A Hunt for the Right Reference

I recently taught at Advanced Critical Care Echocardiography course at the CHEST headquarter in Glenview, IL when I realized this while reading a guideline from ASE.

This famous (?) equation of estimating mean pulmonary artery (PA) pressure by using pulmonary arterial acceleration time (AT) does not have the right reference:

mean PA pressure  = 79 - (0.45 x AT)

Where is it from? This equation is based upon which article? Research? Well, when you read the guideline from the American Society of Echocardiography (ASE), the reference for this equation is written so clearly.

(Reference 81)
Mahan G, Dabestani A, Gardin J, Allfie A, Burn C, Henry W. Estimation of pulmonary artery pressure by pulsed Doppler echocardiography. Circulation 1983;68:367.

The problem is that I could not locate this article. There is no such article in Circulation 1983; 68:367. I know, I know, I did my homework. I went to the Circulation website, went to the past issues, went to the one published on August 1, 1983 Volume 68 Issue 2, and alas, there was no such article. One article ends on page 367, then the next one starts on page 368, neither of which carries this title nor the authors.

https://www.ahajournals.org/toc/circ/68/2

Maybe there might be typos, the year might be wrong, or pages might be wrong. I guess I should be able to find the right reference just googling the equation. After spending hours googling, I gave up. There is some web info about this equation, but no clear mentioning of the reference. Now what? Contact ASE, which I did. First by sending an email like below, then by calling them. Guess what, the staffs at ASE are so kind as to contact the writing group for the 2010 Right Heart guideline immediately upon my calling. Thank you so much. So, I guess I wait for their answer. Stay tuned.

My email to ASE
"While reading your guideline published in 2010 as below, I could not find the reference (81) as shown below.

Rudski LG, Lai WW, Afilalo J, Hua L, Handschumacher MD, Chandrasekaran K, Solomon SD, Louie EK, Schiller NB. Guidelines for the echocardiographic assessment of the right heart in adults: a report from the American Society of Echocardiography endorsed by the European Association of Echocardiography, a registered branch of the European Society of Cardiology, and the Canadian Society of Echocardiography. J Am Soc Echocardiogr. 2010 Jul;23(7):685-713; quiz 786-8. doi:10.1016/j.echo.2010.05.010. PubMed PMID: 20620859.

(Reference 81)
Mahan G, Dabestani A, Gardin J, Allfie A, Burn C, HenryW. Estimation of pulmonary artery pressure by pulsed Doppler echocardiography. Circulation 1983;68:367.

I am afraid there might be an error. Kindly suggest me the right reference. Appreciate your inputs in advance."