![]() ![]() Q > V A (wasted perfusion) dead space alveolar. The ratio of physiologic dead space to tidal volume is usually about 1/3. Derivation of the Alveolar P02-PCO2 Diagram shunt alveolar unit: V A/Q 0, PAO2 40 mm Hg, PACO2 46 mm Hg. Depending on the disease condition, additional mechanisms that can contribute to an elevated physiological dead space measurement include shunt, a substantial increase in overall V'A/Q' ratio, diffusion impairment, and ventilation delivered to unperfused alveolar spaces. Shunt: blood passes from the right side of the heart to the left side of the heart without participating in gas exchange. 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. Dead space: the volume of gas that moves in and out of the lungs without taking part in gas exchange wasted ventilation. 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. When a tidal volume of 6 mL/kg predicted bodyweight is set in patients with COVID-19-associated acute respiratory distress syndrome (ARDS), a respiratory rate of around 25 breaths per min or above should be used to reach a minute ventilation of 150 mL/min per kg predicted bodyweight, to allow adequate CO 2 removal. ![]() Physiologic or total dead space is the sum of anatomic dead space and alveolar dead space. Reduced alveolar ventilation increases PCO2 and lowers PO2. ![]() Dead space is the volume of a breath that does not participate in gas exchange. Patients with a large dead space and VA /Q -mismatch develop hypercapnia, due to hypoventilation. ![]()
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