Ventilation perfusion ratio Flashcards
What machine determines oxygen saturation?
pulse oximeter
What is the calculation to work out how much oxygen is carried by haemoglobin in blood?
1.34ml xHb (g/L) x haemoglobin saturation of O2
1.34ml = amount of oxygen haemoglobin would carry if it was 100% saturated
What % of oxygen carriage is dissolved in the blood? How is this measured?
1.5%
measured in a blood gas - PaO2
Define hypoxemia
reduction in PaO2
Causes of hypoxemia
anatomical shunt (perfusion that bypasses lung)
intrapulmonary shunt (absent ventilation to areas being perfused eg. atelectasis)
VQ mismatching (low ventilation to areas being perfused)
hypoventilation (underventilation of lung units eg. resp muscle weakness, causes hypercapnia and hypoxemia)
diffusion (fibrosis)
reduced barometric pressure
Define hypoxia
local or systemic failure of oxygen delivery
What can cause hypoxia?
poor ventilation
poor circulation
histiotoxic (poisoned mitochondria so oxygen can’t be utilised)
severe anaemia or dyshaemoglobinaemias (not enough Hb/Hb that don’t bind well to oxygen)
Define alveolar ventilation
rate at which new air reaches gas exchange areas
Define dead space air
some air that is breathed never reaches gas exchange areas but fills respiratory passages (eg. nose, pharynx, trachea)
about 150ml
Alveolar ventilation rate equation
VA = Frequency x (VT-VD)
VA = volume of alveolar ventilation/min
Frequency = resp rate
VT = tidal volume
VD = dead space volume
Alveolar air equation
relationship between CO2 and O2 in the alveoli
PAO2 = (PB-PH2O) x FIO2 - (PACO2/R)
PB = barometric pressure
PH2o = water vapour pressure
FIO2 = fraction of O2 in inspired air (0.21)
R = respiratory quotient (0.8)
if perfect transport and no venous admixture, PAO2 = PaO2
What is Aa gradient?
difference between ideal PAO2 and PaO2
(normally less than 15mmHg)
What equation estimates Aa gradient in mmHg?
(Age/4)+4
Why is A-a gradient useful?
can help see what pathological process is driving hypoxemia
Describe expansion of alveoli throughout the lung
alveoli in top of lung are more expanded than those at bottom (due to gravity)
Describe pleural pressure throughout the lung
pleural pressure is less (more negative) at apex than base of lung
with inspiration, pleural pressure decreases further
as inspiration begins, alveoli in lungs are at different lung volumes
Describe pulmonary circulation
begins with right atrium
deoxygenated blood pumped via right ventricle into pulmonary artery
pulmonary artery divides into right and left main artery then enters lung tissue
ends in mesh-like network of capillaries where RBC flow single file through alveolus
capillaries drain into pulmonary venules
finally 2 large pulmonary veins emerge from each lung to empty into left atrium (4 pulmonary veins total)
Why do alveoli not fill with fluid?
normally pulmonary capillaries and lymphatics maintain a slight negative pressure in interstitial spaces
excess fluid will be sucked back into interstitial space from alveoli
What causes diffusion problems in the lung?
fibrosis
What happens to the VQ ratio as you go from the bottom of the lung to the top?
increases
When are ventilation and perfusion matched?
when pulmonary blood flow is proportionally matched to the pulmonary ventilation
results in greatest efficiency for gas exchange
How to calculate VQ ratio for alveolus and a lung
single alveolus = alveolar ventilation/capillary blood flow
lung = total alveolar ventilation/cardiac output
Describe interpretation of VQ ratio
VQ ratio >1 = ventilation exceeds perfusion
VQ ratio <1 = perfusion exceeds ventilation
What is a normal VQ ratio?
0.8 (4L/min / 5L/min)
What happens if areas of low PO2 or high PCO2 detected?
arteries constrict so blood is diverted to better oxygenated areas
mechanism thought to involve inhibition of K channels on smooth muscle cells
Agents that affect pulmonary vascular resistance - dilators
increased PAO2
decreased PACO2
high pH
H2 agonists
PGI2 (prostacyclin)
PGE1
beta-adrenergic agonists
bradykinin
theophylline
acetylcholine
nitric oxide
Agents that affect pulmonary vascular resistance - constrictors
decreased PAO2
increased PACO2
low pH
H1 agonists
thromboxane A2
PGF2alpha
PGE2
alpha-adrenergic agonists
serotonin
angiotensin 2
Describe VQ parameters of a normal lung
VA = normal
Q = normal
VQ = ~1
Describe blood gases in outflow of a normal lung
PO2 = normal
PCO2 = normal
O2 content = normal
CO2 content = normal
Describe VQ parameters of dead space
VA = normal
Q = 0 (no perfusion)
VQ = infinity
Describe VQ parameters of dead space effect
VA = normal
Q = low
VQ = high
Describe blood gases in outflow of dead space effect
PO2 = high
PCO2 = low
O2 content = normal
CO2 content = low
Describe VQ parameters of shunt effect
VA = low
Q = normal
VQ = Low
Describe the blood gases in outflow of shunt effect
PO2 = low
PCO2 = high
O2 content = low
CO2 content = high
Describe the VQ parameters of true/anatomical shunt
VA = 0
Q = normal
VQ = 0
Describe the blood gases in outflow of true/anatomical shunt
PO2 = low
PCO2 = high
O2 content = low
CO2 content = high
