Plzz explain effect of squatting on venous return

NEET PG
#1

1)Plzz explain effect of squatting on venous return

  1. and plzz also explain how hyperventilation increases venous return(as mentioned in tetralogy of fallot)

  2. sir why we are giving alpha agonist in tof treatment?

  3. sir already due to stenosis less blood going to lungs for oxygenation and we are creating shunt between pulmonary and systemic circulation so after creating shunt even less blood will go to lungs and there will be mixing of blood both will lead to cyanosis so why we are doing it?

knee chest position and squatting positions are actually different.

) alpha agonist increases afterload thereby increasing Left ventricle pressure and causing less shunt of blood from Right to Left and thus reducing mixing of blood

  1. Keeping Ductus arteriosus open before surgery so that more blood goes from aorta to pulm artery thus increasing more blood going to lung so more oxygenated blood decreasing cyanosis

Note that fluid always move from an area of higher pressure to lower pressure. Also note that systemic circulation is a high pressure circuit compared to the pulmonary circulation.

Now to clarify your doubts:

  1. Squatting increases preload (venous return) as well as afterload (ie blood pressure). However the increase in after load is more than the increase in preload. So the pressure differential between the systemic and pulmonary circuits is increased. This results in less blood being shunted from right to left ie more blood goes to the lungs resulting in improved oxygenation and a reduction in symptoms.

  2. Hyperventilation results in a rise in mean intrathoracic pressure. Intrathoracic pressure is inversely proportional to venous return, so reduced venous return. However do note that hyperventilation causes CO2 washout resulting in other cardiovascular changes.

  3. Alpha agonists increases myocardial contraction and increases systemic blood pressure resulting in less shunt.

  4. Artificial shunts draws blood away from systemic circulation to pulmonary circulation due to pressure differential and no intervening stenotic valve.