ventilation/perfusion relationships Flashcards
how does concentration of oxygen, carbon dioxide and nitrogen, PO2, PCO2 change from inspiration to expiration
O2 - decreases CO2 - increases N2 - remains the same PO2 - decreases PCO2 - increases
how is the partial pressure of gas calculated
air is saturated in water vapour
water in water vapour exerts water vapour pressure of 47 mmHg
Pb - 47 %
or Pb - %gas
how are arterial blood gas partial pressures measured
using arterial blood samples and a blood gas analyser
how are alveolar PCO2 values measured
by measuring end-tidal values
what is the thickness of alveolar capillary membrane
very thin 0.5 µm (microns)
what is the purpose of the alveolar capillary membrane being very thin
there is rapid, complete equilibration of O2 and CO2 between the alveolar gas and the blood (perfusion rather than diffusion limited)
diffusion is 1.___ proportional to 2.____ and 3.___ and 4.___ proportional to 5.____
- directly
- pressure difference
- surface area
- inversely
- distance
gas exchange in emphysema is reduced by
- pressure difference
- surface area
- increased distance
- resistance
2.reduced surface area
gas exchange in fibrosis a is reduced by
- pressure difference
- surface area
- increased distance
- resistance
3.increased distance
diffusing capacity or transfer factor definition
the extent to which oxygen passes from the air sacs of the lungs into the blood
diffusing capacity or transfer factor calculation
Rate of transfer of gas from lung to blood/ Partial pressure difference = Rate of trans/PACO - PaCO
why carbon monoxide used for diffusing capacity calculation
because the binding to haemoglobin is so strong and the PCO in the blood is zero so the partial pressure difference is the alveolar PCO
how is the diffusing capacity calculated in practice
subject inhales a CO mixture, holds their breath for 10 s, exhales and the alveolar air analysed
CO consumption and alveolar PCO are measured and diffusing capacity is calculated
diffusing capacity and transfer factor calculation units
DLCO = ml/min/kPa
TLCO - mmol/min/kPa
diffusing capacity is depends on what factors
haemoglobin, age, sex
reduced diffusing capacity is because of
lung fibrosis, pneumonia, oedema, emphysema
what has a greater oxygen pressure arterial Po2 or alveolar Po2 and why
arterial (a) blood PO2 is slightly less (95 mmHg) than alveolar (A) PO2 (A-a PO2 gradient) because of venous admixture which is caused by - anatomical shunt, ventilation/perfusion mismatch
if the alveolar (A) is greater than arterial (a) PO2 what does suggest
gradient suggests a problem with gas exchange, i.e., anatomical shunting or with (ventilation/perfusion)V/Q mismatch
respiratory exchange ratio
CO2 production / O2 consumption
why is ventilation/perfusion matching important
for normal gas exchange in the lungs
what conditions of the alveoli and pulmonary capillaries must be in
alveoli must be in close proximity to pulmonary capillaries
average V/Q ratio in lung
0.8
is there a slight mismatch of ventilation to perfusion yes or no
yes
no consistent throughout the lung
ventilation and perfusion at the base of lungs
high ventilation - not stretched, high compliance
higher perfusion -
lower PAO2 and V/Q ratio
ventilation and perfusion at apex of lungs
low ventilation - not stretched, high compliance
lower perfusion - less blood flow
higher PAO2 and V/Q ratio
what would the V/Q ratio look like in respiratory disease
increased V/Q ratio - overventilation and underperfusion
or
decreased V/Q ratio.- underventilation and overperfusion
respiratory diseases that cause high V/Q ratio
increased V/Q ratio - overventilation and underperfusion
embolus and emphysema
how does increased V/Q ratio mean to alveolar Vd
wasted ventilation
how does increased V/Q ratio mean to alveoli
“shunting” where deoxygenated venous blood bypasses the exchange area and enters the left heart causing arterial hypoxaemia
respiratory diseases that cause low V/Q ratio
obstruction - COPD , asthma, bronchitis
small V / Large V
dead space
ventilation and no perfusion V/Q = infinity
V/Q = infinity is indicative of what respiratory disease
pulmonary embolism
true shunt
where blood flows through a region with zero ventilation
examples of shunts
abnormal right-left shunts in the heart, atelectasis, consolidation
what will oxygen therapy improve
oxygen therapy will improve PaO2 with a low V/Q ratio but not with “true shunt
how does overventilation/underperfusion affect alveolar PO2 PCO2
increases alveolar PO2 and decreases PCO2
how does underventilation/overperfusion affect alveolar PO2 PCO2
decreases alveolar PO2 and increases PCO2
how does a decrease in PAO2 and increase in PACO2 affect smooth muscles in airway and pulmonary arterioles
causes relaxation of airway smooth muscle but contraction of pulmonary arterioles
why does expired air have a higher percentage of oxygen than alveolar air
mixing with dead space air prior to exhalation
what is reduced in emphysema and fibrosis
exchange
