Sunday, October 16, 2016

Study group discussion: Dead space

What is dead space?

Physiologic or total dead space is the sum of anatomic dead space and alveolar dead space. Anatomic dead space is the volume of gas within the conducting zone (as opposed to the transitional and respiratory zones) and includes the trachea, bronchus, bronchioles, and terminal bronchioles; it is approximately 2 mL/kg in the upright position. Alveolar dead space is the volume of gas within unperfused alveoli (and thus not participating in gas exchange either); it is usually negligible in the healthy, awake patient.

Flexion of the head decreases dead space. Why?

Flexion of head decreases anatomical dead space. Therefore, physiological dead space will also be decreased.

Neck extension and jaw protrusion can increase the dead space twofold.

Supine position decreases dead space and the dead space increases in upright position. Why?

In upright position, there is decreased perfusion to the uppermost alveoli.

Intubation decreases dead space by 70 ml approx. Why?

The size of the ET tube is smaller than the trachea. Therefore, reduction in the dead space.

Administration of bronchodilator increases dead space. Why?

The conduction zone, from the nose to the respiratory bronchioles, is dead space. Bronchodilators dilate the brochus and bronchioles and not the alveoli, increasing dead space.

Certain anaesthetics, like halothane and sevoflurane, cause bronchodilation. Hence, an important concept and MCQ.

The cause of increased dead space in general anesthesia is multifactorial, including loss of skeletal muscle tone and loss of bronchoconstrictor tone.

That's all!

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