Passage of pathogenic microorganisms through breathing system filters used in anaesthesia and intensive care.

SUMMARY: Invasive ventilation poses a risk of respiratory infection that can be drug-resistant. One means of reducing transmission of infection is the use of a breathing system filter. Filters are intended to be used with dry gas. Current international standards do not require that filters prevent bacterial transfer when wet. It is not known whether microorganisms pass through wet filters, but theory predicts that this might occur. We tested six filters from three different manufacturers. We passed a suspension of microorganisms through the filters using the least pressure necessary, and incubated a sample of the filtrate on blood agar. All the filters tested allowed free passage of both Candida albicans and coagulase-negative staphylococci. The median (IQR [range]) pressure required for fluid to flow across the filter varied greatly between different filter types (20 (0-48 [0-138]) cmH(2)O). We conclude that even large microorganisms pass across moist breathing system filters in conditions that are found in clinical practice.

Auditing the technology used to measure blood pressure.

Patient management requires accurate blood pressure measurement, but there is concern about the accuracy of the equipment. Makes and types of sphygmomanometers and sizes of their associated cuffs used in hospital wards and outpatient departments were recorded and calibration checked. Half the sphygmomanometers were oscillometric, 32% aneroid and 18% mercury. There were faults in 30% of the aneroid devices and 32% of the mercury devices, mostly of cuffs, hoses and connectors. Of the aneroid devices 18% had calibration errors (mostly of only 4 mmHg at pressures above 200 mmHg). Nearly 90% of the aneroid devices had only one cuff size available, suggesting that miss-cuffing might be a serious cause of inaccurate blood pressure measurements. Many oscillometric devices had no evidence of compliance with validation standards. Sphygmomanometers and their cuffs and hoses should be regularly checked. Clinical staff can carry out many of the checks.

Pulmonary artery blood temperature and the measurement of cardiac output by thermodilution.

Thermodilution cardiac output measurement assumes that the temperature within the pulmonary artery is stable during the measurement period. This may not be achieved in clinical practice because of temperature changes that are not solely produced by the thermal indicator. Such temperature changes constitute thermal noise. Thermal noise and how it may interfere with measurement is discussed with reference to both the injectate and the thermal filament methods of thermodilution cardiac output measurement.

Influence of Stiles-Crawford effect apodization on spatial visual performance with decentered pupils.

Using theoretical estimates of the optical-transfer function and line-spread function as image-quality criteria, we predicted the influence of the Stiles-Crawford effect (SCE) on both optical performance of the eye and subjective measurements of transverse aberrations when pupils are decentered. The SCE was modeled as a pupil apodization. The SCE appears to improve image quality by providing compensation for aberrations induced by pupil decentration, but this improvement is usually small. When a criterion of the placement of the image is used as the centroid of the line-spread function, an average SCE reduces the influence of pupil decentration on subjective transverse chromatic aberrations (TCA's) for 5-mm-diameter pupils by 30%. This reduction is much less than that obtained by previous experimental studies of TCA, and possible reasons for this discrepancy are discussed. Decentering the SCE produces an appreciable shift in subjective TCA for 5-mm-diameter pupils of 1.4 arc min per 1-mm decentration (at wavelengths 433 and 622 nm).

Description of a method for neutralising the Stiles-Crawford effect.

The influence of the Stiles-Crawford effect on visual performance can be investigated by filters based on the apodisation model of the Stiles-Crawford effect. We describe the development of practical filters to achieve neutralisation. We present some results of the Stiles-Crawford function showing that the filters work well for expected errors in aligning filters in front of the eye.

Pupil photometric efficiency and effective centre.

We use equations to show pupil photometric efficiency and effective pupil centre when the Stiles-Crawford function is not centred on the pupil of the eye.

Can simulators evaluate systematic differences between oscillometric non-invasive blood-pressure monitors?

BACKGROUND: Oscillometric non-invasive blood-pressure (NIBP) monitors estimate the arterial pressure using model-specific signal processing and algorithms. Hence each model's accuracy must be clinically evaluated. Simulators may assist the evaluation, but their ability to do so has not been verified. OBJECTIVE: To investigate whether simulators can detect systematic differences between NIBP monitors. METHODS: We tested whether a simulator can distinguish between the two different algorithms available in a particular monitor, detect calibration errors and detect systematic differences between monitors that are observed clinically. RESULTS: Simulator evaluation correctly detected 1.8 and 4.2 mmHg systolic and diastolic differences between the two Nellcor N-3100 algorithms (with specified 2 and 5 mmHg differences) but found no difference between their mean arterial pressures (as expected from the specification). Simulator evaluations detected calibration adjustments at 80/50, 120/80 and 200/150 mmHg. Simulator and clinical comparisons of two devices of the same type recording slightly different blood pressures were in close agreement, but simulator and clinical comparisons of three different models (Propaq, Critikon DINAMAP and Datex Cardiocap) were not consistently in agreement. The simulators generated oscillometric pulse shapes different from physiological recordings. CONCLUSION: The results suggest that, although simulators can reveal systematic differences between devices of the same model, they cannot be used to detect systematic differences between different NIBP models. This could be at least partly because the oscillometric pulses generated by the simulators are dissimilar to physiologically recorded pulses.

Beneficial effects of inhaled nitric oxide in hypoxaemic patients after coronary artery bypass surgery.

OBJECTIVE: Arterial oxygenation may be impaired in the early period after open-heart surgery, with an associated increase in ventilation time, morbidity and hospital stay. We tested the hypothesis that inhaled nitric oxide could be a useful therapeutic adjunct in this setting. We sought to establish clinical benefits (if any), safety and the appropriate dose range of inhaled nitric oxide therapy in hypoxaemic patients after coronary artery bypass graft surgery. METHODS: Forty patients who satisfied our definition of post-operative impaired oxygenation were prospectively randomised. The treatment group (n = 20) received nitric oxide in addition to ventilatory support. While the control group (n = 20) was managed only by conventional ventilatory support. Cardio-respiratory parameters and clinical outcome measures were compared. RESULTS: We determined the optimum concentration of inhaled nitric oxide as 20 ppm in the majority (60%) of patients. Treatment improved arterial oxygenation (8.4 +/- 1.4 kPa before, 11.8 +/- 1.5 kPa after 4 h, P < 0.001) and this benefit was sustained with lower oxygen fractions required at 24 h (P < 0.001). A significantly shorter period of mechanical ventilation was required in the treatment group (mean ventilation hours 67.0 +/- 5.9 vs. 85.0 +/- 6.5, P < 0.05), although the study did not have the power to distinguish differences in ITU or overall hospital stay. Nitrous oxide and met-haemoglobin levels did not rise appreciably. CONCLUSION: We have established the safety and efficacy of inhaled nitric oxide, at a dose of between 10 and 30 ppm, in this group of patients. We suggest that nitric oxide and a delivery system are useful adjuvants in a cardiac surgical intensive care unit.

Performance characteristics of a 'to and fro' disposable soda lime canister.

The performance of the Intersurgical disposable soda lime canister was compared to British Pharmacopoeia standards for carbon dioxide absorption and to other carbon dioxide absorber systems. This canister system more than adequately fulfilled the equivalent of the British Pharmacopoeia standard for CO2 absorption. It performed efficiently for over 3 h of continuous use, absorbing 200 ml.min-1 at varying combinations of tidal volume and ventilation rate. Efficiency was not dependent on close matching of tidal volume with canister volume and there was no channelling of gases. Heat was generated by the reaction between soda lime and CO2 and the maximum temperature recorded in the system was 42.1 degrees C. Under clinical conditions this should pose no threat of thermal injury to the patient.