Respiratory function: Dead space

Dead space is the volume of a breath that does not participate in gas exchange. It is ventilation without perfusion. 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. The ratio of physiologic dead space to tidal volume is usually about 1/3.

Factors that increase dead space:

• General anesthesia – multifactorial, including loss of skeletal muscle tone and bronchoconstrictor tone
• Anesthesia apparatus/circuit
• Artificial airway
• Neck extension and jaw protrusion (can increase it twofold)
• Positive pressure ventilation (i.e. increased airway pressure)
• Upright posture as opposed to supine (because of decreased perfusion to the uppermost alveoli)
• Pulmonary embolus, PA thrombosis, hemorrhage, hypotension, surgical manipulation of pulmonary artery tree – anything that decreases perfusion to well-ventilated alveoli
• Emphysema (blebs, loss of alveolar septa and vasculature)
• Age
• Anticholinergic drugs