Effect of changes in inflation and blood volume on regional lung density--a PET study: 2.

Data obtained during positron emission tomographic (PET) imaging of the lung are expressed in terms of lung volume, not lung weight. Therefore changes in PET data may simply reflect changes in regional lung inflation. Normalizing PET data with regional measurements of lung density (rLD) made with positron transmission tomography may be useful, but how rLD is altered by changes in either regional inflation or blood volume has not been evaluated. In 12 supine dogs the mean rLD was 0.32 +/- 0.06 g/ml lung and was not significantly changed by doubling the tidal volume. The mean LD decreased with increasing levels of positive end-expiratory pressure (PEEP) (5-20 cm H2O, but regional changes were consistently present only at levels greater than 5 cm H2O. After sufficient hemorrhage to reduce cardiac output by half, mean rLD only decreased from 0.33 +/- 0.07 to 0.30 +/- 0.06 g/ml lung, while regional extravascular density remained unchanged. Thus, large changes in tidal volume, modest amounts of PEEP, and significant decreases in blood volume produced small, although measurable, decreases in rLD. Therefore, normalization of emission data with transmission derived measurements of LD should not be necessary for interventions that cause rLD to decrease.

PET measurement of regional lung density: 1.

Regional lung density (LD) and lung water (LW) measurements were made with positron transmission and emission tomography (PET) in normal and edematous lung in supine dogs in vivo. A comparison was also made of LD measurements by PET and X-ray CT (used by others to noninvasively assess pulmonary edema). Mean LW was 0.25 +/- 0.06 ml water/ml lung and the mean LD (PET) was 0.32 +/- 0.06 g/ml lung (average ratio of LW to LD was 0.795 +/- 0.041). The LD measurements ranged from 0.25 +/- 0.06 in anterior portions of lung to 0.43 +/- 0.11 g/ml in posterior areas, but the ratio of LW to LD was similar throughout the lung. The LW and LD measurements obtained in both normal and edematous portions of lung were strongly correlated (r = 0.886). Values for LD by PET were consistently higher than values obtained for LD by X-ray CT. These differences are probably due to beam-hardening effects with CT and partial-volume averaging and scattered radiation effects with PET. Nevertheless, PET-LD measurements may be a satisfactory method for following acute changes in LW or for normalizing other PET-derived data.

The effect of regional lung injury or alveolar hypoxia on pulmonary blood flow and lung water measured by positron emission tomography.

We measured regional pulmonary blood flow (rPBF) and extravascular lung water (rEVLW) with positron emission tomography (PET) before and after a lobar lung injury induced by oleic acid in dogs. Changes in rPBF after injury were also compared with those observed after alveolar hypoxia limited to a similar volume of lung. Positron emission tomography techniques for measuring rPBF correlated well with microsphere methods, even in areas of low blood flow (R2 = 0.88). After lung injury, rPBF decreased by 54% from its control value, compared with a 36% decrease in response to alveolar hypoxia. This difference was significant (p less than 0.05). Changes in oxygenation after injury correlated significantly with residual blood flow in the injured area (R2 = 0.69) but not with rEVLW. These data suggest that mechanisms other than hypoxic vasoconstriction may affect rPBF after lung injury, and that individual variation in rPBF to the injured area will have an important influence on oxygenation.

Regional changes in extravascular lung water detected by positron emission tomography.

Regional measurements of extravascular lung water (rEVLW) were made with positron emission tomography (PET) and 15O-labeled radionuclides. The label used to measure the total lung water (TLW) content fully equilibrated with TLW prior to scanning in both dogs with normal and low cardiac outputs, and nearly so in areas of lung made edematous by oleic acid injury (the TLW values used were 97% of maximum values). Regional EVLW measurements made by PET (EVLW-PET) and gravimetric techniques in both normal and edematous lung were closely correlated (r = 0.93), and EVLW-PET increased from an average of 0.20 to 0.37 mlH2O/ml lung (P less than 0.05) after regional lung injury. PET measurements of regional blood volume always decreased [from an average of 0.12 to 0.09 ml blood/ml lung (P less than 0.05)] after cardiac output was lowered by hemorrhage in a separate set of animals. Total EVLW (by thermodye indicator dilution) did not change. Likewise, regional EVLW remained constant in areas below the left atrium but decreased in areas above the left atrium. We conclude that PET measurements are accurate, noninvasive, and reproducible and that regional changes may be detected even when measurements of total EVLW by other methods may fail to change significantly.

Structure of the esophagus in the adult opossum, Didelphis virginiana.

The structure of the esophagus has been studied in the adult opossum, Didelphis virginiana. A thickening of both layers of the muscularis externa occurs at the origin of the esophagus and may represent the upper esophageal sphincter; a massive expansion of the muscularis mucosae occurs in the region of the lower esophageal sphincter. The distribution of striated, mixed and smooth muscle in the muscularis externa differs in the inner and outer layers and elements of the myenteric plexus are found to occur even in the region of striated muscle; however, the ganglia of this plexus become much more prominent as smooth muscle makes its appearance. Esophageal glands are found in the lamina propria where they are confined to the 2 ends. They are especially prominent at the distal end where they are responsible for the formation of permanent transverse folds. Similar glands are found in the submucosa, scattered throughout the length of the esophagus but distally, in the region of the transverse folds, the submucous glands disappear. In both of these layers, the glands contain mucous, serous and myoepithelial cells.