Organ-Based Basic & Clinical Sciences, Respiratory System, Dead Space

Question:

A 49 y/o with multiple medical problems is undergoing hysterectomy, oophorectomy and tumor debulking under general endotracheal anesthesia with arterial line.  After noting a sudden decrease in end-tidal CO₂ (PECO₂) to 30 mmHg, arterial blood gas analysis shows PaCO2 of 45 mmHg.  Which of the following represents dead space ventilation (V_D/V_T)?

1. 15
2. 33
3. 45
4. 66

Correct answer:

B

Key concepts:

The ratio of dead space (V_D) to total ventilation (V_T) can be calculated by a modification of the Bohr equation:

V_D/V_T = (PaCO₂ – PECO₂)/PaCO₂

VD/VT = (45 mmHg – 30 mmHg)/45 mmHg

= 15 mmHg/45 mmHg

= 0.33

Dead space concepts:

1. Dead space is the part of the respiratory system in which there is no gas exchange. This includes anatomic dead space (upper airways through terminal bronchioles) and alveolar dead space (those alveoli in which gas exchange does not occur, often due to lack of perfusion).  Anatomic + alveolar dead space = physiologic dead space.

2. In anesthetized patients, airway devices and the breathing circuit add dead space. Dead space in the breathing circuit includes those parts of the breathing circuit in which bidirectional gas flow occurs—the part between the Y-piece and the patient.

Devices such as the endotracheal tube, face mask, LMA, heat & moisture exchanger (HME), elbows, and extensions between the Y-piece and the patient add dead space.  The photo above, from resource #1 below, shows dead space volume of various devices used in pediatric breathing circuits.

Use of an adult-sized device such as HME in a small pediatric patient will lead to hypercarbia unless accompanied by large increase in minute volume ventilation.

Inhalational anesthesia in itself increases dead space due to bronchodilation.

3. Normal healthy patients have minimal alveolar dead space. Normal anatomic dead space is about 1/3 of the tidal volume in adults.

Dead space is increased by lung disease, such as emphysema, which decreases surface area for gas exchange due to alveolar destruction.  Other causes of increased dead space include under-perfused alveoli, as might occur with severe hypotension, excessive PEEP resulting in barotrauma, or PE.

In early ARDS, higher physiologic dead space (V_D/V_T) is associated with higher mortality.

Resources:

1. Pearsall MF, Feldman JM. When does apparatus dead space matter for the pediatric patient? Anesth Analg 2014;118(4):776–80.
2. Quinn M, St. Lucia K, Rizzo A. Anatomy, anatomic dead space.  StatPearls (Internet)

https://www.ncbi.nlm.nih.gov/books/NBK442016/

3. Intagliata S, Rizzo A, Gossman WG. Physiology, lung dead space. StatPearls (Internet)

https://www-ncbi-nlm-nih-gov.proxy.lib.mcw.edu/books/NBK482501/

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