Pulmonary perfusion & V/Q Flashcards
week 5
Pulmonary vs Systemic circulation
Purpose:
P = Heart –> Lungs O2 and CO2
S= Heart –> body delivery and waste pick up
Pulmonary
Shorter vessels
Decreased pressure
Larger diameter = less resistance
Limited ability to control regional distribution of blood
Blood flow markedly affected by gravity
What physiological variation can cause Pulmonary volume to shift from normal?
- high thoracic pressure = expels blood out
posture (supine → erect will decrease by 1/10)
increased systemic vascular tone forces blood into pulmonary circulation
features of Zone 1
No blood flow in cardiac cycle
Avl cap pressure never exceeds alv air pressure (arterioles crushed)
High PO2 and Low PCO2
PA> Pa > Pv
features of Zone 2
Intermittent blood flow as PA pressur peaks
Pa> PA > PV
features of Zone 3
Continuous blood flow
Pa> PV > PA
What does V and Q stand for?
V = alveolar Ventilation (VA)
Q= Perfusion (CO)
what is the purpose of hypoxic pulmonary vasoconstriction?
to improve V/Q matching**
decreased Blood flow to poorly ventilated Alveloi and redirects blood to ventilated alveoli
What is the normal value of V/Q?
0.8
How does the V/Q ratio change throughout 3 zone model?
progressively decreases from Z 1 –> 3
What is the V/Q ratio?
the relationship between the amount of air reaching the alveoli and the amount of blood flowing through the capillaries surrounding those alveoli.
What are the origins of a High V/Q ratio?
Pulmonary embolism (blood clot in lung, but V normal)
Pulmonary Vascular Disease
Localised Vascular Compression
What are the consequences of high V/Q ratio?
increased VD (less air participating in gas exchange)
Hypoxemia
increased A-a gradient (O not as efficiently transferred)
Hypocapnia
What is actually happening in High V/Q ratio?
More air is reaching the alveoli than there is blood flow to pick up the oxygen.
What is the origin of low V/Q ratio?
Airway obstructions:
Asthma
COPD
Penumonia
Alveolar collapse
Pulmonary Edema
Neuromuscular
What actually occurs in a low V/Q ratio?
enough blood flowing past alveoli, but not enough air reaching to oxygenate
What are the consequences of Low V/Q ratio?
Hypoxemia
Hypercapnia (high CO2)
increased A-a gradient
increased work of breathing
What effect does gravity have on pressures in the lung?
Uneven ventilation and perfusion (due to gravitational gradient in Ppul)
Alveoli on apex = stretched = less compliant
Alveoli on base = squashed due to gravity = better ventilated as has more compliance.
what impact does exercise have on pulmonary blood flow?
Increased blood flow by 4-7 fold
Increased Pulmonary blood flow = increased SA for diffusion
Improved V/Q
Why does PV have a lower PO2?
decreased time in pulmonary circulation
decreased oxygenation
What is a shunt?
Alveoli are perfused but NOT ventilated
What is the impact of a shunt on VQ?
V/Q= 0, decreased PAO2, increased PACO2
What is the impact of a pulmonary Embolism on VQ?
Effect on V/Q: V/Q of the affected alveolus = infinity (dead space), decreased PAO2, increased PACO2
What is a pulmonary embolism?
Foreign fragments (tumour, thrombus) block a blood vessel
What are the 4 starling pressures?
Capillary Hydrostatic Pressure (HPcap)
Capillary Oncotic pressure (OPcap)
Interstitial fluid pressure (HPif)
Interstitial oncotic pressure (OPif)
Which Starling pressures favour filtration?
HPcap, OPif and HPif
Which Starling pressures favour reabsoprtion?
just OPcap
How does LSHF cause a pulmonary Oedema?
LV failure –> backflow into LA and Pul Circulation –> increased HPcap –> favours filtration –> Pulmonary Oedema
How can pneumonia cause pulmonary oedemas?
Infection –> inflammation –> increased pul capillary permeability –> protein and fluid leakage
What is the purpose of Hypoxic pulmonary vasconctriction?
Défense against Shunts
Promotes blood flow to areas of lung that are better ventilated.
What is the impact of lower pressure in Systole and Diastole in the PA on the RV?
decreased afterload (less pressure to overcome = preserves RV contractility, SV and mechanical efficiency)
Describe the systemic and pulmonary Pressure and Resistance.
Systemic
- high P and R = so can get blood to all tissues
Pulmonary
= low P and R = efficient due to short route and needs to protect delicate lung tissue
What is the impact of hypoxia on Pulmonary Vs Systemic Vasculature?
Pulmonary
- Hypoxic Pulmonary Vasoconstriction
- if HPV becomes widespread = increased PA pressure and RSHF
Systemic
- Hypoxic systemic vasodilation
- = drop in BP, circulatory shock
How does chronic pulmonary disease lead to RSHF and Peripheral oedema?
CPD –> chronic hypoxia in lungs –> HPV –> becomes widespread –> increased Res –> increases afterload –> RV hypertrophy to compensate –> overworked RV = RSHF –> blood back flows –> oedema
Why is blood flow more pulsatile in capillaries in zone 2?
due to the cyclical pumping of the heart. Unique pressures of Zone 2 make pulsatile nature more pronounced
Why does a patient with Haemorrhage have Zone 2 –> zone 1?
Decreased Blood Volume:
Reduced Pulmonary Arterial Pressure
What factor can lead to the formatiom of a oedema?
Increased Hydrostatic pressure (hr failure, venous insufficiency, DVT)
Decreased colloid osmotic pressure
increased capillary permeability
lymphatic obstruction (lymphedema)
which areas of the lungs are best
a) ventilated
b) perfused
a- base of lungs
b- bases of lungs
