Anatomic Dead Space Affects Hypoventilation

Understanding anatomic dead space is important to recognizing subtle hypoventilation. Hypoventilation from sedation, pain medications, anesthesia in the immediate postoperative period is common. The most obvious sign is slowing of the rate of breathing. A more subtle sign is that tidal volume becomes shallower. Having a tidal volume close to, or smaller than the patient’s dead space can lead to significant hypercarbia, hypoxia, and respiratory failure. This article discusses the concept of dead space and it’s clinical use in recognizing hypoventilation and preventing hypoxia and hypercarbia. Continue reading

Ventilation Perfusion Mismatch

Alveolar gas exchange depends not only on ventilation of the alveoli but also on circulation of blood through the alveolar capillaries. In other words it depends both on ventilation and perfusion. This makes sense. You need both oxygen in the alveoli, and adequate blood flow past alveoli to pick up oxygen, other wise oxygen cannot be delivered. When the proper balance is lost between ventilated alveoli and good blood flow through the lungs, ventilation perfusion mismatch is said to exist.

The ventilation/perfusion ratio is often abbreviated V/Q. V/Q mismatch is common and often effects our patient’s ventilation and oxygenation. There are 2 types of mismatch: dead space and shunt.

Imbalance between perfusion and ventilation is called ventilation perfusion mismatch. This illustration compares shunt, the perfusion of poorly ventilated alveoli; and Physiologic dead space: the ventilation of poor perfused alveoli.

Shunt is perfusion of poorly ventilated alveoli. Physiologic dead space is ventilation of poor perfused alveoli.

This article will describe how dead space is different from shunt. It will help you understand how you can use these concepts to care for your patient. Continue reading