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 +2  (nbme22#38)

CPAP increases intrapleural pressure as the elevated airway pressure is transmitted to other things in the cavity (lung pushes on pleural space/cavity which can then push on other structures). This can lead to compression of veins, including the vena cava -> decreased venous return -> decreased bp (from decreased preload). This is similar to what happens during valsalva (exertion phases) though the positive pressure is provided by a machine pump instead of abdominal muscles/diaphragm. I think heart rate likely increases instead of decrease as a compensatory response.

Here’s one site that explains it well (the “free” content is enough and probably already exceeds the depth one might need to know) https://thoracickey.com/extrapulmonary-effects-of-mechanical-ventilation/


 +1  (nbme21#25)

To maintain plasma osmolality -> Need to exactly replace all the fluids lost in the day

She cannot concentrate urine above 450 mOsm/kg , so the minimum amount of water required to be excreted by kidneys is 1 (to excrete the 450 mOsm she accumulates per day). The minimum excretion water required is necessarily at max concentration; if you were to say produce diluter urine, say 225mOsm/kg, this would require 2 L of water. The question wants the minimum possible water volume, so we assume she’s concentrating to the max.

1 L losses from kidney + 900 mL insensible + 100 mL in sweat and feces = 2L losses -> need to ingest 2 L of water to replace.





Subcomments ...

submitted by noselex(4),

I understand sleep apnea would cause hypoxia and increase in SNS activity. So treatment would reduce BP and Heart Rate. I was stuck between the two options. Why was the answer blood pressure and not HR? I googled around a bit and found studies that show drop in both HR and BP. I guess BP drops more? Can someone please explain.

madeforupvoting2  CPAP increases intrapleural pressure as the elevated airway pressure is transmitted to other things in the cavity (lung pushes on pleural space/cavity which can then push on other structures). This can lead to compression of veins, including the vena cava -> decreased venous return -> decreased bp (from decreased preload). This is similar to what happens during valsalva (exertion phases) though the positive pressure is provided by a machine pump instead of abdominal muscles/diaphragm. I think heart rate likely increases instead of decrease as a compensatory response. Here’s one site that explains it well (the “free” content is enough and probably already exceeds the depth one might need to know) https://thoracickey.com/extrapulmonary-effects-of-mechanical-ventilation/ +3  
apolla24  I guess since changes in HR are such a transient phenomenon and you only have sustained increase in HR when exercising or like acutely experiencing some medical emergency vs BP that can be elevated for long periods of times with no effects. Therefore an improvement in BP is more important. That’s my take. +