Lung physiology

Question 1

Ventilator issues that cause hyperventilation (leads to resp alkalosis) vs causes of hypoventilation

Answer 1

Causes of hyperventilation:
1) High IPAP
2) High pressure support (IPAP -EPAP)
3) High back-up respiratory rate

–> How to adjust the ventilator setting in hyperventilation:
Reduce the IPAP
Reduce back-up resp rate
(noted that of ventilator memory showed >80% mandatory breaths (and median resp rate = back-up rate), it indicates that the pt is chronically hyperventilate and pt’s ventilation is fully controlled by the ventilator
- Sx of hyperventilation: headache, weakness, cramps, bloated abdomen, compensated resp alkalosis

Causes of hypoventilation:
1) Low pressure support
2) Low EPAP
3) Patient-ventilator asynchrony

–> How to adjust ventilator settingin hypoventilation:
Increase IPAP, reduce EPAP and increase back-up rate

Question 2

A-a gradient
- formula and corresponding causes of hypoxemia

Answer 2

A-a gradient = FiO2 x (atmospheric pressure - H20 pressure) - (PaCO2/0.8) - PaO2
* atmospheric pressure at sea level: 760mmHg
* FiO2 20% under RA
* H2O pressure: 47mmHg
* normal A-a gradient = (Age/4) + 4

Causes of hypoxemia:
Elevated A-a gradient:
1) VQ mismatch (e.g. CHF, PE, ARDS, atelectasis)
2) Shunt (e.g. ASD, pulmonary AVM)
3) Alveolar hypoventilation (e.g. interstitial lung disease, PCP)

Decreased A-a gradient:
1) Hypoventilation (e.g. COPD, CNS disorder, NMD)
2) High altitude