Ventilation - Perfusion Flashcards
What is perfusion?
blood flow to the lung
What is hypoxia?
localized lack of oxygen
What is hypoxemia?
a systemic decrease in O2 content
What are some causes of hypoxemia?
hypoventilation, diffusion impairment, Low PiO2/F1O2, shunt, ventilation-perfusion mis matches
What is the alveolar gas equation?
PaO2 = FiO2 x (Patm - PH2O) - (PaCO2/R)
What is FiO2 in the alveolar gas equation?
the fraction of inspired oxygen - 21% in room air
What is PH2O at room air?
47 mmHg
What is R in the alveolar gas equation?
volume of CO2 produced per unit time/ colume of O2 produced per unit time
What is R normally in the aveolar gas equation?
0.8
What is PaCO2 in the alveolar gas equation?
arterial CO2 pressure
When a patient hypoventilates, what hapens to CO2 in the body?
it builds up (normally 40 mmHg, increases to 80mmHg)
Does hypoventilation affect the A-a gradient?
no; PCO2 increases and PAO2 decreases but it does not affect the gradient
What type of therapy does hypoventilation respond to?
o2 therapy
What is diffusion impairment caused by?
exercise, high altitude, low PiO2, and lung pathologies
Why should thickness impact diffusion?
the oxygen has to diffuse out form the alveoli to the RBC, if you increase the thickness, the diffusion will decrease
How do you fix an impairment caused by increased thickness?
increase the oxygen
What are normal physiologic shunts?
bronchial circulation and thebesian veins
What do thebesian veins do?
drain the myocardium of the heart and some of them drain into the left ventricle
What are some pathological shunts?
arterial-venous anastomoses, absolute intra-pulmonary shunts/true shunts, patent ductus arteriosus, foramen ovale, intraventricular septal defects
Can pathological shunts be fixed by O2 therapy?
no
What must be in sync for optimal gas exchange to occur?
the ventilation-perfusion ratio
What is the effect of more negative intra-pleural pressure at the apex of the lung cause?
greater transmural pressure gradient, larger alveoli has less compliance and less ventilation, poor perfusion
What is the effect of less negative intra-pleural pressure at the base of the lung?
lesser transmural pressure gradient, smaller alveoli, greater compliance, more ventilation, high perfusion
How does a blockage at the apex of the lung affect the V/Q ratio?
it causes it to be 0
How does an alveolar blockage affect the V/Q ratio?
it causes it to be infinate
What level of hemoglobin saturation causes hypoxemia?
60 mmHg or below
What is the physiologic response to V-Q mismatches?
Hypoxic vasoconstriction, brisket disease in cattle, right side heart failure in chicken, COPD, asthma, pulmonary embolism, pneumonia
What does hypoventilation do to PaCO2 and the A-a gradient?
increases PaCO2, A-a remains normal
What does diffusion impairment do to PaCO2 and the A-a gradient?
PaCO2 remains normal, A-a gradient is increased
What does Low PiO2 do to PaCO2 and the A-a gradient?
PaCO2 decreases, A-a remains normal
What does a right-left shunt do to PaCO2 and the A-a gradient?
PaCO2 remains normal, A-a is increased
What does a V-Q mismatch do to PaCO2 and the A-a gradient?
PaCO2 remains normal, A-a increases
