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At least 451 records · Page 25Linked to original sources
PubMed2003

Sources of error in noninvasive pulmonary blood flow measurements by partial rebreathing: a computer model study.

BACKGROUND: Partial rebreathing is a noninvasive method for measuring pulmonary blood flow (PBF). This study examines the systematic errors produced by the partial rebreathing technique utilizing a comprehensive mathematical model of the cardiorespiratory system of a healthy, 70-kg adult male. METHODS: The model simulates tidal breathing through a branched respiratory tree and incorporates the effects on carbon dioxide dynamics of lung tissue mass, vascular transport delays, multiple body compartments, and realistic blood-gas dissociation curves. Four studies were performed: (1) errors produced under standard conditions, (2) effects of recirculation, (3) effects of alveolar-proximal airway partial pressure of carbon dioxide (Pco(2)) differences, and (4) effects of rebreathing time. RESULTS: Systematic errors are less than 10% when the simulated PBF is between 3 and 6 l/min. At 2 l/min, PBF is overestimated by approximately 35%. At 14 l/min, PBF is underestimated by approximately 40%. At PBF of greater than 6 l/min, recirculation causes approximately 60% of the systematic error, alveolar-proximal airway differences cause approximately 20%, and alveolar-arterial differences cause approximately 20%. The standard rebreathing time of 50 s is shown to be excessive for PBF of greater than 6 l/min. At PBF of less than 3 l/min, errors are caused by inadequate rebreathing time and alveolar-arterial gradients. CONCLUSIONS: Systematic errors in partial rebreathing cardiac output measurements have multiple causes. Our simulations suggest that errors can be reduced by using a variable rebreathing time, which should be increased at low PBF so that quasi-equilibrium in the alveoli can be achieved and decreased at high PBF to reduce the effects of recirculation.

Adult↗
PubMed1981

Mathematic coupling of data: a common source of error.

The relationship between two variables may be mathematically coupled if either one or both variables are derived and/or calculated, and this can lead to erroneous results and invalid conclusions. The purpose of this report is to identify four types of mathematic coupling of data. Type 1 coupling involves directional changes in two variables which are mathematically coupled. Type 2 coupling is the functional relationship between two calculated variables which have one or more common component variables. Type 3, the most common type of mathematic coupling, is direct algebraic coupling between two variables, when one or more of the variables is derived and/or calculated. Type 4 is indirect coupling or physiologic coupling. The common problem in each type of mathematic coupling is that one variable either directly or indirectly contains the whole or components of the second variable. Statistical techniques, when properly applied to the relationship between the two variables, further obscure the underlying mathematic coupling, and tend to support the erroneous results. Recognition of mathematic coupling is imperative for correct data analysis and accurate interpretation.

Statistics as Topic↗
PubMed1988

Practical instrumentation and common sources of error.

A thorough comprehension of electrodiagnostic equipment is essential to consistently obtain accurate and reproducible data. Unreliable waveform latencies or morphologies may result from inappropriate filter settings, sensitivity comparisons, sweep speeds, interelectrode separation, cathode/anode reversals and stimulus artifact. A low frequency filter with too high a frequency limit may decrease amplitude, shorten peak latency, decrease the negative spike duration, add a phase and increase total waveform duration. A high frequency filter with too low a cut-off may decrease amplitude and prolong onset and peak latencies. Increasing the amplifier's sensitivity may shorten the onset latency of a response. Sweep speeds that are too slow may omit phases, turns or entire potentials when using digital equipment. If the interelectrode separation is inadequate, waveform morphology and amplitude can be altered. Reversing cathode and anode placement affects latency and velocity determinations. Stimulus artifact may obscure a response and its reduction must be understood. Comparing latencies and amplitudes at different instrument settings is never appropriate and can lead to serious errors and misdiagnoses. A naive approach to instrumentation, therefore, is indefensible.

Electrodiagnosis↗
PubMed1992

Continuous breathing circuit flow and tracheal tube cuff leak: sources of error during pediatric indirect calorimetry.

