Canadian Neighbor Pharmacy: External Radioflux Detection in Clinical Use of Lung Water Measurements
Although the method was designed to measure the rate of tracer protein equilibration between plasma and lung interstitium, it does contain information about interstitial lung water content. In one configuration, the method measures local changes using a portable, focused, scintillation detector. In another configuration, a portable gamma camera is used to obtain a picture of the entire thorax. The chest wall contribution to the external radioflux is a potential source of error, more so with lower energy isotopes (iodine) than with higher energy isotopes (indium). With the focused scintillation detector, repositioning the probe for serial measurements may also be a problem. The time to make a measurement is long in normal subjects (up to one hour or more), but in patients with acute lung injury, the time decreases to about 15 minutes.
The method is moderately invasive because radioactive isotope(s) must be injected intravascularly, and blood samples are needed for calibration purposes. The equipment is portable and only moderately expensive by current standards. The results are reproducible and ought to be sensitive for measuring interstitial water content because intracellular water need not be detected, if the proper tracer is chosen.
Aside from the time to accumulate data, there is a significant time involved with computer calculation. Nevertheless, data could be available in time to be used in treating patients, particularly those in the at-risk group.
Positron Emission Tomography
Positron emission tomography is an accurate method for measuring the distribution of total and extravascular lung density in man in a relatively noninvasive way. The method uses the 511 Kev gamma rays emitted by positron disintegration and can achieve a transaxial tomographic section with a spatial resolution of about 2 cm during a 4 to 10 minute scan.
By judicious selection of position-emitting isotopes, one can measure the blood volume, extravascular and interstitial water volumes, and gas volume. The device can also be used in the transmission mode, like x-ray or computed tomography, from a ring positron source. Latest publications from Canadian Neighbor Pharmacy on twitter online group will help you to grapple with diferent term while reading medical articles.
Currently, the method is complicated, requires some time (30 minutes or more) before data are available, is costly and requires a cyclotron to produce the short-lived positron-emitting isotopes. Therefore, although positron emission tomography appears to be excellent for measuring density and for other studies such as glucose metabolism and for microvascular protein flux, it is not reasonable to expect it to be useful clinically to measure lung water content in the foreseeable future.
Since the Workshop, two publications confirming the views expressed here have appeared.