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/
Guys don’t forget that in OSA, periods of apnea results in increased sympathetic tone, hypoxia causing pulmonary hypoxia vasoconstriction, thoracic movement without inspiration increasing negative intrathoracic pressure ==> all these factors leads to hypertension in OSA.
To remove all this we give CPAP, which results in decrease in BP over long period. (It is not only about increasing intrathoracic pressure and reducing BP)
Although it’s about PPV, this researching helped me understand basic phys, I hope this helps everyone in some way.
Takeaway: During PPV, venous return decreases, cardiac output decreases, and heart pressures decrease in the right side of the heart.
Why does PPV decrease venous return?
Causes for decreased left ventricular output during ventilation:
Normal compensatory mechanisms for maintaining CO and BP during PPV?
Other Physiologic responses:
Guys I have another thought.
Giving other choices are about his other abnormal values (increased creatine, glucose, uric acid), I guess the question is testing us which of these values will improve if his OSA is improved by CPAP.
Since OSA is one of the most important causes of idiopathic hypertension, it should come to us that once his OSA is treated, his hypertension will improve soon.
So "decreased BP" instead of other abnormal values.
(Thanks to my friends for hinting me about this!)
Can someone correct my reasoning here:
I was thinking positive airway pressure will increase alveolar ventilation and decrease hypoxia induced pulmonary vasoconstriction. Thus, RV after load would decrease => more preload to LV and more cardiac output. Then wouldn't BP decrease?
Any help is appreciated. Thanks.
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.
You guys are way overthinking this. Chronic hypoxia causes hypertension due to SNS activation and EPO production.
Increased intracranial pressure that results in Cushing's triad of increased blood pressure, irregular breathing, and bradycardia. Thus, high CO2 induces cushing triad and if you give PP then it will reduced CO2, and then down regulate the sympathetic vasoconstriction. Originally the brain had so much CO2 that it spazzed out and tried to increase the BP in order to push more oxygenated blood to the brain.