The Valsalva & Stroke

[Mike Carlton]

Just found this truly outstanding article. As an anaesthesiologist - or what we Brits call an anaesthetist - I loved the physiology and epidemiology stuff. As a fairly newbie bro I also loved the uncommon sense and advice not to drop the bar on my head. Can I add some "full-frontal nerdity" of my own about intra-thoracic gas volumes during the Valsalva manoeuvre.
The average air volume in the lungs of an adult man at full inspiration is around 5.8 litres, at sea-level atmospheric pressure of 101 kPa (kilo Pascals). (The maths is easier in S.I units and all numbers were pulled from Wikipedia)
The measured rise in intra-oral pressure, a proxy for intra-pleural pressure, during a Valsalva is around 40 mmHg = 5 kPa approx.
Boyle's Law describes the relationship between the pressure P and volume V of a gas at any given temperature: P x V = constant K.
Therefore the volume of gas in the lungs of an adult man during a Valsalva will change as follows: P1 x V1 = P2 x V2, where P1, V1 are at full inspiration and P2, V2 during the Valsalva
Substituting the actual numbers gives 101 x 5.8 = (101 + 5) x V2 or V2 = 5.53 litres.
I'm ignoring any potential change in thoracic volumes due to a shift in the position of the diaphragm caused by possible changes in intra-abdominal volume or due to contraction of muscles of the thoracic cage but there will be a theoretical reduction of around 5% in intra-thoracic volume when we perform a Valsalva.
I'm going to keep on grunting.

[franklie]

Just found this truly outstanding article. As an anaesthesiologist - or what we Brits call an anaesthetist - I loved the physiology and epidemiology stuff. As a fairly newbie bro I also loved the uncommon sense and advice not to drop the bar on my head. Can I add some "full-frontal nerdity" of my own about intra-thoracic gas volumes during the Valsalva manoeuvre.
The average air volume in the lungs of an adult man at full inspiration is around 5.8 litres, at sea-level atmospheric pressure of 101 kPa (kilo Pascals). (The maths is easier in S.I units and all numbers were pulled from Wikipedia)
The measured rise in intra-oral pressure, a proxy for intra-pleural pressure, during a Valsalva is around 40 mmHg = 5 kPa approx.
Boyle's Law describes the relationship between the pressure P and volume V of a gas at any given temperature: P x V = constant K.
Therefore the volume of gas in the lungs of an adult man during a Valsalva will change as follows: P1 x V1 = P2 x V2, where P1, V1 are at full inspiration and P2, V2 during the Valsalva
Substituting the actual numbers gives 101 x 5.8 = (101 + 5) x V2 or V2 = 5.53 litres.
I'm ignoring any potential change in thoracic volumes due to a shift in the position of the diaphragm caused by possible changes in intra-abdominal volume or due to contraction of muscles of the thoracic cage but there will be a theoretical reduction of around 5% in intra-thoracic volume when we perform a Valsalva.
I'm going to keep on grunting.

Wouldn't the volume actually increase due to the increase in temperature from 20 C to 37 C?

P1*V1/T1=P2*V2/T2 because the number of gas molecules (n) is fixed. Or perhaps n would change due to the CO2/O2 exchange taking place. But I'm no doctor.

[Mike Carlton]

Franklie,

as air moves into the lungs it warms up to 37C and stays there whether you're at full inspiration or during a Valsalva.
My original post suggested a 5% reduction in air volume but there's a lot more in the chest than just air: heart, blood vessels, lung tissue and other giblets so the actual reduction in volume of the chest might be only 2-3%. Not that it matters, I just wanted to do the maths.

Mike