OBJECTIVE: To determine whether continuous gas flow in the breathing circuit or an airleak around the tracheal tube cuff will introduce errors into the measurement of oxygen consumption (VO2) with indirect calorimetry. DESIGN: Nonrandomized, controlled trial. SETTING: Experimental laboratory. SUBJECTS: Ten healthy, anesthetized mongrel dogs, weighing 8 to 12 kg. INTERVENTIONS: Data were recorded at seven levels of flow, from 0 to 12 L/min in excess of minute ventilation, through a continuous breathing circuit. Data were recorded at five levels of tracheal tube cuff leak from 0% to 40% of inspiratory minute volume. MEASUREMENTS AND MAIN RESULTS: VO2 was measured using an indirect calorimeter with constant internal gas flow and calculated from results of blood gas analysis, cooximetry, and thermodilution cardiac output determinations at all levels of continuous breathing circuit flow and cuff leak. BP, heart rate, respiratory rate, arterial and mixed venous blood gases, and body temperature were measured to assess stability of cardiopulmonary function. Continuous breathing circuit flow did not affect the accuracy of indirect calorimetry until the total flow reached a critical value (11.5 L/min) that was slightly below the internal flow constant of the metabolic monitor (12.4 L/min). At higher circuit flows, measured VO2 decreased in a linear fashion, while calculated VO2 remained unchanged. Above the critical flow, the error of indirect calorimetry correlated significantly only with the total circuit flow (r2 = .64), not with the exhaled concentration of CO2 (r2 = .005) or the inspiratory-expiratory oxygen difference (r2 = .004). The continuous flow rate at the critical circuit flow was 66 +/- 15% of the subjects' peak inspiratory flow. Increasing tracheal tube cuff leak produced a progressive decrease in measured VO2 but not in calculated VO2. The difference between measured and calculated VO2 was linearly related to the magnitude of the leak (r2 = .56), and was statistically significantly larger at all levels of cuff leak, when compared with measurements during complete cuff seal. CONCLUSIONS: An indirect calorimeter in which measurement of VO2 is based on internal constant flow rather than spirometry can be used to accurately measure VO2 from a continuous-flow breathing circuit, if the total circuit flow is less than the internal flow. This limitation may restrict the use of continuous flow to a level below the subject's peak inspiratory flow. The accuracy of indirect calorimetry cannot be guaranteed for any amount of tracheal tube cuff leak.

Analysis of Variance↗
PubMed1993

Florid mesothelial hyperplasia associated with ovarian tumors: a potential source of error in tumor diagnosis and staging.

Five ovarian tumors were associated with florid mesothelial hyperplasia that in four cases created initial problems in histologic classification and staging of the tumors. In two cases, small nests, cords, and gland-like arrangements of atypical mesothelial cells within the cyst wall of a serous or mullerian mucinous tumor of borderline malignancy were initially considered to possibly represent, or were misdiagnosed as, foci of stromal invasion, lymphatic invasion, or both. In the three other cases--an endometrioid adenocarcinoma, a struma ovarii, and a yolk sac tumor--papillary and sheet-like proliferations of mesothelial cells involved the extraovarian pelvic peritoneum; in one of the cases, necrosis of the mesothelial cells was prominent. These foci of extra-ovarian mesothelial hyperplasia were interpreted as metastatic tumor intraoperatively (three cases) and on initial microscopic examination (two cases). Factors that may have predisposed to the mesothelial hyperplasia included large size of the primary tumor (two cases), the presence of metastatic tumor elsewhere in the pelvis (three cases), ascites (two cases), and coexistent endometriosis (two cases). Distinction of mesothelial proliferation from invasive or metastatic ovarian tumor is obviously important in correct classification, staging, and management of the tumor. Awareness of the occasional occurrence of florid mesothelial hyperplasia in patients with ovarian neoplasms and attention to the histologic, histochemical, and immunohistochemical features of the mesothelial proliferation should facilitate the correct diagnosis.

Adult↗
PubMed1997

Underestimation of the true benefits of antihypertensive treatment: an assessment of some important sources of error.

The treatment of arterial hypertension is based on results from several large prospective randomized intervention trials that demonstrated significant benefits from antihypertensive therapy in terms of reduced cardiovascular morbidity and mortality, mainly from strokes, but also from myocardial infarction, congestive heart failure and other vascular or renal complications. Nonetheless, it is obvious that such studies, for a number of reasons, underestimate the true benefit of lowering elevated arterial pressure. Active treatment given to patients allocated randomly to be administered placebo is an important factor, particularly since active treatment is given preferentially to the placebo patients with the highest risk. Recruitment of low-risk patients into studies is another reason for the underestimation of benefit. Moreover, significant loss of patients to follow-up plays a role. Two recent epidemiological surveys, in Framingham and Göteborg, showed markedly beneficial blood pressure-related changes in those populations, suggesting that the long-term effects of antihypertensive therapy may in fact have been underestimated by intervention studies

Antihypertensive Agents↗
PubMed1993

High activity of semicarbazide-sensitive amine oxidase (SSAO): an important source of errors in the determination of the concentration of dopamine in pig plasma.

