Journal of clinical monitoring
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Comparative Study
Accurate determination of end-tidal carbon dioxide during administration of oxygen by nasal cannulae.
Measurement of end-tidal carbon dioxide tension (PETCO2) by mass spectrometry or infrared capnometry provides a clinically useful approximation of arterial carbon dioxide tension (PaCO2) in intubated patients. Although several devices have been proposed to sample PETCO2 during spontaneous breathing (i.e., unintubated patients receiving supplemental oxygen), thus far no reports have documented their efficacy. This article reports the use of an easily constructed modification of simple nasal cannulae that permits accurate sampling of PETCO2 during oxygen administration to unintubated patients. ⋯ The PaCO2 - PETCO2 gradients were calculated and compared with values obtained in the same patients after intubation and mechanical ventilation. No significant difference was found between the calculated gradients with nasal cannulae (2.09 +/- 2.18 mm Hg) versus intubation (2.87 +/- 2.82 mm Hg). Simultaneous oxygen administration and accurate sampling of PETCO2 may be achieved in unintubated patients by using this easily constructed modification of nasal cannulae.
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Methods used in clinical practice to increase the damping of a transducer hydraulically coupled to an intraarterial blood-pressure monitoring system often decrease the undamped natural frequency of the system. This leads to spuriously high systolic and low diastolic pressure readings. The ROSE damping device is being marketed as a possible solution to the problem. ⋯ Typically it increased the damping coefficient from a minimum of 0.17 +/- 0.01 to a minimum of 0.33 +/- 0.01, while never significantly decreasing the undamped natural frequency. In testing a sample of 25 devices we did observe, however, a wide variability in damping characteristics among different devices. Damping coefficients ranged between 0.19 and 1.20.
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The accuracy of pulse oximeters from fourteen manufacturers was tested during profound brief hypoxic plateaus in 125 subject sets using 50 normal adult volunteers, of whom 29 were studied two to nine times. A data set usually consisted of 10 subjects, and 13 sets were collected between August 1987 and July 1988. In the first 6 sets, six 30-second hypoxic plateaus were obtained per subject at 55 +/- 6% oxyhemoglobin (O2Hb) (range, 40 to 70%). ⋯ Ambiguity values for finger probes (unless specified) with latest data were: Physio-Control, 3.9 (ear, 3.3); Puritan-Bennett, -4.4; Criticare, 5.8 (forehead, 4.7); Kontron, 5.9 (infant probe) and 6.1 (ear, 5.8; forehead, 7.1); Biochem, -6.0; Datex 6.4 (ear, 6.9; forehead, 6.8); Critikon, 8.4; SiMed, 8.6; Marquest, 9.0; Novametrix, 10.2; Invivo, -12.2 (ear, -14.3); Nellcor, -15.1; Ohmeda, -21.2; and Radiometer, -21.2 (ear, -9.6). Linear regression slopes of 36 instruments from twelve manufacturers generally deviated from 1 in proportion to alpha. The data showed substantial differences in bias and precision between pulse oximeters at low saturations, the most common problems being underestimation of saturation and failing precision.
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In clinical practice, the addition of positive end-expiratory pressure (PEEP) into a standard anesthesia circle circuit decreases the delivered tidal volume (DTV) to a patient. We studied the magnitude of the delta DTV/delta PEEP relationship in two commonly used anesthesia systems. In addition, the magnitude of the delta DTV/delta PEEP relationship varies with both pulmonary compliance and volume of gas contained in the patient's breathing system between the ventilator and PEEP valve site, and this was also evaluated. Routine monitoring of expired tidal volume should be used whenever PEEP is added to an anesthesia circuit.
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The pulse oximeter, a widely used noninvasive monitor of arterial oxygen saturation, has numerous applications in anesthesiology and critical care. Although pulse oximetry is considered sufficiently accurate for many clinical purposes, there are significant limitations on the accuracy and availability of pulse oximetry data. This article reviews both the clinical uses of the pulse oximeter and the limitations on its performance. The pulse oximeter is generally acknowledged to be one of the most important advances in the history of clinical monitoring.