Thanks, frodo. I would've thought this was a joke (neck collars, really?) but it's from Dr. Zamboni.
I dug up an article on plethysmography with diagrams of what the neck collar looks like.
Chest. 1991 Aug;100(2):371-5.
Noninvasive measurement of central venous pressure by neck inductive plethysmography.
Bloch KE, Krieger BP, Sackner MA.
Division of Pulmonary Disease, University of Miami School of Medicine, Mount Sinai Medical Center, Miami Beach.
Bedside estimation of the height at which the internal jugular veins collapse, referenced to a standard hemodynamic location, theoretically reflects central venous pressure. This method has never been demonstrated as accurate when compared to invasive CVP measurements because of the great clinical skills and time required to visually identify the internal jugular venous waveform. Since the principles of the bedside method are sound, we utilized them in conjunction with the neck inductive plethysmograph, a device which has the capability of recording internal jugular venous and carotid arterial waveforms. The respiratory distortion of these vascular waveforms was eliminated by employing a digital bandpass filter, making it easy to identify the venous and arterial waveforms on the videoscreen of a personal computer. The upper torso was positioned while observing the videoscreen until the vascular waveform was seen as a mixed arteriovenous waveform, signifying intermittent internal jugular venous collapse. The height of the internal jugular vein above the phlebostatic axis was obtained by external measurement and recorded as CVPni in cm H2O. In 43 patients, 86 percent of CVPni (NIP) values fell within 20 percent CVPi (invasive catheter measurements) over a range of CVP from 0 to 19 cm H2O. In an additional seven patients, CVPi was greater than the height that the upper torso could be elevated and an arterial waveform could not be obtained. Here, CVPni was recorded as the value at least exceeding the value measured. In two other patients, obstruction of an internal jugular vein gave spuriously low values of CVPni. Our study indicates that this new neck inductive plethysmographic method is accurate compared to invasive catheter measurements of CVP and should serve as a safe, noninvasive alternative in situations where such measurements are required.
Full article and diagrams here:
http://chestjournal.chestpubs.org/conte ... 1.full.pdf
For the two with IJV occlusions, one had an indwelling cathether. When it was removed three days later, they measured the values again and they'd gone from abnormal to normal. The other was found to have a partial thrombosis and distended IJV. They measured again after the veins were patent. It's in the last paragraph on page 373 of the article.
What I'm taking from it is that for our exact situation (IJV occlusions when it's pre-venoplasty and occlusion-free IJVs when it's post-venoplasty) the plethysmography shows a measurable difference.