Pulmonary Circulation and Lung Disease- an Overview Flashcards
What is the primary function of the pulmonary circulation
To bring venous blood into contact with the alveoli to facilitate gas exchange
What are the secondary functions of the pulmonary circulation
protect the body from thrombi/ emboli; metabolism vasoactive substances- angiotensin I (via ACE) to angiotensin II blood reservoir (500ml/10%) mobilised in shocked states
How does deoxygenated blood travel to the right atrium
Via systemic system
Describe the pulmonary circulatory system
Carries the same volume of blood as the systemic system but at much lower pressure
Describe resistance in the pulmonary circulation
High flow low pressure circuit= low resistance
What is pressure in the pulmonary ertery
25/8 compared to blood pressure in the aorta which is 120/70
Describe pulmonary artery/ arterioles
Deoxygenated blood, thinner walls than the aorta and more compliant, shorter containing less elastin and smooth muscle thus less ability to constict than thick walled muscular systemic arterioles
Describe pulmonary capillaries
Unlike systemic cap’s frequently arranged as a network of tubular vessels with some interconnections. Mesh network together in the alveolar wall- blood flow as a ‘single sheet’. Cap walls are exceedingly thin. More of a dense cap bed than a network, whole cap bed can collapse if local alevolar pressure exceeds cap pressure
What does the fine meshwork of capillaries around each alveoli result in
An increase in surface area for gas exchange
What happens when you stand up
There is hydrostatic pressure resulting in regional perfusion, therefore there is higher blood flow at the base of the lungs
Why do hydrostatic pressure have much stronger influences on pulmonary blood flow
Pulmonary circulation is low pressure
Describe the pressures in an area with no flow
Alveolar pressure > pulmonary artery pressure > pulmonary venous pressure
What is PA
Alveolar pressure
What is Pa
Pulmonary artery pressure
What is Pv
Pulmonary venous pressure
Describe the pressures in an area with pulsatile flow
Pulmonary artery pressure > alveolar pressure > pulmonary venous pressure
Describe the pressures in an area with continuous flow
Pulmonary artery pressure > pulmonary venous pressure > alveolar pressure
Explain the pressures observed in the lung
Alveolar pressure (PA) at end expiration is equal to atmospheric pressure (0 cm H20 differential pressure, at zero flow), plus or minus 2 cm H2O (1.5 mmHg) throughout the lung. On the other hand gravity causes a gradient in blood pressure between the top and bottom of the lung of 20 mmHg in the erect position (roughly half of that in the supine position). Overall, mean pulmonary venous pressure is ~5 mmHg. Local venous pressure falls to -5 at the apexes and rises to +15 mmHg at the bases, again for the erect lung. Pulmonary blood pressure is typically in the range 25 - 10 mmHg with a mean pressure of 15 mmHg. Regional arterial blood pressure is typically in the range 5 mmHg near the apex of the lung to 25 mmHg at the base
When is no flow observed (Zone 1)
Not observed in a normal healthy human lung. In normal health pulmonary arterial pressure exceeds alveolar pressure in all parts of the lung. It is generally only observed when a person is ventilated with positive pressure. In these circumstances blood vessels can become completely collapsed by alveolar pressure and blood does not flow through these regions. They become alveolar dead space
When does pulsatile flow occur (Zone 2)
Part of the lungs about 3cm above the heart. In this region blood flows in pulses. At first there is no flow because of obstruction at the venous end of the capillary bed. Pressure from the arterial side builds up until it exceeds alveolar pressure and flow resumes. The dissipates the capillary pressure and returns to the start of the cycle
When does continuous flow (Zone 3) occur
Comprises the majority of the lungs in health. There is no external resistance to blood flow and blood flow is continuous throughout the cardiac cycle
In which zone is the ventilation perfusion ratio higher when a person is standing
Zone 1 than zone 3. If you increase pressure to the lungs too much you turn off blood flow to the lungs and therefore worsen hypoxia
What does regional ventilation mean
