PubMed Health⌕ Search

Biomedical subjects

L B Cook

Publications and source records attributed to L B Cook.

18 recordsLinked to original sources
PubMed2001

Extracting arterial flow waveforms from pulse oximeter waveforms apparatus.

A method is described which allows an approximation to the arterial flow waveform to be derived from a pulse oximeter waveform. The observed pulse oximeter waveform is the sum of arterial inflow and venous outflow. These components are separated mathematically. Subtraction of the venous outflow reveals the underlying arterial flow waveform. The assumptions on which the method is based are stated explicitly and discussed.

Algorithms↗
PubMed2001

Arterial flow waveforms from pulse oximetry compared with measured Doppler flow waveforms apparatus.

This study compared derived arterial flow waveforms, extracted from pulse oximeter waveforms, with Doppler flow waveforms. Fingertip pulse oximeter waveforms and radial artery Doppler flow waveforms were measured simultaneously in volunteers. The pulse oximeter waveforms were processed to extract the arterial flow waveforms and these were compared with the measured Doppler waveforms. They were very similar.

Adult↗
PubMed1999

Patient-controlled sedation using propofol: randomized, double-blind dose refinement.

This double-blind, randomized trial compared the onset of sedation with two patient-controlled sedation regimens, allowing a maximum of 16 or 25 mg min-1 propofol. Forty fit young patients presenting for elective surgery were asked to try to put themselves to sleep using the system. Onset times of sedative effect, slurred speech and amnesia were recorded. All patients achieved satisfactory sedation, and none became oversedated. Patients receiving 16 mg min-1 propofol were not reliably sedated within 5 min and took significantly longer to develop slurred speech and amnesia (P < 0.01 for both). We conclude that this maximum infusion rate does not produce amnesia or sedation rapidly enough to be clinically useful. A maximum infusion rate of 25 mg min-1 allowed rapid sedation in all patients without oversedation and may be an acceptable compromise between efficacy and safety.

Adolescent↗
PubMed1997

The safety of patient-controlled sedation.

We have investigated the safety of a previously described method of patient-controlled sedation in 100 healthy patients presenting for elective surgery. The device used 0.33-ml boluses of propofol (10 mg.ml-1) infused over 6 s with no lockout. All patients attempted to put themselves to sleep by pressing a button on the hand-held device. The oxygen saturation, heart rate, sedation score and airway patency were noted every minute. When the patient stopped pressing the button, the anaesthetist took over the handset and continued pressing the button until the patient became unresponsive. The system allowed rapid sedation. Only five patients were still anxious after 2 min. After 3 min, no patient was still anxious and more than 70 patients had slurred speech. When they stopped pressing the button, 11 patients were judged oversedated, of whom two were unresponsive. One patient's oxygen saturation decreased transiently to 84%: no other patient's oxygen saturation decreased below 90%. There were no other significant changes. We conclude that the system studied works well but carries too high a risk of oversedation for unsupervised use.

Adolescent↗
PubMed1997

Rebreathing in the Mapleson A, C and D breathing systems with sinusoidal and exponential flow waveforms.

The degree of rebreathing in Mapleson A, C and D breathing systems for sinusoidal and exponential flow waveforms is analysed mathematically. The effects of altering the I:E ratio and of introducing an expiratory pause are investigated. The results for sinusoidal waveforms closely resemble those for a square wave. Exponential flow waveforms produce results similar to triangular flow waveforms. The Mapleson A system is always the most efficient. The Mapleson C system is efficient when the I:E ratio is 1:1, becoming less efficient with longer expiration and very inefficient with an expiratory pause. The Mapleson D system becomes efficient when the expiratory pause is long.

Anesthesia, Closed-Circuit↗
PubMed1996

The relationship between anaesthetic uptake and cardiac output.

The hypothesis that anaesthetic uptake during maintenance of anaesthesia is related to cardiac output was tested on 21 patients undergoing cardiac surgery. Using a computer-controlled closed breathing system, enflurane was administered to maintain an end-expired concentration of 1%. Cardiac output was measured by thermodilution using a pulmonary artery catheter. A clear qualitative but not quantitative relationship was demonstrated. Changes in anaesthetic requirements at a constant end-expired concentration are a better guide to changes in cardiac output than changes in end-expired carbon dioxide with constant ventilation in patients undergoing cardiac surgery.

Anesthetics, Inhalation↗
PubMed1996

Respiratory pattern and rebreathing in the Mapleson A, C and D breathing systems with spontaneous ventilation. A theory.

A theoretical analysis is presented of the Mapleson A, C and D breathing systems when used in spontaneous respiration. The influence of the respiratory pattern is explained diagrammatically. Simple equations are derived, predicting the fresh gas flow required to prevent rebreathing with different respiratory patterns. Further equations allow the degree of rebreathing caused by inadequate fresh gas flow to be quantified. These are used to examine the effects of different respiratory patterns on the efficiency of the three systems. It is demonstrated that the single most important determinant of efficiency is the duration of the expiratory pause. The nature of the inspiratory and expiratory waveforms is less important and the I:E ratio far less important. The analysis suggests that the Mapleson A system will always be the most efficient of the three systems. The Mapleson C system will be efficient if inspiration is long and the expiratory pause is minimal. The Mapleson D system will be efficient if the expiratory pause is sufficiently long.

Anesthesia, Inhalation↗
PubMed1996

The importance of the expiratory pause. Comparison of the Mapleson A, C and D breathing systems using a lung model.

