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?

• intrathoracic pressure compresses heart, causing blood not to return

Causes for decreased left ventricular output during ventilation:

• Shifting of intraventricular septum to Left due to increased RV volume
• Decreased venous return
• Changes in hearts ability to contract due to positive pressure
• lack of O2 to heart

Normal compensatory mechanisms for maintaining CO and BP during PPV?

• increased HR to compensate for decreased SV
• increased SVR to maintain BP

Other Physiologic responses:

• Decreased cerebral perfusion pressure (body’s response to a fall in CPP is to raise systemic blood pressure and dilate cerebral blood vessels)
• Decreased renal perfusion (Increased ADH, RAAS, and Patient has renal problems so increased creatinine and uric acid)
• Possible malnutrition (increased glucose via gluconeogenesis etc.)

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/

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.

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.

https://pbs.twimg.com/media/CK3J5kZUsAAqSf-.jpg:large