Evaluation of a simple device for regulating intravenous infusions.

Dial-A-Flo, an infusion control device, is assessed for accuracy, in vitro and in vivo. The unit did not permit accurate control and particularly so at lower flow rates. The normal spontaneous variations in venous pressure that occur around an infusion cannula tip, further increase the error of gravity-fed infusions. Dial-A-Flo has no advantage over a standard administration set alone, and should not be used for controlled administration of cardiac drugs.

Continuous flush devices for vascular pressure monitoring.

Six different types of continuous flush devices have been assessed for accuracy in vitro and ease of use in vivo. Only two types of device matched the manufacturers' specifications of performance. The other four gave higher flow rates than expected.

Characteristics of transcutaneous oxygen tension monitors in normal adults and critically ill patients.

The performance characteristics of two transcutaneous PO2 monitors were detailed in eleven healthy adults and eighteen critically ill patients (twelve following cardiopulmonary bypass). There was no significant difference in their performance when blood was used as the calibrating medium. In critically ill patients they proved to be of value for recording short term trends in arterial PO2 rather than for measuring absolute values.

The performance characteristics of an intra-arterial oxygen electrode.

The performance characteristics of intravascular oxygen electrodes used during and after open heart surgery are described. The pO2 recorded continuously by the electrodes correlated extremely well with corresponding measurements made on samples of arterial blood. The results show that intravascular pO2 electrodes are accurate and can be readily used during surgery and intensive care of adult patients.

Characteristics of transcutaneous carbon dioxide tension monitors in normal adults and critically ill patients.

The performance of three electrodes used for transcutaneous carbon dioxide (tcPCO2) monitoring is compared in 15 healthy volunteers and 26 critically ill adults. All three electrodes showed good correlation between tcPCO2 and arterial blood PCO2 (PaCO2) with a correlation coefficient (r) greater than 0.86. There was little difference in the performance characteristics of the three monitors. They may be usefully employed to estimate PaCO2 values when used with a modified calibration recommended by the manufacturers.

Measurement of transcutaneous PCO2 in critically ill patients.

Transcutaneous carbon dioxide tensions (tcPCO2) measured with a heated electrode were compared with arterial carbon tensions (PaCO2) in 28 patients. Seventy-eight observations were made. At a skin electrode temperature of 44 degrees C the tcPCO2 was significantly higher than PaCO2 but changes in PCO2 detected by both measurements were closely correlated. The correlation coefficient between tcPCO2 and PaCO2 was 0.92 (P less than 0.001). The 90% response time of the electrode in vitro was less than 1 min, and in vivo stabilization of the recording occurred in less than 15 min. It was found that the transcutaneous PCO2 electrode can be employed usefully in intensive care monitoring of adult patients especially when weaning them from artificial ventilation.

Continuous tissue pH and transcutaneous PO2 measurement as an index of tissue perfusion in critically ill patients.

Measurements of transcutaneous PO2 (tcPO2) and tissue pH (pHt) were compared with arterial values in 28 critically ill patients. Eleven of the patients were undergoing continuous flow cardiopulmonary bypass and the other 17 were admitted to the Intensive Therapy Unit (ITU) suffering from a variety of medical and surgical disorders. A total of 135 observations were made on the 28 patients. The overall correlation between the two sets of measurements was poor. The correlation coefficient (r) between arterial PO2 (Pa,O2) and tcPO2 was 0.41 (P less than 0.01) and between arterial pH (pHa) and pHt 0.67 (P less than 0.01). However, the value varied greatly in different groups of patients. In those patients with cardiopulmonary failure, the tcPO2 and pHt measurements rapidly and accurately reflected changes in peripheral tissue perfusion, by contrast the core-peripheral temperature deficit did not change significantly. It is concluded that the technique can be used as a sensitive index of peripheral blood flow and may have prognostic significance

Changes in colloid osmotic pressure in patients undergoing cardiothoracic surgery.

Measurements of plasma colloid osmotic pressure (COP) were made during and for 24 h after normothermic continuous flow cardiopulmonary bypass (CPB) in 20 adults exposed to sudden haemodilution by a crystalloid pump prime solution. Haematocrit, albumin and total protein were also measured. Within 5 min of starting bypass the mean COP fell to 45% pre-bypass value and remained at this level until bypass was discontinued. Following bypass, the oncotic pressure rose rapidly during the first hour to 84% of the prebypass value, and then more slowly until it reached the pre-bypass value 24 h later. The trend seen for plasma proteins and haematocrit was similar but the fall was smaller, 65-70% of the pre-bypass values. This significant difference in fall between plasma protein concentration and directly measured COP is explained by their non-linear relationship. Three patients died soon after bypass. They all showed significantly lower values of COP than the mean for the group as a whole at some stage (P less than 0.05).