A physical lung model simulating spontaneous respiration was used to investigate the influence of the respiratory pattern on the efficiency of the Mapleson A, C and D breathing systems. It is shown that the Mapleson A system is always the most efficient breathing system and that its performance is relatively independent of the respiratory pattern. When the expiratory pause is minimal, the Mapleson C system is almost as efficient as the Mapleson A, but becomes ever less efficient as the expiratory pause increases. The Mapleson D system is very inefficient when the expiratory pause is short. With a longer expiratory pause, this system's efficiency approaches that of the Mapleson A. The experimental results are compared with predictions generated by a mathematical model. There is good agreement between the two, validating the mathematics used.

Anesthesia, Inhalation↗
PubMed1996

Measuring rebreathing in spontaneous ventilation. A new lung model.

The performance of breathing systems is tested by means of physical lung models. We describe a new lung model, allowing direct measurement of the dead space/tidal volume ratios produced by any breathing system at different fresh gas flows. The model allows a range of different patterns of spontaneous respiration to be studied. Although rebreathing is measured by capnography, the design ensures that the results are unaffected by changes in carbon dioxide inflow, or even calibration drift in the capnograph.

Anesthesia, Closed-Circuit↗
PubMed1996

Circle systems with a coaxial inspiratory limb. Investigation with a lung model.

A laboratory study of a new circle system is presented. The inspiratory limb encloses a coaxial tube, delivering fresh gas close to the patient, immediately upstream of the inspiratory one-way valve. The system is more efficient than the non-coaxial system and allows the expiratory valve to be situated conveniently at the canister end of the expiratory limb. A disadvantage of low flow anaesthesia is the sluggish response of the system to changes in vapour or oxygen concentrations in the fresh gas flow. This is markedly improved by the new system. The advantages of the system are largely independent of respiratory pattern.

Anesthesia, Closed-Circuit↗
PubMed1994

Pulmonary artery catheterisation.

The introduction of balloon-tipped pulmonary artery flotation catheters revolutionised monitoring of the cardiovascular system and the management of seriously ill patients. Advances in design allow direct measurement of cardiac output and continuous mixed venous oxygen saturation, while intracardiac electrocardiogram monitoring and passage of pacing wires is now possible. The technique of insertion must be meticulous as must the method of obtaining central venous, pulmonary artery, pulmonary artery wedge and pulmonary capillary wedge pressures. Sources of error are highlighted as are methods to avoid and detect them. Correct interpretation of the data is essential for proper patient management and must be taken in light of information obtained by other methods. Complications of insertion are numerous and potentially serious or even lethal. It is mandatory that the information obtained by insertion of a pulmonary artery catheter be used to optimize patient management.

Blood Pressure↗
PubMed1993

True patient-controlled sedation.

A modified patient-controlled analgesia pump provided doses of propofol 3 mg or midazolam 0.1 mg in 0.3 ml, over 5.4 s, with no lockout, during transvaginal oocyte retrieval. Alfentanil 0.2 mg was administered at three points during the procedure, and on request. Patients were randomly assigned to receive either propofol (25 patients) or midazolam (22 patients). The mean age, weight, duration of procedure and dose of alfentanil were similar in both groups. Onset of sedation with propofol or midazolam took 70.6 (SD 22.4) and 106.3 (50.7) s respectively. Mean doses over the first 5 min were midazolam 2.7 (1.2) mg, and propofol 54 (18) mg. Thereafter requirements decreased: midazolam 0.065 (0.065) mg.min-1, propofol 2.1 (1.3) mg.min-1. All patients successfully completed the procedure; none required additional sedation. P-deletion, reaction time, and critical flicker fusion tests revealed similar depression in both groups immediately postoperatively. After 30 min the p-deletion and critical flicker fusion scores were still impaired in the midazolam, but not in the propofol, group.

Analgesia, Patient-Controlled↗
PubMed1992

Fatal tracheal compression after haemorrhage into the axilla.

We report a case of fatal ventilatory obstruction as a result of haemorrhage into the axilla after a comminuted spiral fracture of the upper one-third of the humerus. Guidelines are presented as to the early recognition of such complications and their subsequent management.

Aged↗
PubMed1991

DNA measurement by image analysis of paraffin-embedded breast carcinoma tissue. A comparative investigation.

Measurement of cellular DNA content may provide useful prognostic information in several human neoplasms. The DNA content by image analysis of fresh tissue has been compared with flow cytometry with good correlation, but the use of paraffin-embedded tissue for image analysis has not been studied widely. This study reports the DNA content of 54 breast carcinomas and compares the results of image analysis of paraffin-embedded tissue and results of image analysis and flow cytometry of corresponding fresh tumors. Image analysis of paraffin blocks and fresh tumors showed comparable results in 51 tumors (94%), whereas 47 tumors (87%) were similar by all three methods. The DNA indices from image analysis of fresh and paraffin-embedded tumors showed significant correlation by linear regression analysis (r = 0.96; P less than 0.001). Discordances between image analysis of fresh and paraffin-embedded tumors are the result of technical problems, such as staining and tissue preservation. Discordances between image analysis and flow cytometry reflect the advantages and pitfalls of the two techniques. These data suggest that image analysis of paraffin-embedded tissue is a viable technique that will permit the performance of retrospective studies with long-term follow-up data for the evaluation of the prognostic significance of DNA content.

Breast Neoplasms↗