Pulmonary Vascular Physiology Flashcards
There are 2 types of circulation
Pulmonary circulation and Bronchial circulation
Pulmonary circulation
From Right Ventricle
Receives 100% of cardiac output (4.5-8L/min)
Red cell transit time ≈5 seconds.
280 billion capillaries & 300 million alveoli.
Surface area for gas exchange 50 – 100 m2
Bronchial circulation
2% of Left Ventricular Output
Vessel wall of pulmonary arteries is thin
Vessel wall of systemic arteries is thick
Muscularization of pulmonary arteries is minor
Muscularization of systemic arteries is major
There is no Need for redistribution in pulmonary artery in normal state
There is Need for redistribution in systemic arteries
What is the pressure in the RA? (MmHg)
5
What is the pressure in the RV? (MmHg)
25/0
What is the pressure in the PA? (MmHg)
25/8
What is the pressure in the LA? (MmHg)
5
What is the pressure in the LV? (MmHg)
120/0
What is the pressure in the aorta? (MmHg)
120/80
Left ventricle is larger than hothead right ventricle as
The left ventricle sees higher pressure than the right ventricle
Pouiseuille’s Law
Resistance= (8 x L x viscosity) / (pi x r^4)
Effect of Transmural Pressure on Pulmonary Vessels During Inspiration
Ohms law
V= IR
Pressure across circuit =
Cardiac output x Resistance
Pressure across pulmonary circulation=
mPAP – Left Atrial Pressure x Pulmonary Vascular Resistance
Pressure across pulmonary circulation
mPAP – PAWP = CO x PVR
mPAP (mean pulmonary arterial pressure),
PAWP (pulmonary arterial wedge pressure left atrial pressure),
CO (cardiac output), PVR (pulmonary vascular resistance)
What happens to mPAP on exercise?
On exercise mPAP remains stable in normal subjects but CO increases significantly
How is this possible?
Recruitment and Distention in Response to Increased Pulmonary Artery Pressure
Two types of respiratory failure
Type I
Type II
Type 1 Respiratory Failure
pO2 < 8 kPA
pCO2 <6 kPA
Type II respiratory failure
pO2 < 8 kPA
pCO2 >6 kPA
Causes of Hypoxaemia
Hypoventilation
Diffusion Impairment
Shunting
V/Q mismatch
Hypoventilation causes type II Respiratory failure as
Slide 33
Diffusion Impairment
Gaseous diffusion impairment due to pulmonary oedema
Blood diffusion impairment due to anaemia due to less haemoglobin being present for O2 to then diffuse into
Membrane diffusion due to interstitial fibrosis- membrane can thicken
Physiologic V/Q mismatch
V= ventilation (air going into the alveoli)
Q= Perfusion (amount of blood flow to the alveoli)
The lung is divided into 3 zones- the Ventilation: Perfusion ratio is higher at the apex of the lung. Towards the base of the lung the ventilation perfusion ratio is decreased
Average Ventilation: Perfusion ratio is 0.8. This means there is more perfusion to the lungs and alveoli than there is ventilation. This differs through different parts of the lung and with diseases of the lung.
In zone 1- apex of lung- the blood travelling to here will be decreased because of gravity- decreased perfusion at apex
So In zone 3- increased perfusion
Therefore, at the apex of the lung we have wasted ventilation as all the gas that goes to the alveoli is not exchanged efficiently because theres less perfusion
At the base of the lung there is wasted perfusion as there is too much perfusion relative to ventilation
Ventilation and perfusion is overall higher at the base of the lungs than the apex of the lungs
The part of the lung Above the heart has larger alveoli. There’s reduced pulmonary intravascular pressure because of less blood flow to this area and so less perfusion.
Because of the larger alveoli there’s also lower rates of ventilation. Despite the reduced ventilation the perfusion is much more lower so there’s wasted ventilation
As you go to the the base of the lungs the alveoli becomes smaller because of the difference in intrapleural plush pressure
More perfusion and more ventilation as the smaller alveoli are able to expand more. But ventilation increase isn’t as much as the increase in perfusion so the V:Q is lower here
In zone 1: alveolar pressure is highest
Pulmonary artery pressure is next highest
Pulmonary vein pressure is lowest
In zone 2: Pulmonary artery pressure exceeds alveolar pressure which then exceeds pulmonary ventilation
In zone 3: alveolar pressure exceeds pulmonary ventricular pressure which exceeds pulmonary artery pressure
In summary-
The ventilation increases from the apex of the lung to the base of the lung
But perfusion to the lungs and alveoli increases a lot more from the apex of the lung to the base of the lung
Shunt v V/Q mismatch
V/Q=0 This is termed absolute pulmonary shunt
No oxygen is coming into the alveoli, so overall low oxygen levels leaving the system but there is still a large build up of CO2 as it cannot be breathed out
What is a shunt?
Blood going through the lung but not taking part in gas transfers
Perfusion without ventilation
Slide 43
Causes of shunt- blood going through lung/part of lung but not taking part in any gas transfer
Physiological
Bronchial arteries
Thesebian veins
Intracardiac
Eg VSD - R-L Shunt (Eisenmenger’s Syndrome)
Pulmonary
ArterioVenous Malformation (AVM)
Complete Lobar Collapse
Eisenmenger’s Syndrome
Ventricular Septum defect (VSV)- Hole in septum
When baby is born, blood will be higher in the left ventricle. So if this VSV is not closed then the net direction of flow will be left to right so there will be more blood than normal going through the arteries under higher pressure
The pulmonary arteries, especially small pulmonary arteries, are not designed to cope with this, resulting in damage to them. The pulmonary arteries respond by narrowing down, increasing pressure and increasing resistance
Once the resistance and pressure gets above a certain limit, then the pressure in the right ventricle is similar and sometimes higher than the left ventricle so there is now right to left movement.
So there begins to be deoxygenated blood coming from the SBC into the right atrium and right ventricle and then it starts going from right to left
So the deoxygenated blood is going straight into systemic circulation and is no longer taking part in just transfer
Patients with this syndrome have low oxygen levels
V/Q Mismatch
Physiological
Pulmonary Embolism: a clot occludes the pulmonary capillary pr artery causing reduced perfusion which will subsequently increase the perfusion rate causing a mismatch
Asthma
Pneumonia
Pulmonary Oedema
Hypoxia Pulmonary Vasocontstriction
At high altitude all the vessels narrow down so one of the ventricles has a high pressure
Hypoxic Pulmonary Vasoconstriction
Poorly understood
Local action of hypoxia on pulmonary artery wall
Weak response as little muscle
Aims to improve V/Q matching
Local hypoxia (eg peanut)
Generalised hypoxia (eg altitude)
Diseases of the Pulmonary Circulation
Pulmonary Embolism caused by VQ mismatch
Pulmonary Hypertension caused by increased PVR
Pulmonary AVMs caused by shunt