We noted rapid breakdown at 4 degrees and 20 degrees C of dopamine (DA) (but not of (nor)epinephrine and epinine) in pig plasma, but not in human plasma. The enzyme responsible appears to be a semicarbazide-sensitive amine oxidase (SSAO) because the breakdown can be inhibited by semicarbazide, but not by pargyline, clorgyline, EDTA, or (extra) glutathione. Among catecholamines tested, only DA and 3,4-dihydroxybenzylamine (DHBA, the internal standard of most catecholamine assays using high-performance liquid chromatography (HPLC) with electrochemical detection) were good substrates for the pig plasma SSAO. At 37 degrees C, especially after prolonged storage, all catecholamines break down. This breakdown results from autoxidation since it can be prevented by addition of extra glutathione (but not by semicarbazide) for all catecholamines except DA and DHBA. Breakdown at 37 degrees C of these two compounds cannot be prevented by addition of extra glutathione or semicarbazide, but only by addition of both. For reliable measurements of DA concentrations in pig plasma, blood should be collected in tubes containing not only glutathione, but also semicarbazide. The possibility of similarly high plasma SSAO activity in other species should be investigated further.

Amine Oxidase (Copper-Containing)↗
PubMed2002

Pedicle screws with high electrical resistance: a potential source of error with stimulus-evoked EMG.

STUDY DESIGN: Clinically relevant aspects of pedicle screws were subjected to electrical resistance testing. OBJECTIVES: To catalog commonly used pedicle screws in terms of electrical resistance, and to determine whether polyaxial-type pedicle screws have the potential to create a high-resistance circuit during stimulus-evoked electromyographic testing. SUMMARY OF BACKGROUND DATA: Although stimulus-evoked electromyography is commonly used to confirm the accuracy of pedicle screw placement, no studies have documented the electrical resistance of commonly used pedicle screws. METHODS: Resistance measurements were obtained from eight pedicle screw varieties (5 screws of each type) across the screw shank and between the shank and regions of the screw that would be clinically accessible to stimulus-evoked electromyographic testing with a screw implanted in a pedicle. To determine measurement variability, resistance was measured three times at each site and with the crown of the polyaxial-type screw in three random positions. RESULTS: Resistance across the screw shank ranged from 0 to 36.4 ohms, whereas resistance across the length of the monoaxial-type screws ranged from 0.1 to 31.8 ohms. Resistance between the hexagonal port and shank of polyaxial-type screws ranged from 0 to 25 ohms. In contrast, resistance between the mobile crown and shank of polyaxial-type screws varied widely, ranging from 0.1 ohms to an open circuit (no electrical conduction). Polyaxial-type screws demonstrated an open circuit in 28 of 75 measurements (37%) and a high-resistance circuit (exceeding 1000 ohms) in 5 of 75 measurements (7%). CONCLUSIONS: Polyaxial-type pedicle screws have the potential for high electrical resistance between the mobile crown and shank, and therefore may fail to demonstrate an electromyographic response during stimulus-evoked electromyographic testing in the setting of a pedicle breech. To avoid false-negative stimulus-evoked electromyographic testing, the cathode stimulator probe should be applied to the hexagonal port or directly to the screw shank, and not to the mobile crown.

Alloys↗
PubMed2004

Nonuniform hybridization: a potential source of error in oligonucleotide-chip experiments with low amounts of starting material.

Low amounts of starting material are a significant limitation of gene-expression profiling of microprepared pathologic specimens. Linear RNA amplification has become the method of choice to overcome this problem. Thus, transcriptomal analyses by oligonucleotide-chips or cDNA microarrays are now feasible with labeled complementary RNA generated from total RNA samples in the lower nanogram range. However, in case of oligonucleotide-chips, it has been underestimated so far that individual complementary RNA molecules are shorter in length than and display a 3' bias in comparison to the sequence stretch represented by oligonucleotides on the chip. This can lead to incorrect interpretation of raw data. We have analyzed this problem testing ex vivo-microprepared endothelial cells with Affymetrix GeneChips U133A. Only a small subset of housekeeping genes showed adequate uniform hybridization. We developed a software tool for objective evaluation of oligonucleotide-chips based on automated analysis of as well as normalization to this subset of housekeeping genes. We analyzed the gene expression profile of microprepared lymphatic vascular endothelial cells. We show that optimized normalization prevented exclusion of angiopoietin-2, a lymphatic endothelial marker, from the lymphovascular transcriptome.

Endothelial Cells↗