The lung as a metabolic organ.

Recently, the lung has received increasing attention as a metabolic organ. In this role, the lung modulates the composition of the arterial blood by several mechanisms: removing active substances from the plasma, releasing substances into the plasma, temporarily holding substances from circulation, and activating or inactivating substances that pass through the lungs. In this report, the procedures proposed by different investigators for in vivo noninvasive assessment of the lung metabolic functions are reviewed. Most procedures are based on an estimation of the clearance of plasma amines by the lung endothelial cells. This clearance is assessed by measuring the lung uptake or the extraction fraction of an intravenously (IV) injected radiolabeled amine. Our own procedure, which assesses the number of free pulmonary endothelial amine receptors, is discussed in detail. In our procedure, the number of receptors was computed using the number of injected molecules of amine and determining the lung extraction fraction of the amine during its first pass through the lungs. In goats, using N-isopropyl-p-iodoamphetamine labeled with 123I as the radiopharmaceutical, the total number of endothelial lung amine receptors was found to be 1.589 X 10(20). The methods for studying the lung metabolic functions, which are discussed in this report can be applied in humans to evaluate either physiological or pathological conditions.

Single photon emission computed tomography in the diagnosis of pulmonary thromboembolism.

The use of single photon emission computed tomography (SPECT) as the procedure for screening, assessing the size and number of embolized areas, and evaluating the follow-up of patients with pulmonary thromboembolism (PTE) is reviewed. Details of the technique for acquiring and processing perfusion and ventilation lung SPECTs are presented. The tomographic images produced by SPECT allow the application of a less-expensive diagnostic algorithm than that classically used. This is due to the fact that the SPECT images contain more anatomical and physiological information than similar planar images. The signs of PTE in perfusion SPECT and the significance of mismatch, match, and paradoxical mismatch are discussed and illustrated with examples.

A noninvasive procedure for in vivo assay of a lung amine endothelial receptor.

Lung endothelial N-isopropyl-p-iodoamphetamine (IMP) binding sites were assessed applying principles of competitive binding assay adapted for in vivo measurements obtained by digital imaging. Data were acquired following the method published by Rahimian et al., a modification of the dual indicator dilution technique of Chinard and Crone. Iodine-123 (123I) IMP, the test cellular tracer, and technetium-99m (99mTc) dextran, the reference vascular tracer were imaged during their first pass through the superior vena cava, right heart, lungs, and left heart in West African dwarf goats. The lung fractional extraction of IMP diminished progressively from 0.96 to 0.20 as the amount of IMP in the test tracer boluses was gradually increased from 0.6 to 150 mg. This demonstrated that lung extraction of IMP is by way of a saturable binding system, presumably receptors. The dissociation constant of IMP-lung binding sites reaction was calculated by Scatchard plot and found to be 11.7 mg. The amount of IMP bound at saturation (R), was found to be 30 mg. Assuming that a single molecule of IMP bound a single receptor, the total number of free receptors was computed as the Avogadro's number times R, divided by the IMP molecular weight, and found to be 6.04 X 10(19). Using a computer model, it was determined that the 20 mg per bolus isotherm was the most sensitive for measuring the number of total free receptors (binding sites). This is the first time, to our knowledge, that noninvasive in vivo assessment of receptors in lung has been accomplished. Basically, the method used can be applied in humans and, also, to assess receptors in organs other than the lungs.

Measurement of metabolic extraction of tracers in the lung using a multiple indicator dilution technique.

Selective pulmonary uptake of many natural and synthetic substances has been demonstrated by physiologists and pharmacologists using isolated perfused lung preparations or invasive techniques. It is difficult, however, to relate these laboratory studies to disease processes and to the study of problems encountered in a clinical environment. Our goal was to develop a noninvasive method for studying the pulmonary uptake of tracer substances using available radiotracers, gamma cameras, and computers that would give information similar, if not identical, to that from the invasive laboratory methods, and that could be applied in a clinical setting. The multiple-indicator dilution technique, modified for external counting, is well suited for such studies of pulmonary uptake of tracer substances. In this study, Tc-99m micro sulfur colloid (Tc-99m micro SC) was used as an intravascular reference tracer, N-isopropyl-p-[123I]iodoamphetamine (I-123 IMP) as a cellular test tracer amine, and In-111 DTPA as an extracellular tracer. Calculated first-pass lung uptakes of I-123 IMP and In-111 DTPA were 0.92 +/- 0.04 and 0.17 +/- 0.04, respectively, relative to the reference tracer. Using this approach, the first-pass pulmonary extraction of a variety of radiolabeled test tracers can be measured in a clinical environment in a variety of physiologic settings.

Comparison of supine, upright, and prone positions for liver scans.

We compared liver scan interpretations based on anterior images obtained in the upright, prone, and supine positions. Receiver-operating-characteristic curves were generated for three well trained observers. Results showed that reading the three different views together was more accurate than the reading of any individual image. Furthermore, interpretations based on either the prone or upright view were superior to those using the supine view alone. The prone and upright views should be used more often in liver scanning.

Beta-2-microglobulin as renal diagnostic agent.

Our group has undertaken the determination of serum levels of beta-2-microglobulin in 250 patients. We have compared this diagnostic entity to radioisotopic glomerular filtration rate, serum creatinine, and blood urea nitrogen. We have also analyzed the cost effectiveness of each of these diagnostic entities and combinations thereof. We conclude that beta-2-microglobulin is a simple test to perform; it is specific, sensitive, and accurate. Its cost effectiveness is excellent. We advocate its wide clinical usage.

Dynamic cisternography in normal dogs and in human beings.

Cisternography and ventriculography were performed in five normal anesthetized dogs to study the dynamics of tracer dispersal. Indium 111-diethylenetriaminepentaacetic acid was injected into lateral ventricles, cisterna magna, thoracic subarachnoid space, and lumbar sac. The patterns of dispersal from each injection site support a nonflow dispersal mechanism with a tendency toward ultimate equilibrium distribution throughout the cerebrospinal fluid. Quantitative data in humans complement observations made in dogs. A theory of abnormal tracer movements, based on factors observed to affect tracer dispersal in dog and man, is proposed and applied to the interpretation of abnormal cisternographic patterns seen in clinical situations.