The lower parts of the lung (base) are better ventilated than the top part (apex)
What are the lungs supported by
Only the hilum
What intrapleural pressure exists at the apex of the lungs
Negative intrapleural pressure
Why are the alveolar at the apex larger
Because there is negative intrapleural pressure at the base of the lungs
What happens to the alveoli when you take a deep breath
The alveolar at the base get much bigger as more air goes to the base
Describe intrapleural pressure
It is always below atmospheric pressure during both inspiration and expiration
Describe intrapleural pressure gradients
Exist from the upper lung region to the lower lung region
What are the changes in intrapleural pressure due to
gravity, distribution of weight in the lungs, lungs are suspended at the hilum, lung base weighs more than the apex (increased blood flow), greater negative pressure in the upper regions causes the alveoli in those areas to be more expanded than alveoli in the lower regions, many alveoli are close to or at their total filling capacity
Compare the compliance of alveoli in the upper and lower regions
Compliance of alveoli in the upper regions is lower than compliance of the alveoli in the lower regions
Where is ventilation much greater and more effective
In the lower lung regions
What is a normal V/Q ratio
0.8-1.2 ventilation/ perfusion ratio
What is alveolar minute ventilation
4-6 L
What is normal cardiac output
5 L (500ml per tidal volume)
What does Va/Q determine
The gas exchange in a single unit
What do regional differences in Va/Q ratio cause
A pattern of regional gas uptake
What does Va/Q inequality impair
Uptake or elimination of gases
Where are blood flow and ventilation higher
At the base of the lung than the apex
What happens as blood flow increases
There is a decrease in resting lung therefore the mismatch is greater
What is hypoxaemia
An abnormally low concentration of oxygen in the blood
What are the 4 causes of hypoxaemia
Hypoventilation, diffusion limitation, shunt, Va/Q inequality
What is a capillary shunt
There is an issue present with the alveolus itself e.g. pneumonia
What does pulmonary fibrosis mean
There is a thickening of the alveolar membrane which presents as a shunt-like effect
What happens in pulmonary embolism
Ventilation is not affected but there is hypoxia so the blood is blocked by the embolism resulting in hypoxia
What happens in bronchospasm
There is a mismatch between ventilation and perfusion resulting in hypoxia
What happens in bronchoconstriction
There is gas trapping resulting in dynamic hyperinflation resulting in a barrel chest as you can’t get air out. Gas trapping results in rapid accumulation of carbon dioxide as there is no gas exchange taking place
Describe the pressure of the systemic system
High pressure system, mean systemic pressure 70-105 mmHg, easily measured
Describe the pressure of the systemic system
Low pressure system, mean pulmonary pressure 10-22 mmHg, estimated via ECHO, directly measured with pulmonary artery pressure
Why do we estimate rather than measure pulmonary pressure
Very invasive to directly measure pulmonary artery pressure, therefore we use ECHO to estimate it
What is used to directly measure pulmonary artery pressure
Swan-Ganz catheter
What is PCW
Pulmonary capillary wedge pressure. PCW is an estimate of pressure in the left atrium
What does PCW pressure record changes in
Pulmonary venous pressure and left atrium pressure which has profound effects on gas exchange
What happens if the left side of the heart is failing
There is an increase in pulmonary capillary wedge pressure resulting in fluid leaking out into the interstitial space which decreases the distance over which gas exchange occurs and decreases the area available for gas exchange as the alveoli are filled with fluid
What do you do when a person has pulmonary oedema
You give them oxygen to help improve gas exchange and diuretic to get rid of excess fluid which decreases the amount of fluid around the lungs and reduces the issues of increased distance for gas exchange and reduced surface area
What is Type I ventricular failure
An issue with oxygen but not carbon dioxide levels, carbon dioxide is 19x more soluble than oxygen, therefore in a hypoxic state you have normal carbon dioxide levels
What does pulmonary hypertension result in
