Mechanisms of Hypoxia and Hypercapnia Flashcards
Most common causes of hypoxemia and hypercapnia
Alveolar hypoventilation
- abnormal lung mechanics(COPD, fibrosis)
- CNS depression (stroke, morphine, overdose)
- Weakness of resp muscles (MG)
Diffusion impairment
Right to left shunt - anatomic or physiologic…more hypoxia than hypercapnia
Vent/perf mismatch
Anatomic shunt
Venous blood enters left heart without gas exchange
Normal - part of venous from bronchial enters pumonary vein and LA and joins systemic arterial criculation
Abnormal - tetrology of Fallot
Physiologic shunt
Blood flow perfusiing the lung region that does NOT receive ventilation
Ventilation perfusion ratio
Va = Q normally so Va/Q = 1
In airway obstruction
Va=0 so Va/Q = 0
Right o left shunt is extreme case of V/Q mismatching
Will reach equilbirum with venous blood coming in
In PE
Q=0
Va/Q= infinity
Effect of physiologic shunt and PE on PO2 and PCO2
In physiologic shunt, lower PO2 and higher PCO2 (just venous blood)
In PE, PO2 wayyyyy higher (150), and PCO2 is 0
As V/Q increases, the PCO2, PO2 curve dives
Upright posture and its effect on regional differene of alveolar gas composition
Blood flow and ventilation both increase as you move down the ribs
Equal around rib three
The Va/Q will decrease as you move down…exponentially at first and then levels off
Means more oxygen is at the top
May play a part in the predominance of certain pulmonary diseases in the upper region of the lung
Hypoxemia cause by vent-perfusion mismatch
Acute and chronic lung dz can induce severe mismatch which can lead to hypoxemia
Blood coming from lung region with high V/Q ratio is already fulling saturated with O2, whereas blood from the low V/Q region carries low O2…you are not able to offset the amount
Obstruction effect on V/Q ratio
Will lower it (below 1)
Hypoxia in peripheral tissues
Low PO2 in tissues
O2 delivery = blood vlow * (CaO2)
Hypoxic hypoia
Anemic hypoxia
Hypoperfusion hypoxia
Histotoxic hypoxia
Lung function…low PO2…from hypoventilation, diffusion impairmne, R to L shunt and V/Q mismatch
Low Hb content, severe anemia or CO poisoning
Low cardiac output or insuffient blood flow to deliver O2 to tissue (hemorrhage or HA)
Cyanide poisoning
Measurements of each kind of hypoxia
Hypoxic - low everything
Anemic - normal PaO2 in the arterial blood but low everything else
Hypoperfusion - Normal PaO2 and CaO2
Histotoxic - Normal PaO2 and CaO2 but high PvO2 and CvO2
Cyanosis
Generated by deoxygenated Hbs in the capillaries
When HbO2 sat below 75-80%
NO central cyanosis in pts with anemia
2 types of cyanosis
Abnormal resp system and may be accompanied by peripheral
CV or resp system
Peripheral can also occur alone from inadequate blood circulation
Also in presence of abnormal Hbs