Effect of intermittent pneumatic compression on lower limb skin perfusion.

Transcutaneous oxygen (tcPO2) and carbon dioxide (tcPCO2) measurements were used before, during, and after intermittent pneumatic compression (IPC) to assess lower limb skin perfusion in 21 healthy adult volunteers and 23 elderly patients. The temperature of the sensors was set at 44 degrees C during calibration and maintained throughout all tests. Resting tcPO2 was significantly lower (p less than 0.01) in patients than in controls. In all subjects tcPO2 decreased and tcPCO2 increased during compression. Patients showed a highly significant increase (p less than 0.001) in tcPCO2 during and after the treatment. Results of this study indicate that the symptomatic improvement, in peripheral vascular disease and wound healing following IPC, is not due to improved skin perfusion or enhancement of oxygenation and CO2 removal.

A cam-displacement transducer device for measuring small two-degree of freedom inter-component motion in a prosthesis.

An electromechanical system is described that comprises a cam and two linear displacement transducers for measuring two-degree of freedom. This is aimed at quantifying the extent and pattern of both transverse rotation and longitudinal translation that takes place during amputee gait at what is sometimes termed a "torque-shock absorbing adapter" located between the prosthetic socket and foot. Both types of motion are measured independently by mounting the cam around the adapter and the two transducers orthogonal to one another with their spring loaded plungers moving separately against the cam's perimeter and distal, flat face. The cam geometry should be an arc of a spiral and not a circle for the angular displacement measurement to be linear in response to rotation. Over a range of +/-18 degrees for transverse rotation the maximum deviation of the ideal cam geometry away from an arc of a circle is less than +/-0.5% of the transducers' 10 mm stroke. The transducer's linearity is +/-2% and it could therefore be assumed that this would be the dominant source of error. The theoretical predictions are supported by the calibration results, in which the pattern of errors that are found in the transducers' calibration can also be seen in the calibration of the cam.

Assessment of alternating air mattresses using a time-based interface pressure threshold technique.

Laboratory evaluation techniques for support surfaces have centered largely around interface pressure (IP) measurement, typically analyzing discrete maximum and minimum levels, or calculating the average pressure. Nowadays, alternating pressure air mattresses (APAMs) are used increasingly for the prevention and treatment of pressure sores. Pressure relief (PR) provided by an APAM is time-varying. A computerized system that measures IP, air pressure, and pressure-time characteristics of dynamic support surfaces has been developed for performance assessment. Using this system, IP was recorded continuously and the durations of pressures below three thresholds (30, 20, and 10 mmHg) were calculated automatically. Fifteen sound volunteers were used to evaluate the pressure-relieving characteristics of four APAMs, including one overlay. Results indicated significant differences (p < 0.001) between products when durations below 20 and 10 mmHg thresholds were analyzed, showing some devices were only capable of momentarily relieving pressure. Maximum contact pressures on the sacrum were significantly lower (p < 0.0001) on devices where inflation pressure was adjusted according to the body mass of the subject. With further clinical validation, this tool could assist in the selection of alternating surfaces of any description.

A change for the better? Measuring improvements in upgraded alternating-pressure air mattresses.

This study used measurements of interface pressure over time (the pressure relief index) to investigate improvements made to two alternating-pressure air mattresses. Two older models, the Nimbus 2 (Huntleigh Technology) and Pegasus Airwave (Pegasus Egerton), were compared with two new versions, the Nimbus 3 and Cairwave systems, respectively. Pressure relief was improved in seven out of 12 areas in the Nimbus 3 system, and in four out of 12 areas in the Cairwave. Significant differences in pressure relief index measurements at the heel between the Nimbus 3 and Cairwave products may explain the former's better clinical outcomes in this area, but superior pressure relief index performance at the sacrum did not predict better clinical outcomes. Therefore, different levels of pressure relief may be needed at different body sites. Individual practitioners must decide whether these improvements merit list price increases of 11% and 15%, respectively, and whether other features justify a 20% price difference between the two new systems.

Assessment of alternating-pressure air mattresses using a time-based pressure threshold technique and continuous measurements of transcutaneous gases.

