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PEG "WAVES"

HAROLD Z. LEHRER M.D.

PEG "waves" were found at air-fluid interfaces in the lateral ventricles on early filling roentgenograms in about 75 per cent of patients in this series of 135 consecutive air studies. PEG "waves" were found at atypical sites (other than the lateral ventricles) in 14 cases. PEG "waves" were entirely absent in approximately 22 per cent of patients, the majority of whom were females.

Analysis of the roentgenographic appearance of these PEG "waves" leads to the conclusion that they represent a foam of air bubbles and CSF. The fact that this foam tends to form most often in the lateral ventricles correlates with the very rapid rate at which air has been observed to enter the lateral ventricles through the foramina of Monro during the first stages of ventricular filling at pneumoencephalography. This rapid inrush of air under buoyancy and surface tension forces during early filling is promoted by certain physical factors:

1. The foramina of Monro are relatively large and are approximately vertical in position, along with the anterior wall of the third ventricle, when the patient’s head is slightly flexed.

2. Maximal pressure head as well as maximal ventricular distensibility occur at the beginning of lateral ventricular filling.

It is observed that, when there is asymmetric filling of the lateral ventricles, PEG "waves" are generally more prominent in the lateral ventricle which is less well filled. Furthermore, the "wavelength" of PEG "waves" becomes longer as more air is introduced. Both of these observations can be explained by the tendency of an air-CSF foam to decay within a few minutes with larger bubbles persisting and growing at the expense of smaller ones.

Although it might have been anticipated that an increased spinal fluid protein content should tend to make PEG "waves" more persistent, this effect was not observed. Two plausible explanations for this appears to be that:

1. Proteins are not the only surface active material in CSF.

2. Surface active materials, since they are concentrated at air-fluid interfaces, affect foam formation to a much greater and more uniform degree than one might otherwise expect from their simple bulk concentration.


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