VQ assessment Flashcards
Normal V/Q ratio (ideal)
1
regional abnormalities in VQ
•Upper lobes – ventilated but relatively underperfused (V/Q = 2.5). Lower lobes – perfused but relatively under ventilated (V/Q = 0.6). Normal – slightly less than ideal V/Q ~ 0.8
Local regulation with high VQ
Alveolar Pco2 drops > Increases local airway resistance > Decreases ventilation > Lowers V/Q
Local regulation with low VQ
Alveolar PO2 drops > hypoxic vasoconstriction > decreased perfusion > increased VQ
Describe compesation for low VQ
Increased VQ in other areas occurs, but this can not fully compensate for the low VQ b/c hemoglobin is already near saturation at normal levels of ventilation, so increasing ventilation has a limited benefit.
Describe CO2 levels in VQ mismatch
mild to moderate forms of obstructive diseases that involve VQ mismatch generally do not reduce arterial PCO2, owing to the fact that increases in PCO2 are generally countered by increases in ventilation. There is a limit though
What is dead space
VQ >1 : ventilation of unperfused airway/alveoli. Can be anatomic (air in the trachea, bronchi, bronchioles that does not come into contact with blood) or alveolar (unperfused alveoli) or phsyiologic (reduced efficiency of breathing with altered blood gases)
How does dead space affect PaO2 and PaCO2
Generally doesn’t cause hypoxemia ( low PaO2) unless severe. Can cause increased PaCO2. Dead space decreases with exercise
How can increased dead space increase PaCO2
Rapid and shallow breathing (which increases dead space) decreases alveolar ventilation which allows PaCO2 to increase
Causes of increased deadspace
Anatomic dead space: Rapid, shallow breathing (most of the tidal volume is in the conducting airways). Alveolar dead space: Acute pulmonary embolism, decreased cardiac output. Ventilation in excess of perfusion: ventilators or alveolar septal destruction (emphysema)
What is a shunt
V/Q <1: Blood passing through capillaries that does not get oxygenated
Amount of normal shunt
1-2% shunt is normal b/c bronchopulmonary venous anastomosis
How does shunt affect PaO2 and PaCO2
PaO2 decreases (arterial hypoxemia) and PaCO2 increases (arterial hypercapnea). But note that central chemoreceptors typically compensate for increased PaCO2
How does shunt respond to increased FiO2?
Minimal response in shunt b/c any extra O2 will be taken up by hemoglobin (compared to low VQ which responds to increased FiO2)
Causes of shunt
Filled alveolar space such as in heart failure (transudate) or pneumonia/ARDS (exudate). Anatomic causes include congenital heart disease, pulmonary fistula or vascular lung tumor
