Small animal DXA instrument comparison and validation

Several new peripheral dual-energy X-ray absorptiometry (DXA) devices designed for assessment of bone and body composition in rodents have been developed. We compared the performance (accuracy and precision) of two of these devices, the InAlyzer and the iNSiGHT, to those of an established device, the PIXImus. We measured total body bone mineral content (BMC), bone mineral density (BMD), and body composition (lean and fat mass) on the three DXA devices in 18 male C57Bl/6 J mice (6 each of ages 8, 14, and 24 weeks, weighing 22 to 33 g). DXA body composition measures were compared to whole-body nuclear magnetic resonance (NMR) outcomes. BMC of the femur was also compared to ex vivo micro-computed tomography (microCT). Total body BMD from the InAlyzer and iNSiGHT devices was strongly correlated to that from PIXImus (R2 = 0.83 and 0.82, respectively), but was ~25 % higher than PIXImus. Total body BMC measures by InAlyzer were strongly associated with those from PIXImus (R2 = 0.86), whereas those from iNSiGHT were only weakly correlated (R2 = 0.29). Femur BMC from InAlyzer was strongly correlated with microCT outcomes, whereas iNSiGHT was only weakly correlated. InAlyzer and iNSiGHT fat mass measures were very strongly correlated with PIXImus and NMR outcomes (R2 = 0.91 to 0.97), with slightly weaker associations for lean mass (R2 = 0.81 to 0.76). Short-term precision of InAlyzer and iNSiGHT measurements were excellent, and akin to those from the PIXImus for both body composition and bone measures, ranging between 0.39 and 3.2 %. With faster scan times, closed X-ray source and excellent precision, the new devices are both satisfactory replacements for the now discontinued PIXImus system. However, given the accuracy of the bone and body composition measures, the InAlyzer may be preferable for studies where musculoskeletal changes are the main interest.

NMRES: an artificial intelligence expert system for quantification of cardiac metabolites from 31phosphorus nuclear magnetic resonance spectroscopy

The application of high-resolution 31Phosphorus Nuclear Magnetic Resonance (31P NMR) Spectroscopy in biology and medicine has provided new insights into biochemical processes and also a unique assessment of metabolites. However, accurate quantification of biological NMR spectra is frequently complicated by: (a) non-Lorentzian form of peak lineshapes, (b) contamination of peak signals by neighboring peaks, (c) presence of broad resonances, (d) low signal-to-noise ratios, and (e) poorly defined sloping baselines. Our objectives were to develop an expert system that captures and formalizes 31P NMR spectroscopists' expert knowledge, and to provide a reliable, efficient, and automated system for the interpretation of biological spectra. The NMR Expert System (NMRES) was written in the C and OPS5 programming languages and implemented on a Unix-based (Ultrix) mainframe system with XWindows bit-map graphics display. Expert knowledge was acquired from NMR spectroscopists and represented as production rules in the knowledge base. A heuristic weights method was employed to determine the confidence levels of potential peaks. Statistical and numerical methods were used to facilitate processing decisions. NMR spectra obtained from studies of ischemic neonatal and immature hearts were used to assess the performance of the expert system. The expert system performed signal extraction, noise treatment, resonance assignment, intracellular pH determination, and metabolite intensity quantitation in about 10 s per 4 KB (kilobyte) spectrum. The peak identification success rate was 98.2%. Peak areas and pH estimated by the expert system compared favorably with those determined by human experts. We conclude that the expert system has provided a framework for reliable and efficient quantification of complex biological 31P NMR spectra.

Monitoring of transcutaneous carbon dioxide tension

The authors studied the analytic and clinical characteristics of transcutaneous carbon dioxide monitoring in acutely ill NICU infants. Most infants were premature. The authors used both developmental and commercially available equipment. Monitoring was performed at temperature settings of 44 degrees C. With commercially available equipment, it was found that the least-squares linear regression relationship (r = 0.86, Sy.x = 12.9 torr, n = 100 patients) between transcutaneous PCO2 (y) and arterial PCO2 (x) was: y = -2.8 + (1.86)(x) (in torr). Calibration drift was unacceptably large in about 10% of the 500 monitoring sessions performed. Therefore, the authors conclude that transcutaneous PCO2 monitoring is most appropriate for following short-term trends in PCO2 over a period of one to three hours. Continuous transcutaneous PCO2 monitoring was most useful clinically after extubation when the objectives were careful observation for changes in PCO2 and prevention of reintubation.

Transcutaneous carbon dioxide for short-term monitoring of neonates

We studied transcutaneous pCO2 monitoring in 70 neonates, most of them premature with respiratory distress syndrome. Measurements were at 44 degrees C. Calibration drift was large in some instances. Least squares linear regression analyses of transcutaneous pCO2 (y) vs arterial pCO2 (x) in kilopascals showed, for all observations (n = 516), for one observation randomly selected from each patient (n = 70), and for the first observation from each patient (n = 70): y = -0.28 + 1.80x, y = 0.01 + 1.74x, and y = 0.73 + 1.63x, respectively. Regression lines for individual patients with 14 or more observations each were not coincident (F = 2.80, p less than 0.002). Transcutaneous pCO2 monitoring was most useful clinically as a means of following short-term trends in arterial pCO2 continuously during extubation and afterward when avoiding re-intubation. In view of the potential for error associated with drift, we recommend that intervals between calibrations be limited to about 3 h.

Transcutaneous carbon dioxide for short-term monitoring of neonates

We studied transcutaneous pCO2 monitoring in 70 neonates, most of them premature with respiratory distress syndrome. Measurements were at 44 degrees C. Calibration drift was large in some instances. Least squares linear regression analyses of transcutaneous pCO2 (y) vs arterial pCO2 (x) in kilopascals showed, for all observations (n = 516), for one observation randomly selected from each patient (n = 70), and for the first observation from each patient (n = 70): y = -0.28 + 1.80x, y = 0.01 + 1.74x, and y = 0.73 + 1.63x, respectively. Regression lines for individual patients with 14 or more observations each were not coincident (F = 2.80, p less than 0.002). Transcutaneous pCO2 monitoring was most useful clinically as a means of following short-term trends in arterial pCO2 continuously during extubation and afterward when avoiding re-intubation. In view of the potential for error associated with drift, we recommend that intervals between calibrations be limited to about 3 h.

Unpredictable fluctuations in transcutaneous pCO2 from capillary blood gas determinations

Transcutaneous pCO2 electrode response time was optimized by use of a new electrode filling solution composed of NaCl/NaHCO3 electrolyte buffer (100 and 20 mmol/L, respectively) in an equivolume mixture of glycerol and water. The 95% response time to a step change in pCO2 was 49.9 +/- 2.8 s (mean +/- SD) when there was no spacer between the membrane and glass of the electrode. Use of this filling solution during monitoring of severely ill premature infants with cardiopulmonary disease allowed identification of large, unpredictable transient changes in transcutaneous pCO2, and therefore presumably in arterial pCO2, that occurred during capillary blood gas sampling. The changes, which occurred in 19 of 20 samplings, ranged from -1.06 + 2.53 kPa (-8 to +19 Torr). The maximum relative change observed was +29%. These results indicate that the standard protocol for capillary blood collection induces significant transient fluctuations in blood gas tensions. We believe these fluctuations decrease the reliability of capillary pCO2 values for use in clinical management in patient populations similar to ours.