Overloading the right ventricle which can result in sudden cardiac death
Describe vascular remodelling in pulmonary arterial hypertension
Smooth muscle proliferation/ fibroid necrosis and narrowing of the lumen ultimately leads to right heart failure and death
What does pulmonary vascular resistance equal
PVR= mPAP-PCWP/ cardiac output mPAP= mean pulmonary arterial pressure PCWP= pulmonary capilliary wedge pressure
How do you work out mPAP
(cardiac output x pulmonary vascular resistance) x PCWP
What changes happen to the right ventricle in pulmonary arteriole hypertension
Thickening of the membrane, breakdown of elastin, decrease in radius therefore increase in resistance and in situ microthrombi which creates resistance to blood flow
What does the thickened right ventricular wall in pulmonary hypertension result in
You get a thickened right ventricular wall and the intraventricular septum bows into the left ventricle meaning that the blood pressure in the pulmonary system is higher than in the systemic system
What happens when the pressure in the pulmonary system becomes greater than the pressure in the systemic system
Poses a significant risk of heart failure and death
Define PAH
mPAP >25 mmHg at rest (or > 30 mmHg with exercise) with normal PCWP (240 fynes/s/cm5)
How can the diagnosis of PAH be made
With right heart catheterisation
Describe the management of pulmonary hypertension
‘Targeted therapies’: prostanoid analogues, enothelin receptor antagonists (ERAs), phosphdiesterase-5 (PDE-5) inhibitors. Manage via inhibition. Viagra was used for management as it was designed as a blood pressure tablet. Use warfarin to prevent in situ microthrombi
What can you do if you treat PAH early
You can stop vascular remodelling and vasoconstriction
What is the diagnosis of PAH
Now 8 years, used to be 2. Class also correlates to survival: triple theraphy has increased LE
Describe cor pulmonale
Enlargement and failure of the right ventricle as a response to increased vascular resistance or high lung blood pressure. In cor pulmonale there is an issue with the pulmonary circulatory system meaning that the right ventricle can’t keep up with the work and you get right side heart failure
In order to be classed as cor pulmonale where must the cause originate
In the pulmonary circulation system
What are the two major causes of cor pulmonale
a) tissue damage leading to vascular changes e.g. disease, chemical agents etc. b) hypoxic pulmonary vasoconstriction
Describe the pathophysiology of cor pulmonale
Pulmonary vasoconstriction, anatomical changes in vasculature, increased blood viscocity, idiopathic or primary pulmonary hypertension
Compare left and right side heart failure
Left side heart failure results in pulmonary oedema. Right side heart failure results in peripheral oedema and raised JVP
What are the signs and symptoms of cor pulmonale
SOB which occurs on exertion but when severe can occur at rest, cyanosis, ascites, swelling of the ankles and feet (peripheral oedema), enlargement or prominent neck and facial veins, raised JVP. You get fluid overload of the systemic circulatory system
What is the treatment of cor pulmonale
Elimination of irritant e.g. smoking, correct hypoxia, diuretics
Describe pulmonary vascular resistance
RV pumps mixed venous blood through pulmonary arterial tree. All cardiac output (5L/min) pumped through at a much lower pressure. 10mmHg gradient pulmonary, 100mmHg gradient systemically. Low pulmonary pressure allows blood to flow through the lungs. Pulmonary vascular resistance (PVR) is low- 1/10 of systemic vascular resistance (SVR)
Why is PVR normally so low
R= 8ηl/ πr4 where η= viscosity, l= vessel length, r= vessel radius. Therefore, small changes in radius result in a large change in resistance. An increase in radius by 1 would result in an increase in resistance by 16
What happens to capillaries when you breath in
They become more circular as there is an increase in radius and a decrease in resistance
What happens to capillaries when you breathe out
They become less circular as there is a decrease in radius and an increase in resistance
What happens as you increase pressure in the lungs
Decrease in pulmonary vascular resistance. These changes are able to occur due to a redundancy in the lung capillary bed
What does an increase in cardiac output result in
