Critical Care Medicine-Neurologic Disorders>>>>>Hypoxemia and Oxygen Delivery
Question 1#

A 56-year-old male with advanced idiopathic pulmonary fibrosis presents to the ICU in respiratory distress. He is put on high flow nasal cannula with 50 L flow, 80% FiO₂ . ABG obtained has:

• a pH significant for 7.32
• PaO₂ of 80 mm Hg
• PaCO₂ of 20 mm Hg
• HCO₃ of 30

Given his underlying disease, what is the primary physiological aberration leading to the patient’s hypoxemia?

A. Hypoventilation
B. Reduced inspired oxygen tension
C. Right to left shunt
D. Diffusion limitation

Correct Answer is D

Comment:

Correct Answer: D

The patient may have several physiologic derangements, but given his diagnosis of fibrosis, diffusion limitation may be the primary physiologic aberration leading to hypoxemia. There are several different mechanisms of hypoxemia: hypoventilation, V/Q mismatch, right to left shunt, diffusion limitation, and reduced inspired oxygen tension. Hypoventilation and reduced inspired oxygen tension will lead to reduced PAO₂ , which results in hypoxemia. In the case of hypoventilation, a concurrent rise in PaCO₂ will also be noticed. Increasing ventilation and FiO₂ will improve oxygenation in these cases. 

V/Q mismatch refers to an imbalance between alveolar ventilation and alveolar perfusion. A certain degree of V/Q mismatch exists in normal lungs with V/Q being higher in the apex of the lungs compared to the bases. In pulmonary disease states, the degree of V/Q mismatch will worsen resulting in hypoxemia. In cases of V/Q mismatch, worsening of Aa gradient will be noticed. Right to left shunt results from poorly oxygenated blood passing from the right to left side of the heart without being oxygenated, leading to hypoxemia. Two kinds of right to left shunt exist: anatomic and physiologic. Anatomic shunt refers to when the alveoli are bypassed as in cases of intracardiac shunt or AV malformations. Physiologic shunt refers to when nonventilated alveoli are perfused as in cases of atelectasis.

Diffusion limitation exists when there is a destruction of lung parenchymal tissue, which impairs the movement of oxygen from alveoli to pulmonary capillary. Often times in cases of lung parenchymal disease, V/Q mismatch and diffusion limitation coexists and the exact cause of hypoxemia can be difficult to distinguish.

References:

1. Rodríguez-Roisin R, Roca J. Mechanisms of hypoxemia. Intensive Care Med. 2005;31:1017.
2. Williams AJ. ABC of oxygen: assessing and interpreting arterial blood gases and acid-base balance. BMJ. 1998;317:1213.