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NBME 19 Answers

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 +2 
submitted by aroy3(2),

This patient has Stanford A aortic dissection with bleeding into the pericardial space. This explains the chest pain and low blood pressure 80/40 mmhg. It acts like a cardiac tamponade mainly around the outflow tract of the left ventricles and causes backup of blood in the LV and LA. This increases LA pressure and reduces the pulmonary venous return to left side of the heart.

So pcwp ( an indirect measure of LA pressure) goes up. This pressures back up to the lungs and causes increased diastolic pulmonary pressure and that also backs up to increase RA pressure.

The body sensing decreased blood supply to the tissues ( due to the tamponade effect and low BP) fires reflexly to increase heart rate and vasoconstricts to maintain BP. This explains the increased systemic vascular resistance.

Finally cardiac index is a ratio of cardiac output to body surface area = CO/ BSA. In this case, BSA does not change but due to hypotension, CO reduces. Therefore, the cardiac index also drops

(copied from https://usmlemate.blogspot.com/2019/02/nbme-19-block-2-answers-with-explanation.html)




 +0 
submitted by an_improved_me(18),

More generally, i think PCWP increases due to tamponades ability to cause genererilzed equilibration of heart pressures across all 4 cardiac chambers. From the following link: https://www.mcgill.ca/criticalcare/teaching/files/pericardial-tamponade

"pericardial pressure is distributed equally among all chambers since pericardial fluid is free flowing. This reduces gradients between chambers throughout diastole. This is inconsequential in small effusions, but becomes hemodynamically significant in large effusions, in which greater pressure in the pericardium is transmitted to all four cardiac chambers, resulting in 'equilization of central pressures'"