V D/V T is known to correlate with the severity of lung injury, 8– 12 can be useful as an indicator of lung recruitment versus overdistention in patients with acute lung injury (ALI) and ARDS, 13– 17 may be helpful as a predictor of successful extubation in pediatric 18 and adult patients, 19 and may be useful in diagnosing and assessing the severity of pulmonary embolism. 3 The prognostic value of V D/V T has been linked to mortality risk in ARDS 4– 7 and to other important clinical indices. All conducting airways (anatomical and mechanical dead space), areas of pure shunt (pulmonary capillary perfusion with no ventilation), areas of pure dead space, and the presence of gas exchange units with any degree of inequality of ventilation in relation to perfusion, can contribute to the calculated dead-space ventilation.Īssessing V D/V T in critically ill patients during mechanical ventilation is important for several reasons. The definition of pure dead space is ventilation without perfusion, whereby alveolar gases do not contact blood flowing through the pulmonary capillaries. 2 Physiologic dead-space fraction (dead space divided by tidal volume ), as defined by Bohr and Enghoff, is the sum of anatomic or airway dead space (V D-anat) and alveolar dead space (V D-alv) divided by the V T. Positive pressure ventilation (i.e.Dead-space ventilation, the portion of a tidal volume that does not contribute to gas exchange, was first described and calculated by the Bohr equation in 1891, 1 and later by the Enghoff modification of the Bohr equation in 1938.Neck extension and jaw protrusion (can increase it twofold).General anesthesia – multifactorial, including loss of skeletal muscle tone and bronchoconstrictor tone.

The ratio of physiologic dead space to tidal volume is usually about 1/3. 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.

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. Physiologic or total dead space is the sum of anatomic dead space and alveolar dead space. Dead space is the volume of a breath that does not participate in gas exchange.

Physiological dead space equation