Recruitment of capillaries not being used and therefore distention. As radius of blood vessels increases there is a decrease in pulmonary resistance which promotes blood flow. A decrease in pulmonary vascular resistance means that gas exchange is maintained (such as during exercise). Pressure changes that occur are unique to pulmonary circulation
What does an increase in perfusion pressure cause in systemic circulation
An increase in SVR
Why is PVR normally so low
Resting conditions some capillaries are partially or completely closed (especially at top of lungs) because of low perfusion pressure. Primary cause for decrease in PVR as CO increases. As blood flow increases pressure rises and closed vessels open lowering PVR
Secondary cause is distension due to the fact that capillaries are thin walled and highly compliant
Why is a low PVR useful
A fall in PVR as CO increases opposes the tendency of blood velocity speeding up with increased flow rate maintaining adequate time for gas exchange. Increased surface area for gas exchange. Protective effect against pulmonary oedema (pulmonary oedema is an abnormal collection of fluid in alveolar space impairing gas exchange)
Describe what happens during exercise
CO 5L/min to 25L/min. Without recruitment/ distension rise in CAP pressure which leads to fluid shifts out of the CAP into the alveolar space which minimises the load on the RV. As a result there is a decrease in pulmonary vascular resistance so gas exchange can be maintained during exercise
What does the fact that the left ventricle is cylindrical in shape mean
It is better at pumping out blood at a high pressure
What is regulatory mechanism of hypoxia in the systemic circulation
Local vasodilation
What is regulatory mechanism of hypoxia in the pulmonary circulation
Local vasoconstriction (alveolar hypoxia/ blood hypoxemia)
What is regulatory mechanism of raised CO2 in the systemic circulation
Local vasodilation
What is regulatory mechanism of raised CO2 in the pulmonary circulation
Local vasoconstriction
What is regulatory mechanism of low pH in the systemic circulation
Local vasodilation
What is regulatory mechanism of low pH in the pulmonary circulation
Local vasoconstriction
What does an increase in PO2 resultin
A dramatic increase in blood flow
What does the pulmonary circulation have to allow regional adaptation of blood flow to reduced ventilation
Small precaps and post caps of lungs
Which PO2 determines blood flow
Alveolar PO2 not arterial PO2
What are the beneficial effects systemically of alveolar PO2 determining blood flow
diversion of blood to hypoxic, hypercarbic acidic tissue, high risk of tissue death
What are the beneficial effects pulmonary of alveolar PO2 determining blood flow
Diversion of blood flow away from poorly ventilated areas (hypoxic) to areas well ventilated, improves gas exchange
Describe generalised hypoxa
(severe COPD, severe pulmonary fibrosis, high altitude) results in vasoconstriction throughout the lungs which results in worse gas exchange and is detrimental
Describe regional hypoxic pulmonary vasoconstriction
One-lung ventilation is a common clinical example of regional HPV. HPV in the hypoxic atelectatic lung causes a redistribution of blood flow away from the hypoxic lung to the normoxic lung, thereby diminishing the amount of shunt flow (QS/QT) that can occur through the hypoxic lung. Inhibition of hypoxic lung HPV causes an increase in the amount of shunt flow through the hypoxic lung, thereby decreasing PaO2. Vasoconstriction throughout leading to rise PVR leads to PH and pathophysiological changes in pulmonary blood vessel
What happens if ventricles fail
The heart fails and there is a decrease in PAP
Describe WHO Class I PAH
Symptoms do not limit physical activity. Ordinary physical activity does not cause undue discomfort
Describe WHO Class II PAH
Slight limitation of physical activity. The patient is comfortable at rest, yet experiences symptoms with ordinary physical activity
Describe WHO Class III PAH
Marked limitation of physical activity. The patient is comfortable at rest, yet experiences symptoms with minimal physical activity
Describe WHO Class IV PAH
Inability to carry out any physical activity. The patient may experience symptoms even at rest. Discomfort is increased by any physical activity. These patients manifest signs of right heart failure
