ventilation perfusion ratio Flashcards
what levels of the bronchi does gas exchange occur in
17-23
why is pAO2 (alveolar) greater thsn paO2 (arterial)
diffusion is imperfect and so 100% of the O2 in the alveoli does not get transported into the blood
what does pO2 in an ABG measure and what is this a proxy for
partial pressure of O2 in the artery; proxy for blood leaving the lung
equation for the oxygen-carrying capacity of the blood
1.34mL x [Hb] (g/L) x Hb saturation of oxygen
Each gram of hemoglobin can carry about 1.34 ml of oxygen when fully saturated
what device is used to determine oxygen saturation
pulse oximeter
hypoxemia vs hypoxia
hypoxemia - reduciton in PaO2
hypoxia -local or systemic failure of oxygen delivery
what can cause hypoxemia (5)
reduciton in altitude; hypoventilation; diffusion; shunts; VQ mismatch
what can cause hypoxia (4)
ventilation (paO2 affected); circulation ( ↓CO = ↓O2 to tissue);
histiotoxic (Mt. not working);
sever anaemia/dyhaemoglobinaemias (O2 doesnt bind well)
what is alveolar ventilation
the rate at which new air reavched gas exchange areas
alveolar ventilation rate equation
Va = RR x (Vt-Vd)
Vt = tidal volume
Vd = dead space volume
what is the alveolar air equation
pAO2 = (pB-pH2O) x FIO2 -(pACO2/R)
pB - barometric pressure (usually 150mmHg)
pH20 - water vapour pressure (usually 147mmHg)
FIO2 - fraction of O2 in inspired air (0.21)
R - respiratory quotient (0.8)
what does the alveolar air equation describe
the ideal case of what pAO2 should be if there is perfect transport and no venous admixture => ideally pAO2 = paO2 but paO2 is affected by disease
what is the Aa gradient
the difference between ideal pAO2 and paO2 (should be <15mmHg)
what does an increased Aa gradient indicate
hypoxaemia
how to estimate an Aa gradient of a pt
Age/4 +4
what are 4 types (causes) high Aa gradient
intrapulmonary shunt (e.g. due to atelectesis, pneumonia, ARDS); intracardiac R ->L shunt (e.g. due to PFO, PDA); V/Q mismatch (e.g. due to acute PE, ILD, pneumonia); diffusion limitation (e.g. due to heavy exercise, severe ILD with exercise)
where in the lungs do alveoli receive more tidal volume and why
the base of lungs as these alveoli are underinflated and so more compliant
why are the alveoli smaller at the base
gravity causes increased pleural pressure at the base which causes reduced alveolar volume
why are the alveoli smaller at the base
gravity causes increased pleural pressure at the base which causes reduced alveolar volume
what is diffusion inversely proportional to
membrane thickness
what is diffusion inversely proportional to
membrane thickness
V/Q definition for lung
total alveolar ventilation/cardiac output
if V or Q exceeds the other how does this affect V/Q ratio
if ventilation exceeds perfusion: V/Q >1
if perfusion exceeds ventilaition: V/Q <1
what is a normal V/Q ratio (number)
around 0.8 (i.e. perfusion is slightly higher than ventilation)
how does the graph of V/Q change as rib number decreases (towards apex)
exponentially (y=b^x) i.e. ventilation exceeds perfusion as it goes towards the apex
paO2 values for hypoxemia vs hypoxia
arterial hypoxemia: paO2 <80mmHg (adult at sea level)
hypoxia: paO2 <60mmHg
6 major causes of hypoxemia
1.anatomical shunt (perfusion that bypasses lung)
2. intrapulmonary shunt (absent ventilation to areas being perfused)
3. V/Q mismatching (low ventilation to areas being perfused)
4. hypoventilation (underventilation of lung units)
5. diffusion
6. reduced barometric pressure
V/Q ratio at apex vs base
apex - 3.3
base - 0.8
what is the most common cause of pathological shunts
atelectesis - usually due to obstrution by mucous plug, airway oedema, foreign body, tumour etc.
how do most respiratory diseases cause hypoxemia
V/Q mismatch - individual airways will have varying degrees of abnormal ventilation but perfusion will be normally distributed -> alveolar and end capillary gases will vary according to degree of obstruction
what is the effect of the mixing of high and low V/Q regions on ABGs
combine to give: low O2, high CO2, low pO2, slightly high pCO2 –> peripheral and central chemoreceptors detect this –> increased ventilation –> result in: low O2 content, normal/low CO2 content, low pO2, normal/low pCO2
what mainly regulates pulmonary blood flow
pO2 and pCO2 - not autonomic regulation
how is pulmonary blood flow regulated
arteries constrict so blood is diverted to better oxygenated areas (though to involve K+ ch inhibition on smooth muscle cells)
examples of pulmonary vascular dilators
↓pACO2, ↑pAO2, ↑pH, H2 agonists, bradykinin, Ach, NO, b-adrenergic agonists, PCI2,PGE1
examples of pulmonary vascular constrictors
↓pAO2, ↑pACO2, ↓pH, H1 agonists, PGE2, a-adrenergic agonists, angiotensin II, serotonin
how does hypoventilation affect V/Q ratio and its consequence
V/Q ration ↓ -> underventilation brings less fresh gas to avleoli -> O2 levels in alveoli will decrease and CO2 levels increase