Alternating-pressure air mattresses (APAMs) are used increasingly for the prevention and treatment of pressure sores. Laboratory evaluation techniques have centred largely on interface pressure (IP) measurement, typically analysing discrete maximum and minimum levels, or average pressure. However, since pressure relief varies with time, a time-based analysis technique has been developed for performance assessment. IP was recorded continuously, and the duration of pressures below three thresholds (30, 20 and 10 mmHg) was calculated automatically using a computerized rig. Fifteen healthy volunteers were used to evaluate the pressure-relieving characteristics of four APAMs, including one overlay. Results indicated significant differences (P < 0.001) between products when durations below the 20 and 10 mmHg thresholds were analysed, showing that some devices were only capable of momentarily relieving pressure. Maximum contact pressures on the sacrum were significantly lower (P < 0.0001) on devices whose inflation pressure was adjusted according to subject's body mass. This technique could assist in the selection of alternating or dynamic surfaces of any description with further clinical validation.

Comparison of transcutaneous oxygen and carbon dioxide monitors in normal adults and critically ill patients.

The performance characteristics of two transcutaneous combined PO2 (tcPO2) and PCO2 (tcPCO2) sensors were compared with single tcPO2 and tcPCO2 electrodes in critically ill patients. The relationship between arterial blood gases (PaO2, PaCO2) and transcutaneous values was linear. Correlation coefficients (r) varied from 0.79 to 0.87 for tcPO2 and 0.92 to tcPCO2. The tcPO2 readings were always lower than PaO2 values, but the tcPCO2, with modified calibration of the electrodes, did not differ significantly from PaCO2. There was no significant difference of clinical importance in the performance of the three monitoring systems.

Developments in transcutaneous blood gas monitoring: a review.

Transcutaneous measurements of oxygen (tcPO2) and carbon dioxide (tcPCO2) are useful in the intensive care unit because they provide continuous and non-invasive estimation of arterial blood gases. In patients with compromised peripheral blood flow the cutaneous values will reflect the haemodynamic changes as blood is shunted away. They have been increasingly used in a variety of clinical situations, such as assessment of skin flap viability, drug evaluation, prediction of wound healing and selection of amputation level in peripheral vascular disease. From a review of available literature it appears that transcutaneous monitors have found another application as indicator of local tissue perfusion.

Role of the clinical engineer in patient monitoring services.

The basic role of a clinical engineer is to provide safe and effective management and application of technology within patient care settings. In the UK the total number of such people is still relatively small. Clinical engineering as a profession requires a proper career structure and professional status. This can be achieved only by the support of a professional body such as the Biological Engineering Society.

Comparison of oscillometric and intra-arterial blood pressure and pulse measurement.

Non-invasive oscillometric blood pressure and pulse measured by an Omron HEM-703CP monitor were compared with arterial values obtained from direct measurements of the radial artery. An excellent correlation and agreement was found between the two methods (systolic r = 0.99; diastolic r = 0.97; pulse r = 0.99), although there was some variability among individual subjects. The range of difference between them was 0 to 10 mmHg for systolic and -6 to +5 mmHg for diastolic pressures. When tested on the bench using the Metron QA-1280 non-invasive blood pressure analyser the HEM-703CP monitor rarely exhibited errors exceeding 2-3 mmHg over a measurement range of 50-200 mmHg.

Pressure relief characteristics in alternating pressure air cushions.

In this study a computerised system was used which continuously measured air pressure, interface pressure and pressure-time cycle characteristics of an alternating pressure air cushion (APAC), and calculated the time the interface pressure remained below three chosen thresholds of 20, 40 and 60 mm Hg. Ten healthy volunteers were used to evaluate the pressure relieving characteristics of four APACs. Results indicated significant differences between products when the threshold periods were analysed, showing some devices were not capable of relieving interface pressures below 20 mm Hg. Though deflation pressure decreased to nearer zero, interface pressure did not follow suit.

The biomechanical effects of the inclusion of a torque absorber on trans-femoral amputee gait, a pilot study.

This paper reports on a pilot study investigating the effects on the gait of two transfemoral amputees of to the inclusion of a torque absorber (TA) and its location relative to the knee unit. Both subjects carried out gait tests with a prosthesis with no TA with, a TA proximal to the knee unit and with a TA distal to the knee unit. Three-dimensional gait analysis was carried out to establish the kinematic and kinetic gait parameters of both the prosthetic and sound side. It was found that the TA did not significantly affect the sagittal kinetic and kinematic parameters of the sound or the prosthetic side. However, for one subject the axial rotation of the socket relative to the foot was significantly greater with the TA. It was concluded that by using the methodology of this pilot study, it is possible to investigate the rotations in the transverse plane within the prosthetic limb and pelvis. Further, including a TA may reduce the relative motion between the stump and the socket and therefore may decrease skin breakdown due to diminished shear forces.