such as used in many electronic record keeping systems, are too slow to capture the rapid rates of change seen, and may lead to the assumption that an episode was not recognized promptly or that treatment was not administered in a timely manner.

Surgical helmets and SARS infection.

Performance testing of two brands of surgical helmets indicated that their efficiency at in vivo filtration of sub–micrometer-sized particles is inadequate for their use as respirators. These helmets are not marketed for respiratory protection and should not be used alone for protection against severe acute respiratory syndrome when performing aerosol-generating procedures.

Community needlestick injuries may still be dangerous

The controversy regarding immunisation is longstanding. Records from 1806 concerning a vaccine scare in Northampton give a flavour of events, which strike a contemporary chord. The revelation of Edward Jenner’s 1798 seminal work meant smallpox mortality fell from 31% in unvaccinated children compared to 1.2% in vaccinated.1,2 Northampton General Infirmary made cowpox vaccination a high priority and was proactive in its approach, with free cowpox inoculation being undertaken on the hospital premises from 1804 onwards.3 On 10 January 1806 the Board of Governors dealt with a growing vaccine scare concerning alleged vaccine failure and one in …